Processing fruits: science and technology [2 ed.] 084931478X, 9780849314780, 9781420040074

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Processing Fruits

S e c o n d E d i t i o n

S c i e n c e a n d Te c h n o l o g y

Processing Fruits

S e c o n d E d i t i o n

S c i e n c e a n d Te c h n o l o g y E d i t e d

b y

Diane M. Barrett Laszlo Somogyi Hosahalli Ramaswamy

CRC PR E S S Boca Raton London New York Washington, D.C.

Library of Congress Cataloging-in-Publication Data Processing fruits.—2nd ed. / edited by Diane M. Barrett, Laszlo Somogyi, Hosahalli Ramaswamy. p. cm. Includes bibliographical references and index. ISBN 0-8493-1478-X (alk. paper) 1. Fruit—Processing. I. Barrett, Diane M. II. Somogyi, Laszlo P. III. Ramaswamy, Hosahalli S. TP440.P77 2004 664'.8—dc22

2004049679

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or internal use of specific clients, may be granted by CRC Press LLC, provided that $1.50 per page photocopied is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. The fee code for users of the Transactional Reporting Service is ISBN 0-8493-1478-X/05/$0.00+$1.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe.

Visit the CRC Press Web site at www.crcpress.com © 2005 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-8493-1478-X Library of Congress Card Number 2004049679 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

Preface Fruits are botanically similar plant organs in that they are all composed of seeds surrounded by a juicy, colorful, and aromatic ovary which we humans consume as food. Some things we traditionally consider to be “vegetables”, such as tomatoes, cucumbers, corn, and squash, are actually fruits from the botanical point of view. However, fruits vary widely in their shape, size, color, texture, flavor, nutritional properties, potential for extended shelf-life, and ability to withstand different types of processing. While fruits are delicious, nutritious and therefore desirable components of our diet, they suffer from being extremely perishable. For this reason, it is often advantageous to preserve them for longer shelf-life and easier transport to locations distant to the site of production. Processing also transforms the raw material into new, and perhaps improved, product. This book endeavors to serve as a single source of information about the biology of fruit and a description of the various methods used to preserve fruit. The book covers both traditional methods of preservation, such as canning, freezing, and drying, and looks to the future of novel processes such as high pressure, pulsed electric fields, and ohmic processing. The second half of the book focuses on the major processed fruit products and describes the diverse methods that may be used to preserve them. This book is the second edition of a two-volume book series published in 1996. The first edition separated the biology, principles, and applications into volume one and the major processed products in volume two. When we were asked to consider the editorship of a second edition, we readily agreed because it was clear that an even more detailed and current publication could result given new research and technology. In this second edition, we have introduced new technologies that appear to show promise for the preservation of fruit, such as the production of fresh-cut fruit. There are two brand new chapters addressing these topics. In many cases, we have gone back to the first edition authors and asked for their assistance in updating their chapters. We have added new authors to some chapters, in order to broaden the scope of what was written previously. For this second edition, we decided to put both of the original volumes into one complete package, allowing the reader to consult one succinct resource for anything he or she wants to know about fruit processing. This book is unique in comparison to others currently available because it covers a greater breadth of topics, and includes detailed descriptions of the processing of over 20 different major fruits. No other book published in the last 25 years (with the exception perhaps of the first edition of this series) comes close to providing the same degree of information on methods for the preservation of fruit. We begin the book with a description of different fruit classes, and the principles of preserving fruits in their most fresh-like state, that are stored as whole fruit (under either refrigerated or controlled atmosphere conditions) or lightly processed as fresh-cut fruit. Then the chapters progress through methods that involve increasing degrees of process severity. For example, the production of fruit juice involves mild pasteurization treatments in combination with refrigerated storage, and drying may be carried out in the sun. Freezing typically involves a mild blanching step, while ionizing radiation and canning involve treatments that are relatively more intense. A new chapter on novel processing technologies has been added. The last chapters in Part I of the book describe other topics relevant to the preservation of fruit, e.g., microbiology, food additives, quality assurance, packaging, grades and standards, and residue management. Part II covers more than 22 major processed fruits, including apples, peaches and apricots, sweet and sour cherries, plums, prunes, strawberries, raspberries, cranberries, grapes, oranges and tangerines, grapefruit, lemons and limes, bananas, tropical fruit, coconut, avocadoes, olives, and nuts.

This book will fill a critical need for students and professors, industry personnel, government agents and others working on the preservation of fruit. It is intended as a resource for both individuals familiar with fruit processing who want to expand their horizons, and as an introduction to the diversity of preservation methods for the inexperienced individual. It has been a joy, and a continuing learning experience, for us to be involved in its creation. Diane M. Barrett Laszlo P. Somogyi Hosahalli Ramaswamy

Acknowledgments This book is dedicated to the students, scientists, and food processing industry professionals who strive to produce attractive, nutritious processed fruit products for the international consumer. Diane Barrett is indebted to Dr. Laszlo Somogyi for his initiation of this project, and to her husband Pieter and daughter Jodie for their continuing love, support, and understanding.

Editors Diane M. Barrett graduated with a B.S. in Food Science & Technology from the University of California, Davis, where she is currently on faculty. She received her M.S. in Food Chemistry from the University of Wisconsin, Madison and her Ph.D. in Food Biochemistry from Cornell University, Ithaca, NY. After acquiring her M.S., Dr. Barrett spent four years doing food science research and education in Indonesia as a consultant with the World Bank and U.S. Agency for International Development. At U.C. Davis, Dr. Barrett conducts extension courses for the fruit and vegetable processing industry and carries out applied research. Extension courses include the Better Process Control School, Freezing Technology Workshop, Juice Processing Course, Tomato Processing School, Fresh-Cut Products Workshop and Aseptic Processing & Packaging Workshop. Her research focuses on the effects of raw materials and processing conditions on the quality of fruit and vegetable products. She is particularly interested in the relationship between endogenous enzymes and the color, texture, flavor, and nutritional quality of fruits and vegetables. When she is not harvesting tomatoes or processing fruit, Dr. Barrett enjoys swimming, kayaking, and traveling with her family. Laszlo P. Somogyi specializes in the technical, regulatory, and nutritional aspects of food processing operations, techno-economic studies of food ingredients, and post-harvest handling of fruits. During his 40 years experience in working for and consultation to food industry firms he worked at HuntWesson Foods, Vacu-Dry Company, Finn-Cal Products, and SRI International. Dr. Somogyi has published over 40 technical papers and has contributed to a number of textbooks on subjects of food ingredients, additives, food and beverage processing technologies, fruit quality, and food irradiation, and has been the lead author for SRI’s Chemical Economics Handbook and Specialty Chemical Economics Update, multiclient reports dealing with food additives and flavors and fragrances. Since his retirement from SRI in 1998 he has been participating in U.S. Agency for International Development programs in Egypt and Lebanon and consulting for several food ingredient manufacturers. Dr. Somogyi received a B.S. degree from the University of Agricultural Sciences in Budapest, Hungary, holds M.S. and Ph.D. degrees from Rutgers University; and was a post-doctoral fellow at the University of California, Davis. He was elected as Fellow of the Institute of Food Technologists, and is a member of the American Association of Cereal Chemists, and the American Oil Chemist’s Society. Dr. Hosahalli Ramaswamy is a Professor in the Department of Food Science at McGill University in Montreal, Canada with teaching and research responsibilities in the areas of Food Processing and Post Harvest Technology. He obtained his M.Sc. and Ph.D. in Food Science from the University of British Columbia, Vancouver. Dr. Ramaswamy’s primary research area is thermal and nonthermal processing. In his research activities, he has explored the use of conventional thermal processing as well as thin profile, rotational, microwave, RF, ohmic and aseptic processing. In the area of non-thermal processing, his research focus has been application of high pressure processing for food systems. He has also carried out extensive research in the area of computer modeling, rheology and use of artificial neural networks for process characterization and optimization. Dr. Ramaswamy is a professional member of several organizations such as IFT, CIFST, IFTPS, ASAE and CSAE. He has served as Chair of the Heat Penetration Committee of the Institute for Thermal Processing Specialists, Chair of the Radiation Committee of ASAE, and has been serving as an

associate editor of Transactions of ASAE (ASAE) and Journal of Food Science. He is also on the editorial board of Food Research International, Food Science and Technology Journal, and Journal of Food Process Engineering. He has published over 175 refereed scientific papers and supervised over 30 graduate students. He has received the 1999 W.J. Eva Award by the Canadian Institute of Food Science and Technology, the 2002 John Clark Award of the Canadian Society of Agricultural Engineering, and is a 2002 Fellow of the Association of Food Scientists and Technologists (India).

Contributors P. Alvo (Deceased) McGill University St. Anne Bellevue, Quebec, Canada

Yvan Gariépy McGill University St. Anne de Bellevue, Quebec, Canada

Robert A. Baker USDA-ARS Winter Haven, Florida

Albrecht Höhn AB Enzymes GmbH Darmstadt, Germany

Diane M. Barrett University of California Davis, California

Y.H. Hui American Food & Nutrition Center West Sacramento, California

Norman Berry AVP Baker Ltd. Crawley, West Sussex, U.K.

Charles Huxsoll USDA-WRRC Albany, California

Robert Braddock University of Florida Lake Alfred, Florida

Adel A. Kader University of California Davis, California

Cuiren Chen McGill University–MacDonald Campus St. Anne de Bellevue, Quebec, Canada

Dan Kimball Kimball Consulting Lindsay, California

Sonia Y. de Leon University of the Philippines Quezon City, Philippines

Azriel Kurlaender Rak Consulting Ventura, California

Louise Deschênes Agriculture & Agri-Food Canada St. Hyacinthe, Quebec, Canada Charlotte L. Deuel Medical Ambassadors International Modesto, California

Bor S. Luh Formerly of the University of California Davis, California Jatal D. Mannapperuma University of California Davis, California

Milagros I. Dolores Quezon City, Philippines

Michele Marcotte Agriculture and Agri-Food Canada St. Hyacinthe, Quebec, Canada

Elisabeth Garcia University of California Davis, California

N.R. Markarian CRDH, Agriculture & Agri-Food Canada St. Jean-Sur-Richelieu, Quebec, Canada

Marvin H. Martin Madera, California Mark R. McLellan Texas A&M University College Station, Texas Justin R. Morris University of Arkansas Fayetteville, Arkansas Arun S. Mujumdar National University of Singapore Singapore Brendan A. Niemira USDA-ARS Wyndmoor, Pennsylvania Francois Nolle Valley Research, Inc. South Bend, Indiana Olga I. Padilla-Zakour Cornell University Geneva, New York Mickey E. Parish University of Florida Lake Alfred, Florida Thomas J. Payne San Mateo, California

Hosahalli S. Ramaswamy McGill University St. Anne de Bellevue, Quebec, Canada Cristina Ratti Laval University St. Foy, Quebec, Canada David S. Reid University of California–Davis Davis, California William H. Root Pacific International Technology Walnut Creek, California Ralph Scorza USDA-ARS Kearneysville, West Virginia James P. Smith McGill University St. Anne de Bellevue, Quebec, Canada Pedro Solé Management Consultant Hayward, California Laszlo P. Somogyi Kensington, California Don F. Splittstoesser (Deceased) Formerly of Cornell University Geneva, New York

Anne D. Perera Crop & Food Research Institute of New Zealand Palmerston North, New Zealand

Ken Stewart GusmerCellulo Hillsboro, Oregon

Conrad O. Perera National University of Singapore Singapore

R. Keith Striegler University of Arkansas Fayetteville, Arkansas

Anne Plotto USDA-ARS Winter Haven, Florida

Daqing Sun Valley Research Inc. South Bend, Indiana

G.S.V. Raghavan McGill University St. Anne de Bellevue, Quebec, Canada

Clement Vigneault CRDH, Agriculture & Agri-Food Canada St. Jean-Sur-Richelieu, Quebec, Canada

Yu-Ping Wei Chung-Hua University Hsinchu, Taiwan

Ming-Chang Wu National Pingtung University Pingtung, Taiwan

Randy W. Worobo Cornell University Geneva, New York

Devon Zagory Davis Fresh Technologies, LLC Davis, California

James Swi-Bea Wu National Taiwan University Taipei, Taiwan

Contents PART I Biology, Principles, and Applications Chapter 1

Classification, Composition of Fruits, and Postharvest Maintenance of Quality .......3 Adel A. Kader and Diane M. Barrett

Chapter 2

Refrigerated and Controlled/Modified Atmosphere Storage .....................................23 G.S.V. Raghavan, Clement Vigneault, Yvan Gariépy, N.R. Markarian, and P. Alvo

Chapter 3

Fresh-Cut Fruits..........................................................................................................53 Elisabeth Garcia and Diane M. Barrett

Chapter 4

Juice Processing..........................................................................................................73 Mark R. McLellan and Olga I. Padilla-Zakour

Chapter 5

Enzymes in the Fruit Juice and Wine Industry .........................................................97 Alberecht Höhn, Daqing Sun, and Francois Nolle

Chapter 6

Fruit Preserves and Jams..........................................................................................113 Robert A. Baker, Norman Berry, Y.H. Hui, and Diane M. Barrett

Chapter 7

Drying of Fruits........................................................................................................127 Cristina Ratti and Arun S. Mujumdar

Chapter 8

Fruit Freezing ...........................................................................................................161 David S. Reid and Diane M. Barrett

Chapter 9

Thermal Processing of Fruits ...................................................................................173 Hosahalli S. Ramaswamy

Chapter 10 Novel Processing Technologies for Food Preservation ...........................................201 Hosahalli S. Ramaswamy, Cuiren Chen, and Michele Marcotte Chapter 11 Ionizing Radiation Processing of Fruits and Fruit Products ...................................221 Brendan A. Niemira and Louise Deschênes

Chapter 12 Microbiology of Fruit Products ...............................................................................261 Randy W. Worobo and Don F. Splittstoesser Chapter 13 Direct Food Additives in Fruit Processing...............................................................285 Laszlo P. Somogyi Chapter 14 Quality Assurance, Quality Control, Inspection, and Sanitation.............................339 Conrad O. Perera and Anne D. Perera Chapter 15 Packaging of Fruits and Vegetables .........................................................................355 James P. Smith, Devon Zagory, and Hosahalli S. Ramaswamy Chapter 16 Grades, Standards, and Food Labeling ....................................................................397 Y.H. Hui and Charles Huxsoll Chapter 17 Residual Management in Fruit Processing Plants ...................................................423 Jatal D. Mannapperuma

PART II Major Processed Products Chapter 18 Apples and Apple Processing...................................................................................455 William H. Root and Diane M. Barrett Chapter 19 Peach and Apricot ....................................................................................................481 Ralph Scorza Chapter 20 Sweet Cherry and Sour Cherry Processing .............................................................497 Mark R. McLellan and Olga I. Padilla-Zakour Chapter 21 Plums and Prunes .....................................................................................................513 Laszlo P. Somogyi Chapter 22 Strawberries and Raspberries ...................................................................................531 Charlotte L. Deuel and Anne Plotto Chapter 23 Processing of Cranberry, Blueberry, Currant, and Gooseberry ...............................563 Ken Stewart

Chapter 24 Grape Juice: Factors That Influence Quality, Processing Technology, and Economics..........................................................................................................585 Justin R. Morris and R. Keith Striegler Chapter 25 Oranges and Tangerines ...........................................................................................617 Dan Kimball, Mickey E. Parish, and Robert Braddock Chapter 26 Grapefruits, Lemons, and Limes ..............................................................................639 Dan Kimball, Robert Braddock, and Mickey Parish Chapter 27 Bananas (Processed).................................................................................................657 Pedro Solé Chapter 28 Tropical Fruits ..........................................................................................................679 James Swi-Bea Wu, Ming-Chang Wu, and Yu-Ping Wei Chapter 29 Coconut.....................................................................................................................707 Sonia Y. de Leon and Milagros I. Delores Chapter 30 Avocados...................................................................................................................739 Azriel Kurlaender Chapter 31 Olives ........................................................................................................................751 Elisabeth L. Garcia, Bor S. Luh, and Marvin H. Martin Chapter 32 Nuts...........................................................................................................................765 Thomas J. Payne Index ..............................................................................................................................................811

Part I Biology, Principles, and Applications

Composition of 1 Classification, Fruits, and Postharvest Maintenance of Quality Adel A. Kader and Diane M. Barrett CONTENTS 1.1

1.2

1.3

1.4 1.5

1.6

Classification of Fruits ...........................................................................................................5 1.1.1 Temperate-Zone Fruits.............................................................................................5 1.1.2 Subtropical Fruits ....................................................................................................5 1.1.3 Tropical Fruits..........................................................................................................5 Contribution of Fruits to Human Nutrition ...........................................................................5 1.2.1 Energy (Calories).....................................................................................................5 1.2.2 Vitamins ...................................................................................................................5 1.2.3 Minerals ...................................................................................................................6 1.2.4 Dietary Fiber............................................................................................................6 1.2.5 Antioxidants.............................................................................................................6 Factors Influencing Composition and Quality of Fruits........................................................6 1.3.1 Preharvest Factors....................................................................................................6 1.3.2 Maturity at Harvest and Harvesting Method ..........................................................7 1.3.3 Postharvest Factors ..................................................................................................7 Fruit Maturity, Ripening, and Quality Relationships ............................................................7 Composition and Compositional Changes ............................................................................8 1.5.1 Carbohydrates ..........................................................................................................8 1.5.2 Proteins ....................................................................................................................9 1.5.3 Lipids .......................................................................................................................9 1.5.4 Organic Acids ..........................................................................................................9 1.5.5 Pigments.................................................................................................................10 1.5.6 Phenolic Compounds .............................................................................................10 1.5.7 Volatiles..................................................................................................................12 1.5.8 Vitamins .................................................................................................................12 1.5.9 Minerals .................................................................................................................12 Biological Factors Involved in Postharvest Deterioration of Fruits ...................................13 1.6.1 Respiration .............................................................................................................13 1.6.2 Ethylene Production...............................................................................................13 1.6.3 Transpiration or Water Loss ..................................................................................14 1.6.4 Physiological Disorders .........................................................................................14 1.6.5 Physical Damage ...................................................................................................15 1.6.6 Pathological Breakdown ........................................................................................15

0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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1.7

Environmental Factors Influencing Deterioration of Fruits ................................................16 1.7.1 Temperature ...........................................................................................................16 1.7.2 Relative Humidity..................................................................................................16 1.7.3 Air Movement........................................................................................................16 1.7.4 Atmospheric Composition .....................................................................................16 1.7.5 Ethylene .................................................................................................................16 1.8 Harvesting Procedures .........................................................................................................17 1.9 Postharvest Handling Procedures ........................................................................................17 1.9.1 Dumping ................................................................................................................17 1.9.2 Washing..................................................................................................................17 1.9.3 Sorting....................................................................................................................17 1.9.4 Sizing .....................................................................................................................17 1.9.5 Ripening.................................................................................................................18 1.9.6 Inhibiting Ethylene Action ....................................................................................18 1.9.7 Cooling...................................................................................................................18 1.9.8 Storage ...................................................................................................................18 1.9.9 Food Safety Guidelines .........................................................................................19 1.9.10 Food Security Guidelines ......................................................................................19 1.10 Summary: Keys to Successful Handling of Fresh Fruits....................................................19 1.10.1 Maturity and Quality .............................................................................................19 1.10.2 Temperature and Humidity Management Procedures...........................................19 1.10.3 Physical Damage ...................................................................................................20 1.10.4 Sanitation Procedures ............................................................................................20 1.10.5 Expedited Handling ...............................................................................................20 References ........................................................................................................................................20 Internet Sites ....................................................................................................................................21

The quality of processed fruit products depends on their quality at the start of processing; therefore, it is essential to understand how maturity at harvest, harvesting methods, and postharvest handling procedures influence quality and its maintenance in fresh fruits between harvest and process initiation. Using such information, an appropriate system for harvesting and handling each kind of fruit can be selected and used in conjunction with an effective quality control program to ensure the best quality possible for fresh fruits when processed. Quality attributes of fresh fruits include appearance, texture, flavor, and nutritive value. Appearance factors include size, shape, color, and freedom from defects and decay. Texture factors include firmness, crispness, and juiciness. Flavor components incorporate sweetness, sourness (acidity), astringency, bitterness, aroma, and off-flavors. Nutritional quality is determined by a fruit’s content of vitamins (A and C are the most important in fruits), minerals, dietary fiber, carbohydrates, proteins, and antioxidant phytochemicals (carotenoids, flavonoids, and other phenolic compounds). Safety factors that may influence the quality of fresh fruits include residues of pesticides, presence of heavy metals, mycotoxins produced by certain species of fungi, and microbial contamination. Losses in fresh fruits between harvest and processing may be quantitative (e.g., water loss, physical injuries, physiological breakdown, and decay) or qualitative (e.g., loss of acidity, flavor, color, and nutritive value). Many factors influence fruit quality and the extent of postharvest losses that can occur in the orchard, during transportation, and throughout the handling system (sorting, sizing, ripening, cooling, and storage). The total time between harvesting and processing may also be an important factor in maintaining the quality and freshness of fruit. Minimizing the delays throughout the postharvest handling system greatly reduces quality loss, especially in highly perishable fruits such as strawberries, raspberries, blackberries, apricots, and cherries.

Classification, Composition of Fruits, and Postharvest Maintenance of Quality

5

1.1 CLASSIFICATION OF FRUITS Fruit are commonly classified by growing region as follows: temperate-zone, subtropical, and tropical. Growing region and environmental conditions specific to each region significantly affect fruit quality. Examples of fruit grown in each region are listed below.

1.1.1 TEMPERATE-ZONE FRUITS 1. Pome fruits: apple, Asian pear (nashi), European pear, quince 2. Stone fruits: apricot, cherry, nectarine, peach, plum 3. Small fruits and berries: grape (European and American types), strawberry, raspberry, blueberry, blackberry, cranberry

1.1.2 SUBTROPICAL FRUITS 1. Citrus fruits: grapefruit, lemon, lime, orange, pummelo, tangerine, and mandarin 2. Noncitrus fruits: avocado, cherimoya, fig, kiwifruit, olive, pomegranate

1.1.3 TROPICAL FRUITS 1. Major tropical fruits: banana, mango, papaya, pineapple 2. Minor tropical fruits: carambola, cashew apple, durian, guava, longan, lychee, mangosteen, passion fruit, rambutan, sapota, tamarind

1.2 CONTRIBUTION OF FRUITS TO HUMAN NUTRITION Fruits are not only colorful and flavorful components of our diet, but they also serve as a source of energy, vitamins, minerals, and dietary fiber. The U.S. Department of Agriculture Dietary Guidelines encourage consumers to enjoy “five a day,” i.e., eat at least two servings of fruit and three servings of vegetables each day and to choose fresh, frozen, dried, or canned forms of a variety of colors and kinds of fruits and vegetables. In some countries, consumers are encouraged to eat up to 10 servings of fruits and vegetables per day. For more information access one or more of the following Websites: www.nutrition.gov, www.5aday.gov, and www.5aday.org.

1.2.1 ENERGY (CALORIES) 1. Carbohydrates: banana, breadfruit, jackfruit, plantain, dates, prunes, raisin 2. Proteins and amino acids: nuts, dried apricot, fig 3. Fats: avocado, olive, nuts

1.2.2 VITAMINS 1. Fresh fruits and vegetables contribute about 91% of vitamin C, 48% of vitamin A, 27% of vitamin B6, 17% of thiamin, and 15% of niacin to the U.S. diet. 2. The following fruits are important contributors (based on their vitamin content and the amount consumed) to the supply of indicated vitamins in the U.S. diet: Vitamin A: apricot, peach, cherry, orange, mango, papaya, persimmon, pineapple, cantaloupe, watermelon Vitamin C: strawberry, orange, grapefruit, kiwifruit, pineapple, banana, apple, cantaloupe Niacin: peach, banana, orange, apricot Riboflavin: banana, peach, orange, apple, avocado Thiamin: orange, banana, grapefruit, apple

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Processing Fruits: Science and Technology, Second Edition

1.2.3 MINERALS 1. Fresh fruits and vegetables contribute about 26% of the magnesium and 19% of the iron to the U.S. diet. 2. The following fruits are important contributors to the supply of indicated minerals in the U.S. diet: Potassium: banana, peach, orange, apple, dried fruits such as apricot and prune Phosphorus: banana, orange, peach, fig, raisin Calcium: tangerine, grapefruit, orange Iron: strawberry, banana, apple, orange

1.2.4 DIETARY FIBER 1. All fruits and nuts contribute to dietary fiber. Dietary fiber consists of cellulose, hemicellulose, lignin, and pectic substances, which are derived primarily from fruit cell walls and skin. 2. The dietary fiber content of fruits ranges from 0.5 to 1.5% (fresh weight basis). 3. Dietary fiber plays an important role in relieving constipation by increasing water-holding capacity of feces. Its consumption is also linked to decreased incidence of cardiovascular disease, diverticulosis, and colon cancer.

1.2.5 ANTIOXIDANTS 1. Fruits, nuts, and vegetables in the daily diet have been strongly associated with reduced risk for some forms of cancer, heart disease, stroke, and other chronic diseases. This is attributed, in part, to their content of antioxidant phytochemicals. 2. Red, blue, and purple fruits (such as apple, blackberry, blueberry, blood orange, cranberry, grape, nectarine, peach, plum, prune, pomegranate, raspberry, and strawberry) are good sources of flavonoids and other phenolic compounds that are positively correlated with antioxidant capacity of the fruit. 3. Orange-flesh fruits (such as apricot, cantaloupe, mango, nectarine, orange, papaya, peach, persimmon, and pineapple) and some red-flesh fruits (such as tomato, watermelon, and pink grapefruit) are good sources of carotenoids. Availability of lycopene to humans is increased during tomato processing.

1.3 FACTORS INFLUENCING COMPOSITION AND QUALITY OF FRUITS 1.3.1 PREHARVEST FACTORS 1. Genetic: selection of cultivars, rootstocks. Cultivar and rootstock selection are important because there are often differences in raw fruit composition, postharvest-life potential, and response to processing. In many cases, fruit cultivars grown for fresh market sale are not optimal cultivars for processing. 2. Climatic: temperature, light, wind. Climatic factors may have a strong influence on nutritional quality of fruits. Light intensity significantly affects vitamin concentration, and temperature influences transpiration rate, which will affect mineral uptake and metabolism. 3. Cultural practices: soil type, soil nutrient and water supply, pruning, thinning, pest control. Fertilizer addition may significantly affect the mineral content of fruit, while other cultural practices such as pruning and thinning may influence nutritional composition by changing fruit crop load and size.

Classification, Composition of Fruits, and Postharvest Maintenance of Quality

1.3.2 MATURITY

AT

HARVEST

AND

7

HARVESTING METHOD

Maturity at harvest is one of the primary factors affecting fruit composition, quality, and storage life. Although most fruits reach peak eating quality when harvested fully ripe, they are usually picked mature, but not ripe to decrease mechanical damage during postharvest handling. Harvesting may also mechanically damage fruit; therefore, choice of harvest method should allow for maintenance of quality.

1.3.3 POSTHARVEST FACTORS 1. Environmental: temperature, relative humidity, atmospheric composition. Temperature management is the most important tool for extension of shelf life and maintenance of the quality of fresh fruit. Relative humidity influences water loss, decay development, incidence of some physiological disorders, and uniformity of fruit ripening. Optimal relative humidity for storage of fruits is 85 to 90%. Finally, atmospheric composition (O2, CO2, and C2H4, in particular) can greatly affect respiration rate and storage life. 2. Handling methods: Postharvest handling systems involve the channels through which harvested fruit reaches the processing facility or consumer. Handling methods should be chosen such that they maintain fruit quality and avoid delays. 3. Time period between harvesting and consumption: Delays between harvesting and cooling or processing may result in direct losses (due to water loss and decay) and indirect losses (decrease in flavor and nutritional quality).

1.4 FRUIT MATURITY, RIPENING, AND QUALITY RELATIONSHIPS Maturity at harvest is the most important factor that determines storage life and final fruit quality. Immature fruits are more subject to shriveling and mechanical damage, and are of inferior quality when ripened. Overripe fruits are likely to become soft and mealy with insipid flavor soon after harvest. Fruits picked either too early or too late in the season are more susceptible to physiological disorders and have a shorter storage life than those picked at mid-season. With very few exceptions (e.g., pears, avocados, and bananas), all fruits reach their best eating quality when allowed to ripen on the tree or plant. In general, fruits become sweeter, more colorful, and softer as they mature. However, some fruits are usually picked mature but unripe so that they can withstand the postharvest handling system when shipped long distances. Most currently used maturity indices are based on a compromise between those indices that would ensure the best eating quality to the consumer and those that provide the needed flexibility in transportation and marketing. Fruits can be divided into two groups: (1) nonclimacteric fruits that are not capable of continuing their ripening process once removed from the plant, and (2) climacteric fruits that can be harvested mature and ripened off the plant. Following are examples of each group: Group 1: Berries (e.g., blackberry, cranberry, raspberry, strawberry), grape, cherry, citrus (grapefruit, lemon, lime, orange, mandarin, tangerine), pineapple, pomegranate, lychee, tamarillo, loquat. Group 2: Apples, pear, quince, persimmon, apricot, nectarine, peach, plum, kiwifruit, avocado, banana, plantain, mango, papaya, cherimoya, sapodilla, sapota, guava, passion fruit. Fruits of the first group (nonclimacteric) produce very small quantities of ethylene and do not respond to ethylene treatment except in terms of degreening (degradation of chlorophyll) in citrus fruits and pineapples. Fruits in Group 2 (climacteric) produce much larger quantities of ethylene in association with their ripening and exposure to ethylene treatment will result in faster and more uniform ripening.

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Maturity indices used vary among fruits and often among cultivars within a specific fruit, but generally include one or several (combination indices) of the following: fruit size and shape, overall color, ground color of the skin, flesh color, flesh firmness, soluble solids content, starch content, acidity, and internal ethylene concentration. Listed below are some examples of maturity indices that can be used for selected fruits: Index Elapsed days from full bloom to harvest Size Shape (fullness of fruit shoulders and suture) Dry Weight Color, external Color, internal Firmness Compositional factors Starch content Soluble solids content Acid content Sugar/acid ratio Tannin content

Fruits Apple, pear Most fruits Stone fruits Avocado, kiwifruit All fruits Stone fruits, mango Pome and stone fruits, berries Apple, pear Pome and stone fruits, grapes, kiwifruit Pomegranate Citrus fruits, grape Persimmon

1.5 COMPOSITION AND COMPOSITIONAL CHANGES The flesh of young developing fruits contains very little sugar, and the large amounts of starch, acid, and tannins make them inedible. As the fruits approach maturity, flesh cells enlarge considerably, and sugar content increases while starch, acid, and tannin contents decrease. In addition, certain volatile compounds develop, giving the fruit its characteristic aroma. Chlorophyll degradation (loss of green color) and synthesis of carotenoids (yellow and orange colors) and anthocyanins (red and blue colors) takes place both in the skin and the flesh with fruit ripening. All fruits soften as they ripen due to changes in cell wall composition and structure. In this section, we present an overview of fruit constituents in relation to quality and changes after harvest.

1.5.1 CARBOHYDRATES Carbohydrates are the most abundant and widely distributed food component derived from plants. Fresh fruits vary greatly in their carbohydrate content, with the general range between 10 to 25%. The structural framework, texture, taste, and food value of a fresh fruit is related to its carbohydrate content. Sucrose, glucose, and fructose are the primary sugars found in fruits (Table 1.1), and their relative importance varies among commodities. Sugars are found primarily in the cytoplasm and range from about 0.9% in limes to 16% in fresh figs. Sucrose content ranges from a trace in cherries, grapes, and pomegranate to more than 8% in ripe bananas and pineapple. Such variation influences taste since fructose is sweeter than sucrose, and sucrose is sweeter than glucose. Starch occurs as small granules within the cells of immature fruits. Starch is converted to sugar as the fruit matures and ripens. Other polysaccharides present in fruits include cellulose, hemicellulose, pectin, and lignin, which are found mainly (up to 50%) in cell walls and vary greatly among commodities. These large molecules are broken down into simpler and more soluble compounds as a result of fruit softening. The transformation of insoluble pectins into soluble pectins is controlled, for the most part, by the enzymes pectinesterase and polygalacturonase. Reduced activities of these two enzymes have been associated with reduced juiciness and poor texture in peaches that were ripened after storage at 1∞C for more than 3 weeks.

Classification, Composition of Fruits, and Postharvest Maintenance of Quality

9

TABLE 1.1 Sugar Composition of Selected Fruits Sugar (g/100 ml of Juice) Fruit Apple Cherry Grape Nectarine Peach Pear Plum Kiwifruit Strawberry

Sucrose 0.82 0.08 0.29 8.38 5.68 0.55 0.51 1.81 0.17

± ± ± ± ± ± ± ± ±

0.13 0.02 0.08 0.73 0.52 0.12 0.36 0.72 0.06

Glucose 2.14 7.50 9.59 0.85 0.67 1.68 4.28 6.94 1.80

± ± ± ± ± ± ± ± ±

0.43 0.81 1.03 0.04 0.06 0.36 1.18 2.85 0.16

Fructose 5.31 6.83 10.53 0.59 0.49 8.12 4.86 8.24 2.18

± ± ± ± ± ± ± ± ±

0.94 0.74 1.04 0.02 0.01 1.56 1.30 3.43 0.19

Sorbitol 0.20 ± 0.04 2.95 ± 0.33 ND 0.27 ± 0.04 0.09 ± 0.02 4.08 ± 0.79 6.29 ± 1.97 ND ND

Note: ND = not detected (less than 0.05 g/100 ml). Source: Van Gorsel, H., C. Li, E.L. Kerbel, M. Smits, and A.A. Kader. 1992. Compositional characterization of prune juice. J. Agric. Food Chem., 40: 784–789.

1.5.2 PROTEINS Fruits contain less than 1% protein (vs. 9 to 20% protein in nuts such as almond, pistachio, and walnut). Changes in the level and activity of proteins resulting from permeability changes in cell membranes may be involved in chilling injury. Enzymes, which catalyze metabolic processes in fruits, are proteins that are important in the reactions involved in fruit ripening and senescence. Some of the enzymes important to fruit quality include the following: Enzyme Polyphenoloxidase Polygalacturonase Pectinesterase Lipoxygenase Ascorbic acid oxidase Chlorophyllase

Action Catalyzes oxidation of phenolics, resulting in formation of brown polymers Catalyzes hydrolysis of glycosidic bonds between adjacent polygalacturonic acid residues in pectin; results in tissue softening Catalyzes deesterification of galacturonans in pectin; may result in tissue firming Catalyzes oxidation of lipids; results in off-odor and off-flavor production Catalyzes oxidation of ascorbic acid; results in loss of nutritional quality Catalyzes removal of phytol ring from chlorophyll; results in loss of green color

1.5.3 LIPIDS Lipids constitute only 0.1 to 0.2% of most fresh fruits, except for avocados, olives, and nuts. However, lipids are very important because they make up the surface wax that contributes to fruit appearance and the cuticle that protects the fruit against water loss and pathogens. Lipids are also important constituents of cell membranes. The degree of fatty acid saturation establishes membrane flexibility, with greater saturation resulting in less flexibility. Desaturation of fatty acids can occur upon chilling in chilling-sensitive fruits; in which case membranes undergo a phase change (liquid crystalline to solid gel) at chilling temperatures resulting in disruption of normal metabolism.

1.5.4 ORGANIC ACIDS Organic acids are important intermediate products of metabolism. The Krebs (TCA) cycle is the main channel for the oxidation of organic acids in living cells and it provides the energy required

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Processing Fruits: Science and Technology, Second Edition

TABLE 1.2 Organic Acids of Selected Fruits Organic Acid (mg/100 ml of Juice) Fruit

Citric

Ascorbic

Apple Cherry Grape Kiwifruit Nectarine Peach Pear Plum Strawberry

ND ND tr 730 ± 92 140 ± 39 109 ± 16 ND ND 207 ± 35

tr tr tr 114 ± 6 tr tr tr tr 56 ± 4

Malic 518 727 285 501 383 358 371 294 199

± ± ± ± ± ± ± ± ±

32 20 58 42 67 72 16 24 26

Quinic

Tartaric

ND ND ND 774 ± 57 136 ± 28 121 ± 11 220 ± 2 214 ± 68 ND

ND ND 162 ± 24 tr ND tr ND ND ND

Note: ND = not detected, tr = trace ( PO ) or ( + p CO2 / P, + p O2 / P, + p N2 / P when P < PO ) Subscript “o” indicates the initial conditions that are the ambient conditions outside the chamber.

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If the storage enclosure is perfectly airtight such that the only transfer of gas occurs through the membrane window, then the terms including L(P) may be dropped from the equations. In the limit, if the enclosure is not completely airtight, ambient conditions will prevail regardless of the characteristics of the membrane. Based on the pressure differential between the inside and the outside of the storage enclosure, the leakage rate L(P) can be expressed as: L( P ) = CA c ( Po - P ) n

(2.6)

where L(P) = Ac = C= n=

leakage rate (mol·h–1) wall and ceiling surface area of the storage chamber (m2) leakage flow coefficient (mol·h–1·m–2·kPa–n) leakage flow exponent

The main difficulty in solving this set of equations is that the respiratory functions RCO2 and RO2 are both functions of the gas composition, which evolves with time, and there is really no adequate description of these in a dynamic setting. 2.3.1.1 Design Objective: Approximating Steady State Mathematically, steady-state conditions in a leaking system are defined by Equations 2.3, 2.4, and 2.5 with the left-hand derivatives set at zero. The primary design objective is to achieve and maintain equilibrium at the optimal atmospheric composition for the commodity; however, experimental work has shown that temporal changes in produce metabolism, external pressure fluctuations, and possible changes in airtightness (deterioration of seals, etc.) result in trends and fluctuations in the time histories of gas composition and, consequently, in respiratory activity. The state of the system at equilibrium is, nevertheless, of interest for design purposes. One way to quantify the degree to which the equilibrium concentration of a gas is approximated at a given time is by defining a relative gas difference (RGD): È A( t ) - A av ˘ RGD = ln Í ˙ Î ( A o - A av ) ˚

(2.7)

where A(t) = instantaneous gas concentration at time t (kPa) Aav = average gas concentration (kPa) Ao = initial gas concentration (kPa) If the actual partial pressure of a gas is such that RGD is equal to or less than a preset value of RGD, the gas is said to have stabilized after the initial adjustment phase has passed. The parameters to be considered in design are: 1. 2. 3. 4. 5.

Product response to gas composition Design gas concentrations Membrane area Membrane permeability to the storage gases Leakage rate

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Refrigerated and Controlled/Modified Atmosphere Storage

Of these, the parameters that are considered fixed are the product response to O2 and CO2 and the stabilized (design) gas concentrations. These must be determined through experimentation since no universal models of respiration presently exist. The leakage rate is a function of the physical characteristics of the enclosure, but can be considered adjustable in the sense that vents could be installed in the storage enclosure. The subject of determining the size of perforations (minivents) in semipermeable film packages necessary to produce an effective permeability has been studied by Emond et al.16 An alternative technique is the manipulation of gas fluxes by active control of partial pressure gradients at the membrane interface. This is done by pressurizing the ambient air before allowing it to contact the exterior surface of the membrane, thereby altering the ratios of the partial pressure gradients of O2 and CO2. The membrane area used as a window to the external atmosphere is adjustable and calculation of the required area has been discussed by Gariépy et al.17 The major drawback has been that for a specific set of membrane permeabilities and respiratory responses by a commodity, one can design either for a specified O2 level or for a specified CO2 level but not necessarily for both. Mathematically, if the leakage rate is also fixed, the situation is one of three equations in one unknown. If the leakage rate is considered unknown, then there are three equations in two unknowns. In this case, the constraint that CO2* + O2* + N2* = 1 can be used to find a unique solution in terms of L(P) and Am; however, this implies the manipulation of L(P), which requires some form of active control. For completely passive establishment and maintenance of the desired atmospheric composition, it is important to consider critical levels of each gas relative to the commodity in question and to determine the relative degree of response of the commodity to each in order to choose a design gas concentration. Clearly, there must be a compromise between the degree of active control and the required precision. Since, in general, the response of respiratory activity to O2 levels is more significant if the corresponding attainable CO2 level is not in or near a critical range, calculation of membrane area should be based on stabilizing O2 levels. In the simplest case of perfect airtightness and total compensation of the net gas deficit (resulting from RQs < 1) by N2, stable conditions occur when the following equations are simultaneously satisfied: A m D CO2 ( p CO2 - p CO2 ) dt = R CO2 Mdt

(2.8)

A m DO2 (pO2 - pO2d ) dt = RO2 Mdt

(2.9)

d

where the subscript “d” signifies the design or target gas level. Dividing these two equations results in the condition: D CO2 D O2

=

R CO2 ( p O2 - PO2 ) o

d

R O2 ( p CO2 - p CO2 ) o

(2.10)

d

or

E CO2 ,O2 = RQ

( p O2 - p O2 ) o

d

( p CO2 - p CO2 ) o

(2.11)

d

where, ECO2,O2 is selectivity of the membrane to CO2 over O2. In designing for stability, one should, therefore, know the desired composition and the RQ of the produce at that composition in order to select a membrane with selectivity ECO2,O2 , or given a fixed selectivity (i.e., no other choice of

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Processing Fruits: Science and Technology, Second Edition

membrane), adjust the external pressure so as to change the ratio of partial pressure differentials. For example, if the produce RQ is 0.58 and the design gas levels are 2.5 kPa CO2 and 3.8 kPa O2, ECO2,O2 should be 4.4 for joint stability to occur at the desired composition. 2.3.1.2 Equations for Calculating Membrane Area Required The following equations can be used to calculate the membrane area required to obtain the design O2 concentrations in an airtight and a leaking system, respectively: R O2 M

Am =

(2.12)

D O2 ( p O2 - p O2 ) o

Am =

d

R O2 M - O*2 L( P )

(2.13)

D O2 ( p O2 - p O2 ) o

d

In order to design for CO2, the appropriate substitutions may be made for the terms involving O2 in the above equations. In either case, the steady-state level of the other gas may be computed by using the appropriate form of the above equations, using the membrane area (Am) calculated for the design gas. The possible atmospheric regimes obtained by varying the membrane area for a leaking or airtight system for different RQs are shown graphically in Figure 2.3. In the figure, the membrane area increases from left to right following Equation (2.11) and Equation (2.12) which show that the membrane area increases as the difference between the initial O2 level and the targeted level pO2 d

7

AIRTIGHT ENCLOSURE

CO2 PARTIAL PRESSURE (kPa)

LEAKING ENCLOSURE 6

5

RQ 1.4

4 1.0 3 0.6 2

1

0

0

5

10 15 O2 PARTIAL PRESSURE (kPa)

20

FIGURE 2.3 Possible atmospheric regimes obtainable by varying the membrane area for an airtight and a leaking enclosure for different produce RQs.

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Refrigerated and Controlled/Modified Atmosphere Storage

RSV 0 0.1 −1 − Y(t) −Y − Yo − Y LN

0.3 −2

−3

0.5

−4 −5

−6 −7

0

10

20

30

40

50

TIME (DAYS)

FIGURE 2.4 Effects of the relative stacking volume (RSV) upon the period necessary for O2 to reach steadystate in an airtight enclosure (RCO2 = 3.5 mg·h–1·kg–1, RQ = 1 and O 2 = 5 kPa). d

decreases. For a given CO2 level, the O2 level is higher in a leaking than in an airtight system, the difference diminishing as membrane area increases. The influence of a higher RQ on the possible gas combinations is to increase the CO2 level corresponding to a design O2 level. In this figure, R CO and RQ were constant over the range of O2 levels. 2

2.3.1.3 Factors Affecting Time to Stabilization The time taken for stabilization of gas composition to design levels is important to consider in a passive system and may lead to a decision to use a rapid O2 pull-down technique, particularly for a fruit species with a short storage life. Experiments have shown that the time taken to reach stable gas levels is of the order of days to weeks, depending on commodity respiration, designed gas composition, relative stacking volume (RSV), and airtightness. Clearly, nonoptimal gas levels are not desirable for long periods. Rapid O2 pull-down is the term used for techniques that drop the O2 concentration of the storage atmosphere to optimal immediately after sealing the chamber. It can be most simply achieved by nitrogen flushing. 2.3.1.4 Relative Stacking Volume The RSV, or volume of produce as a ratio of total storage room volume, is essentially a measure of the initial buffering of the membrane–produce interaction by the free air in the storage room. The effect of RSV on time to stabilization of O2 is shown in Figure 2.4. The time to stabilization decreases with RSV; thus; it is important to properly plan the stacking geometry of the storage room. It is important to remember, however, that the air circulation pattern will change according to the geometry and that the ventilation requirements may increase in order to prevent condensation. 2.3.1.5 Design Gas Levels The closer the design gas levels are to ambient, the more rapidly will stabilization occur. In general, the degree of airtightness affects the O2 stabilization time more than the CO2 stabilization time. The following relations derived from a model presented by Deily and Rizvi18 describe the time “t” to stabilization assuming a constant respiration rate and RQ:

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Processing Fruits: Science and Technology, Second Edition

t CO2 =

t O2 =

- p CO2 RGD CO2 d

R CO2 MK

(O 2o - O 2d )RGD O2 R O2 MK

(2.14)

(2.15)

Given that the respiration rate of a commodity cannot be expected to remain constant, it is clear that such equations are most useful for comparative purposes rather than for absolute evaluations for a given commodity. 2.3.1.6 Standard Free Volume The standard free volume (SFV) provides similar information and is the ratio of air per unit volume of commodity. The SFV is typically 1.5 to 3 for warehouse commodity stacking arrangements and depends on room geometry, commodity shape and density, and whether the commodity is packed in bulk or crated. 2.3.1.7 Infiltration Rate The infiltration rate reflects the degree of airtightness of the storage room and is usually defined as the number of complete air changes per day when the room is empty. Air infiltration is due mainly to barometric pressure fluctuations, wind and temperature gradients between the storage room and the environment, and the cyclical operation of the refrigeration system, as well as the room construction and warehouse management strategy. The standard technique for determining the infiltration characteristics of a room consists of pressurizing the interior to 250 kPa and determining the time taken for the pressure to drop to the standard of 125 kPa. If this time is 10 min for example, the room is referred to as a “10-minute room,” which is roughly equivalent to 0.0333 air changes per day. 2.3.1.8 Silicone Membranes Silicone membranes, such as that used by Marcellin19,20 in his work on MA storage of various fruits and vegetables, consist of a fine nylon fabric covered with a thin layer of silicone rubber. Silicone rubbers are high-molecular-weight polymers whose backbone consists of alternate atoms of silicone and O2, to which various organic side groups may be attached to modify the characteristics of the rubbers. The particular polymer used by Marcellin was dimethyl-polysiloxane whose side groups are CH3. The thermal stability of such rubbers is excellent; they have fine water-repellency, good electrical-insulating characteristics, and good chemical resistance but are not strong. At a pressure of 1 atm, the permeability to CO2 is 1750 L·d–1·m–2. The selectivity (ratio of permeabilities) with respect to O2 is 5.5 and to ethylene is 2.5. Further details on these polymers are given by Ash and Ash,21 Roff et al.,22 and Yescombe.23 Commercial packaging systems such as Atmolysair, the Pallet Package, and the Marcellin System have all used this membrane for gas exchange, although in different configurations as will be described in the next section.

2.3.2 COMMERCIAL MODIFIED ATMOSPHERE SYSTEMS 2.3.2.1 The Pallet Package System The first commercial application of the silicone membrane for long-term storage was the Pallet Package System developed by Marcellin.24 This system consists of a pallet box wrapped in heavy

37

Refrigerated and Controlled/Modified Atmosphere Storage

WRAP

AIRTIGHT POLYETHYLENE ENVELOPE

CREATES DIFFUSION MEMBRANE CALIBRATED ORIFICE

PALLET

FIGURE 2.5 Pallet package system.

gauge polyethylene (80 to 150 µm thick) with a silicone membrane window installed for regulation of gas exchange and a calibrated hole for pressure regulation (Figure 2.5). The advantages of the system are ease of manipulation, optimal atmospheric composition without the building modifications and equipment needed for CA storage, and the possibility to market intermediate quantities of produce without disturbing the atmospheric composition of the rest of the produce. However, the disadvantages are the extra space required to provide adequate ventilation to all pallet packages and the time involved in wrapping and installing the proper area of membrane, as well as the extra care needed to avoid damaging the packages. This system was originally developed for the storage of apples and pears, and could accommodate other perishables. 2.3.2.2 The Marcellin System The Marcellin System (shown schematically in Figure 2.6) regulates the atmospheric composition in a storage room via a series of rectangular bags of silicone rubber connected in parallel.25 The

COOLING UNIT

DIFFUSION UNIT O2 CO2 N2

CA STORAGE ROOM

FAN SEAL

FIGURE 2.6 Marcellin system.

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Processing Fruits: Science and Technology, Second Edition

PLASTIC TUBING COOLING UNIT

ATMOPILE AIRTIGHT METAL CASING

OPENINGS DIFFUSION PANELS

CA STORAGE ROOM

OUTSIDE AIR INLET

OUTSIDE AIR OUTLET

BLOWER AND FILTER UNIT

FIGURE 2.7 Atmolysair system.

number of bags depends on the capacity of the storage room, the storage temperature, and the respiration properties of the stored produce. The composition of the headspace is analyzed to determine whether or not more bags should be used. Typical recommendations are 50 m2 of membrane surface area for each 100 t of produce at a bulk density of 200 to 250 kg·m–3 (standard free volume of 2.9 to 3.6) to maintain an atmosphere of roughly 3% O2 and 3% CO2. The units can be installed inside or outside the cold room. 2.3.2.3 The Atmolysair System The Atmolysair System (Figure 2.7) consists of a set of gas diffusion panels enclosed in an airtight container with two separate airflow paths and a control panel.26 The panels are arranged side by side so that the exterior air and the modified air from the storage chamber flow on opposite sides of the membrane without direct mixing. Two centrifugal blowers operated by a timer maintain air circulation. The gas composition is analyzed periodically such that the time of blower operation may be adjusted. The advantages of this system over the Marcellin System are ease of operation, better gas exchange due to control of both air streams from the airflow paths, and the potential for complete automation. The surface area of the membrane required is computed as previously outlined. This system has been successfully implemented by several Canadian cabbage producers.25

2.3.3 CONTROLLED ATMOSPHERES (ACTIVE ESTABLISHMENT) There are several ways to actively control the atmospheric composition inside a storage chamber. Each has its merits and demerits. Furthermore, most of the active methods have higher operation and maintenance costs than the passive and semipassive (partial control) technologies. One may divide active systems into O2-control and CO2-control systems. The O2 systems are: external gas generators, liquid nitrogen atmospheric generators, gas separators, and hypobaric storage. The CO2 control systems are lime scrubbers, water scrubbers, activated charcoal scrubbers, and molecularsieve scrubbers. C2H4 control systems should also be mentioned since these can be important in application to fruit such as apples. Typically, these store well enough under CA to accommodate the marketing period of small and intermediate orchards without resorting to MAs, but which are susceptible to problems of quick ripening if C2H4 is not removed.

39

Refrigerated and Controlled/Modified Atmosphere Storage

COOLING UNIT HEAT BURNER

CA STORAGE ROOM

FUEL CONDENSER SCRUBBER

AIR INLET

FIGURE 2.8 External gas generator.

2.3.3.1 Oxygen Control Systems 2.3.3.1.1 External Gas Generators External gas generators operate either on the open-flame (Figure 2.8) or catalytic burner principle to remove O2 from the incoming air. Heat may be recovered or the exhaust air may be water-spray cooled before being blown into the storage room. The disadvantages of these types of system are that fuel is required and a CO2 scrubber must also be used to remove excess CO2. The advantages of these systems are the flexibility of operation and the rapidity with which O2 is depleted in the initial stages of storage, compared to a membrane-only system. The catalytic system is preferable in ensuring better combustion and fewer plant-toxic or plant-activating combustion products. External burners are recommended for rapid O2 pull-down. Catalytic burners are more expensive to install when used in recirculating configuration but have lower operating costs. However, flushing systems are easier to maintain since they do not require the frequent adjustments characteristic of catalytic burners. If the storage room is well-sealed, relative to the standard free volume and commodity respiration activity, it is often only necessary to operate the burners during the initial O2 reduction stage. Thereafter, infiltrating air combined with commodity respiration and operation of the CO2 scrubber can maintain close to optimal conditions. 2.3.3.1.2 Liquid Nitrogen Atmospheric Generators Flushing the storage room with sprayed liquid nitrogen is an excellent method for establishing the CA condition rapidly. The spraying heads that pulverize the liquid N2 are best placed in front of evaporator blowers. An O2 sensor is used to determine whether further spraying is needed to maintain the recommended O2 level. Lime bags are used to absorb excess CO2. 2.3.3.1.3 Gas Separator Systems The two commercially available gas separator systems are the pressure-swing adsorption system (PSA) and the hollow-fiber membrane system (HFM). Both systems are used primarily for rapid O2 pull-down and can be used in flushing or recirculating modes to control the CO2 level during storage. PSA systems work on the following principle (Figure 2.9): A stream of air is compressed, filtered, and circulated through a molecular sieve that selectively absorbs O2. The airstream leaving the unit consists mainly of N2. O2 is then released from the sieve by depressurizing the vessel and is vented to the outside. Typically, two units are mounted in parallel such that absorption and

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Processing Fruits: Science and Technology, Second Edition

PARTICULATE FILTER

O2 SURGE ANALYZER VESSEL

COOLING UNIT MOLECULAR SIEVE ABSORPTION VESSEL

VENT

VENT

CA STORAGE ROOM

AIR COMPRESSOR AIR INLET

COALESCING FILTER

REFRIGERATION DRYER

FIGURE 2.9 Pressure-swing absorption system.

desorption may be carried out without affecting the flow of N2-rich air. The opening and closing sequences of the valves are microprocessor-controlled. Depending on the level of pressurization (usually 700 kPa) and the temperature, the purity of the N2 gas is between 90 and 99.9%. According to Bartsch and Blanpied,27 the energy consumption of the PSA in apple storage applications is 1 kW·h·m–3 of apples. HFM systems are based on hollow fibers made of semipermeable membranes. Air compressed to about 700 kPa is heated to 38∞C, filtered, and forced into the HFM unit. Since the membranes are more permeable to O2 and CO2, these gases pass to the external-flow stream and vent to the atmosphere, while the N2-rich stream continues to the storage room and purges O2 (Figure 2.10). The energy consumption in CA storage of apples is about 1.5 kW·h·m–3 per m3 of apples.27

GAS SEPARATOR COALESCING FILTERS

N2 HEATER OXYGEN STREAM ENRICHED

COMPRESSOR

COOLING UNIT

CA STORAGE ROOM AIR INLET

FIGURE 2.10 Hollow-fiber membrane system.

Refrigerated and Controlled/Modified Atmosphere Storage

41

2.3.3.1.4 Hypobaric Storage Low-pressure storage by means of a vacuum pump has the advantages of ease of manipulation of the effective O2 concentration and RH level of the storage enclosure. CO2, C2H4, and other volatiles of metabolism are removed. This offers also the advantage of permitting the storage of noncompatible commodities. However, due to the low required pressures (1.3 to 13 kPa), there are significant costs associated with storage room structure and airtightness quality needed for efficient pump operation. Moreover, commodities requiring high CO2 levels cannot be accommodated, and some quality characteristics can be affected (e.g., flavor). 2.3.3.2 CO2 Control Systems CO2 control is affected by scrubbing systems. The five reagents used are: caustic soda, water, hydrated lime, activated charcoal, and molecular sieves. Monitoring of the atmospheric composition is necessary to control the scrubbing system. 2.3.3.2.1 Scrubbing CO2 with Caustic Soda Caustic soda (NaOH) was one of the first reagents used commercially for CA storage applications and was used in mixture with water. The solution was circulated in open tubes and thus absorbed CO2, the residence time being controlled according to the required removal rate. Its use was discontinued due to its corrosiveness and the potential danger in handling. However, dry caustic material has been proposed as a reasonable alternative.28 2.3.3.2.2 Scrubbing CO2 with Hydrated Lime Perhaps the simplest method of controlling CO2 levels is through hydrated lime scrubbers (Ca(OH)2). The scrubber is an insulated and sealed plywood box, usually containing between half to the total amount of lime required to remove the CO2 produced during the entire storage period. The box is connected to the CA room and airflow to it may be left to natural convection or controlled by forced-airflow using blowers and dampers. The choice between natural convection and forced flow depends on the commodity respiration characteristics, the room airtightness quality, and the RSV or SFV. The efficiency of the lime bag drops with time as it becomes saturated with CO2. The level of efficiency is thus easily determined by simply weighing the bag. A 22 kg bag of hydrated lime weighs 30 kg at its maximum CO2 holding capacity. It is recommended that bags no heavier than 25 kg be used, since crusting of the outer layer may occur in larger bags, thus reducing the efficiency of CO2 absorption. 2.3.3.2.3 Scrubbing CO2 with Water There are two types of water scrubbers for CO2. One type is the brine scrubber in which brine is pumped over evaporator coils, solubilizes CO2, and is pumped to an external reservoir (Figure 2.11) and then to an aerator that causes the CO2 to escape to the atmosphere. Dry evaporator coils are used to prevent corrosion. Smock et al.29 modified the system (Figure 2.12), using two aerators, one internal and one external. Both systems regulate CO2 levels efficiently, the controls needed being water flow-rate adjustment through the aerators and operating time. It is recommended that hydrated lime be used with this system to help regulate CO2 at the beginning of storage when CO2 evolution is high, thus eliminating the need for a higher capacity scrubber. The scrubber should be designed on the basis of the quantity, respiration rate, and CO2 level required by the commodity. In application to apple storage, Pflug30 recommended a circulation rate of 100 l·h–1 per ton of apple stored, based on a recommended atmosphere of 5% CO2 and 2% O2, and a temperature of 1∞C. The scrubbing capacity is 0.02 m3 of CO2 removed per m3 of water circulating through the scrubber. This is four times lower than the theoretical scrubbing capacity of water under these conditions: that is, 0.086 m3 CO2 per m3 of water with a CO2 differential of 5%.

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Processing Fruits: Science and Technology, Second Edition

FAN BRINE

AERATOR

COOLING UNIT

CA STORAGE ROOM

LIME PUMP BRINE RESERVOIR

FIGURE 2.11 Brine-water CO2-scrubber.

COOLING UNIT WATER

AERATOR

CA STORAGE AERATOR ROOM

LIME PUMP

WATER RESERVOIR

FIGURE 2.12 Modified brine-water CO2-scrubber equipped with two aerators.

2.3.3.2.4 Scrubbing CO2 with Activated Charcoal and Other Molecular Sieves Activated charcoal and molecular sieve scrubbers are units filled with the respective absorbers, two blowers, and four timer-controlled valves (Figure 2.13). Air from the CA room is circulated to the scrubber, where CO2 is absorbed. The CO2-depleted air is returned to the storage room. When the absorbent is saturated, outside air is circulated through the scrubber and back to the outside to remove the CO2. If a molecular sieve is used, it is also heated to speed up the reactivation process. These systems have very low operating costs and the absorbent lifetime is about 5 years. 2.3.3.3 Ethylene Control Systems Ethylene may be removed from the storage atmosphere by a heated catalyst system such as that developed by Blanpied,31 and tested in a MacIntosh apple storage facility. The unit is shown in Figure 2.14. Storage air is blown through two electrically-heated ceramic banks in which the ethylene is oxidized to yield CO2 and water vapor. The air leaving the unit is cooled before being returned to the storage room. It was found that the scrubber removed 87% of the C2H4 produced by the stored apples, but required an airflow of 142 l·sec–1. The ceramic packing configuration could be modified to facilitate airflow and reduce the required fan power (which was 7.5 kW).

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SOLENOID VALVE COOLING UNIT

ACTIVATED CHARCOAL CONTAINER

CONTROL UNIT

CA STORAGE ROOM

SOLENOID VALVE

FIGURE 2.13 Activated charcoal CO2-scrubber. ELECTRIC HEATER INSULATION

FORWARD

CATALYSIS CERAMIC PACKING

REVERSE

COOLING UNIT

DAMPER MOTOR

FLOW REVERSING VALVE

BLOWER FAN

CA STORAGE ROOM

FIGURE 2.14 Heated catalyst system for C2H4 scrubbing.

An alternative to the heated catalyst system is the use of an absorbent bead scrubber that contains aluminum silicate spheres mixed with potassium permanganate (KMnO4), which is used as an absorbency indicator. The beads are held in a container through which the storage air is circulated. As the beads become saturated, the KMnO4 turns the beads from purple to brown, thus simplifying maintenance. The number of units required depends on the size of the storage room and the expected C2H4 production from the stored commodity. However, more data are needed to determine the C2H4 production rates in a system with C2H4 removal. It is likely that C2H4 production rates diminish with removal since autocatalycism is reduced. About 280 kg of beads are recommended in two units (each equipped with a 280 l·sec–1 fan) for a 360 m3 storage room for MacIntosh apples.

2.3.4 AUTOMATED CA CONTROL SYSTEMS In recent years, better quality requirements and advances in computer technology have led modern, large CA storage facilities to adopt automated control systems.32 Automation of a CA storage

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facility provides many advantages. They have become essential not only to control the process but also to ensure optimum product quality and efficiency.33 The difficulty in effectively controlling the environment in a CA storage room is due to its dynamic behavior. Physiological response of the stored produce, CO2 and O2 regulating equipment, building airtightness and integrity, cyclical operation of the refrigeration system, and changes in weather are all major elements affecting the gas balance. For these reasons, the process must be carefully studied before an attempt is made to automate the control process. 2.3.4.1 Control Strategy Aside from temperature and humidity control, the major challenge resides in achieving and stabilizing the O2 and CO2 levels to the desired concentrations (setpoints). At regular time intervals, the O2 and CO2 levels are measured and compared to their respective setpoints. The control system attempts to reduce the difference between setpoints and the process variables and decides whether or not to operate the gas regulation equipment. The ability of the control system to effectively decide on the type and magnitude of the interventions is based on the control strategy. A properly designed control system is robust and stable, with adequate response time and proper damping. 2.3.4.2 Control Devices Typical devices used to control the operation of the gas regulating equipment in CA control systems include on/off switches, and proportional (P), proportional-integral (PI), or proportional-integralderivative (PID) controllers. PID controllers are by far the most popular feedback controllers used in controlling automated industrial processes, and a good review can be found in vanDoren.34 More advanced controllers such as adaptive, fuzzy logic, knowledge-based, and artificial intelligence controllers can also be used in CA control systems. Advanced controllers are generally more intelligent and can dynamically optimize the process by a deduction process. These self-learning and self-tuning controllers can monitor the controlled system and adjust controller parameters automatically.35 Usually designed to provide dedicated control over one parameter, they are sturdy, reliable and commercially available in a multitude of configurations. One drawback is their limited ability to exchange information. This could be of concern in CA storage facilities equipped with certain types of regulating equipment. Morimoto and Hashimoto36 propose a new breed of intelligent control systems for the automation of storage processes based on a “speaking fruit” approach (SFA). This approach takes into account the physiological status of the stored commodity to optimize the storage environment. It consists of a fuzzy logic controller and two optimizers. Neural networks identify and describe fruit responses, and genetic algorithms determine the optimal setpoints. These setpoints are then passed to the fuzzy controller to control the environment. The concept was applied to the control of the RH level in an orange storage facility and was found superior to a basic on/off control. 2.3.4.3 PC-Based Systems Personal computer (PC)-based systems can also be used to provide direct on-line control of the gas levels in CA storage facilities. A PC-based system generally consists of a computer, control software, a data acquisition system (DAQ), communication ports, and switching devices. PC-based systems offer great flexibility since in addition to controlling the gas levels, the software can be programmed to compute on-line parameters, to manage interaction between process variables, and automatically log valuable information for later retrieval and analysis. It is important to plan for contingencies in cases of power or equipment failure or emergencies in order to minimize down time and avoid costly spoilage. Also, the software should allow for manual control of all equipment. Alarms should be installed to warn of any critical changes in gas levels in the storage room. Implementation of automatic emergency paging features are highly recommended.

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2.3.4.4 Commercial Systems Dedicated systems for monitoring and control of the environment in CA storage facilities are now available commercially. Most suppliers provide technical support to assist the storage owner in specifying and installing the most suitable equipment for the particular application. The equipment usually consists of a microprocessor-based controller, a CO2/O2 gas analyzer, temperature sensors, and devices to control the operation of the gas regulating equipment. Most of these systems can be interfaced with a PC to allow the storage operator to enter the required control parameters and monitor the process.

2.3.5 INSTRUMENTATION Automation of CA storage facilities requires the utilization of on-line instruments to measure the quality of the environment surrounding the stored produce. Variables that are routinely measured include temperature, relative humidity, and gas concentrations. Vibrations, dust, moisture, and temperature fluctuations affect sensor performance. Therefore, verification of the state and accuracy of the instruments should be made in accordance with manufacturers’ specifications or on an annual basis, if not specified. 2.3.5.1 Temperature Temperature is the easiest parameter to measure in the storage environment. The most common sensors are: the RTD, the thermocouple, the thermistor, and the integrated circuit (IC). All will perform well under CA storage conditions, and the selection of the appropriate sensor should be based on the desired accuracy, precision, and output signal, as well as its temperature range, the installation location, and cost.37,38 2.3.5.2 Relative Humidity Measurements of relative humidity (RH) can be accomplished by several means including sling spychrometers (wet bulb and dry bulb temperatures), synthetic hair hygrometers (changes in length of a moisture sensitive fiber), dew point hygrometers (temperature at which the air becomes saturated and begins to condense), thin film capacitive sensors (changes in capacitance of a polymer film), resistive sensors (changes in resistance of a polymer), and new micromachined RH sensor chip (organic or polymer sensing film with a Wheatstone Bridge piezoresistor circuit).39,40,41 These instruments vary widely in performance and cost. Electronic RH sensors are most commonly used in CA storage facilities. It should be remembered that monitoring and controlling RH above 95% in an environment maintained at a temperature close to the freezing point is extremely difficult. 2.3.5.3 Oxygen The level of oxygen in a CA storage environment can be monitored with either a paramagnetic oxygen analyzer or with an electrochemical fuel cell. With the paramagnetic oxygen analyzer, the level of oxygen is determined by measuring the magnetic properties of oxygen in the air sample.42 Although high in cost, this instrument can provide an accuracy of up to 0.1% after calibration. With the electrochemical cell, oxygen is converted by the fuel cell into a current proportional to the concentration of oxygen in the air sample.43 Recent technological improvements have increased the accuracy of these cells tremendously, and they are now frequently used in CA storage facilities. 2.3.5.4 Carbon Dioxide Measurement of the CO2 level is normally done by infrared absorption. This spectroscopic method is based on the absorption of nondispersive infrared radiation. The reduction in the radiation is a

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function of the wavelength and provides a measure of the concentration of carbon dioxide in the air sample.43,44

2.4 PRECOOLING There are many methods of cooling horticultural products before storage or loading for shipment: room cooling, forced-air cooling, vacuum cooling, hydrocooling, package icing, and top icing.45 Some commodities can be cooled by several methods, but most commodities respond best to one cooling method. The precooling requirements of a given commodity, and therefore the method used, are largely determined by product physiology in relation to harvest maturity and ambient temperature at harvest time.46 Highly perishable commodities must be precooled as soon and as rapidly as possible after harvest. In general, such commodities are those harvested in late spring, summer, and early fall. Precooling is not as important for late-season crops (such as winter apple varieties) or low-respiration commodities. In this section, only cooling methods that apply to fruits will be discussed.

2.4.1

ROOM COOLING

Room cooling is probably the most widely used refrigeration technique due to its versatility and low cost. However, room-cooled produce must be tolerant of slow heat removal since much of the cooling is by heat conduction through the container walls. This technique is not considered to be a true precooling method. It involves placing field containers in a room cooled by a refrigeration system. The ventilation system must provide good air circulation through and around the containers (Figure 2.15). Cold air from the evaporator enters the room near the ceiling, moves horizontally under the ceiling, and then sweeps past the produce containers in returning to the evaporator. For adequate heat removal, the air-flow velocity must be 1 to 2 m·sec–1, with the moving cold air in good contact with all container surfaces.45 Container disposition is, therefore, very important in the handling sequence. Room cooling is best applied to produce harvested at the end of the season45 or with a relatively long storage life. It should also have a low respiration rate and a low harvest temperature. The main advantage of room cooling is that produce can be cooled and stored in the same room without being transferred. However, it is too slow for most commodities and can result in excessive water loss. Rehandling may be needed for better use of the storage space after cooling is achieved.47

FIGURE 2.15 Schematic of the air circulation pattern during air cooling.

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FIGURE 2.16 Forced-air tunnel cooling.

Furthermore, the addition of warm produce into a cold room may result in condensation on already cooled produce.48

2.4.2 FORCED-AIR COOLING Forced-air cooling consists of using fans to pull refrigerated air through the container vents by vacuum so that warm air is swept away by the cold airflow.49 Two basic designs are used: the forced-air tunnel and the cold wall. Mitchell et al.48 observed that forced-air pressure cooling is four to ten times faster than conventional room cooling, but two to three times slower than hydrocooling or vacuum cooling. The product cools as a result of the convective action of refrigerated air sweeping away warm air. Pulling rather than blowing air through the containers is a better option since “short-circuiting” (i.e., refrigerated air that, when the containers are under vacuum, flows directly to the fan without first going around the product) is reduced. Ideally, the ventilation system must deliver between 0.5 and 3 L·sec–1·kg–1 of warm product.49 The forced-air tunnel system (Figure 2.16) consists of a row of palletized containers or bins placed on either side of an exhaust fan, leaving an aisle between the rows. The aisle and the open end are then covered to create an air plenum tunnel. The exhaust fan creates negative air pressure within the tunnel. Cold air from the room then moves through the openings in and between containers toward the low-pressure zone, thus sweeping heat from the product.45 The investment for this method is minimal: a ventilation unit and a covering linen. However, the method also returns warm air into the room, which may condense on the produce. It is possible to eliminate the use of a fan and the condensation problem if one end of the tunnel is put directly on the air intake of the cooling system. In this case, the produce containers must be rearranged in order to prevent dehydration after precooling is achieved. The cold wall cooling system (Figure 2.17) uses a permanent air plenum, formed by the construction of a “dummy wall” equipped with exhaust fans. It is often located at one end or side of a cold room, with the exhaust fans designed to move air over the cooling system. Openings are located along the room side of the plenum against which stacks or pallet loads of containers can be placed. Dampers ensure that airflow is blocked except when a pallet is in place. Each pallet starts cooling as soon as it is in place, thus eliminating the need to await deliveries to complete a tunnel.47 This is a good system for operations in which the produce arrives at variable times. It is

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COOLING SYSTEM

FIGURE 2.17 Cross-sectional view of a cold-wall forced-air cooling unit.

also versatile, in that the treatment time can be adjusted for each product; however, rehandling of the containers is also necessary as soon as cooling is achieved.

2.4.3 HYDROCOOLING Cold water is an old and effective method for quickly precooling a wide range of fruits and vegetables in bins or in bulk. Hydrocoolers can be based on immersion or showering.45 In a typical shower-type hydrocooler (Figure 2.18), cold water is pumped from a bottom reservoir to an overhead perforated pan. The water showers over the commodity, which may be in bins or boxes or loose on a conveyer belt passing beneath. Water leaving the product may be filtered to remove debris, then passed over refrigeration coils or ice, where it is recooled.45 WATER PERFORATED PAN

PRODUCE PUMP BOTTOM RESERVOIR

COOLING SYSTEM

FIGURE 2.18 Schematic view of a typical hydrocooler.

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The evaporator of the refrigeration system recools used water. It is usually located in one of the two reservoirs of the precooler. When the evaporator is on the top reservoir, it frees the bottom reservoir, thus facilitating cleaning which must be done regularly due to the amount of dirt washed off the produce. Since the top reservoir is empty when the pump is off, it cannot be used as an ice reservoir. If the evaporator is located in the bottom reservoir, it can accumulate ice between cooling periods and, thus, considerably increases the refrigeration capacity for a given compressor capacity; it is a great advantage when the produce deliveries vary considerably. Ice may also be used if it can be added to water when the cooling capacity of the system is not enough to keep water temperature near 0∞C.45 Cooling speed is generally greater with a water system than with an air system, and there is no product dehydration. The produce and the packing and packaging materials must be tolerant to wetting, resistant to water impact, and tolerant of chlorine or other chemicals that are used to sanitize the hydrocooling.45 The produce must also be able to tolerate prolonged exposures to a temperature of 0∞C. Hydrocooling operations can require rehandling of the produce before packing or storage, thus increasing labor costs.

2.5 ECONOMIC CONSIDERATIONS CA and MA technologies prolong the storage time of many kinds of fruits and vegetables, and optimal atmospheres often reduce the proliferation of pathogens. However, the economic viability of investing in such technology depends on local, regional, and national socioeconomic conditions, as well as on national and international trade policy. Moreover, the economics of such an endeavor depends on the scale of production and the organizational structure of producer, wholesaler, and retailing groups. In some regions, producers work independently while in others they are organized in cooperatives. Wholesale buying may also be done on an individual or cooperative basis, as with retailing. From region to region and country to country, there are important differences in supply, demand, and seasonality, thus making it nearly impossible to present a reasonable economic analysis of the wisdom of investing in such technology. If the storage goals mentioned in the introduction to this chapter are taken to be worthwhile incentives — that is, they should reduce the use of chemicals in storage, extend the period of availability of a nutritionally superior product, and create the possibility of making quality products available to a wider range of income groups — then the investment is worthwhile. If, however, the decision is to be made on purely profit-making grounds, the analysis is complex. Insofar as CA and MA technology may offer a marketing advantage to one or more interest groups for certain products and in certain regions, widespread adoption of the technology will tend to remove the advantage and make the technology less attractive on those grounds alone. A recent example of this notion is the almost negligible price differential between conventional horticultural produce and so-called certified organic produce. In the early 1980s, the organically-certified product was worth about double the conventional product. Today, market conditions have resulted in a negligible advantage in implementing “organic” techniques.

ACKNOWLEDGMENTS The authors are grateful to NSERC, FCAR, and CORPAQ for their financial assistance, and to Valérie Orsat and Bernard Goyette for their help in preparing this chapter.

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REFERENCES 1. Bohling, H. and Hansen, H. Storage of white cabbage (Brassica oleacea var. capitata) in controlled atmosphere. Acta Hortic., 62: 49–54. 1977. 2. Smock, R.M. Controlled atmosphere storage of fruits. Hortic. Rev., 1: 301–336. 1979. 3. Soderstrom, E.L. and Brandl, D.G. Controlled atmospheres to reduce postharvest insect damage to horticultural crops. In: Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities. pp. 207–212. Proceedings of the Fourth National Controlled Atmosphere Research Conference, Raleigh, NC. July 23–26, 1985. 4. Gariépy, Y., Raghavan, G.S.V., and Thériault, R. Controlled atmosphere storage of celery with the silicone membrane system. Int. J. Refrig., 9: 234–240. 1986. 5. Kader, A.A. Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol., 40(5): 99–104. 1986. 6. Yong, R.E., Romani, R.J., and Biale, J.B. Carbon dioxide effects on fruit respiration. II. Response of avocados, bananas and lemons. Plant Physiol., 37: 416–422. 1961. 7. Kader, A.A. 1985. A summary of CA requirements and recommendations for fruits other than pome fruits. In: Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities. p. 445. Proceedings of the 4th National Controlled Atmosphere Research Conference, Raleigh, NC. July 23–26, 1985. 8. Day, N.B., Skura, B.J., and Powrie, W.D. Modified atmosphere packaging of blueberries: microbiological changes. Can. Inst. Food Sci. Techn. J., 23(1): 59–65. 1990. 9. Lépine, Y. Strawberry handling in Québec. Thesis. Agricultural Engineering Department, Macdonald Campus of McGill University, Ste-Anne-de-Bellevue (QC). 1989. 10. El Kazzaz, M.K., Summer, N.F., and Kader, A.A. Ehtylene effects on in vitro and in vivo growth of certain postharvest fruit-infecting fungi Citrus sinensis, Valencia oranges, Fragaria chiloensis var. ananassa, strawberries. Phytopathol., 998–1001. 1983. 11. Farber, J.M. Microbiological aspects of modified atmosphere packaging technology — A review. J. Food Protect., 54(1): 58–70. 1991. 12. Skrzynski, J., Fica, J., and Dilley, D.R. The effect of ethylene removal during controlled atmosphere storage of Red Delicious, Empire, Idared and Law Rome apples. In: Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities. p. 115–126. Proceedings of the 4th National Controlled Atmosphere Research Conference, Raleigh, NC. July 23–26, 1985. 13. Kader, A.A. An overview of the physiological and biochemical basis of CA effects on fresh horticultural crops. In: Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities. pp. 1–9. Proceedings of the 4th National Controlled Atmosphere Research Conference, Raleigh, NC. July 23–26, 1985. 14. Ulrich, R. Controlled atmosphere storage, Part 2. In Postharvest Physiology, Handling and Utilization of Tropical and Subtropical Fruit and Vegetables. AVI Publishing, Westport, CT. 1975. 15. Plasse, R. Vegetable storage, respiration, and design criteria in a membrane storage system. Thesis. Agricultural Engineering Department, Macdonald Campus of McGill University, Ste-Anne-de-Bellevue (QC), 215 pp. 1987. 16. Emond, J.P., Castaigne, F., Toupin, C.J., and Desilets, D. Mathematical modelling of gas exchange in modified atmosphere packaging. Trans. ASAE, 34(1): 239–245. 1991. 17. Gariépy, Y., Raghavan, G.S.V., Thériault, R., and Munroe, J.A. Design procedure for the silicone membrane system used for controlled atmosphere storage of leeks and celery. Can. Agric. Eng., 30(2): 231–236. 1988. 18. Deily, K.R. and Rizvi, S.S.H. Packaging of fresh peaches in polymeric films. J. Food Process. Eng., 5: 23–41. 1981. 19. Marcellin, P. Emballages de matière plastique pour la conservation des fruits en atmosphère controlée. Rev. Prat. Froid Condition. Air., 10: 23–29. 1972. 20. Marcellin, P. Application des membranes de polymères à la conservation des fruits et légumes en atmosphère controlée. Proceeding of the 4th International Congress of Food Science and Technology. Valencia, Spain. pp. 91–99. 1974. 21. Ash, M. and Ash, I. Encyclopedia of Plastics, Polymers, and Resins. Vol. 3. Chemical Publishing Company, New York. 1983.

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22. Roff, W.J., Scott, J.R., and Pacitti, J. Handbook of Common Polymers, Fibers, Films, Plastics and Rubbers. CRC Press, Cleveland, OH. 1971. 23. Yescombe, E.R. Plastics and Rubbers: World Source of Information. Applied Science Publishing, London. 1976. 24. Marcellin, P. Principe et réalisation des atmosphères controlées. In Perspectives Nouvelles dans la Conservation des Fruits et Légumes. CRESALA, Montreal (QC). pp. 88–97. 1978. 25. Marcellin, P. and Leteinturier, J. Premières applications des membranes de caoutchouc de silicone à l’entreposage des pommes en atmosphère controlée. Int. Inst. Refrig., Paris. pp. 1–9. 1967. 26. Raghavan, G.S.V., Gariépy, Y., Thériault, R., Phan, C.T., and Lanson, A. System for controlled atmosphere long-term cabbage storage. Int. J. Refrig., 7(1): 66–71. 1984. 27. Bartsch, J.A. and Blanpied, G.D. Air separator technology for controlled atmosphere storage. A workshop sponsored by Cornell Cooperative Extension. Cornell University. Ithaca, NY, 1988. 28. Bartsch, J.A. and Blanpied, G.D. Refrigeration and controlled atmosphere storage for horticultural crops. Northeast Regional Agricultural Engineering Service, Cornell University, Ithaca, NY. NRAES J. Paper No. 22. 1990. 29. Smock, R.M., Creasy, L.L., and Blanpied, G.D. Water scrubbing in CA rooms. Cornell University, Ithaca, NY, Paper No. s-508. 1960. 30. Pflug, I.J. Oxygen reduction in CA storages: A comparison of water vs. caustic soda absorbers. Mich. Agr. Expt. Stat. Q. Bull., 43(2): 455–466. 1960. 31. Blanpied, G.D. Handbook for Low Ethylene CA Storage of McIntosh and Empire Apples. Pomology Department, Cornell University, Ithaca, NY. 1985. 32. Bishop, D.J. Application of new techniques in CA storage. Symposium on New Application of Refrigeration to Fruit and Vegetable Processing. June 8–9, Istanbul, Turkey. 323–331. 1994. 33. Mittal, G.S. Computer-based instrumentation: sensors for in-line measurements. In Computerized Control Systems in the Food Industry. Marcel Dekker, New York. pp. 13–53. 1997. 34. vanDoren, V.J. Basics of proportional-integral-derivative control. Control Engineering. 45(3): 135–142. 1998. 35. Seborg, D.E., Shah, S.L., and Edgar, T.F. Adaptive control strategies for process control: a survey. AIChE paper presented at the AIChE Diamond Jubilee Meeting, Washington, D.C. November 1983. 36. Morimoto, T. and Hashimoto, Y. An intelligent control for greenhouse automation, oriented by the concepts of SPA and SFA — an application to a postharvest process. Comput. Electron. Agri., 29(1–2): 3–20. 2000. 37. Simpson, J.B. and Pettibone, C.A. Temperature. In Instrumentation and Measurement for Environmental Sciences. Bailey, W.M., Ed. ASAE Special Publication 82–13. St. Joseph MI, 6–01 to 6–16. 1983. Chap. 6. 38. Desmarais, R. and Breuer, J. How to select and use the right temperature sensor. Sensors Online (www.sensorsmag.com) 18(1): 11. 2001. 39. White, G.M. and Ross, I.J. Humidity. In Instrumentation and Measurement for Environmental Sciences. Bailey, W.M., Ed. ASAE Special Publication 82–13. St. Joseph MI, 8–01 to 8–12. 1983. Chap. 8. 40. Fenner, R., Kleefstra, M., and Zdankiewicz, E. New micro-machined water vapor sensor for home appliance applications. Paper presented at the Appliance Manufacturer Conference and Expo 2000. Cincinnati, OH, Sept. 11–13, 2000. 2000. 41. Roveti, D.K. Choosing a humidity sensor: A review of three technologies. Sensors Online (www.sensorsmag.com) 18(7): 9. 2001. 42. Anonymous. Application of Oxygen Analyzers. Analytical Specialties (www.analyzer.com) Houston TX. 77058. 43. Anonymous. Gas Analyzer for IR-absorbing Gases and Oxygen 7MB233. Siemens Ultramat23 Instruction Manual. Karlsruhe, Germany 3–5. 1997. 44. Anonymous. Infrared analysis. Analytical Specialties (www.analyzer.com) Houston TX, USA 77058. 45. Kader, A.A. Postharvest Technology of Horticultural Crops. 2nd ed. Cooperative Extension University of California, Davis, CA. Publication No. 3311. 1992. 46. ASHRAE. Methods of precooling fruits, vegetables and ornamentals. In: Refrigeration Systems and Applications Handbook. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA. Chap. 11. 1986.

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Processing Fruits: Science and Technology, Second Edition 47. Mitchell, F.G. Cooling methods. In: Postharvest Technology of Horticultural Crops. Kader, A.A. Ed., 2nd ed. Cooperative Extension University of California, Davis, CA. Publication No. 3311. pp. 56–63. 1992. 48. Mitchell, F.G., Guillou, R., and Parsons, R.A. Commercial Cooling of Fruits and Vegetables. Manual 43. University of California. Division of Agricultural Sciences. 1972. 49. Fraser, H.W. Forced-Air Rapid Cooling of Fresh Ontario Fruits and Vegetables. Ministry of Agriculture and Food. Toronto, ON. AGDEX 202–736. 1991.

3 Fresh-Cut Fruits Elisabeth Garcia and Diane M. Barrett CONTENTS 3.1 3.2 3.3

Introduction ..........................................................................................................................53 Effects of Fresh-Cut Processing on Produce Physiology ...................................................54 Unit Operations in Fresh-Cut Fruit Preparation..................................................................55 3.3.1 Receiving, Inspection, and Storage of Raw Material ...........................................56 3.3.2 Cleaning and Disinfection .....................................................................................56 3.3.3 Peeling, Deseeding, Trimming, Coring, and Cutting ...........................................56 3.3.4 Washing and Cooling ............................................................................................56 3.3.5 Dewatering .............................................................................................................57 3.3.6 Packaging and Distribution ...................................................................................57 3.4 Quality Aspects of Fresh-Cut Fruits....................................................................................57 3.4.1 Importance of Cultivar Selection ..........................................................................57 3.4.2 Optimum Degree of Ripeness for Processing.......................................................59 3.4.3 Microbial Spoilage ................................................................................................60 3.4.4 Browning Control ..................................................................................................61 3.4.5 Prevention of Textural Losses ...............................................................................63 3.4.6 Appearance and Sensory Quality ..........................................................................64 3.4.7 Nutritional Aspects ................................................................................................65 3.5 Shelf Life Extension of Fresh-Cut Fruit .............................................................................65 3.5.1 Temperature Management .....................................................................................66 3.5.2 Modified Atmosphere Packaging ..........................................................................66 3.5.3 Humidity ................................................................................................................68 3.5.4 Edible Coatings......................................................................................................68 References ........................................................................................................................................69

3.1 INTRODUCTION Over the past 25 years the consumption of fresh fruit in the U.S. was reported to have grown by about 26%, according to the Economic Research Service-U.S. Department of Agriculture (Pollack, 2001). However, this does not reflect all fruit because the report does not include melon consumption, which was part of the vegetable category statistics. Over the same period, the U.S. consumption of fresh vegetables and melon has increased by 33%. In addition to awareness of the health benefits of a diet rich in fruits and vegetables, several factors are attributed to these changes, such as rising income, increased production, greater diversity and availability (of domestic and imported produce, on and off season), product convenience, storage, and transportation, among others (Pollack, 2001). Nevertheless, according to a USDA survey (1994 to 1996), the recommended number of at least two daily fruit servings is currently met by only 23% of the American population (USDA, 1998). Frequently, convenience is an important factor in a consumer’s selection of produce, and fruit are no exception. From 1997 to 1999, over 40% of the total fruit intake was consumed as juice, 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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63% of which was solely orange juice (accounting for 86% of orange consumption). Another trend observed since the mid-1970s is that Americans are eating less canned produce, and more fresh and frozen items. Although over the past two decades the consumption of frozen fruit increased 36%, this constitutes a small fraction of all fruit consumed in the U.S. During the last two decades, another category of fruit product has been introduced in the market: fresh-cut fruit. Fresh-cut products have also been referred to as “lightly processed,” “minimally processed,” “fresh-processed,” “partially processed,” or “preprepared” products. According to the International Fresh-Cut Produce Association (IFPA), Fresh-cut produce is defined as any fresh fruit or vegetable or any combination thereof that has been physically altered from its original form, but remains in a fresh state. These fruits and vegetables have been trimmed, peeled, washed, and cut into 100% usable product that is largely bagged or prepackaged to offer consumers high nutrition, convenience, and value while still maintaining freshness (IFPA, 2002).

The idea of minimal processing of fruit has been around in different parts of the world for some time, but at a different scale. Consumers could find convenient produce items already washed and peeled or either, cut on site at a local market or street stall. No special packaging or refrigeration was used; these products were directed toward immediate consumption. In the U.S., fresh-cut produce first appeared in retail markets in the 1940s, but second-quality, misshapen produce was used; quality was unpredictable and shelf life was limited. In the mid-1970s, fast-food chains were using shredded fresh-cut lettuce and chopped onions; in the mid 1980s, salad bars opened, and fresh-cut produce start replacing canned products (Garrett, 2002). Today, vegetables (mainly salads) constitute the main segment (~70%) of fresh-cut products. In addition to the convenience, there are other reasons for the success of fresh-cut produce, such as the absence of waste material. When utilizing fresh-cut produce, 100% is consumable, and there is also a substantial decrease in labor required for home produce preparation (e.g., no need for washing, trimming, etc.) and waste disposal. The amount of waste in peeling and coring fruit can be quite elevated; in pineapple it frequently exceeds 50% of the fresh fruit weight (including the crown). These advantages make fresh-cut produce well accepted items in the foodservice sector, commercial salad bars, and fastfood outlets (Price and Floros, 1993). Examples of fresh-cut fruit available in the U.S. for retail and foodservice distribution are sliced apples, peaches, strawberries, oranges, grapefruit, mangoes, melons, watermelons, pineapple, citrus segments, and fruit salads. Fresh-cut fruit tend to be more perishable than the commodity from which they were prepared. The expected shelf life of fresh-cut fruit is around 7 to 8 d, in contrast to 10 to 14 d for fresh-cut vegetables (Kader, 2002). The limited commercial success of fresh-cut fruit is related to their perishability and consequently short shelf life.

3.2 EFFECTS OF FRESH-CUT PROCESSING ON PRODUCE PHYSIOLOGY Whereas the processing of fruits generally extends their shelf life, as seen in the case of canning, freezing, and drying, fresh-cut processing increases the perishability of fruits (Cantwell and Suslow, 1999). The physical wounding of tissue caused by the preparation (peeling, cutting, slicing, dicing, etc.) of the fresh-cut products leads to many physical and physiological responses (Brecht, 1995; Saltveit, 1997). There is an increase in respiration rate and ethylene production, which impacts fruit quality and shelf life. Increased respiration rate is related to elevated cellular metabolism, which contributes to faster quality deterioration. Table 3.1 shows examples of respiration rates of fresh-cut fruit as compared to intact fruit. Respiration rates increase with the degree of injury or processing and with storage temperature. In a study with papayas (Paull and Chen, 1997), fruit were longitudinally halved, one half fruit was kept with seeds, and the other half was deseeded.

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Fresh-Cut Fruits

TABLE 3.1 Respiration Rates of Fresh-Cut Fruit and Whole Fruits Stored at Different Temperatures Respiration (mg CO2/kg/h) Fruit Kiwi Peach Muskmelon

Honeydew

whole slices whole slices large, whole large, slices small, whole small, slices whole slices

0∞C

5∞C

10∞C

3.2 7.2 4.0 6.0 4.8 3.7 3.1 2.7 1.4 2.3

4.6 11.6 8.1 10.0 8.6 7.0 6.3 4.2 4.6 3.0

8.6 23.3 15.0 18.6 14.7 12.2 13.6 9.8 5.2 8.3

Source: From Watada, A.E., Ko, N.P., Minott, D.A. 1996. Factors affecting quality of fresh-cut horticultural products. Postharvest Biol. Technol. 9: 115–125.

Opposite halves of the same fruit were used. The wound caused by slicing and deseeding led to increases in ethylene production five- and ninefold higher in the halves with seeds and in the deseeded halves, respectively, as compared to the intact fruit. The injury caused by wounding through slicing and deseeding of papaya fruit also led to yellowing of the fruit skin and softening of the mesocarp. Wound-related production of ethylene is higher in fruit at the preclimacteric and climacteric stages than in postclimacteric period (Abeles et al., 1992). The response to injury may also be affected by the maturity stage of fruit. In a study by Cantwell and Suslow (1999), cantaloupe melon pieces processed from fruit at different maturity stages had similar respiration rates, but ethylene production was much higher in pieces from riper fruits. The removal of protective epidermis or skin of the fruit leads to changes in gas diffusion, accelerates water loss, and favors microbial contamination. It is important to prevent cut-surface desiccation, which negatively impacts product appearance. Another consequence of fresh-cut fruit processing is cell disruption, which allows previously compartmentalized enzymes to come in contact with their substrates, leading to undesirable reactions such as development of enzymatic browning and accelerated softening.

3.3 UNIT OPERATIONS IN FRESH-CUT FRUIT PREPARATION In order to maintain the highest possible quality of a fresh-cut fruit, it is important to observe adequate handling of the produce before preparation. It is necessary to keep produce under adequate storage conditions (temperature and relative humidity) and use gentle handling to minimize bruising and other physical injuries. Strict temperature control is the single most important factor in maintaining quality throughout the fresh-cut preparation and distribution. All unit operations should be carried out in cold processing rooms (£5∞C), and cleanliness is very important to ensure a highquality and safe product. It is of utmost importance to use sanitary equipment and adopt Good Manufacturing Practices (GMP) in combination with Hazard Analysis Critical Control Point (HACCP) (IFPA, 1996a, 1997; Beuchat, 2000). The overall steps involved in the preparation of fresh-cut fruit follow.

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3.3.1 RECEIVING, INSPECTION,

AND

STORAGE

OF

RAW MATERIAL

Fruit has to be inspected and evaluated according to high standards of safety and quality. It is well known that no processing can increase the quality of a diseased or misshapen ingredient. Therefore, it is very important that while awaiting processing, fruit should be kept under refrigeration (in general, 1 to 5∞C) according to produce specifications. If a commodity is sensitive to chilling injury, it requires appropriate storage conditions, separate from nonsensitive fruit. For example, symptoms of chilling injury are seen in pineapple stored for prolonged periods at temperatures below 12∞C (Paull and Rohrbach, 1985). Whereas some fruits, such as pears and apples, can be stored for extended periods of time while waiting to undergo processing, the effects of such storage on freshcut products may be undesirable. Gorny and coworkers (2000) observed significant shelf-life reduction in fresh-cut pears prepared from fruit that had been kept for increased storage periods at 1∞C in air or controlled atmosphere (2% O2 + 98% N2).

3.3.2 CLEANING

AND

DISINFECTION

With many fruit that have a smooth surface, such as apples and pears, the use of cold clean water may be enough for a first wash. However, some fruit may harbor a high microbial population on their external surface. Cantaloupes were one of three imported produce with the greatest incidence of pathogen contamination in a survey carried out by the Food and Drug Administration (FDA, 2002). Such fruit should be washed, scrubbed, and dipped in solutions of disinfectants. The most commonly used sanitizer is chlorinated water, but other possible chemicals for disinfection are hydrogen peroxide, surfactants, peroxyacetic acid, trisodium phosphate, and ozone, among others (Heard, 2002; Beuchat, 2000; Cherry, 1999; Sapers and Simmons, 1998). The efficacy of chlorine and hydrogen peroxide treatment on the native microflora of cantaloupes, as well as on Escherichia coli population, was compared by Ukuku and coworkers (2001). Hydrogen peroxide proved to be more efficient against the surface microflora of cantaloupe, while chlorine was more effective against E. coli ATCC 25922.

3.3.3 PEELING, DESEEDING, TRIMMING, CORING,

AND

CUTTING

In contrast to vegetables, which in many instances can be peeled and cut mechanically (such as carrots), for most fruit whose texture is soft, the removal of skins and rinds is carried out by hand. It is very important to use clean and sharp knives during this operation. The use of blunt blades causes excessive physical injury (tissue crushing) to the fruit tissue adjacent to the cut, accelerating quality deterioration. Fresh-cut cantaloupe melon prepared with sharp and blunt knives were compared (Portela and Cantwell, 2001). Melon pieces prepared using blunt knives had increased ethanol levels, off-odors, and electrolyte leakage compared to sharp-cut pieces. The marketable visual quality of sharp-cut melon lasted much longer; blunt-cut pieces developed a translucency, which is a common visual defect that indicates cell disruption in commercially prepared freshcut cantaloupe. With citrus, enzymatic peeling has been reported as an alternative to hand peeling (Bruemmer et al., 1978; Pretel et al., 1996, 1998). Whole citrus fruit can be vacuum infused with solutions of pectic enzymes that will eliminate flavedo and albedo, with limited damage to the juice vesicles. After this treatment, peeled fruit have to be rinsed by immersion in water for removal of the enzyme solution.

3.3.4 WASHING

AND

COOLING

During the cutting steps there is release of tissue fluids that should be removed to avoid undesirable microbiological or chemical reactions. It is imperative to rinse the cut surfaces of the fruit. At this stage the use of cold water (~0∞C) accomplishes the washing of the cut surfaces as well as cooling of the fruit pieces. For safety reasons, chlorinated water (usually 50 to 100 ppm) is frequently used.

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Fresh-Cut Fruits

All the equipment used in the processing (cutting tools and mats) are a potential source of contamination and require adequate sanitation. In some cases it is necessary to use dipping solutions containing processing aids that will help prevent nonmicrobial quality deterioration such as surface browning and softening (Garcia and Barrett, 2002).

3.3.5 DEWATERING Excess moisture picked up during the washing operation should be removed prior to packaging. This step helps to prevent growth of microorganisms that remained after produce disinfection. Due to their delicate texture, fresh-cut fruit require passage through semi-fluidized beds with forced air to remove moisture.

3.3.6 PACKAGING

AND

DISTRIBUTION

In order to ensure the longest possible shelf life for fresh-cut fruit, it is important to choose appropriate packaging materials and storage conditions. Temperature is always a critical factor in the shelf life of fresh-cut fruit. The importance of temperature control is related to food safety and extension of fresh-cut life by slowing respiration rate and preventing quality deterioration. As an example, the shelf life of sliced pineapple is reported to range from a few hours at 20∞C and to greater than 5 weeks at 1∞C (O’Hare, 1994). Throughout the unit operations involved in preparation and up to consumption, a cold chain should be maintained; ideally temperatures should not exceed 5∞C, although preferably they should be closer to 1∞C (IFPA, 1997). During the distribution of fresh-cut products, it is also important to avoid rough handling. Very often products are subjected to shock and vibration stress, which cause injury, leading to more rapid quality loss. In a study of fresh-cut watermelon cubes, results showed increased juice leakage and darkening of product vibrated in noncompartmentalized packages. Authors suggested that the placement of compartments that decrease movement of fruit pieces during transit can improve overall quality by reducing vibration (Fonseca et al., 1999).

3.4 QUALITY ASPECTS OF FRESH-CUT FRUITS Fresh-cut fruit combine the convenience of a 100% usable product with the fresh-like quality characteristics of fresh fruit, e.g., appearance, flavor, and texture. The initial appeal of a convenient fresh-cut product will only be ensured of continued acceptability if the product quality meets the consumer’s quality criteria including appearance, product shelf-life, and relationship between perceived value and cost. Moreover, the product value as food also depends on its nutritional quality and its safety (Kader, 2002). Several preharvest factors affect the quality of a commodity, such as genetic background, agronomic practices, environmental conditions during cultivation, and stage of maturity at harvest. Final product quality is influenced to a great degree by postharvest handling and fresh-cut preparation, from the initial unit operations up to distribution. As a result of the injury (wound) brought by cutting, cells are broken and undesirable reactions may result. Fresh-cut fruit may undergo surface browning, tissue softening, loss of flavor, and other deterioration reactions. Moreover, the leakage of nutrient-rich tissue fluids can potentially lead to microbial spoilage; peeling and removal of protective tissues lead to water loss and, consequently, quality defects such as surface drying, loss of gloss, and shrinkage are more noticeable.

3.4.1 IMPORTANCE

OF

CULTIVAR SELECTION

A high-quality fresh-cut product starts with fruit of superior raw material quality and the appropriate selection of fruit cultivars (cv.). In fact, cultivar selection is one of the most important considerations.

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TABLE 3.2 Enzymatic Browning of Peara and Apricotb Purees Prepared from Different Fruit Cultivars Harvested at Commercial Maturity Pear Cultivar Abbe Fetel Comice Conference Guyot P 2198 Williams 6.30.100

DLa –16.2 –15.1 –14.1 –9.9 –15.3 –13.3 –4.8

Apricot Cultivar

DLa

Bebeco Cafona Canino Henderson Moniqui Polonais P. Tyrinthe R. Fournes R. Roussillon

5.3 11.8 16.7 26.3 21.4 16.8 3.7 17.8 17.8

Reflectance measurements expressed as DL, difference in lightness between oxidized and nonoxidized purees. a

Source: From aAmiot, M.J., Tacchini, M., Aubert, S., Oleszek, W. 1995. Influence of cultivar, maturity stage, and storage conditions on phenolic composition and enzymatic browning of pear fruits. J. Agr. Food Chem. 43(5): 1132–1137; bRadi, M., Mahrouz, M., Jaouad, A., Tacchini, M., Aubert, S., Hughes, M., Amiot, M.J. 1997. Phenolic composition, browning susceptibility, and carotenoid content of several apricot cultivars at maturity. HortScience 32(6): 1087–1091.

The most suitable cultivars should be selected, or developed, for compatibility with fresh-cut processing, packaging, and distribution. At this stage, many characteristics may be considered, such as fruit shape or size, flesh thickness, susceptibility to pre- and postharvest pathogens, susceptibility to browning, desiccation, texture loss, etc. (Romig, 1995). Different fruit cultivars may vary in their respiration rates when cut for processing. A comparison of four pear cultivars commonly available in the U.S. (cv. Bosc, Bartlett, Anjou, and Red Anjou) showed that pears of the cv. Bartlett showed no difference in respiration rate when stored at 10∞C in air and 90 to 95% relative humidity. However, slices prepared from the cv. Bosc had a 35% increase, cv. Anjou a 64% increase, and cv. Red Anjou a 232% increase in respiration in relation to the intact fruit of the same cultivar. The evolution of carbon dioxide in all cultivars was between two- and fourfold greater when temperature was increased from 0∞C to 10∞C (Gorny et al., 2000). Varying composition of different cultivars will affect final product quality. The browning tendency of different fruit cultivars is exemplified for pears and apricots in Table 3.2. The pear cultivars evaluated in this study showed almost a fourfold range in their difference in lightness (DL value) between fresh and oxidized purees. Apricot cultivars showed a similar range of difference in browning tendency. Bartlett pear slices had a large decrease (82%) in flesh firmness, in contrast with cultivar Anjou (12% decrease in firmness) after 6 d at 10∞C (Gorny et al., 2000). Variations in the shelf life (ranging from 2 to 12 d at 0∞C) of different cultivars of sliced peaches and nectarines are shown in Figure 3.1. Evaluation of shelf life was based on appearance; products developed surface browning, pit cavity breakdown, and limited dehydration, resulting in loss of sheen and gloss at the cut surface (Gorny et al., 1999).

59

Fresh-Cut Fruits

Peaches Nectarines Arctic Queen August Red September Red Sparkling Red Summer Diamond Summer Fire Summer Grand Zee Grand 0

2

4

6

8

10

12

14

Marketable shelf-life (days)

August Lady Cal Red Carnival Elegant Lady Flavorcrest O’Henry Red Cal Red Sun Ryan Sun Snow Giant Snow King Summer Lady Tra Zee 0

2

4

6

8

10 12 14

Marketable shelf-life (days)

FIGURE 3.1 Genotypic differences in shelf life of fresh-cut peaches and nectarines kept in air at 0∞C and 90 to 95% relative humidity. (From Gorny, J.R., Hess-Pierce, B., Kader, A.A. 1999. Quality changes in freshcut peach and nectarine slices as affected by cultivar, storage atmosphere and chemical treatments. J. Food Sci. 64(3): 429–432.)

Many agronomic (soil type, fertilizer application, water supply, etc.) and environmental (climate and rainfall) factors also affect fruit quality. Sucrose is the main soluble sugar in muskmelon flesh, and adverse weather conditions in the weeks prior to harvest can have a negative effect on fruit sweetness. Short-term shading (5 d at ~ 50% reduced daylight) of muskmelon plants prior to harvesting promoted accumulation of acetaldehyde and ethanol and reduction of sucrose content in melon flesh (Nishizawa et al., 1998). Cultivars may also respond differently to conditions adopted in modified atmosphere packaging. Different cultivars of strawberries showed wide variation in response when exposed to 20% carbon dioxide for up to 9 d. Some cultivars (such as Honeoye and Kent) accumulated large levels of fermentation products associated with off-flavor development, such as acetaldehyde and ethanol. Depending on the commodity, different traits may be relevant in selecting cultivars for freshcut preparation. While tendency to brown is important in fresh-cut apples and pears, there may be more interest in other quality attributes such as texture retention in sliced kiwifruit, absence of seeds in watermelon for fresh-cut use, reduced off-flavor production in strawberries, and a balance between sweetness and acidity in pineapple.

3.4.2 OPTIMUM DEGREE

OF

RIPENESS

FOR

PROCESSING

Quality and shelf life of fresh-cut fruit are affected by maturity stage. While a fresh-cut fruit is expected (by the consumer) to be at its optimum ripeness, with the best sensory attributes of flavor, taste, and firmness, this is usually not the best maturity stage for handling and processing. Often a fully ripe fruit may be at an advanced stage of softening for fresh-cut preparation, whereas an immature fruit may not present adequate eating quality (Kader and Mitcham, 1998). Synthesis of both flavor and color compounds occur relatively late in fruit maturation; therefore an early harvest may result in poor flavor and color. Shorter shelf life results from harvest of either immature or overripe fruit that are more susceptible to physiological disorders than fruit harvested at the proper maturity (Kader, 2002). In a study of sliced pears (cv. Bartlett), ripe fruit had a shelf life of ~2 d at 0∞C, in contrast with partially ripe and mature-green slices with a shelf life of 8 d. However, the eating quality of mature-green fruit was compromised due to poor aroma and lack of juiciness (Gorny et al., 2000). Likewise, using quality scores based on appearance, firmness, and taste, honeydew melon cubes prepared from fruit at the immature to mature threshold stage (8.8% soluble solids) were rated as poor after storage of 3 d at 10∞C, and as fair to poor after 7 d at 5∞C. Melon

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Processing Fruits: Science and Technology, Second Edition

cubes from very mature fruit (13% soluble solids) were rated as good to fair after 7 d at 5∞C and fair after 3 d at 10∞C. Fresh-cut products from immature fruit did not have honeydew taste or aroma and deteriorated faster than product from mature fruit (Watada and Qi, 1999).

3.4.3 MICROBIAL SPOILAGE Although it is considered that whole fresh produce, for the most part, are among one of the safest foods (Brackett, 1987), the safety of fresh-cut products requires a great deal of attention. The processing that fruit undergo turn them more vulnerable to microbiological risks, and the sensory quality is meaningless if the product is unsafe (Brackett, 1992). Along the chain of steps involved in the production of fresh-cut products, from the growth of the raw material to the processing and distribution, there is potential for microbiological risk. Microbial contamination may (1) be of public health significance due to the presence of human pathogens, or (2) decrease the shelf life of the fresh-cut fruit due to spoilage microorganisms, and in addition render a major economic impact. Appropriate care should start in the farm environment. Contamination may result from soil, poor agronomic practices such as use of raw or improperly treated animal manure as fertilizer, irrigation with contaminated water, animal contact, etc. During fruit picking and packing, contamination may occur through contact with food handlers who may be carriers of microbial pathogens. In general, all fruit arrive at the receiving section of the food processing plant with a microbial load. In the initial steps of processing, with peeling and cutting there is removal of protective barriers (skin, peel, rind) and release of nutrient-rich cellular fluids, conditions that provide increased risk for microbial contamination of the product. After some outbreaks occurred due to the consumption of cantaloupe from salad bars, scientists concluded that microorganisms present in the melon rind were most likely introduced into the fruit during cutting of the melons (Madden, 1992). The normal microflora found on the surface of produce is diverse, but it generally includes a variety of fungi, mostly harmless bacteria and different species of yeasts. It is expected that different microorganisms will be present in produce that grows on top of the soil vs. a tree-borne fruit. Fungi constitute the majority of spoilage microorganisms found in fruits (Brackett, 1987). However, human pathogens may be present in the fruit microflora. Clostridium botulinum, Clostridium perfringens, and Bacillus cereus can be isolated from soils free of fecal contamination. Listeria monocytogenes can be found in decaying vegetation and soil, as can coliforms of nonfecal origin such as Enterobacter spp. and Klebsiella spp. Enteric microorganisms, among them E. coli, are linked to soils contaminated with feces, improperly composted manure or sewage, and contaminated irrigation water. In agricultural environments, animals (domestic and wild) constitute another source of pathogenic bacteria. Salmonellae are found in the intestine of many animals and are abundant in fecal material and sewage (Francis et al., 1999). Birds that feed at sewage works can be a vector for L. monocytogenes and enteric bacteria into farm environment. Enteric bacteria can be transmitted to flowers during pollination by insects (NACMCF, 1999). Viruses such as the Hepatitis A and Norwalk can also be found in foods. Fresh-cut product contamination may also occur during harvesting, processing and handling as a result of inadequate personal hygiene among employees, poor sanitation of the processing facilities, dirty cutting tools and work surfaces, and reuse of wash water or ice. In addition, due to certain characteristics of the fresh-cut fruit processing and packaging, there are additional risks. While one of the initial steps in the processing of fresh-cut products is sanitation, not all microorganisms are eliminated. After breaking the fruit surface integrity with peeling and cutting, rapid bacterial growth can take place. However, the low pH of most fruits does not support the growth of most pathogens (Brackett, 1987). For example, the pH of pineapple ranges between 3.5 and 4 (Siriphanich, 1994). Nevertheless, high pH values are common in different types of melons (Lund, 1992), such as honeydew (pH 5.2 to 5.6), cantaloupe (pH 6.2 to 6.5), and watermelon (pH 6.3 to 6.7), and these fruit are particularly susceptible to microbial contamination and growth.

Fresh-Cut Fruits

61

Storage temperature is a very important factor in controlling microbial growth. Although storage under refrigeration temperatures deters the growth of many microorganisms, some can grow at these conditions, as shown for L. monocytogenes (Nguyen-The and Carlin, 1994). With the use of certain types of packaging (such as in modified atmosphere packaging) and long periods of storage, some pathogenic microorganisms may attain numbers high enough to pose a health risk. This could result from the growth of a pathogen population under conditions where there is a suppression of the natural microflora that under normal circumstances would play a positive role in competing with pathogens. The long storage periods allow time for available pathogens to grow, and significant microbial populations may develop. Additionally, if temperature abuse occurs, anaerobic atmospheres can develop inside packages during extended storage (Francis et al., 1999). While these conditions do not favor the growth of some microorganisms, C. botulinum can grow under high CO2. Product appearance or sensory characteristics may not be affected and the potential health risk may not be perceived by the consumer (IFPA, 1996b). Following the strict guidelines of GMP and a well-designed HACCP (Hazard Analysis Critical Control Points) plan are fundamental recommendations of the International Fresh-Cut Produce Association (1996a, 1997) in order to produce a microbiologically safe product.

3.4.4 BROWNING CONTROL Surface discoloration is probably the most common quality defect of fresh-cut fruit and the factor most limiting shelf life. During the peeling and cutting operations, cells are broken, and their contents include previously compartmentalized enzymes that are suddenly freed to come in contact with their substrates. A group of enzymes called polyphenol oxidases (PPO), which occur in particularly high amounts in fruits such as banana, apple, pear, avocado, and peach, are responsible for the discoloration referred to as enzymatic browning. Enzymatic browning may be controlled through the use of physical and chemical methods; often both are employed. Physical methods commonly used include reduction of temperature and oxygen, and use of modified atmosphere packaging or edible coatings. Chemical methods depend on either treatment with compounds that inhibit polyphenol oxidase, remove its substrates (oxygen and phenolics), or function as preferred substrates. Various antibrowning agents can be used in postcutting dip solutions. An overview on preservative treatments for fresh-cut products was recently published (Garcia and Barrett, 2002). Ascorbic acid is probably the most common antibrowning agent selected for use in fresh-cut fruit. The drawback of ascorbic acid is that it confers only temporary protection because it is oxidized in the process of preventing browning. While the inhibitory effect of ascorbic acid on browning is due to its reductant action on quinones, other common types of antibrowning agents available for use in fresh-cut fruits include acidulants (e.g., citric acid, malic acid) and chelators (e.g., EDTA), which are often used in combination in browning prevention. Antibrowning treatments commonly include ascorbic acid or its isomer erythorbic acid (d-isoascorbic acid), citric acid, EDTA, cysteine, and its derivatives. Research papers have presented results on 4-hexylresorcinol, which has shown effective control of enzymatic browning in fruits (Sapers, 1993); however, 4hexylresorcinol is currently only approved for use in the prevention of shrimp discoloration (McEvily et al., 1991). No single antibrowning treatment can control enzymatic browning of all fresh-cut fruit because there is not just one possible solution for a particular fruit product. This can be illustrated by the examples shown in Table 3.3, where various antibrowning treatments selected by different authors for use in fresh-cut pears are presented. Although there has been commercial interest in the development of fresh-cut pear products, pear fruit presents some particular challenges, such as its high susceptibility to enzymatic browning and tissue softening. Fruit ripeness stage proved to be an important factor in the control of browning in pears. Overripe pears are generally prone to more severe discoloration (Sapers and Miller, 1998; Dong et al., 2000); optimal flesh firmness for

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Processing Fruits: Science and Technology, Second Edition

TABLE 3.3 Examples of Treatments Suggested for Controlling Enzymatic Browning in Fresh-Cut Pears Pear

Firmness

Antibrowning Treatment

Bartlett Anjou

44.5 N 4.0 kg

1% CaCl2 + 0.5% O2 atmosphere 4% Sodium erythorbate, 0.2% CaCl2 100 ppm 4-Hexylresorcinol 1-min dip; storage at 4∞C Modified Atmosphere Packaging (MAP)

Bartlett

5.7 kg

4% Sodium erythorbate, and/or 0.2% CaCl2 and/or 4-Hexylresorcinol MAP

Bartlett

27–45 N

2% Ascorbic acid 1% Calcium lactate 1-min dip

Anjou Bartlett Bosc

36–45 N 45–67 N 27–45 N

0.01% 4-Hexylresorcinol 0.5% Ascorbic acid 1% Calcium lactate 2-min dip Storage at 2–5∞C in pouches partially vacuumed

Anjou Bartlett Bosc

Slices: 22 N 36 N 22 N

0.01 M 4-Hexylresorcinol 0.5 M Isoascorbic acid 0.05 M Potassium sorbate pH 5.5 30-sec dip Storage at 5∞C, 14 d

Bartlett

45–58 N

2% Ascorbic acid 1% Calcium lactate 0.5% Cysteine pH 7.0 5-min dip at 20∞C

Bosc

Comments

Slightly underripe fruit. Use of 0.25% NaCl holding solution after cutting With Anjou pears, neutral dip (pH 7.7) more effective than acidic (pH 3.3) dip Less firm fruit browned more, and soft Bartlett pears developed tissue breakdown Shelf life at least 14 d (Anjou and Bartlett) Trials with 0.2% cysteine did not inhibit browning, and induced red or pink discoloration 4-Hexylresorcinol prevented core tissue browning Treatment not effective on Bosc pears Product observed for 2 d at 20∞C; surface color was not significantly different from that at the start of experiment Partially ripe fruit was peeled Shelf life of 30 d for Bartlett and Bosc, and 15–20 d for Anjou pears Sensory panelists detected difference in taste as compared with water-treated control Browning inhibitor of both cut surface and edges of the slices Less inhibition in Bartlett pears In Anjou initial firmness (21–52 N) did not affect browning control When acidic (pH 3.7) dip was used, a pinkish-red discoloration was observed Dip alone was unable to control browning at the edges of flesh beneath the skin Sensory panelists did not perceive difference in taste

Reference Rosen and Kader (1989) Sapers and Miller (1998)

Gorny et al. (1998)

Dong et al. (2000)

Buta and Abbott (2000)

Gorny et al. (2002)

Note: Unless indicated, firmness was measured in whole fruit, and fresh-cut products were prepared from unpeeled fruit. N = Newtons.

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Fresh-Cut Fruits

6 banana

Firmness (N)

5 4

kiwi 0 24 hr 48 hr

3 2 1 0 0

2

20 0 Ethylene in air (ppm)

2

20

FIGURE 3.2 Changes in firmness of sliced kiwifruit and bananas kept at 20∞C in atmosphere of 0, 2, or 20 ppm of ethylene in air. (From Watada, A.E., Ko, N.P., Minott, D.A. 1996. Factors affecting quality of freshcut horticultural products. Postharvest Biol. Technol. 9: 115–125.)

fresh-cut processing needs to be determined in order to obtain a product of high quality and long shelf life. In addition, fruit cultivar is a very important factor to consider; as discussed previously, the browning potential can vary greatly among cultivars (Amiot et al., 1995). Pear fruit size was also shown to affect surface discoloration in pears. Gorny and coworkers (2000) found that pear slices prepared from small-size fruit discolor more than slices cut from larger fruit. This may indicate a difference in maturity and activity of the polyphenol oxidase enzyme.

3.4.5 PREVENTION

OF

TEXTURAL LOSSES

Fruit texture is perceived by the consumer prior to taste. When biting into a piece of apple, crunchiness is perceived before juiciness. Softening or loss of tissue firmness is a quality defect that compromises the shelf life of many fresh-cut fruits. Examples of changes in the firmness of sliced kiwifruit and banana during storage in various ethylene concentrations are shown in Figure 3.2. Selection of appropriate fruit cultivars is important in avoiding apple fruit prone to mealiness, for instance, or choosing cv. with attractive characteristics of juiciness and crispness. However, textural defects may develop after preparation of the fresh-cut product. Figure 3.3 illustrates pear

FIGURE 3.3 Ripe fresh-cut pears (cv. Bartlett) a few minutes after cutting. Note the translucency at the edges of the pear pieces.

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pieces prepared from ripe fruit, where translucent edges were noticeable just a few minutes after cutting. Fruit had been selected based on its superior eating qualities (flavor, sweetness, juiciness, and texture), but it was evidently inadequate for fresh-cut processing. In fresh-cut kiwifruit, flesh softening is the most noticeable change after cutting (Varoquaux et al., 1990); the cut surface darkens not due to enzymatic browning, but rather to the appearance of a translucent water-soaked tissue (Agar et al., 1999). The loss of firmness increases with storage temperature and time, while removal of ethylene from the storage atmosphere improves retention of slice texture. The most common treatment used to improve texture retention is to dip fruit pieces in calcium solutions, as described for strawberries (Main et al., 1986), and pears and strawberries (Rosen and Kader, 1989). The firming effect of calcium is attributed to the formation of complexes with polygalacturonic acid residues in the middle lamella and cell wall (Van-Buren, 1979). Both calcium chloride and calcium lactate are frequently used. However, trained sensory panels have judged that calcium chloride imparts a bitter flavor to fresh-cut cantaloupe (Luna-Guzmán and Barrett, 2000). Moreover, typical melon flavor was better detected in cantaloupe treated with 1% calcium lactate as compared with 1% calcium chloride. In this study, we found that the initial firming effect of both calcium chloride and calcium lactate on melon cylinders was the same; however calcium lactate–treated samples tended to maintain higher firmness during storage. A combined treatment using low-temperature blanching prior to dipping in calcium solution has been associated with a decrease in pectin esterification by pectinmethylesterase, creating potential sites for cross-linking with calcium (Stanley et al., 1995). A study of the effect of combined calcium chloride dips and heat treatment on firmness of fresh-cut cantaloupe carried out in our group showed improved firmness as compared with a calcium dip alone. However, it may be that rather than pectinmethylesterase activation, the results observed could be related to a membrane or turgor pressure effect. No significant difference was observed in the amount of bound calcium between samples treated at 60, 40, or 20∞C (Luna-Guzmán et al., 1999). The texture of fresh-cut apples has been reported to improve with the application of heat treatments to apples prior to slicing (Kim et al., 1994). Three apple cv. (Delicious, Golden Delicious, and McIntosh) that had been kept in cold storage (2∞C, 90% RH) for less than 2 months were treated for 1.75 h in a water bath at 45∞C, then held overnight at 2∞C. Fruit were sliced and stored at 2∞C for 21 d in unsealed polyethylene bags. Whereas untreated control samples exhibited a steady loss of flesh firmness during storage, heat-treated samples showed an initial increase in firmness (up to 7 d for cv. Golden Delicious and up to 14 d for cv. Delicious), followed by a decrease in firmness. At 7 d of storage, the difference in firmness between heat-treated and control samples were 12% for cv. McIntosh, 34% for Golden Delicious, and 28% for Delicious. The greatest difference observed was with cv. Delicious at 14 d of storage, where the heat-treated sample was ~40% firmer than the untreated control.

3.4.6 APPEARANCE

AND

SENSORY QUALITY

The most appealing attributes of fresh-cut products include their perception of freshness, taste and flavor, in addition to convenience. When assessing quality, consumers take product appearance into consideration as a primary criterion. Appearance can be characterized by size, shape, color, gloss, condition, and absence of defects. However, product color contributes more than any other single factor (Kays, 1999). While appearance may have a major impact on consumers, sensory quality (taste, aroma, texture) is what will ensure consumers’ repeated acceptance of a fresh-cut product. Most of the quality indexes used by both academia and industry for these products are visual, based on appearance; little research has been done on sensory quality of fresh-cut produce (Beaulieu and Baldwin, 2002). General methods for determining quality characteristics of fresh produce are described elsewhere (Mitcham and Kader, 1998).

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The relative importance of each quality characteristic depends on the commodity; for instance, gloss is relevant for peach slices but may not be for persimmon wedges. When the optimum ripeness stage for fresh-cut preparation occurs before full ripening and fruit flavor has yet to completely develop, even a fresh-cut product with very good appearance will lack the characteristic flavor of a perfectly ripe fruit. In addition, some flavor components may disappear after processing, or in other cases off-flavors may develop due to either physiological changes in the fruit tissue or microbial spoilage after packaging. Nevertheless, it has been reported that when off-flavors of microbial origin are detected, product appearance is likely to be compromised (Huxsoll et al., 1989).

3.4.7 NUTRITIONAL ASPECTS In addition to their sensory appeal, for nutritional reasons fruit should be an important part of a healthy diet. Although it is generally low in calories due to its high moisture content, fruit supplies vitamins, minerals, phytonutrients, and fiber to the diet. One of the most significant contributions to our diet comes from the high content of vitamin C and precursors of vitamin A (carotenoids) found in fruits. While citrus fruits are well known as sources of vitamin C, kiwifruit, melons, tomatoes, and berries are also significant sources of this vitamin. Good sources of carotenoids include yellow-fleshed peaches and nectarines, mangoes, papaya, and persimmons. Apples and pears are good sources of fiber, as are many berries (Margen et al., 1992). While it has been assumed that fresh-cut products would have lowered nutrient content than the commodities they are derived from (Klein, 1987; McCarthy and Matthews, 1994), some experimental data do not indicate this. The shelf life of fresh-cut persimmons and peaches was limited before major losses of carotenoids were detected (Wright and Kader, 1997b). The same was observed with the vitamin C content of strawberries and persimmons (Wright and Kader, 1997a). However, significant oxidation of ascorbic acid was determined when washing fruit slices with chlorinated water (100 ppm sodium hypochlorite) in comparison to washing with water. An intermediate level of oxidized ascorbic acid, or dehydroascorbic acid, was obtained when washing the whole fruit in chlorinated water before slicing it. Fresh-cut kiwifruit stored for 6 d at different temperatures showed a gradual decrease in total vitamin C content with increased temperature. While a loss of 8% in relation to initial vitamin C level in kiwifruit occurred at 0∞C, there was a decrease of 13 and 21% at 5 and 10∞C, respectively. In ethylene-free storage, ascorbic acid levels were threefold higher than in control slices stored in air (Agar et al., 1999). Losses of ascorbic acid in whole pears stored under controlled atmosphere conditions (low O2 and high CO2) revealed that in the cv. Rocha, losses occurred mainly during long-term storage, while in the cv. Conference, most of the ascorbic acid decreased when pears were transferred to controlled atmosphere storage. It is a common practice to store apples and pears under controlled conditions for extended periods of time prior to processing. Lowered levels of ascorbic acid in the fruit flesh will increase susceptibility to browning (Veltman et al., 2000). In recent years there has been a great interest in phytochemicals due to a variety of potential health benefits they may have. This is an emerging field of research, and very little information is available on concentrations of such compounds in different plant tissues, and even less on possible effects of cultivar, agronomic practices, maturity stage, and postharvest handling on the retention of phytochemicals. In a study with whole apples stored at 1∞C for 2 months, the cv. Granny Smith and Delicious had increased antioxidant levels of up to 10-fold, but in longer storage substantial decreases in antioxidants were reported (Curry, 1997).

3.5 SHELF LIFE EXTENSION OF FRESH-CUT FRUIT Once the negative consequences brought about by injuries caused by fresh-cut processing are recognized, it is important to search for means of extending the shelf life of fresh-cut fruits. The

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objective is to maintain a fresh-like appearance, flavor, and nutritional quality of the product while ensuring its safety. Among the most common strategies used to extend the shelf life of fresh-cut fruits are temperature management and modified atmosphere packaging. Other benefits can be found with the application of edible coatings. It is important to consider that the consumer perceives a fresh-cut product as fresh-like produce, just minimally prepared to be ready for eating. It is not expected that many chemicals are added to such products or that they are treated in ways that are not considered wholesome by the public. The perception of fresh-cut products as “natural” is part of its appeal. In a survey on the perception of convenience products, consumers revealed the desire that such products maintain fresh characteristics longer without the use of food preservatives (Bruhn, 1994).

3.5.1 TEMPERATURE MANAGEMENT Temperature control is very critical in prolonging the shelf life of fresh-cut products, serving to minimize losses due to wound-related responses and microbial spoilage. At low temperatures, respiration rates and enzymatic activities are reduced, and general metabolic rates are also lowered, extending product shelf life. The growth of some spoilage microorganisms and foodborne pathogens is decreased under the low-temperature conditions recommended for fresh-cut fruit processing and storage. The results presented in Table 3.1 show the effect of temperature on respiration rates in some fruits. Fresh-cut fruit ideally should be kept close to 0∞C, even for chilling-sensitive fruit, because the quality deterioration that results from storage at nonchilling temperatures is more damaging than that resulting from chilling injury (Watada et al., 1996). Rapid cooling of fresh-cut produce reduces respiration and deterioration rates, as well as microbial spoilage. However, it is of paramount importance to maintain product under refrigeration throughout the processing and distribution, and up to consumption.

3.5.2 MODIFIED ATMOSPHERE PACKAGING Modified atmosphere packaging (MAP) can extend the shelf life of produce by minimizing water loss, reducing respiration and ethylene production rates, decreasing metabolic activity, and reducing microbial growth and decay. The widely used MAP aims at the creation of an ideal gas composition inside the package. The modification of the atmosphere surrounding the fresh-cut produce can be achieved either through the establishment of an active modified atmosphere (gas flushing) in the package or be generated over time by the fruit pieces respiring in the sealed package. Despite additional costs, active MAP has been preferred because of the limitations in regulating a passively established atmosphere (Kader and Watkins, 2000). In modified atmospheres, O2 levels are generally reduced and CO2 is increased; high levels of CO2 lead to a decreased sensitivity of plant tissues to ethylene (Kader et al., 1989). When O2 levels are lowered, respiration of produce begin to decrease, and generally continues to decrease with lowering O2 levels down to a level where anaerobic respiration takes place. Among the responses of plant tissue to low O2 levels include reduction in respiration, reduced ethylene synthesis and perception, reduced chlorophyll degradation, reduced cell wall degradation, and reduced phenolics oxidation. Negative responses to low O2 include induction of fermentation, accumulation of acetaldehyde, ethanol and lactate, and reduced biosynthesis of aroma compounds (Beaudry, 2000). In the establishment of safe atmospheres it is important to determine product tolerance to low O2 levels and high CO2 levels. Watkins (2000) reviewed the effects of high CO2 on produce. In general, fruit tolerances for high concentrations of CO2 and low O2 vary for the same commodity between fresh-cut products and the intact fruit. It is important to experimentally determine the appropriate gas composition for each particular product, which extends the product shelf life and also prevents the damaging

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effects of low levels of O2 or high levels of CO2. These may lead to anaerobic respiration and increase susceptibility to decay. An adequate MAP should help achieve a decrease in respiration while preventing anaerobic respiration. In the design of the optimal MAP, it is of utmost importance to know the ideal atmosphere for each particular fresh-cut fruit product and its respiration rate under a certain temperature. In addition, it is necessary to avoid temperature abuse during storage; otherwise the atmospheric composition inside the package changes, which negatively impacts the product. Recommended atmospheres for storage of selected fresh-cut fruits were reviewed by Gorny (1997). Another important aspect in the application of modified atmosphere packaging is the appropriate selection of packaging material. Common packages include flexible film pouches, rigid trays overwrapped with flexible films, and rigid plastic containers with gas diffusion windows. The most common polymeric film used in MAP is oriented polypropylene; other examples are low-density polyethylene (for bagged products), monolayer polyvinyl chloride (for overwrapped trays), and blends of low- and medium-density polyethylene with ethylene vinyl acetate; microperforated films have been studied as a way of overcoming anaerobic conditions in the package (Al-Ati and Hotchkiss, 2002). A comparison of different systems for storage of fresh-cut cantaloupe was carried out using the cv. Athena, considered adequate for fresh-cut processing (Bai et al., 2001). Cantaloupe cubes were stored either in (1) pouches where they were allowed to develop a “natural” modified atmosphere (nMAP) through produce respiration, or (2) packages that were flushed with a gas mixture containing 4 kPa of O2 plus 10 kPa of CO2 (fMAP), or (3) packages where the package film was perforated with a needle (PFP). In cantaloupe melons, quality deterioration is primarily related to the development of tissue translucency; salable quality may be limited when the level of translucency is greater than 20%. Although shelf life of cantaloupe cubes (Figure 3.4) was prolonged under both nMAP (9 d) and fMAP (12 d) conditions, a faint off-odor was detected in all samples by day 12, with the exception of one of the three replicate trials for the fMAP. Fresh-cut fruits do not respond to MAP as well as young vegetative fresh-cut tissues; the effects of MAP on control of senescence rate have been only marginally effective. Many factors influence the atmospheric composition at equilibrium inside the package. Film type, thickness, area, weight 100 PFP nMAP

Translucency (%)

80

fMAP 60

40

20

0 0

2

4

6

8

10

12

Storage (days)

FIGURE 3.4 Changes in tissue translucency of cantaloupe cubes stored in packages with overlap film perforated with 10 holes of 1.5 mm (PFP), packages containing a naturally formed modified atmosphere (nMAP), and packages in which the internal atmosphere was flushed with a gas mixture of 4 kPa O2 plus 10 kPa CO2 prior to storage. (From Bai, J.-H., Saftner, A.E., Watada, A.E., Lee, Y.-S. 2001. Modified atmosphere maintains quality of fresh-cut cantaloupe (cucumis melo L.). J. Food Sci. 66(8): 1207–1211.)

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of fresh-cut fruit in the package, temperature, relative humidity, and product respiration rate are all important. In spite of the advances occurring in film technology, currently available films are not considered as satisfying the requirements of fresh-cut fruit. Current films meet the needs of either CO2 or O2 because they allow permeability of CO2 at higher rates than O2, which constitutes a shortcoming of MAP for fresh-cut fruit application (Al-Ati and Hotchkiss, 2002). More research is needed in establishing the most appropriate atmospheres for fresh-cut fruits, which are expected to vary with cultivar, growing location, and duration of storage prior to processing (Gorny, 1997).

3.5.3 HUMIDITY Humidity is another important factor that should be controlled in fresh-cut fruit product handling and storage. Although water is the most abundant (80 to 90% of the fresh weight) component of fruit tissues, even small changes (~5%) in the water content of fruit are detrimental to quality. In fresh-cut products the removal of skin or rind and cutting lead to exposure of a large surface as seen in fruit slices, cubes, and wedges, which are prone to great water loss. Surface dehydration occurs quickly and has a negative impact on product appearance, resulting in a product exhibiting less gloss, greater wrinkling, wilting, or flaccidness. However, poor appearance is not the only possible defect resulting from water loss; cellular metabolism may be affected, fruit ripening may be accelerated, and ripening related softening significantly impacted (Paul, 1999). Desiccation may also favor the development of microorganisms that tolerate low moisture, such as fungi (Brackett, 1987). In the dewatering operation, it is important to avoid excessive water loss. Moreover, reduction of moisture loss can be achieved by decreasing the capacity of the surrounding air to hold water. This is done by lowering the temperature, increasing the relative humidity, or creating a barrier to water loss. The last mentioned is commonly done through the use of polymeric films in packaging; moisture retention can also be attained with edible coatings, as exemplified with fresh-cut papaya, where desiccation is a major problem (Siriphanich, 1994). However, in film-packaged products, water is formed as a result of respiration, and it condenses inside the package and becomes available for microbial growth (Al-Ati and Hotchkiss, 2002). This is another point demonstrating the need to avoid product temperature abuse and maintain lower respiration rates.

3.5.4 EDIBLE COATINGS The use of edible coatings is another method of extending the shelf life of fresh-cut fruit. Edible coatings consist of thin layers of protective materials applied to the surface of the fruit as a replacement for the natural protective tissue (epidermis, peel). Edible coatings are used as a semipermeable barrier that help reduce respiration, retard water loss and color changes, improve texture and mechanical integrity, improve handling characteristics, help retain volatile flavor compounds, and reduce microbial growth. It is possible to create a modified atmosphere enrobing fresh-cut produce in edible coating. (Baldwin et al., 1995; Baldwin et al., 1996; Nisperos and Baldwin, 1996). Different food additives can be incorporated into coating formulation, such as coatings with antioxidants (Baldwin et al., 1995). Control of surface browning in apples by ascorbic acid was improved when it was incorporated into an edible coating formulation compared to dipping. A carboxymethylcellulose-based coating did not control enzymatic browning of cut apples, but when such a coating was combined with additives (antioxidant, acidulant, and preservative), browning control was superior to dipping the fresh-cut produce in solutions with the same additives (Baldwin et al., 1996). Examples of browning inhibition in apple slices have been described for different edible coatings (Avena-Bustillos and Krochta, 1993; Kinzel, 1992). Edible coatings made from apple puree to which lipids and beeswax were added helped control moisture loss and browning of fresh-cut apples (McHugh and Senesi, 2000). Coated and wrapped (“apple wraps”) fresh-cut apple products were compared; apple pieces were either coated with (dipped in) a solution of 70% apple puree, 27%

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vegetable oil, 1.5% ascorbic acid, and 1.5% citric acid or wrapped in a film of the same composition. Moisture loss was significantly reduced in apple wraps during storage at 5∞C for 12 d. While coatings inhibited browning by 80% over a period of 3 d, wraps showed a 100% inhibition of browning for up to 10 d at 5∞C, in addition to maintaining texture, fruity flavor, and odor. It is important to understand the need for an integrated approach in the processing of fresh-cut fruits. In order to ensure prolonged shelf life, all steps should be carried out taking into consideration the requirement for superior fruit quality of cultivars selected for fresh-cut, adoption of good sanitation practices, gentle handling throughout processing, adequate temperature management, and appropriate choice of packaging and storage conditions. All these factors should be seen as a way of ensuring not only extended shelf life, but also a convenient high quality product that is safe, nutritionally sound, and appealing to the senses.

REFERENCES Abeles, F.B., Morgan, P.W., Saltveit, M.E. 1992. Ethylene in Plant Biology. 2nd ed., Academic Press, San Diego, CA. Agar, I.T., Massantini, R., Hess-Pierce, B., Kader, A.A. 1999. Postharvest Co2 and ethylene production and quality maintenance of fresh-cut kiwifruit slices. J. Food Sci. 64(3): 433–440. Al-Ati, T., Hotchkiss, J.H. 2002. Application of packaging and modified atmosphere to fresh-cut fruits and vegetables. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 305–338. Amiot, M.J., Tacchini, M., Aubert, S., Oleszek, W. 1995. Influence of cultivar, maturity stage, and storage conditions on phenolic composition and enzymatic browning of pear fruits. J. Agr. Food Chem. 43(5): 1132–1137. Avena-Bustillos, R.J., Krochta, J.M. 1993. Water vapor permeability of caseinate-based edible films as affected by pH, calcium crosslinking and lipid content. J. Food Sci. 58: 904–907. Bai, J.-H., Saftner, A.E., Watada, A.E., Lee, Y.-S. 2001. Modified atmosphere maintains quality of fresh-cut cantaloupe (cucumis melo L.). J. Food Sci. 66(8): 1207–1211. Baldwin, E.A., Nisperos-Carriedo, M.O., Baker, R.A. 1995. Use of edible coatings to preserve quality of lightly (and slightly) processed products. CRC Crit. Rev. Food Sci. Nutr. 35(6): 509–524. Baldwin, E.A., Nisperos, M.O., Chen, X., Hagenmaier, R.D. 1996. Improving storage life of cut apple and potato with edible coating. Postharvest Biol. Technol. 9(2): 151–163. Beaudry, R.M. 2000. Responses of horticultural commodities to low oxygen: limits to the expanded use of modified atmosphere packaging. HortTechnology 10(3): 491–500. Beaulieu, J.C., Baldwin, E.A. 2002. Flavor and aroma of fresh-cut fruits and vegetables. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 391–425. Beuchat, L.R. 2000. Use of sanitizers in raw fruit and vegetable processing. In: Minimally Processed Fruits and Vegetables: Fundamental Aspects and Applications. S.M. Alzamora, M.S. Tapia, A. López-Malo (Eds.), Aspen, Gaithersburg, MD, pp. 63–78. Brackett. R.E. 1987. Microbiological consequences of minimally processed fruits and vegetables. J. Food Qual. 10: 195–206. Brackett, R.E. 1992. Shelf stability and safety of fresh produce as influenced by sanitation and disinfection. J. Food Protect. 55(10): 808–814. Brecht, J.K. 1995. Physiology of lightly processed fruits and vegetables. HortScience 30(1): 18–22. Bruemmer, J.H., Griffin, A.W., Onayemi, O. 1978. Sectionizing grapefruit by enzyme digestion. Proc. Florida Sta. Hortic. Soc. 91: 112–114. Bruh, C. 1994. Consumer perception of quality. In: Minimal Processing of Foods and Process Optimization: An Interface. R.P. Singh, F.A.R. Oliveira (Eds.), CRC Press, Boca Raton, FL, pp. 493–504. Buta, J.G., Abbott, J.A. 2000. Browning inhibition of fresh-cut ‘Anjou’, ‘Bartlett’, and ‘Bosc’ pears. HortScience 35(6): 1111–1113. Cantwell, M., Suslow, T. 1999. Fresh-cut fruits and vegetables: Aspects of physiology, preparation, and handling that affect quality. Fresh-Cut Workshop, Sept. 14–16, University of California–Davis.

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Cherry, J.P. 1999. Improving the safety of fresh produce with antimicrobials. Food Technol. 53(11): 54–57. Curry, E.A. 1997. Effect of postharvest handling and storage on apple nutritional status using antioxidants as a model. HortTechnology 7: 240–243. Dong, X., Wrolstad, R.E., Sugar, D. 2000. Extending shelf life of fresh-cut pears. J. Food Sci. 65(1): 181–186. FDA 2002. FDA’s Produce Safety Activities. U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, http://www.cfsan.fda.gov/~dms/prodact.html. Fonseca, J.M., Rushing, J.W., Testin, R.F. 1999. Shock and vibration forces influence the quality of fresh-cut watermelon. Proc. Fla. State Hortic. Soc. 112: 147–152. Francis, G.A., Thomas, C., O’Beirne, D. 1999. The microbiological safety of minimally processed vegetables. Int. J. Food Sci. Technol. 34(1): 1–22. Garcia, E., Barrett, D.M. 2002. Preservative treatments for fresh-cut fruits and vegetables. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 267–303. Garrett, E.H. 2002. Fresh-cut produce: tracks and trends. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 1–10. Gorny, J.R. 1997. A summary of CA and MA requirements and recommendations for fresh-cut (minimally processed) fruits and vegetables. CA ’97: Seventh International Controlled Atmosphere Research Conference. University of California, Davis, pp. 30–66. Gorny, J.R., Cifuentes, R.A., Hess-Pierce, B., Kader, A.A. 2000. Quality changes in fresh-cut pear slices as affected by cultivar, ripeness stage, fruit size, and storage regime. J. Food Sci. 65(3): 541–544. Gorny, J.R., Gil, M.I., Kader, A.A. 1998. Postharvest physiology and quality maintenance of fresh-cut pears. Acta Hortic. 464: 231–236. Gorny, J.R., Hess-Pierce, B., Cifuentes, R.A., Kader, A.A. 2002. Quality changes in fresh-cut pear slices as affected by controlled atmospheres and chemical preservatives. Postharvest Biol. Technol. 24: 271–278. Gorny, J.R., Hess-Pierce, B., Kader, A.A. 1999. Quality changes in fresh-cut peach and nectarine slices as affected by cultivar, storage atmosphere and chemical treatments. J. Food Sci. 64(3): 429–432. Heard, G.M. 2002. Microbiology of fresh-cut produce. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 187–248. Huxsoll, C.C., Bolin, H.R., King, A.D. Jr. 1989. Physicochemical changes and treatments for lightly processed fruits and vegetables. In Quality Factors of Fruits and Vegetables. J.J. Jen (Ed.). ACS Symp. Ser. No 405, Washington, DC, chap. 16 pp. 203–215. IFPA. 1996a. Model HACCP plan: a hazard analysis critical control point system food safety program for fresh-cut produce. International Fresh-Cut Produce Association. IFPA. 1996b. Food Safety Guidelines for the Fresh-Cut Produce Industry. 3rd ed. International Fresh-Cut Produce Association, Alexandria, VA. IFPA. 1997. Fresh-Cut Produce Handling Guidelines. 2nd ed. International Fresh-Cut Produce Association, Alexandira, VA. IFPA, 2002. International Fresh-Cut Produce Association. http://www.fresh-cuts.org. Kader, A.A. 2002. Quality parameters of fresh-cut fruit and vegetable products. In: Fresh-Cut Fruits and Vegetables: Science, Technology, and Market. O. Lamikanra (Ed.), CRC Press, Boca Raton, FL, pp. 11–20. Kader, A.A., Mitcham, B. 1998. Methods for determining quality of fresh horticultural commodities. In: Freshcut Products: Maintaining Quality and Safety, Postharvest Outreach Program, University of California, Davis. Kader, A.A., Zagory, D, Kerbel, E.L. 1989. Modified atmosphere packaging of fruits and vegetables. CRC Crit. Rev. Food Sci. Nutr. 28(1): 1–30. Kader, A.A., Watkins, C.B. 2000. Modified atmosphere packaging — Toward 2000 and beyond. HortTechnology 10(3): 483–486. Kays, S.J. 1999. Preharvest factors affecting appearance. Postharvest Biol. Technol. 15: 233–247. Kim, D.M., Smith, N.L., Lee, C.Y. 1993. Apple cultivar variations in response to heat treatments and minimal processing. J. Food Sci. 58: 1111–1114, 1124. Kim, D.M., Smith, N.L., Lee, C.Y. 1994. Effect of heat treatment on firmness of apples and apple slices. J. Food Process. Pres. 18: 1–8. Kinzel, B. 1992. Protein-rich edible coatings for food. Agr. Res. (May): 20–21.

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Klein, B.P. 1987. Nutritional consequences of minimal processing of fruits and vegetables. J. Food Qual. 10: 179–193. Luna-Guzmán, I., Barrett, D.M. 2000. Comparison of calcium chloride and calcium lactate effectiveness in maintaining shelf stability and quality of fresh-cut cantaloupes. Postharvest Biol. Technol. 19: 61–72. Luna-Guzmán, I., Cantwell, M., Barrett, D.M. 1999. Fresh-cut cantaloupe: effects of CaCl2 dips and heat treatments on firmness and metabolic activity. Postharvest Biol. Technol. 17: 201–213. Lund, B.M., Baird-Parker, T.C., Gould, G.W. 2000. The Microbiological Safety and Quality of Food. Aspen, Gaithersburg, MD. Madden, J.M. 1992. Microbial pathogens in fresh produce — the regulatory perspective. J. Food Protect. 55(10): 821–823. Main, G.L., Morris, J.R., Wehunt, E.J. 1986. Effect of preprocessing treatments on the firmness and quality characteristics of whole and sliced strawberries after freezing and thermal processing. J. Food Sci. 51: 391–394. Margen, S., and editors of UC Berkeley Wellness Letter. 1992. The Wellness Encyclopedia of Food and Nutrition. New York, Rebus, pp.187–189. McCarthy, A., Matthews, R.H. 1994. Nutritional quality of fruits and vegetables subject to minimal processes. In: Minimally Processed Refrigerated Fruits and Vegetables, R.C. Wiley (Ed.), Chapman & Hall, New York, pp. 313–326. McEvily, A.J., Iyengar, R., Otwell, W.S. 1991.Sulfite alternative prevents shrimp melanosis. Food Technol. 45(9): 80–86. McHugh, T.H. and Senesi, E. 2000. Apple wraps: a novel method to improve the quality and extend the shelf life of fresh-cut apples. J. Food Sci. 65(3): 480–485. Mitcham, B., Kader, A.A. 2000. Methods for determining quality of fresh horticultural commodities. In: FreshCut Products: Maintaining Quality and Safety, Postharvest Horticultural Series No. 10, Section 3b, Unversity of California, Davis. NACMCF, 1999. United States National Advisory Committee on Microbiological Criteria for Foods. Microbiological safety evaluations and recommendations on fresh produce. Food Control 10: 117–143. Nguyen-the, C., Carlin, F. 1994. The microbiology of minimally processed fresh fruits and vegetables. CRC Crit. Rev. Food Sci. Nutr. 34(4): 371–301. Nishizawa, T., Taira, S., Nakanishi, M., Ito, M., Togashi, M., Motomura, Y. 1998. Acetaldehyde, ethanol, and carbohydrate concentrations in developing muskmelon fruit (Cucumis melo L. cv. Andesu) are affected by short-term shading. HortScience 33(6): 992–994. Nisperos, M.O., Baldwin, E.A. 1996. Edible coatings for whole and minimally processed fruits and vegetables. Food Aust. 48(1): 27–31. O’Hare, T.J. 1994. Respiratory Characteristics of Cut Pineapple Tissue. 1994 Report. Postharvest Group, DPI, Queensland, Australia. Paull, R.E. 1999. Effect of temperature and relative humidity on fresh commodity quality. Postharvest Biol. Technol. 15: 263–277. Paull, R.E., Chen, W. 1997. Minimal processing of papaya (Carica papaya L.O and the physiology of halved fruit. Postharvest Biol. Technol. 12: 93–99. Paull, R.E., Rohrbach, K.G. 1985. Symptom development of chilling injury in pineapple fruit. J. Amer. Soc. Hortic. Sci. 110: 100–105. Pollack, S.L. 2001. Consumer demand for fruit and vegetables: the U.S. example. In: Changing Structure of Global Food Consumption and Trade. A. Regmi (Ed.), ERS WRS No.01-1, pp. 49–54. Portela, S.I., Cantwell, M.I. 2001. Cutting blade sharpness affects appearance and other quality attributes of fresh-cut cantaloupe melon. J. Food Sci. 66(9): 1265–1270. Pretel, M.T., Lozano, P., Riquelme, F., Romojaro, F. 1996. Pectic enzymes in fresh fruit processing: optimization of enzymic peeling of oranges. Process. Biochem. 32: 43–49. Pretel, M.T., Fernández, Romojaro, F., Martinez, A. 1998. The effect of modified atmosphere packaging on ‘Ready-to-Eat’ oranges. Lebensm. Wiss. Technol. 31: 322–328. Price, J.L., Floros, J.D. 1993. Quality decline in minimally processed fruits and vegetables. In: Food Flavors, Ingredients and Composition. G. Charalambous (Ed.), Elsevier, Amsterdam, pp. 405–427. Radi, M., Mahrouz, M., Jaouad, A., Tacchini, M., Aubert, S., Hughes, M., Amiot, M.J. 1997. Phenolic composition, browning susceptibility, and carotenoid content of several apricot cultivars at maturity. HortScience 32(6): 1087–1091.

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Romig, W.R. 1995. Selection of cultivars for lightly processed fruits and vegetables. HortScience 30: 38–40. Rosen, J.C., Kader, A.A. 1989. Postharvest physiology and quality maintenance of sliced pear and strawberry fruits. J.Food Sci. 54(3): 656–659. Saltveit, M.E. 1997. Physical and physiological changes in minimally processed fruits and vegetables. In: Phytochemistry of Fruits and Vegetables. F.A. Tomás-Barberán, R.J. Robins (Eds.), Clarendon Press, Oxford, U.K., pp. 205–220. Sapers, G.M. 1993. Browning of foods: control by sulfites, antioxidants, and other means. Food Technol. 47(10): 75–84. Sapers, G.M., Simmons, G.F. 1998. Hydrogen peroxide disinfection of minimally processed fruits and vegetables. Food Technol. 52(2): 48–53. Sapers, G.M., Miller, R.L. 1998. Browning inhibition in fresh-cut pears. J.Food Sci. 63(2): 342–346. Sapers, G.M., Miller, R.L., Pilizota, V., Mattrazzo, A.M. 2001. Antimicrobial treatments for minimally processed cantaloupe melon. J. Food Sci. 66(2): 345–349. Siriphanich, J. 1994. Minimal processing of tropical fruits. In: Postharvest Handling of Tropical Fruits. B.R. Champ, E. Highley, G.I. Johnson (Eds.), ACIAR, Canberra, Australia, pp. 127–137. Stanley, D.W., Bourne, M.C., Stone, A.P., Wismer, W.V. 1995. Low temperature blanching effects on chemistry, firmness, and structure of canned green beans and carrots. J. Food Sci. 60: 327–333. Ukuku, D.O., Pilizota, V., Sapers, G.M. 2001. Influence of washing treatment on native microflora and Escherichia coli population of inoculated cantaloupes. J. Food Safety 2191): 31–47. USDA, 1998. CSFII/DHKS 1994–96 Data Set, Documentation and Technical Support Files: The 1994–96 Continuing Survey of Food Intakes by Individuals and the 1994–96 Diet and Health Knowledge Survey. Agricultural Research Service, Riverdale, MD (CD-ROM: Accession No. PB98-500457, National Technical Information Service, Springfield, VA). Van-Buren, J.P. 1979. The chemistry of texture in fruits and vegetables. J. Texture Stud. 10: 1–23. Varoquaux, P., Lecendre, I., Varoquaux, F., Sounty, M. 1990. Change in firmness of kiwifruit after slicing. Sci. Aliment. 10: 127–139. Veltman, R.H., Kho, R.M., van Schaik, A.C.R., Sanders, M.G., Ooterhaven, J. 2000. Ascorbic acid and tissue browning in pears (Pyrus communis L. cvs. Rocha and Conference) under controlled atmosphere conditions. Postharvest Biol. Technol. 19(2): 129–137. Watada, A.E., Ko, N.P., Minott, D.A. 1996. Factors affecting quality of fresh-cut horticultural products. Postharvest Biol. Technol. 9: 115–125. Watada, A.E., Qi, L. 1999. Quality of fresh-cut produce. Postharvest Biol. Technol. 15: 201–205. Watkins, C.B. 2000. Responses of horticultural commodities to high carbon dioxide as related to modified atmosphere packaging. HortTechnology 10(3): 501–506. Watkins, C.B., Manzano-Méndez J.E., Nock, J.F., Zhang, J., Maloney, K.E. 1999. Cultivar variation in response of strawberry fruit to high carbon dioxide treatments. J. Sci. Food Agric. 79: 886–890. Wright, K.P., Kader, A.A. 1997a. Effect of slicing and controlled-atmosphere storage on ascorbate content and quality of strawberries and persimmons. Postharvest Biol. Technol. 10: 39–48. Wright, K.P., Kader, A.A. 1997b. Effect of controlled-atmosphere storage on the quality and carotenoids content of sliced persimmons and peaches. Postharvest Biol. Technol. 10: 89–97.

4 Juice Processing Mark R. McLellan and Olga I. Padilla-Zakour CONTENTS 4.1 4.2 4.3 4.4

4.5

4.6 4.7

4.8

4.9 4.10 4.11 4.12

Introduction ..........................................................................................................................74 Fruit Quality and Quality Products .....................................................................................74 Juice Production...................................................................................................................74 Extraction Processes ............................................................................................................75 4.4.1 Pome Fruit and Small Fruit Processing ................................................................76 4.4.1.1 Disintegration .......................................................................................76 4.4.1.2 Hot Break Process ...............................................................................77 4.4.1.3 Mash Enzyme Treatment .....................................................................77 4.4.1.4 Enzymatic Liquefaction .......................................................................78 4.4.1.5 Extraction Equipment ..........................................................................79 4.4.2 Citrus Processing ...................................................................................................80 Clarification ..........................................................................................................................80 4.5.1 Enzymatic Preparation...........................................................................................80 4.5.2 Mechanical Separation ..........................................................................................81 4.5.2.1 Using Decanters and Finishers ............................................................81 4.5.2.2 Centrifugation ......................................................................................81 4.5.2.3 Diatomaceous Earth Filtration.............................................................81 4.5.2.4 Cross-Flow Membrane Filtration ........................................................83 Concentration .......................................................................................................................86 Pasteurization and Nonthermal Processes ...........................................................................87 4.7.1 Hot Filling and Product Pasteurization for Shelf-Stable Juices ...........................87 4.7.2 Aseptic Juice Processing .......................................................................................87 4.7.3 Refrigerated Juices ................................................................................................88 4.7.4 Nonthermal Processes............................................................................................88 Packaging .............................................................................................................................88 4.8.1 Cans........................................................................................................................89 4.8.2 Glass.......................................................................................................................89 4.8.3 Semirigid and Flexible Packages ..........................................................................89 Fruit Nectars.........................................................................................................................90 Waste Treatment...................................................................................................................90 Juice Authentication To Prevent Adulteration .....................................................................91 Standard Tests for Identification of Hazes in Clear Juices .................................................91 4.12.1 Microbial Growth ..................................................................................................91 4.12.2 Starch .....................................................................................................................92 4.12.3 Protein–Tannin Complex .......................................................................................92 4.12.4 Tannin Haze ...........................................................................................................92 4.12.5 Gum Haze ..............................................................................................................92 4.12.6 Metal Ions ..............................................................................................................93

0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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4.13 Fruit Juice Safety and HACCP............................................................................................93 References ........................................................................................................................................94

4.1 INTRODUCTION Juice and juice products represent a very important segment of the total processed fruit industry. U.S. retail sales of fruit juice products in the year 2000 accounted for $15 billion and were projected to reach $18 billion by 2005. The per capita consumption in the U.S. of fruit juices (as single strength equivalent) was 36 l in 2000, whereas the average European consumption was 23.4 l/person. The most popular juices were orange and apple. Total sales in Europe for the same year were reported as 9100 million liters, representing a 10% increase over the 1995 levels (The Food Institute, 2002; Anon., 2001). Juice products are being marketed as refrigerated, shelf-stable, and frozen, in a variety of packages with increased emphasis on functionality, health attributes, new flavors or blends, and in some cases fortified with vitamins and minerals.

4.2 FRUIT QUALITY AND QUALITY PRODUCTS •

•

• • •

High-quality juice operations are dependent upon a source of high-quality raw material. No matter how good the process is, starting with poor-quality fruit for juice production will lead to a poor-quality product. Often, the quality of the fruit is dependent upon the stage of maturity or the level of ripening. Typical assessments of fruit ripening include sugar concentration, acidity, starch content, color, flavor, and firmness (Hulme, 1971). As is often the case in unit operations, efficiency drives the harvesting process, and in terms of harvester operations, this usually means some form of mechanical harvesting. Quality can be preserved with mechanical harvesting; however, extra care needs to be exercised in the design and use of harvesters. In general, all handling of the fruit should be done with sensitivity to the potential of bruising and contamination of the fruit. Special care must be made in transport of the fruit through the plant so that large drops or other impacts are avoided. Storage facilities must be optimized for the type and maturity of the fruit. Although general cooling of fruit at controlled relative humidity is standard for extending storage life of the fruit, controlled atmosphere can be used to maximize shelf life of fruits such as apples. Starting with good quality, sound fruit is important but so, too, is the cleanliness of the process operations. In all phases of the juice production, design and assurance of clean and safe operations are important. Daily cleaning and sanitation of the plant with routine shutdowns will assure maintenance of a clean operation and prevent buildup of trouble spots.

4.3 JUICE PRODUCTION The process starts with sound fruit, freshly harvested from the field or taken from refrigerated or frozen storage (Figure 4.1). Thorough washing is usually necessary to remove dirt and foreign objects and may be followed by a sanitation step to decrease the load of contaminants. Sanitizing is especially important for minimally processed juices that rely on hygienic conditions to ensure the safety of perishable products. Sorting to remove decayed and moldy fruit is also necessary to make sure that the final juice will not have a high microbial load, undesirable flavors, or mycotoxin contamination. For most fruits, preparation steps such as pitting and grinding will be required prior to juice extraction. Heating and addition of enzymes might also be included before the mash is transferred to the extraction stage. Juice extraction can be performed by pressing or by enzymatic treatment followed by decanting. The extracted juice will then be treated according to the

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Fruit (fresh or thawed) Cleaning/Washing

Sorting/culling

Sanitizing

Preparation for Extraction (pitting, crushing, heating, enzymatic treatment)

Juice Extraction (Pressing, decanting)

Cloudy Juice (coarse filtering, centrifuging)

Clarification (Depectinizing, centrifuging, filtering)

Concentration (evaporation, reverse osmosis, freeze-concentration) Pasteurization (or equivalent non-thermal treatment) Pasteurization

Filling and Storage (shelf-stable, refrigerated, frozen) Single Strength Juice

Filling and Storage (shelf-stable, refrigerated, frozen) Juice Concentrate

FIGURE 4.1 General diagram for juice production.

characteristics of the final product. For cloudy juices, further clarification might not be necessary or may involve a coarse filtration or a controlled centrifugation to remove only larger insoluble particles. For clear juices, complete depectinization by addition of enzymes, fine filtration, or highspeed centrifugation will be required to achieve visual clarity. The next step is usually a heat treatment or equivalent nonthermal process to achieve a safe and stable juice and final packaging if single-strength juice is being produced. For a concentrate, the juice is fed to an evaporator to remove water until the desired concentration level is obtained. Other processes used for water removal include reverse osmosis and freeze concentration, which are best suited for heat-sensitive juices. The concentrate is then ready for final processing, packaging, and storage (Downing, 1996).

4.4 EXTRACTION PROCESSES Once the fruit is delivered for processing, the critical operation of juice extraction begins. In general, juice extraction should be done as rapidly as possible so as to minimize oxidation of the juice by naturally present enzymes.

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AND

SMALL FRUIT PROCESSING

Generally, most pome fruit and small stone fruit can be used for juice extraction. No peeling is needed. Small stone fruit such as apricots and plums might have to be destoned (pitted) depending on the grinding–extraction equipment selection. Cherries, although containing a pit, may be pressed with the pit intact. Breakage of the pit will release benzaldehyde, the familiar aroma of maraschinotype cherries. 4.4.1.1 Disintegration The juicing process starts with the crushing step to break down the cell tissue. Grain sizes of 5 to 8 mm diameter are recommended for presses while grain sizes of 3 to 5 mm are desirable for decanters (Hamatschek et al., 1995). Hammer mills are devices used to crush the whole fruit in preparation for pressing. Hammer mills consist of heavy stainless steel bars spinning from a common axis under high-speed rotation. The fruit is disintegrated until it passes out through a screen of a specific size mounted in the bottom of the mill. With firm fruit, a small screen size should be used, and the mash will be of a finer particle size. Mash from firm fruit will press more easily, and the smaller particle size will allow greater yields. Softer fruit presses with more difficulty, and a larger particle size in the mash will enhance ease of pressing. Thus, for example, softer apples require a larger screen size in the hammer mill. Grinding mills offer an alternative method to disintegrating fruit. In the earlier models, the fruit was drawn past fixed knives mounted on a rotating cylinder. Control of the grind was accomplished by adjusting the depth of the knives and, thus, the size of the cut from the fruit. Grinding disk mills offer more flexibility and improved performance. In the Bucher-Guyer unit (Model CM 50; Switzerland) the fruit is transported by a feed screw to the grinding area. The screw pressurizes the fruit against a rotating disk equipped with grinding knives in a star pattern, and the milled fruit exits via an adjustable discharge slot. The process can be controlled by adjusting the feeder speed, the rotating speed of the grinding disk, the width of the product discharge slot (up to 10 mm), or by changing the knife size (Pinnow, 2000). Better yield is obtained due to the easy adjustment corresponding to the fruit ripeness at the time of operation. Grating mills are used in small juice operations to produce uniformly sized fruit pieces. Fruit is fed to a rotating–grating disk with fixed aperture, and the shredded fruit is discharged at the bottom. Fruit must be relatively firm with small seeds or pitted. Stemmer/crushers are used in grape juice processing to remove residual stems, leaves, and petioles from grapes and to perform the initial crush of the fruit after its arrival at the plant. These units are designed around a perforated rotating drum, with holes approximately 2.5 cm in diameter. In the process of traversing the rotating drum, grapes are caught by the perforated drum and knocked from the stems. Individual grapes are broken open or crushed in the process and dropped through the drum. Stems, leaves, and so forth continue on to the center of the drum and are discharged at the end for waste. Grapes are generally put through the crusher in order to gently express the juice and free up the flesh, yet still not break the seeds. Breakage of the seeds releases increased amounts of phenolics, adding to the astringency of the juice. Stoned fruit mills are used for plums, damsons, and apricots to crush the fruit without breaking the stones to avoid juice flavor changes and storage instability. Hard rubber-lobed wheels rotate simultaneously, forcing the fruit down and separating most of the flesh from the intact stone (Downes, 1995). Turbo extractors can be used for extraction of juice and puree from fruits and vegetables. The cold extractor unit from Bertocchi (Italy) has a feeding section with a variable speed screw and a cutting head; a softening section consisting of a stator and rotor (rotopulse); and an extraction area equipped with a rotor with paddles and a perforated cylindrical screen that continuously turns the

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product by centrifugal force. The extractor can be adjusted by changing the feeding speed, the rotor speed, the gap between the rotor and the screen, and the screen size. The fruit can be protected from oxidation by the injection of nitrogen gas or antioxidant solution to the cutting area through built-in openings. 4.4.1.2 Hot Break Process In order to maximize juice yield and color–flavor extraction, a hot break process is often used. The most common use is in grape juice processing, but other fruits such as cherries, plums, and berries may also benefit. Increased interest in highly colored juices, rich in phenolic compounds with associated health benefits, is driving the development of better techniques to preserve the functional components while maximizing the extraction. Typically crushed fruit or mash passes through a large bore, tubular heat exchanger where it is heated to 50 to 60∞C. This stage, known as the hot break process, is designed to extract a large amount of color and assist in maximizing the yield. To the hot fruit a pectolytic enzyme can be added, and in some cases such as red grape juice processing, kraft (wood pulp) paper is also added prior to pressing to serve as a press aid. The type of enzyme used is critical, as some pectolytic enzyme solutions may have some side effects that could destroy the juice color (Helbig, 2001). The addition of press aid to the mash provides coarseness and channels for the juice to exit (Hurler and Wey, 1984). Alternative press aids include rice hulls, bleached kraft-fiber sheets or rolled stock, and ground wood pulp. Ideally, a press aid should have relatively long fibers that can be separated with a minimum breakage of those fibers for maximum effectiveness. In addition, press aids should neither impart off-flavors to the juice nor remove the fruit’s flavor. If the juice is going to be extracted by decanting or centrifugation, then there is no need for press aids. 4.4.1.3 Mash Enzyme Treatment This step might not be used for the production of high quality, single-strength, cloudy and clear juices, where the preservation of the fresh flavor is imperative. Soluble pectin found in fresh juice is a result of physical breakup of the cells and the activity of pectolytic enzymes that are primarily located in the cell wall of the fruit. This soluble pectin is the cause of much difficulty in extraction due to increased juice viscosity and the lubrication it affords the press cake, resulting in reduced extraction effectiveness. Typically, the fruit mash is heated to 45 to 50∞C followed by the addition of pectolytic enzyme preparations at the dosage recommended by the enzyme supplier. Reaction time can take up to 1 to 2 h. Depectinization is designed to reduce the viscosity and slipperiness of the pulp and thus permit the effective use of decanters and presses with proper press aids as needed. It is especially useful in processing mature and stored fruit that results in low juice yield. Several depectinizing tanks are employed so that a continuous flow may be maintained to the presses or decanters. The pectic substances that need to be broken down are the structural polysaccharides in the middle lamella and the primary cell walls of fruit tissue. These pectic substances are polymeric chains with a backbone consisting of straight sections of a-D-1, 4-galacturonan regions with a dispersion of 1, 2 linked a-L-rhamnosyl residues with an alternating rhamnogalacturonan chain (Selvendran, 1985). The alternating side chains composed of neutral sugars give the pectin structure a “hairy” character (De Vires et al., 1986). The predominant sugars of pectin are D-galactose and L-arabinose. Found to a lesser degree is xylose in monomeric and oligomeric side chains (McNeil et al., 1984). Treatment of the mash with enzymes is expected to increase the yield, reduce the processing time, and improve the extraction of important or valued components of the fruit. It has been reported that the enzyme costs amount to about one third of the profits gained from the increased juice yield (Possmann, 2000).

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4.4.1.4 Enzymatic Liquefaction Fruit liquefaction is not new. Commercial enzyme preparations from Aspergillus niger with highpectinase activity for liquefaction of fruit have been studied. Effective dosages (125 to 150 g/t) for total liquefaction of apple mash during production of apple juice have been derived (Grassin and Fauquembergue, 1993). Liquefaction is accomplished using both pectolytic and cellulolytic enzymes in combination, taking advantage of observed synergistic effects (Voragen et al., 1980).

O

COOCH3

COOCH3

OH O

OH O

O OH

O

COOH OH O

O

COOH OH O

O OH

O OH

OH Polygalacturonase

+ H 2O

Pectin Esterase

O

COOH

COOH

COOH

OH O

OH O

OH O

O OH

4.4.1.4.1

O

O

OH

COOH OH O

O

OH OH

OH

OH

Commercial pectolytic enzyme preparations containing predominantly polygalacturonase (PG) and pectin and pectate lyase (PLs) are utilized in this processing step. The PG activity is targeted at the glycosidic bonds by -elimination. The enzyme treatment usually included a pectin esterase (PE) in order to improve the activity of the polygalacturonase (Pilnik et al., 1973).

O

COOH

COOH

OH O

OH O

O OH

COOCH3 O

O

4.4.1.4.2

O OH

Pectin lyase

COOH

COOH

OH O

OH O

O

OH OH

OH O

O OH

OH

Pectate lyase

O

COOCH3

OH O

H

OH

O

COOCH3

COOCH3

OH O

OH O

O

OH OH

H

OH

Commercial cellulase products capable of hydrolyzing nonpectin polysaccharides such as cellulose, glucans, and xylans are used to fractionate the cell walls and liquefy the remaining solids. Progress of the liquefaction of the pomace can be monitored by high performance liquid chromatography (HPLC) analysis of the hydrolysate for free monomeric and oligomeric sugars. The sugar content in the juice and the yield increase significantly due to the complete hydrolysis of macromolecules. The juice produced from liquefaction has a different flavor and composition compared to pressed juice. The process of fruit liquefaction for juice production is regulated in many countries with strict limitations on final usage and labeling declaration. It is mainly used for production of juice concentrate.

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4.4.1.5 Extraction Equipment 4.4.1.5.1 Rack and Frame Hydraulic Press The hydraulic rack and frame press is a very common batch press system found in small juice operations. It was the primary method of fruit juice-pressing operations for many years. Heavy cotton or nylon cloths are filled with a set amount of mash and then folded to produce what is called a cheese. The individual cheese is stacked and separated by a wooden, stainless steel, or plastic spacer platen. The combined stack is then compressed using a hydraulic ram, during which the juice is expressed. The process delivers good yield but is labor intensive. 4.4.1.5.2 Horizontal Piston Press Probably one of the most successful press systems in the fruit juice market is the Bucher horizontal piston press (Bucher-Guyer AG, Zurich, Switzerland). This press is capable of pressing berries, stone fruit, and vegetables. It operates in batch mode with loads of up to 14 t/filling. Flexible drainage elements covered with a nylon filter cloth carry the expressed juice out to a manifold. Usually, the juice has a very low level of suspended solids, more typical of a rack and frame press. The Bucher-Guyer Press is a highly automated pressing system used in a batch pressing operation. Generally, this system consists of a rotatable basket or cylinder with a hydraulic ram used for juice expression. Within the cylinder are fabric-covered flexible rubber rods with longitudinal grooves in them that allow the juice to transport easily to the discharge port. 4.4.1.5.2 Bladder Press The Willmes Press is a commonly used system for grape juice pressing. It is a pneumatic-based system that consists of a perforated, rotatable, horizontal cylinder with an inflatable rubber tube (air bag) in the center. The cylinder is filled with grape mash through a door on the cylinder wall, which is rotated to the top position. After filling, the press is rotated to ensure even filling. During this rotation, the air bag is filled, creating the mash compression action. The bag is then collapsed, and the cylinder is rotated. The rotation and pneumatic compression of the mash is repeated many times with increasing air pressure. 4.4.1.5.3 Belt Press The continuous belt press, such as the Frontier Technology Inc. system, is effective for grape and apple juice processing. In belt presses, a layer of mash is pumped onto the belt entering the machine. Press aid may be added for improved yield and reduced suspended solids. The belt is either folded over or another belt is layered on top of the one carrying the mash. A series of pressurized rollers compress the enveloped mash. Expressed juice is caught in drip pans. The cake is discharged from the last pressure roller (Figure 4.2). Fruit Mash Juice Collection Zone Press Cake (Pomace) Fruit Mash

FIGURE 4.2 A continuous belt press.

Spent Press Cake

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4.4.1.5.4 Screw Press A typical screw press consists of a reinforced, stainless steel cylindrical screen enclosing a large bore screw with narrow clearance between the screw and the screen. Breaker bars are located between the screw intervals in order to disrupt the compressing mash. Back pressure is provided at the end of the chamber and is usually adjustable. Capacities for screw presses with diameters of 30.5 and 41 cm are 5,080 kg and 15,240 kg/h, respectively (Bump, 1988). 4.4.1.5.5 Decanter Centrifuge In addition to sieving technology, the separation of juice from the mash can be performed by sedimentation through increased gravity in a decanter. Centrifugal force is used to accelerate the settling of higher density insoluble particles present in the juice. Enzyme-treated mash is best suited for juicing by decanters as the reduced viscosity and higher temperatures result in faster and more effective separation. Decanters are versatile units that can process a wide variety of fruit and vegetable mashes including hard-to-press mash from ripe or heated pears (Schobinger, 1999).

4.4.2 CITRUS PROCESSING The extraction of orange juice is based on removal of the available juice (50% by weight) in the orange. Two major types of orange juice extractors are used on the market. An FMC Citrus Juice Extractor (FMC Corporation) is designed with a squeezing process in which a hole is cut in the fruit and the juice and flesh are squeezed out of the orange. Another extractor is the Brown Extractor (Automatic Machinery Corp.) where a reaming process is used. The fruit is cut in half, and the juice and flesh are reamed out of each half. Following the extraction, the raw juice is strained to remove heavy solids, using a finisher. Although finishers can vary in design and operation, in general, all are expected to remove seeds, bits of peel, and heavy flesh from the juice. Typical models use a spinning screw that compresses the juice solids against a screen prior to ejecting the solids from the finisher. If low-pulp juice is desired, the juice may be processed with a centrifuge (Pecoroni and Gunnewig, 2001). Following the finishing step, juice from various lots is blended for optimal flavor and quality. The citrus juice is then deaerated and deoiled for standardization of juice quality. A pasteurization process not only ensures destruction of spoilage organisms, but it also inactivates the pectic enzymes responsible for juice separation. From this point on, the juice is ready for processing into chilled, aseptic, and hot-filled frozen concentrate or other products.

4.5 CLARIFICATION Solids separation in juice processing is a unit operation, often requiring multiple steps and possible pretreatment. Clarification is required for the production of clear juices.

4.5.1 ENZYMATIC PREPARATION Clarification is a process by which the semistable emulsion of colloidal plant carbohydrates that support the insoluble cloud material of a freshly pressed juice is “broken” such that the viscosity is dropped and the opacity of the cloudy juice is changed to an open splotchy look. This can be accomplished in one of two general ways: enzymatically and nonenzymatically. It has been suggested that by using pectinase enzymes, the pectin coat surrounding the protein particulates in this emulsion is broken down (Figure 4.3). This process allows the particles to aggregate and then drop to the bottom of the tank (Yamasaki et al., 1963). Detailed reviews of enzyme use with apples have been written by Kilara and others (Kilara and Van Buren, 1989; Kilara, 1981; Kulp, 1975).

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−− − − − −− ++++++ − +− − − ++ −− + +++ −− + + − + − −− − − − − − − −− ++++++ − +− − − ++ −− + +++ −− −− +++ − − − −− − − − −− +++++ − +− − ++ −+ −− + +++ −− Pectin −− +++ − − − Protein

Stable pectin suspension due to electrostatic forces of repulsion.

− − +++ − ++ ++ −− +− + + ++ − −−++++−−−− − −

−− − − −−+++++ − + + − − ++ + −− + ++ −−−+++ −− −− −

−− − − −−+++++ − + + − − ++ + −− + ++ −−−+++ −− −− −

Juice treated with pectic enzymes. Exposure of opposite charged surfaces.

−− + − +++ ++ − + + − + − + + + − + −− +++ ++−−− ++++−−− −+ − − + − −− + ++ −− + −−++++++ −−− ++ ++ −− + ++ −− −−−− ++ −−− + + −− +++ − −− −−−

Formation of floc due to agglomeration via electrostatic attraction.

FIGURE 4.3 A suggested theory of floc formation during enzyme treatment of the juice.

Nonenzyme clarification involves breaking the emulsion by other means, the most common of which is heat (Smock and Neubert, 1950). Other techniques include addition of gelatin, casein, and tannic acid–protein combinations (Kilara and Van Buren, 1989). Additionally, the use of honey and combined honey–pectinase treatments have been found to be effective clarification agents (Kime, 1982). It is believed that the proteinaceous component of honey is responsible for a synergistic effect when honey and pectinase are used in combination (McLellan et al., 1985).

4.5.2 MECHANICAL SEPARATION 4.5.2.1 Using Decanters and Finishers A high-solids stream can be partially clarified using decanters and finishers. Both pieces of equipment operate on the same principle with a spinning central cone, drum, and set of paddles pushing the juice through a screen of some type. The unit is typically mounted horizontally, and throughput is relatively high. Total suspended solids may be reduced to 1% or less during operation, depending upon the characteristics of the feed stream and operating conditions of the separator. 4.5.2.2 Centrifugation A very common unit used for removal of juice-insoluble solids is the centrifuge. A centrifuge places the juice under high gravimetric force induced by centrifugal action. This is effective in producing a juice that is opaque but free of visible solids. Modern centrifuges are highly automated and run continuously with timed solids ejection. Centrifuges with a high force of gravity are capable of producing clear juice under optimized conditions. Operation of the centrifuge must be done in a way that minimizes the introduction of excessive oxygen in the product. Possible remedies include the use of inert gas. 4.5.2.3 Diatomaceous Earth Filtration The use of filter aids in filtration operations is one of the most traditional techniques to achieve a clarified juice. It is not as popular as in the past due to safety restrictions in handling the material and cost of waste disposal. It involves a three-step operation in which a precoat of filter aid is built up on a filtration element (paper, cloth, or screen), and then filtration is conducted using the continuous addition of filter aid to the juice (Figure 4.4). This continuous addition is called body feed. Finally, the built-up cake is removed, and the entire cycle is started again.

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FIGURE 4.4 Component parts of a diatomaceous earth filter.

The basic system utilized in one of these operations included the following elements: a filter feed pump, a body feed injection pump, the precoat mixing tank, the body feed mixing tank, the filter element, connecting pipes, and openings or method for filter cake removal. 4.5.2.3.1 Pressure Filtration The final filtration process is typically accomplished through the use of a diatomaceous earth (DE) filtration system. This system utilizes suspension of diatomaceous earth in the juice as a means of renewing the filter surface in a plate and frame filter. When done properly, this filtration process is highly efficient and very effective. Its success can be highly operator dependent. The rate and effectiveness of filtration is dependent upon the use of the appropriate grade and amount of DE. In the filtration process, suspended filter aid deposits on a precoated layer as the filtration progresses. This deposition is permeable and prevents the extended buildup and subsequent clogging of the filter by the suspended solids. The following are examples of filtration equipment: •

•

•

•

•

The simplest of the pressure filtration type is the filter press. The cost is typically lower than other types of pressure filters. Although some of the systems are partially automated, it does require a significant amount of manual labor. The system can be dismantled easily for inspection and cleaning. Filter cakes can be easily washed from the system once disassembling has progressed. In the filter press, the amount of unfiltered liquid is relatively low once the shutdown process is terminated. Another type of pressure filter is the cylindrical element filter. In this system, tubular elements are suspended vertically in a closed tank system. Juice enters from the base of the system and filters through the elements, and the filtrate exits from the top of the system. Wash down and automation of this system are relatively straightforward. A third type of pressure filter is a vertical leaf filter. It is a low-cost system because of the inherent simplicity of its design. It offers an easy cake removal system and can be automated. A modified version of the leaf filter is the horizontal tank vertical leaf filter that accommodates a very large area of filtration surface, up to 2000 ft2 (180 m2). Filter leaves can easily be removed, inspected, and repaired. Due to the size of this equipment, costs can be very significant. A rotating leaf filter, in which the filtration elements are circular leaves suspended on a central axis, is another type of pressure filter. The leaves are rotated only during cleaning and discharging, which allows for an automated and rapid cake removal and cleanup system. Another type of pressure filter is a horizontal rotating leaf filter, essentially identical to the vertical rotating leaf system, except that it is available in much smaller models.

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Removal of the rotation capability leaves a horizontal leaf filter. This filter, although lowcost, must be manually disassembled for cleaning. It is usually used only for polishing operations, with liquids containing very low solids, giving relatively long filtration cycles. 4.5.2.3.2 Rotary Vacuum Filtration This filtration technology is dealt with separately here because of the unique character of the filtration system. This system has a large rotating element creating a vacuum that draws onto the drum element a buildup of precoat and body feed on the cake. Juice is fed into the base of the tub where vacuum action draws the juice through the filter cake. Once the cake is coated, a knife blade removes solids and filter aid material from the element itself. This system can deal with large amounts of suspended solids. It has a relatively high capital cost, including maintenance of the system. These drum filters can be up to 1000 ft2 (90 m2). Use of the rotary vacuum precoat (RVP) filter is called for when test filtration rates drop to near zero within seconds after only a very small amount of fluid has passed through the filter cake. It is particularly effective for small, slimy material that may blind a filter cake. Other conditions, in which particles are so small that they pass in significant numbers before a thick-enough cake is formed are appropriate for considering an RVP filter. The rotary drum is precoated with a thickness of up to 6 in. (15 cm). This precoat must be put on uniformly, with a variability of no more than ±1/4 in. (6 mm). The precoat has to be firm and should be applied at maximum vacuum, at maximum drum speed (2 rpm), and with a drum submergence of 10 to 15% of the drum diameter. For a 6 in. (15 cm) precoat thickness, precoating should take about 1.5 h. Precoat slurry should be approximately 7 to 10%. Possible cake cracking can be minimized with a row of flat sprays at 10 to 15 psig (69 to 103 kPa). Operation of the RVP filter is affected by many factors, including filter aid grade, vacuum level, drum speed, knife advance rate, and drum submergence level, as well as all of the other typical factors involved with precoat and bodyfeed filtration. Tests should be conducted to arrive at optimum filtration conditions. The first test should be to determine the coarsest grade of precoat filter aid that shows no penetration of solids into the precoat. The second test should be to determine filtration rates vs. submergence time of the drum. The third and final set of tests measures penetration of solids into the precoat at various knife advance rates. 4.5.2.4 Cross-Flow Membrane Filtration Membrane filtration is a pressure-driven technology with pore sizes ranging from 100 mol wt cutoff (MWCO) to 5 µm, offering a wide spectrum of applications for the juice industry. Membrane filtration processes include reverse osmosis, nanofiltration, ultrafiltration, and microfiltration. For juice clarification, ultrafiltration and microfiltration are now commonly used, representing membranes with pore sizes from 10,000 MWCO to 0.6 µm. Different configurations and membrane materials are available in the market to fit specific separation applications. Although tubular polymeric membranes were the first to be used in juice clarification, hollow fiber membranes are currently used, also. Advantages of membrane filtration over traditional clarification methods include reduced processing time, increased juice yield, elimination of filter aid and filter presses, better product quality, and reduced enzyme usage (Cheryan, 1998). 4.5.2.4.1 Polymeric Membranes Ultrafiltration (UF) is a membrane filtration process that separates particles based on molecular weight (Milnes, 1984; Cheryan, 1986). The process uses a cross-flow method of operation, as opposed to depth filtration used in DE filtration (Figure 4.5). It can be utilized to clarify apple juice, as well as other fruit juices (Heatherbell et al., 1977). It has been used commercially for this purpose in several plants in Europe, the U.S., and South Africa (Cheryan, 1998; O’Sullivan et al., 1988; Möslang, 1984).

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Feed membrane surface

Polarized layer

Low permeate passage

Depth Filtration (Perpendicular Flow) Feed

Retentate

membrane surface

High permeate passage Cross-flow Filtration

FIGURE 4.5 A comparison of cross-flow filtration with depth filtration.

Polymeric ultrafiltration membranes, with a wide range of pore size ranging from 500 to 750 kDa MWCO, are available in the market (Swientek, 1986). The juice manufacturer prefers to buy the largest possible effective pore size to minimize the filtration time. The decision is then based on two factors: quality of the ultrafiltered juice and filtration rates. There are very few studies that deal with both factors at the same time. Baumann et al. (1986) reported that apple juice ultrafiltered with membranes of pore sizes between 10 kDa and 0.22 mm presented large variations only in color. Flow rates were not reported, and the storage stability at room temperature of the prepared juices was not studied. In the juice operation, the UF system can be set up as a single pass batch configuration or as a feed-and-bleed system (Figure 4.6). In the standard batch system, the membrane is run in a closed system where the permeate is continually drained out of the system. The juice retained by the membrane is identified as the retentate. The ratio of the initial volume of cloudy juice to the final volume of permeate is called the concentration factor. Generally, the final concentration factor is achieved when the centrifugal or spin solids level of the retentate is in the range of 40 to 80%, depending on the design of the system. As with any system, the initial solids level of the juice depends upon the type of press utilized, the application of any mash enzyme treatment, the use of filter aid, and the degree of depectinization, as well as the variety and condition of the fruit brought into the operation. Typically, the initial solids level of juice and streams entering the UF systems used nowadays range from 2 to 5% solids. The rate of permeate flow from the UF system is dependent on two primary variables: the temperature of the feed stream and the degree of depectinization. Typical flux rates for UF can range at room temperature from 15 gal/ft2/d (630 l/m2/d) for a raw juice to 25 gal/ft2/d (1050 l/m2/d) for a depectinized juice. By elevating the temperature to 125∞F (52∞C), feed streams will range from 30 gal/ft2/d (1260 l/m2/d) for raw juice up to a level of 50 gal/ft2/d (2100 l/m2/d) for depectinized juice. Several studies have been published concerning the quality of the ultrafiltered fruit juice. Möslang (1984) compared apple juice clarified by conventional procedures, i.e., gelatin and pectinase treatment followed by DE filtration with juice ultrafiltered with 18 kDa MWCO tubular

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Single pass processing Feed tank Retentate

Permeate

Feed pump

Retentate Feed tank

Batch operation

Permeate

Feed pump

Feed tank

Recycle loop

Feed

Feed and bleed

Permeate

Retentate

FIGURE 4.6 Various modes of operation for juice ultrafiltration.

membranes. The only difference noted was for higher calcium and magnesium in the DE-filtered juice. Drake and Nelson (1987) compared the quality and storage stability of ultrafiltered apple juice with plate-and-frame filtered juice using a 50 kDa MWCO membrane. Results showed that the ultrafiltered juice was as acceptable as the filtered one, even though sensory flavor was rated lower. The juice maintained a very low turbidity through 12 months of storage at 1, 21, and 32∞C. 4.5.2.4.2 Inorganic Membranes Inorganic membranes for cross-flow filtration of foods offer advantages to processors, such as resistance to abrasion and chemical tolerance, thus opening the range of applications and facilitating the cleaning cycles. They are more common in microfiltration processes with pore sizes of 0.1 to 0.6 µm. They can be operated at high temperatures and pressures, and they are autoclavable and have virtually unlimited life. Their major drawback is the high cost per membrane area compared to polymeric membranes. Ceramic membranes are used widely in the dairy industry and moderately in the beverage industry, the latter due in part to the lack of information and to low reported flow rates (Merin and Daufin, 1989). Baumann et al. (1986) compared the quality of apple juice clarified with microfiltration membranes (0.2 µm) made of polysulfone and alumina. They concluded that the filtered apple juice was sterile and presented no significant differences due to the membrane material. The steady-state permeate fluxes (flow rate per membrane area) reported for both modules were in the range of 100 to 120 l/h/m2 at 16 to 23∞C. Another study by Wu et al. (1990) compared the quality of apple juice clarified using two polymeric ultrafiltration membranes (5 and 50 kDa) and one ceramic microfiltration element (0.1 µm). The results showed that the microfilter produced a darker and more turbid juice, as was expected, due to the much larger pore size of the ceramic element. They reported that the average permeate flux of the ceramic membrane was 112 l/h/m2, but no operating parameters were specified to compare against other studies.

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Ben Amar et al. (1990) examined the clarification of apple juice using two mineral membranes of 0.2-µm pore size: a ceramic membrane and a carbon membrane. The effect of the temperature, inlet velocity, time, transmembrane pressure, and pectinase treatment on the permeate flux, as well as methods to reduce or control fouling, were studied on test benches. Optimum operating parameters were temperatures of 50 to 55∞C, transmembrane pressure of 3 to 3.5 bar, and inlet velocity of 4 to 4.5 m/s, giving juice flux from 100 to 150 l/h/m2. Permeate backflushing for 2 to 5 sec at 6.5 to 7 bar every 2 to 5 min was beneficial for the ceramic membrane (average flux increased to 200 l/h/m2) but had no effect on the carbon membrane. The use of pulsating entry flow (1.33 Hz frequency, 84 cm3 displaced volume) increased the permeate flux from 200 to 250 l/h/m2 for the ceramic membrane and from 175 to 225 l/h/m2 for the carbon membrane. The increase in flux obtained by the use of backflushing and pulsating flow seemed to indicate that substantial improvement in juice fluxes could be achieved by further optimizing the operating conditions to minimize membrane fouling and polarization through higher inlet velocities. In addition, these experiments were carried out by changing pressures while recycling the juice, so the results did not account for the potential decrease in flux due to fouling that could occur at the previous set of experimental conditions. The trials also tested each operating parameter separately, which made it impossible to evaluate any interactions that could affect the permeate flux. The performance of a 0.2-µm ceramic membrane for clarification of depectinized apple juice was studied. The results showed that the flux is higher at high feed velocities (14.6 m/sec) and high temperatures (50∞C), with the transmembrane pressure being a positive factor only at high temperatures. The juice flux at optimal conditions was between 400 and 500 kg/h/m2. Filtration of juice with pectin resulted in flux decreases of 40 to 50% compared to depectinized juice. Periodic backflushing during processing at optimal conditions, i.e., high temperatures, high feed velocity at the membrane surface, and low pressure, did not increase the juice flux significantly (Padilla and McLellan, 1989, 1993).

4.6 CONCENTRATION Fruit juice concentration can offer significant advantages to the processor; by concentrating the juice, the bulk is reduced, thereby reducing storage volume requirement and transportation costs. Concentration allows a more complete deposition of insoluble solids and tartrates in grape juice. Storage of the cold concentrate is less likely to exhibit yeast growth because of the high sugar concentration and also if frozen concentrate is the primary end product. Storage as a concentrate prior to canning and freezing is an appropriate process. Hartel (1992) and Rao (1989) have published reviews that cover the technical aspects of concentration of fruit juices. An indepth discussion concerning fluid-food evaporators, energy use in the concentration of fluid foods (Schwartzberg, 1977), principles of freeze concentration and its economics (Thijssen, 1974), and engineering principles of aroma recovery (Bomben et al., 1973) have been presented in the literature. Fruit juice is concentrated by controlled evaporation of water, the major constituent of the juice. Because of possible loss of aroma constituents, the first major step is stripping of volatiles from which aromas can be recovered. Stripping is primarily handled by partial evaporation. Alternatively, a steam-stripping process could be employed. Many different evaporator designs can be employed for juice concentration including: evaporators with single-pass recirculating concentrators, single or multiple stages, single or multiple effect, and with or without a surface mixing type of condenser (Hartel, 1992). The word stage is used to identify the flow of juice through the system. Single-strength juice enters the first stage, and final concentrate exits the last stage. Effect describes the path of steam through the system. The first effect of an evaporator system receives steam from a boiler, and the second effect receives steam and vapors boiled off from the first effect.

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A Schmidt Sigmastar multieffect evaporator is commonly used for grape juice concentration. The removed volatiles from this system are concentrated 100- to 150-fold and added back to the concentrate prior to storage. Concentration of “clean” juice streams is also possible using a combination of reverse osmosis and evaporation. Cross-flow membrane systems are ideally suited for this application due to the self-cleaning turbulence effect. The reverse osmosis technology is effective in concentrating a lowsolids juice (7 to 8∞ Brix) two- or threefold. Afterwards, evaporation technology would be appropriate. Modified reverse osmosis systems can achieve a 50 to 60∞ Brix concentrate (Cross, 1989).

4.7 PASTEURIZATION AND NONTHERMAL PROCESSES 4.7.1 HOT FILLING

AND

PRODUCT PASTEURIZATION

FOR

SHELF-STABLE JUICES

Pasteurization of the finished juice is accomplished by hot-filling or bottled juice pasteurization. Hot filling is conducted by passing the final juice through a heat exchanger and raising the temperature such that the temperature of the juice filling the container reaches a recommended fill temperature of 88 to 95∞C. A holding time of at least 3 min is normally applied to the juice prior to cooling. This hot-fill process is adequate for highly acidic beverages such as apple, cranberry, cherry, and grape juice. The maximum pH allowed for the hot-fill process is normally 4. A shelf life of high-quality retention can range from 9 to 12 months in glass bottles or in high-barrier containers, though, typically, juice movement through U.S. markets to consumers will take only 3 to 6 months. Postbottling pasteurization is also applicable to acidic juices and is currently used for carbonated juices. A continuous tunnel pasteurizer or a batch system can be selected to perform the heating and cooling operations.

4.7.2 ASEPTIC JUICE PROCESSING This process requires that the product be commercially sterile at the time of packaging. For each product, a commercial sterilization process should be determined and verified by a process authority. Additionally, the package itself must be free of any microorganisms at the time of filling, and, finally, the filling and sealing process must be done such that no recontamination is possible (Figure 4.7). Use of high-temperature, continuous processes can produce a sterile product with a minimal effect on product quality. This is because spoilage organisms are more sensitive to increasing temperatures than many quality characteristics. Packaging materials for aseptic processing are generally cheaper than those used in traditional hot-fill operations. Package sterilization procedures include heat, chemicals (such as hydrogen peroxide), high-energy radiation, or a combination of these (Gavin and Weddig, 1995). Aseptic processing is not a new concept. It was commercially practiced in Europe with the filling of milk during the 1950s; however, the widespread use of this technology has awaited the development of more suitable packaging designed for the process. Today, a wide variety of rigid and flexible materials are available for retail, institutional (food service), and bulk packaging. The aseptic process itself requires an effective, continuous, and closed loop system for heating the product. This might include tubular, plate and scraped-surface heat exchangers, or direct steam injection and steam infusion. For fruit juice products, the most heat-resistant microbial component for process determination should be heat-resistant molds identified with the specific product being processed (Cousin and Rodriguez, 1987). An alternative method appropriate for high-acid juice sterilization might be sterile filtration. This technology would be used for fully clarified juice with no remaining particulate matter. Membranes with a pore size small enough to exclude microorganisms (< 0.45 mm) would be required.

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The Aseptic Process

Commercial Sterilization of Foods-cooled to ambient temperature (Closed System)

Sterilization of the Container

Product Packaged in Sterile Environment at Ambient Temperature

Room Temperature Product Storage Extended Shelf life

FIGURE 4.7 A layout of the component steps for an aseptic operation.

4.7.3 REFRIGERATED JUICES The consumer demand for fresher juices has created the need to develop milder pasteurization procedures that enable the retention of nutrients and fresh flavor profiles. This category includes minimally processed and unheated juices with limited refrigerated shelf life. The main concern is to apply a process that will render the juice safe by eliminating microorganisms and contaminants that could pose a health risk to the consumer. Pasteurization temperatures in the 70 to 90∞C range for a few seconds are typically used. If the juices are not pasteurized, careful control of the microbiological quality of the raw fruit and final juice, and proper sanitizing procedures to eliminate pathogens from the fruit prior to juice extraction are necessary (see section “Fruit Juice Safety and HACCP”). Extended shelf life of refrigerated juices can be achieved by combining pasteurization with clean-fill operations. Addition of chemical preservatives, if permitted by law, will also extend the life of the product. Typical compounds used in the juice industry are potassium sorbate and sodium benzoate at levels not to exceed 0.1% as regulated by the FDA in the U.S.

4.7.4 NONTHERMAL PROCESSES Alternatives to thermal pasteurization have become a reality in the food industry. The production of cold-treated juices offers the advantage of fresh-like characteristics with extended shelf life. Technologies that are being utilized or developed include ultraviolet light (UV), high-pressure processing, pulsed electric fields, electron beam irradiation, high carbon dioxide processing, and chemical sterilants (Knorr et al., 2002; Odebo, 2001, Worobo et al., 1998). Iu et al. (2001) demonstrated that a combination of pulsed electric fields and heat treatments was very effective against E. coli O157:H7 in apple cider. The use of UV at 14 mJ/cm2, a method approved by the FDA to deliver safe refrigerated apple juice, has been described by Tandon et al. (2003). The UVtreated juice retained the fresh flavor and was favorably compared to juice pasteurized at 71∞C for 6 sec.

4.8 PACKAGING The primary functions of food packaging are to retard or prevent the loss of quality, to contain the food adequately, and to give protection against environmental contamination (Crosby, 1981; Paine

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and Paine, 1983). Packaging also provides the consumer with information, helps sell the product, and adds convenience. A package should not interact adversely with the food contained within it. The packaging systems used for juices have two primary goals: to retain a hermetic environment such that recontamination is unlikely and to minimize further quality degradation due to oxygen permeation into the product. New consumer expectations are driving the juice industry to offer a variety of choices based on flavors, freshness, health, environmental stewardship, convenience, closure features, shapes, and sizes. For shelf-stable juices, glass, cans, and cartons are still the dominant packaging types, although plastic (PET and HDPE) bottles layered with an oxygen barrier are becoming popular. The expected trend is for the PET bottles to eventually dominate the aseptic juice market. Plastic containers are increasing in the fresh and extended shelf-life refrigerated category, where the carton containers have been strong. Active packaging, where oxygen absorbers are used to extend the life of a product, can improve the utilization of polymeric materials. Oxygen absorbing packaging materials offer barrier properties superior to other products currently available in the marketplace (Castberg et al., 2000).

4.8.1 CANS Cans are made of steel or aluminum. Steel cans are coated to prevent corrosion from acidic juices. Using cans for juice has fallen into disuse due to concerns over flavor loss and interaction with the can coatings. A properly made and closed metal can is an absolute barrier between the canned product and the external environment. Cans provide high-strength, retortable containers for processed foods. Processing standards for canned foods are well defined; however, highly acidic foods such as fruits may corrode can interiors, causing extensive detinning, particularly at the product headspace interface. Aluminum cans are heavily used in beverages. For frozen juice concentrate, composite cans are typically used. They are made of spiral wound laminated board with metal or plastic ends, although all-plastic containers are also available.

4.8.2 GLASS Glass enjoys a continued and strong use in this industry. The primary advantages of glass are its chemical inertness, clarity, and heat resistance. In food applications, its transparency is considered to be a significant marketing advantage, conveying the image of a quality product (Osborne, 1980). Its heat resistance ensures that containers will not deform during hot filling; however, glass containers are subject to thermal shock and may shatter. Failure may also result from frictional stresses caused by repeated bottle contact along a packaging line. In addition, glass is heavier than most other packaging materials.

4.8.3 SEMIRIGID

AND

FLEXIBLE PACKAGES

A wide variety of polymeric materials are available for the manufacture of single or multicompound packages. Because plastic and carton packages are lighter in weight than glass or metal containers, they provide economic advantages in terms of lower shipping costs (Kashtock, 1988). Production is also less energy intensive than for glass or metal cans. Flexible films are more efficiently stored as roll stock prior to packaging of products. Rigid tubs can be nested to minimize space utilization; however, the higher permeability of some plastics may lead to reduced product shelf life. Difficulty in recycling multilayer packaged can also be a limitation. Product–package interactions are a major concern in the use of plastic polymers for food packages (Hotchkiss, 1988; Gray et al., 1987). Possible interactions include: • • •

Chemical and physical interactions between the food and polymer Absorption or scalping of flavor notes or other components from the food Migration of components from the plastic to food

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Foods may actively leach components from the surrounding polymer, or components may migrate independently (Briston and Katan, 1974). The permeability of the packaging material to gases, water vapor, and volatiles also impacts the packaged foods. Various mathematical models have been developed to describe migration phenomena (Gray et al., 1987). Low-molecular-weight components of plastics that may migrate into foods include polymerization residues and processing aids such as antioxidants, antiblock agents, lubricants, and plasticizers (Crosby, 1981; Harte and Gray, 1987). Considerations in selecting a composite polymer packaging material include the compatibility of the material with the food to be packaged; the tolerance levels of the food to uptake or loss of oxygen, water, and volatile food components; and requirements of the package based on the established food tolerance levels (Brown, 1987).

4.9 FRUIT NECTARS Fruit nectar is a blend of sugar syrup, fruit acid, and very high-solids juice or puree, generally containing most of the solids from the fruit. In the U.S., a standard of identity for fruit nectars has been defined in the Federal Register (Anon., 1968). Usually, these nectars are produced with fruit that are either low in flavor as a single-strength, clarified juice or too strong in flavor and need dilution. The process of making nectars generally starts with a softening and cooking of the fruit flesh followed by a pulping and finishing of the puree. Acidified sugar syrup is added, and the entire mixture is hot-filled into appropriate containers. Typical nectar products include apricot, peach, pear, prune, mango, cranberry, and individual and blended citrus.

4.10 WASTE TREATMENT Large quantities of liquid and solid waste are produced from the manufacture of fruit juices. The disposal of these materials is regulated by law and can represent a significant cost in the juice plant operation. When possible, reduction in waste generation and processing of the waste effluents into value-added products represent the best alternatives. In rural agricultural areas, where land is available, the simplest solution is to spray the fields with liquid waste and to use the solid waste as soil conditioner and animal feed. For disposal on land, the wastewater must be screened through 10 to 20 mesh screens to remove silt and other insoluble solids. The pH of the wastewater is usually adjusted to 6.4 to 8.4 because the pH outside this range will render some nutrients in the wastewater inaccessible to plants. The most commonly used methods for waste disposal on land are sprinkler irrigation and surface irrigation. Possible value-added products from solid waste are fuels, proteins, and biochemicals. In other cases, the effluents will need to be processed substantially to be safely discharged into the public systems (Hang, 2000). The effluents from the processing lines consist of three broad categories: wash water (low polluted), process water (high polluted), and residual solids (extremely high polluted). Lowpolluted liquid effluents, with biological oxygen demand (BOD) of less than 200 mg/l, should be treated in aerobic systems such as aerated lagoons. Anaerobic biological treatment systems are better suited for high-polluted process water with BOD up to 20,000 mg/l. Efficiencies of 70 to 95% reduction of BOD are expected with these systems. For higher reductions, the treated effluent might need to be processed in an aerobic system (Koevoets, 2002). The activated sludge process is another widely used system for wastewater treatment. The treatment unit consists of a bioreactor that provides an environment for converting soluble waste solids into insoluble microbial cells under aerobic conditions and a clarifier where microbial cells are allowed to settle. The settled cells or sludge may be returned to the bioreactor or wasted. The removal of BOD by this method

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ranges from 80 to 99% depending on the waste characteristics, loading rate, and other operating conditions (Hang, 2000). Solid waste poses a more difficult dilemma due to reduced opportunities to dispose of the solids. Fruit solids can be used as animal feed or fertilizer, and apple and citrus solids have been studied extensively for this use. Not only have cattle been fed these, but to a lesser extent, sheep, pigs, horses, and deer have also used fruit solids as a feed supplement. If necessary, solid fruit waste can be disposed of in local landfills (Koevoets, 2002). Proper composting offers an excellent opportunity to dispose of a waste product and produce a valuable end product. The challenge in this is getting a substrate that offers adequate aeration such as wood chips, ground newspaper, hay, or already composed material to mix with the waste. The application of liquefaction to fruit pomace as a waste reduction process and to increase juice yield is an option being utilized by some manufacturers.

4.11 JUICE AUTHENTICATION TO PREVENT ADULTERATION Extensive texts have been written on this subject and should be referenced for complete details (Hammond, 2001; Brause, 1998; Nagy et al., 1988). Adulteration has been and will continue to be a serious problem due to the incentives to adulterate fruit juice, the primary one being economics, even though adulteration can lead to legal issues that include the potential for imprisonment. The extent and type of adulteration have changed over the years from a simple sweetening and diluting of the juice to a high-tech chemical masking of the adulterated product. This sophistication leads to the need for a more detailed understanding of the unique chemistries of these fruit systems.

4.12 STANDARD TESTS FOR IDENTIFICATION OF HAZES IN CLEAR JUICES When working with juice systems, a number of tests are available that can help identify problem situations, especially those related to juice stability. Van Buren (1988) has discussed these tests in detail.

4.12.1 MICROBIAL GROWTH A most obvious question to ask when faced with a clear juice that has turned cloudy is whether the cloud is microbial in nature. Microscopic examination should answer this question quickly. Probably the most common type of microbial growth in fruit juice is due to yeast contamination. Lactic acid and acetic acid bacteria are also able to grow in acid foods such as fruit juices (Beech and Carr, 1977). Finally, molds of the genus Byssochlamys, Talaromyces, and Neosartorya produce ascospores that can survive the thermal process given to fruit juices. Fruit commonly comes into contact with the soil, which is a common source of ascospore contamination. Mold growth is usually in the form of a localized mycelial mass; however, under certain conditions, the growth may be broken up to produce a generalized haze (Splittstoesser and King, 1984). A microscope capable of 500 to 1000 times magnification should allow for identification of specific microorganisms. In each of these cases, the finding of microbial growth should cause immediate concern as to the adequacy of the process, the cleanliness of the equipment, and possible tampering with the bottle. The yeast and bacteria that can grow in fruit juice possess little heat resistance. Their presence, therefore, would indicate a gross underprocessing or leakage following the process. Heavily contaminated cooling water can be the source of non-heat-resistant microorganisms (Gavin and Weddig, 1995). Spoilage of fruit juice by heat-resistant molds is rare. The level of ascospore contamination is usually low, and the washing of fruit prior to milling and pressing will eliminate many of the spores.

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The polishing–filtration treatments will also remove ascospores. As a result of the latter, “natural style” juice that has a cloud is more susceptible to contamination by ascospores than clear juice.

4.12.2 STARCH Starch buildup is an easy, quick problem to identify and address. The first question that might come to mind is the stage of maturity of the fruit at harvest. Was it a relatively early harvested fruit used in making the fruit juice in question? Is the fruit prone to production of starch? If so, then the possibility exists that a bottling haze is due to starch. In fact, heavy starch accumulation in early harvest can lead to a white powder precipitate in the tank bottoms of early pressed juice. A relatively simple test for soluble starch consists of combining juice with an iodine test solution to determine the presence of starch. The iodine test solution can be made by dissolving 0.1 g of iodine crystals and 2 g of potassium iodide in 100 ml of distilled water. Warm a 1-ml sample of juice in a test tube by immersing in a boiling water bath for 5 min. Cool the sample and add 1 ml of iodine test solution. A blue color is a positive reaction for soluble starch. If this problem is confirmed, all starch precipitate should be filtered from the juice, and then the soluble starch should be treated with amylase.

4.12.3 PROTEIN–TANNIN COMPLEX Protein–tannin complexes are a common cause of turbidity in bottled apple juice and other juices. In this situation, the tannins (polyphenolic compounds) act as bridges and glue to aggregate the haze-activating protein chains together. A complete explanation of the haze formation is given by Siebert (1999). These hazes can usually be decreased by heating the juice, but then the haze reforms rapidly again. The most common cause of this haze is proline-rich proteins such as gelatin or enzymes that are added in excess quantities by the processors. In clarified juice without any protein addition, there are seldom any significant levels of proteins because natural tannins will precipitate them prior to filtration (Yokotsuka et al., 1978). A test for protein in the juice can be made, and if appreciable amounts of protein exist, then additional high-molecular-weight tannin can be added to help precipitate the excess protein. Removal of protein can also be handled by ultrafiltration or by the addition of bentonite and silica sol. To test for protein, make a tannic acid test solution by dissolving 1 g of tannic acid in 20 ml of 95 % ethanol and enough distilled water to makeup a volume of 100 ml. Add 1 ml of the tannic acid solution to 10 ml of juice. Observe for haze formation. An increase of ten or more nephelous turbidity units (NTU) indicates a relatively high level of protein.

4.12.4 TANNIN HAZE Tannin hazes can be formed in juices with small amounts of protein or starch. Tannin hazes can also form slowly as the polyphenolic molecules polymerize to form large moieties. Their growth and formation are maximized in apple juice by low pH and high storage temperature. Tannin complexes will tend to form after long-term storage. For normal bottling, distribution, and sales histories, this type of haze is seldom a significant issue. Various methods for determining tannin content are described by Amerine and Ough (1980).

4.12.5 GUM HAZE Gums can also be possible contributors to haze formation. These hazes form under cool conditions and settle very slowly. Determination of this type of haze usually requires an analysis of the hyrdolyzed sugars present in the precipitate. If the analysis found an appreciable amount of arabinose, galactose, or xylose, then the sediment probability indicates plant gums as the cause. Treatment of the juice with hemicellulose enzymes may be able to solve this problem by helping to digest the gum material.

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4.12.6 METAL IONS Hazes can also be caused by metal ions in the juice. Typical levels of iron in apple juice are 3.7 mg/kg and of copper, 0.2 mg/kg. If quantities of these metals rise to levels five times or more than these natural levels, they could be a causative agent for haze (Gebhardt et al., 1982; Kilara and Van Buren, 1989). Sources of these metals would usually be from corroding pipes or fittings, or the presence of tools or scrap metals in tanks or the flow stream of the juice. Treatment is mainly preventative, in the sense that all sources of copper, brass, bronze, and iron should be eliminated from the process stream. To test for iron and copper, test solutions need to be made up. A ferrocyanide solution should be made by dissolving 0.5 g potassium ferrocyanide in 100 ml of distilled water. A 3 N HCI solution should be made by diluting 258 ml of 36% HCI to 1 l distilled water. To test for the presence of copper, add 2 ml of the ferrocyanide solution to 20 ml of juice. A brown-red flocculant indicates the presence of copper. To test for the presence of iron, add 2 ml of 3 N HCI to 20 ml of juice : a blue color resulting indicates the presence of iron.

4.13 FRUIT JUICE SAFETY AND HACCP Fruit juices were considered safe due to their high acid content until a number of outbreaks in the 1990s became associated with the consumption of fresh apple and orange juices. The pathogens responsible for the illnesses were Salmonella spp., Escherichia coli O157:H7 and Cryptosporidium parvum. As a result of these outbreaks, the FDA issued a final rule in 2001 titled “Hazard Analysis and Critical Control Point (HACCP); Procedures for the Safe and Sanitary Processing and Importing of Juice” (21 CFR Part 120). The rule requires processors and importers of juices to establish a HACCP plan to minimize the risk of juice contamination with biological, chemical, or physical hazards. The HACCP plan must be developed individually for each processing establishment by a team of knowledgeable individuals that includes persons trained in juice HACCP. Any company producing 100% juice or juice puree used in manufacture of juices and beverages is required to comply with this regulation. The law requires the juice to be treated with a process that achieves at least a 100,000-fold decrease in the number of pertinent pathogens likely to occur in the juice. This requirement is known as the 5-log reduction performance standard. The pertinent pathogen that may occur in the juice is the most heat-resistant microorganism of public health concern. For apple juice, the processors must address critical control points to eliminate the risk of E. coli O157:H7 and C. parvum. For orange juice, the most likely risk comes from Salmonella spp. Approved processing methods to control microbiological hazards are thermal pasteurization (minimum of 71 to 73∞C for 6 to 11 sec) and UV light. Citrus processors have the option of treating the surface of the fruit prior to juice extraction because it is unlikely that the pathogens enter intact, sound fruit under current industry processing practices. Proper monitoring, verification, and validation procedures are necessary to ensure that the HACCP plan is effective. In addition to microbial hazards, the rule also established the maximum level of patulin allowed in a juice at 50 ppb. Patulin is a metabolite of certain fungi such as Aspergillus clavatus, Aspergillus claviforme, Byssochlamys fulva, Penicillium patulum, and Penicillium expansum and, as a mycotoxin, could pose a health risk if found in significant quantities in fruit juices. Patulin is typically associated with unsound, rotting apples (Stott and Bullerman, 1975) and must be prevented from entering the juice by culling infected fruit before juice extraction. There is one exemption in the HACCP rule for retail establishments. A retail establishment is an operation that provides juice directly to consumers. Even though these operations are not required to implement a HACCP plan and therefore might not pasteurize or UV-treat the juice, they must label fresh juices with a warning statement that describes the risk of consuming untreated juices (Juice HACCP Alliance, 2002). Strict sanitary conditions, good agricultural practices, adherence

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to current good manufacturing practices, and utilization of clean healthy fruit are necessary to minimize the presence of microbiological and chemical contaminants in freshly squeezed juices. Other bacterial spore formers have been associated with acidic fruit juices. For a more complete reference to all of these food safety issues in juice processing, the reader is referred to Chapter 10 in this book by D.F. Splittstoesser and R.W. Worobo.

REFERENCES Amerine, M.A. and Ough, C.S. 1980. Wine and Must Analysis. John Wiley & Sons, New York. Anon. 2001. Steady growth in European fruit juice market. Fruit Process., 11(12): 494–495. Anon. 1968. Canned fruit nectars: order establishing definitions and standards of identity. Fed. Reg., 33: 6862–6864. Baumann, G., Strobel, B., and Gierschner, K. 1986. Microfiltration and ultrafiltration of apple juice — comparison of inorganic/organic membranes and conventional deep-bed filters. Flüssiges Obst., 53:251. Beech, F.W. and Carr, J.G. 1977. Cider and perry. In Economic Microbiology, Vol. 6, Alcoholic Beverages, A.H. Rose (Ed.), Academic Press, London, pp. 139–313. Ben Amar, R., Gupta, B.B., and Jaffrin, M.Y. 1990. Apple juice clarification using mineral membranes: fouling control by backwashing and pulsating flow. J. Food Sci., 55: 1620. Bomben, J.L., Bruin, S., Thijssen, H.A.C., and Merson, R.L. 1973. Aroma recovery and retention in concentration and drying of foods. Adv. Food Res., 20: 1–111. Brause, A. 1998. Detection of apple juice adulteration. Fruit Process., 8(7): 290–297. Briston, J.H. and Katan, L.L. 1974. Plastics in Contact with Food. Food Trade Press, London. Brown, W.E. 1987. Selecting packages and composite barrier systems for food packages. In Food ProductPackage Compatibility Proceedings, J.I. Gray, B.R. Harte, and J. Miltz (Eds.), Technomic Publishing, Lancaster, PA. Bump, V.L. 1988. Apple processing and juice extraction. In Processed Apple Products, D.L. Downing, (Ed.), AVI Publishing, New York, p. 3. Castberg, H.B., Fredsted, L.B., and Rysstad, G. 2000. Marketing and packaging of fruit juices and noncarbonated juice based beverages. Fruit Process., 10(4): 130–135. Cheryan, M. 1986. Ultrafiltration Handbook. Technomic Publishing, Lancaster, PA. Cheryan, M. 1998. Ultrafiltration and Microfiltration Handbook. Technomic Publishing, Lancaster, PA. Cousin, M.A. and Rodriguez, J.H. 1987. Microbiology of aseptic processing and packaging. In Principles of Aseptic Processing and Packaging, P.E. Nelson (Ed.), Food Processors Institute, Washington, D.C. Crosby, N.T. 1981. Food Packaging Materials: Aspects of Analysis and Migration of Contaminants. Applied Science Publishers, London. Cross, S. 1989. Membrane concentration of orange juice. Proceedings of the Florida Horticultural Society, 102: 146–152. De Vires, J.A., Voragen, A.G.J., Rombouts, F.M., and Pilnik, W. 1986. Structural studies of apple pectin with pectolytic enzymes. In Chemistry and Function of Pectins, M.L. Fishman and J.J. Jen (Eds.), American Chemical Society, Washington, D.C., pp. 38–48. Downes, J.W. 1995. Equipment for extraction of soft and pome fruit juices. In Production and Packaging of Non-Carbonated Fruit Juices and Fruit Beverages, 2nd ed., P.R. Ashurst (Ed.), Blackie Academic & Professional, London, pp. 197–220. Downing, D.L. 1996. Canning of juices, fruit drinks and water. In A Complete Course in Canning and Related Processes 13th ed., Book III Processing Procedures for Canned Food Products. CTI Publications, Baltimore, MD. Drake, S.R. and Nelson, J.W. 1987. Apple juice quality as influenced by ultrafiltration. J. Food Qual., 9: 399. Gavin, A. and Weddig, L.M. 1995. Canned Foods — Principles of Thermal Process Control, Acidification and Container Closure Evaluation. The Food Processors Institute, Washington, D.C. Gebhardt, S.E., Cutrufelli, R., and Matthews, R.H. 1982. Composition of Foods: Fruits and Fruit Juices. Agricultural Handbook 8–9. U.S. Department of Agriculture. Grassin, C. and Fauquembergue, P. 1993. Enzymatic liquefaction of apples. Flüssiges Obst., 60(7); supplement Fruit Proc. 3(7): 242–245.

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95

Gray, J.I., Harte, B.R., and Miltz, J. (Eds.). 1987. Food Product-Package Compatibility Proceedings. Technomic Publishing, Lancaster, PA. Hamatschek, J., Buhler, K.H., Schottler, P., and Gunnewig, W. 1995. Separators and Decanters for the Production of Fruit and Vegetable Juices. Technical scientific documentation No. 18. Westfalia Separator AG, Oelde, Germany. Hammond, D.A. 2001. Synergy between liquid chromatographic-pulsed amperometric detection and capillarygas chromatographic methods for the detection of juice adulteration. J. AOAC Int., 84(3): 964–975. Hang, Y.D. 2000. Waste management: fruits and vegetables. In Encyclopedia of Food Science and Technology. John Wiley & Sons, New York. Harte, B.R. and Gray, J.I. 1987. An overview of food component interaction during processing and storage. In Food Product-Package Compatibility Proceedings, J.I. Gray, B.R. Harte, and J. Miltz (Eds.), Technomic Publishing, Lancaster, PA. Hartel, R.W. 1992. Evaporation and freeze concentration. Handbook of Food Engineering, Marcel Dekker, New York, chapt. 8: 343–393. Heatherbell, D.A., Short, J.L., and Strübi, P. 1977. Apple juice clarification by ultrafiltration. Confructa, 22(5–6): 157. Helbig, J. 2001. Production of colour-intensive and colour-stable coloured juices. Fruit Process., 11(9): 342–347. Hotchkiss, J.H. 1988. Food and Packaging Interactions. ACS symposium series, Series No. 365 ACS, Washington, D.C. Hulme, A.C. 1971. The Biochemistry of Fruits and Their Products, Vols. 1 and 2, Academic Press, New York. Hurler, A. and Wey, R. 1984. Technique of grinding and making the mash. Confructa Stud., 28: 125–130. Iu., J., Mittal, G.S., and Griffiths, M.W. 2001. Reduction in levels of Escherichia coli O157:H7 in apple cider by pulsed electric fields. J. Food Prot., 64(7): 964–969. Juice HACCP Alliance. 2002. Juice HACCP Training Curriculum, 1st Edition. Developed by the Juice HACCP Alliance as recognized by the Food and Drug Administration. Kashtock, M.E. 1988. New packaging for processed foods: Opportunities and challenges. In Food and Packaging Interactions, J.H. Hotchkiss (Ed.), American Chemical Society, Washington, D.C. Kilara, A. 1981. Enzymes and their uses in the processed apple industry. In Proceedings of the Processed Apple Institute. Research Seminar at The Pennsylvania State University, University Park, PA. Kilara, A. and Van Buren, J. 1989. Clarification of apple juice. In Processed Apple Products, D.L. Downing (Ed.), Van Nostrand Reinhold, New York. Kime, R.W. 1982. Clarification of fruit juice with honey. United States Patent 4,327,115. Knorr, D., Ade-Omowaye, B.I.O, and Heinz, V. 2002. Nutritional improvement of plant foods by nonthermal processing. Proceedings of the Nutrition Society 61(2): 311–318. Koevoets, W.A.A. 2002. Reliable and sustainable treatment solution for fruit processing effluents. Fruit Process., 12(3): 102–106. Kulp, K. 1975. Carbohydrases. In Enzymes in Food Processing, G. Reed (Ed.), Academic Press, New York. McLellan, M.R., Kime, R.W., and Lind, L.R. 1985. Apple juice clarification with the use of honey and pectinase. J. Food Sci., 50: 206. McNeil, M., Darvill, A.G., Frey, S.C., and Albersheim, P. 1984. Structure and function of the primary cell walls of plants. Ann. Rev. Biochem., 53: 625–663. Merin, U. and Daufin, G. 1989. Separation processes using inorganic membranes in the food industry. Paper presented at the 1st International Conference on Inorganic Membranes, Montpellier, France, July 3–6. Milnes, B.A. 1984. The application of ultrafiltration to apple juice processing. New York State Agricultural Experimental Station Special Report, No. 54, D.L. Downing (Ed.), Geneva, NY. Möslang, H. 1984. Ultrafiltration in the fruit juice industry. Confructa, 28(3): 219. Nagy, S., Attaway, J.A., and Rhodes, M.E. 1988. Adulteration of Fruit Juice Beverages. Marcel Dekker, New York. Odebo, U. 2001. Fresher under pressure — a fully commercial cold pasteurization method for fruit products. Fruit Process., 11(6): 220–221. Osborne, D.G. 1980. Glass. In Developments in Food Packaging — 1, S.J. Paling (Ed.), Applied Science Publishers, London. O’Sullivan, T., Simons, and Epstein, A. 1988. For winemakers — a simplified introduction to ultrafiltration. Vineyard Winery Management, 3: 65.

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Padilla, O.I. and McLellan, M.R. 1989. Molecular weight cut-off of ultrafiltration membranes and the quality and stability of apple juice. J. Food Sci., 54(5): 1250–1254. Padilla, O.I. and McLellan, M.R. 1993. Optimization and modeling of apple juice cross-flow microfiltration with a ceramic membrane. J. Food Sci., 58(2): 369–374, 388. Paine, F.A. and Paine, H.Y. 1983. A Handbook of Food Packaging. Leonard Hill, Glasgow. Pecoroni, S. and Gunnewig, W. 2001. Citrus technology — improved quality and economics through the use of high performance centrifuges. Fruit Process., 11(2): 46–48. Pilnik, W., Rombouts, F.M., and Voragen, A.G.J. 1973. On the classification of pectin depolymerases: Activity of pectin depolymerases on glycol esters of pectate as new classification criterion. Chem. Microbiol. Technol., 2: 122–128. Pinnow, D. 2000. Higher yields by means of an optimised mash preparation. Fruit Process., 10(9): 346–350. Possmann, P. 2000. European way of apple processing. Fruit Process., 10(3): 88–93. Rao, M.A. 1989. Concentration of apple juice. In Processed Apple Products, D.L. Downing (Ed.), Van Nostrand Reinhold, New York. Schobinger, U. 1999. Progress in fruit juice technology during the last 50 years — survey. Fruit Process., 9(7): 275–281. Schwartzberg, H.G. 1977. Energy analysis in food process operations. Food Technol., 31: 49–88. Selvendran, R.R. 1985. Developments in the chemistry of pectic and hemicellulosic polymers, J. Cell. Sci. Suppl., 2: 51–88. Siebert, K.J. 1999. Effects of protein-polyphenol interactions on beverage haze, stabilization, and analysis. J. Agric. Food Chem., 47(2): 353–362. Smock, R.M. and Neubert, A.M. 1950. Apples and Apple Products. Interscience Publishers, New York. Splittstoesser, D.F. and King, A.D., Jr. 1984. Enumeration of Byssochlamys and other heat resistant molds. In Compendium of Methods for the Microbiological Examination of Foods, M.L. Speck (Ed.), American Public Health Association, Washington, D.C., pp. 203–210. Stott, W.T. and Bullerman, L.B. 1975. Patulin: A mycotoxin of potential concern in foods. J. Milk Food Technol., 38: 695–705. Swientek, R.J. 1986. Ultrafiltration’s expanding role in food & beverage processing. Food Process., 4: 71. Tandon, K., Worobo, R.W., Churey, J.J., and Padilla-Zakour, O.I. 2003. Storage quality of pasteurized and UV treated apple juice. J. Food Process. Preserv., 27(1): 21–34. The Food Institute. 2002. The Almanac of the Canning, Freezing, Preserving Industries 2001-2002. 84th ed. Elmwood Park, NJ. Thijssen, H.A.C. 1974. Freeze concentration. Advances in Preconcentration and Dehydration of Foods, A. Spicer (Ed.), John Wiley & Sons, New York, pp. 115–149. Van Buren, J.P. 1988. The sources and prevention of turbidity in apple juice. In Processed Apple Products, D.L. Downing (Ed.), VNR/AVI Publishing, Westport, CT. Voragen, A.G.J., Heutink, R., and Pilnik, W. 1980. Solubilization of apple cell walls with polysaccharidedegrading enzymes. J. Appl. Biochem., 2: 452–468. Worobo, R.W., Churey, J.J., and Padilla-Zakour, O. 1998. Apple cider: treatment options to comply with new regulations. J. Assoc. Food Drug Off., 62(4): 19–26. Wu, M.L., Zall, R.R., and Tzeng, W.C. 1990. Microfiltration and ultrafiltration comparison for apple juice clarification. J. Food Sci., 55: 1162. Yamasaki, M., Yasui, T., and Arima, K. 1963. Pectic enzymes in the clarification of apple juice. 1. Study on the clarification in a simplified model. Agric. Biol. Chem., 28: 779. Yokotsuka, K., Kato, A., and Kushida, T. 1978. Microdetermination of proteins in juices and wines by trichloroacetic acid (perchlori acid)-dye method. J. Ferment. Technol., 56: 606.

in the Fruit Juice and 5 Enzymes Wine Industry Alberecht Höhn, Daqing Sun, and Francois Nolle CONTENTS 5.1 5.2

Introduction ..........................................................................................................................97 Enzymes for Apple Juice Production ..................................................................................99 5.2.1 Juice Clarification ................................................................................................100 5.2.2 Mash Treatment ...................................................................................................101 5.2.3 Advanced Maceration ..........................................................................................102 5.3 Enzymes for Berry Processing ..........................................................................................103 5.4 Enzymes for Citrus Processing..........................................................................................105 5.4.1 General Citrus Processing ...................................................................................105 5.4.2 Citrus Processing Enzymes .................................................................................106 5.4.2.1 Oil Extraction Enzymes.....................................................................106 5.4.2.2 Pulpwash Enzymes ............................................................................106 5.4.2.3 Cloudifier Production Enzymes.........................................................107 5.4.2.4 Clarified Citrus Juice Production ......................................................108 5.5 Enzymes in Wine Making..................................................................................................108 5.5.1 General Remarks .................................................................................................108 5.5.2 Use of Pectolytic Enzymes..................................................................................108 5.5.2.1 Mash Treatment .................................................................................108 5.5.2.2 Must Treatment ..................................................................................109 5.5.3 Use of Proteases ..................................................................................................109 5.5.4 Other Enzyme Applications in Wine Making.....................................................111 References ......................................................................................................................................111

5.1 INTRODUCTION The use of technical enzymes has been an essential part of the entire technology of fruit juice production from the beginning. In the early 1930s, the first steps were made to process fruits into juices that could be stored for a longer time without the danger of alcoholic fermentation or other forms of undesired spoilage. The purposes of using enzymes in fruit juice or wine are (1) to increase extraction of juice from raw material, (2) to increase processing efficiency such as pressing and solids settling or removal, and (3) to render the final product sparkling clear and attractive. Fruit juices were and still are considered to be an excellent alternative to alcoholic beverages like beer or wine. Starch-degrading enzymes for fruit juice clarification and, later, pectolytic enzymes represented the first enzymatic products that were successfully used in fruit juice production. New processing technologies have been made feasible by new enzyme preparations. For example, yields of juice from apples have been pushed higher and higher, making modern fruit processing more efficient and economical. Typical of the enzyme activity of commercial fruit juice processing 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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activity at different temperatures

activity at different pH values % relative activity

% relative activity

100

100

80

80

60

60

40

40 20

20 pH value 0

0 5 7 4 6 50 1 3 20 30 40 10 2 reaction time: 90 min - temperature: 55°C reaction time: 90 min - pH value: 3.5 properties of pectinases

°C 60

FIGURE 5.1 Pectinase activity — dependence on pH and temperature.

enzymes is their dependence on reaction conditions like pH and temperature (Figure 5.1). Most of them show their highest activity at a pH range of 3.5 to 4.5. As can be seen in the general production line depicted in Figure 5.2, there are two possibilities for using enzymes in clear juice processing: mash treatment and juice clarification. In the mash treatment stages of juice production, the objective is to obtain as much juice as possible from the fruit mash. Pectolytic enzymes help considerably at this stage. At the clarification stage, pectolytic enzymes help to clarify juice and make juice filtration much easier. In the citrus industry, on the other hand, special pectolytic enzymes are used in the pulp wash process and to reduce viscosity in order to avoid gel formation of pectin during concentration and to recover citrus oil. Traditionally, the term pectinases includes the three enzymes pectin methyl esterase, pectin lyase, and polygalacturonase. Sufficient hydrolysis of pectin is the primary key to increase yield and clarity of clear juice. The early model of the structure of pectin was that of a smooth chain of partially esterified polygalacturonic acid. Therefore, it was believed that as long as there is sufficient pectinases, i.e., pectin lyase, pectin methyl esterase, and polygalacturonase, the pectin molecule could be totally hydrolyzed. More recent studies of the structure of pectin molecule have concluded that the structure of pectin is composed of smooth regions and hairy regions (Figure 5.3). The smooth region is called homogalacturonan, which is partially esterified polygalacturonic acid. The hairy region is complex and may be composed of rhamnogalacturonan I, rhamnogalacturonan II, and other galacturonic acidcontaining polysaccharides. There are a large number of neutral sugar branches attached to the hairy regions of a pectin molecule. To hydrolyze both the smooth region and the hairy regions, and therefore to achieve total hydrolysis of the pectin molecule, requires the activity of more enzymes than the three basic pectinase enzymes mentioned. Based on our current knowledge of the pectin molecule, the following enzymes may be necessary to achieve total hydrolysis of pectin: pectin lyase, pectin methyl esterase, polygalacturonase, arabinanase, rhamnogalacturonase, rhamnananase, galactanase, and other glycanses. Different fruits have different types of pectin, in which the weight, composition, and structure of the hairy region are different. In principle, commercial enzyme preparations for juice processing should be formulated in such a way that they cover necessary enzyme activities to achieve a high grade of hydrolysis of the targeted fruit or vegetable pectin. The same principle applies to the white wine making process because, before the fermentation step, grapes need to be crushed and solids need to be removed just like in any other juice making process. In addition, as recently as in 2002, a protease was found to be of important use in hydrolysis of a heat labile protein in white wine. In the case of red wine making, the situation is different

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FRUITS WASHING, SELECTING

STEMMING, DESTONING, PEELING

BLANCHING IF NECESSARY

CRUSHING, GRINDING, MILLING MASH (HEATING) ENZYME TREATMENT

PRESSING

DECANTING FINISHING

TURBID JUICE

PUREE

ENZYME TREATMENT ULTRAFILTRATION

FLASH HEATING

FLOCCULATION CLARIFICATION STABILIZATION

CENTRIFUGING

FILTRATION

CLEAR JUICE

CLEAR JUICE

CLOUD STABLE JUICE

FIGURE 5.2 General fruit juice line.

Hairy Region

Smooth Region

Hairy Region

FIGURE 5.3 Pectin structure.

from white wine making because red wine is fermented on the skin. Still, certain commercial pectinases are specifically useful in red color extraction and in a process called thermovinification.

5.2 ENZYMES FOR APPLE JUICE PRODUCTION Apples are by far the most important fruit for the production of clear juices and clear juice concentrates worldwide. The most important producers of apple juice concentrates are the U.S., China, Chile, and Argentina and, in Europe, Germany, Italy, and Poland.

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aroma recovery

juice from the press

pectinase amylase

aroma

enzyme treatment

fine filter

clarification

bentonite gelatin silica sol

pre-filter

FIGURE 5.4 Schematic of apple juice clarification.

5.2.1 JUICE CLARIFICATION The principle of apple juice clarification can be taken as representative for juices from other fruits as well. Apple juices that are running off the press are always cloudy due to the presence of a wide range of natural colloidal polysaccharides, cellulosic fragments from skin and pulp, and other small particles like protein fragments or polyphenols and tannins with protein. The polysaccharides are part of the intercellular cement or pectin, which holds together the plant tissue, whereas proteins, polyphenols, and other compounds are constituents of the plant material. During crushing of the fruit and the dejuicing process, these constituents enter the juice in solubilized and unsolubilized colloidal form. The most important of these high molecular substances is pectin, which is responsible for a series of technical problems encountered during processing. As a cloud stabilizer, it impedes the clarification of juices, which leads to increased viscosity and gelling and thus can be easily scorched to burn the juice in the evaporator. Pectin can also complex with calcium to form hazes and precipitates in later processing steps. Pectolytic enzymes are used in order to facilitate the clarification process. Through enzymatic depectinization, the juice can be clarified and filtered easily. Starch can often be found in apple juices. It is normally present in ripe fruit in the form of microscopically small, insoluble granules. When the juice is heated, for example, for aroma recovery or during pasteurization, the starch becomes hydrated and gelatinized. However, after filtration of the juice, the starch once again becomes insoluble as it retrogrades and precipitates. This causes an unattractive postbottling clouding of the juice or concentrate. Complete enzymatic degradation of the starch by means of special amylases is only possible if the juice was previously heated to at least 85∞C. The heat treatment hydrates and gelatinizes the starch and makes it ready for enzymatic degradation. This production process in Figure 5.4 describes the hot-clarification process, which is by far the most frequently used for the production of clear apple juice concentrate worldwide. In this process, enzyme treatment and subsequent fining are carried out in the juice coming from the aroma recovery where the juice has been heated up briefly in order to remove the aroma and to solubilize the starch. The aroma has to be removed before concentration and stored separately due to quality reasons. The de-aromatized juice has a temperature of 50 to 55∞C after passing through a heat exchanger where it is cooled down by preheating the juice coming from the press. This temperature range is almost ideal for enzyme activity. Compared to cold clarification of apple juice, which is done at ambient temperature and takes 6 to 8 h, the hot clarification usually requires

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a time period of only 2 to 3 h. This process and the higher temperatures during processing practically exclude the danger of alcoholic fermentation or other forms of undesired microbial activity. Only after the enzyme treatment has been completed is the process ready to be continued. Fining agents like bentonite, gelatin, and silica sol may be added for further flocculation and precipitation of cloudy material. Compared to the fining procedure, ultrafiltration is a relatively new technology in the fruit juice industry. The benefits of ultrafiltration are as follows: 1. 2. 3. 4.

No clarifying and fining agents are necessary. Continuous processing is possible. Less tank space is required. There is a saving of labor costs because of automatic controls.

In respect to the enzyme application, both the fining process and ultrafiltration process require complete enzymatic breakdown of pectin and starch or other polysaccharides. The practical application of enzymes for apple juice clarification is fairly simple. Liquid enzymes, which are by far the most widespread in fruit juice technology, are simply poured into the juice tank while filling up the tank with juice. The correct dosage is usually determined with the help of pretests in the laboratory. A mechanical stirrer in the tank leads to a good distribution of the enzyme. Granulated enzyme preparations have to be diluted in cold tap water before they are added to the juice. After the enzyme reaction, which has to be checked with specific test methods, the juice is ready for clarification, filtration, and concentration. The most important test method for pectin degradation is the so-called alcohol test. The test is simple and rather reliable; therefore, it is well-suited to be used during production. The test is carried out in the following way. Two parts of acidified alcohol are added to one part of juice. Gel formation in the presence of pectin can be observed 10 to 15 min after addition of the alcohol. Using this method, even very small amounts of pectin can be detected; therefore, this test has become very important as a quality control tool in international trade with clear juice concentrates. The degradation of starch is tested by using the iodine test. The addition of a 1% iodine solution to the juice sample provokes a typical bluish-purple coloration of the juice if starch is present. The disappearance or absence of this color is proof that starch was completely degraded.

5.2.2 MASH TREATMENT Whereas enzymes for clarification were part of apple juice production right from the beginning, the use of enzymes for apple mash treatment is fairly recent. There are two main reasons for using pectinases for mash treatment: higher juice yields and an increase of the press capacity and efficiency. Many stored apples are processed, particularly in the late part of the processing season. The quality of this raw material for juice production is not as good as that of fresh fruit. The longer the storage period, the greater is the change in composition of the apple tissue: pectin is converted from its insoluble form, the so-called protopectin, into soluble pectin. The latter leads to increased mash viscosity and impedes the flow of the juice. The whole pressing process takes much longer. Measures to increase the juice yield are often indispensable to producing juice with profit. Enzymatic hydrolysis of the soluble pectin by means of a special mash enzyme removes these drawbacks and provides a better yield. The quantity of juice can be increased significantly. In years with a good harvest, the main emphasis is on the maximum use of the press capacity. By using a special apple mash enzyme, large quantities of raw material can be processed with ease. The whole pressing capacity, expressed by hourly throughput, is stepped up considerably. The use of a mashing enzyme to increase the throughput could help avoid the need to purchase additional expensive presses. Due to the high investment costs for juice presses, the increase of the press capacity is a very strong argument for using mash enzymes.

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Special pectinase for apple mash treatment

apple mash enzyme continuous dosage into the mill

Time: 15–60 min Temperature: 10–30°C

belt press

horizontal press

One pressing step

two pressing steps

FIGURE 5.5 Processing line — apple mash treatment.

The enzyme is usually added as a 1 to 2% dilution into the mill. Optimal distribution is essential for a good effect of the mash enzyme. The principle is shown in Figure 5.5. Extraction of the pomace with water after the first pressing can further increase the total yield. Addition of mash enzyme in this extraction process, together with warm water, is necessary to wash out residual sugar that can still be found in the pomace after one press run. This water extract can be processed only into clear apple juice concentrate. By combining pressing and extraction processes in the production of apple juice concentrate with the help of enzymes, yields of more than 90% as measured by total brix are realistic. Furthermore, enzymatic mash treatment facilitates cleaning of the presses. Especially when stored apples have been pressed, a lot of residue remains in the pomace, making it sticky and difficult to remove. Much time has to be used cleaning the presses, which results in a decrease in the total processing capacity.

5.2.3 ADVANCED MACERATION Decanters are horizontal centrifuges that are able to separate juice from mash with high contents of solids. Progress in the construction of decanters and the availability of specially designed advanced maceration enzymes have supported this new development. As is known from various studies and tests, it is preferable to achieve a rapid drop in mash viscosity to 50% or 60% of the initial value. This is only possible with enzymatic treatment. Enzymes used in this application are a family of products able to perform advanced pulp maceration before juice separation in decanters or can be used to perform advanced pomace maceration, i.e., first pressing on belt press, then liquefaction of residual pomace before separation in a decanter (Figure 5.6) or a Bucher press. Those processes result in much better yield, but require a different technology from the classical mash treatment. This technology is not popular in some parts of the world because people do not like the whole apple to be totally macerated into juice. Some in the industry have a concern that the liquefaction process may introduce undesirable components into the juice. For example, the presence of trace levels of cellobiose in juice has been attributed to the use of enzymes in the liquefaction process. However, it has been found that cellobiose, the breakdown product of cellulose, is present in commercial applesauce where no enzymes have been employed. This misperception is increased when the process is incorrectly described as “total liquefaction.” It should be pointed out that none of the enzymes used in either the mash or liquefaction processes are capable of total

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apples mill pips and cores

finisher reaction tank

pomace

decanter

liquefaction enzyme 200–300 ppm 1–3 hrs. reaction time

juice fining filtration concentration

FIGURE 5.6 Processing line — mash liquefaction.

liquefaction of the entire fruit. In fact, in the liquefaction process about 25% of the mash is discarded as waste after juice extraction is completed. In the liquefaction process the enzymes function to assist in the physical separation of the juice from the pomace and to reduce the viscosity of the juice so that it can be efficiently separated from the pomace by the decanter. The end result is that additional juice is recovered in the liquefaction process that may not be extracted during traditional pressing operations. The differences between the “classical” mash treatment and advance maceration are as follows:

Dejuicing system Enzyme dosage Reaction time Reaction temperature Enzyme activities Process

Mash Treatment

Advanced Mash Maceration

Horizontal press 50–100 ppm 30–60 min 10–30∞C Pectinase Batchwise

Decanter 100–300 ppm 1–3 h 30–50∞C Pectinase, Hemicellulase Continuous

A further advantage of advance maceration is the saving of an extra step for juice treatment with enzymes. The enzyme dosages for the advanced maceration are usually high enough to achieve complete pectin and starch degradation of compounds already in the mash. Due to the fact that the alcohol test cannot be carried out with the mash itself, it is necessary to separate the serum (juice) from the solids by centrifuging a small sample of mash in a lab centrifuge. The supernatant is taken to check whether pectin and starch have been degraded completely and the mash is ready for further processing. When alcohol and iodine tests are negative, indicating that no residual pectin or starch is present, the highly macerated mash is ready to be extracted with the decanter.

5.3 ENZYMES FOR BERRY PROCESSING Compared to apples, the pectin content in some berry fruits is much higher (Figure 5.7). When berry fruits are crushed without using pectolytic enzymes, the soluble pectin causes gelling of the mash before the juice can run off the press (Figure 5.8). Adding pectolytic enzymes to the mash prevents the formation of the pectin gel and makes it possible to produce good quality berry juices for a reasonable price. The enzymes that are used for processing berry fruits have to meet special requirements, which are caused by the production technology and the character of the raw material.

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FIGURE 5.7 Pectin content of apples compared with black currants. Dosage to the mash: 80–300 g/t (Blackberries) 40–150 g/t (Others) mash tanks pectinase for berry processing

roller mill

heater

control: alcohol test

pomace extraction

belt press

enzyme treatment and fining tanks filtration enzyme application for berries and stone fruits

FIGURE 5.8 Processing line — berry juice production.

The berry mash is usually heated after crushing. Special tubular heat exchangers have to be used for this production step. Heating the mash can improve extraction of the cell wall material and color. Hot pressing of black currant or cranberry pulp after a 1 h enzyme treatment at 55∞C is widely practiced. The process is preferred to cold clarification because of better yield and retention of good color, aroma, and vitamin C. The pH value of most berry juice is very low. As an example, a typical blackcurrant or raspberry juice has a pH of 2.8. Anthocyanins are the largest group of water soluble pigments responsible for most of the red, pink, and blue color of fruits. Berry color is primarily given by anthocyanins. Most commercial pectolytic enzymes contain certain anthocyaninase side activity, which degrade anthocyanins into a colorless form called anthocyanidin. To ensure berry juice color stability, the level of anthocyaninase side activity must be minimal. A good pectolytic enzyme preparation for berry processing therefore needs to possess the following characteristics: heat tolerance and low pH tolerance, with little or no anthocyaninase activity. In addition, it was found that juices from blanched currants contain a substantially larger amount of alcohol-precipitable material (pectin, polysaccharide, and protein) and that an acid fungal protease improved the action of pectinase in the secondary clarification of the juice. Cellulase and hemicellulase may be useful in color extraction from berry fruits.

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5.4 ENZYMES FOR CITRUS PROCESSING Among citrus juices, orange juice is by far the most important. Lemon, lime, grapefruit, and tangerines are also processed as juice. Lemon juice is still a by-product of cold pressed lemon oil. Bergamotes and other citrus species are processed just for their essential oil. Brazil is by far the biggest producer of orange juice, followed by the U.S., Greece, Italy, Spain, and South Africa. The three biggest producers of lemon juice are Argentina, Spain, and Italy. One of the characteristics of citrus juice is that it is to be consumed mainly as a cloudy drink. Although the biggest percentage of that juice is made of diluted first extraction juice that does not require enzyme treatment most of the time, there is an appreciable production of by-products that are made from the pulp and the peels, and these are marketed either as second-quality juice or as natural cloudifiers. The processes involved in the production of such by-products require specific enzymatic treatments that are designed to extract sugars and reduce juice viscosity in order to allow concentration without destabilizing the juice cloud. Some citrus juices, especially lemon juice, are to be marketed as clarified juice. In that specific case, the enzymatic treatment is designed to depectinize the juice before filtration and concentration.

5.4.1 GENERAL CITRUS PROCESSING After the fruits are washed and sorted out, the juice extraction is performed on specific juicing equipment. Two technologies are widely used: The FMC system extracts the juice at the same time an oil emulsion is obtained by spraying water on the fruit, while Brown technology is designed to extract the oil emulsion first before the fruit is squeezed. The first extraction therefore results in the production of a pulpy juice on one hand and an oil-in-water emulsion on the other hand. Juice yields in this step are generally within the 40 to 50% (w/w) range. The pulpy juice then goes to a series of finishers where the pulp content is reduced, and the de-pulped juice can be concentrated as cloudy citrus juice. While orange juice is marketed mostly as 60 or 65∞ brix concentrate, cloudy lemon juice is sold based on its citric acid content. Generally, it is concentrated to 400 or to 530 gpl (grams citric acid per liter). It is important to note that citrus fruits usually have substantial endogenous pectin methyl esterase content. This enzyme is able to destabilize the juice cloud through the production of calcium pectates that rapidly precipitate. It is therefore of premium importance to pasteurize the juice as soon as possible after squeezing to denature that pectin methyl esterase. The oil-in-water emulsion generally goes through two steps of centrifugation in order to isolate the essential oil. It is then stored at cold temperatures to remove the waxes before it can be sold. Enzymes are used to increase the oil recovery. The by-products resulting from the general oil extraction process are mainly peel, pulp, and core. The peel can be dried to be used as cattle feed or for pectin extraction. However, a new trend is to produce natural cloudifiers from those peels (sometimes added to pulp) that are used in fruit based drinks such as lemonades. Enzymatic treatment is of primary importance in this process. The peel and pulp are finely crushed and water is added. Sometimes, pasteurization can be performed at this step in order to destroy fruit pectin methyl esterase and to solubilize more pectin. Then an enzymatic maceration is performed in order to extract sugar and to reduce the molecular weight of the pectin. The macerated mash then goes to a centrifugal separator (decanter) to separate the juice from the remaining pulp. This step is often performed at 80∞C in order to make the separation easier. Then the juice is centrifuged and concentrated. Later the key enzymatic treatment linked to that process will be described. The pulp is generally used to produce a cloudy concentrated pulpwash juice. The process consists of counter-current sugar extraction with leaching water. The resulting juice has a high soluble pectin content that impedes its concentration. An enzymatic treatment has to be performed

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PEELS (RAGS/PULP) Water (1:1)

Grinding Heating 45°C

pectinase 40–60 ppm

Enzyme Reaction approx. 60 min

Finisher

Extract Liquid

pectinase 10–30 ppm

Enzyme Reaction 20–30 minutes continuous Viscosity Control

Only if enzymatic Pulp treatment has not been applied

Pasteurization Centrifuge Concentration CLOUDY PEEL PRODUCTS

FIGURE 5.9 Citrus oil recovery process.

to allow its concentration without gelling. In some cases, an initial enzymatic maceration of the pulp plus water mix is performed in order to increase the sugar recovery from the pulp.

5.4.2 CITRUS PROCESSING ENZYMES 5.4.2.1 Oil Extraction Enzymes Due to the low price of orange essential oil, this application is seldom used in orange processing. However, it is economically justified to use enzymes in lemon, tangerine, grapefruit, and bergamote oil production. Enzymatic cocktails based on pectolytic enzymes and cellulytic enzymes are widely used to increase the oil recovery. These enzymes are added to the oil-in-water emulsion before centrifugation or between the two centrifugation steps (Figure 5.9), and help in breaking down the emulsion, resulting in easier separation and increased yield. These enzymes are mainly used when the extraction water is recirculated to the extraction system after it is de-oiled. However, even in open systems, an important yield increase can be obtained. Typical yield increases vary from 5 to 10%, and the oil recovery can go up to around 80%, based on fruit oil content, if enzymes are properly used. 5.4.2.2 Pulpwash Enzymes As mentioned above, pulpwash juice has a high soluble pectin level. The soluble pectin tends to gel when the sugar concentration increases during the concentration step. Pectolytic enzymes can

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be used to reduce the molecular weight of pectin, thus decreasing dramatically their ability to gel. However, since the juice should remain cloudy, this enzymatic treatment must be designed to reduce pectin molecular weight without causing juice clarification. Although most pectolytic enzymes can achieve the pectin molecular-weight reduction in the process conditions, using common pectolytic enzymes can be risky. If the enzyme dosage is properly calculated and the contact time is completely under control, an acceptable viscosity reduction can be achieved with acceptable cloud stability. However, in industrial conditions, reaction temperature and contact time can seldom be completely under control, and common enzymatic products can easily induce clarification or cloud loss if contact time is longer than expected or if the reaction temperature is not exactly as expected. This is why very specific enzymatic products have to be used in that application, so that the pulpwash juice cloud remains stable when enzymatic treatment conditions vary within an acceptable industrial range. The most efficient viscosity reduction with stable cloud effect can be achieved with pectolytic enzymes that have been previously purified from some other undesirable activities that render cloud instability. Specific hemicellulases also have a beneficial effect on viscosity reduction, without inducing any risk of cloud instability. An adequate combination of those specific pectolytic enzymes and hemicellulases allow achievement of fast and efficient viscosity reduction, without initiating any cloud instability when contact time and temperature are reasonably under control. Such enzymes are designed to allow processors a sufficient margin to cope with process variability. It has to be noted that the same enzymatic products can also be used, although at much lower dosages, to decrease first extraction juice viscosity. In some cases, the first extraction juice tends to gel at the high Brix levels that are required for the market place. A light enzymatic treatment with adequate pulpwash enzymes can allow a dramatically reduced juice viscosity, thus eliminating the gelling risk and making the processing easier. Lower concentrate viscosity means easier pumping, even higher concentration capacity, etc. 5.4.2.3 Cloudifier Production Enzymes These enzymes are designed to solubilize solids from the peels and to decrease juice viscosity in order to make concentration possible (Figure 5.10). Specific enzymatic blends are required in order to have an acceptable soluble substance extraction, reduce viscosity while maintaining the cloud stability, and prevent concentrate gelling. Specific pectinases, with high cellulase side activity content, as well as specific hemicellulases are necessary to achieve this goal. Careful control of processing conditions such as particle size, maceration temperature, and contact time are of premium importance to achieve a good result. citrus fruits

Water recycling

oil extraction emulsion (1–3% oil) desludger pectinase proteinase

Water

cream (60–80% oil)

filter

make up tank pectinase

polisher dewaxing citrus oil essence

FIGURE 5.10 Processing of natural cloudy peel concentrate — extraction process.

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5.4.2.4 Clarified Citrus Juice Production While orange juice clarification can be achieved with most common pectinases, acid juices such as lemon and lime juice will dramatically reduce the activity of the above mentioned common pectolytic enzymes. It is therefore of premium importance to use acid-stable pectolytic enzymes to economically depectinize lemon and lime juice, whose pH can be as low as 2. Most pectolytic enzymes have an optimal pH varying between 4.5 and 3.8. At pHs lower than 3.5, the remaining activity is reduced dramatically and the activity at pH values lower than 2.5 can be quite low. Only a very small number of pectolytic enzymes can retain enough activity at those low pHs to perform economical pectin degradation, and therefore allow acceptable filtration and trouble free concentration of the filtered juices. Side activities present in such pectinases are also of premium importance to allow production of haze free concentrates. In the case of using ultrafiltration, the hemicellulase content of the pectolytic enzymes used is also of premium importance to achieve good ultrafiltration rates. Last but not least, in some cases, the use of acid-stable proteases can be useful in achieving perfect concentrate stability. Other applications of enzymes in the citrus industry have been developed on a large scale, such as juice debittering. Naringinase, for instance, is able to convert bitter naringin into sweet prunin, thus reducing the bitter taste of grapefruit juice. In this section, we emphasized the importance of hemicellulase activity working together with pectolytic enzymes. In fact, the “hemicellulase” works against the pectin hairy region; therefore, strictly speaking it should be considered as part of the group of pectolytic enzymes. Generally speaking, enzymes are very useful tools in citrus juice processing. Specific processes can be implemented using specific enzymes in many applications, and new alternatives are being investigated on a daily basis. It would be difficult in one chapter to mention all the possible applications, and all the investigations going on. One should refer to specialists in enzyme companies for specific needs.

5.5 ENZYMES IN WINE MAKING 5.5.1 GENERAL REMARKS Many years after the fruit juice industry’s recognition of the value of applying enzymes, wine makers have also come to acknowledge their usefulness. In many wine growing countries, enzyme use had long been difficult or impossible for reasons of tradition or because of legal regulations. In Germany, the Wine Act of 1971 opened the door for the industrial use of enzymes by expressly permitting the addition of pectinases. In Switzerland, the U.S., Australia, and South Africa, proteases and b-glucanases are also approved for use in wine treatment, and their approval is currently being discussed in Europe as well. Meanwhile, enzymes have become an indispensable element of processing, both from the aspect of quality and of cost reduction.

5.5.2 USE

OF

PECTOLYTIC ENZYMES

Depending on the quality of the grapes and the processing method used, pectolytic enzymes can be used for various purposes. Their use in the treatment of grape mash, must, and hazy young wines is generally acknowledged. 5.5.2.1 Mash Treatment Treatment of the de-stemmed grape mash is particularly recommended for some white grape varieties that are difficult to press or for better flavor enhancement of bouquet grapes. With dessert grapes and similar types, enzymatic mash treatment is, in fact, almost indispensable to press the juice at an acceptable rate. Enzymatic liquefaction of grapes containing pectin improves so-called

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preliminary juice extraction, i.e., free-run juice from the mash before pressing. This juice, which runs without any mechanical pressure, is considered to be of superior quality and is usually used for the production of high-quality wines. Similar to fruit juice production, the total juice yield is also increased. In red and rose wine making, enzymatic mash treatment speeds up the color extraction. Pectolytic enzymes open up the fruit tissue, and thereby enhance pigment diffusion. In the mash fermentation process, which is considered to be the “classical” red wine technology, the fermentation period is reduced. Drawing off can be performed earlier and without loss of color and wine quality, which is a great advantage for automated processes. In thermovinification, the mash is heated up in order to extract the color before alcoholic fermentation. The juice that runs off the press contains most of the anthocyanins from the skin, which is responsible for the typical red wine color. The juice is then fermented as in the production of white wine. In this technology, the enzyme unfolds its full activity in the optimum temperature range of 45 to 65∞C, thereby improving the color extraction, preliminary dejuicing, and pressing. 5.5.2.2 Must Treatment Musts that are coming from the press contain considerable amounts of soluble pectin, which stabilizes the suspended matter and prevents it from settling rapidly. Preclarification is a very important measure in quality wine production because it reduces the content of undesired microorganisms and makes the alcoholic fermentation easier to control. Therefore, it must be the aim to remove the soluble pectin until the musts subjected to fermentation contain a certain minimum content of suspended matter. Pectolytic enzymes hydrolyze pectin quickly, so that it takes only a short reaction time for the preclarification process, either as natural sedimentation or with the help of mechanical separation. In the case of normal settling, enzymatic must treatment leads to more effective clarification and a smaller lees volume. After a reaction time of 4 to 6 h, a compact lees has formed, from which the supernatant can be drawn off with ease. After fermentation, young wines can be clarified and filtered easily, so that no further enzymatic treatment is required. Red wines from thermovinification process usually cause problems during clarification and filtering because their natural enzyme system has been destroyed by heating. To avoid problems during clarification and filtering, it is therefore imperative to add enzymes either to the mash or to the juice obtained from the press.

5.5.3 USE

OF

PROTEASES

Brilliance and clarity are important features for a wine to be perceived as good quality. The formation of haze or precipitate in wine after it is bottled causes consumers to be suspicious as to the quality of the wine. For consumers, the formation of haze or precipitates in wine indicates that the wine may be of poor quality or microbiologically spoiled. The aggregation of heat-labile proteins to form precipitate in white wine has been a concern in wine making for many years. Wine makers have attempted to use a number of approaches for the removal of wine proteins. The most commonly used preventative measure involves addition of bentonite, a montmorillonite clay that principally consists of aluminum and silicone oxides. The adsorption of compounds, including wine proteins, on bentonite is principally through a cation-exchange action. A protease was recently developed to replace bentonite to prevent a heat instability problem in white wine. The protease can be added at various stages of white wine fermentation (Figure 5.11). The benefits of using this protease instead of bentonite include but are not limited to: 1. Recovering more wine. Bentonite retains large volumes of wine, resulting in a significant loss in wine volume that is very difficult to recover. Using protease results in no loss of wine.

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Obtain Fruit

Extract Juice Add Protease Add Yeast

Fermentation

Remove Solids

Bottle

FIGURE 5.11 Using protease in white wine making.

2. Improving wine quality. Large amounts of added bentonite have the potential for changing the sensory characteristics of the wine by removal of desirable aroma or flavor and other components contributing to wine body. Using this enzyme will not affect wine sensory characteristics. 3. Reducing operation cost associated with bentonite application. 4. Positive environmental impact. Unlike the use of bentonite, the use of protease will not raise the issue of disposal because there is nothing to dispose of in such a case. Possibly due to low substrate concentration and inhibiting factors in wine environment, the action of protease in white wine stabilization should be considered a slow and constant process. This is different from the effect of bentonite, which eliminates protein rather quickly by absorption. The role of protease in white wine protein stabilization may not be to eliminate unstable grape protein preemptively but rather to act upon unstable grape proteins to hydrolyze them into smaller peptides and keep them from giving haze or precipitate during prolonged storage. Even though the enzyme can be applied at virtually any stage of white wine making for the purpose of heat stabilization, researchers found some other benefits when applying the protease at the beginning of fermentation. It was found that juice treated with the enzyme did not produce a significant amount of foam during fermentation compared with the control (Figure 5.12). This can help to save tank capacity requirement and to prevent the contamination problem. The benefit can be seen more obviously when fermentation is at relatively high temperatures or with grape varieties that tend to foam severely. For low temperature fermentation, faster and more complete fermentation was achieved by using enzymes. The enzyme-treated fermentation produced 0.1 to 1.5% more alcohol than the control. Moreover, the enzyme can consistently shorten fermentation time by at least a day and reduce the risk of “stuck fermentation,” that is, fermentation stopping before all sugars are converted to alcohol by yeast. Most white wines are fermented cold (13 to 15∞C) for development of better aroma, fruitiness, and richer mouth feel. A “stuck” wine will soon develop volatile acid (vinegar) and hydrogen sulfide. Understanding and eliminating stuck fermentation is the most active research project in wine industry and universities worldwide. There are many theories, but no solutions. This enzyme is a good candidate to be used universally to prevent stuck fermentation so that wine makers can perform cold fermentation with confidence.

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Control

Crystalzyme Vino-ProTM 180 ppm

Enzymes in the Fruit Juice and Wine Industry

FIGURE 5.12 Antifoaming effect of protease in wine fermentation.

5.5.4 OTHER ENZYME APPLICATIONS

IN

WINE MAKING

Enzyme treatment in wine production can also be done after fermentation. Most wine makers prefer to do an enzyme treatment in the mash or in the must. The reason is that, at this early stage, the reaction conditions for the enzyme are more favorable than after fermentation when alcohol, sulphur dioxide, and cold storage temperatures have an enzyme-inhibiting effect and therefore require long reaction times for the enzyme treatment. Nevertheless, due to some technical problems, the enzyme treatment can only be carried out after fermentation. At this production step, problem-solving enzymes are needed, so it is often more effective to use enzymes that are provided with defined secondary activities instead of standard pectinases. In the treatment of wines from grapes that were infected with noble rot during ripening. This fungus, Botrytis cinerea, converts a certain part of the grape sugar into a polysaccharide that is known as b-glucan. Although these wines are of superb quality due to their high concentration of sugar and flavor compounds, they cause serious problems when they are filtered. When sheet filters are used, particularly, many sheets have to be used, resulting in high filtration costs. Furthermore, used filter sheets are solid waste and must be disposed of. These filtering problems can only be solved by hydrolyzing the high molecular b-glucan with the aid of specific b-glucanases. Another example is pectinases with b-glucosidase side activity for natural essence release. In some white wine varieties, the content of bound terpenes is rather high. These substances are a potential bouquet reservoir and can be released when enzymes with b-glucosidase activity are added to the wine immediately after fermentation. Especially in bouquet grapes like Morio Muskat, Gewuerztraminer, and Scheurebe, this treatment leads to a noticeable enhancement of the characteristic fragrance, which can be proved, both sensorially and with analytical methods.

REFERENCES Braddock, R.J., By-products of citrus fruit, Food Technol., 49, 74, 1995. Browska, J., Application of enzymes in the production of pulpy carrot juice, Fruit Process., May, 162, 2000. Clahorst, S., Enzymes solutions for fruit processors, Food Product Design, Jan., 73, 2003.

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Ducroo, P., Arabans in apple juice, Flüssiges Obst., 54, 265, 1987. Endo, A., Studies on the enzymic clarification of apple juice, Agric. Biol. Chem., 29, 129, 1965. Fugelsang, K.C., Gump, B.H., and Zoecklein, B.W., Review of protein stability in wine, Vineyard Winery Manage., Sept./Oct., 28, 1988. Fundira, M., Blom, M., Pretorius I.S., and Van Rensburg P., Comparison of commercial enzymes for the processing of marula pulp, wine, and spirits, J. Food Sci., 67, 2346, 2002. Garzon, G.A. and Wrolstad, R.E., Comparison of the stability of pelargonidin-based anthocyanins in strawberry juice and concentrate, Food Chem. Toxicol., 67, 1288, 2002. Labib, A.S., Enzymes in guava fruit processing, Fruit Process., Sept., 13, 1999. Lagace, L.S., Survey of yeast acid proteases for effectiveness of wine haze reduction, Am. J. Enol. Vitic., 41, 147, 1990. Van der Maarel, M.J.E.C., van der Veen, B., Uitdehaag, J.C.M., Leemhuis, H., and Dijkhuizen, L., Properties and applications of starch-converting enzymes of the a-amylase family, J. Biotechnol., 94, 137, 2002. Miller, G.C., Amon, J.M., Gibson, R.L., and Simpson, R.F., Loss of wine aroma attributable to protein stabilization with bentonite or ultrafiltration, Aust. Grapegrower Winemaker, April, 46, 1985. Modra, E.J. and Williams, P.J., Are protease active in wines and juices? Aust. Grapegrower Winemakers, April, 42, 1988. Nakamura, T., Hours, R.A., and Sakai, T., Enzymeatic maceration of vegetables with protopectinases, J. Food Sci., 60, 468, 1995. Paigh, J.G., Enzyme formulations for optimizing juice yields, Food Technol., 49, 79, 1995. Pinik, W., Enzymes in the beverage industry, in Use of Enzymes in Food Technology, Dupuy, P., Ed., Lavoisier, Paris, 1982, 425. Sun, D. and Harris, J., Effective use of protease in winemaking, U.S. patent pending, 2002. Underkofler, L.A., Enzymes, in Handbook of Food Additives, 2nd ed., Furia, T.E., Ed., CRC Press, Boca Raton, FL, 1980, p. 57. Uriaub, R., Enzymes in Citrus Processing. Rohm GmbH, Darmstadt, Germany, 1992, 32. Waters, E.J., Wallace, W., and Williams, P., Identification of heat-unstable wine proteins and their resistance to peptidases, J. Agric. Food Chem., 40, 1514, 1992. Wightman, J.D., Price, S.F., Watson, B.T., and Wrolstad, R.E., Some effect of processing enzymes on anthocyanins and phenolics in pinot noir and cabernet Sauvignon wines, Am. J. Enol. Vitic., 48, 39, 1997. Yamasaki, M., Kato, A., Chu, S.Y., and Arima, K., Studies on the clarification of apple juice, Agric. Biol. Chem., 31, 552, 1967.

6 Fruit Preserves and Jams Robert A. Baker, Norman Berry, Y.H. Hui, and Diane M. Barrett CONTENTS 6.1 6.2

Introduction ........................................................................................................................113 Pectin ..................................................................................................................................114 6.2.1 Pectin Structure....................................................................................................114 6.2.2 Pectin and Gelation .............................................................................................115 6.2.3 Effect of Methylation on Function......................................................................115 6.2.4 Low-Methoxyl Pectins.........................................................................................116 6.2.5 Interaction with Sugar Cosolute..........................................................................117 6.2.6 Effect of pH .........................................................................................................117 6.3 Processing Technology.......................................................................................................118 6.3.1 General Considerations........................................................................................118 6.3.2 Plate Evaporation Process ...................................................................................118 6.3.3 Vacuum Batch Process ........................................................................................120 6.3.4 High-Pressure-Treated Fruit Preserves and Jam .................................................121 6.4 Federal Standards...............................................................................................................121 6.4.1 21 CFR 150.160 Fruit Preserves and Jams.........................................................121 6.5 Phytonutrient Properties of Fruit Preserves and Jams.......................................................123 References ......................................................................................................................................124

6.1 INTRODUCTION Historically, jams and jellies may have originated as an early effort to preserve fruit for consumption in the off-season. As sugar for their manufacture became more affordable, the popularity and availability of these fruit products increased (Anon., 1983). Jellies, jams, preserves, and marmalades are primarily distinguished by the form in which their fruit component is incorporated. In jellies, only strained fruit juice is used, while jams are made with crushed or ground fruit material. Preserves are made with whole fruit (if sufficiently small) or large pieces of fruit (Ahmed, 1981). Marmalades are basically clear jellies in which slices or shreds of (usually) citrus peel are suspended. Regardless of their form, all are sugar–acid–pectin gels or low-methoxyl pectin–calcium gels. Their structure, appearance, and mouthfeel result from a complex interaction between pectin level and functionality, PH, sugar type and content, setting temperature, and, in the case of low-methoxyl pectin gels, calcium content. Originally, jam or jelly production relied on the native pectins of incorporated fruit for gel formation. Fruit was cooked with sugar, extracted acids, and pectins, and if the proper balance of sugar level, pH, and pectin content were achieved, a satisfactory jelly was obtained; however, modern manufacturing requirements for uniform gel strength and appearance preclude reliance on fruit component pectins, which may vary in content and quality, depending on fruit maturity and variety. In spite of the current availability of other gelling agents, pectin remains the universal 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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choice for jams and jellies, in part because of its presence as a natural fruit ingredient and also because of the characteristic consistency that pectin imparts to a gel. Pectins of known quality and gelling capacity (usually derived from citrus or apple by-products) are added to jelly and jam formulations to achieve a desired gel strength. It is estimated that 80 to 90% of commercial pectin production, which totals 6 to 7 million kg, is used in the production of jellies and jams (Crandall and Wicker, 1986). Although the basic steps in the production of jams remain the same, there is a continuous demand for improvement in processing performance and product quality. Important aspects to consider include close cooperation between preserve manufacturers and engineers to meet changing consumer preferences (e.g., preserves with higher fruit content or low-sugar jams), optimization of the preserve and jam manufacturing process, new developments in processing equipment and automatic controls, and strict sanitation controls. The next section gives an updated account on pectin and gelation.

6.2 PECTIN 6.2.1 PECTIN STRUCTURE An understanding of pectin structure is a prerequisite to defining its role in gel formation, and much research has focused on elucidating the finer points of this molecule’s primary, secondary, and tertiary structures. Pectin is a plant cell-wall polysaccharide consisting of a linear chain composed of D-galacturonic acid residues linked via a(1-4)-glycosidic bonds. At intervals in this homogalacturonan chain are regions in which the polymer backbone consists primarily of alternating units of galacturonic acid and rhamnose (Toman et al., 1976). Such regions are described as “hairy,” because, within these areas, short side chains of xylose occur, interspersed with larger, more elaborately branched side chains of arabinose and galactose. Approximately 95% of pectin’s galacturonic acid residues are located in the “smooth,” or homogalacturonan, regions (de Vries et al., 1986). Xylose residues in the hairy regions are primarily attached to galacturonic acid residues, while most galactose side chains are attached to rhamnose residues of the backbone (Schols et al., 1990). These arabinogalactan-rich regions are located at regular intervals along the pectin chain, but this regularity may vary depending on the pectin source. Recent work suggests that the hairy regions of apple pectin comprise, at least in part, smaller repeating units (Colquhoun et al., 1990). In commercial pectins, rhamnose and pentose linkages of these hairy regions may be partially hydrolyzed by acids used in extraction (de Vries, 1988; Mort, 1993a). As a long-chain polymer, pectin’s molecular weight (MW) may vary widely, depending on source and method of extraction. Its MW is also susceptible to reduction by chemical, enzymic, and even physical treatments. Average MWs of commercial pectins may range from 5 ¥ 104 to 1.8 ¥ 105 (Ahmed, 1981; Fishman et al., 1991; Smit and Bryant, 1969), but milder extraction procedures can yield pectins of considerably higher MWs (Kontominas and Kokini, 1990; Miyamoto and Chang, 1992). In addition, pectin is an association colloid and can aggregate to form even larger macromolecular-sized units (Fishman et al., 1992). Galacturonic acid residues of the pectin backbone are esterified to varying degrees, usually with methanol; the extent to which this occurs is referred to as the degree of esterification (DE) or methoxyl content. There is some evidence of a repeating structure in the distribution of these sites as well, with a suggested repeating unit of five esterified galacturonate residues and one free acid residue (de Vries, 1988). This would imply a DE of 83% for unaltered intercellular pectin; however, extraction and processing of pectins alter these in situ ratios, such that commercial pectins may range from 20 to 70% DE. Certain pectins may be naturally low in ester content, such as those from cotton cell cultures (Mort et al., 1993b) or sunflower head residues (Miyamoto and Chang, 1992).

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Recent studies have claimed that esterified residues in pectin are randomly distributed (Garnier et al., 1993; de Vries et al., 1986) or nonrandomly distributed (Mort, 1993a, 1993b). Using liquid hydrofluoric acid (HF) solvolysis to cleave cultured cotton cell pectin at the rhamnose inserts of the pectin backbone, Mort (1993a, 1993b) isolated two homogalacturonan fractions, one approximately 40% esterified and the other 10 to 15%. Isolation of residues differing in esterification suggested a nonrandom distribution in the original pectin. However, ion-exchange chromatography showed a random distribution of charge in a citrus pectin that had undergone acid-catalyzed deesterification (Garnier et al., 1993). Such disparate findings may, in part, be due to the length of galacturonate residues being examined or to differences in pectin source. From the above discussion, it should be apparent that pectin is an extremely heterogeneous material and any discussion of pectin’s molecular weight or degree of esterification refers only to the average for the molecular species present in solution. Such average values alone will not define the functionality of a particular pectin. Different methods of preparation may result in pectins identical in average MW or DE but with differing MW and DE distributions, which will, in turn, confer entirely different properties (Baker, 1979).

6.2.2 PECTIN

AND

GELATION

Pectin gels have been aptly described as intermediate between a solid and a liquid state, consisting of a three-dimensional network of pectin immobilizing the aqueous component (Oakenfull, 1987). The solvent water, pH, and accompanying cosolutes (usually sugar) influence the intermolecular forces contributing to gel structure; conversely, the gel structure prevents the aqueous phase from separating. In high methoxyl pectin solutions, gelation occurs via noncovalent bonding between adjacent polymer chains, with both hydrogen bonding and hydrophobic interactions between juxtaposed chains contributing to formation of junction zones (Oakenfull, 1987). Bonding between adjacent pectin chains occurs primarily at these junction zones, which can range from 18 to 250 galacturonic-acid units (Oakenfull and Scott, 1984). Although hydrogen bonding in a higher methoxyl (70% DE) pectin is approximately double that of the hydrophobic contribution, hydrogen bonding alone is insufficient to initiate gelation. Thus, hydrophobic interaction between adjacent ester methyl groups, which is enhanced by cosolutes such as sucrose, is essential to gel formation (Oakenfull and Scott, 1984). More recent work with high-methoxyl pectin gels utilizing ethanol, 1-butanol, and dioxan as cosolutes confirmed the dependence of junction-zone formation on hydrophobic interactions (Brosio et al., 1993). Although formed by noncovalent bonds, these junction zones can be of sufficient strength to prevent gel melting without polymer breakdown (May and Stainsby, 1986).

6.2.3 EFFECT

OF

METHYLATION

ON

FUNCTION

Of the features influencing pectin gelation, the degree of esterification may arguably be the most significant. Pectins can be modified chemically to produce a continuum of esterification levels, from 0% (pectic acid) to almost 100%. In practical terms, for food use, the upper limit of esterification in commercial pectins is set by the naturally occurring ester content of pectin in the raw material source. Although raw material origin may provide an upper bound to esterification levels, both DE and gel-forming capacity of pectins can be altered by extraction time, temperature, and pH (Woodmansee and Baker, 1954; Aravantinos-Zafiris and Oreopoulou, 1992). Pectin functionality is altered by changes in DE, and a broad distinction is made between high-mehoxyl pectins (DE values from 55 to 80%) and low-methoxyl pectins (DE values below 50%); however, it should be understood that this is an arbitrary distinction, and pectins with DE values near 50% will display features intermediate between those of high and low DE values. Commercial low-methoxyl pectins usually have DE values ranging from 20 to 50% because pectins with excessively low DE will precipitate rather than gel.

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High-methoxyl pectins gel only under acidic conditions and when the sugar (sucrose) content is at least 55% (Oakenfull and Scott, 1984). Low pH suppresses dissociation of free carboxylic acid groups, reducing their electrostatic repulsion (Watase and Nishinari, 1993), whereas sugars stabilize hydrophobic interactions between the methyl ester groups (Oakenfull and Scott, 1984; Brosio et al., 1993; Rao et al., 1993). Junction-zone size and the standard free energy of gelation increase as the degree of esterification increases, being proportional to the square of the DE (Oakenfull and Scott, 1984). As pectin ester content is lowered to 50%, jelly strength increases, but only at progressively lower pH values (Smit and Bryant, 1968). Commercial high-methoxyl pectins are further divided into rapid set (20 to 70 sec), medium set (100 to 150 sec), and slow set (180 to 250 sec) categories, depending on the time required for a gel to form under standard conditions. Gel-setting time is a function of DE, with rapid-set pectins possessing a DE of 72 to 75, medium-set pectins a DE of 68 to 7l, and slow-set pectins a DE of 62 to 66 (Crandall and Wicker, 1986). Such distinctions are of value in processing where speed of gelation can influence product quality. Rapid-set pectins are useful in the manufacture of jams and ensure uniform dispersal of fruit pieces and prevent floating. When flotation of fruit is not a problem, such as in clear jellies, then slow-set pectins are preferable because they allow entrained air bubbles to rise before gelation. Esterification levels of pectins can also have an effect on the flavor perception of jellies. For a substance to be tasted, it must contact the taste buds; therefore, if a gel delays diffusion of the substance (flavor) to the taste bud surface, taste perception may be controlled not by the taste reaction but by diffusion. Guichard et al. (1991) found that, at similar gel consistencies, highmethoxyl pectin reduced taste intensity more than low-methoxyl pectin.

6.2.4 LOW-METHOXYL PECTINS Low-methoxyl pectins, those with DE of 50% or less, are also able to form gels but by an entirely different mechanism than that described previously. These pectins do not require high levels of sugar or low pH to initiate gelations, but gel in the presence of divalent cations such as calcium. Such divalent cations form associations between sequences of charged species on adjacent chains. As stated earlier, pectins with DE in the upper range of this class (45 to 50% DE) exhibit properties intermediate between high- and low-methoxyl pectins; that is, they can also form acid–sugar gels (Smit and Bryant, 1968). However, the low pH requirements for such gels preclude their use in most foods. Low-methoxyl pectins with DEs near the upper limit of the range require some sugar for gelation, a further indication of their intermediate properties (Oakenfull, 1987). Low-methoxyl pectin’s ability to form gels with less sugar content allows the production of dietetic jams or jellies, whereas their ability to gel with calcium at higher pH enables them to produce gels in acid sensitive foods such as milk. The free carboxyl groups available on the galacturonic acid residues of low-methoxyl pectin may form calcium bridges (Figure 6.1) with adjacent pectin polymers. This results in a stronger gel network and firmer structure, and less occurrence of syneresis. Low-methoxyl pectins can be generated from high-methoxyl pectins by various treatments, including acid-, base- or enzyme-catalyzed deesterification. Acid-catalyzed deesterification can be done simultaneously with initial extraction of pectin from the raw material source (Woodmansee, 1954). Such treatment tends to lower the MW of pectin, but methods have been developed to produce high-MW, low-DE pectins (Wiles and Smit, 1971). In addition, some sources of pectins (e.g., sunflower) are naturally low in DE, and careful extraction of these pectins provides highMW, low-DE pectins (Chang and Miyamoto, 1992; Miyamoto and Chang, 1992). While acid- and base-catalyzed deesterification tends to randomly remove methyl esters, enzyme-catalyzed deesterification with pectin methylesterase cleaves esterified sites nonrandomly, producing blocks of completely deesterified galacturonic acid units interspersed with unmodified original material. Activation energies for low-methoxyl pectin gels are much lower than for high-methoxyl pectin

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Methyl esterified,

Free carboxylic acid,

No Ca binding

Ca binding

FIGURE 6.1 Pectin polymers indicating methylation (left) which prevents calcium binding or free carboxylic acid (right) that may bind calcium.

gels, suggesting that shorter sections of the polymer backbone are involved in the gelation process in low-methoxyl pectin gels (Garnier et al., 1993).

6.2.5 INTERACTION

WITH

SUGAR COSOLUTE

As stated earlier, high-methoxyl pectins will not form a gel with less than 55% sugar as cosolute. As pectin DE levels drop below 50%, the amount of sugar becomes less significant because lowmethoxyl-calcium gelation becomes the predominant gelling mechanism; however, even in lowmethoxyl pectin gels, the addition of sugar can increase gel strength and reduce syneresis (Axelos and Thibault, 1991). For high-methoxyl pectin gels, any of a number of other sugars, alcohols, and polyols will permit gelation. From a practical standpoint, it may be advantageous to substitute other sugars for sucrose, either because of cost, to reduce the likelihood of crystallization, or for flavor modification (Ahmed, 1981). Partial replacement of sucrose with other sugars such as maltose, glucose syrups, or high-fructose syrups altered the setting times and certain rheological properties of model gels (May and Stainsby, 1986). For example, addition of maltose reduced the setting time and extended the pH range of gelation, whereas fructose delayed setting time. Partial or complete replacement of sucrose with other sugars alters the water activity of the system and can modify the hydrophobic interactions contributing to gelation. The use of dietary fiber as a thickener was evaluated in peach jam (Grigelmo-Miguel and Martin-Belloso, 2000). Commercial amidated pectin was either partially or totally substituted by peach dietary fiber, and jams with soluble solids contents of 40, 45, 50, and 55∞ Brix were produced. Jam color was unaffected because both the dietary fiber and puree originated from peaches. The rheological behavior of the jams with substituted dietary fiber was pseudoplastic in nature, similar to that of conventional jams. The sensory characteristics of the high peach dietary fiber jams were also deemed as acceptable as conventional jams. Therefore, the authors concluded that such a jam might serve as a dietary supplement with no adverse effects.

6.2.6 EFFECT

OF PH

Control of pH is critical to successful gel formation with pectins, particularly high-methoxyl pectins. Low pH increases the percentage of unionized carboxyl groups, thus reducing electrostatic repulsion between adjacent pectin chains. High-DE, rapid-set pectins will gel at higher pHs than lower-DE, slow-set pectins; however, this difference is slight, with the optimum pH for slow-set pectins being

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about 3.1 and for rapid set pectins 3.4 (Crandall and Wicker, 1986). Substitution of other sugars for sucrose, by modifying hydrophobic interactions between chains, allows gels to be formed at somewhat higher pHs (May and Stainsby, 1986). Because they rely on calcium bonding to effect gelation, low-methoxyl pectins can form gels at higher pHs than high-methoxyl pectins. Gels can be made at pH values near neutrality (Chang and Miyamoto, 1992; Garnier et al., 1993), an advantage in producing dairy-based products.

6.3 PROCESSING TECHNOLOGY 6.3.1 GENERAL CONSIDERATIONS This section concerns the manufacture of jams. The four necessary ingredients in manufacturing jams include fruit, pectin, sugar, and acid. Optional ingredients include spice, buffering agents, preservatives, and antifoaming agents. The exact process selected will depend upon the type of product to be manufactured, raw materials available, and scale of production. Essentially, the main stages in jam manufacture are as follows: • • •

Blending together of ingredients Evaporation to the desired total solids level Heat treatment to pasteurize the product

Traditionally, all of the ingredients were blended together at the first stage of the process; however, with modern demands for high consistency of quality, it has been common to add some critical ingredients at later stages in the process. Thus, for example, citric acid may be added at a late stage to give precise pH control. Volatile flavoring may also be added after the evaporation stage to avoid evaporative loss. While it is possible with a simple atmospheric batch evaporation to carry out the whole process in one vessel, the requirements for consistent, economic, high-speed production and improved product quality mean that this technique has been generally superseded, except for very small-scale manufacture. Most modern plants are based on low temperature or vacuum evaporation, which may necessitate the addition of an extra pasteurizing stage to give a product of suitable microbiological quality to allow prolonged storage. In general, the main factors to be considered when selecting process are as follows: • • • • • •

What What What What What What

is the required total solids (TS) level in the finished production? is the required fruit percentage? requirement is there for fruit particle integrity? will the fruit content be? are the required production rates? will the range of ingredients be?

Although it is not possible to discuss in detail all of the possible process variables, the following two processes cover the main needs of the larger scale jam manufacturer.

6.3.2 PLATE EVAPORATION PROCESS This process is particularly suitable for jams that do not have any large particles in suspension but where there is a need for a high-quality product and good process economy. It may be used for

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Fruit Preserves and Jams

PLATE EVAPORATION PROCESS CONDENSATE

FRUIT PULP SUGAR PECTIN

STEAM

DEAERATOR

STEAM

PRE-MIX

VESSELS

PRE-HEATER

CONDENSATE

VAPOR SEPARATOR

BUFFER

IN-LINE MIXER

TANK

PLATE EVAPORATOR

S.S.H.E.

COOLANT

ACID/FLAVOR DOSING

FILLER

FIGURE 6.2 A typical processing plant.

the production of standard consumer products and is particularly suitable for special bakery and confectionery jams with very high solids levels and for jams for portion packing. Figure 6.2 shows a typical plant. The first stage is the mixing together of ingredients that would normally include fruit pulp, sugar, pectin, and possibly corn syrup (liquid glucose). The various ingredients may be weighed or metered into the batch premix vessels. Two vessels are provided so that one may be used for the recipe preparation while the other is feeding the process. Thus, with flip/flop operation, a fully continuous feed is possible. Although in most cases this type of batch premix system is used, where long runs on a single recipe are likely, a continuous inline metering and blending system may be more appropriate. The premix then goes through a Paraflow plate heat exchanger where it is heated by condensate and steam. If sulphited fruit pulp is included in the recipe, the hot mix enters a desulphiting column. At the top of the column is a variable orifice spray device. This allows the production of a good spray pattern with large surface area exposure to steam, which rises up the vessel to give effective stripping of sulphur dioxide. The opening of the nozzle may be adjusted to take into account both the velocity and viscosity of the feed. A second trimming stage of desulphiting takes place in the premix held in the bottom of the vessel by steam being sparged up through the mix. A level-control device allows the residence time in the vessel to be controlled according to the particular recipe’s sulphite content. Where nonsulphited pulps are used, this stage may be omitted. The hot mix is now fed to the APV plate evaporator, which is held under vacuum. This is a rising and falling film-type evaporator. Flashing occurs, giving good distribution in the wide gap plates, and further heating by steam passing through alternate plates ensures that very rapid evaporation takes place, minimizing loss of fruit volatiles and giving a product with a good natural fruit color. Typically, the temperature of evaporation is 60 to 65∞C (140 to 149∞F).

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A vapor recompressor (thermocompressor) may be used to recompress the product vapor with incoming high-pressure steam. This reduces the evaporation steam requirement by almost 50%. Further steam economy is achieved by utilizing the steam condensate in the Paraflow for the partial preheating of the premix. The concentrated jam and resulting vapor are discharged through a large rectangular port to a snail shell separator where jam is separated from the vapor. The vapor passes to the condenser, which may be of the spray or surface type. The jam is extracted by a rotary pump and passes through an inline refractometer. This is used to monitor the solids level, and may be used to control the steam rate to the evaporator and thus accurately control the finished product concentration. Inline pH metering allows control of citric acid solution addition to give the desired acidity level. Postprocess addition of any flavorings avoids volatile loss during the evaporation. The product now passes through a scraped surface cooler before passing to buffer storage and on to filling. This type of process may also be used for certain types of particulate-bearing products where the particulate material is presyruped in a strong sugar syrup. With this technique, it is possible to manufacture products with very good particle identity and a surrounding liquid phase of sparkling clarity.

6.3.3 VACUUM BATCH PROCESS Figure 6.3 shows the general details of this process, which is suitable for the production of highquality jams containing fruit pieces or whole fruit. Evaporation is carried out at low temperature, reducing thermal degradation. Postprocess pasteurization allows the process to be used for jams of a wide range of TSs with or without preservatives. It is ideal for linking to aseptic or ultraclean filling equipment. The various ingredients are metered or weighed into the premix vessel, which is jacketed for the preheating of the mix and dissolution of sugar. A scraped surface agitator and baffle combination in this vessel gives both good heat transfer and rapid but gentle blending together of the various ingredients. After mixing, the premix is drawn under vacuum into the vacuum cooking vessel. This has zoned dimple panel heating elements so that there is efficient heat transfer using the low-speed scraper agitator without problems of burn-on even in preparing part batches. Rapid vacuum evaporation takes place at 60 to 65∞C (140 to 149∞F), and the vapor is separated in a cyclone separator with any product carryover being returned to the vessel. Normally, the vapor is condensed in a surface and may be recovered for use in recipe makeup, in-place cleaning, and so on; however, where required, a partial condensation volatile recovery system may be added for recovery of volatile flavorings. After completion of this rapid low-temperature cooking process, the product is transferred using top-filtered air pressure to the buffer tank. From here, it is pumped to an APV scraped-surface heat VACUUM

VACUUM BATCH PROCESS

VAPOR EVAPORATOR

METERS CORN SYRUP SUGAR SYRUP PECTIN

COOLING WATER

FRUIT

METERING PUMP

PREMIX VESSEL

FIGURE 6.3 Processing details.

VACUUM BOILING PAN

BUFFER TANK

PUMP

FILLER

SCRAPED SURFACE HEAT EXCHANGER

121

Fruit Preserves and Jams

exchanger flash-pasteurizing plant prior to filling. This stage is necessary because the low-temperature vacuum evaporation may not be effective in killing any spoilage microorganisms present in the ingredients. The rapid pasteurization usually takes place at 85 to 95∞C (185 to 203∞F), depending on the TS and acidity of the recipe. Pasteurizing is under pressure to avoid any volatile loss and, after a short hold time, is followed by rapid cooling to the required filling temperature. This process may also be adapted for use during the fruit harvesting period for the pasteurization of fruit pulp or fruit in syrup, which can then be filled into aseptic tanks or containers for later use in recipes. While the two systems described above may not be suitable for every circumstance, they have proved effective for a wide range of applications throughout the world. As mentioned at the beginning, it is important to carefully analyze the requirements and then select the most appropriate process accordingly.

6.3.4 HIGH-PRESSURE-TREATED FRUIT PRESERVES

AND JAM

Jam preparation using a new method (e.g., the application of high pressures) has been studied extensively in Japan in recent years. In a study by Horie et al. (1991), the use of high pressure (4000 to 6000 kg/cm2) resulted in a fresh fruit jam of superior color and flavor. In addition, vitamin C retention of fresh strawberries was 95% in pressure-treated jam. The authors reported that these pressure levels resulted in reduced levels of Zygosaccharomyces rouxii, Saccharomyces cerevisiae, Staphylococcus and Salmonella. In addition, a taste panel preferred the high-pressure-treated jam over the heat-processed product. Due to the presence of residual enzymes, refrigeration of the jam is required. Pressure-treated jam and preserves are commercially available in Japan and offer the consumer a more colorful, flavorful, and perhaps nutritionally sound product.

6.4 FEDERAL STANDARDS U.S. federal standards and definitions do not differentiate between preserves and jams. A preserve is minimally 45 parts prepared fruit with 55 parts of sugar and is concentrated to 65% or higher solids, resulting in a semisolid product. Jellies are similar to preserves, with 45 parts of clarified fruit juice and 55 parts of sugar, resulting in a minimum of 65% solids. Both categories can utilize a maximum of 25% corn syrup for sweetness, as well as pectin and acid to achieve the gelling texture required. Fruit butters are prepared from mixtures containing not less than 5 parts by weight of fruit to 2 parts of sugar. The following is the 1994 U.S. Food and Drug Administration standard of identity for fruit preserves and jams (21 CFR 150.160).

6.4.1 21 CFR 150.160 FRUIT PRESERVES

AND JAMS

(a) The preserves or jams for which definitions and standards of identity are prescribed by this section are the viscous or semisolid foods, each of which is made from a mixture composed of one or a permitted combination of the fruit ingredients specified in paragraph (b) of this section and one or any combination of the optional ingredients specified in paragraph (c) of this section which meets the specifications in paragraph (d) of this section, and which is labeled in accordance with paragraph (e) of this section. Such mixture, with or without added water, is concentrated with or without heat. The volatile flavoring material from such mixture may be captured during concentration, separately concentrated, and added back to any such mixture, together with any concentrated essence accompanying any optional fruit ingredient. (b)(1) The fruit ingredients referred to in paragraph (a) of this section are the following mature, properly prepared fruits that are fresh, concentrated, frozen, and/or canned:

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Group I: Blackberry (other than dewberry), black raspberry, blueberry, boysenberry, cherry, crabapple, dewberry (other than boysenberry, loganberry, and youngberry), elderberry, grape, grapefruit, huckleberry, loganberry, orange, pineapple, raspberry, red raspberry, rhubarb, strawberry, tangerine, tomato, yellow tomato, youngberry Group II: Apricot, cranberry, damson, damson plum, fig, gooseberry, greengage, green-gage plum, guava, nectarine, peach, pear, plum (other than greengage plum and damson plum), quince, red currant, currant (other than black currant) (2) The following combinations of fruit ingredients may be used: (i) Any combination of two, three, four, or five of such fruits in which the weight of each is not less than one fifth of the weight of the combination; except that the weight of pineapple may be not less than one tenth of the weight of the combination. (ii) Any combination of apple and one, two, three, or four of such fruits in which the weight of each is not less than one fifth and the weight of apple is not more than one half of the weight of the combination; except that the weight of pineapple may be not less than one tenth of the weight of the combination. In any combination of two, three, four, or five fruits, each such fruit is an optional ingredient. For the purposes of this section the word “fruit” includes the vegetables specified in this paragraph. (c) The following safe and suitable optional ingredients may be used: (1) Nutritive carbohydrate sweeteners (2) Spice (3) Acidifying agents (4) Pectin, in a quantity which reasonably compensates for deficiency, if any, of the natural pectin content of the fruit ingredient (5) Buffering agents (6) Preservatives (7) Antifoaming agents, except those derived from animal fat (d) For the purposes of this section: (1) The mixture referred to in paragraph (a) of this section shall be composed of not less than: (i) In the case of a fruit ingredient consisting of a Group I fruit or a permitted combination exclusively of Group I fruits, 47 parts by weight of the fruit ingredient to each 55 parts by weight of the saccharine ingredient, and (ii) in all other cases, 45 parts by weight of the fruit ingredient to each 55 parts by weight of the saccharine ingredient. The weight of the fruit ingredient shall be determined in accordance with paragraph (d)(2) of this section, and the weight of the saccharine ingredient shall be determined in accordance with paragraph (d)(5) of this section. (2) Any requirement with respect to the weight of any fruit, combination of fruits, or fruit ingredient means: (i) The weight of fruit exclusive of the weight of any sugar, water, or other substance added for any processing or packing or canning, or otherwise added to such fruit. (ii) In the case of fruit prepared by the removal, in whole or in part, of pits, seeds, skins, cores, or other parts; the weight of such fruit, exclusive of the weight of all such substances removed therefrom. (iii) In the cases of apricots, cherries, grapes, nectarines, peaches, and all varieties of plums, whether or not pits and seeds are removed therefrom, the weight of such fruit, exclusive of the weight of such pits and seeds. (iv) In the case of concentrated fruit, the weight of the properly prepared fresh fruit used to produce such concentrated fruit. (3) The term “concentrated fruit” means a concentrate made from the properly prepared edible portion of mature fresh or frozen fruits by removal of moisture with or without the use of heat or vacuum, but not to the point of drying. Such concentrate is canned or frozen without the addition of sugar or other sweetening agents and is identified to show or permit the calculation of the weight of the properly prepared fresh fruit used to produce any given quantity of such concentrate. The volatile flavoring material or essence from such fruits may be captured during concentration and

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separately concentrated for subsequent addition to the concentrated fruit either directly or during manufacture of the preserve or jam, in the original proportions present in the fruit. (4) The weight of any optional saccharine ingredient means the weight of the solids of such ingredient. (5) The soluble-solids content of the finished jam or preserve is not less than 65%, as determined by the method prescribed in Official Methods of Analysis of the Association of Official Analytical Chemists, 13th ed. (1980), section 22.024, under “Soluble Solids by Refractometer in Fresh and Canned Fruits, Jellies, Marmalades, and Preserves — Official Final Action,” which is incorporated by reference, except that no correction is made for water-insoluble solids. Copies may be obtained from the Association of Official Analytical Chemists, 2200 Wilson Blvd., Suite 400, Arlington, VA 22201-3301, or may be examined at the Office of the Federal Register, 1100 L St. NW, Washington, D.C. 20408. (e)(1) The name of each preserve or jam for which a definition and standard of identity is prescribed by this section is as follows: (i) If the fruit ingredient is a single fruit, the name is “Preserve” or “Jam,” preceded or followed by the name or synonym whereby such fruit is designated in paragraph (b) of this section. (ii) If the fruit ingredient is a combination of two, three, four, or five fruits, the name is “Preserve” or “Jam” preceded or followed by the words “Mixed fruit” or by the names or synonyms whereby such fruits are designated in paragraph (b) of this section, in the order of predominance, if any, of the weights of such fruits in the combination. (2) Each of the optional ingredients specified in paragraphs (b) and (c) of this section shall be declared on the label as required by the applicable sections of Part 101 of this chapter, except that: (i) The name(s) of the fruit or fruits used may be declared without specifying the particular form of the fruit or fruits used. (ii) When the optional ingredients listed in paragraphs (c) (3), (4), and (5) of this section are declared on the label, the declaration may be followed by the statement “used as needed” on all preserves or jams to which they are customarily, but not always, added to compensate for natural variations in the fruit ingredients used.

6.5 PHYTONUTRIENT PROPERTIES OF FRUIT PRESERVES AND JAMS Fruits are widely revered for their macronutrient properties; indeed, they serve as a primary source of vitamins and minerals in the diets of many people. Recent attention has focused on the phytonutrients naturally present in many fruits. These compounds include flavonoids (including polyphenolics and anthocyanins), carotenoids, and other antioxidant compounds that epidemiological studies show play a role in prevention of cancer and heart disease. Many of these nutritional components are also responsible for the color of fruit crops; for example, the flavonoids may be yellow, red, blue, and purple colors whereas carotenoids are typically yellow, orange, and red. Jams have been determined to contain flavonoid profiles similar to those of the natural fruit. Tomas-Lorente et al. (1992) analyzed the phenolic compounds in a number of commercial jams as a means of determining the authenticity of the fruit jam. They compared between three and six commercial preparations of apricot, peach, plum, strawberry, sour orange, apple, and pear jam for both total phenolics and specific phenolic compounds. It was interesting to note that, even when there were significant differences in total phenolics content, a single pattern of individual phenolics was determined. The total phenolic content was influenced by cultivar of fruit, maturity stage of fruit, and the industrial process utilized.

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REFERENCES Ahmed, G.E. 1981. High methoxyl pectins and their uses in jam manufacture — A literature survey. The British Manufacturing Industries Research Association, Scientific and Technical Surveys (127):July. Anon. 1983. Jams and jellies. Institutional Distribution, 19(12): 218, 222–224. Aravantinos-Zafiris, G. and Oreopoulou, V. 1992. The effect of nitric acid extraction variables on orange pectin. J. Sci. Food Agric., 60: 127–129. Axelos, M.A.V. and Thibault, J.F. 1991. The chemistry of low-methoxyl pectin gelation. In The Chemistry and Technology of Pectin. R.H. Walter (Ed.), Academic Press, New York, pp. 109–118. Baker, R.A. 1979. Clarifying properties of pectin fractions separated by ester content, J. Agric. Food Chem., 27: 1387–1389. Brosio, E., Delfini, M., Di Nola, A., D’Ubaldo, A., and Lintas, C. 1993. 1H and 23Na NMR relaxation times study of pectin solutions and gels. Cell. Mol. Biol., 39: 583-588. Chang, K.C. and Miyamoto, A. 1992. Gelling characteristics of pectin from sunflower head residues. J. Food Sci., 57: 1435–1438, 1443. Colquhoun, I.J., de Ruiter, G.A., Schols, H.A., and Voragen, A.G.J. 1990. Identification by n.m.r. spectroscopy of oligosaccharides obtained by treatment of the hairy regions of apple pectin with rhamnogalacturonase. Carbohydr. Res., 206: 131–144. Crandall, P.G. and Wicker, L. 1986. Pectin internal gel strength: Theory, measurement and methodology. ACS Symposium Series, 310: 88-102, American Chemical Society, Washington, D.C. de Vries, J.A., Voragen, A.G.J., Rombouts, F.M., and Pilnik, W. 1986. Structural studies of apple pectin with pectolytic enzymes. ACS Symposium Series, 310:38-48, American Chemical Society, Washington, D.C. de Vries, J. 1988. Repeating units in the structure of pectin. In Gums and Stabilizers for the Food Industry. G.O. Phillips, P.A. Williams, and D.J. Wedlock (Eds.), IRL Press, Washington, D.C., pp. 25–29. Fishman, M.L., Gillespie, D.T., Sondey, S.M., and El-Atawy, Y.S. 1991. Intrinsic viscosity and molecular weight of pectin components. Carbohydr. Res., 215: 91–104. Fishman, M.L., Cooke, P., Levaj, B., Gillespie, D.T., Sondey, S.M., and Scorza, R. 1992. Pectin microgels and their subunit structure. Arch. Biochem. Biophys., 294: 253–260. Garnier, C., Axelos, M.A.V., and Thibault, J.-F. 1993. Dynamic viscoelasticity and thermal behaviour of pectincalcium gels. Food Hydrocolloid., 5(112): 105–108. Grigelmo-Miguel, N. and Martin-Belloso, O. 2000. The quality of peach jams stabilized with peach dietary fiber. Eur. Food Res. Technol., 211: 336–341. Guichard, E., lssanchou, S., Descourvieres, A., and Etievant, P. 1991. Pectic concentration, molecular weight, and degree of esterification: Influence on volatile composition and sensory characteristics of strawberry jam. J. Food Sci., 56: 1621–1627. Horie, Y., Kimura, K., Ida, M., Yosida, Y., and Ohki, K. 1991. J. Agric. Chem. Soc. Jpn., 65(6): 975–980. Kontominas, M.G. and Kokini, J.L. 1990. Measurement of molecular parameters of water soluble apple pectin using low angle laser light scattering. In Flavors and Off-Flavors: Proceedings of the 6th International Flavor Conference, Dev. Food Sci., Elsevier Scientific Publication, 24: 375–384. May, C.D. and Stainsby, G. 1986. Factors affecting pectin gelation. In Gums and Stabilisers for the Food Industry. G.O. Phillips, D.J. Wedlock, and P.A. Williams (Eds.), Elsevier Applied Science, London, pp. 515–523. Miyamoto, A. and Chang, K.C. 1992. Extraction and physicochemical characterization of pectin from sunflower head residues. J. Food Sci., 57: 1439–1443. Mort, A.J., Qiu, F., and Maness, N.O. 1993a. Determination of the pattern of methyl esterification in pectin. Distribution of contiguous nonesterified residues. Carbohydr. Res., 247: 21–35. Mort, A.J., Maness, N.O., Qiu, F., Otiko, G., An, J., West, P., and Komalavilas, P. 1993b. Extraction of defined fragments of pectin by selective cleavage of its back-bone allows new structural features to be discovered. J. Cell. Biochem., issue S17A, 9 pp. Oakenfull, D. and Scott, A. 1984. Hydrophobic interaction in the gelation of high methoxyl pectins. J. Food Sci., 49: 1093–1098. Oakenfull, D. 1987. Gelling agents. CRC Crit. Rev. Food Sci. Nutr., 26: 1–25. Rao, M.A., Van Buren, J.P., and Cooley, H.J. 1993. Rheological changes during gelation of high-methoxyl pectin/fructose dispersions: Effect of temperature and aging. J. Food Sci., 58: 173–176, 185.

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Schols, H.A., Posthumus, M.A. and Voragen, A.G.J. 1990. Structural features of hairy regions of pectins isolated from apple juice produced by the liquefaction process. Carbohydr. Res., 206:117–129. Smit, C.J.B. and Bryant, E.F. 1968. Ester content and jelly pH influences on the grade of pectin. J. Food Sci., 33: 262–264. Smit, C.J.B. and Bryant, E.F. 1969. Changes in molecular weight of pectin during methylation with diazomethane. J. Food Sci., 34: 191–193. Toman, R., Karacsonyi, S., and Kubackova, L. 1976. Studies on pectin present in the bark of white willow (Salix alba L.). Structure of the acidic and neutral oligosaccharides obtained by partial acid hydrolysis. Cellulose. Chem. Technol., 10: 561–565. Tomas-Lorente, F., Garcia-Viguera, C., Ferreres, F., and Tomas-Barberan, F.A. 1992. Phenolic compounds analysis in the determination of fruit jam genuineness. J. Agric. Food Chem., 40: 1800–1804. Watase, M. and Nishinari, K. 1993. Effects of pH and DMSO content on the thermal and rheological properties of high methoxyl pectin-water gels. Carbohydr. Polymers, 20(3): 175–181. Wiles, R.R. and Smit, C.J.B. 1971. Method for producing pectins having high resistance to breakage and high capability for gelling in the presence of calcium. U.S. Patent No. 3,622,559. Woodmansee, C.W. and Baker, G.L. 1954. The preparation of calcium pectinates and the effect of the degree of esterification on their gel properties, University of Delaware Agricultural Experiment Station Bulletin No. 305, Newark, DE.

7 Drying of Fruits Cristina Ratti and Arun S. Mujumdar CONTENTS 7.1 7.2

Introduction ........................................................................................................................127 Drying Characteristics of Fruits ........................................................................................129 7.2.1 Properties of Fruits ..............................................................................................129 7.2.1.1 Sorptional Equilibrium ......................................................................129 7.2.1.2 Density and Thermal Properties ........................................................130 7.2.1.3 Shrinkage............................................................................................132 7.2.1.4 Mass Transfer.....................................................................................135 7.2.1.5 Dielectric Properties...........................................................................136 7.2.1.6 Radiation ............................................................................................136 7.2.2 Drying Kinetics....................................................................................................139 7.2.2.1 Air-Drying..........................................................................................139 7.2.2.2 Freeze-Drying ....................................................................................141 7.2.3 Quality Considerations ........................................................................................141 7.3 Types of Dryers..................................................................................................................142 7.3.1 Classification and Selection.................................................................................142 7.3.2 Specific Drying Systems .....................................................................................144 7.3.2.1 Conventional Hot-Air Drying ............................................................144 7.3.2.2 Solar Drying.......................................................................................146 7.3.2.3 Microwave Drying .............................................................................147 7.3.2.4 Osmotic Dehydration .........................................................................147 7.3.2.5 Explosion Puffing...............................................................................148 7.3.2.6 Freeze-Drying ....................................................................................149 7.3.3 Novel Dryers........................................................................................................150 7.4 Design Considerations .......................................................................................................150 7.4.1 Heat and Mass Balances......................................................................................150 7.4.2 Energy Aspects ....................................................................................................152 7.4.2.1 Heat Pump Dryers .............................................................................152 7.5 Concluding Remarks..........................................................................................................152 7.6 Nomenclature .....................................................................................................................153 7.6.1 Subscripts.............................................................................................................153 7.6.2 Greek Symbols ....................................................................................................154 References ......................................................................................................................................154

7.1 INTRODUCTION Two definitions of fruit (botanical and commercial) can be found in the literature. Botanically, a fruit is “a ripened ovary” usually developing as part of the plant that contains the ovules that become the seeds of the plant. This definition is frequently too strict for the idea of fruit that people normally 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

127

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FRUITS

Whole

Sliced/Chopped / Particulate

Pastes/ Suspensions

1. Sun drying

1. Through circulation

1. Spray dryer

2. Solar drying in cabinets

2. Conveyor

2. Drum dryer

3. Freeze dryer

3. Solar dryer

3. Through circulation dryers

4. Tray dryer 5. Vacuum dryer

FIGURE 7.1 Various types of dryers for drying of fruit.

have; for example, it does not include apples or strawberries in its classification (Wills et al., 1981). The other definition of fruit can be stated as “the parts of a tree or bush that contain seeds and are often eaten for their usually sweet flesh” (Longman Dictionary of Contemporary English, 1990). Under this more common definition, the tomato is eaten as a vegetable because it is not sweet although it is technically a fruit because it contains seeds. Fruits are essentially dessert foods (Wills et al., 1981). Artificial drying of fruit is an important method of preservation and production of a wide variety of products. Storage technologies are now available to keep fruit fresh over extended periods of time without appreciable change in the original physical form of the fruit. Drying, on the other hand, changes the physical and biochemical form of the fruit leading to shrinkage and change of colour, texture, taste, and so on. If the water activity is reduced to appropriate levels (depending on the fruit variety and sugar content), the dried product can have a shelf life exceeding 1 year if properly packaged. Fruit may be dried as a whole (e.g., grapes, various berries, apricot, plum, etc.), in sliced form (e.g., banana, mango, papaya, kiwi, etc.), in puree form (e.g., mango, apricot, etc.), as leather, or as a powder by spray or drum drying. Depending on the physical form of the fruit (e.g., whole, paste, slices), different types of dryers must be used for drying. Figure 7.1 illustrates the wide assortment of dryers that may be found in practice for drying of fruits. Fruits are characterized by the following features, which must be taken into account when selecting the type of dryer as well as the operating conditions: 1. Very high initial moisture content 2. High temperature sensitivity (color, flavor, texture, nutritional value subject to thermal deterioration) 3. High susceptibility to microbial attack 4. High sugar content (problems of stickiness, fermentation, etc.) 5. Presence of a “skin” on some fruits that has poor permeability for water or moisture (e.g., grapes, blueberries) It is clear from the above list that the drying of fruit is necessarily a very slow process carried out under gentle drying conditions, leading to large dryers for a given throughput. Where feasible, solar drying often provides the most cost-effective drying technique although open sun drying is practiced widely in California and the tropical countries. Pretreatment of the feed to speed the drying process and use of additives to avoid biochemical damage during the extended drying period

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are often necessary. The type of treatment and additives used depends on the species of fruit being dried. No generalization can be made. The objective of this chapter is to provide a brief overview of the principles of drying and to discuss some applications in the drying of selected fruits. Some thermophysical property data useful in the design of such dryers are presented. Finally, examples of various dryers used commercially for drying of fruit are presented. For details, the reader is referred to the literature cited.

7.2 DRYING CHARACTERISTICS OF FRUITS 7.2.1 PROPERTIES

OF

FRUITS

7.2.1.1 Sorptional Equilibrium Fruits are highly hygroscopic materials. The water contained in a fruit is bound to the solid matrix such that the vapor pressure it exerts is lower than that for pure water at the same temperature. Sorptional equilibrium data for water vapor–foodstuff systems are used to describe the hygroscopic properties of a product. This information is required in numerous processes involving foods. As pointed out by Wolf et al. (1985), useful information on the sorption and desorption enthalpies, as well as microbiological stability of the product, can be obtained from the sorptional data. In drying, desorption equilibrium moisture content determines the final moisture content to which a material can be dried using the specified drying conditions. Also, such data are needed for estimation of the drying times and energy requirements (Shatadal and Jayas, 1992). The ratio of the water vapor pressure in a food material to that for pure water at the same temperature is termed water activity, aw. Sorptional equilibrium is the relationship between the water activity of a solid with its water content and temperature: aw = aw ( X , T )

(7.1)

Values of aw as functions of X (water content) and T (temperature) for various fruits and fruit powders are given in Table 7.1 and Table 7.2. A typical plot of Equation (7.1) for foodstuffs exhibits a sigmoidal shape. This plot is shown in Figure 7.2 for sorptional equilibrium of prunes at different temperatures (Maroulis et al., 1988). Usually, an increase of temperature results in a decrease of the amount of water adsorbed. The effect of temperature on isotherms can be well represented by the Clausius–Clapeyron relationship (Crapiste and Rotstein, 1986; Okos et al., 1992): DHs È ∂ ln aw ˘ Í ∂(1 / T ) ˙ = - R Î ˚

(7.2)

where Hs is the heat of sorption. A plot of the heat of sorption for selected fruits is shown in Figure 7.3 (Román et al., 1982; Kapsalis, 1987). Other factors that affect the sorption isotherms are chemical composition, physical structure, and pretreatment of the sample among others related to the way of handling and drying the product (Van der Berg, 1986). For design or simulation of processes, an empirical correlation of the sorptional data is convenient because a continuous rather than discrete representation is required to perform the calculations. A large number of empirical correlations have been proposed in the literature to represent the isotherms (Brunauer et al., 1938; Halsey, 1948; Henderson, 1952; Chung and Pfost, 1967). They require extensive experimental data at each temperature in order to evaluate the parameters in the model. The number of correlations that allow for a variation with temperature as well is much smaller. Table 7.3 summarizes some of them. There are some requirements that a good

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TABLE 7.1 Sorptional Data of Selected Fruits aw Fruit

X (%)

Apricot

7.75 14.79 24.2 60.2 10 15 20 10 15 20 6.25 9.01 24.7 6.10 12.8 15.88 26.13 52.8 92.8

Grapefruit

Peach

Persimmon

Plum

5∞C

20∞C

25∞C

30∞C

45∞C

60∞C

90∞C

0.10 0.35 0.60 0.90 0.46 0.52 0.60

Source Ginzburg and Savina (1982)

0.46 0.52 0.60 0.40 0.50 0.60

0.10 0.30 0.40 0.60 0.80 0.90

0.50 0.59 0.65 0.40 0.50 0.60 0.25 0.40 0.70

0.66 0.71 0.75

Okos et al. (1992)

0.50 0.65 0.78

Okos et al. (1992)

Ginzburg and Savina (1982) Ginzburg and Savina (1982)

correlation must achieve (Ratti et al., 1989). For example, the correlation must predict accurately the effect of water content and temperature, and give a good estimation of the partial derivatives (∂X/∂aw)T and (∂X/∂T)aw and also of the heat of sorption. Following these criteria, the correlations from Table 7.3 that represent the sorption data on fruits rather well are those proposed by Iglesias and Chirife (1976), Crapiste and Rotstein (1986) and Ratti et al. (1989). However, the Guggenheim–Anderson–de Boer (GAB) equation is the correlation the most used to represent sorption data of foodstuffs. A list of GAB coefficients for several fruits can be found in Rahman (1995). 7.2.1.2 Density and Thermal Properties The design and optimization of any process in which the transfer of heat is involved requires knowledge of the density, as well as of the thermal properties, of the materials being processed. The density of fresh fruits is typically in the range 865 to 1067 kg/m3, whereas for frozen fruits it is between 625 and 801 kg/m3 (Lewis, 1987). Some other thermophysical properties of fresh fruits are presented in Table 7.4. It is important to note that the product composition, as well as temperature, changes during drying. In addition, fruits being materials of biological origin, their chemical compositions are not fixed but vary with parameters like variety, maturity, or the location where they are grown. Figure 7.4 shows the specific heat and thermal conductivity of selected fruits as a function of water content (Ratti, 1991; Lozano et al., 1979). Density and specific heat of apple juice (Constenla et al., 1989) as a function of concentration and temperature, are presented in Figure 7.5. Several literature sources present empirical equations to model these properties (Heldman, 1975; Choi and Okos, 1986; Singh and Mannapperuma, 1990; Singh, 1992). The most common equations are those that describe the properties of the material in terms of their component fractions and the properties of individual components. The effect of change on the property when there is a

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TABLE 7.2 Sorptional Data of Selected Fruit Powders aw Fruit

X (%)

Banana

2.47 5.50 15.62 26.89 34.93 2.06 6.78 19.54 29.34 56.17 2.13 6.82 17.00 29.42 49.20 2.18 6.81 15.34 29.23 45.73 0.09 0.34 2.22 6.82 23.0 0.42 1.08 4.13 17.94 44.35 3.53 7.64 23.57 33.69 70.91 3.68 7.75 18.87 34.03 64.13 2.45 7.41 17.82 34.45 55.52

Blueberry

Mango

Strawberry

5∞C

15∞C

25∞C

37∞C

0.10 0.30 0.60 0.80 0.90

Source Siddappa and Nanjundaswamy (1960)

0.11 0.34 0.65 0.76 0.88

Khalloufi et al. (2000a)

0.11 0.33 0.61 0.76 0.86 0.11 0.33 0.58 0.75 0.84 0.10 0.30 0.70 0.80 0.90

Siddappa and Nanjundaswamy (1960)

0.32 0.54 0.75 0.79 0.91 0.11 0.34 0.65 0.76 0.88

Khalloufi et al. (2000a)

0.11 0.33 0.61 0.76 0.86 0.11 0.33 0.58 0.75 0.84

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80 15 C 70 30 C

X (dry basis, percent)

60

45 C

50

60 C

40 30 20 10 0 0

0.2

0.4

0.6

0.8

1

aw

FIGURE 7.2 Sorptional equilibrium of prunes at different temperatures. (From Maroulis, Z.B., E. Tsami, D. Marinos-Kouris, and G.D. Saravacos. 1988. Application of the GAB model to the moisture sorption isotherms for dried fruits. J. Food Eng., 7, 63–78.) 80 75 Pinneaple Banana Apple Grapefruit Water

∆Hs (kJ/mol)

70 65 60 55 50 45 40 0

5

10

15

20

25

30

35

X (dry basis, percent)

FIGURE 7.3 Heat of sorption of selected fruits. (From Román, G.N., Rotstein, E., and Urbicain, M.J. 1982. Kinetics of water vapor desorption from apples. J. Food Sci. 44(1), 193–197; Kapsalis, J.G. 1987. Influences of hysteresis and temperature on moisture sorption isotherms. In L.B. Rockland and L.R. Beuchat (Eds.), Water Activity: Theory and Applications to Food, IFT Basic Symposium Series, Marcel Dekker New York, pp. 173–214.)

phase change is taken into account in these models. The reader may refer to the references cited for more details. 7.2.1.3 Shrinkage Considerable changes in the physical structure of the product, such as reduction in volume and decrease in internal porosity can be found during drying (Krokida and Maroulis, 1997; Jankovié,

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TABLE 7.3 Correlations for Sorptional Equilibrium Source

Equation

Ratti et al. (1989) Crapiste and Rotstein (1986) GAB, Guggenheim (1966) Iglesias and Chirife (1976) Pfost et al. (1976) Thompson (1972)

ln aw = -k1X-k2 + k3 exp(-k4X)Xk5 ln pwo ln aw = -k1(1/T - 1/k2)w-k3 exp(-k4w) X/Xm = ckaw/(1 - kaw)(1 - kaw + ckaw) ln aw = -exp(k1 + k2T)Xk3 ln aw = -(k1/R(T + k2))exp(-k3X) ln (1 - aw) = -k1(T + k2)Xk3

TABLE 7.4 Thermal Properties of Selected Fresh Fruits Product

Ta (∞C)

ktb

Cpafc

Cpbfc

Apple Apricot Grapefruit Orange Pineapple Plum Strawberry

2 to 36 — 30 30 — — -14 to 25

0.393 — 0.450 0.431 0.549 0.551 0.675

3.651 3.684 3.818 3.751 3.684 3.718 3.852

1.892 1.905 1.955 1.930 1.905 1.918 1.968

a

Range of temperature for kt. kt is given in [W/m K]. Source: From Singh, R.P. 1992. Heating and cooling processes for foods. In D.R. Heldman and D.B. Lund (Eds.), Handbook of Food Engineering, Marcel Dekker, New York, pp. 247–255; Mohsenin, N.N. 1980. Thermal Properties of Foods and Agricultural Materials, Gordon and Breach, New York, pp. 161. c Cp is given in [J/kg ∞C]. The subscripts af and bf mean above and below freezing. Source: From Rao, M.A. 1992. Transport and storage of food products. In D.R. Heldman and D.B. Lund (Eds.) Handbook of Food Engineering, Marcel Dekker, New York, pp. 225–245. b

1993; Ratti, 1994). Shape and size changes during drying modify both dimensions and transport properties of individual particles and also thickness and porosity of the packed bed in the dryer. Volume changes of individual particles are usually expressed as a bulk shrinkage ratio of sample volume at any time to initial volume, (V/Vo). Experimental data on shrinkage of fruits during airdrying reported in previous studies (Kilpatrick et al., 1955; Lazar and Farkas, 1971; Lozano et al., 1980, 1983) showed shrinkage of different foods only as a function of total water content. In practice, however, this phenomenon is also dependent on drying conditions (Van Arsdel, 1963; Ratti, 1994). There are few reported models for shrinkage (Kilpatrick et al., 1955; Suzuki et al., 1976; Lozano et al., 1980, 1983; Ratti, 1994) and most of them fit the data only under specific operating conditions. Constant porosity and no shrinkage are often stated as key assumptions in the model for dryer design. However, to properly predict the drying time and average drying rates, it is necessary to know the relationship between porosity and average water content of the bed of particles. Although scarce, some experimental data on changes in bed volume and porosity of a packed bed during airdrying of pieced fruits, and as a function of average water content, can be found in the literature (Ratti, 1994). Another parameter also important in the design of dryers is the particle surface

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4.5 4

Cp (J/g°C) or Kt (mW/cm K)

3.5 3 2.5 2 1.5

Cp, apple Cp, peach k (apple, 22°C) k (apple, 80°C)

1 0.5 0 0

2

6

4

8

10

12

14

X (dry basis)

FIGURE 7.4 Thermal properties of apple and peach. (From Ratti, C. 1991. Design of Dryers for Vegetable and Fruit Products. PhD. thesis (in Spanish). Universidad Nacional del Sur, Bahía Blanca, Argentina; Lozano, J.E., Urbicain, M.J., and Rotstein, E. 1979. Thermal conductivity of apples as a function of moisture content. J. Food Sci., 44(1), 198–199.) Cp 䡲 30°C ● 50°C ⽧70°C 多 90°C ρ ▫ 20°C 䡩 40°C 〫60°C 夞 80°C 1.1

1.4 ▫

1.2

多 ⽧ ● 䡲

多 ⽧ ● 䡲

1.1

1

0.9 0

多 ⽧ ● 䡲

▫ ▫ 䡩 〫 夞

䡩〫 夞

20

▫ 䡩 〫 夞

多 ⽧ ● 䡲

▫ 多 〫 䡩 ⽧ 夞 多 ● 䡲 ▫ ⽧ 多 ⽧ ● 䡩 䡲 〫 夞 ● 䡲

40

▫ 䡩 〫 夞

1

0.9

多 ⽧ ● 多 䡲 ⽧ 多 ● ⽧ 䡲 ● 䡲

60

0.8

多 ⽧ ● 䡲

Cp (cal /g°C)

Density (g/cm3)

1.3

䡩 〫 夞

0.7

0.6 80

Concentration (°Brix)

FIGURE 7.5 Density and specific heat of apple juice. (From Constenla, D.T., Lozano, J.E., and Crapiste, G.H. 1989. Thermophysical properties of clarified apple juice as a function of concentration and temperature. J. Food Sci., 54(3), 663–668.)

area-to-volume ratio (av ). This parameter has been shown to be practically independent of the drying conditions but to be dependant on the sample geometry and type of foodstuff (Ratti, 1994). Figure 7.6 presents experimental data for av related to the initial value for slices and cylinders of apples. The solid state of water during freeze-drying protects the primary structure and the shape of the products with minimal reduction of volume. Less than 10% of shrinkage is normally expected in final products due to freeze-drying as compared to 80 to 90% when air-dried. Volumetric changes due to freeze-drying have been quantified as 8% for whole strawberries and less than 2% for slices

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Drying of Fruits

3 2.8 Slices Cylinders

2.6 2.4 av /avo

2.2 2 1.8 1.6 1.4 1.2 1 0

0.2

0.4

0.6

0.8

1

X /Xo

FIGURE 7.6 Area/Volume ratio for slices and cylinders of apple. (From Ratti, C. 1994. Shrinkage during drying of foodstuffs. J. Food Eng., 23(1), 91–105.)

(Shishehgarha et al., 2002). These results are similar to those obtained by Jancovié (1993) for raspberries and blackberries. Although volume reduction is not usually pronounced, in freeze and spray drying of high-sugar content foodstuffs such as fruits or fruit juices, collapse is a frequent problem if operational variables are not well set (Roos, 1995; Slade and Levine, 1991). This phenomenon occurs when the solid matrix of the foodstuff can no longer support its own weight, leading to drastic structural changes shown as a marked decrease in volume, increase in stickiness of dry powders, loss of porosity, and change in color, etc. (Chuy and Labuza, 1994; Levi and Karel, 1995; Roos, 1995). In the case of air-drying, volume reduction is usually accompanied by wrinkles, deformation, and even change in color, indicating a certain degree of collapse in air-dried products. 7.2.1.4 Mass Transfer Diffusion coefficients are often used to characterize mass transfer during drying. A rigorous and realistic approach to the phenomena taking place in biological multiphase materials (e.g., fruits) during air-drying was developed by Crapiste et al. (1988). This theory was defined in terms of a reduced dimensionless coordinate that follows the movement of the nonaqueous material due to shrinkage. On this basis, the water transport equation is given by ∂W * 1 ∂ È ∂W * ˘ = 2 D ∂t Lo ∂Z ÍÎ r ∂Z ˙˚

(7.3)

where Dr is effective reduced diffusivity, a function of temperature, water content, shrinkage, and also of the water transport mechanism within the material. The evaluation of Dr becomes extremely complex if the effect of all variables is to be taken into account. In addition, the experimental measurement of Dr is almost impossible due to the lack of reliable instrumentation required to measure the water content profiles inside the solid. A mathematical model must be used to determine Dr . A simple but very useful way to estimate an approximate value of Dr is to suppose this property to be constant over a certain range of water content, so within that range, Equation (7.3) can be written as

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Processing Fruits: Science and Technology, Second Edition

∂W * Dr ∂ 2 W * = 2 ∂t Lo ∂Z 2

(7.4)

The solution of Equation (7.4) for internal mass transfer, planar geometry, and uniform initial concentration can be found in Crank (1967):

W - We 8 = 2 1 - We p

•

 i=0

È D (2i + 1) 2 p 2 t ˘ exp Í- r ˙ 4 L2o ˚ Î (2i + 1) 2

(7.5)

For long drying times, the first term of the series is adequate to represent the series, so Equation (7.5) is reduced to È - D p 2t ˘ W - We 8 = 2 exp Í r 2 ˙ 1 - We p Î 4 Lo ˚

(7.6)

The effective diffusivity can be obtained from experimental drying data through a linear regression of the linearized form of Equation (7.6). Table 7.5 shows a compilation of effective diffusivity values for selected fruits. The same table also indicates the water content ranges over which the reported values were obtained. The functionality of Dr with temperature can be represented through the well-known Arrhenius relationship DE ˘ Dr = Dro exp ÈÍÎ RT ˚˙

(7.7)

where DE is called activation energy for diffusion. Typical values of DE for different fruits are given in Table 7.6. 7.2.1.5 Dielectric Properties Moist solids such as foodstuffs are classified as “lossy dielectric materials” (Schiffmann, 1987). Such materials absorb electromagnetic energy and convert it into heat. There are two properties that characterize dielectric heating: the relative dielectric constant e¢ and the loss factor e≤. The loss tangent or dissipation factor is then defined as tan d =

e ¢¢ e¢

(7.8)

Table 7.7 shows a compilation of these dielectric properties for selected fruits (Nelson, 1983, 1980; Mohsenin, 1984). Details can be found in the literature cited. Values of the loss factors are important if dielectric (microwave or radio frequency) heating is used to enhance drying rates. This is not always desirable in view of its potentially negative effects on product quality. 7.2.1.6 Radiation Specifically for infrared (IR) drying, knowledge of the material properties may be the clue to accomplish a safe and efficient process because radiation properties of both the radiator and the

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TABLE 7.5 Effective Diffusivity of Selected Fruits Product

Temp (∞C)

Apple: McIntosh Granny S.

Red Delicious

Avocado

Banana chip Coconut albumen

Pear Raisin

66 30 30 71 76 40 40 51 51 61 61 31 43 56 58 58 58 25 45 60 80 100 66 25

Water Content

Dr(m2/s)

Reference

— X > 0.15 X < 0.15 — — Dry zone Wet zone Dry zone Wet zone Dry zone Wet zone 14.7% 14.7% 14.7% 4.7% 7.9% 9.2% 0.0096–0.0102 0.6 (initial) 0.6 (initial) 0.6 (initial) 0.6 (initial) 6.5 (initial) 0.27 (initial)

1.1 e-9 2.6 e-10 4.9 e-11 1.6 e-9 3.6 e-9 1.9 e-10 9.4 e-10 2.2 e-10 1.3 e-9 5.4 e-10 1.7 e-9 1.1 e-10 2.1 e-10 3.3 e-10 1.8 e-9 1.3 e-9 1.2 e-9 2.911 e-11 4.6 e-11 7.7 e-11 10.4 e-11 11.8 e-11 9.63 e-10 4.167 e-11

Labuza and Simon (1970) Rotstein and Cornish (1978) Román et al. (1979) Alzamora (1979) Ratti (1991)

Alzamora et al. (1979)

Alzamora (1979)

Hong et al. (1986) Bimbenet et al. (1985)

Saravacos and Charm (1962) Lomauro et al. (1985)

TABLE 7.6 Activation Energy for Diffusion in Selected Fruits Product Apple: Red Delicious Apricot Avocado Coconut albumen Grapes

Composition

Activation Energy (kJ/mol)

Dry zone Wet zone —

42.26 24.4 22–26

— X > 0.4 X < 0.4 —

39.71 13.0 33.9 43.0

Reference

Ratti (1991) Berna et al. (1990); Abdelhaq and Labuza (1987) Alzamora et al. (1979) Bimbenet et al. (1985) Berna et al. (1991)

material to be dried must be matched in order to obtain the most efficient results (Ratti and Mujumdar, 1995). Again, quality parameters determine the suitability of radiant heating. Emissivity (e), absorptivity (a), reflectivity (r), and transmissivity (t) are the four key radiative properties of a material. The relative magnitudes of a, r, and t depend not only on the material, its thickness, and surface finish, but also on the wavelength of the radiation applied (Kreith, 1965). The emission of electromagnetic waves is a material property.

Pear

—

—

79.7

Pineapple pieces

Peach

57.6

86 88 91 92

w%

Apple Peach Watermelon Cantaloupe Concentrated orange juice

Product 23 23 23 23 50 75 100 125 50 75 100 125 -3.9 -6.7 -9.4 -15.0 -3.9 -9.4 -15.0

T (∞C)

—

—

—

—

0.840 0.944 0.847 0.912

Tissue Density (g/cm3)

TABLE 7.7 Dielectric Properties of Several Fruits

7.48 3.22 4.10 1.77 13.6

3.0 1.97 0.50 0.19 4.56

9.8 3.84 5.79 2.58 11.2 5.9

3.56 0.82 0.61 0.29 5.85 2.94

e’’

1.5 GHz e’

e’’

1.0 GHz e’

4.56 3.14 10.4 5.70 4.17

8.45 0.85 0.38 6.0 2.30 1.12

4.22

e’’

2.0 GHz e’

58.3 58.9 55.2

e’

12.7 15.2 13.6

e’’

2.45 GHz

43

e’ 9

e’’

2.5 GHz

Dielectric properties

54.1 53.5 52.0 49.9 62.0 56.5 51.4 46.0

15.7 15.2 15.7 16.9 11.0 8.9 8.5 8.4

e’’

2.8 GHz e’

49.4 50.4 50.4

e’

5.2 2.6 3.7

e’’

11.7 GHz

49.9 49.9 49.1

e’

1.8 1.3 1.2

e’’

22.0 GHz

138 Processing Fruits: Science and Technology, Second Edition

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Drying of Fruits

The material to be dried by IR should have low reflectivity in order to minimize the power required to heat it. In drying thick moist materials such as foodstuffs, it is necessary to have a reasonable level of transmissivity in order to avoid excessive heating and thermal damage to the product surface. It is important to point out that, if the absorptivity of a material is low, its transmissivity is high and vice versa. Properties like absorptivity and transmissivity of moist materials are not readily found in the literature. In addition to the dependency with wavelength and thickness, these properties also depend on the water content. One of the most extensive reports on experimental values of these properties for foodstuffs can be found in Ginzburg (1969). The variation of absorptivity of moist materials with wavelength is difficult to estimate without experimental data. Foodstuffs, for example, are complex mixtures of different large biochemical molecules and polymers, inorganic salts, and water (Sandu, 1986), and the absorption bands of each of these constituents are not the same. Generally, many fully wet materials have their minimum absorptivity at those wavelengths where water has its maximum transmissivity; this points out the important role that water plays in radiation absorption. For many materials, transmissivity is generally higher at lower wavelengths (Ginzburg, 1969). As drying proceeds, the material being dried undergoes a change in its radiation properties, increasing its reflectivity and, consequently, lowering its absorptivity at low water contents. It is then possible to adequately change the temperature of the emitter in order to improve the absorption of radiation during drying. Further, transmissivity decreases with increase of layer thickness, while absorptivity is increased. An approximate way of representing experimental transmissivity data as a function of thickness is presented by Ginzburg (1969). A compilation of experimental data about the variation of transmissivity of foodstuffs with thickness, maturity, water content, and wavelength can be found in Mohsenin (1984).

7.2.2 DRYING KINETICS 7.2.2.1 Air-Drying Air-drying curves generally have two well-defined periods: constant rate period and falling rate one. The constant rate period is characterized by an almost free water evaporation from the surface of the solid and may be predicted by the following well-known heat and mass transfer relationships: nw = kg( pws - pw• ) ms (1 + X )C p

dT = hg As (Tg - T ) - nw As DHs dt

(7.9)

(7.10)

Note that the classical drying curve is obtained under constant drying conditions. On the other hand, the falling rate period is an extremely complex phenomena and it is not well understood yet where the control for mass transfer occurs within the particle. The constant rate period is governed by external heat and mass transfer rates and is thus not dependent on the material being dried. Drying of food particles is a complex problem involving simultaneous mass and energy transport in a hygroscopic, shrinking system. In general, the constant rate period does not appear in the drying of these materials, and in addition, the equations that represent the mass transfer during the falling rate period demand additional calculations if the effect of shrinkage and the dependence of diffusivity with water content and temperature are to be taken into account. The heat transfer rate is not appreciably affected by the change in mass transfer, and since the Biot number for heat transfer generally remains small during the whole drying process (Bruin and Luyben, 1980), Equation (7.10) is still valid for the falling rate period.

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Processing Fruits: Science and Technology, Second Edition

Drying rate, -dX/dt (kg/(kg sec))

1.7E-04

0.5 m/sec

1.5E-04

1 m/sec

1.3E-04

2 m/sec

1.1E-04 9.0E-05 7.0E-05 5.0E-05 3.0E-05 1.0E-05 0

5

10

15

20

25

30

X (kg water/kg dry matter)

FIGURE 7.7 Drying of prunes at different air velocities. (From Bimbenet, J.J., Daudin, J.D., and Wolf, E. 1985. Air Drying Kinetics of Biological Materials. In A.S. Mujumdar (Ed.), Drying ’85, Hemisphere, New York, pp. 178–185.)

Figure 7.7 shows the effect of air velocity on the drying rate of prunes (Bimbenet et al., 1985). The effect of temperature on the drying kinetic of slices of apple is presented in Figure 7.8 (Ratti, 1991). In summary, the influence of external factors is as follows: An increase in temperature or velocity or a decrease in air humidity results in an increase in the drying rate. As the dimensions of the particle increase, the drying rate decreases. Another characteristic of the drying curves is the initial warming-up period that occurs when the temperature of the solid at the start of the process is lower than the dew point temperature of the air-water vapor mixture; therefore, condensation of water vapor occurs upon initial exposures. Fick’s diffusion law is commonly used to represent the drying kinetics in the falling rate period; however, this simple model is not always adequate to represent the complex process of drying. Any simple theory idealizes the process and generally cannot be used safely in the design of dryers (Keey, 1980). On the other hand, there are some complex theories that represent the drying process from the microscopic standpoint of mass and heat transfer between each phase inside the food particles (Gekas, 1992; Crapiste et al., 1988). This careful approach, which uses the volume 1

X /Xo

0.9 0.8

61°C

0.7

51°C

0.6

40°C

0.5 0.4 0.3 0.2 0.1 0 0

100

200

300

400

500

t (min)

FIGURE 7.8 Drying kinetics of slices of apple. (From Ratti, C. 1991. Design of Dryers for Vegetable and Fruit Products. PhD. thesis (in Spanish). Universidad Nacional del Sur, Bahía Blanca, Argentina.)

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Drying of Fruits

averaging method to solve the governing transport equations, is too complex for practical use. For practical purposes, it is often useful to use a lumped-parameter model supported by carefully designed experimentation at laboratory scale. The so-called characteristic drying curve (Van Meel, 1958) is a semiempirical approach, which minimizes the experimental data required to evaluate a kinetic drying model. This effectively allows scale-up of kinetic data to other operating conditions than those at which the original data were taken. This theory defines a characteristic function f that is independent of drying conditions but is dependant on the product and water content: nw = fnwc

(7.11)

Numerous simplifying assumptions are implicit in the definition of Equation (7.11). Application of Equation (11) to food systems presents several problems because it is derived for nonhygroscopic, nonshrinking systems. A new model is needed for vegetables and fruits (Ratti and Crapiste, 1992). This model is represented by the following equation: nw =

kg[aw pws - pw• ] [1 + (F / Xo ) Bimrs ]

(7.12)

in which the parameter F must be obtained empirically. This parameter was shown theoretically and experimentally to be independent of drying conditions and particle geometry, and in the case of apple, potato, and carrot, the results of the regression yield (Ratti and Crapiste, 1992): F = 5.320e - 3( X / Xo ) -1.079

(7.13)

7.2.2.2 Freeze-Drying Freeze-drying is a process by which the product is first frozen and then the ice from the frozen material is removed by sublimation, usually under conditions of low pressure and temperature. The sublimation leaves place to a dry porous layer that continuously recedes during the process. Several theoretical models that can be used to represent food freeze-drying kinetics can be found in the literature (Lombraña et al., 1997; Lombraña and Izkara, 1996; Liapis and Bruttini, 1995a; Mellor, 1978; Karel, 1975). However, in most cases adjustable parameters are needed to match the model predictions with experimental data (Sheehan and Liapis, 1998; Sadikoglu and Liapis, 1997; Sharma and Arora, 1995; Millman et al., 1985). In other cases, no comparison with experimental data is presented (Liapis and Bruttini, 1995b; Nastaj, 1991). Khalloufi et al. (1999) have recently developed a model that has proved valuable in predicting experimental freeze-drying kinetics data for fruits and vegetables.

7.2.3 QUALITY CONSIDERATIONS Special attention on quality is required in processes dealing with foodstuffs. As mentioned by Karel (1991), the quality of a dehydrated product depends strongly on the quality of the raw materials, but their processing and storage must be carried out, minimizing any undesirable change in quality. Figure 7.9 shows that the required quality characteristics of a foodstuff may be different, depending on the end use of the product. The quality characteristics may be separated into two major groups (Salunkhe et al., 1991): sensory and hidden. Color, gloss, size, shape, defects, texture, flavor, and aroma belong to the former group, and the nutritive value and the presence of adulterants and toxic components belong to the latter group. Attention has recently been focused on degradation of nutraceutical compounds

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Processing Fruits: Science and Technology, Second Edition

ease of cultivation

GROWER

size

disease resistance

yield color texture

PROCESSOR

flavor

CONSUMER

nutritive value presence of toxins

FIGURE 7.9 Quality characteristics of foodstuffs.

during drying. Such is the case for tomato drying, in which the deterioration of lycopene has been extensively studied (Zanoni et al., 1999; Shi et al., 1999; Inakuma et al., 1998; Yoon et al., 1997). The whole process of drying involves also the treatment of foodstuffs before and after dehydration — for example, transport, peeling, cutting, and storage — so quality restrictions have to be taken into account in all these sub-processes. The important changes occurring in a foodstuff due to dehydration are (Bruin and Luyben, 1980) the influence of drying on microorganisms, enzymatic and chemical conversions, and physical changes. The estimation of these changes as a function of the wide, diverse range of conditions that can be expected for individual foods is extremely complicated (Karel and Flink, 1983). The application of the knowledge learned in the area of glass transition of polymers to food systems succeded in improving understanding and predicting the behavior of foodstuffs during processing (Schenz, 1995). Glass transition temperature, Tg, can be defined as the temperature at which an amorphous system changes from the glassy to the rubbery state (Karmas et al., 1992; Roos and Karel, 1991b). Recently, Tg of food products has been pointed out to be responsible for the deterioration mechanisms during processing, and an indicator of food stability (Chirife and Buera, 1995; Roos, 1995; Slade and Levine, 1991; Kerr et al., 1993; Roos and Karel, 1991a; Schenz, 1995; Peleg, 1995). It has also been reported that when temperature of some processes exceeds Tg, the quality of foodstuffs is seriously altered (Peleg, 1996). Although numerous, most of the published articles on glass transition and water content of foodstuffs are limited to the study of liquid model foods such as solutions of pure sugars (Roos, 1995; Slade and Levine, 1991). There is little literature concerning the glass transition of solid foodstuffs such as fruits, vegetables, and meats as a function of water content. Figure 7.10 shows the glass transition temperature of several fruit powders (Khalloufi et al., 2000b) as a function of water content, showing a strong relationship between variables. Process-quality relationships could be a solid basis to optimize both existing and novel dehydration methods (Nijhuis et al., 1996). However, although this parameter is important in analyzing the impact of processing on the final quality of a product, quantifiable expressions between quality parameters and glass transition have not been found yet. For more details on this topic, the reader can refer to the references cited.

7.3 TYPES OF DRYERS 7.3.1 CLASSIFICATION

AND

SELECTION

Over 200 types of dryers are available commercially to process the diverse physical forms of foods and feeds and impart the required product characteristics. Menon and Mujumdar (1987) have given detailed schemes for classification of dryers based on

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Drying of Fruits

340 Strawberry

320

Blueberry 300 Tg (K)

Raspberry 280

Blackberry

260 240 220 200 0

0.1

0.2

0.3

w (water content, wet basis)

FIGURE 7.10 Glass transition temperature of several fruit powders as a function of water content. (From Khalloufi S., El Maslouhi, Y., and Ratti, C. 2000b. Mathematical model for prediction of glass transition temperature of fruit powders. J. Food Sci., 65(5), 842–848.)

1. Mode of processing (batch, continuous, and semi-batch) 2. Operating pressure (vacuum, atmospheric, and high pressure) 3. Mode of heat transfer (conduction, convection, radiation, dielectric, and a combination of different modes) 4. Adiabatic or nonadiabatic (e.g., with heat exchangers immersed within the dryer) 5. Physical form of feed (e.g., granular, pasty, continuous web, etc.) 6. Physical state of product being dried (e.g., stationary, moving, conveyed, fluidized, dispersed, spouted, vibrated, atomized, stirred, coated on inert surfaces, etc.) This classification scheme readily gives an idea of the large number of potential configurations for dryers. The selection of the dryer for a specific task, however, is governed by a number of criteria that limit the possible candidate dryers significantly. Among the important criteria to be considered are: 1. Physical form of feed and product. For example, if it is a paste or slurry, the choice of dryer is limited to spray, drum, or fluidized beds of inert particles. The dryer must be able to handle the physical form of the feed as well as the product. 2. Desired quality of product. 3. Heat-sensitivity of product (e.g., mode of heat transfer and maximum permissible product temperature must be selected to be consistent with the physical form of the feed). For extremely heat-sensitive products, it may be necessary to use a vacuum dryer or even a freeze dryer. 4. Energy efficiency. Some dryer types are inherently more energy-efficient than others. For example, contact (conduction or direct) dryers are more efficient than convection dryers; however, they are not suitable for all types of feeds. 5. Production rate (e.g., small, medium, large). For drying of fruits and vegetable materials, conventional types of dryers are commonly used because of their simplicity of construction and operation, as well as low cost. Most such products

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Processing Fruits: Science and Technology, Second Edition

TABLE 7.8 Selection of Dryers Based on the Form of Feed Form of Feed Particulate or granular

Large cubes or slices

Paste-like (or suspension)

Possible Dryer Types

Form of the Product

Packed bed dryer Fluid bed dryer Conveyor dryer Spouted bed dryer Tray (batch or continuous) Rotary Vibrated bed Flash (only if surface moisture is present) Freeze dryer Packed bed dryer Conveyor dryer Tunnel dryer Impinging jet (jet-zone) Vibrated bed Tray (batch or continuous) Freeze dryer Spray dryer Drum dryer Fluid or spouted bed of inert solid particles

Particulate or granular

Cubes/slices (deformed)

Powder

are seasonal in nature, which means that dryers are operated only over a fraction of the year. Thus, the units must have low capital costs for economic reasons. Also, fruits and vegetables may be dried in their original form (e.g., grapes, slices or cubes of papaya, mango, apple, pineapple, etc.) or in processed form (e.g., puree of mango, papaya, etc.). Depending on the physical form of the feed, different dryers must be used. The presence of sugar in most fruits leads to the danger of undesirable caramelization during drying if the operating temperature in the dryer is chosen incorrectly. Also, the hygroscopicity and stickiness of a product may lead to problems of deposits in dryers, as well as caking during storage. These and other factors must be considered in selecting both the dryer type and the operating conditions. Table 7.8 is a brief, simple summary of types of dryers one may use, depending on the form of the feed. Numerous other variants are possible (see Menon and Mujumdar [1987] for details). It is important to carry out laboratory or pilot tests to ensure feasibility of handling the product in a given dryer. For example, because of their particle size, shape, or stickiness characteristics, some particulate materials may not fluidize at all. In such cases, it is useful to test if back-mixing the feed with the dried product makes the mixture fluidizable (or dispersible for a flash dryer).

7.3.2 SPECIFIC DRYING SYSTEMS 7.3.2.1 Conventional Hot-Air Drying In this type of drying, the heat needed for drying is provided by convection with hot air in contact with the product. The most common of these dryers used for fruits are: kiln, cabinet, tunnel, and continuous belt. The kiln dryer is basically an oven heated by gas burners in which the product is placed in slotted trays. This equipment is operated in a batch mode and is still widely used to dry slices of apples and several other fruits. It takes from 6 to 8 h to dry a batch of slices or rings of apples to a final moisture content of 14 to 40% (Somogyi and Luh, 1986).

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The cabinet dryer is also a type of batch equipment, but there is a forced flow of air along or through the trays where the material is placed. This air is heated indirectly, generally by steam coils, after entering the cabinet. The operation can be controlled, and the drying is more uniform in this case than in the kiln dryer. This unit is more suitable for small-scale operations of pieced fruits (Salunkhe et al., 1991), and although the cost of the equipment is low, its operating (labor) cost is high. The thermal efficiency is low as well. The tunnel dryer is the most commonly used equipment to dry fruits. It is similar to the cabinet dryer but the trays are made to move along a tunnel in which hot air flows in parallel or counter flow to the product. This type of dryer is efficient yet simple. It is more suitable to industrial application because they can handle larger production rates. This method is often used to dry apricots, peaches, pears, apples, figs, dates, and so on. The continuous belt dryer is formed by an endless (permeable and impermeable) belt on which the product to be dried is placed and carried through a counter- or co-current flow of hot air. The main advantage of this dryer is the ease of automatic continuous operation that minimizes labor requirements (Salunkhe et al., 1991). Conventional hot-air drying is the main process used in obtaining fruit leather. A flow diagram for the fruit leather process is shown in Figure 7.11. Apples, apricots, berries, cherries, nectarines, peaches, pears, pineapples, and strawberries have been determined to be the best fruit sources to obtain this type of product (Reynolds, 1993). Fruits should first be selected for their ripeness (ripe or slightly overripe fruits are required) and afterwards washed, peeled, and cut into chunks. The fruit pieces are pressed until a smooth puree is obtained. In order to obtain the desired texture and to avoid fruit darkening, water and ascorbic acid are added to the puree. The fruit puree is poured SELECTION

WASHING

PEELING

CUTTING

Ascorbic Acid

FRUIT MASHING

POURING

DRYING

STORING

FIGURE 7.11 Flow diagram for the fruit leather process.

Liquid

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into trays that are placed in a kiln or cabinet dryer at 60∞C (maximum temperature to avoid case hardening) until the fruit leather is dry (Reynolds, 1993). 7.3.2.2 Solar Drying The drying of fruits and vegetables using solar energy has been used since antiquity in order to preserve foods. There are two main types of drying using solar energy: direct and indirect. In direct solar drying, commonly called open-sun drying or just sun-drying, the product is placed directly on trays in open air where only the incident solar radiation (or incident and reflected radiation) is used as the energy source for drying. Such dryers have the following main disadvantages (Imre, 1987): large area requirement, inability to control degradation of product by biochemical or microbiological reactions, insect infestation, and so on. In addition, the drying is uneven in the tray, and the drying time is extremely long. For instance, drying apricots to 24% moisture requires almost 50 h using direct sun-drying; this decreases to 31 h when the dryer is improved with the use of reflected solar radiation (Bolin and Salunkhe, 1982). In the case of peaches, the drying time to a final moisture content of 24% is 150 h due to the large fruit size, and it decreases by 45% when reflected radiation is also used (Bolin and Salunkhe, 1982). The indirect solar dryers use solar energy to heat the drying air in special heat exchangers (solar collectors), and then this air flows through or over the product by natural or forced convection. Figure 7.12 shows one of the most common and simple solar dryers for vegetables and fruits — the cabinet dryer (Kalra and Bhardwaj, 1981). It is composed of a solar collector followed by a packed-bed-type dryer. The air conditions at the inlet of the collector (air humidity and temperature) vary with time, depending on the hour of the day and other factors like weather conditions; therefore, the air conditions at the entrance of the dryer also vary in time required. A description of more complicated solar dryers can be found in Imre (1987). The main advantage of these dryers is that the energy source is renewable, free, and nonpolluting (Imre, 1987). On the other hand, this type of drying is only useful in climates with a hot sun and dry atmosphere (Somogyi and Luh, 1986) over extended periods. The problem of periodicity in solar energy can be solved with the storage Air Exit

Batch Dryer

Solar Collector

Air Inlet

FIGURE 7.12 Cabinet solar dryer. (From Kalra, S.K. and Bhardwaj, K.C. 1981. Use of simple solar dehydrator for drying fruit and vegetable products. J. Food Sci. Technol. (India), 18, 23.)

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of part of this energy (e.g., rock storage), but any improvement in this type of equipment must be done carefully because the technoeconomics are not always favorable. Fruits commonly dried with solar energy include figs, prunes, peaches, apricots, and to a large extent grapes from which raisins are obtained. An updated review of these processes can be found in Jayaraman and Das Gupta (1992). Note that a backup fossil-fuel-fired heater is needed when the product is subjected to biochemical or microbiological degradation during periods of no insolation (e.g., night) or poor insolation (e.g., cloudy, rainy days). Thermal storage adds to the cost of solar drying very significantly. 7.3.2.3 Microwave Drying Conventional drying of foods is a slow process. Attempts to enhance this process have been made, but only few of them are applied presently in the industry. Microwave heating increases the temperature of the interior, wetter parts of the solid. In addition, moisture transport to the evaporation surface is also enhanced by an internal pressure gradient. Microwave heating has three main advantages (Van Arsdel et al., 1973): (1) a penetrating quality that leads to uniform drying (conventional drying may cause damage to the surface of the product; it is dry and uses high temperatures compared to the interior); (2) selective adsorption by liquid water, which leads to a uniform moisture profile within the particle; and (3) ease of control due to the rapid response of such heating. Advantages (1) and (2) increase the quality of the final product and make it easier to be rehydrated. But on the other hand, such equipment is complicated and expensive, and, unfortunately, no industry uses it for fruit dehydration (Somogyi and Luh, 1986). Work done on the finish drying of fruits demonstrated a synergistic effect between hot-air and microwave drying (Salunkhe et al., 1991). Apple, mango, and pineapple were investigated recently for microwave drying under vacuum (Xiufan et al., 1993). The rate of drying and the core temperature were measured in this work. The experimental results showed that the geometry of the samples and the power used have a strong influence on the microwave drying rate of fruits, as well as on product quality. 7.3.2.4 Osmotic Dehydration Hot-air drying notably reduces the quality of the processed foodstuffs (changes in color, shape, losses of aroma and nutrients, etc.). Osmotic dehydration is an alternative technology to reduce the water content, as well as to improve the quality of the final product. This process is being used in industry to dehydrate fruits, vegetables, meat, and fish, but the industrial application is still limited. Osmotic dehydration involves the immersion of cut foods in concentrated solutions of sugars and salts. A flux of water out of the food and of other solutes into the foodstuff develops due to the difference in osmotic pressure. The product thus loses some water to the external solution. Ponting et al. (1966) first applied osmotic dehydration for reduction of weight by up to 50% of apples prior to vacuum drying. The osmotic dewatering rate can be enhanced by increasing the concentration of the osmotic solution or the temperature. Rogachev and Kislenko (1972) coupled alternate heating and cooling with acoustic fields to increase osmosis. More recently, Shi and Maupoey (1993) applied vacuum (100 mb) treatment to increase water transport during osmosis in 65∞Brix sugar solutions for dehydration of pineapple and apricot cubes. They claimed higher dehydration rates at lower temperatures, thus improving the quality of dehydrated fruit products. Sugar gain is related to the biological characteristics of fruits; it is unaffected by application of vacuum. Grabowski and Mujumdar (1992) have proposed a novel scheme for solar-assisted osmotic dehydration of fruits. Solar energy is used to dry the osmotically dehydrated slices of fruits and also to concentrate the osmoticum. Figure 7.13 shows the flow diagram for osmotic dehydration of fruits. Mango, banana, pear, apple, cranberries, and strawberries (as whole or cut in pieces) are the usual fruits transformed by

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FRUIT (whole or pieces)

PRE-TREATMENT (blanching, SO2, etc.)

OSMOTIC DEHYDRATION

DRAINING

Fruits

Syrup

AIR-DRYING

STORING

FIGURE 7.13 Flow diagram for the osmotic dehydrated fruit process.

this process. Fruits can be pretreated if necessary to avoid darkening. The osmotic medium is commonly a sugar solution with a fruit/syrup ratio of 1/3–5 at a temperature between 20 to 50∞C and slightly agitated (Garrote et al., 1992). The duration of the process and syrup concentration are specific to the fruit being processed (Dauthy, 1995). After osmotic dehydration has been completed (reducing up to 50% of the initial water content), the fruits are drained and placed into a hot air dryer to achieve the final desired moisture content. Syrup solution can be recuperated and after the sugar concentration has been adjusted, it could be used again for osmotic dehydration. 7.3.2.5 Explosion Puffing The incorporation of explosion puffing into a hot-air dehydration process facilitates faster dehydration and leads to a final product with a highly porous structure that is capable of rapid rehydration. This process has long been applied to rice and wheat breakfast cereals and now is being applied to fruit and vegetables (Salunkhe et al., 1991). Explosion puffing has been successful with apples, blueberries (Somogyi and Luh, 1986), and banana slices (Saca and Lozano, 1992). The process starts with conventional hot-air drying of the fruit pieces until a certain water content is attained. Then the pieces are placed in a closed chamber, the gun, where the pressure is increased by means of superheated steam (Figure 7.14, Saca and Lozano, 1992). After some time, the pressure is suddenly released by opening the lid of the gun. Due to the high temperature and the sudden decompression, the water in the particles flashes explosively. Then the solids are placed again in the air drying equipment to be dried to the desired final water content. Due to the highly porous structure after puffing, the drying time is reduced notably. The key parameters that affect this process are the initial water content of the particles entering the explosion puffing process, the temperature and pressure of the steam in the gun, and the dwell

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OPENING MECHANISM

O-RING

GUN CYLINDER THERMOCOUPLES SAMPLES STEAM

LID

TEFLON TRAY

GAS BURNER

FIGURE 7.14 Schematic of an explosion-puffing dryer. (From Saca, A.S. and Lozano, J.E. 1992. Explosion puffing of bananas. Int. J. Food Sci. Technol. 27, 419–426.)

time inside the gun. The optimal initial water content is between 20 to 30% for apples, 19 to 30% for blueberries (Somogyi and Luh, 1986), and 27 to 38% for banana slices (Saca and Lozano, 1992). In the latter case, it was found that the explosion puffing process decreased by 25% the drying time of the whole process, with an increase of porosity of the product from 10.3% to 46.8%. This fact makes the product easily rehydratable. 7.3.2.6 Freeze-Drying Freeze-drying (also called lyophilization) is characterized by heating (i.e., conduction, radiation, radio-frequency, microwaves, etc.) a moist frozen product in a vacuum chamber maintained at an absolute pressure below the vapor pressure of the ice within the product (Hanson, 1976). When the material is heated, the ice sublimes. The main advantages of this drying method are related to the high quality of the final product compared with other drying methods. These advantages can be summarized as follows (Okos et al., 1992; Jayaraman and Das Gupta, 1992; Salunkhe et al., 1991; Somogyi and Luh; 1986): (1) high flavor and aroma retention; (2) high retention of nutritional value; (3) minimal shrinkage; (4) minimal change in shape, color, and appearance; (5) practically no damage in structure and texture; (6) porous final structure; and (7) easily rehydratable. As was pointed out by Salunkhe et al. (1991), the low temperature of this process leads to a decrease in the kinetics of all the degradative processes (nonenzymatic browning, protein deterioration, and enzymatic reaction, etc.). The industrial application of freeze-drying to a wide range of fruits has been limited by its main disadvantage: the high capital and operating costs. In addition, the final product has to be properly packaged in special materials to avoid oxidation and moisture pickup (Jayaraman and Das Gupta, 1992), which also increases the cost of the final product. Pilot and industrial scale freeze dryers are carefully described in Liapis (1987) and Liapis and Bruttini (1995a). Among the industrial freeze dryers, the most common are the tray and tunnel type. In general, they are only of the batch type. This drying technique has been applied successfully to raspberries, strawberries, peaches, cherries, and figs (Shishehgarha et al., 2002; Salunkhe et al., 1991; Somogyi and Luh, 1986), but the commercial application has been limited to exotic fruit and orange juices (Jayaraman and Das Gupta, 1992). It has been shown that this process is viable because of the higher quality of production of fruit powder for retail consumption (Holdsworth, 1986).

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7.3.3 NOVEL DRYERS Kudra and Mujumdar (1994) have extensively surveyed scores of novel drying techniques, some of which are applicable to dehydration of fruits in sliced, diced, or pureed forms. In principle, dielectric drying, which provides preferential heat to the polar free water molecules volumetrically, should be a desirable mode of heating for heat-sensitive products. In actual fact, this process tends to be highly capital-intensive and requires expensive electrical energy, as noted earlier. For products that are nonfluidizable, e.g., apple cubes, strawberries, and blueberries, it is possible to employ the so-called technique of two-component fluidization. For example, Mujumdar and Macdonald (1993) have shown that it is possible to fluidize whole grapes and blueberries in beds of granulated sugar. Using hot air as the fluidizing and heating medium, the hygroscopicity of sugar helps improve the mass transfer rate from the fruit pieces. Heat transfer rates from fine particles to immersed bodies is known to be of an order-of-magnitude greater than that to air flow. It is found that the sugar-to-grape weight ratio should be at least six to ensure smooth fluidization. In principle, this idea could be extended to other fruits. Care is needed to ensure that the sugar does not granulate and eventually defluidize the bed. For “weak” fruits (e.g., blueberries), fluidization leads to structural damage and, hence, is not recommended. For products in sliced form, the so-called jet-zone dryer is used, wherein downwardly diverted jets of hot air impinge on a bed of the product conveyed on a vibratory or oscillatory conveyor. The product achieves a state of pseudo-fluidization and is yet handled gently (see Kudra and Mujumdar, 1995). In a novel process tested recently by Mujumdar and Macdonald (1993), blueberries were dried in a two-step process. First, the berries were treated in a warm dilute solution of ethyl oleate in caustic soda to make the skin permeable to water and solute flow and then immersed for several hours in a saturated sugar syrup. The osmotically dehydrated berries were dried in a vacuum dryer (100 mm Hg pressure) at low temperatures (-20 to 20∞C). The berries retained their color and physical form and had enhanced taste due to some sugar uptake. Better optimization of the process conditions for osmotic dehydration followed by freeze-drying has been shown through experimentation to yield high quality dried blueberries for special applications, such as additives to cereals, desserts, etc. Sometimes fruit purees may be added as flavoring components to products such as mashed potato. The mixtures may be dried in spray, drum, flash, or the novel impinging stream dryers (Kudra et al., 1995). In the latter, the feed is dispersed finely in hot air jets turned in opposite directions for very rapid and efficient drying to produce fine powders directly. Such dryers are not currently available commercially.

7.4 DESIGN CONSIDERATIONS 7.4.1 HEAT

AND

MASS BALANCES

The main problem in the design of dryers for biological materials arises when the diverse physicochemical changes that occur in the product during the process have to be taken into account. As an example, the effect of shrinkage is often neglected due to the complexity in the derivation and solution of the corresponding model equations. Ratti (1991) developed a model for packed bed batch dryers, which includes the effect of shrinkage and the variation of the product properties with water content and temperature for application to fruit and vegetable products. The transient heat and mass balances equations are expressed in appropriately defined moving coordinates to account for particle shrinkage during drying. The key assumptions employed in this model are: 1. One-dimensional transport of heat and mass 2. Uniform air velocity distribution in the dryer (plug flow of drying air)

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TABLE 7.9 Heat and Mass Balances for a Batch Dryer Partial Differential Equations System Mass balance in the gas phase: nw av (1 - e ) 1 Gs r s (1 - e ) ∂Y È ∂Y ˘ ÍÎ ∂t ˙˚ = ra e SLo r a e r s,o (1 - e o ) ∂L L

(1)

Mass balance in the solid: nw av È ∂X ˘ ÍÎ ∂t ˙˚ = - r L s

(2)

av È ∂Ts ˘ Í ∂t ˙ = r (1 + X )Cp [hg (Tg - Ts ) - nw DH s ] Î ˚L s sh

(3)

Energy balance in the solid:

Energy balance in the gas phase: hg av (1 - e ) È ∂Tg ˘ 1 Gs r s (1 - e ) ∂Tg (Tg - Ts ) Í ˙ =r a eCpah SLo r a e r s,o (1 - e o ) ∂L Î ∂t ˚ L

(4)

Boundary and Initial Conditions Initial Profile (t = 0): Ï X = Xo Ô ÔTs = Tso L=0Ì Y Y Ô = go ÔTg = Tgo Ó

Ï X = Xo ÔT = T Ô s so Lπ0Ì Y Y (T ) Ô = sat so ÔTg = Tso Ó

(5)

Boundary Conditions: L = 0 Y = Ygo, Tg = Tgo

3. Adiabatic system (well-insulated dryer) 4. Purely convective heat transfer 5. Negligible conduction heat transfer between particles in the bed and negligible contact diffusion 6. Shrinking particles Table 7.9 summarizes the governing differential heat and mass balance equations and the initial and boundary conditions for constant inlet air drying conditions. Most of the parameters and properties needed to complete the model have been referred to in previous sections. An extension of this model for varying inlet air conditions can be found in the literature (Ratti and Mujumdar, 1993). Models for different types of dryers can be found in the literature (Keey, 1978; Mujumdar, 1987). Models for drying of foodstuffs in particular also exist, e.g., concurrent-flow packed bed and cascading rotary dryers (Bakker-Arkema, 1986), spray dryers (Sokhansaj and Jayas, 1987; Bakker-Arkema, 1986), or freeze dryers (Okos et al., 1992; Sokhansaj and Jayas, 1987). In most cases, the reader must carefully revise the model assumptions in order to apply these models to drying of highly deformable shrinking material such as fruits. Recently, a model for a tunnel

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dehydrator for tomatoes was developed (Unadi et al., 2002). This model was used to determine the performance of such a dryer in different modes, as well as the influence on the quality of dried tomatoes. The authors claimed that counterflow mode gives the highest throughput, the lowest specific energy consumption, and satisfactory quality of dried tomatoes. 7.4.2

Energy Aspects

A dryer is an energy-consuming piece of equipment (Strumillo and Lopez-Caicedo, 1987), and when applied to foodstuffs, it becomes more energy consuming because mild operating conditions are needed in order to obtain a final product with acceptable quality. Convective drying requires electric energy and heat. For instance, drying of apples up to 20% moisture content requires 5 MJ/kg of evaporated water (Lewicki and Lenart, 1992), whereas only 3% of the energy needed is electric. An interesting description of the energy consumption during osmo-convection drying of fruits and vegetables, together with operating data, can be found in the previous reference. This author claimed that depending on the design of an osmo-convective process, the savings in energy consumption can be as high as 75% compared with regular convection drying because osmotic dehydration reduces the amount of water that must be thermally removed. The most highly energy-consuming process is obviously freeze-drying. This operation has three main energy-consuming steps: freezing, sublimation, and water vapor removal. Fruits are generally frozen by cold air prior to dehydration (Salunkhe et al., 1991). It is known that the freezing temperature and the freezing rate are important factors that affect the quality of the final product in terms of its structure (Jayaraman and Das Gupta, 1992). Fast freezing leads to small crystals so new techniques applied to fruit freezing are based on their immersion in liquid CO2, freon, or liquid N2 (Salunkhe et al., 1991). The second step is carried out under vacuum, and the heat for sublimation is provided by conduction or by microwave heating in order to accelerate the process. The process of water removal depends on the size, shape, and tortuosity of the pores (Somogyi and Luh, 1986), so the faster the freezing rate, the slower the water removal. 7.4.2.1 Heat Pump Dryers Drying of heat-sensitive products (e.g., fruits and vegetables) requires large amounts of lowtemperature heat. Use of high-grade energy sources (e.g., gas, oil, electricity, etc.) for such tasks leads to low energetic efficiencies. Heat pumps can take advantage of the energy of high-grade energy sources. Indeed, benefits of heat pumps increase as the target air temperature for heating approaches the ambient temperature. Several different configurations are possible to couple heat pumps with conventional convective dryers. They may be single-pass or of a recirculating type. Aveces-Saborio (1993) has presented some simulation results on energy consumption in heat pump dryers and recirculating conventional dryers. Stromman (1986) has discussed energy-optimal temperature differences in heat-pump dryers. Pendyala et al. (1986) have evaluated the economics of heat pump-assisted drying systems. Zylla and Strumillo (1987) have reviewed heat pumps in drying. Recently, Jia et al. (1993) developed, modeled, and tested a heat pump-assisted microwave dryer for vegetables (ginger, carrots, etc.) and showed that energy consumption in such systems is comparable to conventional drying while the product quality may be enhanced. They also showed that the specific moisture evaporation rate (SMER) is strongly dependent on the product being dried. While no tests are reported on fruits, it is likely the product quality may be better at a lower cost.

7.5 CONCLUDING REMARKS Industrial drying of fruits is a technology based both on engineering and empiricism. Experimental evaluation of the effects of dryer operating variables on the drying kinetics (which govern the dryer

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size), as well as the quality of the product, are keys to successful design and economic drying of fruits. Energy consumption is often a secondary consideration to quality, but it is possible to enhance efficiency without compromising on quality through a fundamental understanding of the phenomena involved. Drying of biological materials such as fruits is extremely complex since it is accompanied by biochemical and physical changes that govern product quality, such as color, flavor, shape, size, taste, etc. Extensive laboratory and pilot testing is needed before a full-scale dryer can be designed without prior experience with the same feedstock.

7.6 NOMENCLATURE As = surface area [m2] av = particle surface area/volume [m -1] aw = water activity [–] Bimrs = mass Biot number (see Ratti and Crapiste, 1992) [–] Cp = specific heat [J/kg K] c = constant of GAB equation, Table 7.3 Dr = effective diffusivity [m2/sec] f = Van Meel’s characteristic drying function [–] hg = heat transfer coefficient [J/m2 sec K] k = constant of GAB equation, Table 7.3 kg = mass transfer coefficient [kg water/m2 sec kPa] ki = regression constants, Table 7.3 kt = thermal conductivity [W/m K] L = characteristic length [m] ms = dry mass [kg] nw = water mass flux [kg water/m2 sec] nwc = water mass flux in the constant rate period [kg water/m2 sec] pw = water vapor pressure [kPa] pwo = pure water vapor pressure [kPa] R = universal gas constant [J/mol K] T = temperature [K] t = time [sec] V = volume [m3] X = water content (dry basis) [kg water/ kg dry matter] Xm = constant of GAB equation, Table 7.3 W = dimensionless water content (dry basis) relative to the initial = X/Xo [–] W* = local dimensionless water content w = water content (wet basis) [kg water/ kg total matter] Z = reduced moving coordinate [–]

7.6.1 SUBSCRIPTS af bf e g o s •

= = = = = = =

above freezing below freezing equilibrium gas initial at the surface at the bulk

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7.6.2 GREEK SYMBOLS DE DHs e¢ e≤ r F

= = = = = =

activation energy for diffusion [kJ/mol] heat of sorption [kJ/mol] relative dielectric constant [–] loss factor [–] density [kg/m3] generalized drying parameter (see Ratti and Crapiste, 1992) [–]

REFERENCES Abdelhaq, E.H. and T.P. Labuza. 1987. Air drying characteristics of apricots. J. Food Sci., 52(2), 342–345. Acevez-Saborio, S. 1993. Analysis of energy consumption in heat pump and conventional dryers. Heat Recov. Syst. CHP., 13(5), 419–428. Alzamora, S.M. 1979. Thesis. Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires, Argentina. Alzamora, S.M., J. Chirife, P. Viollaz, and L.M. Vacarezza. 1979. Heat and mass transfer during air drying of avocado. In A.S. Mujumdar (Ed.), Drying ’80, Vol. I: Developments in Drying, Science Press, NJ. Bakker-Arkema, F.W. 1986. Heat and mass transfer aspects and modelling of dryers — A critical evaluation. In D. MacCarthy (Ed.), Concentration and Drying of Foods, Elsevier Applied Science, London, pp. 165–202. Berna, A., C. Roselló, J. Cañellas, and A. Mulet. 1990. Drying kinetics of apricots. In W.E.L. Spiess and H. Schubert (Eds.), Engineering and Food. 1. Physical Properties and Process Control, Elsevier, Amsterdam, pp. 628–636. Berna, A., C. Roselló, J. Cañellas, and A. Mulet. 1991. Drying kinetics of a Majorcan seedless grape variety. In A.S. Mujumdar and I. Filková (Eds.), Drying ’91, Elsevier, Amsterdam., pp. 455–462. Bimbenet, J.J., J.D. Daudin, and E. Wolf. 1985. Air Drying Kinetics of Biological Materials. In A.S. Mujumdar (Ed.), Drying ’85, Hemisphere, New York, pp. 178–185. Bolin, H.R. and D.K. Salunkhe. 1982. Food dehydration by solar energy. CRC Crit. Rev. Food Sci. Nutr., April, 327. Bruin, S. and K.Ch.A.M. Luyben. 1980. Drying of food materials: A review of recent developments. In A.S. Mujumdar (Ed.), Advances in Drying, Vol. 1, Elsevier Science, Amsterdam, pp. 155–213. Brunauer, S., P.H. Emmet, and E. Teller. 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc., 60, 309. Chirife, J. and M. Buera. 1995. A critical review of some non-equilibrium situations and glass transition on water activity values of foods in microbiological growth range. J. Food Eng., 25, 531–552. Choi, Y. and M.R. Okos. 1986. Effects of temperature and composition on the thermal properties of foods. In M. Le Maguer and P. Jelen (Eds.), Food Engineering and Process Applications, Vol. 1: Transport Phenomena, Elsevier Applied Science, London, pp. 93–101. Chung, D.S. and H.B. Pfost. 1967. Adsorption and desorption of water vapour by cereal grains and their products. Trans. ASAE, 10, 549. Chuy, L.E. and T.P. Labuza. 1994. Caking and stickiness of dairy-based food powders as related to glass transition. J. Food Sci., 59: 43–46. Constenla, D.T., J.E. Lozano, and G.H. Crapiste. 1989. Thermophysical properties of clarified apple juice as a function of concentration and temperature. J. Food Sci., 54(3), 663–668. Crank, J. 1967. The Mathematics of Diffusion. Oxford University Press, London. Crapiste, G.H. and E. Rotstein. 1986. Sorptional equilibrium at changing moisture contents. In A.S. Mujumdar (Ed.), Drying of Solids, Wiley Eastern. Crapiste, G.H., S. Whitaker, and E. Rotstein. 1988. Drying of cellular material. Parts 1 and 2. Chem. Eng. Sci., 43(11), 2919–2936. Dauthy, E.M. 1995. Fruit and Vegetable Processing. FAO Agricultural Services Bulletin No.119, Food and Agriculture Organization of the United Nations, Rome.

Drying of Fruits

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Garrotte, R.L., E. Silva, and R.A. Bertone. 1992. Osmotic concentration at 5 and 25∞C of pear and apple cubes and strawberry halves. Lebensm.-Wiss.u.-Technol., 25, 133–138. Gekas, V. 1992. Transport Phenomena of Foods and Biological Materials, CRC Press, Boca Raton, FL, pp. 150–160. Ginzburg, A.S. 1969. Application of Infra-red Radiation in Food Processing, Chemical and process engineering series, Leonard Hill, London. Ginzburg, A.S. and I.M. Savina. 1982. Mass and Moisture Transfer Characteristic of Food Products (in Russian), Lyegkaya Pischevaya Promyshlennost, Moscow. Grabowski S. and A.S. Mujumdar. 1992. Solar-assisted osmotic dehydration. In A.S. Mujumdar (Ed.), Drying ’92, Elsevier Science, Amsterdam, pp. 1689–1700. Guggenheim, E.A. 1966. Applications of Statistical Mechanics, Clarendon Press, Oxford. Halsey, G. 1948. Physical adsorption on non-uniform surfaces. J. Chem. Phys., 16, 931. Hanson, L.P. 1976. Commercial processing of fruits. Food Technology, Review No. 30, Noyes Data Corporation, New Jersey, pp. 41–100. Heldman, D.R. 1975. Food Process Engineering, AVI Publishing, Westport, CT pp. 96–103. Henderson, S.M. 1952. A basic concept of equilibrium moisture. Agr. Eng., 33, 29. Holdsworth, S.D. 1986. Advances in the dehydration of fruits and vegetables. In D. MacCarthy (Ed.), Concentration and Drying of Foods, Elsevier Applied Science, London, pp. 293–303. Hong, Y.C., A.S. Bakshi, and T.P. Labuza. 1986. Finite element modeling of moisture transfer during storage of mixed multicomponents dried foods. J. Food Sci., 51(3), 554–558. Iglesias, H.A. and J. Chirife. 1976. Prediction of the effect of temperature on water sorption isotherms of food materials. J. Food Technol., 11, 109. Imre, L.L. 1987. Solar Drying. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying, 1st ed., Marcel Dekker, New York, pp. 357–417. Inakuma, T., M. Yasumoto, M. Koguchi, and T. Kobayashi, 1998. Effect of drying methods on extraction of lycopene in tomato skin with supercritical carbon dioxide. J. Jpn. Soc. Food Sci. Technol., 45(12), 740–743. Jankovié, M. 1993. Physical Properties of Convectively Dried and Freeze-dried Berrylike Fruits. A publication of the Faculty of Agriculture, Belgrade, 38(2), 129–135. Jayaraman, K.S. and D.K. Das Gupta. 1992. Dehydration of fruits and vegetables — recent developments in principles and techniques. Drying Technol., 10(1), 1. Jia, X., C. Shave, and P. Jolly. 1993. Study of heat pump assisted microwave drying. Drying Technol., 11(7), 1583–1616. Kalra, S.K. and K.C. Bhardwaj. 1981. Use of simple solar dehydrator for drying fruit and vegetable products. J. Food Sci. Technol. (India), 18, 23. Kapsalis, J.G. 1987. Influences of hysteresis and temperature on moisture sorption isotherms. In L.B. Rockland and L.R. Beuchat (Eds.), Water Activity: Theory and Applications to Food, IFT Basic Symposium Series, Marcel Dekker New York, pp. 173–214. Karel, M. 1975. Heat and mass transfer in freeze-drying. In S.A. Golblithm, L. Rey, and W.W. Rothamayr (Eds.), Freeze-Drying and Advanced Food Technology, Academic Press, New York. Karel, M. 1991. Physical structure and quality of dehydrated foods. In A.S. Mujumdar and I. Filková (Ed.), Drying ’91, Elsevier Science, Amsterdam, pp. 26–35. Karel, M. and J.M. Flink. 1983. Some recent developments in food dehydration research. In A.S. Mujumdar (Ed.), Advances in Drying, Vol. 2, Elsevier Science, Amsterdam, pp. 103–153. Karmas, R., M.P. Buera, and M. Karel. 1992. Effect of glass transition on rates of nonenzymatic browning in food systems. J. Agric. Food Chem., 40, 873–879. Keey, R.B. 1978. Introduction of Industrial Drying Operations. Pergamon Press, Oxford. Keey, R.B. 1980. Theoretical foundations of drying technology. In A.S. Mujumdar (Ed.), Advances in Drying, Vol. I, Hemisphere, Amsterdam, pp. 1–20. Kerr, W.L., M.H. Lim, D.S. Reid, and H. Chen 1993. Chemical reaction kinetics in relation to glass transition temperatures in frozen food polymer solutions. J. Sci. Food Agric., 61, 51–56. Khalloufi S., Y. El Maslouhi, and C. Ratti. 2000b. Mathematical model for prediction of glass transition temperature of fruit powders. J. Food Sci., 65(5), 842–848. Khalloufi S., J. Giasson, and C. Ratti. 2000a. Water activity of freeze-dried berries and mushrooms. Can. Agric. Eng., 42(1), 51–56.

156

Processing Fruits: Science and Technology, Second Edition

Khalloufi, S., J.-L. Robert, and C. Ratti. 1999. Mathematical simulation of food freeze-drying. ASAE Annual International Meeting, Toronto, Canada. Kilpatrick, P.W., E. Lowe, and W.B. Van Arsdel. 1955. Tunnel dehydrators for fruit and vegetables. Adv. Food Res., 50, 385. Kreith, F. 1965. Principles of Heat Transfer. International Textbook Company, Scranton, PA. Krokida, M.K. and Z.B. Maroulis, 1997, Effect of drying method on shrinkage and porosity. Drying Technol., 15(10), 2441–2458. Kudra, T. and A.S. Mujumdar, 1995. Novel dryers/special drying technologies. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying, 2nd ed., Marcel Dekker, New York. Kudra, T., A.S. Mujumdar, and V.L. Melster. 1995. Impinging stream dryers. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying, 2nd ed., Marcel Dekker, New York. Labuza, T.P. and I.B. Simon. 1970. Food Technol., 24, 712. Lazar, M.E. and D.F. Farkas. 1971. The centrifugal fluidized bed: 2. Drying studies on pieces-form foods. J. Food Sci., 36, 315. Lewicki, P. and A. Lenart. 1992. Energy consumption during osmo-convection drying of fruits and vegetables. In A.S. Mujumdar (Ed.), Drying of Solids, International Science Publisher, New York, pp. 354–366. Lewis, M.J. 1987. Physical Properties of Foods and Food Processing Systems. Ellis Horwood Chichester, U.K. Levi, G. and M. Karel, 1995. Volumetric shrinkage (collapse) in freeze-dried carbohydrates above their glass transition temperature. Food Res. Int. 28(2), 145–151. Liapis, A.I. 1987. Freeze Drying. In A.S. Mujumdar (Ed.) Handbook of Industrial Drying, Marcel Dekker, New York, pp. 295–326. Liapis, A.I. and R. Bruttini. 1995a. Freeze drying. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying, 2nd ed., Marcel Dekker, New York. Liapis, A.I. and R. Bruttini. 1995b. Freeze drying of pharmaceutical crystalline and amorphous solutes in vials: dynamic multi-dimensional models of the primary and secondary drying stages and qualitative features of the moving interface. Drying Technol., 13(1, 2), 43–72. Lomauro, C.J., A.S. Bakshi, and T.P. Labuza. 1985. Moisture transfer properties of dry and semimoist foods. J. Food Sci., 50, 397–400. Lombraña, J.I., C. De Elvira, and M.Villarán 1997. Analysis of operating strategies in the production special foods in vials by freeze drying. Int. J. Food Sci. Technol., 32, 107–115. Lombraña, J.I. and J. Izkara 1996. Experimental estimation of effective transport coefficients in freeze drying for simulations and optimization purposes. Drying Technol., 14(3, 4), 743–763. Longman Dictionary of Contemporary English. 1990. Longman Group, Essex, U.K. Lozano, J.E., E. Rotstein, and M.J. Urbicain. 1983. Shrinkage, porosity, and bulk density of foodstuffs at changing moisture contents. J. Food Sci., 51, 113. Lozano, J.E., M.J. Urbicain, and E. Rotstein. 1979. Thermal conductivity of apples as a function of moisture content. J. Food Sci., 44(1), 198–199. Lozano, J.E., M.J. Urbicain, and E. Rotstein. 1980. Total porosity and open-porosity in the drying of fruits. J. Food Sci., 45, 1403. Maroulis, Z.B., E. Tsami, D. Marinos-Kouris, and G.D. Saravacos. 1988. Application of the GAB model to the moisture sorption isotherms for dried fruits. J. Food Eng., 7, 63–78. Mellor, J.D. 1978. Fundamentals of Freeze Drying. Academic Press London. Menon, A.S. and A.S. Mujumdar. 1987. Drying of solids: Principles, classification, and selection of dryers. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying, 1st ed., Marcel Dekker, New York, pp. 3–46. Millman, M.J., I.A. Liapis and J.M. Marchello. 1985. An analysis of lyophilization process using a sorptionsublimation model and various operational policies. J. AIChE, 31(10), 1594–1604. Mohsenin, N.N. 1980. Thermal Properties of Foods and Agricultural Materials, Gordon and Breach, New York, p. 161. Mohsenin, N.N. 1984. Electromagnetic Radiation Properties of Foods and Agricultural Products. Gordon and Breach, New York. Mujumdar, A.S. (Ed.). 1987. Handbook of Industrial Drying. 1st ed., Marcel Dekker, New York. Mujumdar, A.S. and A. Macdonald. 1993 (August). Private Communication, McGill University, Montreal, Canada. Nastaj, J.F. 1991. A mathematical modelling of heat transfer in freeze drying. Drying, 91, 405–413. Edited by Mujumdar, A.S. and Filkova, I., Elsevier, Amsterdam.

Drying of Fruits

157

Nelson, S.O. 1980. Microwave dielectric properties of fresh fruits and vegetables. Trans. ASAE., 23, 1314. Nelson, S.O. 1983. Dielectric properties of some fresh fruits and vegetables at frequencies of 2.45 to 22 GHz. Trans. ASAE., 26, 613. Nijhuis, H.H., E. Torringa, H. Luyten, F. René, P. Jones, T. Funebo, and T. Ohlsson. 1996. Research needs and opportunities in the dry conservation of fruits and vegetables. Drying Technol., 14(6), 1429–1457. Okos, M.R., G. Narsimhan, R.K. Singh, and A.C. Weitnauer. 1992. Food dehydration. In D.R. Heldman and D.B. Lund (Eds.), Handbook of Food Engineering, Marcel Dekker, New York, pp. 437–562. Peleg, M. 1995. Description of mechanical changes in foods at their glass region. In G.V. Barbosa-Canovas and Welti-Chanes (Eds.) Food Preservation by Moisture Control: Fundamentals and Applications. Technomic Publishing, Lancaster, PA. Peleg, M. 1996. On modeling changes in food and biosolids at and around their glass transition temperature range. Crit. Rev. Food Sci. Nutr., 36,49–67. Pendyala, V.S. Devotta, and V.S. Patwardhan, 1986. The economics of heat pump assisted drying systems. J. Heat Recov. Sys., 6, 433–442. Pfost, H.B., S.G. Maurer, D.S. Chung and G.A. Milliken. 1976. Summarizing and Reporting Equilibrium Moisture Data for Grains. A.S.A.E. Paper 76-3520. 1976 ASAE Meeting, St Joseph, MI. Ponting, J.D., G.G. Watters, G.G. Forrey, R.R. Jackson, and R. Stanley. 1966. Osmotic dehydration of fruits. Food Technol., 20, 125. Rahman S. 1995. Food Properties Handbook. CRC Press, Boca Raton, FL. Rao, M.A. 1992. Transport and storage of food products. In D.R. Heldman and D.B. Lund (Eds.) Handbook of Food Engineering, Marcel Dekker, New York, pp. 225–245. Ratti, C. 1991. Design of Dryers for Vegetable and Fruit Products. PhD. thesis (in Spanish). Universidad Nacional del Sur, Bahía Blanca, Argentina. Ratti, C. 1994. Shrinkage during drying of foodstuffs. J. Food Eng., 23(1), 91–105. Ratti, C. and G.H. Crapiste. 1992. A generalized drying curve for shrinking food materials. In A.S. Mujumdar (Ed.), Drying ’92, Elsevier Science, Amsterdam, pp. 864–873. Ratti, C., G.H. Crapiste, and E. Rotstein. 1989. A new water sorption equilibrium expression for solids foods based on thermodynamics considerations. J. Food Sci., 54(3), 738–742. Ratti C. and A.S. Mujumdar. 1993. Fixed-bed batch drying of shrinking particles with time varying drying air conditions. Drying Technol., 11(6), 1311. Ratti, C. and A.S. Mujumdar. 1995. Infrared drying. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying. 2nd ed., Marcel Dekker, New York, pp. 567–588. Reynolds, S. 1993. So Easy to Preserve, Bulletin 989, 3rd ed., Cooperative Extension Service. The University of Georgia, College of Agricultural and Environmental Sciences. Rogachev, V.I. and I.I. Kislenko. 1972. Combined Methods of Decreasing Water Activity in Fruits under Preservation. Acta Aliment., Hung. 2, 245. Román, G.N., E. Rotstein, and M.J. Urbicain. 1979. Kinetics of water vapor desorption from apples. J. Food Sci. 44(1), 193–197. Román, G.N., M.J. Urbicain, and E. Rotstein. 1982. Moisture equilibrium in apples at several temperatures: experimental data and theoretical considerations. J. Food Sci. 47(5), 1484–1488. Roos, Y. 1995. Phase Transitions in Foods. Academic Press, London. Roos, Y. and M. Karel. 1991a. Applying state diagrams to food processing and development. Food Technol., 45, 66–70, 107. Roos, Y. and M. Karel, 1991b. Plasticizing effect of water on thermal behavior and crystallization of amorphous food models. J. Food Sci., 56, 38–43. Rotstein, E. and A.R.H. Cornish. 1978. A transport phenomena analysis of air drying of foodstuffs. In A.S. Mujumdar (Ed.), Proceedings of the 1st International Symposium on Drying, Montreal, Canada, Science Press, Princeton, NJ. Saca, A.S. and J.E. Lozano. 1992. Explosion puffing of bananas. Int. J. Food Sci. Technol. 27, 419–426. Sadikoglu, H. and A.I. Liapis. 1997. Mathematical modelling of the primary and secondary drying stages of bulk-solution freeze-drying in trays: parameter estimation and model discrimination by comparison theoretical results with experimental data. Drying Technol., 15(3, 4), 791–810. Salunkhe, D.K., H.R. Bolin, and N.R. Reddy. 1991. Storage, Processing, and Nutritional Quality of Fruits and Vegetables, 2nd ed., Vol. 1 and 2. CRC Press, Boca Raton, FL.

158

Processing Fruits: Science and Technology, Second Edition

Sandu, C. 1986. Infrared radiative drying in food engineering: a process analysis. Biotechnol. Progr., 2(3), 109–119. Saravacos, G.D. and S.E. Charm. 1962. A study of the mechanism of vegetable and fruit dehydration. Food Technol., 16, 78. Schenz, T.W. 1995. Glass transitions and product stability — an overview. Food Hydrocolloid., 9(4), 307–315. Schiffmann, R.F. 1987. Microwave and dielectric drying. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying, Marcel Dekker, New York, pp. 327–356. Sharma, N.K. and C.P. Arora, 1995. Influence of product thickness, chamber pressure and heating conditions on production rate of freeze-dried yoghurt. Int. J. Refrig., 18(5), 297–307. Shatadal, P. and D.S. Jayas. 1992. Sorption isotherms of foods. In A.S. Mujumdar (Ed.), Drying of Solids, Science Publishers, New York, pp. 433–448. Sheehan P. and A.I. Liapis. 1998. Modeling of primary and secondary drying stages of freeze drying of pharmaceutical products in vials: numerical results obtained from solution of dynamic and spatially multi-dimensional lyophilization model for diffrent operational policies. Biotechnol. Bioeng., 60(6), 712–728. Shi, X.Q. and P.F. Maupoey. 1993. Vacuum osmotic dehydration of fruits. Drying Technol., 11(6), 1429. Shi, J., M. Le Maguer, Y. Kakuda, A. Liptay, and F. Niekamp, 1999. Food Res. Int., 32(1), 15–21. Shishegarha, F., J. Makhlouf, and C. Ratti. 2002. Freeze-drying characteristics of strawberries. Drying Technol. Siddappa, G.S. and A.M. Nanjundaswamy. 1960. Equilibrium relative humidity (ERH) relationships of fruit juices and custard powders. Food Technol., 14(10), 533–537. Singh, R.P. 1992. Heating and cooling processes for foods. In D.R. Heldman and D.B. Lund (Eds.), Handbook of Food Engineering, Marcel Dekker, New York, pp. 247–255. Singh, R.P. and J.D. Mannapperuma. 1990. Developments in food freezing. In H.G. Schwartzberg and M.A. Rao (Eds.), Biotechnology and Food Process Engineering, Marcel Dekker, New York, pp. 309–329. Sokhansaj, S. and D.S. Jayas. 1987. Drying of Foodstuffs. In A.S. Mujumdar (ed.) Handbook of Industrial Drying. 1st edition,. Marcel Dekker, New York, pp. 517–554. Somogyi, L.P. and B.S. Luh. 1986. Dehydration of Fruits. In J.G. Woodroof and B.S. Luh (Eds.), Commercial Fruit Processing, 2nd ed., AVI Publishing, Westport, CT, pp. 353–405. Slade, L. and H. Levine. 1991. Beyond water activity: recent advances based on an alternative approach to the assessment of food quality and safety. Criti. Rev. Food Sci. Nutr. 30, 115–360. Stromman, I. 1986. Energy optimal temperature differences in heat pump dryers. In A.S. Mujumdar (Ed.), Drying of Solids, John Wiley & Sons, Chichester, U.K. Strumillo, C. and Lopez-Caicedo, C. 1987. Energy aspects in drying. In A.S. Mujumdar (Ed.), Handbook of Industrial Drying, 1st ed., Marcel Dekker, New York, pp. 823–862. Suzuki, K., K. Kubota, T. Hasegawa, and H. Hosaka. 1976. Shrinkage in dehydration of root vegetables. J. Food. Sci., 41, 1189. Thompson, T.L. 1972. Temporary storage of high-moisture shelled corn using continuous aeration. Trans. ASAE, 15, 333. Unadi, A., R.J. Fuller, and R.H. Macmillan. 2002. Strategies for drying tomatoes in a tunnel dehydrator. Drying Technol., 20(6). Van Arsdel, W.B. 1963. Food Dehydration. Vol. 1. Principles. AVI Publishing, Westport, CT. Van Arsdel, W.B., M.J. Copley, and A.I. Morgan. 1973. Food Dehydration, 2nd ed., AVI Publishing, Westport, CT. Van der Berg, C. 1986. Water activity. In D. MacCarthy (Ed.), Concentration and Drying of Foods, Elsevier Applied Science, London, pp. 11–36. Van Meel, D.A. 1958. Adiabatic convection batch drying with recirculation of air. Chem. Eng. Sci., 9, 36. Wills, R.H.H., T.H. Lee, D. Graham, W.B. McGlasson, and E.G. Hall. 1981. Postharvest: An Introduction to the Physiology and Handling of Fruit and Vegetables. AVI Publishing, Westport, CT. Wolf, W., W.E.L. Spiess, and G. Jung. 1985. Sorption Isotherms and Water Activity of Food Materials, Elsevier, New York, pp. 1–5. Xiufan, G., M. Grüneberg, and H. Schubert. 1993. Influence of Types of Water Bindings on the Microwave Heating of Foods (in German). Symposium of Internationl Seminar, University of Karlsruhe, pp. 137–147.

Drying of Fruits

159

Yoon, K.Y., K.S. Kim, K.H. Lee, and S.R. Shin. 1997. Changes of quality in the osmotic dehydration of cherry-tomatoes and optimisation of the process. J. Korean Soc. Food Sci. Nutr. 26(5), 866–871. Zanoni, B., C. Peri, G. Giovanelli, R. Nani, and B.J. Bieche. 1999. Study of oxidative heat damage during tomato drying. Acta-Hortic., 487, 395–399. Zylla, R. and C. Strumillo. 1987. Heat pumps in drying. In A.S. Mujumdar (Ed.), Drying ’87, Hemisphere, New York.

8 Fruit Freezing David S. Reid and Diane M. Barrett CONTENTS 8.1 8.2

Introduction ........................................................................................................................162 The Freezing Process .........................................................................................................162 8.2.1 The Influence of Temperature Change................................................................162 8.2.2 The Freezing Profile ............................................................................................162 8.2.3 The Freezing Process Described by Phase Diagram ..........................................163 8.2.4 Freezing in Tissue Systems .................................................................................164 8.2.4.1 Freezing in the Presence of Cells......................................................165 8.2.5 The Freezing Process and Freezing Damage......................................................165 8.2.5.1 Osmotic Damage................................................................................165 8.2.5.2 Solute-Induced Damage.....................................................................165 8.2.5.3 Structural Damage .............................................................................166 8.3 Industrial Freezing Methods ..............................................................................................166 8.3.1 Freezers ................................................................................................................166 8.3.2 Prefreezing Handling and Preparation ................................................................167 8.4 Freezing Methods for Specific Fruits ................................................................................167 8.4.1 Apple....................................................................................................................167 8.4.2 Apricot .................................................................................................................167 8.4.3 Avocado ...............................................................................................................168 8.4.4 Berries ..................................................................................................................168 8.4.5 Cherry ..................................................................................................................168 8.4.6 Coconut ................................................................................................................168 8.4.7 Cranberry .............................................................................................................169 8.4.8 Dates ....................................................................................................................169 8.4.9 Figs.......................................................................................................................169 8.4.10 Mango ..................................................................................................................169 8.4.11 Melon ...................................................................................................................169 8.4.12 Papaya ..................................................................................................................169 8.4.13 Peach ....................................................................................................................169 8.4.14 Pineapple..............................................................................................................169 8.4.15 Plum .....................................................................................................................170 8.4.16 Rhubarb................................................................................................................170 8.4.17 Strawberry............................................................................................................170 8.4.18 Tomato .................................................................................................................170 8.5 Effects of Freezing on Nutritional Components ...............................................................170 References ......................................................................................................................................171

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8.1 INTRODUCTION Most fruits have limited harvest periods. In order to have extended availability, some form of storage and preservation is needed. A variety of preservation systems exist, each of which results in an extended shelf life. Freezing provides a significantly extended shelf life and has been successfully employed for the long-term preservation of many fruits. In this chapter, we will discuss the application of freezing preservation to fruits. In order to do so, it is necessary to first discuss the freezing preservation process briefly and to consider the special problems of preservation that are presented by fruits.

8.2 THE FREEZING PROCESS Freezing involves the use of low temperatures. In general, reactions take place at slower rates as temperature is reduced. One of the more common temperature dependences of rate is expressed by the Arrhenius equation: log K = const – EA/RT where K is the reaction rate, EA is the activation energy, R is the gas constant, and T is the absolute temperature. Based on this equation, the reduction in rate can be quantified through the activation energy. All other things being equal, therefore, storage at a low temperature would give an extended storage life, and the lower the temperature the better. All other things are not equal, however. The Arrhenius expression describes the temperature dependence of reaction rates when the mechanism of the reaction does not significantly change. It also describes single reaction rates and does not necessarily describe the temperature dependence of a series of reactions that have different individual temperature dependences. Furthermore, the Arrhenius expression does not describe reaction rates where the direction of the equilibrium changes, for example, phase change, where the favored form is temperature-dependent. For all of these reasons, it is necessary to examine the effect of temperature change in rather more detail.

8.2.1 THE INFLUENCE

OF

TEMPERATURE CHANGE

As temperature is lowered, many processes will slow. In tissue systems, there may be changes in membrane and organelle properties that produce altered metabolic pathways. Should this happen, the mere act of chilling can produce product quality loss, known as chilling damage (Lyons, 1973; Wilson, 1987), If chilling damage is not a problem, refrigeration close to the freezing point can lead to a significant extension of shelf life. Freezing and the freezing point are the next sources of complications to the simplified “lower temperatures give longer storage” picture. Why should this be? Freezing implies phase change. The aqueous component of the tissue separates into at least two phases, one of which is ice. Because some of the water has separated out as ice, the remaining liquid phase has to have increased solute concentrations. The presence of ice and the increase in solute concentration can have significant effects upon the state of the tissue that is being frozen (Brown, 1979; Reid, 1983). Let us therefore follow a freezing process in some detail, from this particular viewpoint.

8.2.2 THE FREEZING PROFILE In order to freeze, heat must be removed from the product. Figure 8.1 shows a typical freezing profile for a point close to the surface of a product. In region A, the temperature is falling but is still above the freezing point. At the surface, as cooling progresses, the temperature will reach the freezing point. Due to difficulty in seeding ice, freezing does not immediately initiate. The

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temperature

A

C

B D

time

FIGURE 8.1 Schematic cooling curve. See text for an explanation of segments.

temperature continues to fall. At some point, seeding (or nucleation) initiates freezing, and the temperature rises to close to the freezing point. This is region B in the plot. Closer to the center of the product, this undercooled region is not seen, and freezing initiates at the freezing point. As heat continues to be removed, the temperature now falls more slowly. The reason for the slower fall, given that the rate of heat removal is unchanged, is that heat is released by the phase change from water to ice. This heat, termed latent heat, is additional to the sensible heat loss that accompanies temperature change. This region of slower temperature change due to the heat release of ice formation is region C in the plot. As the process continues, the rate of ice formation decreases, the contribution of latent heat decreases, and the temperature begins to fall more rapidly. This is region D. Many workers, e.g., Persson and Lohndahl (1993), have labelled these regions as follows: region A — prefreezing, regions B and C — freezing, region D — reduction to storage temperature.

8.2.3 THE FREEZING PROCESS DESCRIBED

BY

PHASE DIAGRAM

Another view of this freezing process can be obtained through the use of a simplified phase diagram. If we assume that in the initial freezing process only ice will separate out, we can utilize the schematic binary phase diagram of Figure 8.2 to help describe the process. The product composition

FIGURE 8.2 Schematic phase diagram for a binary system. See text for an explanation of labels.

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is assumed to be represented by X. Initially, as the product is cooled, the composition stays unchanged. Segment PQ represents this initial cooling and corresponds to region A of Figure 8.1. Region B, the period of undercooling prior to the initiation of crystallization, is represented by the short section QR that lies below the liquidus curve (the liquidus curve shows the concentration dependence of the melting point, indicating the one temperature at which a solution of a given composition and ice can coexist in equilibrium). When freezing initiates, the system separates into two phases: (1) ice, represented by the left axis (i.e., 100% water) and (2) a more concentrated solution, where the concentration is defined by the liquidus coordinate for that temperature. This is represented by the short curve from R to Q¢, where Q¢ is a port on the liquidus curve QT. The curved segment of the liquidus from Q¢ to S illustrates the change in concentration of the nonice matrix as we move through region C. The segment of the liquidus from S to T is region D, where the rate of ice formation has reduced. A useful property of a phase diagram of this type is that it allows for the estimation of the amount of ice at any temperature. For example, the line S¢ S at temperature T1 crosses the lute PX at point L. For our system of overall composition X, at temperature T1, the ratio of the amount of ice to the amount of solution of composition represented by point S is simply S/LS¢. The overall composition is still X. This diagram shows that, as the temperature decreases, the amount of ice increases, and the composition of the unfrozen phase increases. At some temperature and liquid phase composition, a second phase may start to separate out, yielding what is termed a eutectic mix. This would happen at point E, where the solubility curve of the crystallized material intersects the liquidus. Solute crystallization does not always take place, due to kinetic constraints (Franks, 1982). Should this prove to be the case, the unfrozen phase continues to cool until it crosses a kinetic threshold and becomes effectively solid (in a glassy state). Since this glassy phase is produced by the freeze-concentration process just described, it is often referred to as the glassy state of the maximally freeze-concentrated matrix. The temperature of transformation to this glassy matrix can be measured and has some significance for frozen storage stability because it has been suggested that, once the unfrozen matrix enters the glassy state, the rate of change in storage will significantly reduce (Levine and Slade, 1989; Slade and Levine, 1991). The relevance of this in producing freezing is discussed briefly in Reid (1990). Current research in my laboratory seeks to establish this critical temperature for many frozen products, including fruits.

8.2.4 FREEZING

IN

TISSUE SYSTEMS

Up to this point, the discussion has been of freezing in a uniform system. Consider now the freezing process as it might occur in plant tissue. In addition to the complexities introduced by the formation of ice as temperature is lowered, an additional complexity is introduced. In plant tissues there are cells with cell walls. In other words, there are at least two distinct environmental situations. There are cell interiors that are individual, separate entities, and there are the extracellular spaces in between that exist in a connected network. This additional state, “inside or outside,” interacts with the phase state of the aqueous system. The interaction depends upon the properties of the cell wall boundaries. If the cell wall and cell membranes are intact and in functional form, they provide a barrier that is permeable only to certain small molecules, including water. If this barrier is somehow damaged, molecular movements through the barrier become much easier. Damage to the barrier can result from a variety of causes. In processing, the most common cause of damage is heat treatment, such as might be applied in blanching. If the barrier is intact, it will allow the process of osmosis (or selective water transfer) to occur between the cell contents and the external environment of the cell. This assumes particular relevance if the external environment is changing due to ice formation. A damaged barrier does not support osmotic processes. The process of osmosis is the passage of solvent through a barrier permeable to solvent but not to solute, in such an amount as to tend to equalize the concentration of the solutions on either side of the barrier.

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8.2.4.1 Freezing in the Presence of Cells Let us look at the freezing process again, taking into account the presence of cells. The first cooling, A, is the same. Once we reach regions B and C, the presence of cells changes the detailed picture. Ice forms external to cells, in general, because even if some ice growth initiates within a cell, it can only reach another cell by growing into the external matrix between cells. If ice is in the external matrix and the cell wall barrier is intact and effective, ice does not penetrate into the cell. Because water can permeate through the membrane, an osmotic process will occur. Water will leave the cell, forming additional extracellular ice and, at the same time, increasing the concentration of the internal cell contents in the direction of the concentration of the external unfrozen matrix. As the temperature falls and the external unfrozen matrix concentration increases as described by the liquidus line in the phase diagram, the concentration of the internal medium will tend to increase in the same manner. The maximum rate at which water can leave the cell is important to the effectiveness of this process, as this governs the maximum rate at which the concentration of the internal medium can increase. If water cannot be exported sufficiently rapidly, the internal contents will be more dilute than required for equilibrium (described by a coordinate point below the liquidus). The contents are therefore undercooled by an amount described by the difference between t h e liquidus temperature coordinate for the actual internal solution composition and the actual internal temperature. If the undercooling exceeds a threshold value characteristic of the particular tissue, internal seeding of ice and consequent ice growth may occur. Once ice forms within the cell, the concentrations of the internal and external unfrozen matrices match, and there is no longer an osmotic driving force for water transfer. The system-dependent variation in cross-barrier water transport rates of the osmotic process accounts, in part, for the differences between fast freezing and slow freezing. In fast freezing, there is insufficient time to remove the water from the cell through osmosis. The cell contents undercool and seed, and ice forms within the cell. In slow freezing, there is enough time to r e m ove t h e appropriate amount of water from the cell. The concentration of the cell contents increases sufficiently rapidly to prevent the cell contents from being significantly undercooled. Ice does not form within the cell. Note that, if any c h a n g e should occur on freezing that would prevent this water from returning to the cell on thawing, then the water will become a source of drip loss. It may be for this reason that drip loss is often more marked in slowly frozen fruits. Fast freezing and slow freezing are therefore operational definitions, and the threshold freezing rate separating fast freezing from slow freezing will be system dependent.

8.2.5 THE FREEZING PROCESS

AND

FREEZING DAMAGE

8.2.5.1 Osmotic Damage When heat is removed rapidly, ice forms rapidly. These ice crystals tend to be small. Because the ice grows rapidly, the concentration of the external unfrozen matrix rises rapidly. Osmotic transfer of water is limited: the cells freeze internally, and little water translocates. In slow cooling, the ice forms slowly, external to the cells, and there is sufficient time for a large amount of osmotic transfer of water from the cells. This results in cell shrinkage that can damage the membranes (Meryman, 1971; Steponkus, 1984). A considerable amount of water translocates. D u e t o c e l l wa l l damage consequent upon the freezing process, this water does not return to the cells on thawing but, rather, becomes drip loss. 8.2.5.2 Solute-Induced Damage In addition to this cell shrinkage mechanism for damage, primarily linked to the extensive cellular dehydration accompanying slow freezing, there are other mechanisms of damage. The high-solute concentrations of the unfrozen matrix, in particular the high salt concentrations, can cause damage to many polymeric cell components and may kill the cell (Mazur, 1977). To prevent this, some

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form of solution-based protection might be needed (Meryman et al., 1977). A typical method for reducing salt concentration-induced damage is to add sugars to the aqueous phase that is undergoing “freeze-concentration.” Note that these sugars must actually be incorporated into the solution that is freezing. It is not enough to add the sugar to the overall system. The concentration effect is present whether freezing is fast or slow. 8.2.5.3 Structural Damage In fast freezing, additional to the concentration effect, the formation of ice within the cell may cause damage to the delicate organelle and membrane structures of the cell. As one consequence, enzyme systems may be dislocated. This may result in uncontrolled enzyme action, leading to a variety of effects, including the production of off-flavors. Prevention of such enzyme-mediated damage can be achieved by utilizing blanching, a prefreezing heat treatment that denatures the enzymes and, hence, terminates their catalytic activity; however, it has to be remembered that blanching, because it is a heat treatment, will influence the semipermeable properties of the cell membrane and also destroy cell turgor. Cell turgor is an important component of the eating quality of many fruits. It is produced by the internal pressure of the cell contents. Lack of turgor is perceived as softness and lack of crispness and juiciness. Where turgor is an important product characteristic, blanching may not be an acceptable procedure, and other steps may be necessary to control enzymically initiated degradative processes. Blanching is not the only cause of reduced turgor. If cells become leaky or lose some of their contents, turgor is reduced or destroyed, and the texture of the fruit becomes much softer (Brown, 1977; Mohr, 1971). A loss of turgor caused by freezing is particularly evident in fruits such as strawberry (Szczesniak and Smith, 1969). Loss of turgor due to processing procedures is of most relevance to fruits that are customarily eaten raw, rather than fruits that are customarily cooked. Cooking, a more severe thermal treatment than blanching, destroys turgor so that the retention of turgor through earlier processing procedures is not necessary. Familiarity with the molecular picture of the freezing process is necessary if we are to appreciate the sources of the freezing damage that results in a reduction in consumer-perceived quality in comparison to the fresh raw product. Through an awareness of the mechanisms of damage it is possible to identify whether careful design and control of the freezing processes applied to the product might avoid or minimize some of the quality degradation.

8.3 INDUSTRIAL FREEZING METHODS 8.3.1 FREEZERS It is now appropriate to consider the industrial freezing processes that are applied to fruit products. In order to remove heat, the product must be brought into contact with a cold medium. This can be cold air, in a blast freezer, cryogenic liquids or gases in a cryogenic freezer, or cold surfaces in a plate freezer. The heat transfer mechanism in cold air is through convection. This can be assisted by blowing the air over the product. Cryogenic gases are similarly blown past the product and, due to their lower temperature, result in more rapid heat transfer. Cryogenic liquids have a higher thermal density and remove heat more rapidly due to an improved heat transfer capability. Heat transfer to a cold surface tends to be by conduction, and its effectiveness depends on the quality of the thermal contact. Reid (1991) discusses briefly the many types of commercial freezers that exist and that can be utilized in the freezing of plant tissues. This discussion includes consideration of the methods by which large quantities of the product can be exposed to freezing conditions, utilizing both batch and continuous methods. A more extensive discussion of freezers is given by Persson and Londahl (1993). The choice of appropriate freezer is, in part, governed by the product size and, in part, by the required freezing rate. Small fruits, which need to be frozen rapidly, might be frozen in a low-temperature, high air velocity blast freezer or in a cryogenic freezer to produce

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an individually quick frozen (IQF) product. Fruits packed in large cans or drums, on the other hand, will not freeze quickly due to heat transfer limitations and so are usually frozen in a cold room with a high-capacity cooling unit and reasonable air circulation. Note that it is poor practice to freeze such product in a cold storage room. A separate room should be set aside for product freezing. Retail packs of approximately rectangular brick shape are often frozen in plate (i.e., conduction) freezers. This method is reasonably successful, provided that the package is well filled. Air spaces within the package will slow down heat removal significantly.

8.3.2 PREFREEZING HANDLING

AND

PREPARATION

Prior to entering the freezer, the fruit must be prepared appropriately. Methods of preparation are specific to the individual fruit, but there are some common factors that can be identified. The preparation begins at harvest. Poor handling at this stage can irreversibly degrade final product quality. Methods of harvesting are discussed in several major textbooks, for example, Woodroof and Luh (1975) and Tressler et al. (1968). Acceptable harvesting methods are designed to minimize mechanical damage to the fruit. After harvest, cleaning will be required. The method of cleaning is fruit specific. Sorting and trimming will also be required to remove undesirable materials. Many fruits are peeled, or they may be sliced. A variety of specially designed pieces of equipment exists for these tasks. It is often necessary to remove field heat rapidly prior to these process steps, especially for highly perishable fruits, if unacceptable damage is to be avoided. The processor has the responsibility of delivering the fruit to the freezing equipment at as high a quality level as is practical.

8.4 FREEZING METHODS FOR SPECIFIC FRUITS In this section, details of the processing procedures for selected fruits are given. The information summarized in this section comes from a variety of sources, including Woodroof and Luh (1975), Tressler et al. (1968), and TRRF (1993).

8.4.1 APPLE Not all varieties of apple result in an acceptable product when frozen, whether the end use is for the bakery trade or for other purposes; however, some cultivars of apple are suitable for freezing. These are sorted, washed, peeled, cored, and sliced. The sliced fruit is treated to minimize enzymatic browning. This can be achieved by application of antioxidant solutions, including some proprietary treatments. One common treatment is salt brining, soaking slices in a 1% salt solution in order to remove intercellular air. Ascorbic acid solutions can also be used, but these tend to be expensive. A surface blanch, using steam or boiling water, can also be employed, though it results in a softened slice that may not be suited for some uses. The treated slices in a ratio of five parts fruit to one part dry sugar are frozen in large 30- to 50-lb containers in a blast freezer below –10∞F. Some apples may be frozen using a dehydrofreezing process where about 50% of the water is removed from the apple slices by standard drying equipment prior to freezing the slices.

8.4.2 APRICOT Though some apricots are frozen whole for later processing, the major proportion of apricots are usually frozen as peeled apricot halves. This enhances the tendency for browning and, therefore, requires steps to be taken to minimize browning. The apricots are peeled, halved and pitted, and dipped in ascorbic acid solution to minimize browning, or blanched for a short time to inactivate the enzymes. The halves are packed in sugar or sugar syrup prior to freezing at a 3:1 or 4:l ratio of fruit to sugar. Air blast freezers are adequate. It is best to freeze the apricots on trays or on a

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belt prior to packing into barrels or 30-lb containers. This helps minimize discoloration. Storage should be below 0∞F. For good retention of ascorbic acid, storage should be at –20∞F.

8.4.3 AVOCADO Avocados present a challenge to the commercial freezer due to their high oil content that readily becomes rancid, and also because of a very active oxidative browning system. Pureed avocado is a successful product. Preservation life is enhanced by lowering the pH of the puree to below 4.5 through the addition of lemon juice, lime juice, and salt. Packaging under nitrogen also enhances shelf life. Vacuum packaging has also been employed. Any reasonably rapid freezing method can be employed. Storage should be around 0∞F for a reasonable shelf life.

8.4.4 BERRIES Many varieties of berries are frozen. Berries can be frozen in syrup or as individual berries. As individual berries, they may be tray frozen or IQF frozen on a belt in an air blast or cryogenic freezer. Individually frozen berries will be discussed after bulk methods for freezing for retail or the processing trade. Red raspberries for retail are packed in an approximately 50% syrup, in the proportion of six parts berry to four parts syrup, and 10- and 16-oz containers are used. Any reasonably rapid freezing method may be employed. Black raspberries are used for further processing and are packed in 30-lb containers or larger. In order to successfully freeze berries in a large container, the following procedures have been shown to be necessary (TRRF Commodity Storage Handbook, 1993). 1. 2. 3. 4.

The temperature of the fruit should not exceed 60∞F at the time of filling. The containers should be moved to the freezer as quickly as possible. The temperature on entering the freezer should be below 70∞F. Freezer conditions (air temperature < –15∞F and airflow velocity high) should allow for the center of the container to reach a temperature of 32∞F or less within 48 h. 5. Freezing should be continued until the center temperature is 0∞F. This should take no more than 4 to 5 d. 6. Storage should be below 0∞F. Reasons for these recommendations were discussed previously in the text. Blackberries, boysenberries, loganberries, and others are frozen utilizing the same procedures as have been described for raspberries. Blueberries are frozen in 20-lb containers, with steps being takers to minimize or eliminate air in the package.

8.4.5 CHERRY The major portion of cherries frozen are tart cherries, though some sweet cherries are also frozen. The procedures for freezing are essentially the same. Tart cherries are harvested when bright red, sweet cherries when mature. The cherries are held and transported in ice-cold water, which reduces losses due to crushing and bruising and makes the fruit firmer for pitting. Fruits are size graded, pitted, packed with sugar in large cans, and frozen in a blast freezer.

8.4.6 COCONUT Shredded coconut can be frozen without any particular preparation. The rate of freezing is not critical so long as cooling is sufficiently rapid to minimize microbiological contamination. Storage, in large containers, is at 0∞F.

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8.4.7 CRANBERRY Cranberries are frozen at 0∞F using conventional techniques. The majority of the frozen crop is used for processing.

8.4.8 DATES Fresh dates may be frozen. The use of a good moisture-proof and vapor-proof wrapping is recommended to prevent moisture loss during freezing or storage.

8.4.9 FIGS Figs can be frozen as whole fruit in heavy syrup or as sliced fruit as four parts fruit to one part water. Standard freezing methods are employed. Storage temperatures should be below 0∞F.

8.4.10 MANGO Mango is frozen as slices in syrup. The syrup contains ascorbic acid to inhibit polyphenol oxidaseinduced browning. In addition, mango puree is a significant frozen product. Purees can be single or double strength. Storage should be at or below 0∞F. Browning can be a significant problem at higher storage temperatures due to nonenzymic browning.

8.4.11 MELON Melon is frozen when the texture is firm enough to allow for cutting of cubes or balls that retain their integrity. If too ripe, a very mushy product will result because fully thawed melon loses considerable texture. Melon is usually frozen in syrup.

8.4.12 PAPAYA Papaya puree is prepared from ripe papaya. Steamed fruit can be sliced and crushed, and the pulp can be separated from the skin. The acidified pulp is passed through a heat exchanger to inactivate enzymes before cooling and freezing to –10∞F.

8.4.13 PEACH In general, freestone peaches are used for freezing. Yellow fleshed varieties are preferred for better texture and lower susceptibility to oxidative browning. Fruit for freezing is usually harvested while still firm and then ripened under control. The peaches are pitted, peeled, and sliced prior to freezing. The usual pack is in syrup (one part syrup to five parts peach) containing around 250 ppm ascorbic acid to help protect against browning. Freezing is usually in packages; 32- to 40lb packs are common. Larger barrels are also available. Freezing methods are, in general, as described for other bulk frozen fruit products. Some lQF slices are frozen for special markets. Storage should be at temperatures below 0∞F if extended shelf life is required. The limiting change is the browning.

8.4.14 PINEAPPLE Pineapple for freezing is prepared in the same way as pineapple for canning. Rectangular chunks are filled in syrup into cans or bulk containers and frozen. The cans are frozen in a blast tunnel, the bulk containers in a blast freezer. The Smooth Cayenne variety should be frozen. The Red Spanish variety has a tendency to develop off-flavors on freezing.

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8.4.15 PLUM A small volume of purple plums and prunes are frozen for institutional markets and for further processing. The fruit is halved, pitted, and packed in syrup in barrels. Freezing is by standard methods. Storage is at or below 0∞F.

8.4.16 RHUBARB Rhubarb freezes easily and requires no special treatment, though a short blanch can extend the storage life significantly. Rhubarb can be frozen with or without sugar. Stalks are trimmed to fit the package. Storage life at 0∞F is at least 6 months.

8.4.17 STRAWBERRY Not all varieties of strawberry freeze well. The selection of varieties for freezing should be made in conjunction with agricultural advisors familiar with the production state. Strawberries are frozen in several forms, depending on the final end use. Most strawberries are frozen as a raw material for use in further processing. Depending on the final product, different freezing procedures might be appropriate. For use in jam manufacture or ice cream, strawberries are packed in syrup and frozen. This can be in 30-lb tins or 50-gal barrels. The strawberries may be sliced and sugared for this process. The procedures described under berries are appropriate. Because strawberries are even more fragile than many other berries, it is recommended that the critical times be shorter. For example, a core temperature of 15∞F should be reached in no more than 24 to 36 h. Storage should be at 0∞F or below for a reasonable shelf life. Flavor and color are lost rapidly if the storage temperature is too high. Prefreezing treatments may be applied to strawberries to stabilize the integrity of the tissues. Suutarinen et al. (2000) studied the effects of various calcium chloride or sucrose prefreezing treatments on the textural integrity and drip loss in frozen strawberries. These authors examined CaCl2 concentration of the dipping solution (1, 5.5, or 10 g/l), dipping time (0.25, 7.625, or 15 min) and solution temperature (25, 37.5, or 50∞C) and found the greatest firmness resulted from the combination of 5.5 g/l CaCl2 applied for 7.625 min at 37.5∞C. Crystallized sucrose was compared to dips in water–sucrose solutions (350 and 700 g sucrose/l) and dipping times of 1 and 15 min were utilized. Sucrose prefreezing treatments also resulted in greater cellular integrity, with those strawberries sprinkled with crystalline sucrose having the highest firmness. IQF methods are used to produce frozen whole strawberries for both institutional and retail trade. Freezing utilizes air blast, liquid nitrogen, or carbon dioxide belt freezers. Storage of IQF fruit should be at a stable, low temperature to prevent clumping of the berries (due to moisture migration) and loss of the lQF character.

8.4.18 TOMATO Whole tomatoes are not an item of frozen commerce. They lose turgor and, hence, texture on freezing and are no longer suited to the uses common for fresh tomatoes. Chopped or pureed whole tomatoes can be frozen and stored for 6 to 9 months at 0∞F for use in further processing. Other tomato products such as purees, sauces, and pastes can readily be frozen. They are commonly employed as ingredients in other frozen products. Freezing provides an advantage of color stability compared to other storage methods.

8.5 EFFECTS OF FREEZING ON NUTRITIONAL COMPONENTS Consumers are becoming increasingly aware of the importance of nutritional components in their diets, and the potential for fruit and vegetables in particular, to provide beneficial health effects.

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Fruits are a relatively significant source of various antioxidant compounds, including the polyphenolics, carotenoids, and vitamins. Preservation of fruit by freezing, and the effect of this process on various antioxidant components, have been the subject of many recent investigations. A select group of publications on this topic will be highlighted in this section. Asami et al. (2003b) evaluated the effects of storage at refrigeration and frozen temperatures on the concentration of total phenolics in clingstone peaches. Maturity stage III peaches of the Ross variety were peeled, pitted, sliced, and frozen at –12∞C for a period of 3 months. There appeared to be a statistically significant increase (P < 0.05) in total phenolic content following freezing, and this higher content was retained after 2 and 3 months of frozen storage. It was postulated that the freezing process may have resulted in cellular disruption and more facilitated extraction of phenolics. The effect of freezing and frozen storage on raspberry phytochemicals and volatiles was the subject of two manuscripts by de Ancos and colleagues (de Ancos et al., 2000a, 2000b). These authors compared two early-season and two late-season raspberry cultivars and found differential effects of freezing. In the early-season cultivars, freezing resulted in increased anthocyanin content, while in the late-season cultivars, which initially had higher concentrations of anthocyanins, freezing caused an overall reduction. The authors suggested that the preservation of anthocyanins during freezing depends on the pH of the fruit, organic acid content, sugar concentration, initial anthocyanin concentration, and initial cyaniding-3-glucoside content. They did not find a relationship between polyphenol oxidase activity and anthocyanin content. De Ancos and colleagues (2000b) also found that freezing had a slight effect on ellagic acid, vitamin C, and total phenolics, depending on the raspberry cultivar. Free radical scavenging capacity was decreased as a result of the freezing process, anywhere from 4 to 26%, again related to cultivar. Frozen storage of raspberries at –20∞C for a 1-year period did not appear to affect total phenolics or free radical scavenging capacity, but did cause a decline in ellagic acid vitamin C. In another study of the effects of freezing on raspberry phenolics, ellagitannins, flavonoids, and antioxidant capacity (Mullen et al., 2002), these authors found that the antioxidant capacity of the fruit and vitamin C levels were not affected by freezing. The raspberry cultivar used in this study differed from those evaluated by de Ancos, however, and this may have affected the results. Freezing preservation of fruit and vegetables is less destructive toward some antioxidant compounds, in particular total phenolics and ascorbic acid, than other means of preservation. One illustration of this is a recent publication (Asami et al., 2003a) in which Marionberries, strawberries, and corn were preserved using freezing, freeze-drying, and air-drying methods. The highest levels of both total phenolics and ascorbic acid (reduced form) were consistently found in the extractions of frozen samples, followed by those of freeze-dried and then air-dried samples. Freezing may cause some damage to cell structure, and application of a drying procedure following freezing, even though this is under vacuum at reduced temperatures, may result in even greater losses of beneficial nutrients. Air-drying at temperatures above 60∞C may result in oxidative condensation or decomposition of thermolabile compounds, such as (+)catechin and ascorbic acid. Therefore, the presence of total phenolics and ascorbic acid in the air-dried products was lower than that in either frozen or freeze-dried products.

REFERENCES Asami, D.K., Hong, Y.-J., Barrett, D.M., and Mitchell, A.E. 2003a. A comparison of the total phenolic and ascorbic acid contents of freeze-dried and air-dried Marionberry, strawberry, and corn grown using conventional, organic, and sustainable agricultural practices. J. Agric. Food Chem., 51, 1237–1241. Asami, D.K., Hong, Y.-J., Barrett, D.M., and Mitchell, A.E. 2003b. Processing-induced changes in total phenolics and procyanidins in clingstone peaches. J. Sci. Food Agric., 83, 56–63. Brown, M.S. 1977. Texture of frozen fruits and vegetables. J. Texture Stud., 7: 491–494. Brown, M.S. 1979. Frozen fruits and vegetables: Their chemistry, physics, and cryobiology. Adv. Food Res., 25: 181–235.

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de Ancos, B., Ibanez, E., Reglero, G., and Cano, M.P. 2000a. Frozen storage effects on anthocyanins and volatile compounds of raspberry fruit. J. Agric. Food Chem., 48, 873–879. de Ancos, B., Ibanez, E., Reglero, G., and Cano, M.P. 2000b. Ellagic acid, Vitamin C, and total phenolic contents and radical scavenging capacity affected by freezing and frozen storage in raspberry fruit. J. Agric. Food Chem., 48, 4565–4570. Franks, F. 1982. The properties of aqueous solutions at subzero temperatures. In Water: A Comprehensive Treatise, Vol. 7, F. Franks (Ed.), Plenum, New York, pp. 215–338. Levine, H. and Slade, L. 1989. A food polymer science approach to the practice of cryostabilization technology. Comments Agric. Food Chem., 1: 315–396. Lyons, J.M. 1973. Chilling injury in plants. Annu. Rev. Plant Physiol., 24: 445–466. Mazur, P. 1977. The role of intracellular freezing in the death of cells cooled at supraoptimal rates. Cryobiology, 14: 251–272. Meryman, H.T. 1971. Osmotic stress as a mechanism of freezing injury. Cryobiology, 8: 489–500. Meryman, H.T., Williams, R.J., and Douglas, M.St.J. 1977. Freezing injury from solution effects and its prevention by natural or artificial cryoprotection. Cryobiology, 14: 287–302. Mohr, W.P. 1971. Freeze thaw damage to protoplasmic structures in high moisture edible plant tissue. J. Texture Stud., 2: 316–327. Mullen, W., Stewart, A.J., Lean, M.E.J., Gardner, P., Duthie, G.G., and Crozier, A. 2002. Effect of freezing and storage on the phenolics, ellagitannins, flavonoids, and antioxidant capacity of red raspberries. J. Agric. Food Chem., 50: 5197–5201. Persson, P.O. and Londahl, G. 1993. Freezing Technology. In Frozen Food Technology, C.P. Mallett (Ed.), Blackie Academic and Professional, London, pp. 20–58. Reid, D.S. 1983. Fundamental physiochemical aspects of freezing. Food Technol., 37: 110–115. Reid, D.S. 1987. The freezing of food tissues. In The Effects of Low Temperature on Biological Systems, B.W.W. Grout and G.J. Morris (Eds.), Edward Arnold, London, pp. 478–488. Reid, D.S. 1990. Optimizing the quality of frozen foods. Food Technol., 44: 78–84. Reid, D.S. 1991. Freezing. In Vegetable Processing, D. Arthey and C. Dennis (Eds.), Blackie, Glasgow, Scotland, pp. 102–122. Reid, D.S. 1993. Basic physical phenomena in the freezing and thawing of plant and animal tissues. In Frozen Food Technology, C.P. Mallett (Ed.), Blackie Academic and Professional, Glasgow, Scotland, pp. 1–19. Slade, L. and Levine, H. 1991. Beyond water activity: Recent advances based on an alternative approach to the assessment of food quality and safety. Crit. Rev. Food Sci. Nutrition, 30: 115–360. Steponkus, P.L. 1984. Role of the plasma membrane in freezing injury and cold acclimation. Annu. Rev. Plant Physiol., 35: 543–584. Suutarinen, J., Heiska, K., and Autio, K. 2000. The effects of calcium chloride and sucrose prefreezing treatments on the structure of strawberry tissues. Lebensm.-Wiss. Technol., 33: 89–102. Szczesniak, A.S. and Smith, B.J. 1969. Observations on strawberry texture, a three pronged approach, J. Texture Stud., 1: 65–89. Tressler, D.K., van Arsdel, W.B., and Copley, M.J. (Eds.). 1968. The Freezing Preservation of Foods. 4 volumes, AVI, Westport, CT. TRRF. 1993. Commodity Storage Handbook. The Refrigeration Research Foundation, Washington, D.C. Wilson, J.M. 1987. Chilling injury in plants. In The Effects of Low Temperatures on Biological Systems, B.W.W. Grout and G.J. Morris, (Eds.), Edward Arnold, London, pp. 271–292. Woodroof, J.G. and Luh, B.S. 1975. Commercial Food Processing. AVI, Westport, CT.

9 Thermal Processing of Fruits Hosahalli S. Ramaswamy CONTENTS 9.1 9.2

Introduction ........................................................................................................................174 Thermal Process Considerations........................................................................................174 9.2.1 Thermal Resistance of Microorganisms and Enzymes.......................................175 9.2.1.1 Oxygen, pH and Temperature Sensitivity .........................................175 9.2.1.2 Microflora in Canned Fruits ..............................................................176 9.2.1.3 Composition of Fruits ........................................................................177 9.2.1.4 Microbial Destruction Kinetics .........................................................177 9.2.1.5 Heat Penetration Curve......................................................................180 9.2.1.6 Heat Penetration Parameters..............................................................181 9.3 Thermal Process Calculations............................................................................................181 9.3.1 Original General Method.....................................................................................181 9.3.2 Improved General Method ..................................................................................183 9.3.3 The Formula Methods .........................................................................................183 9.3.3.1 Ball’s Method.....................................................................................183 9.3.3.2 Stumbo’s Method ...............................................................................184 9.4 Process Calculation Methodology for Pasteurization of Fruits ........................................184 9.5 Fruit Canning Operations...................................................................................................187 9.5.1 Raw Material Selection .......................................................................................188 9.5.2 Washing................................................................................................................188 9.5.3 Sorting/Grading....................................................................................................189 9.5.4 Blanching .............................................................................................................189 9.5.5 Prevention of Fruit Browning..............................................................................190 9.5.6 Peeling/Preparation ..............................................................................................191 9.5.7 Filling...................................................................................................................191 9.5.8 Exhausting and Vacuum ......................................................................................193 9.5.9 Can Seaming and Closing ...................................................................................193 9.5.10 Container Coding.................................................................................................194 9.5.11 Retort Operations.................................................................................................194 9.5.12 Cooling.................................................................................................................195 9.5.13 Labeling and Storage...........................................................................................195 9.6 Quality Control of Canned Fruits......................................................................................196 9.7 Grades for Canadian and U.S. Canned Fruits...................................................................196 9.8 Nomenclature .....................................................................................................................198 References ......................................................................................................................................199

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9.1 INTRODUCTION Since ancient times, several techniques have been used to preserve fruits and their products: drying, concentration, freezing, fermentation, and chemical preservation by use of vinegar, wine, sugars, and spices. Thermal processing is relatively a more recent technique used for fruit preservation, but it has proven to be one of the most effective (Woodroof and Luh, 1986). Nicholas Appert, in 1809, was awarded a prize by the French government for developing a new method for heat preservation of food. The method has since been recognized as Appertization (named after Appert), sterilization, or more commonly, canning. Appert utilized his method to preserve more than 50 kinds of foods, some of which included fruits, vegetables, and meats. In the early stages of the method, food was filled into wide-mouthed glass bottles or jars and carefully corked, after which they were cooked in boiling water. The process time varied according to the product type. Although the method was successful in producing shelf-stable foods which did not undergo spoilage, the scientific basis for the preservation was not really known until almost half a century later, when another Frenchman, Louis Pasteur, discovered that food spoilage was caused by microorganisms which are destroyed at elevated temperatures. The technique which bears his name, pasteurization, as we know today, refers to a relatively mild heat treatment intended to destroy pathogenic microorganisms in foods, providing short-term extension of shelf life under refrigerated conditions. By the turn of the century, significant progress achieved in the microbiology of food spoilage, understanding of thermal destruction kinetics of microorganisms, and heating behavior of packaged foods led to scientific approaches in thermal process calculations. The intervening period also saw significant developments in the manufacturing of thermal processing equipment in the form of improved versions of retort systems. Next in the line of process equipment were continuous systems for thermal sterilization of food cans and glass jars and systems for handling high-temperature short time (HTST) processes in batch or continuous modes in still or rotary autoclaves. Developments such as aseptic processing and packaging, thin profile processing, fully automated agitating retorts, and retort systems based on different media have revolutionized the food industry in the last few decades. New processes such as combined methods technology (Alzamora et al., 1993) are continually being introduced, especially for heat-sensitive products, such as delicate fruits and fruit juices, to preserve overall color, flavor, and other quality attributes. Fruits, in general, are commodities with special organoleptic properties which must be carefully preserved when establishing operating conditions for a thermal process. Although the quality of canned fruits is generally considered to be inferior to that of frozen fruits, the total U.S. per capita consumption of canned fruits in 1992–1993 was reported to be approximately four times that of dried and frozen fruits together (Almanac, 1993). Per-capita consumption of frozen and dried fruit was 4.66 and 3.24 lb (product weight basis), respectively, whereas the total was 14.40 lb for canned fruits. A 1989 survey conducted in 2000 American households (Market Reserve Corporation of America Information Service) indicated that the greatest per-capita consumers of canned fruit were those in the 0 to 5 and +55 age groups. The tendency for canned fruit consumption lies in lower income groups and those who demand more convenience type foods. The California canned-fruit industry, which is one of the most competitive leaders in the industry, has an estimated value of $600 million for canned peaches, pears, apricots, and fruit cocktail (Moulton, 1992).

9.2 THERMAL PROCESS CONSIDERATIONS Canning is the most commonly used technique to heat-sterilize foods in order to prevent microbiological and enzymatic spoilage. A variety of foods are canned such as meats, fish, poultry, fruits, vegetables, dairy, and vegetable products. Heat processes used for these foods are dependent on the type of food, its chemical composition and type(s) of microorganism(s) that cause spoilage or

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public health concern, in addition to properties related to container material, shape, and size as well as the properties related to the heating medium. Any thermal process for a food should be designed to achieve three basic objectives, the most important being to reduce the number of microorganisms to statistically small levels, whether they are of public health concern or of the spoilage type which cause off-flavors and odors. The second objective is to create an environment in the container which would suppress the growth or activity of spoilage type microorganisms by utilizing one or more of the following methods: (1) oxygen removal, (2) pH control, and (3) control of storage temperature. The third objective is to assure an adequate or hermetic seal of the container to prevent recontamination following processing and during storage. Thermal processing is not intended to completely sterilize the packaged food. Such an approach might produce a stable product, but it will be at the expense of severe destruction of product quality. The success of thermal processing depends on selectively destroying the microorganisms of spoilage and public health concern while creating an environment around the product to minimize the growth and activity of other microorganisms. In order to determine the extent of heat treatment, several factors must be known (Fellows, 1988): (1) The type and the heat resistance of the target microorganism, spore, or enzyme present in the food; (2) The pH of the food; (3) The storage conditions following the process; (4) The heating conditions; and (5) The thermophysical properties of the food, and container shape and size.

9.2.1 THERMAL RESISTANCE

OF

MICROORGANISMS

AND

ENZYMES

9.2.1.1 Oxygen, pH and Temperature Sensitivity Target microorganisms and enzymes for thermal destruction in a food vary according to the type of food and its composition. Thus, these target components and their respective thermal resistance determine the thermal process itself. The first step in designing the thermal process is to verify the type of test microorganism or enzyme upon which the process should be based. Several factors should be considered in this aspect. For example, hermetic conditions that are used to package foods with extremely low oxygen levels (under vacuum such as in sous vide products) provide an atmosphere which does not support the growth of microorganisms that require oxygen (obligate aerobes) to cause food spoilage or public health problems. Further, the spores of obligate aerobes are less heat resistant than the microbial spores that grow under anaerobic conditions (facultative or obligate anaerobes). The growth and activity of these anaerobic microorganisms are largely pH dependent. Consequently, the most important factor affecting microbial spoilage is acidity, and heat processing requirements for various foods depend mainly on pH. From a thermal processing standpoint, foods are categorized into three pH groups: (1) high-acid foods (pH < 3.7), (2) acid or medium-acid foods (3.7 < pH < 4.5), and (3) Low-acid foods (pH > 4.5). The pH classification of thermally processed foods is extremely important, especially for the determination of processing requirements and specific criteria. The establishment of the resulting thermal process is generally based on Clostridium botulinum, which is a highly heat-resistant, rodshaped, spore-forming, anaerobic pathogen. The mere presence of the pathogen alone in a product does not constitute a health hazard; however, concern in this respect is warranted if the spores are permitted to germinate in favorable conditions for growth and produce a deadly botulism toxin. It has been generally recognized that C. botulinum does not grow and produce toxin below a pH of 4.6, which is the pH of most fruits and fruit products. Berries and most fermented foods are included in this category because they are considered high-acid foods (pH < 3.7). Consequently, the designated pH to separate the low acid group from the acid group is set at 4.5 such that in the mediumand high-acid foods (pH < 4.5), C. botulinum would not pose any potential threat. On the other hand, in the low-acid foods (pH > 4.5), the most heat-resistant spore former, C. botulinum, may cause foodborne illness and death following production of the fatal toxin. This can easily occur under anaerobic conditions that prevail inside a sealed container, provided, of course, the pathogen

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is not destroyed by the heat treatment. There are other microorganisms, for example, Bacillus stearothermophilus, Bacillus thermoacidurans, and Clostridium thermosaccolyaticum, that are more heat resistant than C. botulinum. These are generally thermophilic in nature (optimal growth temperature ~ 50 to 55∞C) and hence are of little concern if the processed cans are stored at temperatures below 30∞C. The phrase minimal thermal process was introduced by the U.S. Food and Drug Administration in 1977 and defined as “the application of heat to food, either before or after sealing in a hermetically sealed container, for a period of time and at a temperature scientifically determined to be adequate to ensure the destruction of microorganisms of public health concern.” For low-acid foods, C. botulinum is designated the microorganism of public health concern. Due to its high heat resistance, temperatures of 115 to 125∞C are commonly employed for the processing of these foods. Thermal processes for acid foods and medium-acid foods are usually based on the destruction of heatresistant spoilage-type vegetative bacteria, yeasts, or molds, or the inactivation of enzymes at temperatures below 100∞C. Thus, thermal processes for such foods are normally carried out in boiling water. If not inactivated following thermal processing, several heat-resistant enzymes in fruits (including peroxidase, pectinesterase, lipoxygenase, catalase, and polyphenoloxidase) may cause undesirable quality changes in the final canned fruit product during storage, especially related to color, texture, and flavor attributes. For the thermal processing of acid foods (including pasteurization), the inactivation of these enzyme systems is often used as a basis because they usually possess a higher thermal resistance than the microorganisms present in the food. As peroxidase is known to have a very high heat resistance, its destruction is often used as an adequate marker for the destruction of all heat-resistant enzymes present in the food. Nevertheless, the heat resistance of enzymes varies with the fruit, its variety, pH, and TSS (total soluble solids) (Ranganna, 1986). Despite the established processing criteria for acid foods and high-acid foods, fruits that receive a heat treatment such as blanching or cooking have recently been designated as potentially hazardous by the Food and Drug Administration Retail Foods Branch (Madden, 1992). All fruits are subject to a variety of conditions during growth, harvest, and distribution, all of which provide sources of microbial contamination. Although most raw fruits possess a protective barrier that prevents penetration of potentially pathogenic microbes, it is recognized that thermal processing may destroy this barrier, allowing potential pathogens to penetrate the produce and to grow in conditions conducive to their growth. In order to prevent contamination and foodborne illnesses, it is recommended that chlorinated water be used to remove soil from produce and that proper refrigeration, storage, and shipping conditions be maintained. 9.2.1.2 Microflora in Canned Fruits Spoilage of fruits in cans is mainly caused by microorganisms that are sensitive to heat in humid conditions and are readily destroyed at temperatures between 60 and 100∞C within a few minutes. Spore-forming bacteria such as Bacillus thermoacidurans and Clostridium pasteurianum may be found in acid foods but are destroyed by sterilization temperatures. The former is an aerobic thermophile which produces flat sour. The latter is a spore-former anaerobic saccharolytic gas producer. Nonsporulating bacteria include Lactobacillus and Leuconostoc that occur in stewed fruits and sweet and sour cucumbers and that cause can-bulging due to carbon dioxide production. They are the two most resistant types of nonsporulating bacteria that require heating to 88∞C or a hold at 64.5∞C for 10 min. Yeasts that are of major concern are those that have low heat resistance and that cause spoilage in canned foods only in cases of gross underprocessing or can leakage resulting from faulty seams. A 5-min exposure to 66∞C destroys living forms and the same exposure time at 80∞C destroys spores. Molds in canned fruits are quite insignificant because some are destroyed together with spores in 30 min at 65 to 70∞C, the effect being greater in anaerobic conditions. Some molds

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possess an extremely high resistance to heat in canned acid foods, and these can be found in the species Byssochlamys, Paecilomyces, and Phialophora. Byssochlamys fulva is exceptionally heat resistant as it breaks down pectinous materials disintegrating fruit and sometimes producing gas. Its limited ability to grow in the presence of air permits growth in products such as sterilized compotes, jams, and fruit preserves. It has been found that B. fulva is more heat resistant in foods containing malic acid, citric acid, and tartaric acid as compared to foods containing higher quantities of lactic acid and acetic acid (Kyzlink, 1990). Obviously, the heat process for the preservation of fruits will not necessarily kill spore-forming microorganisms such as C. botulinum that will not normally grow in acidic conditions but may nevertheless still be present in the acidic food as viable bacterial spores even after the product has undergone an adequate thermal process for the destruction of aciduric bacteria, yeasts, and molds. The thermal resistance of microorganisms in fruits is dependent upon various factors such as the amount and type of sugar present, the pH, and type of acid (Desrosier, 1977). Organic acids have a detrimental effect on microorganisms due to the toxicity of the hydrogen ion and the undissociated molecule. Lower pH levels are more toxic to bacteria, which explains the addition of an acidulant to adjust the pH to a standard value often allowing for a shorter sterilization time. In some instances, however, decreasing the temperature is also possible. Different acids possess various levels of effectiveness in lowering the heat resistance of microorganisms. This order is lactic > citric > acetic. Based on the pH of the product, the order is acetic > citric > lactic (Ranganna, 1986). 9.2.1.3 Composition of Fruits The acids present in fruits are advantageous for preservation, especially because they have a bacteriostatic effect. In good-quality fruit ready for processing, the organic acids most frequently encountered are citric, malic, and tartaric acids, although quinic acid is sometimes present in considerable quantities in apples and pears (Kyzlink, 1990). In blueberries, 40% of the organic acids present is quinic acid. Lactic and acetic acid are also present in varying amounts. Malic acid is present in significant amounts in stone fruits such as cherries, apples, and apricots. Citric acid is found in berries, red currants, strawberries, raspberries, and citrus fruits. Organic acids reach peak levels in fruit just as they reach the stage of ripeness. Consequently, normal pH values in fruits correspond to an organic acid content of between 0.3 and 3.1% (averaging approximately 1%). Malic acid, for example, reaches a peak of 3% in apples at ripeness. The organic acid content will tend to decrease in several types of fruit at the end of the ripening period through conversion to sugars. Thus, the acid content decreases while sugar content increases. During storage, the acid is consumed through respiration. Fruits are rich in sugars; however, their levels depend on a variety of factors, such as species, soil, location, and the ripening stage. Sugar levels generally range between 0.5 and 25%. At first when the fruit is detached from the mother plant, sugar levels decrease due to the increase in respiration rate (cells consume sugar). In the maturing fruit, the total sugar content rises due to two main reasons: (1) hydrolysis of polysaccharides and (2) formation of sugars as secondary products following acid conversion. 9.2.1.4 Microbial Destruction Kinetics In order to establish the thermal processing schedule consisting of an appropriate heating time at a specified temperature, the thermal destruction rate of the test microorganism or enzyme must be determined under the conditions that normally prevail in the container. In addition, the temperature dependence of the microbial destruction rate or enzyme inactivation rate must be known in order to integrate the destruction or inactivation effect through the temperature profile, mainly due to the come-up time (CUT) required to achieve processing temperatures.

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Number of Survivors

105

104

103 D 102

101 0

2

4

6

8

10

Heating Time (min)

FIGURE 9.1 A typical survivor curve.

Survivor curves and D value: The thermal destruction of microorganisms generally follows a first-order reaction or a logarithmic order of death. The logarithm of the surviving number of microorganisms following a heat treatment at a particular temperature is plotted against heating time to give a straight line “survivor” curve. Thus, the same percentage of surviving microorganisms will be destroyed in each successive unit of time irrespective of the number present. The microbial destruction rate is defined as a decimal reduction time (D value) which is the heating time in minutes at a given temperature required to result in one decimal reduction (or 90% destruction) in the surviving microbial population. This is depicted graphically as the time between which the survival curve passes through one logarithmic cycle (Figure 9.1). The same concept may be extended to enzyme inactivation kinetics where a heat-resistant enzyme in a fruit product is subjected to a thermal process at a particular temperature. The resulting plot of residual enzyme activity vs. time would yield the D value of the enzyme, which is the heating time in minutes required to inactivate 90% of total enzyme activity. Thermal death time (TDT) and D value: The thermal death time (TDT) is the heating time required to cause microbial death or destruction and is obtained by subjecting the microbial population to a series of heat treatments at a given temperature and testing for survivors. Theoretically, complete destruction of a microbial population or inactivation of enzymatic activity is not possible because a decimal fraction of the population would ultimately survive even after numerous D values. The microbial death would indicate the failure to show positive growth in a subculture media after heating. The TDT value differs from the D-value approach in that the former depends on the initial microbial load, whereas the D value does not. In practice, calculated fractional survivors are treated by a probability approach; for example, a surviving population of 10–8/unit would indicate one survivor in 108 units. Thermal inactivation time (TIT) of enzymes: The heat resistance of enzymes is dependent on a variety of factors, some of which include the fruit and its variety, the pH, and TSS. A thermal inactivation curve is established in a manner similar to that of the TDT curve for bacteria. This is done by subjecting the food sample to a series of heat treatments at a specific temperature and testing for residual enzyme concentration. When there is no measurable residual activity, the enzyme is considered inactivated and the time taken as TIT. Since most enzyme systems in fruits (peroxidase, pectinesterase, and polyphenoloxidase) possess higher thermal resistance than microorganisms, the calculation of the process time for canned fruits is based on the TIT of the most heat-resistant enzyme present in the product. Temperature dependence and z value: The D value of a particular microorganism or enzyme is strongly dependent on the processing temperature. Increased heating temperatures

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D Value, TDT or TIT (min)

103

102

101 z 100 220

230

240

250

260

270

Temperature (°F)

FIGURE 9.2 A typical thermal resistance curve.

clearly yield lower D values. A thermal resistance curve with logarithm of D values plotted against temperature would yield a temperature sensitivity indicator, z value, which is defined as the temperature range that results in a 10-fold change in D values. Graphically represented (Figure 9.2), the z value indicates the temperature range between which the D value curve passes through one logarithmic cycle. Lethality concept: The effectiveness of the thermal process in killing microorganisms in a food product may be denoted by the F value or the lethality. The lethality or sterilizing value of a thermal process is a measure of the effectiveness of the heat treatment. In order to compare the relative sterilizing capacities of different heat processes, a unit of lethality has been established as the equivalent heating of 1 min at a reference temperature (such as 250∞F for sterilization or 180∞F for pasteurization). Heating times at other temperatures (FT) can be converted to equivalent minutes (Fo) at the reference temperature using Equation (9.1): Fo = FT 10(T – To)/z

(9.1)

In thermal processes which involve a food product passing through a time–temperature profile, the lethal effects at each time–temperature combination may be integrated using the following equation: Fo = Ú 10(T – To)/z dt

(9.2)

The resulting combined lethality is known as the process lethality and may be represented by the symbol Fo. The term always applies to a specific location in the product container or the slowest heating point (cold spot). For conduction heating foods, the cold spot is the thermal center, whereas for convection heating foods, it is located approximately 1/10th the length of the can from the bottom. Generally, it is assumed that if the cold spot receives an adequate process lethality, then all other points within the container will have received an equal or greater process lethality. From a microbiological safety point of view, the assurance of a minimal lethality at the cold spot is of utmost importance, whereas from a quality standpoint it is desirable to minimize the overall destruction throughout the container. The criteria for the adequacy of a thermal process is based on microbiological considerations, especially for low-acid foods for which the minimal criterion for processing is the destruction of C. botulinum spores. It has been established that this minimum process should be severe so as to reduce the population of C. botulinum through at least 12 decimal reductions (bot cook). A decimal

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FIGURE 9.3 Schematic of thermal processing principles.

reduction time of 0.21 min at 250∞F is normally assumed for C. botulinum (Stumbo, 1973). A 12decimal reduction would thus be equivalent to an Fo value of 12 ¥ 0.21 = 2.52 min. Therefore, the minimal process lethality (Fo) required for all low-acid foods is 2.52 min; however, several lowacid foods are processed beyond this minimum value. An Fo value of 5 min is perhaps more common for these foods simply due to the occurrence of more heat-resistant spoilage-type of microorganisms that are not of public health concern. In the case of acid foods such as fruits, the criterion for the adequacy of the process is based more specifically on the reduction in the number of spoilage-causing bacteria and the inactivation of the most heat-resistant enzymes present. The two objectives of thermal process calculations are (1) to estimate how much destruction a given process will accomplish and (2) to arrive at an appropriate process time required to accomplish the desired level of destruction or inactivation (in the case of enzymes). In order to attain these two objectives, thermal destruction kinetics and the product heating profile must be obtained. The basis for establishing a thermal process can be represented as shown in Figure 9.3. Oxygen is intentionally excluded from the cans so as to eliminate aerobic bacterial growth. The pH of food differentiates the severity of the process (sterilization vs. pasteurization). Processed cans are recommended to be stored in a cool place to reduce spoilage by thermophilic bacteria. 9.2.1.5 Heat Penetration Curve In order to establish a thermal process schedule, the temperature history of the product and the thermal resistance characteristics of the test microorganism (z and Fo) are required. The temperature history of the product undergoing the process depends on several factors:

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181

1. The heating process: still vs. agitated cook; in-package vs. aseptic processing 2. The heating medium: steam, water (immersion or spray) with or without air over pressure, steam/air 3. The heating conditions: retort temperature, initial temperature, loading pattern 4. Product type: solid, semisolid, liquid, particulate liquid; thermophysical properties of the product 5. Container type, shape, and size The heating process may include in-package, which is the conventional way (e.g., canning), or aseptic processing, which involves heating and cooling a liquid product (with or without particulates), and then filling into sterile packages and sealing under aseptic conditions. The latter process generally offers a higher quality product due to rapid heating and cooling. Container agitation is another method that promotes higher quality and shorter process times through mixing of the contents and more rapid, uniform heating (Eisner, 1988). The product type greatly influences the mode of heat transfer and the temperature history of the product. Due to a variety of ingredients in many heat processed foods, packaged foods on the market rarely heat by pure conduction or pure convection. Generally, conduction-heating products that are tightly packed and quite viscous do not move within the container during heating or cooling. They exhibit an initial lag followed by a straight-line heating curve. Examples include creamy soups, purees, fruit jams, and fruit compotes. Convection heating products include those which are more loosely packed and may contain particulates. The semilog curve is generally steeper than that of conduction-heating products. Examples include thin soups with vegetables, peas in brine, and fruit slices or halves in syrup solution. Some products show a broken heating behavior. They heat by convection at the beginning of the process and then change to conduction following a certain temperature. Examples include products containing starch or other thickening agents. Basically, process calculation methods can be divided into two major types: (1) general methods and (2) formula methods. General methods, the first scientific approaches to calculate the adequacy of thermal processes, are based on destruction data of a particular test organism and are involved in a graphical or numerical integration procedure whereby time–temperature data during heat processing is used to integrate lethal effects at the cold spot in the container. Common temperature measuring devices known as thermocouples are placed at the appropriate cold spot and used to gather time–temperature data necessary for the calculations. Formula methods utilize heat penetration data obtained from experimental time–temperature data. 9.2.1.6 Heat Penetration Parameters Heat penetration parameters are obtained from a plot of the temperature difference (Tr – T during heating and T – Tw during cooling, on log scale) against heating time (on linear scale). The key parameters are the heating rate index, fh, and heating and cooling lag factors, jch and jcc. Details on calculating the various parameters are given in Nomenclature (see Figures 9.4 and 9.5):

9.3 THERMAL PROCESS CALCULATIONS The purpose of the thermal process calculations is to arrive at an appropriate process time under a given set of heating conditions to result in a given process lethality or, alternately, to estimate the process lethality of a given process.

9.3.1 ORIGINAL GENERAL METHOD This method is mostly of academic interest today; however, it has laid the foundation for all subsequent process calculation methods. Originally described by Bigelow et al. (1920), it is based

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Temperature Difference, Tr−T (log scale)

Processing Fruits: Science and Technology, Second Edition

2.25

log (Tr−Tpih)

2.00

jch =

Tr−Tpih Tr−Tih

log (Tr−Tih)

1.75 1.50 1.25 1.00 fh 0.75 0

2

4

6 8 10 12 14 16 18 20 Heating Time (min)

Temperature Difference, T−Tw (log scale)

FIGURE 9.4 Heating curve and heating parameters.

2.50

log (Tpic−Tw) jcc =

2.25

Tpic−Tw Tic−Tw

log (Tic−Tw)

2.00 1.75 1.50 1.25 fc 1.00 0

2

4

6 8 10 12 14 16 18 20 Cooling Time (min)

FIGURE 9.5 Cooling curve and cooling parameters.

on a graphical procedure for integrating the lethal effects of the time–temperature data at the cold spot in a container obtained for a given processing condition. It was based on the destruction data of a selected microorganism. The destruction kinetics are integrated over the heating process. In the original method, the heat penetration data is first obtained as a time–temperature curve. The temperatures are converted to lethal rates (reciprocal of TDT at the existing product temperature) to get a lethal rate vs. time curve. The area under the lethal rate curve so obtained represents the integrated sterilization value of the process which indicates the extent of destruction at the cold spot after the entire thermal process. Thus, a sterilization value of one indicates that the effect is the same as heating a spore suspension at any given temperature to a time equal to the TDT at that temperature. The sterilization value or value of unity is the minimal requirement with respect to a particular test microorganism. If the resulting unity value is greater than one or less than one, it indicates that the product is overprocessed and underprocessed, respectively. Thus, in the former case, the heating time should be reduced, and in the latter it should be increased. In this way, the process is repeated until the desired sterilization value is obtained and the corresponding process time is noted. This method is rather tedious and is somewhat limited to the experimental setup.

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Consequently, it has undergone several modifications that resulted in the development of the improved general method.

9.3.2 IMPROVED GENERAL METHOD The improved general method suggested by Ball (1923) employs the same graphical approach as the original method and the process calculations are quite similar. However, the improved general method is based on a hypothetical thermal destruction curve or reference TDT curve of the test microorganism with an Fo value of 1 min at the reference temperature. The z value is conventionally taken as 18∞F for sterilization and 10∞F for pasteurization; however, this value may vary depending on the product. The lethal rate or L value at any temperature may be calculated using the following equation: L = 10(T To)/z

(9.3)

The L value is the number of minutes at To equivalent to 1 min at temperature T. The time–temperature data from the heat penetration test are integrated with respect to the L values as calculated above. Generally, a numerical integration approach is later employed to facilitate computer calculation of the area under the curve that represents the accumulated lethality or process lethality (Fo) as shown by the following equation: Process lethality = Fo = Ú L dt

(9.4)

Briefly, the sum of the lethal effects of the entire process (heating and cooling) is expressed as the number of minutes at a reference temperature. Limitations of the general methods include tedious adjustments that need to be made with the process calculations.

9.3.3 THE FORMULA METHODS 9.3.3.1 Ball’s Method The formula methods are more versatile than the general methods and can accommodate several variables. They are based on characterizing heat penetration data and combining kinetic data with heat penetration parameters. Ball’s formula method in particular is one of the most widely used techniques for thermal process calculations. The method is based on the following equation derived from the heat penetration curve (using the same symbols as detailed earlier): B = fh log (jch • Ih/gc)

(9.5)

Heat penetration data is arranged in such a way that the log of the difference between the retort temperature and the product center temperature during heating is plotted against time. After an initial hyperbolic heating lag, the heating process is denoted by a logarithmic straight line. The time required for the straight-line portion of this curve to pass through one log cycle is the heating rate index, fh, and the lag factor is jch. The cooling curve is also constructed in a similar fashion, where the log of the difference between the product temperature and the cooling water temperature is plotted against the time after the steam is shut off and the product starts to cool. The cooling rate index, fc, and cooling lag factor, jcc, are calculated in a manner similar to fh and jch. Ball’s formula method is based on several assumptions: 1. Based on the reference TDT curve concept, the F value (or U) at the retort temperature is equivalent to the product of the desired process lethality Fo and Fi.

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2. 3. 4. 5. 6.

The The The The The

z value is 18∞F. heating rate index is equivalent to the cooling rate index. CUT effectiveness is 42%. cooling curve is initially hyperbolic followed by logarithmic. cooling lag factor may be approximated by an average jcc value of 1.41.

Several implications of Ball’s assumptions include: 1. 2. 3. 4.

The calculations are not valid for z not equal to 18∞F. If fc > fh (cooling is slower than heating), then there is a possibility of overprocessing. If fh > fc (heating is slower than cooling), an underestimation of the process is possible. If jcc > 1.41, there is a greater lag and the process is safe; however, overprocessing is quite likely. 5. If jcc < 1.41, the product is cooling much faster causing underprocessing since insufficient lethality is contributed by the cooling phase.

Despite some inaccuracies in the development of this method (Merson et al., 1978), Ball’s method is still the most widely used method for thermal process calculations in the food industry. 9.3.3.2 Stumbo’s Method Stumbo’s method is similar to Ball’s method in the sense that it is based on heating and cooling curves and assumes that fh is equal to fc. The difference lies in its being versatile in accounting for the lethality contributed by the cooling phase when the cooling lag factor jcc is not equal to 1.41, as assumed by Ball. Stumbo’s method accommodates several jcc values and z values as outlined in published tables (Stumbo and Longley, 1966). Stumbo’s method is very flexible; it can accommodate the destruction of bacterial spores and vegetative cells or nutrients, all of which possess different thermal resistances (D and z values). All types of thermal processes may be calculated; for example, the z value of a typical pasteurization process is 10∞F (vegetative bacteria), that of a sterilization process is 18∞F (C. botulinum), and that of nutrients is typically 40 to 50∞F. The steps followed are essentially similar to Ball’s procedure with the calculation of the process time and process lethality using similar equations. Table 9.1 shows the fh/U vs. g relationships when z = 10∞F necessary for typical pasteurization processes.

9.4 PROCESS CALCULATION METHODOLOGY FOR PASTEURIZATION OF FRUITS Heat processing is quite clearly the most destructive method of fruit processing. Since many fruits are high in organic acids such as lactic, acetic, and citric acid, their overall pH is quite low and they require much less rigorous heat treatments as compared to low-acid foods such as meats. In order to establish a processing schedule for canned fruits subjected to pasteurization, the most heat-resistant microorganism of public health concern in the container must be identified and evaluated for its thermal destruction rate. In most cases, however, it is the most heat-resistant enzyme system and vegetative bacteria that are used as the basis for establishing the thermal process. The thermal inactivation time for the particular enzyme, which is usually peroxidase, should be adequate to provide a microbiologically safe fruit product. In contrast to destruction of spores during sterilization processes, pasteurization processes target vegetative cells only. The D and z values of these vegetative cells are much smaller than those of spores. This decreased thermal resistance permits the use of lower processing temperatures. With reference to processing of fruits or pasteurization process, first, the reference temperature is changed from 250∞F to a lower value, typically 180∞F. In addition, the reference z value is changed

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TABLE 9.1 fh/U Relationships When z = 10∞F Values of g When j of Cooling Curve Is fh/U

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0.20 0.50 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00

2.68–05 2.01–02 0.282 1.14 1.83 2.33 2.71 3.01 3.25 3.47 3.67 3.84 5.22 6.27 7.14 7.87 8.51 9.07 9.56 10.0 10.4 13.0 14.3 15.2 15.8 16.3 16.8 17.1 17.4

2.78–05 2.21–02 0.294 1.17 1.88 2.41 2.81 3.15 3.43 3.68 3.90 4.11 5.67 6.81 7.72 8.49 9.15 9.72 10.2 10.7 11.1 13.7 15.2 16.1 16.8 17.4 17.8 18.2 18.5

2.88–05 2.40–02 0.305 1.19 1.92 2.48 2.92 3.29 3.61 3.89 4.14 4.38 6.12 7.34 8.31 9.10 9.78 10.4 10.9 11.4 11.8 14.5 16.0 17.0 17.8 18.4 18.9 19.3 19.7

2.98–05 2.60–02 0.317 1.21 1.97 2.55 3.03 3.43 3.78 4.10 4.38 4.64 6.57 7.88 8.89 9.72 10.4 11.0 11.6 12.0 12.5 15.2 16.8 17.9 18.7 19.4 19.9 20.4 20.8

3.07–05 2.79–02 0.329 1.24 2.01 2.63 3.14 3.57 3.96 4.30 4.62 4.91 7.01 8.41 9.48 10.3 11.1 11.7 12.2 12.7 13.1 16.0 17.7 18.8 19.7 20.4 21.0 21.5 22.0

3.17–05 2.99–02 0.340 1.26 2.05 2.70 3.24 3.72 4.13 4.51 4.85 5.17 7.46 8.95 10.1 11.0 11.7 12.3 12.9 13.4 13.8 16.8 18.5 19.7 20.6 21.4 22.1 22.6 23.1

3.27–05 3.18–02 0.352 1.29 2.10 2.77 3.35 3.86 4.31 4.72 5.09 5.44 7.91 9.48 10.7 11.6 12.3 13.0 13.6 14.1 14.5 17.5 19.3 20.6 21.6 22.4 23.1 23.7 24.3

3.36–05 3.38–02 0.364 1.31 2.14 2.85 3.46 4.00 4.49 4.93 5.33 5.70 8.35 10.0 11.2 12.2 13.0 13.6 14.2 14.7 15.2 18.3 20.1 21.5 22.6 23.4 24.2 24.8 25.4

3.46–05 3.57–02 0.376 1.33 2.19 2.92 3.57 4.14 4.66 5.14 5.57 5.97 8.80 10.6 11.8 12.8 13.6 14.3 14.9 15.4 15.9 19.0 21.0 22.4 23.5 24.5 25.3 25.9 26.6

Source: Adapted from Stumbo, C.R. 1973. Thermobacteriology in Food Processing. 2nd ed. Academic Press, Orlando, FL.

from 18∞F to 10∞F. Process calculations can then be performed either using the general or formula methods. A typical calculation using the numerical integration technique is shown in Table 9.2. The formula methods may also be employed using one of two approaches. The first approach is based on the criterion of achieving a certain minimal center point temperature in the product, for example 185∞F or 85∞C. Using the heat penetration parameters and the above criterion, the process time can easily be calculated using Ball’s formula: B = fh (log jchIh – log gc)

(9.6)

The parameters fh and jch are obtained from the heat penetration data, and Tih and Tr are known. The value for gc is also known because gc = Tr – 185 (using the above criterion). An example calculation for peach processing is shown in Table 9.3. The other approach using the formula method is based on Ball and Stumbo methods using a reference temperature of 180∞F and z value of 10∞F. Examples of process calculation for achieving an F10180 = 10 min are shown in Table 9.4 to Table 9.7.

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TABLE 9.2 Process Calculation by the Improved General Method (Numerical Integration Technique)a Time (min)

TEMP (∞F)

L = 10(T–180)/10

L¥t

SL¥t

–4 –2 0 (CUT) 1 2 3 4 5 6 7 8 9 10 11 12

80 91 130 145 160 170 180 188 195 190 185 170 160 140 130

0.000 0.000 0.000 0.000 0.010 0.100 1.000 6.310 31.62 10.00 3.160 0.100 0.010 0.000 0.000

0.000 0.000 0.000 0.000 0.010 0.100 1.000 6.310 31.62 10.00 3.160 0.100 0.010 0.000 0.000 FO, min

0.000 0.000 0.000 0.000 0.010 0.110 1.110 7.420 39.04 49.04 52.20 52.30 52.31 52.31 52.31 52.31

Example: pasteurization of canned apple slices Tr = 212∞F (boiling water). a

TABLE 9.3 Process Calculation Using Ball’s Formula Approacha 1. fh 2. jch 3. Retort temperature (Tr) 4. Initial temperature (Ti) 5. Ih = Tr – Ti 6. jchIh 7. log (jchIh) 9. Target temperature (Tc) 10. g = Tr – Tc 11. log (g) 12. log (jchIh) – log (g) 13. B = fh [log (jchIh) – log (g)] Conveyor speed = tunnel length/ process time (B) = 50/14.4 = 3.47 ft/min

17.6 min 1.16 212∞F 60∞F 152∞F 176 2.25 185∞F 27∞F 1.43 0.82 14.4 min

Example: Pasteurization of pouch-packed peach slices in syrup. Pouches subjected to pasteurization in a steam tunnel (Tr = 212∞F; 50-ft-long tunnel). The calculation computes the process time (conveyor speed) to achieve a target temperature of 185∞F in slices. a

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TABLE 9.4 Calculation of Process Time Using Ball’s Method

TABLE 9.5 Calculation of Process Lethality Using Ball’s Method

1. 2. 3. 4. 5. 6. 7. 8. 9 10. 11. 12.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

jch fh Process Lethality (Fo) Retort Temperature (Tr) Initial Temperature (Ti) Ih = Tr – Ti jch.Ih log (jch·Ih) Fi = 10[(180–Tr)/10] fh/U = fh/(F0 ¥ Fi) From Ball Table obtain log g B = fh [log (jch·Ih/g)]

1.46 30 min 40 min 200∞F 60∞F 140∞F 204.4 2.310 0.01 75 1.328 29.5 min

jch fh Process time Retort temperature (Tr) Initial temperature (Ti) Ih = Tr – Ti jch·Ih log (jch·Ih) Fi = 10[(180 – Tr)/10] log (g) = [log (jch·Ih) – B/fh] From Ball Table obtain fh/U Fo = fh/[fh/U ¥ Fi)

1.60 25 min 30 min 200∞F 60∞F 140∞F 224.0 2.350 0.01 1.150 24.367 102 min

TABLE 9.6 Calculation of Process Time by Stumbo’s Method 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13.

jch 1.5 fh 10.6 min Process lethality (Fo) 40 min Retort temperature (Tr) 200∞F Initial temperature (Ti) 56∞F Ih = Tr – Ti 144∞F jch·Ih 216 log (jch·Ih) 2.334 z 10∞F Fi = 10[(180 – Tr)/10] 0.01 fh/U = fh/[F0 – Fi) 26.5 jcc 1.8 From Stumbo’s Table for z = 10∞F (jcc = 1.8), obtain g value by interpolation fh/U g value 25.0 9.26 30.0 10.02 Interpolate 26.2 9.488 B = fh [log (jch·Ih/g) ] 14.4 min

9.5 FRUIT CANNING OPERATIONS The peach is the fruit which is canned most frequently and the major processors are the U.S. and Greece. In 1990, the U.S. canned peach production was 50% larger than that of Greece despite Greece being the largest exporter of canned peaches and apricots in the world. Compared to 1989 figures, the world production of peaches is expected to increase by 10% by the year 1995 (Moulton, 1992). The fruit is often inspected not only at the beginning of the process but also at various checkpoints during processing. Due to variations in the canning process for different types of fruits,

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TABLE 9.7 Calculation of Process Lethality by Stumbo’s Method 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15.

jch 1.75 fh 15.0 min Process time 20.0 min Retort temperature (Tr) 200∞F Initial temperature (Ti) 56∞F Ih = Tr – Ti 144∞F jch·Ih 252 log (jch·Ih) 2.401 z 10∞F Fi = 10[(250 – Tr)/z] 0.01 B/fh 1.333 log (g) = log (jch·Ih) – B/fh 1.0168 g 11.69 jcc 1.6 From Stumbo’s Table for z = 10∞F (jcc = 1.6), obtain fh/U by interpolation fh/U g value 50 11.57 60 12.33 Interpolate 51.7 11.69 51.8 Fo = fh/[fh/U) ¥ Fi] 29.0 min

the following operations are described in general for all fruits which are canned, with particular reference to specific fruits whenever appropriate. The flowchart for the typical operation is shown in Figure 9.6.

9.5.1 RAW MATERIAL SELECTION The quality of processed fruit depends largely on the quality of incoming fruit, and this in turn depends on how the fruit is harvested, handled, and stored (Prussia and Woodroof, 1986). Harvesting at the proper maturity is an important step in thermal processing of fruits. Plums, grapes, whole olives, gooseberries, brandied peaches, and maraschino cherries are harvested and processed in the “firm-ripe” stage. Exceptions include bananas, pears, and some apples that, when harvested at a mature stage, produce a higher quality processed product than those harvested at the “soft-ripe” stage (Wiley and Tolby, 1960). However, most types of fruits are canned in the “mellow-ripe” stage to capture maximum natural nutrients, flavor, aroma, and color. Different ingredients may be added such as spices, salt, sugar, colors, flavors, and nutrients (e.g., vitamin C) to compensate for the underdeveloped full natural flavor, aroma, and nutrient loss. Fruits that are needed to produce juices, purees, preserves, marmalades, and sauces are processed in the “soft-ripe” stage because flavor and aroma are more important than fruit texture (Prussia and Woodroof, 1986).

9.5.2 WASHING Fruits are washed with water to remove dust, dirt, and mold spores that will affect their color, aroma, and flavor. This process should be carried out thoroughly to also ensure the removal of the heat-resistant micromycete N. fischeri, which has been linked to mold formation in canned fruits

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Raw Material

Filling

Washing

Preparation (peeling, slicing, etc.)

Exhausting (vacuum)

Storage

Sorting/Grading

Blanching

Sealing

Thermal Processing

Labeling

Cooling

FIGURE 9.6 Typical fruit canning operations.

(Jesenka et al., 1991). In addition, the water is also used to cool the produce by removing the field heat following harvest. Fruits that are peeled, such as peaches, pears, apples, and apricots, are seldom washed before peeling (Prussia and Woodroof, 1986). On the other hand, washing after peeling removes vitamins and minerals and should be kept to a minimum. The volume of water required varies with the method of preparation of the fruit for canning and the kind of fruit (Lopez, 1987). Detergents are frequently used in the wash or rinse water. The water temperature should be kept low because it will keep the fruit firm and reduce leaching. The effectiveness of the washing operation depends on the amount, temperature, acidity, hardness, and mineral content of the water and the force at which it is applied. High pressure sprays are sometimes used; however, caution should be used so as to not injure the fruit. Browning may take place before peeling due to bruising, but is accelerated once the skin is broken or the tissue cells are ruptured (Prussia and Woodroof, 1986).

9.5.3 SORTING/GRADING This operation ensures the removal of inferior and/or damaged produce. An inspection belt may be used in addition to trained personnel who detect poor quality produce unsuitable for canning. Sorting labor costs may be reduced by new technologies that are noninvasive, such as magnetic resonance imaging (Moulton, 1992). In certain cases, produce may be sorted by its difference in optical properties for the determination of various chemical and physical measurements such as chlorophyll, yellow pigments, and brix (Dull et al., 1980). Sugar content may be measured using high-resolution magnetic resonance (Cho et al., 1991), and quality evaluation at this stage may also include the glycoside content of raw stone fruits such as cherries, peaches, and apricots, which may be associated with cyanogenesis in the final canned product (Voldrich and Kyzlink, 1992). Acceptable fruits are then size-sorted, where they are mechanically passed over a screen with different sizes of holes or slits. Undersized fruit may be sorted out and used as concentrate or for baby food (Moulton, 1992). Fruits with aesthetic defects may be used for juices and concentrates.

9.5.4 BLANCHING The blanching of fruits or a “partial cook” is an important step in the canning process, especially with respect to overall final quality evaluation. The product is usually immersed in hot water (88 to 99∞C) or exposed to live steam. It is an energy-intensive unit operation and can account for as

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high as one third of the total energy required for processing. Most of the water-soluble nutrients, such as ascorbic acid, are lost in this operation. During the blanching process, it is imperative that certain enzymes that have the potential to cause flavor and textural changes be inactivated. The process involves a brief heat treatment applied to most vegetables but also to some fruits in order to inactivate oxidative enzyme systems such as catalase, peroxidase, polyphenoloxidase, ascorbic acid oxidase, and lipoxygenase. When the unblanched tissue is disrupted or bruised and exposed to air, these enzymes come in contact with substrates, causing softening, discoloration, and the production of off-flavors. Since this action can potentially occur during the period prior to heat processing, it is most often standard practice to blanch fruits in order to prevent quality deterioration. Although the primary purpose of blanching is enzyme inactivation, especially with preservation techniques such as freezing and dehydration, there are several other benefits: blanching initially cleanses the product, decreases the microbial load, and preheats the product before processing. The mild heat treatment also softens the fruit, which facilitates compact packing in the can. At the same time, intercellular gases in the raw fruit are expelled, preventing excessive pressure buildup in the container and allowing for an improved heat transfer during heat processing. Consequently, a higher vacuum can be achieved in the final product as well as a reduction in internal can corrosion. For fruits that are blanched, the most common methods are conventional water and steam blanching. Water blanching is generally of the immersion type or could be a spray as the product moves on a conveyor. Steam blanching often involves belt or chain conveyors upon which the product moves through a tunnel containing live steam. Other methods less frequently used include hot gas blanching and microwave blanching. Adequacy of blanching is usually assessed based on inactivation of one of the heat-resistant enzymes (peroxidase or polyphenol oxidase).

9.5.5 PREVENTION

OF

FRUIT BROWNING

Some fruits cannot be subjected to blanching due to delicate tissues that may be disrupted during the process. They undergo alternate treatments that are used to prevent the oxidative browning which can occur due to exposure to oxygen, especially during peeling and slicing operations. Oxidative browning is caused by the action of oxidase with catechol tannins and is an important problem, especially in fruits such as peaches, apples, bananas, cherries, nectarines, apricots, grapes, and persimmons. Pineapples, tomatoes, and melons are not as prone to browning. The most common means of preventing browning in fruits are the following: SO2 or sulfite treatment: A 2000- to 4000-ppm SO2 solution (sodium or potassium metabisulfite) may be used to dip the fruits for approximately 2 to 5 min. SO2 may also be used as a gas dissolved in water (sulfurous acid) for dipping or in the gaseous form in fumigation chambers as is commonly done for grapes to control fungal growth prior to dehydration. The Food and Drug Administration has recommended a maximum residual sulfur dioxide level of 300 ppm in fruit juices and 2000 ppm in dried fruit (IFT, 1993). Acids: Acids used to raise acidity (acidifying antioxidants) include citric, fumaric, tartaric, acetic, phosphoric, ascorbic, and citraconic acid (Woodroof and Luh, 1986). For example, acidification with citric acid is especially important for sliced fruit. The cut surfaces may be protected from browning by immersion into a 1 to 2% solution of citric acid. Antioxidants: Ascorbic acid is a commonly used antioxidant in most fruit juices and canned fruits. Prestano and Manzano (1993) found ascorbic acid to be an effective inhibitor of peroxidase in selected fruits such as kiwi. It may be used on its own, in dry sugar/citric acid/ascorbic acid mixes, or in syrups. Citric acid is sometimes necessary because it acts as a stabilizer for the ascorbic acid. Usually, very small amounts are used that do not affect the taste of the final canned product. Ascorbic acid acts to reduce quinones, which are generated by polyphenol oxidase–catalyzed oxidation of polyphenols, back to phenolic compounds, preventing their conversion to brown pigments (IFT, 1993).

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Sugars: Although the main purpose of sugar is to sweeten the fruit, it is also used to prevent browning in peeled and sliced fruit, whether it is in dry or syrup form. It acts in inhibiting oxidation by partially excluding air in the tissues. This ability seems to increase with pH. In many cases, the sugar is mixed with ascorbic acid and citric acid as an effective agent against loss of texture, color, and flavor. Sapers et al. (1989) evaluated the performance of a variety of novel browning inhibitors in raw apple and the open cut surfaces of apples. They demonstrated that ascorbic acid-2 phosphate (AAP) and triphosphate (AATP) inhibited browning for apple surfaces but not in juices. Effective treatments for juice alone included cinnamate, benzoate, and combinations of beta-cyclodextrin with ascorbic acid, AAP, or ascorbyl palmitate. Recently, the addition of chitosan in filtered apple and pear juices has been found to be successful in the prevention of enzymatic browning (Sapers, 1992).

9.5.6 PEELING/PREPARATION Many different types of equipment are used in peeling and/or coring fruits. Some peeling and coring methods are advantageous for only certain kinds of fruits due to their structure or physical resistance to bruising. Lye peeling: Lye peeling is a common method of peeling for various fruits such as peaches, nectarines, apricots, and pears. Lye is an aqueous solution of approximately 10 to 15% caustic soda (sodium hydroxide) or potassium hydroxide. The operation requires an ample water supply, lye, and a heat source. The lye peeler is a heated tank containing the hot lye solution (60 to 90∞C) that allows for the passage of fruit at a specific rate. The caustic lye solution dissolves the fruit skin. Several variables should be controlled at all times, namely lye concentration and temperature, product holding time, and agitation in the caustic solution. Larger fruits require more water than smaller fruits during lye peeling. Following the treatment, the fruits are subjected to a water wash with pressure sprays to remove the peel and remove the lye from the surface. An acid dip is used (citric acid) after washing to neutralize any remaining traces of caustic soda. This method is advantageous in that there is a relatively low loss in usable fruit tissue as compared to other methods, and it is economical. Mechanical peeling: Mechanical peelers are also very popular with fruits, especially apples and pears. The fruit should be free of bruises and other blemishes in order to ensure the efficient operation of the peeling and coring (if necessary) operation (Downing, 1989). Either the fruit is made to rotate against a stationary knife or vice versa. The operation may consist of a continuous system to peel, core, and slice in a high-speed, continuous manner. Peeled fruits are washed with high-pressure water and then mechanically cored. Coring is usually done before slicing. If there is a delay after peeling and coring, fruit may be held in a 2 to 3% salt solution to prevent browning, treated in one of the ways previously mentioned. Examples of fruits that usually require peeling before canning include peaches, apples, tomatoes, and pears. Generally, fruits that are not peeled prior to canning retain more nutrients as compared to peeled fruits.

9.5.7 FILLING Filling of the cans is usually done by automatic machines, although it may be done by hand for very soft fruits that have a tendency to bruise easily. Mechanical fillers are adjusted to dispense each can a predetermined volume of fruit from a chamber and add a given amount of syrup or juice. These quantities must be uniform in order to ensure accurate and constant fill weights for the final product. Some fruit products undergo a “hot fill” whereby they enter the can at a sufficiently high temperature (approximately 80 to 85∞C). This preheating of the contents before filling, which drives out much of the dissolved gases in the product, may be done for the removal of air and the production of a vacuum. In such cases, a headspace is not necessary. Generally, these products are not processed

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at high sterilization temperatures and hence space is not needed for the expansion of the can contents, which may cause a strain on the seal. Canned applesauce is an example of a “hot-fill” product. Some fruit products require a headspace in the can, especially those that are processed in an agitating retort. The headspace bubble in the can is crucial for the movement of the contents during agitation. The amount of headspace in a can is important; insufficient space may cause can ends to bulge, whereas excessive space can cause underprocessing and even collapse of cans during processing as well as lead to can corrosion during storage due to insufficient vacuum. Type of pack: Fruits are available for canning in a variety of forms: whole fruit, slices, halves, sauce, purée, juice, or in mixed fruit packs such as fruit cocktail. Peaches are usually canned as halves, slices, or in fruit cocktail. Pears canned as halves or slices are also known as “grade pack,” which is used to distinguish them from pears added to fruit cocktail. This unit operation involves the addition of prepared fruit and the covering liquid, when necessary, into the can. Type of covering liquid: During thermal processing of canned fruits, heat is first transferred from the heating medium (steam or water) to the container surface and then to the covering liquid. The covering liquid may include syrups, water, mixtures of fruit juices, and water or fruit juices alone. Heat from the covering liquid is then transferred to the fruit product. Besides facilitating the heat transfer to fruits, the covering media also serves to sweeten the product and improve the quality characteristics (odor and color), as well as to fortify the nutrients. The most common syrups used in canned fruit products are sucrose syrup (cane or beet sugar), corn syrup, invert sugar syrup, dextrose, and high-fructose corn syrup (Lopez, 1987). The different designations for syrups range from light fruit juice syrups to extra heavy syrups. Their designations followed by their brix measurement are: extra heavy syrup (E): 22 to 35∞; heavy syrup (H) 18 to 22∞; light syrup (L) 14 to 18∞, and light fruit juice syrup or water (W) < 14∞ (Lopez, 1987). Other options include canning in natural fruit juices (Vyas and Joshi, 1981). Syrup strengths may be verified using a refractometer or a Brix hydrometer. Recent market trends, however, have found a growing popularity toward packing in concentrated juice of the same fruit with no sugar added (Moulton, 1992). According to Canadian labeling laws, when fruits are packed without sugar, the label may indicate “no sugar added” or “unsweetened.” Juices such as grape juice and apple juice, which are labeled “vitamin C added,” must contain no less than 18 mg and 35 mg ascorbic acid, respectively. This vitamin fortification is usually done in order to offset the minimal loss during unit operations and storage. Container specifications and types: In the U.S., Canada, and a majority of other countries, can dimensions are expressed using two numbers, each number consisting of three digits. The first digit of both numbers refers to whole inches, and the second and third digits together refer to the additional fraction of the dimension expressed in sixteenths of an inch. The first three-digit number refers to the can diameter and the second three-digit number refers to the can height. For example, a can with the dimensions 401 ¥ 411 would be 4 1/16 in. in diameter (from the outside edge of both double seams) and 4 11/16 in. in height (outside edge of both terminal seams). Specifications for dimensions of glass containers are similar to cans, except that specific numbers for jars are assigned to the equivalent can dimensions. Glass containers are used frequently for products such as applesauce and cherries. Three numbers are used for rectangular cans to denote the dimensions of length, width, and height, respectively. In addition to the traditional can that is recyclable but costly with respect to its requirements for tin, steel, and aluminum, other types of containers are competing for an advantage in the processed fruit industry. Plastic-ring six-pack carriers for beverages, thinner wall metal cans, and paper-based composites (e.g., paper, plastic, and metal foil laminates) will undoubtedly be used on a larger scale in the future due to current concern for convenience, cost, and environmental issues (Moulton, 1992). Popular new ideas include individual aseptically processed laminated paper juice packs and vacuum-packed fruit without a cover syrup or liquid. Container sizes: In the California canned fruit industry, the most popular consumer sizes are the buffet (8 oz), the No. 303 (16 oz), and the No. 2 1/2 (1 lb 13 oz). Canned fruits in Canada are

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usually packed in standard sizes of 5 fl oz (142 ml), 10 fl oz (284 ml), 14 fl oz (398 ml), 19 fl oz (540 ml), 28 fl oz (796 ml), 48 fl oz (1.36 l), and 100 fl oz (2.84 l). For information on “Suggested Net Quantity Statements for Fruits” refer to The Almanac (1993). Suggested container sizes are listed for a variety of canned fruits with different levels of syrup (from light to extra heavy) along with the imperial and metric measures for net capacity. A table with label measurements for various can sizes are also included in The Almanac, with the common name of the can, can size from 202 ¥ 204 to 603 ¥ 700, and label size. Can lacquer: Fruits such as apricots, grapefruit, peaches, pears, and pineapples need an acidresisting lacquer that is mostly used for high-acid foods. Highly colored fruits such as boysenberries, raspberries, strawberries, and red plums also require it, as do cans for jams made from fruits high in anthocyanin pigments. Two kinds of lacquered cans are available: an acid-resisting (AR) lacquer (for acid foods, mostly fruits) and a sulfur-resisting lacquer (SR), for nonacid foods such as vegetables, beans, and meat. The “R” enamel protects the natural pigment of highly colored fruits such as dark colored berries and cherries. Peaches and pears that are packed in cans completely enameled inside will be somewhat darker in color and slightly different in flavor than if packed in cans lined with plain tin (Lopez, 1987).

9.5.8 EXHAUSTING

AND

VACUUM

The primary reason for exhausting and vacuum is to create an anaerobic environment in the can that would inhibit microbial spoilage. Vacuum treatment also removes occluded gases from fruit tissue, which is necessary in order to increase its specific gravity. Generally, three methods of can exhausting are used in order to remove headspace gas and produce a vacuum. The conventional technique is thermal exhausting, which involves the passage of cans through a steam chamber or exhaust box. The steam replaces the air inside the can and it is sealed while still hot. Vacuum is created in the can following condensation of the steam. This process is very energy intensive due to excessive steam requirements. Another available method is “steam-flow” or “steam-vac” closing where live high-pressure steam is injected into the can headspace just prior to closing (approximately 5 to 8 min at 212∞F) (Moulton, 1992). Thus, all of the air is quickly replaced with steam, which will condense and form a vacuum following steaming. Steam-vac closure, when combined with hot fill, usually assures a very high vacuum reading of 200 to 300 mmHg (e.g., processed apple products). Steam-flow closing is relatively efficient, less energy intensive, and less expensive than thermal exhausting. High-speed mechanical vacuum sealing is also commonly used for fruits. In this method, cans filled cold with fruit and syrup are passed into a clincher that clinches the cans (first operation roll seam) but does not form an airtight seal. The cans are then subjected to a vacuum for only a short period of time. This practice will remove only the free headspace air but not all dissolved gases within the product. An advantage of this method is that it eliminates the need for exhausting of cans as a separate unit operation and saves processing space. Vacuum can-closing machines may pose potential problems for fruits packed in syrup. Because of the excess liquid in the can, the vacuum applied may draw with it some of the liquid out of the can because the dissolved air in the can is removed at the same time. This may occur with peach and pear halves packed in syrup, and sliced and diced fruit (Lopez, 1987). In order to prevent such loss of liquid, a prevacuumizing step before vacuum closing is employed, where a vacuum is drawn first on fruit alone in the can, and then, while still under vacuum, the syrup is added (Lopez, 1987). The filled cans are then subjected to the vacuum sealer with practically no dissolved air in the can and no subsequent loss of syrup.

9.5.9 CAN SEAMING

AND

CLOSING

Can container should be closed immediately after filling to prevent excessive cooling of the surface of the product. Modern can seaming machines operate at speeds as high as 300 cans per minute.

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Liquid products may be sealed in cans at speeds of up to 1600 per minute (Lopez, 1987). The double seaming operation is critical for the assurance of an hermetic seal and good keeping quality of the final product during storage. Faulty seaming can result in deformations in the can during processing and eventually recontamination. Glass jars are closed with a screw cap.

9.5.10 CONTAINER CODING All containers should be coded for recall purposes in case of problems in the stored product, such as spoilage, contamination, or consumer complaints. The code should provide necessary information such as the canning plant where packed; the day, year, and hour packed; the packing period; and the line on which the product was packed (Lopez, 1987). This practice should be carefully followed along with the filing of adequate production and shipping records.

9.5.11 RETORT OPERATIONS The simplest pasteurization equipment is a water bath maintained at an appropriately high temperature (Lund, 1975). Because pasteurization temperatures are normally below 100∞C (212∞F), acid food products may be pasteurized in the same type of equipment used for blanching, making the procedure quite convenient. Full crates containing fruits packed in syrup or fruit juice are placed in steam-heated water in large steel tanks. After the required process, cold water is added to the tank for cooling or the crates are lifted up and immersed in a cold-water tank for cooling. Continuous water bath pasteurizers consist of a long tank through which cans move along on a belt. Alternately, the containers can travel through a tunnel along a conveyer belt and are subjected to continuous water sprays. As they travel, the cans pass through several temperature zones from preheating and pasteurization to the final cool. In the pasteurization section, steam at atmospheric pressure (212∞F) is sometimes injected for quicker heating. The moderated temperatures that are possible in these systems make them ideal pasteurizers for fruit products in glass jars that are sensitive to thermal shocks. Another type of unit which is used for pasteurization of fruit, fruit juices, and tomatoes is the continuous, agitating, atmospheric cooker (Lund, 1975). Its operation is similar to that of the more conventional high-pressure continuous cookers, but the operation is limited to atmospheric pressure to accomplish pasteurization. These agitated processes are used to uniformly cook products throughout the container. Product agitation is also possible in batch-type rotary retorts in which cans are often subjected to end-over-end agitation. For small-scale processing of fruit juices, low-cost pasteurizers with a simple design may be suitable (Crandall et al., 1990). Aseptic or ultra-high temperature (UHT) processing has become a success story for fruit beverages, purees, and juices containing small particles. In this process, the food and the packaging material are sterilized separately and then assembled under sterile conditions. The product is first subjected to heat by passing the liquid product through a shell and tube or plate heat exchanger and held for sufficient time in holding tubes to complete the required pasteurization treatment. Following the required treatment, the product is then passed through another heat exchanger where it is cooled. The filling and sealing operations are then performed in presterilized containers (laminated cartons) under aseptic conditions. Plate-type indirect heat exchangers are extensively used for such a purpose. The juice flows through one side of a wall while the heating medium (steam or hot water) flows on the other side. The plate is designed in such a way that it yields extremely high rates of heat transfer. Tubular heat exchangers are also available for pasteurization purposes. For more viscous products such as cream, yogurt, and salad dressings, and for those containing small particulate matters, scraped surface heat exchangers are used so as to prevent surface fouling problems and promote rapid heat transfer. Because the packaging material must be sterilized prior to filling, the material should possess physical properties that permit this application as well as that of hermetic sealing. One type of

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195

composite package that is extensively used as a packaging material for aseptic processing is a paperboard–foil–plastic laminate also known as “Tetra-Pak®.” This type of package consists of a series of six layers of materials, with polypropylene as the outermost layer, followed by “Surlyn®,” aluminium foil, polyethylene, paperboard, and polyethylene again as the innermost layer. Polypropylene and polyethylene act as heat sealing surfaces, whereas aluminium foil acts as a barrier material, protected from mechanical damage by the paperboard. This composite package in its entirety acts as a barrier to oxygen, light, moisture, and microorganisms, and it meets all of the requirements necessary for successful application of the aseptic process, including heat sealability and strength. Prior to filling, a common sterilization method for this packaging material is the use of hydrogen peroxide employed in combination with heat or ultraviolet radiation treatment. For maximum nutrient retention, high-temperature short-time process (HTST) pasteurization methods are often recommended if heat-resistant enzymes such as peroxidase or pectinesterase are not present in the product. This is because increases in processing temperatures will cause faster destruction rates for the microbial population as compared to nutrients. The presence of heatresistant enzymes, which have a higher heat tolerance than microorganisms, would not always permit fruits, for example, to be subjected to such a process.

9.5.12 COOLING After thermal processing, the contents of the can should be cooled to an average temperature of about 35 to 40∞C (Jackson, 1979). Storage at higher temperatures will cause loss of normal color, and darkening or pink discoloration (stack burn). If cans are cooled too far below the average temperature, they will remain wet and rusting may result due to insufficient surface drying. Water used for cooling should be noncorrosive, low in bacterial and yeast content, and chlorinated for measurable free-chlorine residual detected at the discharge end of the cooler. The cooling water should also be chlorinated with 2 ppm of available chlorine to preclude infection of the can contents by spoilage microorganisms.

9.5.13 LABELING

AND

STORAGE

Following cooling, cans are labeled for identification purposes. Adequately processed cans usually ensure an acceptable canned fruit quality on the retail market for at least 1 year. Storage temperature has been found to be the most important variable in the maintenance of an acceptable product with minimal flavor, color, and textural and nutritive changes. High storage temperatures of 37∞C or higher cause brown discoloration of fruit products, especially canned peaches (Luh and Phithakpal, 1972). The discoloration was accompanied by a decrease in leucoanthocyanin content and may be related to polyphenol oxidase activity. The corrosion of internal lining of canned peaches is also related to storage temperatures. In the warehouse, fruit products should be kept at approximately 20∞C and subjected to good ventilation to protect cans from condensation of moisture. High-temperature storage at approximately 35∞C for orange juice can cause the formation of degradation products leading to malodorous properties (Tatum, 1975). Furthermore, the browning index caused by storage temperature abuse of orange juice may be predicted by CIE tristimulus values (Robertson, 1981). Common storage temperatures for canned fruit seldom average above 70∞F (22∞C) (Woodroof and Luh, 1986). Above 80∞F (27∞C), gradual softening occurs and freezing causes greater changes. Canned fruits should not be allowed to freeze. Freezing may cause distortion of the can seams and eventually lead to microbial spoilage. The most important causes of quality deterioration of canned fruits are very slow chemical changes that take place during storage, resulting in changes in nutritive value, flavor, color, and texture. However, based on estimated kinetic parameters for modeling quality changes, it may be possible to predict color degradation of juices or other liquid foods during processing prior to storage (Cohen et al., 1994).

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Table 9.8 lists commonly canned fruits along with selected operations and their recommended allotted time for processing.

9.6 QUALITY CONTROL OF CANNED FRUITS The color, flavor, and texture of fruits are affected by thermal processing. In the thermal processing of fruits, sacrificing color and flavor are inevitable in order to provide longer shelf life and more convenience. Because heat preservation is the most destructive method of fruit processing, the processor’s goal should be to minimize flavor and color changes while ensuring a safe product. Canned fruits should be regularly examined for four parameters that have the potential to change following a thermal treatment: pH, added color, metallic contamination, and vacuum (Young and How, 1986). Commercial juices such as grapefruit, pear, and orange may be analyzed for bacterial contamination during processing using D-alanine (Gandolfi et al., 1994). Textural properties of fruits are important as they can predict the overall acceptability of the fruits for consumption. Texture loss in thermally processed fruits can be attributed to degradation of pectic substances (Wiley and Thompson, 1960). Fruits selected for thermal processing should be firm enough to withstand high temperatures, although substances such as calcium lactate may be added to grapefruit sections in juice or syrup to decrease firmness loss (Baker, 1993). During processing, the juice of the fruit will dilute the syrup. Consequently, the density of the final syrup (the “cutout” syrup) will be lower than that of the added syrup. For example, a syrup added before processing with a 40∞ Brix will usually “cut out” at 22 to 25∞ Brix. The cutout syrup will depend upon various factors: strength of the added syrup, the type of fruit, its ripeness, soluble solids content, and the ratio of fruit to syrup (Young and How, 1986). In order to preserve the color of processed fruit, selection of fruit with optimal pigmentation is recommended. Undesirable compounds such as melanoidins and melanins are formed in browning reactions. The major pigments in fruits are chlorophylls, carotenoids, and anthocyanins. Chlorophylls are lipid soluble pigments that disappear during fruit ripening. Carotenoids are subdivided into xanthophylls and carotenes. Xanthophylls are yellow (as in Golden Delicious apples and bananas), and carotenes are red (such as lycopene, the major pigment in watermelon and pink grapefruit flesh) and orange (the B-carotene found in orange and apricots). Anthocyanins are water soluble pigments responsible for the red, blue, and purple of many fruits, flowers, and vegetables. All these pigments related to color are destroyed during thermal processing. The chlorophylls are degraded to brown pheophytins (Gold and Weckel, 1959; Schwartz and von Elbe, 1983), the carotenoids are converted to epoxides, and the anthocyanins are rapidly degraded. During heat processing, anthocyanins react quite readily with the metal walls of nonlacquered cans. Thus, it is necessary to lacquer the cans to protect both the product and the can, as the color will usually pass out into the syrup during processing. There are two types of discoloration associated with anthocyanins. The first type occurs when leucoanthocyanins are converted to anthocyanins during canning. This causes the characteristic pink discoloration of pears (Luh et al., 1960) and the excess red color in peaches. Other fruits that undergo pink discoloration include guava, lychee, and banana. The second type involves enzymic browning, which can be prevented by blanching and the addition of ascorbic acid, citric acid, and malic acid. Heat processing may also cause the formation of various zinc complexes such as in canned kiwifruit slices (Cano and Marin, 1992).

9.7 GRADES FOR CANADIAN AND U.S. CANNED FRUITS Most heat-processed fruit in Canada is sold by grade. The standards are established by the Processed Fruit and Vegetable Regulations of the Canada Agricultural Products Standards Act. The products are graded on a variety of quality factors such as flavor and aroma, tenderness and maturity, color, consistency of texture, appearance of packing media, uniformity of size and shape, and freedom from defects and foreign matter (Agriculture Canada, 1982).

E E E E

E

E E E E E E E E

PL PL E* E*

PL

PL PL PL E* E* E* E* EE* MR MR MR

MR MR MR L L L

MR L

MR MR L L

MR

Type Steeld

30–60 30–60 120–180

240–300

20–45

120–480

Blanching Time (S)

3

4 8

3–10 10 4 10

303

4

8–10 12–15 6–8

8 10

15–40

5

401

10

16 12

45–110

8

603

60 93 82

82 88 82

82 71 71 77

35 35

7–8

15 15

30–50

10

20

10

88 88 82 82 82 82 99

303

Fill Temp (∞C)

Include blackberries, blueberries, dewberries, huckleberries, loganberries, and raspberries. PL = plain, E* or EE* = inside side strip of enamel. E = single coat enamel, EE = double coat enamel. Type L = used for strongly corrosive fruits; Type MR = used for mildly corrosive fruits. For No. 2 cans only (307 ¥ 409). Packing media: E = extra heavy syrup, H = heavy syrup, L = light syrup, W = water or artificially sweetened.

E

PL

Coating Endsc

Can Size

Exhaust Time (min)

45 45

20 17–19 10 16–20 8–12 30–55 20 10 12 25 25 7–10 12–15e 20

10

401

80 70

28–35 35

35 35

30–70

35 20–30 20 25–30

10

603

35 35 35

20 20

20 25 10 25

10

45 45 45

25 25

20 25 15 25

10

32 oz

Jar Size 16 oz

Process Time (min) Can Size

E, E, E, E, E, E,

H, L, W H, L, W H, L, W H, L, W H, W H, L, W

E, H, W E, H, L, W L, W

E, H, L, W E, H, L, W E, H, L, W

Packing Mediad E/H/L/W

Source: American Can Co. (1949); Hurst (1984); Woodroof, J.G. and Luh, B.S. 1986. Commercial Fruit Processing. 2nd ed. AVI Publishing, Westport, CT. pp. 678.

f

e

d

c

b

a

Applesauce Apples In syrup Apricots Berriesa Cherries Currants (Black) Figs Fruit cocktail Grapefruit Lychee Peaches Pears Pineapples Plums (dark) Prunes (dried) Strawberries Tomatoes Cold pack Hot pack Tomato juice

Fruit

Enamel Bodyb

Type of Can

TABLE 9.8 Conditions for Processing of Fruits in Boiling Water

4.1–4.6 4.1–4.6 3.9–4.4

3.4–4.2 3.8–4.6 3.5–4.1 2.8–3.0 2.5–4.2 3.0–3.9

3.6–4.0 3.0–3.5

3.4–3.5 3.4–4.4 3.0–4.2 3,6–4.0

3.2–4.2

pH Range

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There are three general grades: Canada Fancy, Canada Choice, and Canada Standard. Canada Fancy fruits are free from blemishes, clean, and of good color and uniform size. Canada Choice fruits have slight variations in size, color, and maturity, but are almost completely free of blemishes. For example, Canada Fancy and Canada Choice are the grades used for apple juice and apple juice from concentrate. Canada Standard fruits are mainly used for sauces and puddings because they may be broken and more or less ripe. The appropriate grade must appear on the main part of the label. The U.S. grade standards for fresh and processed fruits are under the jurisdiction of the U.S. Department of Agriculture, Food Safety and Quality Service (FSQS). Manufacturers and packers frequently employ them in quality control work. The standards for grades usually vary for each commodity. They include the definition of the product, the style of pack (whole, halves, quarters, slices, dices, pieces, or mashed), and the grade (Woodroof, 1986). The U.S. grades include the following: U.S. Grade A or U.S. Fancy, U.S. Grade B or U.S. Choice, U.S. Grade C or U.S. Standard, U.S. Grade D or U.S. Substandard. Kader (1985) published a list of various quality factors for processing fruits listed in the U.S. grade standards and the California Food and Agricultural Code. Examples of quality factors include color uniformity, freedom from decay and defect, maturity based on soluble solids content, firmness, and shape. The above factors vary depending on the type of fruit.

9.8 NOMENCLATURE B = Thermal process time. Ball’s Process time (BB) = Corrected for come-up period (steam on to steam off minus 0.6l) CUT = Come-up period of the retort. D = Decimal reduction time. D value = time required to result in 90% reduction in microbial population. F = Thermal death time. fc = Cooling rate index. It is the time required for the straight-line portion of the cooling curve (Figure 9.5) to pass through one complete log cycle. It is also the negative reciprocal slope of the cooling rate curve. fh = Heating rate index. It is the time required for the straight-line portion of the heating curve (Figure 9.4) to pass through one complete log cycle. It is also the negative reciprocal slope of the heating rate curve. Fo = Process lethality. g = Difference between the retort temperature and food temperature (T) at time t — (Tr – T). gc = The value of g at the end of heating or beginning of cooling period (Tr – Tic). Ic = Difference between the cooling water temperature and food temperature at the start of the cooling process (Tic – Tw). Ih = Difference between the retort temperature and food temperature at the start of the heating process (Tr – Tih). jcc = Cooling rate lag factor. A factor that, when multiplied by Ic, locates the intersection of the extension of the straight-line portion of the semilog cooling curve and the vertical line representing the start of the cooling process — (Tw – Tpic)/(Tw – Tic). jch = Heating rate lag factor. A factor that, when multiplied by Ih, locates the intersection of the extension of the straight-line portion of the semilog heating curve and the vertical line representing the effective beginning of the process (0.6l) = (Tr – Tpih)/(Tr – Tih). l = Come-up period. In batch processing operations, the retort requires some time to reach the operating condition. The time from steam on to when the retort reaches Tr is called the comeup period. 0.6l = Effective beginning of the process. The retort come-up period varies from one process to the other and from one retort to the other. In process evaluation procedures, about 40% of

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this come-up period is generally considered as time at retort temperature because the product temperature increases even during this period. In order to accommodate this, the effective beginning of the process is moved left a distance “0.4l” from the time the retort reached Tr or moved right 0.6l from steam on. Pt = Operator’s Process time (time after come-up period; Pt = B – 0.4l). Tic = Initial food temperature at start of cooling period. Tih = Initial food temperature at the start of heating period. Tpic = Pseudo-initial temperature during cooling. Temperature indicated by the intersection of the extension of the cooling curve and the vertical line representing the start of cooling. Tpih = Pseudo-initial temperature during heating. Temperature indicated by the intersection of the extension of the heating curve and the vertical line representing the effective beginning of the process (0.6l). Tr = Retort temperature. Tw = Cooling water temperature. Z = Temperature sensitivity indicator of D or F. Temperature range in which D or F changes by a factor of 10.

REFERENCES Agriculture Canada. 1982. Canada’s Food Grades. Publication 1720E. Minister of Supply and Services Canada. Almanac of the Canning, Freezing, Preserving Industries, 77th Annual, Vol. 1 and 2. 1993. Edward E. Judge & Sons, Westminster, MD. Alzamora, S.M., Tapia, M.S., Argaiz, A., and Welli, J. 1993. Application of combined methods technology in minimally processed fruits. Food Res. Int. 26: 125–130. Baker, R.A. 1993. Firmness of canned grapefruit sections improved with calcium lactate. J. Food Sci. 58(5): 1107–1110. Ball, C.O. 1923. Thermal Process Time for Canned Food. Bulletin 37, Vol. 7, Part 1. National Research Council, Washington, D.C. Bigelow, W.D., Bohart, G.S., Richardson, A.C., and Ball, C.O. 1920. Heat Penetration in Processing Canned Foods. Canners National Association. Bulletin 16L. Cano, M.P. and Marin, M.A. 1992. Pigment composition and color of frozen and canned kiwi fruit slices. J. Agric. Food Chem. 40: 2141. Cho, S.I., Bellon, V., Eads, T.M., Stroshine, R.L., and Krutz, G.W. 1991. Sugar content measurement in fruit tissue using water peak suppression in high resolution 1H magnetic resonance. J. Food Sci. 56(4): 1091–1094. Cohen, E., Birk, Y., Mannheim, C.H., and Saguy, I.S. 1994. Kinetic parameter estimation for quality change during continuous thermal processing of grapefruit juice. J. Food Sci. 59(1): 155–158. Crandall, P.G., Upadhyaya, J.K., and Davis, K.C. 1990. Portable, low cost equipment for small scale fruit juice processing. Int. J. Food Sci. Technol. 25: 583–589. Desrosier, N.W. 1977. Elements of Food Technology. AVI Publishing, Westport, CT. Downing, D.L. 1989. Processed Apple Products. Van Nostrand Reinhold, New York. Dull, G.G., Birth, G.S., and Magee, J.B. 1980. Nondestructive evaluation of internal quality. Hortic. Sci. 15: 60–63. Eisner, M. 1988. Introduction into the Technique and Technology of Rotary Sterilization. Private Author’s Edition, Milwaukee, WI. Fellows, P. 1988. Food Processing Technology: Principles and Practices. Ellis Horwood, Chichester, U.K. Gandolfi, I., Palla, G., Marchelli, A., Dossena, A., Puelli, S., and Salvadori, C. 1994. D-alanine in fruit juices: a molecular marker of bacterial activity, heat treatments, and shelf life. J. Food Sci. 59(1): 152–154. Gold, H.J. and Weckel, K.G. 1959. Degradation of chlorophyll to pheophytin during sterilization of canned green peas by heat. Food Technol. 13: 281–286. IFT. 1993. Browning of foods: Control by sulfites, antioxidants, and other means. A Publication of the Institute of Food Technologists’ Expert Panel on Food Safety and Nutrition. Scientific Status Summary. Food Technol. 47(10): 75–84.

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Jackson, J.M. 1979. Canning procedures for fruits. In: Fundamentals of Food Canning Technology. Jackson, J.M. and Shinn, B.M., Eds. AVI Publishing, Westport, CT. Jesenka, Z., Pieckova, E., and Sepitkova, J. 1991. Thermoresistant propagules of Neosartorya fischeri; some ecologic implications. J. Food Protect. 54(8): 582–584. Kader, A.A. 1985. Standardization and inspection of fresh fruits and vegetables. In: Postharvest Technology of Horticultural Crops. The Regents of the University of California, Division of Agriculture and Natural Resources, Davis, CA. Kyzlink, V. 1990. Principles of Food Preservation. Elsevier Science, New York. Lopez, A. 1987. A Complete Course in Canning and Related Processes. 12th ed. Book 1, 2 and 3. The Canning Trade, Baltimore, MD. Luh, B.S., Leonard, S.J., and Patel, D.S. 1960. Pink discoloration in canned Bartlett pears. Food Technol. 14: 53–56. Luh, B.S. and Phithakpal, B. 1972. Characteristics of polyphenoloxidase related to browning in cling peaches. J. Food Sci. 37: 264–268. Lund, D.B. 1975. Heat processing. In: Principles of Food Science, Part 2: Physical Principles of Food Preservation. Fennema, O.R. and Lund, D.B., Eds. Marcel Dekker, New York. Madden, J.M. 1992. Microbial pathogens in fresh produce — the regulatory perspective. J. Food Protect. 55(10): 821–823. Merson, R.L., Singh, R.P., and Carroad, P.A. 1978. An evaluation of Ball’s Formula method of thermal process calculations. Food Technol. 32(3): 66. Moulton, K. 1992. Maintaining the Competitive Edge in California’s Canned Fruit Industry. UC Agricultural Issues Center, University of California, Berkeley. Prestamo, G. and Manzano, P. 1993. Peroxidases of selected fruits and vegetables and the possible use of ascorbic acid as an antioxidant. Hortic. Sci. 28(1): 48–50. Prussia, S.E. and Woodroof, J.G. 1986. Harvesting, handling, and holding fruit. In: Commercial Fruit Processing. 2nd ed. AVI Publishing, Westport, CT. Ranganna, S. 1986. Handbook of Analysis and Quality Control for Fruit and Vegetable Products. Tata McGrawHill, New Delhi. Richardson, P.S. 1987. Flame sterilization. Int. J. Food Sci. Technol. 22: 3–14. Robertson, G.L. 1981. Relationship between color and brown pigment concentration in orange juices subjected to storage temperature abuse. J. Food Technol. 16: 535–541. Sapers, G.M. 1992. Chitosan enhances control of enzymatic browning in apple and pear juice by filtration. J. Food Sci. 57: 1192–1193. Sapers, G.M., Hicks, K.B., Phillips, J.G., Garzarella, L., Pondish, D.L., Matulaitis, R.M., McCormack, T.J., Sondey, S.M., Seib, P.A., and El-Atawy, Y.S. 1989. Control of enzymatic browning in apple with ascorbic acid derivatives, polyphenol oxidase inhibitors, and complexing agents. J. Food Sci. 54(4): 997–1002, 1112. Schwartz, S.J. and von Elbe, J.H. 1983. Kinetics of chlorophyll degradation to pyropheophytin in vegetables. J. Food Sci. 46: 636–637. Stumbo, C.R. 1973. Thermobacteriology in Food Processing. 2nd ed. Academic Press, Orlando, FL. Stumbo, C.R. and Longley, R.E. 1966. New parameters for process calculation. Food Technol. 20(3): 341. Tatum, J.H. 1975. Degradation products formed in canned single-strength orange juice during storage. J. Food Sci. 40: 707. Voldrich, M. and Kyzlink, V. 1992. Cyanogenesis in canned stone fruits. J. Food Sci. 57(1): 161–162, 189. Vyas, K.K. and Joshi, V.K. 1981. Canning of fruits in natural fruit juices. I. Canning of peaches in apple juice. J. Food Sci. Technol. 18(1): 39–40. Wiley, R.C. and Thompson, A.H. 1960. Influence of variety, storage, and maturity on the quality of canned apple slices. Proc. Am. Soc. Hortic. Sci. 75: 61–84. Wiley, R.C. and Tolby, V. 1960. Factors affecting the quality of canned apple sauce. Proc. Am. Soc. Hortic. Sci. 76: 112–123. Woodroof, J.G. and Luh, B.S. 1986. Commercial Fruit Processing. 2nd ed. AVI Publishing, Westport, CT. pp. 678. Young, C.T. and How, J.S.L. 1986. Composition and nutritive value of raw and processed fruits. In: Commercial Fruit Processing. 2nd ed. Woodroof, J.G. and Luh, B.S., Eds. AVI Publishing, Westport, CT.

Processing 10 Novel Technologies for Food Preservation Hosahalli S. Ramaswamy, Cuiren Chen, and Michele Marcotte CONTENTS 10.1 10.2

Introduction ........................................................................................................................201 Advanced Thermal Processing Technologies ....................................................................202 10.2.1 Thin Profile Processing .......................................................................................202 10.2.2 Agitation Processing............................................................................................204 10.2.3 Aseptic Processing...............................................................................................205 10.2.4 Microwave Heating..............................................................................................208 10.2.5 Radio-Frequency Heating....................................................................................209 10.2.6 Ohmic Heating.....................................................................................................210 10.3 Nonthermal Processing Technologies................................................................................211 10.3.1 High-Pressure Processing (HPP).........................................................................211 10.3.1.1 Principles and Advantages of HPP....................................................211 10.3.1.2 Advantages of HPP............................................................................212 10.3.1.3 Applications of HPP ..........................................................................213 10.3.2 Pulsed Electric Fields (PEF) ...............................................................................214 10.3.2.1 Effects of Process Parameters............................................................215 10.4 Conclusions ........................................................................................................................216 References ......................................................................................................................................217

10.1 INTRODUCTION One of the most important objectives of food processing is to obtain shelf-stable and nutritious food products that can be stored and handled under normal conditions. To ensure the stable shelf life the foods to be processed have to be commercially sterilized to destroy both pathogenic microorganisms that cause the food to become unsafe for consumption as well as the microorganisms responsible for spoilage. So that they can provide valuable nutrients, foods after processing must continue to remain safe and healthy during storage. Thermal processing, one of most widely used sterilization technologies, is still the dominant technique in food industries. As detailed in Chapter 9, thermal processing can be successfully used to produce safe and shelf-stable foods. Usually, the heat resistance of nutrients and other quality components is much larger than that of microorganisms. This fact is taken advantage of in high temperature short time (HTST) processing techniques and in the optimization of several thermal processing technologies, such as retort pouch processing, agitation processing, and aseptic processing. In recent years, advanced processing techniques such as microwave heating and ohmic heating, as well as related computer control 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

201

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technologies, have made it possible for food industries to produce foods with optimal qualities using the idea of deliverable rapid temperature profiles. However, it is a fact that all thermal processing techniques, however advanced, cannot fully eliminate nutrient degradation. Hence, several advanced nonthermal processing technologies such as high-pressure processing and electrical pulse processing have received attention in recent years as alternatives to conventional thermal processing for producing higher quality products. In this chapter, the concepts involved in some novel processing techniques are introduced.

10.2 ADVANCED THERMAL PROCESSING TECHNOLOGIES Advanced thermal processing technologies are aimed at optimizing both food quality and process time. It is well known that the destruction rates (D values) of most common spoilage and pathogenic microorganisms during thermal processing are more sensitive to changes in temperature than to changes of other quality factors; thus, in theory, the use of high temperatures can not only help achieve a shorter process time but can also promote better food quality. However, this theory is based on the assumption that the temperature distribution inside the food during processing is uniform and rapidly changing. For liquid foods, especially for those with low viscosity such as milk and fruit beverages, this assumption can be easily realized with the application of various types of heat exchangers and pumps. However, for solid foods, especially those stored in larger containers, neither rapid change nor uniformity is easy to achieve since heat is transferred by conduction, which has a limited heat transfer rate. Therefore, different thermal processing methods have been developed based on the characteristics of foods and packages. For example, the traditional HTST or ultra high temperature (UHT) techniques, with processing temperatures in the range of 130 to 145∞C, are used for liquid foods. Such techniques are not appropriate for solid foods since they heat slowly and a large temperature gradient exists between the product at the container surface and that at the center; therefore, by the time the required sterility is achieved in the center, the product bulk on the outer regions gets overprocessed, resulting in greater quality destruction. Thus, the product quality is unfavorably affected with HTST or UHT processing techniques. The concept used to improve quality and shorten process time for solid foods is called thin profile processing: reducing package thickness by employing thinner containers such as retortable pouches or thin profile semirigid containers and then applying an optimal temperature profile for a given package size. On the other hand, in agitation retort processing, the product packaged in a given container is agitated during processing to induce mixing of the container liquid and particulate foods. In aseptic processing all three concepts are combined. In the following text, these typical advanced thermal processing techniques are further discussed.

10.2.1 THIN PROFILE PROCESSING Thin profile packages, also called retortable flexible containers, include retort pouches or semirigid containers, and were developed in the U.S. in the early 1950s. Compared with cylindrical containers, the pouch transfers heat faster to the critical point because of its characteristic thinner profile. This fast transfer permits the required amount of heat for sterilization to be applied to the critical point with minimal overcooking of the product bulk near the peripheral container areas. Thus, thin profile containers can potentially provide higher quality retention for solid foods than can conventional cylindrical containers. Retortable flexible containers are laminated structures that are thermally processed like a can; they are shelf-stable and have the convenience of frozen boil-in-the-bag products. The structure of the retortable pouch is usually a 3-ply laminate composed of 0.012 mm polyester film and adhesive laminated to 0.0089 or 0.018 mm aluminum foil that is laminated to a 0.076 mm polypropylene film. The polyester film is used for high temperature resistance, toughness, and printability. The polyester may be reverse-printed by embedding ink between the film and the foil. The aluminum foil provides the barrier properties (light, gas, microorganisms, etc.)

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for extended shelf life. The polypropylene layer facilitates heat sealing. With the development of high-oxygen barrier films such as ethylene-vinyl alcohol copolymer (EVOH) and polyvinylidenechloride (PVDC), a nonfoil retortable pouch that uses a polyester/polyurethane/adhesivemodified high density polyethylene structure has been used for retort packaging items requiring shelf life of 3 months or less, or for products held under refrigeration. The main structure of semirigid containers is a polypropylene/EVOH or PVDC/polypropylene sandwich with appropriate tie layers and fillers. Alternate materials include high-impact polystyrene, crystallized polyester, and polycarbonates. The shelf life of products packaged in semirigid containers can be up to 12 months but the actual life obtained varies with the food product. Retortable flexible containers can be processed in batch retorts or continuous retorts, and in static mode or agitating mode. The heating medium could be saturated steam, steam–air mixtures, or hot water with overriding air pressure. From the heat transfer rate point of view, saturated steam has the highest coefficient, hot water the next highest, and steam–air mixtures have the lowest coefficient. Japanese and European systems tend to favor steam–air mixtures or saturated steam, but water systems are widely used in the U.S. The key reasons for selecting water as the heating medium are familiarity with the system, accessibility, and controllability. The primary consideration in retort operations is the provision of overriding pressure during both heating and cooling. Such pressure is important during heating to provide better heat transfer rates by keeping the heat transfer surface (the pouch walls) in contact with the food material (thereby preventing the expansion of residual gases and swelling of the containers). During cooling, overriding pressure is absolutely necessary to prevent package collapse that can occur due to internal pressure in the container far exceeding the retort pressure. (The latter follows the reduction of steam that accompanies the onset of cooling.) Overriding air pressure is most commonly used in retort operations, hence most retorts for pouch processing are based on either steam–air mixtures or steam-heated water, with air overpressure. Among the major critical factors that affect thermal processing of retort containers are the following: • • • • • • • • • • • • • •

Minimum headspace Product consistency Maximum filling or drained weight Initial temperature Processing temperature Processing time Temperature distribution Container orientation Residual gas in headspace and in food Processing and racking systems Processing medium Product heating rates Materials from which the pouch rack is constructed Divider sheet hole sizes and spacing

Products packed in retortable flexible containers include meats, sauces with or without particulates, soups, fruits and vegetables, specialty items like potato salad, bakery products, and pet foods. Generally, any product currently packaged in cans or glass jars may also be packaged in flexible containers. The major advantages of retortable flexible containers can be summarized as follows (Lopez, 1997): 1. A reduction of 30 to 40% in processing time is possible, with energy savings. 2. The quality losses during processing such as taste, color, flavor, and nutrients can be reduced significantly compared to quality losses in conventional canning processing methods.

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3. Pouches and containers do not corrode externally and there is a minimum of product–container interaction. 4. Use of flexible containers could potentially eliminate or at least greatly reduce the incidence of dented cans, which is the single largest spoilage factor in supermarkets today, accounting for 49% of the total spoilage. 5. The cost to the canner will be less than that with the conventional rigid-type containers. 6. Food can be heated in a microwave oven. 7. Flexible containers are safer in that consumers would not cut themselves with metal cans or be faced with broken glass when jars break.

10.2.2 AGITATION PROCESSING Rotary retort thermal processing makes use of mechanical agitation to increase heat transfer during processing, reducing process time and improving production quality. Clifcorn (1950) reported that heat penetration to the cold spot could be achieved much faster if the container (with a liquid or semiliquid product and normal headspace) was agitated by end-over-end rotation of the sealed container during processing and if higher temperatures were used. It was apparent that sensory characteristics were considerably improved over those of conventionally processed foods. The effects of various parameters (e.g., process temperature, rotational speed, liquid viscosity, particle properties, volume fraction occupied by the particles, etc.) on heat transfer have been studied by many researchers (see Sablani and Ramaswamy, 1995). The rotary retort processing method can be used for liquid, semiviscous, or particulate foods and has many advantages compared to the still retort processing method. These advantages are summarized as follows (Eisner, 1988): 1. When processing products of greater viscosity or heat sensitivity, higher retort temperatures of 126 to 138∞C and precise timing could be used without the problem of overcooking and scorching. 2. Many more products having a liquid-phase component could be improved by a quicker temperature come-up of the container contents than could be improved under the conditions of conventional still retort temperatures. 3. A rapid HTST method for sterilizing products would preserve the color, flavor, and texture as well as the nutritive value of the thermoprocessed foods. 4. Greater flexibility in specifying sterilization times and temperatures would be available for tailoring the sterilization process to fit various products to several container sizes, thus allowing processes with the same sterilization effect (F value) to be selected on the basis of the degree of cooking (C value). 5. Many of the more viscous, semiliquid, or even semifirm heat-sensitive products can be sterilized in large containers without overcooking. 6. Heat-resistant microorganisms are more easily and thoroughly destroyed without damaging the product, even in large container sizes. Based on the principles of rotary retort processing, three types of rotation (axial, end-over-end, and circular) have been developed and applied in both batch and continuous rotary retorts (Figure 10.1). The continuous rotary retort consists of two cylindrical shells: one shell is used for processes such as heating and cooling, and the other is used for storage of hot water to save energy consumption and speed the come-up heating. One of the early retort systems employing the principle of agitation was the FMC continuous pressure cooker (Sterilmatic). Continuous retorts offer additional advantages over batch-type retorts due to increased production throughput and lower labor costs. In the Sterilmatic system, cans are admitted into the retort through a mechanical pressure lock and conveyed through the retort by

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mrw2

mrw2

w

mg

mg

w

n r

r mg

mg

mrw2

mg mrw2

FIGURE 10.1 The schematics of different types of can rotation.

means of a horizontal helical spiral. The cans remain at the periphery of the helix, and each can rotates about its own axis for a portion of each rotation. By operating the reel at 5 rpm, each can rotates about 100 rpm around its own axis as it moves through the retort. During the bottom third of the rotational cycle, the can is rolled against the shell of the retort. As the can rolls, the headspace travels along the cylindrical contours of the can, resulting in agitation of the contents. Rotational retorts have also been manufactured in Europe. Hermann Stock (Stock Machinfabrik, Hamburg, Germany) developed rotational retorts as early as 1952. The Stock Rotomat (Stock America) comes in simulator, pilot, and production models, with overhead heating tanks to eliminate temperature come-up times, and offers both axial and end-over-end agitation. The heating medium can be steam, water spray, or water immersion with air overpressure. Lagarde Autoclaves (France) manufactures retorts based on heating using steam–air mixtures. The retort design employs a powerful turbo fan at the back of the retort to create turbulence within the heating medium. While the company’s early retorts were mainly static, it has recently released fully automated rotary systems. Barriquand (France) manufactures retorts based on steam–air and the rapid circulation of reheated water using the water spray principle. A typical retort that can be used for processing thin profile packages as well as for agitation processing is shown in Figure 10.2.

10.2.3 ASEPTIC PROCESSING Aseptic processing and packaging consist of presterilizing of the product before filling it into a presterilized package that is then closed in a sterile atmosphere. The sterilization of the aseptically manufactured food product is accomplished by continuous heating under a HTST or UHT regime. Normally, heating temperatures for UHT sterilization are in the range of 135 to 145∞C, and at these temperatures “residence” times to obtain the desired lethality are only from about 2 to 45 sec. The general process schematic of aseptic processing is shown in Figure 10.3. The concept of aseptic processing originated to solve several problems associated with conventional in-container sterilization of foods: low rate of heat penetration to the slowest heating point in the container, the long processing times required to deliver the required lethality, destruction of the nutritional and sensory characteristics of the food, low productivity, and high energy costs (Smith et al., 1990). Aseptic processing was initiated in 1927 at the American Can Company research department in Maywood, IL, under the direction of C. Olin Ball. The result was the development of the HCF (heat, cool, fill) process for which Ball was granted a patent in 1936. The HCF process was targeted at pumpable liquid and semiliquid foods. In 1938 two HCF units

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FIGURE 10.2 A pilot scale retort for thin-profile packages and agitation processing (Stock Rotomat). Courtesy of Stock America.

Raw material

Heating

Packaging material

Holding

Cooling

Sterilization

Filling

or

Filling

Holding

Cooling

Product

FIGURE 10.3 The general process schematic of aseptic processing.

Packaging material

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were installed for the commercial production of a chocolate-flavored milk beverage. Ball is considered the pioneer of aseptic processing (Mitchell, 1988). The Avoset process, another development in the field of aseptic processing, is credited to George Grindrod at the former Avoset plant located in the San Joaquin Valley in California (Ball and Olson, 1957). This process is unique in that the filling and closing area was treated to eliminate bacteria and further protection was accomplished by ultraviolet (UV) lamps (Mitchell, 1988). The area was also enclosed by a wall with an opening for conveying the finished product. Sterilization was achieved by heating to temperatures of 127 to 138∞C. The Avoset process is no longer in operation but nevertheless was another milestone in the development of aseptic processing. Real progress in the commercial development of aseptic processing technology for foods began with the invention of the Martin–Dole process in the late 1940s in California by the Dole Engineering Company (Lopez, 1987). The process could be used for the sterilization of any low- or high-acid fluid. The technical success of the process was not, however, accompanied by the expected high level of exploitation, the reason being that the package, a metal can, could not be differentiated from conventional in-container sterilized foods (Dennis, 1992). Product heating and cooling were achieved by heat exchangers based on the principles of HTST sterilization, while the containers were sterilized by superheated steam (Mitchell, 1988). The first commercial Dole system was installed in the early 1950s in California for the production of soups (split pea soup) and sauces (white, cheese, and Hollandaise). Another historic development was brought about in the early 1960s by Loelinger and Regez of Switzerland who successfully used hydrogen peroxide (H2O2) to sterilize flexible packaging materials used in the manufacture of milk containers (Lopez, 1987). Obtaining better quality products than those from conventional processing was the main objective of the Martin–Dole process, whereas extending the shelf-life of fresh milk without refrigeration using low-cost containers was the goal of Loelinger and Regez. Compared with conventional canning, the advantages of aseptic packaging of food products are summarized as follows (Lopez, 1987): 1. A higher quality product is obtained by using HTST sterilization especially for largecapacity containers. 2. It is possible to use a wide variety of packaging materials, and many package sizes and shapes are possible. 3. The sterilization and packaging procedures used are continuous. 4. Product heating and cooling equipment is designed with a high ratio of surface area to product volume, thereby providing very efficient heat transfer. 5. The package need not be designed to withstand high temperatures, unlike in product sterilization. However, aseptic processing and packaging also has some limitations such as: 1. It needs large capital investment and a complex control system. 2. The product must be relatively homogeneous fluid that can be pumped. 3. A given system is designed for a limited range of product types. Current aseptic processing and packaging technology in North America are primarily limited to liquid foods but there is considerable interest in the extension of the technology to low-acid liquid foods containing large particulates (Dignan et al., 1989; Heldman, 1989; Lund, 1993; Ramaswamy et al., 1995). There were two problems in applying this technology to particulate foods. The first was to come up with a mechanical means of physical handling that would maintain proper distribution and particle integrity, and ensure commercial sterility with minimal quality loss. This problem was successfully solved with equipment such as scraped-surface heat exchangers (SSHEs) or tubular heat exchangers with a displacement pump (Lee and Singh, 1990). The second

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problem — the real challenge — was the establishment of a microbiologically safe process for particulates without sacrificing the essential quality factors. The key to solving this problem is to develop a reliable method to measure the temperature of moving particulates in aseptic processing systems. So far, several methods of temperature measurement have been studied such as biological validation techniques, moving thermocouple methods, liquid crystal technique, time temperature integrators, melting point indicators, and relative velocity methods (Sastry, 1991).

10.2.4 MICROWAVE HEATING Heating rate remains one of the major limitations for the optimization of conventional thermal processes in which the heat is transferred through both conduction and convection, although advanced equipment such as rotary retorts and scraped-surface exchangers, etc., have been developed. Microwave heating is one of the volumetric heating methods that has the potential to lead to a quantum change in the ability of the food processor to achieve the heating rates necessary to deliver profiles that could improve current UHT process routes (Mullin, 1995). Microwaves used in the food industry for heating are of ISM (industrial, scientific and medical) frequencies (2450 or 900 MHz, corresponding to 12 or 34 cm in wavelength). In this frequency range the dielectric heating mechanism dominates up to moderated temperatures. Polar molecules, the dominant water, try to align themselves with the rapidly changing direction of the electric field. The energy to achieve this alignment is taken from the electric field. When the field changes direction, the molecule “relaxes” and the energy previously absorbed is dissipated into the surroundings, that is, directly inside the food. This means that the water content of the food is an important factor in the microwave heating performance of foods. The penetration ability of microwaves in foods is limited. For normal wet foods the penetration depth from one side is approximately 1 to 2 cm at 2450 MHz. At higher temperatures heating due to the electric resistance of dissolved ions also plays a role in influencing the heating mechanisms, normally further reducing the penetration depth of microwaves. The limited penetration depth of microwaves implies that the distribution of energy within the food can vary. Thus, an important requirement of microwave equipment and microwave energy applications in the food industry is the ability to correctly control heating uniformity (Ohlsson, 1983). It may be expected that a lower frequency system will lead to more uniform heating, However, this has not been the case in the areas of pasteurization and sterilization, where all the commercial systems use a frequency of 2450 MHz (Mullin, 1995). Microwave heating has been widely applied in industrial food applications such as defrosting or thawing of frozen foods, drying, blanching, and pasteurization. Sterilization using microwaves has been investigated for many years but the commercial introduction of this technique has only come about in the last few years in Europe and Japan. Microwave pasteurization and sterilization promise to give very quick heat processing that should lead to small quality changes due to the thermal treatment according to the HTST principle. However, it has turned out that very high requirements of heating uniformity must be met in order to fulfill these quality advantages (Ohlsson, 1991). Pasteurization with microwave heating can also be used with pumpable foods. Microwaves are directed at the tube in which the food is transported, thereby heating the tube directly across the cross section. Again, uniformity of heating must be ensured, which requires selection of the correct dimensions for the tube diameter and proper design of the applicators (Ohlsson, 1983). The destruction kinetics of microorganisms such as Saccharomyces cerevisiae, Lactobacillus plantarum and Escherichia coli as well as the inactivation of enzymes under continuous microwave heating have been reported (Tajchakavit and Ramaswamy, 1998; Koutchma and Ramaswamy, 2000). A further application of microwave heating is drying in combination with conventional hot-air drying or vacuum drying. Often microwaves are primarily used for moving water from the wet interior of solid-food pieces to the surfaces, relying on the preferential heating of water by microwaves. Applications can be found for pasta, vegetables, and various cereal products, where puffing

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up by rapid expansion of the interior of the food matrix can be accomplished using microwave energy (Tempest, 1996). The lethality of microwaves has been debated for over 50 years, on the focus of the debate being the nonthermal effects of microwave heating on microorganisms. However, it is still too early for scientists to reach a definitive conclusion on this subject. Of course, this lack of definitiveness will not adversely affect the application of the microwave heating technique in food processing. Currently, the main barriers to further microwave processing applications are the relatively high cost of microwave production lines, and the lack of knowledge about the dielectric behavior of foods, which is important for the design and optimization of microwave processes.

10.2.5 RADIO-FREQUENCY HEATING Radio Frequency (RF) heating or capacitative dielectric heating is an innovation that stands out among several techniques that are based on electro-technologies such as ohmic heating, radiative or microwave dielectric heating, inductive/ohmic combinations, inductive heating, and radiative magnetic heating. All these inject heat energy from the electromagnetic source directly into the body of the product, initiating volumetric heating throughout. RF heating technology involves applying a high-voltage AC signal to a medium placed between parallel electrodes (one of them being grounded) set up as a capacitor. Heating occurs when an AC displacement current flows. High-frequency heating is accomplished as a combination of dipole heating and electric resistance heating from the movement of the dissolved ions. (When the water polar molecules in the medium try to align themselves with the applied AC electric field and thereby interact with neighboring molecules, the result is lattice and frictional losses as they rotate.) RF heating differs from higher-frequency electromagnetic radiative-microwave heating. Wavelengths in the microwave range (12 cm and 34 cm at 2450 MHz and 915 MHz, respectively) are generally comparable to the dimensions of the sample, and microwave heating occurs in a metal chamber with resonant electromagnetic standing wave modes as in a microwave oven or waveguide. In the microwave frequency range the dielectric heating mechanism dominates up to moderate temperatures. The water content of the foods is an important factor for microwave heating performance. For normal wet foods the penetration depth from one side is approximately from 1 to 2 cm at 2450 MHz. Because RF heating uses longer wavelengths than microwave heating, electromagnetic waves in the RF spectrum can penetrate deeper into the products so that there is no overheating or dominance of hot or cold spots, a common problem with microwave heating. RF heating also offers simple uniform field patterns as opposed to the complex nonuniform standing wave patterns in a microwave oven. Ohmic heating or electric resistant heating relies on direct ohmic conduction losses in a medium and requires the electrodes to be in direct contact with the medium; the current cannot penetrate a plastic film or air gap. Ohmic heating gives direct heating because the product acts as an electrical resistor. The heat generated in the product is due to the loss in electric resistance. RF heating has advantages over low-frequency ohmic heating as the medium — which is enclosed inside an insulating plastic package or surrounded by an air gap — can be heated without the electrodes coming into to direct contact with the medium. The performance of RF heating is also less dependent on smooth contact between the product and the electrodes. RF heating is a very attractive processing method to provide safe and high-quality food products because of the rapid and uniform heating patterns and the large penetration depth. There are several promising applications of RF heating in the food industry due to the potential this technique has to improve the quality of food products. The use of RF heating also can result in reduced energy consumption, which can be considered as a great advantage over the traditional methods of heating. The main applications of RF heating are: heating packaged bread, blanching vegetables, thawing frozen foods, baking and postdrying snacks, cooking, pasteurizing, and sterilizing processed meat products. RF technology in food processing has not been fully explored and offers considerable

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scope for further research. Commercial RF heating systems for food pasteurization or sterilization are not known to be in use, although such systems have been researched over the years (Bengtsson and Green 1970; Houben et al., 1991; Wig et al., 1999). The primary advantage of RF heating is improved uniform heating, which was demonstrated for in-package sterilization of foods in large packages using radio frequency at 27.12 MHz, although enhanced edge heating continued to be an issue (Wig et al., 1999).

10.2.6 OHMIC HEATING Ohmic heating, also called electric resistance heating, is a direct heating method that uses the food itself as a conductor of electricity that is taken from mains that are 50 Hz in Europe and 60 Hz in the U.S. The benefits of ohmic heating are numerous. The most important is that heating is very rapid and uniform. The process is ideal for shear-sensitive products. Ohmic heating can heat the food continuously without needing the hot heat transfer surface of a scraped-surface heat exchanger (SHHE) or a tubular heat exchanger (that may foul). The process is quiet in operation as rotating parts are not used in the system to ensure temperature uniformity. Unlike microwave heating, the depth of heat penetration in the food is virtually unlimited. A high level of control and automation ensure safety during the operation. Finally, it is easier to design a heating time/temperature profile that will ensure sterility because heat is generated within the solids independent of thermal conductivity through the liquid. The concept of ohmic heating technology is not new; early applications date back to 1897. During the last century, attempts were made to use this technique in several food processing applications (de Alwis and Fryer, 1990; Palaniappan and Sastry, 1990). However, several technical problems such as product contamination from improper contact between the electrodes and the food product, adhesion of the product to the electrodes, a suitable control system, etc., became major barriers to the early development of the ohmic heating technique. Recent developments in related sciences and technologies such as materials science, computer control, and aseptic processing have stimulated renewed interest in the ohmic heating method on the part of food scientists and engineers since the 1980s. The most recent industrial achievements in ohmic heating are the development of the ELECSTER process for the pasteurization of milk (which is based on the electropure process) and the APV Baker ohmic heating technology for the sterilization of particulate foods (Skudder, 1991). The latter achievement was recently recognized as a commercial breakthrough at the annual meeting of the Institute of Food Technologies (IFT) in 1996. The ohmic system of APV Baker has been used for the pasteurization and sterilization of a number of food products, and the resulting quality has been excellent. The majority of these installations are for the production of fruit products in Japan (Tempest, 1996). In France the CTCPA (Center Technique de la Conservation des Produits Agricoles) and UTC (Universitè Technologique de Compiègne) jointly installed an APV pilot plant unit in 1995 to help European food companies develop sterilization processes for liquids containing particles (Zubber, 1997). Other cooking operations using electric resistance heating involve rapid cooking of potatoes and vegetables for blanching in industry and preparing foods in institutional kitchens. Recently there has been a great deal of research on the application of this technique to food processing for a variety of purposes. Naveh et al. (1983) reported that ohmic heating was applied to thaw foods. A fully automated prototype for ohmic thawing of fish blocks based on the control of current flow within the product to eliminate the formation of hot spots was designed and tested by Roberts et al. (1998). Huang et al. (1997) investigated the feasibility of using a batch-type ohmic heater to coagulate fish proteins obtained from frozen fish mince wash water. As a rapid method, the ohmic heating principle was also used to design a laboratory apparatus for fat analysis in meat products (Piette and Jacques, 1997). It has been recognized that the ohmic heating system will play an important role in food processing areas such as aseptic processing of food products containing large particulates (Marcotte, 1999).

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10.3 NONTHERMAL PROCESSING TECHNOLOGIES Consumer requirements for foods are constantly changing. Today consumers demand foods that are both fresh and natural. Therefore the steps used to process foods should be designed to preserve their natural quality. Hence nonthermal processing techniques such as high-pressure processing (HPP), pulsed electric field, and ozone treatment have been attracting more attention from food scientists and engineers in recent years not only because of their food preservation capabilities but also because of their potential to achieve some interesting functional effects.

10.3.1 HIGH-PRESSURE PROCESSING (HPP) New and alternative food processing methods, and novel combinations of existing methods are continually being investigated by the industry in pursuit of producing better quality foods more economically. HPP offers one such technique for the pasteurization of foods or for modification of their functional properties and has been highlighted in several recent IFT symposia. In 2002 Avure Technologies, Inc. was awarded the prestigious IFT Industrial Achievement Award for its efforts to develop and commercialize high-pressure processing equipment. High-pressure technology has traditionally been used in nonfood areas on a relatively large scale for the production of ceramic, carbide and steel components, and superalloys, where inert gases or water are used as the pressure medium. As liquid compression results in a small volume change, high-pressure machines using water do not present the same operating hazards as machines using compressed gases. Furthermore, the application of hydrostatic pressure to food results in instantaneous and uniform transmission of the pressure throughout the product independent of the product volume. The hydrostatic treatment is unique in that the effects do not follow a concentration gradient nor do they change as a function of time. Other advantages include the absence of chemical additives and operation at low or ambient temperatures so that the food is essentially raw (Farkas, 1986). Presumably, hydrostatic pressure is a physical treatment that will not cause extensive chemical changes in the food system. Once the desired pressure is reached, it can be maintained without the need for further energy input. Liquid foods can be pumped to treatment pressures, held, and then decompressed aseptically for filling as with other aseptic processes. The application of HPP to food preservation started around 1900 when Hite and his coworkers investigated its effects on food microorganisms by subjecting them to pressures of 650 MPa and found a reduction in the viable numbers of microbes. They also applied HPP to other products such as meat, fruits, and vegetables. The higher resistance of microbial spores to pressure destruction was demonstrated by Timson and Short (1965). The first commercial products processed by HPP were offered in Japan in 1990. Since then, numerous academic and industrial activities on HPP have been conducted, especially in Japan and Europe. Currently, some typical commercial pressure-treated products in Europe or the U.S. are orange juice by UltiFruit, Pernod Ricard Company, France; acidified avocado puree (guacamole) by Avomex Company in the U.S. (Texas and Mexico), and sliced ham (both cure-cooked and raw-cooked) by Espuna Company, Spain. There is no doubt that HPP opens another interesting and promising dimension for food processing because it not only inactivates microorganisms but also provides opportunities for the development of new value-added food products. However, there are two major limitations to the further development of HPP. First, the database of pressure processing kinetic parameters such as D and z values in thermal processing is not comprehensive enough to ensure the reliability of HPP as an alternative to thermal processing. Second, developing a continuous-pressure food processor remains an engineering challenge. 10.3.1.1 Principles and Advantages of HPP There are two principles that underlie the development of an HPP system. The first is the Le Chatelier–Braun principle that governs the influence of high pressure on biomolecules. This

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FIGURE 10.4 The isostatic principle in high pressure processing. From Barbosa-Cánovas, G.V., GongoraNieto, M.M., Pothakamury, U.R., and Swanson, B.G. 1999. Preservation of Foods with Pulsed Electric Fields. Academic Press, London, pp. 1–9, 76–107, 108–155.

principle implies that under equilibrium conditions any event (such as a chemical reaction, phase transformation, molecular transformation, etc.) that is accompanied by a decrease in volume is reinforced by an increase in applied pressure, while an event accompanied by an increase in volume is suppressed by a pressure increase. Generally, high pressure affects noncovalent bonds (such as hydrogen, ionic, and hydrophobic bonds) substantially as such bonds are usually very sensitive to pressure; it follows that low-molecular-weight food components (i.e., those responsible for nutritional and sensory characteristics) are not affected by pressure whereas high-molecular-weight components (whose tertiary structure is important for functionality determination) are pressuresensitive. Some specific covalent bonds, however, are modified by pressure. The second principle that plays a key role in HPP systems is Pascal’s law or the isostatic rule, which states that the transmittance of pressure is uniform and instantaneous (Figure 10.4). This makes HPP independent of the size and geometry of the sample and also shortens processing time. Moreover, overprocessing, which is a major concern for the thermal processing method, is prevented. Compared with conventional thermal processing, HPP has a number of unique advantages. First, it can produce much higher quality “fresh-like” foods because HPP destroys microorganisms at lower temperatures and most of the quality components are not affected. Second, HPP is environmentally friendly since it requires only electrical energy for pressure build up. Moreover, HPP can easily be combined with other treatments such as heating, supercritical CO2, and other methods to increase its efficiency. 10.3.1.2 Advantages of HPP HPP has many advantages: 1. It does not break covalent bonds; therefore, the development of flavors alien to the products is prevented, maintaining the natural qualities of the products. 2. It can be applied at room temperature thereby reducing the amount of thermal energy needed for food products during conventional processing. 3. Because HPP is isostatic (applied uniformly throughout the food), the food is preserved evenly throughout its mass without any particles escaping treatment.

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4. High pressure is not time- or mass-dependent; it reduces processing time by acting instantaneously. 5. HPP is independent of the size and geometry of the food. 6. The process is environment-friendly since it requires only electrical energy and there are no waste products (Thakur and Nelson, 1998). However, much like any other process, HPP also has certain limitations. These are: 1. Food enzymes and bacterial spores are very resistant to pressure and very high pressure is required for their inactivation. 2. Residual enzyme activity and the presence of dissolved oxygen results in enzymatic and oxidative degradation of certain food components. 3. Most pressure-processed foods need low-temperature storage and distribution to retain their sensory qualities (Thakur and Nelson, 1998). 10.3.1.3 Applications of HPP HPP has been applied in various areas of the food industry such as food preservation (pasteurization and sterilization), starch gelatinization, enzyme inactivation (blanching), osmotic drying enhancement, pressure-shift freezing, and thawing enhancement. Generally, HPP has been shown to be more effective against vegetative bacteria (Hoover et al., 1989). However, more recently, by combining higher pressures from 700 to 1000 MPa and higher temperatures from 70 to 90∞C, HPP has been successfully applied to the sterilization of low-acid foods. In Japan, hydrostatic pressure has been used to induce the gelation of different kinds of surimi. Excellent gels have been produced from pollack, sardine, skipjack, and tuna at 4000 kg/cm2 and from squid at 6000 kg/cm2. Pressureinduced gels from marine species were smoother and more elastic than those produced by heat and were considered to be organoleptically superior. In meats, pressures in the range of 1000 to 1500 kg/cm2 have been used to tenderize prerigor beef (Elgasim and Kennick, 1980; MacFarlene, 1985). High pressure has also been used to improve the functionality of foods. For example, pressurization of rice grains at 400 MPa or higher significantly increased their water uptake (Hayashi and Hayashida, 1989). Because high-pressure processing affects only noncovalent bonds, food quality factors such as color, flavor, and nutrients are minimally affected (Knorr 1995). Strawberry jams prepared by high pressurization at pressures from 4000 to 6000 kg/cm2 not only maintain their original, fresh fruit color and flavor but also retain up to 95% of the vitamin C originally present in the fresh fruit. Pressure-processed jams were preferred to heat-processed jams. However, high pressure may cause unwanted browning of fresh foods through activation of enzyme reactions. Knorr (1995) concluded that high pressure accelerated enzymatic browning of potatoes and fruit jams. The texture of several fruits and vegetables have been shown to undergo slight firming as a result of high pressure treatment. This effect can be used to advantage in inducing hardening without using chemical additives to protect texture and form damaged by overcooking and also in shortening the time required for the cell membrane to collapse. (It is this collapse that causes the hardness.) Eshttiaghi et al. (1994) reported that HPP blanching significantly reduced the leaching of nutrients and minerals compared to the leaching that occurred with conventional treatment. They also found that pressure-treated vegetables that were subjected to freezing had higher drying and dehydration rates. Fuchigami et al. (1995) subjected cabbage to pressure-shift freezing using pressures of 100 to 700 MPa and a temperature of –20∞C and found that there was an increase in both textural and histological attributes of pressure-frozen samples when compared to the results from traditional freezing. Seyderhelm et al. (1996) showed that application of a pressure as high as 900 MPa for 2 min at 45∞C was sufficient to completely inactivate commercial pectin methyl esterase (PME) in Tris buffer at pH 7.0. Basak (2001) investigated the pressure-induced inactivation kinetics of PME at

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pH levels of 3.2 and 3.7 in both single-strength and concentrated orange juices. Results showed the biphasic nature of pressure-induced inactivation of PME in both juices, characterized by an instantaneous pulse effect and a first-order rate hold-time effect. By a combination treatment of pressure cycle, hold time, pressure level and pH, 90% inactivation of PME was achieved. Pressurepasteurized single-strength and concentrated orange juice had a shelf-life of several weeks under refrigerated storage conditions (4∞C). Basak and Ramaswamy (1998) investigated the effects of HPP on the texture of fruits (pear, apple, pineapple, and orange) and vegetables (carrot, celery, and green and red pepper) and found that the net effect of pressure on texture was dependent on the magnitude of the pressure, instantaneous loss and pressure-hold time. At low pressure levels the initial loss in texture was recovered mostly during the pressure-hold period, and no such recovery of structure was noted during the standing period (at atmospheric pressure) after pressure treatment. High pressure had no adverse effect on the color of the product immediately following pressurization (Eshtiaghi et al., 1994). Prestamo et al. (1999) reported that the browning of apples due to pressure processing was completely arrested by a combined treatment of high pressure and ascorbic acid. Rastogi and Niranjan (1998) found that water and solute of pressure-pretreated pineapple had a significantly higher diffusion rate than in control during osmotic dehydration.

10.3.2 PULSED ELECTRIC FIELDS (PEF) High-intensity pulsed electric field (PEF) processing involves the application of pulses of high voltage (typically from 20 to 80 kV/cm) to foods placed between two electrodes. PEF treatment is conducted at ambient, subambient, or slightly above ambient temperatures for less than 1 sec, as a result of which the energy loss due to heating of foods is minimized. PEF technology is considered superior to traditional heat treatment of foods because it maintains food quality by avoiding or greatly reducing detrimental changes to the sensory and physical properties of foods. Some important aspects of pulsed electric field technology are: generation of high electric field intensities, design of chambers that ensure foods are uniformly treated with a minimum increase in temperature, and the design of electrodes that minimize the effect of electrolysis. The large field intensities are achieved by storing a large amount of energy in a capacitor bank (a series of capacitors) from a DC power supply, which is then discharged in the form of high voltage pulses (Zhang et al., 1995). Studies on energy requirements have concluded that PEF is an energy-efficient process compared to thermal pasteurization particularly when a continuous system is used (Qin et al., 1995a, 1995b). PEF has been mainly applied to preserve the quality of foods, such as to improve the shelf-life of bread, milk, orange juice, apple juice and liquid eggs. The use of PEFs for inactivating microorganisms is another promising nonthermal processing method that can be used for pasteurization and possibly also sterilization, with the integration of other processing parameters such as pH, ionic strength, temperature, and high-pressure processing (Jeyamkonan et al., 1999). Sale et al. (1970) reported a systematic study on the effect of PEFs on the inactivation of microorganisms. They found that the electric field caused an irreversible loss of the membrane’s ability to function as a semipermeable barrier between the bacterial cell and its environment and that this was the cause of cell death. They developed the following model for the prediction of microbial inactivation under PEFs:

Êt ˆ s=Á ˜ Ë tc¯

-

E -E C k

where s is the survival ratio of the numbers of microorganisms after and before treatment; t is the treatment time, which is the product of the number of pulses and pulse width (ms); and Ec, tc, and

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k are constants determined empirically. From this model, it can be seen that for a given process, the inactivation level of microorganisms is only dependent on two parameters: electric strength, E, and treatment time, t. It should be noted that the constants Ec, tc, and k are determined by other process parameters such as properties of the electric pulse, medium and treatment chamber, and microorganism type and growth stage. The results reported on the effects of these factors are reviewed below. 10.3.2.1 Effects of Process Parameters Pulse properties include shape, polarization and frequency. The effects of pulse shapes on the inactivation rate were investigated by Qin et al. (1994). They used square, exponential, and oscillatory voltage pulse shapes for experiments, and the results showed that square wave pulses were more efficient than exponential or oscillatory pulses. They also reported that bipolar pulses were more efficient than monopolar pulses because the alternating stress produced by bipolar pulses resulted in structural fatigue of the membrane and in increased susceptibility of the cell membrane to electrical breakdown. The pulse frequency does not affect the inactivation ratio (Hulsheger et al., 1983). Temperature, electrical conductivity, ionic strength, and pH are usually used to describe medium properties. Pothakamury et al. (1996) reported that increasing medium temperature resulted in a higher inactivation ratio since it decreased the breakdown of the transmembrane potential (TMP) of the cell membrane in addition to causing thermal effects. The effects of medium conductivity on the inactivation rate were studied by Jayaram et al. (1993), who found higher inactivation rates in lower conductivity mediums. This was because lower medium conductivity increased the difference in ionic concentration between the cytoplasm and the medium, thereby facilitating an increased flow of ionic substances across the membrane. The authors believed that this caused a drain on cell energy reserves and eventually weakened the membrane structure, making the membrane susceptible to pulse application. Vega-Mercado et al. (1996) studied the effects of both ionic strength and pH level of the medium and concluded that higher ionic strength reduced the inactivation ratio, while reducing the pH level from neutral increased the inactivation ratio. The sensitivity of different microorganisms to PEF treatment was investigated by Sale et al. (1970). They reported that yeasts were more sensitive than vegetative bacteria to PEF. Qin et al. (1995a, 1995b) investigated the inactivation of E. coli and S. cerevisiae by PEF and also obtained the similar result that yeasts were more sensitive than gram-negative bacteria. In addition, it was found that the growth stage of microorganisms also affected the inactivation rate during PEF processing (Hulsheger et al., 1983; Pothakamury et al., 1996). Results indicated that cells in the logarithmic growth phase were more sensitive to PEF processing than those in the stationary and lag phases. The chamber properties discussed here are electrode configurations and modes of operation. There are five types of electrode configurations used for PEF treatment: parallel plate, wire–cylinder, rod–rod, rod–plate, and coaxial cylinders (Zhang et al., 1995). Parallel plate and coaxial electrode configurations (Figure 10.5) have been used in most of the studies. The former configuration produces a uniform distribution of electric field strength and are simple in design while the latter provides a smooth and uniform product flow and is suitable for industrial applications (Zhang et al., 1995). In terms of operation modes, Zhang et al. (1995) reported that a 9-log reduction of E. coli was obtained using stepwise PEF in a static parallel-plate treatment chamber with 16 pulses at each step. Qin et al. (1995a, 1995b) found that a continuous treatment chamber was more efficient that a static treatment chamber due to the same processing reason mentioned above. However, the results obtained by Martin-Belloso et al. (1997) demonstrated that there was no significant difference between continuous and static treatments.

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Electrode

Filling/withdrawing ports

Treatment region (a) A parallel plate chamber

Product exit

Aluminum support attached to the chamber body with screws

Chamber body

Product intake ports Electrode plastic support attached to the chamber body with screws High voltage electrode

− + High voltage (b) A coaxial electrode chamber

FIGURE 10.5 Configuration designs of both parallel plate and coaxial electrode chambers.

Pulsed electric field processing has been demonstrated to have the potential to be a food processing alternative to conventional thermal processing. However, most of the reported studies have been conducted at the laboratory level, and differences between results from various researchers still exist. Hence, before this technique can be applied for industrial purposes, further research at the commercial level is necessary.

10.4 CONCLUSIONS New and alternative food processing methods and novel combinations of existing methods are continually being sought by industry in pursuit of producing better quality foods more economically. Hence, new innovations, technologies, and concepts continue to emerge. Over the years, many traditional technologies such as thermal processing have been optimized for producing better quality foods with the use of aseptic processing, microwave, RF, and Ohmic heating. New technologies such as high-pressure processing and pulsed electrical field treatment have emerged as nonthermal alternatives. These new technologies, although emerging as strong alternatives to conventional processing, still need to be rigorously tested and proved to be safe and commercially viable.

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REFERENCES Ball, C.O. and Olson, F.C.W. 1957. Sterilization in Food Technology. McGraw Hill, New York. Barbosa-Cánovas, G.V., Gongora-Nieto, M.M., Pothakamury, U.R., and Swanson, B.G. 1999. Preservation of Foods with Pulsed Electric Fields. Academic Press, London, pp. 1–9, 76–107, 108–155. Basak, S. (2001). Studies on high pressure processing of orange juice: enzyme inactivation, microbial destruction and quality changes, process verification and storage. Ph.D. thesis, Food Science Department, McGill University, Montreal, Canada. Basak, S. and Ramaswamy, H.S. 1998. Effect of high hydrostatic pressure processing (HPP) on the texture of selected fruits and vegetables. J. Texture Stud., 29: 587–601. Bengtsson, N.E. and Green, W. 1970. Radio-frequency pasteurization of cured hams. J. Food Sci., 35: 681–687. Clifcorn, I.E. 1950. A new principle for agitating in processing canned foods. Food Technol., 4: 11. de Alwis, A.A.P. and Fryer, P.J. 1990. The use of direct resistance heating in the food industry. J. Food Eng., 11: 3–27. Dennis, C. 1992. HTST processing — scientific situation and perspective of the industry, in Processing and Quality of Foods, Vol. 1, P. Zeuthenet, J.C. Cheftel, C. Eriksson, T.R. Gormley, and K. Paulus, Eds. Elsevier Applied Science, London. Dignan, D.M., Berry, M.R., Pflug, I.J., and Gardine, T.D. 1989. Safety considerations in establishing aseptic processes for low acid foods containing particulates. Food Technol., 43(3): 118–21, 131. Eisner, M. 1988. Introduction into the Technique and Technology of Rotary Sterilization. Private Author’s Edition, Milwaukee, WI. Elgasim, E.A. and Kennick, W.H. 1980. Effect of pressurization of prerigor beef muscles on protein quality. J. Food Sci., 545: 1122–1124. Eshtiaghi, M.N., Stute, R., and Knorr, D. 1994. High pressure and freezing pretreatment effects on drying, rehydration, texture and color of green beans, carrots, and potatoes. J. Food Sci., 59(6): 1168–1170. Farkas, D.F. 1986. Novel processes — ultra high pressure processing, in Food Protection, C.W. Felix, Ed. Lewis Publishers, Ann Arbor, MI, p. 393. Fuchigami, M., Katoh. N., and Teramoto, A. 1995. Effect of Pressure-Shift Freezing on Texture, Pectic Composition, and Histolgical Structure of Carrots. International Conference on High Pressure Bioscience and Biotechnology. Kyoto, Japan, November 5–9. Hayashi, R. and Hayashida, A. 1989. Increased amylase digestibility of pressure-treated starch. Agric. Biol. Chem., 53: 2543. Heldman, D. 1989. Establishing aseptic thermal processes for low acid foods containing particulates. Food Technol., 43(3): 122–124. Hoover, D.G., Metrick, C., Papineau, A.M., Farkas, D.F., and Knorr, D. 1989. Biological effects high hydrostatic pressure on food microorganisms. Food Technol., 43(3): 99–107. Houben, J., Schoenmakers, L., van Putten, E., van Roon, P., and Krol, B. 1991. Radio-frequency pasteurization of sausage emulsions as a continuous process. J. Microwave Power Electromagnetic Energy. 26(4): 202–205. Huang, L., Chen, Y., and Morissey, M.T. 1997. Coagulation of fish proteins from frozen fish mince wash water by ohmic heating. J. Food Process Eng., 20: 285–300. Hulsheger, H., Potel, J., and Niemann, E.G. 1983. Electric field effects on bacteria and yeast cells. Radiat. Environ. Biophys., 22: 149–162. Jayaram, S., Castle, G.S.P., and Margaritis, A. 1993. The effects of high field DC pulse and liquid medium conductivity on survivability of Lactobacillus brevis. Appl. Microbiol. Biotechnol., 40: 119–122. Jeyamkondan, S., Jayas, D.S., and Holley, R.A. 1999. Pulsed electric field processing of foods: a review. J. Food Prod., 62: 1088–1096. Knorr, D. 1995. Hydrostatic pressure treatment of food: microbiology, in New Methods of Food Preservation, G.W. Gould, Ed. Blackie Academic and Professional, Glasgow, U.K. Koutchma, T. and Ramswamy, H.S. 2000. Combined effects of microwave heating and hydrogen peroxide on the destruction of Escherichia coli. Food Sci. Technol., (LWT), 33(1): 30–36. Lee, J.H. and Singh, P.K. 1990. Mathematical models of scraped surface heat exchangers in relation to food sterilization. Chem. Eng. Comm., 87: 21–51. Lopez, A. 1987. A Complete Course in Canning and Related Processes, Book 1, 2, 3. The Canning Trade, Baltimore, MD.

218

Processing Fruits: Science and Technology, Second Edition

Lund, D.B. 1993. The system and its elements, in Principles of Aseptic Processing and Packaging, 2nd ed., P.E. Nelson and J.V. Chambers, Eds. The Food Processors Institute, Washington, D.C., pp. 3–30. MacFarlane, J.J. 1985. High pressure technology and meat quality, in Developments in Meat Sciences — 3, R.A. Lawrie, Ed. Elsevier Applied Science, New York, pp.154–166. Marcotte, M. 1999. Ohmic Heating of Viscous Liquid Foods. Ph.D. thesis, Food Science Department, McGill University, Montreal, Canada. Martin-Belloso, O., Vega-Mercado, H., Qin, B.L., Chang, F.J., Barbosa-Canovas, G.V., and Swanson, B.G. 1997. Inactivation of Escherichia coli suspended in liquid egg using pulsed electric fields. J. Food Process. Preser., 21: 193–208. Mitchell, E.L. 1988. A review of aseptic processing. Adv. Food Res., 32: 1–37. Mullin, J. 1995. Microwave processing, in New Methods of Food Preservation, G.W. Gould, Ed. Blackie Academic and Professional, Glasgow, U.K. Naveh, D., Kopelman, I.J., and Mizrahi, S. 1983. Electroconductive thawing by liquid contact. J. Food Technol., 10: 282–288. Ohlsson, T. 1983. Fundamentals of microwave cooking. Microwave World, 4(2): 4. Ohlsson, T. 1991. Microwave processing in the food industry. European Food and Drink Review, 3. Palaniappan, S. and Sastry, S.K. 1990. Effects of electricity on microorganisms: a review. J. Food Process. Preser., 14: 394–414. Patterson, M.F., Quinn, M., Simpson, R., and Gilmour, A. 1995. Sensitivity of pathogens to high hydrostatic pressure treatment in phosphate-buffered saline and foods. J. Food Protect., 58: 524–529. Piette, J.P.G. and Jacques, L. 1997. Evaluation of a rapid fat analyzer, based on ohmic heating, for the classification of ground beef. Fleischwirtschaft, 77(1): 54–56. Pothakamury, U.R., Vega, H., Zhang, Q., Barbosa-Canovas, G.V., and Swanson, B.G. 1996. Effect of growth stage and processing temperature on the inactivation of E.coli by pulsed electric fields. J. Food Protect., 59: 1167–1191. Prèstamo, G., Sanz, P.D., Fonberg-Broczek, M., and Arroya, G. 1999. High pressure response of fruit jams contaminated with Listeria monocytogenes. Lett. Appl. Microbiol., 28: 313–316. Qin, B.L., Zhang, Q., and Barbosa-Canovas, G.V. 1994. Inactivation of microorganisms by pulsed electric fields of different voltage waveforms. IEEE Trans. Dielec. Electr. Insul., 1: 1047–1057. Qin, B.-L., Chang, F.-J., Barbosa-Cánovas, G.V., and Swanson, B.G. 1995a. Nonthermal inactivation of S. cerevisiae in apple juice using pulsed electric fields. Lebensm. Wiss. Technol., 28(6): 564–568. Qin, B., Pothakamury, U.R., Vega, H., Martin, O., Barbosa-Cánovas, G.V., and Swanson, B.G. 1995b. Food pasteurization using high intensity pulsed electric fields. J. Food Technol., 49(12): 55–60. Ramaswamy, H.S., Abdelrahim, K.A., Marcotte, M., and Clavier, P. 1995. Residence time distribution (RTD) characteristics of meat and carrot cubes in starch solutions in a vertical scraped surface heat exchanger (SSHE). Food Res. Int., 28: 273–84. Rastogi, N.K. and Niranjan, K. 1998. Enhanced mass transfer during osmotic dehydration of high pressure treated pineapple. J. Food Sci., 63(3): 508–511. Roberts, J.S., Balaban, M.O., Zimmerman, R., and Luzuriaga, D. 1998. Design and testing of a propotype ohmic thawing unit. Computers and Electronics in Agriculture 19: 211–222. Sablani. S.S. and Ramaswamy, H.S. 1995. Fluid/particle heat transfer coefficients in cans during end-overend rotation. Lebensm. Wiss. Technol., 28(1): 56–61. Sale, A.J.H., Gould, G.W., and Hamilton, W.A. 1970. Inactivation of bacterial spores by hydrostatic pressure. J. Gen. Microbiol., 60: 323–334. Sastry. S. (1991). Liquid to particle heat transfer coefficient in aseptic processing. In Advances in Aseptic Processing Technologies, R.K. Singh and P.E. Nelson Eds. Elsevier Applied Science, London. Seyderhelm, I., Boguslawsky, S., Michaelis, G., and Knorr, D. 1996. Pressure induced inactivation of selected food enzymes. J. Food Sci., 61(2): 308–310. Skudder, P.J. 1991. Industrial application of the ohmic heating for the production of high-added value products. Proceedings of the VTT Symposium No. 19 Technical Research Center of Finland Espoo 15. pp. 47–52. Smith, J.P., Ramaswamy, H.S., and Simpson, B.K. 1990. Developments in food packaging technology. Part I: processing/cooking considerations. Trends Food Sci. Technol., 1: 106–109. Tajchakavit, S. and Ramaswamy, H.S. 1998. Enhanced destruction of spoilage microorganisms in apple juice during continuous flow microwave heating. Food Res. Int., 31(10): 713–722.

Novel Processing Technologies for Food Preservation

219

Tempest, P. 1996. Electroheat Technologies for Food Processing. Bulletin of APV processed food sector, PO Box 4, Crawley, West Sussex, U.K. Thakur, B.R. and Nelson, P.E. 1998. High-pressure processing and preservation of food. Food Rev. Int., 14(4): 427–447. Timson, W.J. and Short, A.J. 1965. Resistance of microorganisms to hydrostatic pressure. Biotechnol. Bioeng., 7: 139–159. Vega-Mercado, H., Pothakamury, U.P., Chang, F.J., Barbosa-Canovas, G.V., and Swanson, B.G. 1996. Inactivation of Escherichia coli by combining pH, ionic strength, and pulsed electric fields hurdles. Food Res. Int., 29: 119–129. Wig, T., Tang, J., Younce, F., Hallberg, L., Dunne, C.P., and Koral, T. 1999. Radio Frequency Sterilization of Military Group Rations. Presented at the AIChE Annual Meeting. Zhang, Q., Barbosa-Canovas, G.V., and Swanson, B.G. 1995. Engineering aspects of pulsed electric field pasteurization. J. Food Eng., 25: 261–281. Zubber, F. 1997. Le chauffage ohmique: une nouvelle technolgie pour la stabilization des plats cuisinès. Viandes Prod. Carnès 18(2): 91–95.

Radiation Processing 11 Ionizing of Fruits and Fruit Products Brendan A. Niemira and Louise Deschênes CONTENTS 11.1 11.2

Introduction ........................................................................................................................222 Ionizing Radiation Physics and Technologies...................................................................223 11.2.1 Gamma Rays........................................................................................................224 11.2.2 Accelerated Electron Beam (E-Beam) ................................................................225 11.2.3 X-Rays .................................................................................................................225 11.2.4 General Mode of Action......................................................................................226 11.3 Chemical Effects of Ionizing Irradiation in Foods ...........................................................227 11.3.1 Macromolecules...................................................................................................227 11.3.2 Small Molecules ..................................................................................................227 11.3.3 Protection in Complex Foods..............................................................................227 11.4 Biological Effects of Ionizing Irradiation in Foods ..........................................................227 11.4.1 Principal Targets ..................................................................................................227 11.4.2 Sensitivity of Organisms .....................................................................................228 11.5 Technical Aspects of Food Irradiation...............................................................................228 11.5.1 Dosimetry.............................................................................................................228 11.5.2 Detection of Prior Irradiation ..............................................................................229 11.6 Irradiation Facilities ...........................................................................................................230 11.7 Controlling Regulations: Commodities, Dose Limits, and Purposes ...............................232 11.8 Applications of Irradiation to Fruit ...................................................................................233 11.8.1 Delay of Ripening ...............................................................................................234 11.8.2 Disinfestation .......................................................................................................234 11.8.3 Reduction of Microbial Load ..............................................................................234 11.9 Response of Fruit Tissue to Radiation Treatment.............................................................235 11.9.1 Wound Response .................................................................................................235 11.9.2 Delay of Ripening ...............................................................................................236 11.9.3 Postharvest Disease and Storage Losses.............................................................236 11.9.4 Physiological Disorders .......................................................................................237 11.10 Quality of Irradiated Fruits ................................................................................................237 11.10.1 Texture, Color, and Sweetness ............................................................................237 11.10.2 Flavor and Aroma ................................................................................................237 11.10.3 Nutritional Quality...............................................................................................238 11.10.4 Influence of Variety/Cultivar ...............................................................................239 11.10.5 Quality Control ....................................................................................................239 11.11 Irradiation of Fruit Juices and Pulps .................................................................................239 11.11.1 Microbiology of Irradiated Juices and Pulps ......................................................239 11.11.2 Sensory Properties of Irradiated Juices and Pulps..............................................240

0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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11.12 Fruits Currently Being Irradiated ......................................................................................242 11.12.1 Strawberry............................................................................................................242 11.12.2 Mango ..................................................................................................................243 11.12.3 Papaya ..................................................................................................................243 11.13 Combination Treatments ....................................................................................................244 11.13.1 Mild Heat .............................................................................................................244 11.13.2 Modified Atmosphere Packaging (MAP)............................................................244 11.14 Irradiation of Packaging Material......................................................................................245 11.14.1 Types of Plastics Used in Fruit Packaging .........................................................246 11.14.2 Effects of Irradiation on Packaging Plastics .......................................................247 11.14.3 Regulation of Packaging Materials for Use with Irradiated Products ...............248 11.15 Cost-Benefit of Fruit Irradiation ........................................................................................248 11.16 The Future of Irradiation Processing of Fruits..................................................................250 Acknowledgments ..........................................................................................................................251 References ......................................................................................................................................252

11.1 INTRODUCTION Food irradiation is a physical treatment in which food is exposed to ionizing radiation, i.e., radiation of sufficient energy to expel electrons from atoms and to ionize molecules. This radiation may be in the form of high-energy photons (gamma rays or x-rays) or accelerated electrons in the form of an electron beam (e-beam). Foods treated with ionizing radiation have consistently been shown to be wholesome and nutritious [1–9]. Numerous reviews have been written on food irradiation [10–14], including reviews that specifically address the irradiation of fruit and vegetable products [15–17]. Despite the extensive body of evidence and the virtual consensus among researchers, acceptance of irradiated foods by the general public has historically (i.e., pre-2001) been comparatively low [18–20]. This reluctance had been ascribed primarily to a failure on the part of food scientists and food safety experts to adequately educate consumers about the benefits of food irradiation and to dispel persistent fallacies about the process, such as the myth that irradiated food becomes radioactive [21]. Frenzen et al. [22] found that 45.9% of 10,767 U.S. adults surveyed had never heard of food irradiation. Irradiated meats, fruits, and vegetables have been commercially available in the U.S. since 1992, although in limited markets. In 2000, food processors in the U.S. began a major effort to introduce irradiated ground beef and papayas to the marketplace, which led to a steady growth in the capacity of food irradiation facilities in the U.S. In October 2001, following bioterrorist attacks in which spores of Bacillus anthracis were mailed to public figures in the U.S., the U.S. Postal Service began using e-beam technology to irradiate letters and parcels to eliminate bacterial spores. For several weeks, the benefits and technical details of the application of ionizing radiation to a consumer product as an antimicrobial measure were discussed at length in a variety of broadcast, print, and Internet news outlets. As a statement of the safety of the process, the reports typically cited the use of irradiation on food, and usually quoted assessments from the Centers for Disease Control and Prevention, which said that irradiation is a safe and effective technology that can prevent many foodborne diseases [2], and from the U.S. Food and Drug Administration (FDA), which determined that the process is safe and effective in decreasing or eliminating harmful bacteria [7]. This media exposure provided an unprecedented public educational program. A poll of 1008 U.S. adults conducted in November 2001 showed a markedly increased level of support for food irradiation — with 52% of the participants agreeing that the government should require irradiation of foods to ensure a safe food supply — as compared to only 11% of consumers polled in 2000 who stated a willingness to buy irradiated foods [23].

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This shift in consumer attitude toward irradiation as a tool to increase the safety of consumer products is mirrored by a change in the way irradiation is regarded by food processors and researchers. Rather than as a tool to extend product shelf life, much of the latest research on food irradiation, particularly with regard to irradiation of fruit and vegetable products, has been on the elimination or attenuation of food-borne pathogens such as Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella [16, 17, 24, 25]. Early studies with irradiated produce typically used relatively high doses to completely eliminate spoilage bacteria and fungi [26]. The doses employed often exceeded the maximum radiation tolerances of vegetable commodities tested, resulting in loss of quality [27, 28]. Irradiation was, therefore, generally regarded as unsuitable for application to fresh produce [29–31]. However, low dose irradiation, i.e., less than 3 kGy (0.3 Mrad), is currently seen as one of several potential sanitary approaches to fruit and vegetable processing [16, 25, 32]. Recent research has shown that bacteria may be internalized in fruits and vegetables beyond the reach of surface sanitizers [33, 34]. The penetrability and subsurface antimicrobial efficacy of irradiation suggest that it can play an important role in the sanitization of produce. The United Nations Food and Agriculture Organization (FAO) and International Atomic Energy Agency (IAEA) recently initiated a cooperative research program on the use of irradiation to ensure hygienic quality of fresh, precut fruits and vegetables and other minimally processed food of plant origin [35]. This program brings together food scientists from 15 nations, including Brazil, Canada, China, India, the U.K., and the U.S., to share expertise, methodologies, and research data. The various research programs will investigate the effect of irradiation on a variety of human pathogens (e.g., E. coli O157:H7, L. monocytogenes, Salmonella spp., and Shigella spp.) associated with a number of whole and cut fruits, including apple, apricot, blueberry, cantaloupe, jackfruit, mango, peaches, pineapple, pomello, and watermelon. Additional research topics will include other vegetables, modified atmosphere packaging (MAP), sensory response, processing conditions, effect on shelf life and storage, etc. It is expected that the results of this multiyear research program will be used by food processors to design proper protocols for the irradiation treatment of fresh and minimally processed produce, as well as to improve the safety and quality of these fruits and vegetables. This chapter will discuss the use of ionizing radiation in the processing of fruits and fruit products such as juices, ciders, and fruit pulp. While the focus is on fruits and fruit products, valuable insights may be obtained by an examination of the research conducted on irradiation of vegetable products, and these studies will be discussed where appropriate. The key technologies used for irradiation will be compared, and the current state and extent of commercial fruit irradiation will be summarized. Also discussed will be matters of quality control in irradiation processing of fruits, and ways that other treatments may be combined with irradiation in a “hurdle”-type synergistic control program, as well as the regulations and scientific data concerning packaging materials. In addition to a review of the relevant scientific information on radiation microbiology and physiology, this chapter will present summaries of the national and international regulations concerning which fruit commodities may be irradiated, the permitted purposes for irradiation, and the maximum doses allowed by key exporting and importing nations.

11.2 IONIZING RADIATION PHYSICS AND TECHNOLOGIES The level of treatment received by a food product — the radiation dose — is defined as the quantity of energy absorbed during exposure. The international unit of treatment is the gray (Gy). One gray represents one joule of energy absorbed per kilogram of irradiated product, the equivalent of 100 rad or 0.1 Mrad. The energy absorbed depends on the mass, bulk density, and thickness of the food. Each kind of food has to absorb a specific dose of radiation for the desired results to be achieved. An excessive dose may damage the food and make it unacceptable for consumption,

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TABLE 11.1 Technologies for the Production of Ionizing Radiation Factors

Electron Beam

X-Ray

Gamma

Source

Electrons are generated on an emission coil and accelerated to high energy, 5–10 MeVa. Extensive cooling equipment required

Radioactive decay of cobalt-60 (2.5 MeV) or cesium-137 (0.51 MeV). Cooling equipment required

Mechanism

High-energy electrons cleave water molecules, creating oxygen and hydroxyl radicals that damage DNA membranes. Direct cleavage of DNA also occurs During operation, > 2 m concrete or ~0.7 m steel/iron/lead. When source is powered off, no radiation is emitted Seconds

Created when high-energy electrons (up to 5 MeV) strike a metal plate (e.g., tungsten or tantalum alloys); typical conversion efficiency is 5–10%. Extensive cooling equipment required High-energy photons stimulate atoms within target to release high-energy electrons that cleave water molecules into radicals. Direct cleavage of DNA also occurs During operation, > 2 m concrete or ~0.7 m steel/iron/lead. When source is powered off, no radiation is emitted Seconds

Shielding required

Speedb Penetrabilityc

6–8 cm, suitable for relatively thin or low-density products. Passes from multiple angles may be required

30–40 cm, suitable for all products

High-energy photons stimulate atoms within target to release high-energy electrons that cleave water molecules into radicals. Direct cleavage of DNA also occurs > 5 m water or > 2 m concrete or ~0.7 m steel/iron/lead. Source cannot be turned off, shielding of source must be the default position Minutes (depending on source strength) 30–40 cm, suitable for all products

MeV = million electron volts. Speed of dose delivery. The desired dose will vary depending on the target organism and commodity irradiated. c Penetrability in food products of average density approximating 1g/cm 3. This figure will vary for individual commodities due to localized variation in density associated with bone, voids, fibrous matter, etc. a

b

while an inadequate dose will fail to achieve the desired effects. The qualitative terminology used to describe doses is not firmly established, but generally, doses may be characterized as low (< 3 kGy), medium (> 3 and < 10 kGy), or high (> 10 kGy). The three types of ionizing radiation that are used for commercial food processing are gamma rays, e-beam, and x-rays. An overview of the advantages and disadvantages of each is presented in Table 11.1. Each of the technologies, whether high-energy photon or high-energy electron, induces ionization of molecules in the food target, leading to the generation of radicals, breakage of DNA, and other radiochemical effects. While these types of radiation are energetic enough to ionize atoms in the food being treated, it should be noted that they are not energetic enough to induce radioactivity in the target. This process is discussed in greater detail in the Section, “General Mode of Action.” Ozone and heat are generated during the irradiation process for each of the technologies; proper ventilation and temperature control are therefore especially important for the irradiation of fruits to avoid product degradation from these secondary effects [29].

11.2.1 GAMMA RAYS Gamma rays are high-energy photons produced by the disintegration of radioactive isotopes. The isotopes of significance in food irradiation processing are cobalt-60 and, less commonly, cesium-

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137. Gamma radiation is generated when cobalt-60 (half-life of 5.27 years) disintegrates. It first produces an unstable intermediate isotope of nickel-60 with emission of beta-radiation and then disintegrates further into stable nickel-60 with emission of two gamma rays of average energies of 1.17 and 1.33 MeV, respectively. Disintegration of cesium-137 (half-life of 30.17 years) to barium137 produces gamma rays with an energy of 0.66 MeV. The preferred radioisotope for food irradiation has historically been cobalt-60. Gamma rays from cobalt-60 are more energetic than those from cesium-137, but due to its much shorter half-life, cobalt60 irradiators must be replenished with fresh radioisotope (recharged) much more frequently than cesium-137 irradiators. After 10.5 years of operation, a cobalt-60 irradiator will have approximately 25% of its initial strength, while a cesium-137 irradiator will have retained approximately 80% of its initial strength. Despite the need for more frequent recharging, the key factors that have made cobalt60 the preferred radioisotope include the higher initial cost of cesium-137 and the water solubility of the CsCl form of cesium-137 used in irradiators, a point of significant environmental concern. Gamma rays have excellent penetrability and are suitable for irradiating large food items such as pallet- or crate-sized packages of product. The shielding required for a gamma irradiator is roughly comparable to that required for an e-beam or x-ray irradiator (Table 11.1). However, the stigma attached to radioactive material, particularly in a food processing facility, as well as the always-on nature of the source, has led to only limited adoption of gamma sources for food processing. The time required for processing is dependent on the dose desired and the strength of the source. Longer processing times may necessitate some form of temperature control, particularly in temperature-sensitive products such as fruits.

11.2.2 ACCELERATED ELECTRON BEAM (E-BEAM) E-beams are produced by electronic equipment that uses a linear accelerator or a cyclotron accelerator to impart a high velocity, and therefore a high kinetic energy, to a stream of electrons. The accelerators used in commercial irradiators intended to treat food produce a focused beam of electrons with an energy of up to 10 MeV. The processing dose is delivered as a pulse of electrons, and the full dose is delivered quickly, taking typically less than 5 sec. The electrons are aimed at the target with a cone-shaped guide. Higher doses may be delivered by repeated exposure. Despite the intensity of the beam, the short exposure time for product being irradiated generally prevents any significant rise in temperature during processing. The penetrability of the electron beam is lower than that of gamma rays or x-rays (Table 11.1), so product must be packaged in relatively thin cartons treated from multiple sides or some combination of these. The increased handling (e.g., stacking and unstacking pallets of produce, multiple passes, etc.) may result in decreased throughput compared to gamma ray or x-ray processing, as well as increased opportunities for postprocess handling damage to the product. With regard to construction of new irradiation facilities, a significant commercial advantage of e-beam irradiators over gamma irradiators is the electronic nature of the radiation source, which enables it to be completely deactivated — i.e., when it is off, it is off. It is important to note, however, that an electron accelerator does not resemble a home microwave oven; as high-power, industrial-scale electronic devices, electron accelerators require specialized training and maintenance for their operation, and the shielding required for an operating e-beam unit is comparable to that required for a gamma irradiator.

11.2.3 X-RAYS X-rays are high-energy photons and are the form of ionizing irradiation most familiar to the general public due to their widespread medical applications, although at much lower energies. The highenergy x-rays used in food irradiation are generated from high-energy electrons and thus require the same type of accelerator systems used in an e-beam irradiator. The beam of electrons is directed at a high-density metal target. The electrons are absorbed by the metal atoms and the high-energy

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photons (x-rays) are emitted. The efficiency of this process is relatively low, typically 5 to 10%, depending on the specific alloy used in the metal target plate. Alloys of tungsten, tantalum, and various types of stainless steel have been used, and they vary in durability, cost, and efficiency. The energy of the x-ray output is dependent on the energy of the originating e-beam and the conversion efficiency. The vast majority of the energy that is directed at the metal plate is converted into heat, making the proper design of cooling subsystems in x-ray irradiators critical. Also of concern are neutrons that are occasionally knocked from the high-density metal plate by the ebeam. At high energy levels, these neutrons can induce a low level of short-lived radioactivity in the target [36]. This neutron generation is less likely, although not impossible, with the lowerdensity foodstuffs targeted directly in e-beam irradiators. Shielding for an x-ray irradiator is comparable to that required for the other two types of units. X-rays, like gamma rays, are high-energy photons, have a similar penetrability (Table 11.1), and can be used for bulk packages or higher-density foodstuffs. However, x-rays share with e-beam the commercial advantage of being produced electronically and are therefore able to be completely inactivated in a power-off state. The design of some commercial e-beam irradiators includes the ability to convert from e-beam to x-ray operation as needed by installation and removal of the metal target plate. While x-rays would seem to be an attractive middle ground between gamma and ebeam irradiators, the low energy conversion efficiency, the extensive heat build-up in the metal plate, and the low, but measurable, neutron scattering are drawbacks for commercial application.

11.2.4 GENERAL MODE

OF

ACTION

A stream of high-energy photons, either gamma rays or x-rays, can energize electrons within the atoms of the target. These electrons may leave the atom completely (ionization), or the energy of the electrons may rise to a higher level within the atom (excitation). Both processes may yield free radicals, i.e., atoms with unpaired electrons on their outer shell. The stream of high-energy electrons in an e-beam interacts with the atoms directly to create free radicals. These free radicals are very reactive because their unpaired electrons may pair up with the outer shell electrons of the atoms that make up cellular components. Because water makes up the bulk of mass of foods, particularly fresh fruits and vegetables, the water molecule is most frequently affected. The majority of the absorbed energy from the ionizing radiation treatment goes into the creation of hydrogen and hydroxyl radicals from water molecules [26]. The interaction of these free radicals with the organic molecules of the food is the main mode of action of ionizing radiation. Under conditions of limited free water, such as in dried or frozen products, radicals are produced with less efficiency and have reduced mobility. In these products, higher doses become necessary for microbial control [37, 38]. In most discussions of food irradiation, a recurrent question arises regarding the mode of action of high-energy photons (gamma or x-ray) vs. high-energy electrons (e-beam), and the possibility of differential antimicrobial efficacy or effect on product sensory attributes or physiology [39]. It is tempting to make a blanket statement that, based on the physics of energy transfer in the process of generating radical molecules, one would expect to see no appreciable difference in irradiated product based on the means of irradiation. However, direct-comparison experiments to verify this contention with fresh produce are lacking. Papaya treated with gamma radiation tended to be firmer than fruit similarly treated (0.4 or 0.6 kGy) with x-rays; significant, though small, differences in aroma, texture, and color were detectable to one set of triplicate sensory panels but not to another [40]. In the available direct-comparison studies of meats, high-energy photons and high-energy electrons tend to have similar, but not quite identical, effects. E-beam and gamma (1.5 or 3 kGy) were similarly effective at eliminating Salmonella typhimurium from refrigerated beef steaks, but e-beam was not as effective as gamma in the elimination of Pseudomonas fluorescens (a common spoilage bacterium found on meats and produce) [41]. Additional direct comparisons of the various irradiation methods are therefore warranted, especially studies that involve sensitive products such as fruits and vegetables.

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11.3 CHEMICAL EFFECTS OF IONIZING IRRADIATION IN FOODS Although irradiation shows considerable promise for extending shelf life and maintaining microbiological and sensory qualities of fruits, the literature is rich in contradictions about the results of irradiation of specific fruits. This leads to difficulty in standardizing the treatment. The success of the treatment depends on numerous factors. The most important ones are commodity and cultivar, dose of radiation, degree of maturity, physiological status of the fruits, temperature and atmosphere during and after treatment, pre- and postharvest practices, and susceptibility of the microorganisms to be controlled to radiation.

11.3.1 MACROMOLECULES Nucleic acids, because of their large size, are the main targets of free radicals generated by gamma rays. They may be affected in various ways, leading to breaks in one or both strands of DNA or to intra- or extramolecular cross-linking. Polysaccharides, cellulose, pectins, and starches may be partly depolymerized. Proteins are relatively little affected, although reduction of disulfide bonds may lead to inactivation of active sites and conformational changes in enzymes.

11.3.2 SMALL MOLECULES Sugars may be hydrolyzed or oxidized when subjected to gamma radiation. Free amino acids can be deaminated. The small molecules most affected are the polyunsaturated fatty acids. Free radicals react with polyunsaturated fatty acids, producing unstable hydroperoxides and a range of further degradation products. The effect of irradiation on vitamins has been studied extensively. Certain vitamins (A, B12, C, E, K, thiamine), particularly those with anitoxidant activity, are degraded when irradiation is carried out in the presence of oxygen. Loss of vitamin C is commonly overestimated in the literature because ascorbic acid is oxidized to dehydroascorbic acid that is still active as vitamin C. Dehydroascorbic acid is, for the greater part, reduced back to ascorbic acid in living plant tissues. Only when dehydroascorbic acid is further oxidized to diketoglutonic acid is it irreversibly lost as vitamin. In general, small molecules, including vitamins, are little affected by low-level irradiation, and to a lesser extent, by thermal processing.

11.3.3 PROTECTION

IN

COMPLEX FOODS

Antioxidants added to solutions are known to protect sugars, vitamins, and proteins, as well as suspended bacteria from the effects of ionizing radiation [42, 43]. It may therefore be theorized that naturally occurring antioxidants may serve to protect plant tissue from radical molecules. However, the role of natural antioxidants in determining radiation tolerance of fruits and vegetables has not been addressed directly. Kader [15] and Dupont et al. [44] grouped fruits and vegetables according to their tolerance for irradiation. Products that are more radiation tolerant are, in some cases, also reported to be higher in antioxidant concentration, and lower radiation tolerance was similarly associated with low antioxidant concentration [45, 46]. However, methodological differences in how the produce was evaluated in the various studies prevent definitive conclusions. It is important to note that each commodity, and even each cultivar, has its own limits of tolerance. The dose to be applied should be preliminarily tested on a sample.

11.4 BIOLOGICAL EFFECTS OF IONIZING IRRADIATION IN FOODS 11.4.1 PRINCIPAL TARGETS The principal targets of the radical molecules, with respect to biochemical function, are nucleic acids and membrane lipids. Alterations in DNA will affect gene expression and the biosynthesis

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of various enzymes and will interfere with cell division. DNA is generally more sensitive to damage from radicals during active replication or transcription [47]. Bacterial plasmid DNA may also be damaged, disrupting and inactivating plasmid-encoded genes; pathogenic bacteria that carry plasmid-encoded virulence genes may therefore have a reduced virulence after irradiation [48]. Alterations in membrane lipids, particularly polyunsaturated lipids, lead to the perturbation of the membrane and to deleterious effects on various membrane functions — most important, permeability. Proteins are scarcely affected, and enzyme inactivation is rarely observed when irradiated in situ. In pure solution, peptide bonds and sulfhydryl groups, often involved in enzyme activity sites, may hydrolize or oxidize into disulfide intra- or inter-peptide bridges. However, the activity of membrane-associated enzymes may be affected as a secondary effect of membrane lipid degradation.

11.4.2 SENSITIVITY

OF

ORGANISMS

The physiological effects of ionizing radiation on spoilage organisms and human pathogens have been reviewed by Monk et al. [49] and Diehl [26]. Fungi are, typically, more resistant to radiation than bacteria [49, 50]. D10 values, i.e., the amount of radiation necessary to effect a 90% (1-log) reduction, have been reported for yeasts and molds in the range of 1 to 3 kGy [51–53] as opposed to D10 of 0.3 to 0.7 kGy for pathogenic bacteria on produce and fruit juices [54–57]. Human pathogenic viruses are usually more resistant to radiation than bacteria or fungi. The doses required to achieve meaningful virus population reductions, as a rule, result in loss of sensorial quality of the produce [27, 30, 31, 49]. D10 for plant pathogenic fungi has been shown to range from 0.4 to 1.1 kGy [58]. Low dose irradiation has been shown to suppress some plant pathogenic fungi responsible for storage losses [16]. However, the relative resistance of fungi suggests that irradiation is best suited to control of bacterial pathogens, as opposed to viral or fungal pathogens. A concern expressed by various authors [59] is the possibility of increased human or plant pathogen growth in storage following irradiation due to reduced interspecies competition. Although studies of the effect of low radiation doses on the microbial ecology of produce are lacking, studies of irradiated meats have tended to support the position that while irradiation of a product reduces the total population of bacteria on the sample, it does not lead to increased recolonization by pathogens [59, 60, 61]. However, Matches and Liston [62] found that Salmonella grew more rapidly on irradiated vs. nonirradiated fish fillets. Carlin et al. [63] showed that the growth of L. monocytogenes inoculated onto previously disinfected leaves of endive (Cichorium endiva) was significantly enhanced. While this study [63] relied on chemical disinfection rather than irradiation, the result indicates that the interaction of native microflora and bacterial pathogens such as L. monocytogenes is an important subject for future study.

11.5 TECHNICAL ASPECTS OF FOOD IRRADIATION 11.5.1 DOSIMETRY Measurement of the dose of radiation absorbed by the food (dosimetry) is the underlying key to effective use of irradiation. It is important to stress that the quality of the dosimetry will determine the quality of the irradiated product. Material near the outside of a carton or package tends to receive a somewhat higher dose than the product in the center, owing to its proximity to the radiation source. The ratio of the maximum amount absorbed (typically, near the perimeter) to the minimum absorbed (typically, near the center) is referred to as the max/min ratio. In an ideal, perfectly uniform treatment system, this value would be 1; in practicality, max/min ratios of 1.5 to 2 are more typically obtained. The max/min ratio is influenced by the bulk density of the product being treated, the penetrability of the radiation type, the strength of the source, and localized density variations within the product (voids, pits, etc.). A Monte Carlo simulation of a single apple’s dose absorption profile suggests that even within a single piece of fruit, the max/min ratio may be in

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excess of 2.4 due to the topographical complexity of apples [64]. A process that has a high max/min ratio may result in one part of a shipment receiving too high a dose (resulting in violation of regulations, sensory damage to product, etc.) while another part receives too low a dose (resulting in surviving insects or bacteria, inadequately delayed ripening or sprouting, etc.). Thus, design of effective irradiation protocols begins with a preliminary 3-D mapping of the doses absorbed by the product in each part of the package, when it is packaged in the type of carton and packing arrangement expected to be used commercially. Dosimetry is important for establishing the desired dose for each commodity, for obtaining data required for approval of a petition by regulatory agencies, and for developing quality control procedures in the irradiation plant. Dosimetry is also used to determine the configuration of the irradiation field in the facility. Mclaughlin et al. [65] have reviewed the technical aspects of dosimetry in detail. Although the absorbed dose can be calculated, experimental dosimetry is preferred. The energy absorbed is measured by placing dosimeters (radiation sensors) inside the lot being irradiated. In this way, the distribution of the energy absorbed and the minimum and maximum dose can be determined. The choice of dosimeter is influenced by several factors, including environment (humidity, temperature, and light) stability under the process conditions, dose range, and dose rate. It is therefore important to have a quality control procedure to test the dosimeters. An example of a chromatographic dosimeter commercially available for routine measurement of absorbed dose is the Harwell YR (Didcot, U.K.). This is a radiation-sensitive polymethylmethacrylate-based product hat darkens on exposure to ionizing radiation. Chromatographic dosimeters are read with a spectrophotometer, and the extent of color change is calibrated to absorbed dose. Electron paramagnetic resonance (EPR) is a sensitive dosimetry technique suitable for research purposes. The dosimeter consists of an alanine pellet that undergoes a measurable shift in paramagnetic spin state following exposure to ionizing radiation. With proper storage, the dosimeter pellet can hold the new electron spin state for months, providing an opportunity for repeated reference to the same set of processed dosimeters.

11.5.2 DETECTION

OF

PRIOR IRRADIATION

Although irradiation of various food products has been approved by various nations, detection methods are not required for clearance. Development of methods to detect irradiation and to determine the dose absorbed are, nevertheless, important for a number of reasons, e.g., differences in national legislation, control of international trade, quality control, and consumer reassurance. Regulatory authorities and processors are the two groups that are most interested in the development of such methods. Whenever attempts have been made to detect irradiation on an experimental basis, the task has proven most difficult. The changes are not radiation specific. It is probably not possible to develop a detection method for foods in general [66]; however, it is possible to devise such methods for specific food groups. Detection of irradiation in food has been reviewed in detail by Raffi [67] and Delincee [66]. Detection of irradiation has been investigated for a large number of fruits including banana, mango, papaya, and strawberry [Figure 11.1; 68]. These methods were usually based on chemical, physical, and biological changes in fruits. Some methods studied are listed below. After irradiation, carbohydrates may react to give acids and carbonyl compounds. Den Drijver et al. [69] detected glucosone (D-arabinohexos-2-ulose) in an irradiated sugar mixture solution mimicking the sugar composition of the mango, and not in the untreated controls. Studies with apples showed significant changes in starch and pectins [70, 71]. Detection of irradiation based on DNA analysis is promising and widely applicable; however, more research is needed to obtain results that are both specific and reliable [68]. Inhibition of seed germination (half-embryo test) has been used as an indication of exposure to radiation for citrus fruits [72]. It should be noted, however, that alfalfa seeds treated with 5 kGy did not show a significant change in germination rate, suggesting that caution should be exercised with this method of detection [73].

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FIGURE 11.1 Fruits investigated for detection of radiation treatment. (Adapted from IAEA, Analytical Detection Methods for Irradiated Foods, IAEA-TECDOC-587, Vienna, 1991.)

EPR was shown to be reliable with strawberry achenes exposed to a dose of 1 kGy or more [74] and can be used with blueberry, fig, raspberry, and red currant. EPR can also detect prior irradiation of alfalfa seeds (D.W. Thayer, personal communication). The test may not be reliable with all fruits, as shown with plums [75] and with citrus and grapes [68]. Detection of radiationinduced hydrocarbons by gas chromatography or mass spectrometry has been successfully employed in irradiated (1 kGy) perilla (Perilla frutescens) seeds [76]. Rahman et al. [77] showed that the irradiation dose used for quarantine treatment induced a significant decrease of the size of the supraesophageal ganglion of insects. Measurement of luminescence is the first detection method used routinely for inspection purposes. The method, used in Germany, is based on the determination of light emitted by oxidizing substances formed and trapped in irradiated tissue. It can be used with produce containing solid inclusions, such as achenes and adhering minerals. Chemiluminescence, resulting from oxidation reactions after irradiation, was used to detect irradiation of spices, dried herbs, and fruits [78, 79]. Thermoluminescence is released after heating of irradiated products. It is more reliable than chemiluminescence since it is less influenced by interferences from test conditions [68]. This approach gave good results with strawberries [79]. Thermoluminescence has shown promise in detecting irradiation treatment of spices [81] and potatoes [82]. Two methods that showed some potential with meat were tested on fruits with limited success. Changes induced to the microflora by irradiation were too influenced by pre- and postharvest conditions to be reliable. Determination of o-tyrosine in proteins was attempted with strawberries but showed high background values and artifacts [83, 84]. Changes in the content of phenolic compounds [85] may not be specific to radiation because such changes are common under stress conditions. Presently, the more promising irradiation detection methods for fruits seem to be EPR and thermoluminescence. A particular challenge is the low dose of irradiation, 1 kGy or less, with which fruits are treated. Further investigation, standardization, and development of instrumentation will eventually lead to practical inspection methods. It should be noted that ex post facto dosimetry is difficult; while these and other detection methods may be able to indicate that a given product was irradiated, it is unlikely to determine when the product was so treated, or when the dose was delivered [26].

11.6 IRRADIATION FACILITIES Although e-beam, x-ray, and gamma radiation have essentially identical effects on food, irradiation facilities differ greatly with respect to design and physical arrangement. The vast majority of gamma-based facilities use cobalt-60 as the radionucleotide source. The majority of new irradiation

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Product Carrier (with food cartons)

Palletized Food products

Product Carrier Tracks

Concrete Radiation Shield (cut-away view)

Robotic De-palletizer Product Carriers

Source Assembly Track (vertical to horizontal)

Cobalt 60 Storage Pool

FIGURE 11.2 Semiautomated cobalt-60 irradiator specially designed for processing food products. (From Nordion International Inc., Kanata, Ontario. With permission.)

facilities under construction in the U.S. are e-beam or dual-use e-beam and x-ray facilities. Several new e-beam facilities have been built in the U.S. in the last decade for irradiation of meat and poultry products, shell eggs, and Hawaiian fruits destined for the U.S. mainland, such as papaya. In 2001, the U.S. Postal Service contracted to purchase eight e-beam systems for irradiation of mail in conjunction with antiterrorism efforts, with options to purchase as many as 12 additional facilities. Thus, in terms of market share and processing capacity, e-beam technology may benefit from increased familiarity with the technology and the economies of scale that come with mass production of the equipment. Figure 11.2 depicts a typical cobalt-60 automatic carrier irradiator specifically designed for processing food products (Nordion International Inc., Kanata, Ontario). This unit is suitable for processing large volumes (up to 8 million ft3 annual throughput) in continuous operation. Produce is loaded into boxes on a conveyer. The boxes move around the cobalt-60 source within a thick concrete enclosure shielding the environment from the gamma rays. The source is lowered into a pool of water when not in use. The dose applied is controlled by regulating the speed of the conveyer. In 2001, a joint venture between an American manufacturer of e-beam/x-ray equipment (SureBeam, San Diego, CA) and a Brazilian irradiation processing corporation (Tech Ion, Manaus, Brazil) was announced [86]. This joint venture is expected to facilitate irradiation of fruit and vegetables produced in Brazil, with the stated goals of reducing postharvest losses in storage and transshipment, and improving disinfestation. A gamma-based irradiation facility is currently in operation in Manaus, Brazil. Fruits irradiated at this facility are destined primarily for the large population centers of Sao Paulo and Rio de Janeiro in the southeast [87]. As part of a significant effort to promote and facilitate food irradiation, the government of Brazil recently established the Food Irradiation Center of Excellence in Rio de Janeiro [86]. Near the city of Rio de Janeiro, a large-scale irradiation facility is under construction, which is designed to incorporate e-beam, x-ray, and gamma-ray processing capacity at a single site [35]. This facility is expected to be operational in 2002 with an accelerator of 15 kW power at 10 MeV, with an expected capacity of 120,000 t/year. The processing capacity of the x-ray processing unit is expected

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to be 80,000 t/year. The cobalt-60 source strength is planned to be 600,000 Ci with a capacity of 120,000 t/year. This facility will process fruits and vegetables intended for domestic consumption, as well as for export. An x-ray facility near Hilo, HI, is currently irradiating fresh fruits intended for shipment to the mainland U.S., with particular focus on the New York, Washington, Oregon, California, and Minnesota markets. The fruits treated include papaya, rambutan, lychee, atemoya, longan, and star fruit [88]. The fruits are treated with 0.25 to 0.40 kGy for disinfestation purposes; an extended shelf life is an additional benefit to the treatment. The projected production capacity of this facility is 12,500 t/year.

11.7 CONTROLLING REGULATIONS: COMMODITIES, DOSE LIMITS, AND PURPOSES Each nation issues its own regulations concerning the commodities that it allows to be irradiated, the maximum (and in some cases, the minimum) dose to be applied, and the purposes for which a product may be irradiated. In many cases, the intended purpose of the irradiation treatment will determine the maximum dose that can be legally applied. Regulations concerning the approval and inspection of food irradiation plants are also nation specific. It is expected, however, that as food processing operations, irradiation facilities will be held to the highest standards of cleanliness and safety. The regulations governing products, dose limits, purposes, etc., can range from very restrictive to very permissive. The International Consultative Group on Food Irradiation (ICGFI) is a group of experts from 46 nations organized under a joint FAO/IAEA/WHO collaboration to review and disseminate scientific information regarding food irradiation. ICGFI maintains an online listing of the regulatory limits governing food irradiation for a variety of developed and developing nations [89]. This section will provide an overview of some key nations with regard to regulations dealing with fruits and vegetables. With changing consumer attitudes toward irradiated foods, the implementation of irradiation on food is increasing worldwide. The controlling regulations in many nations are currently being reviewed and revised, and the current state of affairs may therefore differ somewhat from the information presented herein. U.S. regulations permit the irradiation of dry or dehydrated vegetable-derived spices, seasonings, and flavorings, as well as coloring agents (e.g., paprika) to doses of up to 30 kGy [90]. Approved sources are gamma rays from cobalt-60 or cesium-137; accelerated electrons from a machine source, not exceeding 10 MeV (to eliminate the risk of inducing radioactivity); or x-rays from a machine source, not exceeding 5 MeV. The regulatory limit for fresh fruits and vegetables is 1 kGy, limited to the specific uses of disinfestation and inhibition of produce growth and maturation [90]. For comparison, irradiation to control food-borne pathogens is permitted at higher doses for shell eggs (3 kGy), seeds used for growing sprouts (8 kGy), fresh and frozen poultry (3 kGy), fresh meats (4.5 kGy), and frozen meats (7 kGy). Irradiated foods must bear the internationally recognized radura logo (Figure 11.3) along with the statement “treated with (or by) irradiation.” In 1999, a coalition of U.S. food processors petitioned the U.S. FDA to amend U.S. regulations to allow doses of up to 4.5 kGy for a wide variety of refrigerated and ready-to-eat meat and vegetable products, including juices, and doses up to 10 kGy for frozen meat, vegetable, and juice products. Elimination of human pathogens is the primary goal of these requested dose limits; the potential for extension of shelf life is regarded as a secondary goal [91]. Many European nations (e.g., Austria, Germany, Ireland) have approved only dry herbs, spices, and vegetable seasonings for irradiation, with a maximum dose of 10 kGy. Canada has approved for herbs, spices, and seasonings (max 10 kGy), and allows irradiation of onions and potatoes for sprout suppression (max 0.15 kGy). Mexico will allow any type of fresh fruit to be treated with up to 1 kGy for quarantine treatment or to delay ripening, and up to 2.5 kGy to extend shelf life.

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FIGURE 11.3 The radura — the internationally recognized logo identifying foods that have been irradiated.

Mexican dried fruit may be treated with 1 kGy for disinfestation but with up to 10 kGy for microbial control. The U.K. allows fresh or dried fruit to be treated for disinfestation (max 2 kGy). India has approved irradiation disinfestation of dates and mangoes (max 0.75 kGy). Japan has approved treatment of a single product, potato, for sprout inhibition (max 0.15 kGy). Australia allows irradiation of herbs and spices for disinfestation (max 6 kGy) or microbial control (max 30 kGy). Brazil possesses the most open regulations concerning food irradiation. Prior to 2001, a specified list of fruit commodities were permitted for irradiation [87]. This list included avocado, banana, orange, lemon, and other fruits important for domestic consumption and the export market. The maximum dose permitted was 1 kGy, although the treatment could be applied for a variety of purposes, including disinfestation, delay of ripening, shelf-life extension, or (in combination with a heat treatment) control of microbial load. A joint FAO/IAEA/WHO study [92] examined the wholesomeness of food irradiated above 10 kGy. In that study, it was concluded that food may be safely irradiated to any dose sufficient to achieve the desired physiological or microbial outcome, without appreciable loss of nutritional adequacy. Based on this recommendation, the Brazilian government dramatically revised its regulations governing food irradiation [89]. As of 2001, any food product in Brazil, such as fruit, vegetable, meat, poultry, and fish, etc., may be irradiated to any dose for any purpose. The only limit on the dose applied is based on the product’s physiological tolerance for irradiation from a quality or marketability standpoint. Brazil’s “any product, any dose, any purpose” stance, although based on the scientific recommendations of respected international authorities, is markedly more open than most other nations. Brazilian products that are intended for export must still meet the regulatory requirements of the importing nations, and are therefore, as a matter of practice, subject to dose limits and purpose restrictions. Although efforts are underway to encourage Brazil’s trading partners to lift restrictions on irradiated foods (J.F.B. Medieros, personal communication), dramatic changes to the regulations of importing nations are not expected in the near future.

11.8 APPLICATIONS OF IRRADIATION TO FRUIT Fresh produce may be irradiated for a number of purposes, including inhibition of sprouting in the case of some vegetables, and, of more significance for irradiated fruit, delay of ripening, insect

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disinfestation, and reduction of microbial load. Of these, the most important effect of irradiation in food processing, whether meats and poultry or fresh fruits and vegetables, is reduction of microbial load. While this has traditionally meant elimination of produce spoilage pathogens so as to extend shelf life [93, 94], the elimination of human pathogens is also of increasing importance [16, 17, 25]. Refrigeration and modified-atmosphere packaging are currently the primary means of preserving fresh produce [20]. Fresh produce is generally more sensitive to ionizing radiation than meat or poultry products. Unlike meat, fresh produce is living tissue that respires, maintains water relations with its environment, and, in many cases, may continue synthesis of secondary metabolites [95]. Irradiation can alter these processes, leading to changes in firmness, aroma, color, or taste [25, 30]. Plant physiology and surface-associated microbial ecology can be altered by irradiation; these effects are seen most clearly during subsequent refrigerated handling and storage [27, 82, 96]. In general, ionizing radiation causes less alteration to fresh produce than other preservation techniques with equivalent benefits, e.g., thermal treatment. However, irradiation does not replace these other methods; it supplements them. Irradiated fresh produce, like all irradiated foods, remain subject to the basic rules of good manufacturing practice for preservation of quality and food safety [2].

11.8.1 DELAY

OF

RIPENING

Ripening of fruits can be delayed by doses of 0.2 to 0.5 kGy. The underlying mechanism of the effect is not well understood. It involves interference with biochemical processes that are part of ripening and senescence [97]. Ethylene production by Gala apples was reduced by gamma irradiation (0.44 kGy) [98]. Most studies on fruit irradiation have been carried out with gamma rays. E-beams do not penetrate enough to act on metabolism and increase shelf life of certain fruits, e.g., avocado [99]. Tolerance varies with degree of maturity, as discussed by many authors. Delay in ripening of climacteric fruits requires that the treatment be applied before the onset of the climacteric increase in ethylene production [29, 97]. The need to treat at an early stage of maturity brings about a loss of quality, and may cause abnormal ripening and uneven coloring; however, in the case of tomato, very early treatment may accelerate ripening due to ethylene production in response to wounding [100].

11.8.2 DISINFESTATION Disinfestation, the control of arthropod pests, can be achieved by doses up to 3 kGy. Sterilization to prevent reproduction of the insects during or after storage can be achieved by doses of 0.03 to 0.2 kGy. Doses sufficient to kill insects outright are in the range of 1 to 3 kGy. This application of irradiation is among the most promising since fumigation with chemicals like ethylene dibromide (prevented in the U.S. since 1984) is increasingly questioned. Irradiation is probably the most effective quarantine treatment for control of fruit flies and other insects in a number of commodities, including mango and papaya. Other quarantine treatments exist but are not easily applicable to all fruits. Cold treatment requires several days and is therefore not useful for fruits with short shelf lives. Heat treatment occasionally causes physiological disorders and discoloration.

11.8.3 REDUCTION

OF

MICROBIAL LOAD

The efficacy of ionizing radiation to reduce the microbial load is dependent on a number of factors. Plant fruits and stems typically support 103 to 106 colony-forming units (cfu) per gram of plant tissue [20, 101]. These organisms come from the environment in which the plants are grown, including the soil, water, air, manure, and compost, as well as from postharvest handling, processing, and shipping [24]. It is increasingly recognized that bacteria may become internalized in fresh produce and survive for days or weeks, reducing the efficacy of traditional, surface-oriented antimicrobial measures such as chemical rinses and washes [33, 102–105].

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The vast majority of plant-associated microorganisms are nonpathogenic bacteria. These bacteria often form biofilms that can further reduce the efficacy of antimicrobial measures [106–108]. Human pathogens such as E. coli and Salmonella have been observed to form durable biofilms on industrial surfaces [109, 110]. The extent to which enteric bacteria, as well as the psychotrophs L. monocytogenes and Yersinia entericolitica, participate in preexisting phytoplane biofilms formed by nonpathogenic bacteria is not known [108, 111]. Ionizing radiation can penetrate sheltered areas of fruits, and, in the case of gamma and x-ray, penetrate internal structures to inactivate bacteria. As has been discussed, however, the radiation types differ in penetrability, most significantly in higher-density products. In a study of sliced cantaloupe inoculated with human pathogens (106 cfu/g), after radiation treatment with 3 kGy from a 5-MeV e-beam, Draughon et al. [112] were able to recover Salmonella and Listeria, but not Staphylococcus. The authors concluded that for the 1-cm thick slices examined, an e-beam dose of greater than 3 kGy would be necessary to completely eliminate Salmonella or Listeria. Vegetables have been the object of numerous studies of the elimination and recovery of human pathogenic bacteria following irradiation. Total aerobic plate count and inoculated L. monocytogenes were reduced by ~4 logs on precut bell pepper following a dose of 1 kGy. The pathogen regrew to initial levels within 4 d on peppers subsequently stored at 15 or 10∞C, but remained low on peppers stored at 5∞C [113]. The authors concluded that irradiation followed by refrigeration effectively suppressed pathogen growth throughout the useful shelf life of the produce. E. coli and L. monocytogenes were effectively eliminated (> 5 logs) from diced celery by 1 kGy [28]. Also in that study, total aerobic counts were determined, following irradiation or one of three conventional treatments (i.e., acidification, blanching, or chlorination). Acidification reduced initial counts to a degree comparable to that of 1 kGy, but by the end of the storage period (22 d), the aerobic population following conventional treatments had regrown to equal (acidification) or exceed (blanching and chlorination) the untreated controls, while that of irradiated (0.5 and 1 kGy) celery remained significantly lower throughout the study. The D10 value for human or plant pathogenic bacteria can range from 0.3 to 0.7 kGy [54–57]. Achieving a 5-log reduction, then, would require between 1.5 kGy and 3.5 kGy. Fresh produce will generally tolerate doses of 2 kGy, although variation exists. This suggests, therefore, that irradiation will most likely play a role as one of several options in preserving fruit quality and microbial safety. These other options might include mild thermal treatment, modified atmosphere packaging, ozone treatment, among other processes. A complete discussion of combination treatments is presented later in this chapter.

11.9 RESPONSE OF FRUIT TISSUE TO RADIATION TREATMENT 11.9.1 WOUND RESPONSE Plant tissues are relatively sensitive to ionizing radiation and show a typical wound response, characterized by a transient increase in respiration and ethylene production, even at low doses. Such a wound response was shown after irradiation of strawberries [114]. The rate of respiration increased linearly with increasing dose of gamma irradiation. The effect was transient and the rate of respiration decreased back to preirradiation levels within 24 h for the lowest dose, 0.3 kGy, but slower with an increasing dose. The low dose of 0.3 kGy was insufficient to control mold development, as indicated by the progressive increase in rate of respiration in control and 0.3 kGy irradiated fruits. This progressive mold development could be prevented by treatment with a fungicide and by irradiation at 1 kGy. Ethylene production also increased after irradiation, but in contrast with respiration, it reached a maximum at 1 kGy. The latter observation suggests that irradiation beyond 1 kGy caused membrane damage since ethylene production is membrane associated. Similar results were obtained by Larrigaudiere et al. [115] with cherry tomatoes. Other typical wound (and stress) responses are (1) stimulation of phenol biosynthesis, particularly stim-

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ulation of the first enzyme in the pathway phenylalanine-ammonia lyase (PAL) [116] that predisposes the tissue to enzymatic browning, and (2) an increase in enzymatic defenses against free radicals scavenged by peroxidase, catalase, and superoxide dismutase [117]. Notwithstanding the information presented herein and the extensive body of research on the subject of irradiation of fresh horticultural products (by 1983, more than 1150 publications had been compiled [118]), not enough is known about the reaction of these live tissues to treatment. Of particular interest is the interaction of irradiation with commonly used agricultural practices such as application of hormone or hormone-like chemicals.

11.9.2 DELAY

OF

RIPENING

The mechanisms whereby irradiation delays ripening in several fruits are not fully understood at the present time. The effect is most associated with climacteric fruits, i.e., fruits that show a transient increase in their rate of respiration and ethylene production at the onset of ripening. The majority of fruits of tropical origin are climacteric, and they have, therefore, a short shelf life. They are also sensitive to low temperature, which is the main preservation method for produce. The biochemical basis for the delay in ripening is discussed by Thomas [119]. Irradiation was shown to influence the respitory pattern of the fruits [97] and to cause a shift from glycolysis toward pentose phosphate shunt in bananas and toward the glyoxylate cycle in bananas and mangos [119]. Dubery and coworkers [97] showed that irradiation did not truly delay ripening of mango, but that it interfered with some biochemical processes, including respiration, involved in senescence. Gamma irradiation promoted respiration in Gala apples that had been treated with an ethylene action inhibitor (1methylcyclopropene) and subsequently stored at 20∞C for 3 weeks [120]. An important mechanism whereby the treatment slows down the ripening in climacteric fruits is not fully understood but seems to be the reduction of ethylene biosynthesis and of sensitivity of the tissue to ethylene, probably related to alterations in membrane physical characteristics [29, 100, 121]. The inhibition of ripening of irradiated Bartlett pears was not reversed by subsequent exposure to ethylene, supporting the suggestion of inhibited ethylene action in the fruit tissue [122]. The production of ethylene and a variety of volatile esters and alcohols was inhibited in irradiated Gala apples by doses as low as 0.44 kGy [98].

11.9.3 POSTHARVEST DISEASE

AND

STORAGE LOSSES

According to Dubery et al. [97], the physiological status of the fruits at the time of irradiation is probably the most important factor influencing the response of the tissues. This status is, in turn, influenced by the preirradiation history of the fruits, e.g., mechanical injury, season, and humidity at the time of harvest. The response of each individual batch of fruits is therefore difficult to predict. One important element of success is rapid precooling of the produce and irradiation at the lowest temperature tolerable by the fruits. The ripening inhibition in fruits that have been treated with low doses of ionizing radiation has a notable side effect with regard to plant pathogens. A dose of radiation that is too low to control fungal development directly may, still, indirectly result in reduced fungal disease. Fruits progressively lose their resistance to plant pathogens with ripening. Fruits that have had their ripening delayed retain a higher level of host resistance to fungal plant pathogens. As the spoilage fungi are unable to establish themselves, secondary bacterial plant pathogens such as Erwinia spp. are also less able to become established on the fruit, and overall microbial development will be delayed as an added benefit. As with all applications of irradiation to food products, the treatment functions best as a terminal processing step in a system where postirradiation recontamination is avoided. Also, to maximize the efficacy of the process, preharvest cultural practices should reduce, as much as possible, the initial microbial load on the fruit surface. It is preferable, therefore, that the fruits be packaged before radiation treatment. A brief discussion of the effects of irradiation on packaging materials is presented later in this chapter.

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11.9.4 PHYSIOLOGICAL DISORDERS When exposed to doses beyond their limits of tolerance, fruits suffer physiological disorders that cause undesirable symptoms, mainly tissue softening and enzymatic browning [15, 123]. Tissue softening is caused by partial depolymerization of cell wall polysaccharides, mainly cellulose and pectins, and by damage to cell membranes, which results in loss of intracellular water [30, 124, 125]. Enzymatic browning is an indication of decompartmentation due to damage to membranes, bringing phenolic substrates in contact with polyphenoloxidases [121]. A calcium dip pretreatment prevented loss of firmness and texture in irradiated diced tomatoes [126]. Also, as alteration of the physical characteristics of cell membranes results from oxidative attack on polyunsaturated fatty acids of membrane lipids by oxygen-free radicals, irradiating in an atmosphere with reduced oxygen contact can reduce these effects. However, under these conditions, efficiency of the treatment is also reduced. A high carbon dioxide atmosphere was shown to protect tissues from radiationinduced loss of membrane proteins [127].

11.10 QUALITY OF IRRADIATED FRUITS 11.10.1 TEXTURE, COLOR,

AND

SWEETNESS

Broad statements regarding the effect of ionizing radiation on fruit quality are difficult to make. It is generally true that irradiation tends to make fruits softer and sweeter, primarily through hydrolysis of pectins and release of sugars following depolymerization of carbohydrate polymers [30, 124]. Softer fruit are more susceptible to damage during handling and shipping. This fact, in addition to the desire to reduce the possibility of postirradiation recontamination, reinforces the role of irradiation as a terminal, postpackaging process step. Strawberries softened following a dose of 1 or 2 kGy, and chemical analysis showed an association with an increase in water-soluble pectin and a decrease in oxalate-soluble pectin [30]. Irradiation enhanced the sweetness of strawberries by reducing titratable acidity in comparison with the unirradiated controls [51]. Titratable acidity was similarly decreased in stored apples following irradiation [120]. Gibberellic acid-treated grapefruit tolerated 0.3 kGy with little loss of quality of the fruit, pulp, or juice; however, pitting of the skin, softening of the fruit, and loss of juice quality rose to unacceptable levels following treatment with 0.6 kGy [128]. Firmness of papaya, rambutan, and Kau orange declined following x-ray treatment (0.75 kGy) [129]. Two blueberry cultivars, i.e., Brightwell and Tifblue, did not differ in their response to gamma radiation, with no negative impact after 0.5 kGy; both cultivars showed softening and loss of quality following 1 kGy [130]. The flavor and texture of Sharpblue blueberries were considered acceptable following electron beam irradiation (1 kGy) [131], but the flavor and texture of Climax blueberries similarly irradiated (1 kGy) [132] declined significantly. Color, pH, postharvest decay rate, and other agronomic factors related to the irradiated blueberries were reportedly not affected by dose level in these studies. However, climacteric fruits that have been irradiated when mature, but before the fruits have ripened, may not ripen normally and may develop uneven coloring and skin discoloration [15]. As in the case of grapefruit, effects on skin color and blemishes can be dose dependent [128]. Fresh-cut cantaloupe melons were slightly bleached, softened, and off-flavored following 3 kGy, but these sensory attributes were unaffected by 1 kGy [133].

11.10.2 FLAVOR

AND

AROMA

Radiation-induced alteration of flavor and aroma is variable, and is influenced by the product tested, dose, fruit maturity, cultivar, storage conditions, and other agronomic production factors. With regard to taste, a distinction is made here between sweetness, which results from the balance of sugar content and acidity, and flavor resulting from the complex combinations of volatile, low

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C

Ethylene (log µmol . kg−1 . h−1)

1

5 4 3 2

0.1

1 0.01 B 1

0 kGy 0.5 kGy 1 kGy 1.5 kGy

D

0.8 0.4

0.1

0.01

0 1.2

Total volatile compounds (µmol . kg−1 . h−1)

A

0.0 0 5 10 15 20 Days at 20°C after irradiation

0 5 10 15 20 Days at 20°C after irradiation

FIGURE 11.4 Production of ethylene (A, B) and total volatile compounds (C, D) of Gala apples during ripening. The fruit treated with (B, D) and without (A, C) 0.5 mL MCP were exposed to 0, 0.44, 0.88, and 1.32 kGy gamma irradiation and kept at 20∞C. Vertical bars represent standard deviation. (From Fan, X. and Thayer, D.W., Quality of irradiated alfalfa sprouts, J. Food Prot., 64(10), 1574–1578, 2001. With permission.)

molecular weight compounds. This aspect of fruit quality may more accurately be referred to as the flavor–aroma complex as the two aspects are inseparably linked in fresh fruit. Modifications of taste had been reported by Kader [15] in the case of decreased acidity of strawberries and astringency in persimmons. The odor, color, and overall appearance of irradiated mango proved as acceptable to a sensory panel as nonirradiated controls immediately after treatment; the treated fruit remained acceptable after 50 d of refrigerated storage, while nonirradiated controls became unacceptable after less than 30 d [58]. Aroma and flavor of papaya, rambutan, and Kau orange were generally more intense following x-ray treatment (0.75 kGy) [129], although changes in other sensory characteristics were product specific. Alteration of aroma is to be expected because many fruit aroma volatiles originate from the breakdown of polyunsaturated fatty acids, which are among the main targets of irradiation-generated free radicals. Fan et al. [98] examined the ability of Gala apples to generate volatile compounds after having been treated with gamma radiation, 1-methylcyclopropene (MCP), or both. MCP is an ethylene action inhibitor, and is used agronomically to delay ripening. Figure 11.4 shows the concentrations of ethylene and total volatile compounds generated by the apples in this study. In non-MCP-treated fruit, irradiation inhibited production of most esters and some alcohols in a dosedependent response. In MCP-treated fruit, irradiation had little difference in ester and alcohol production over nonirradiated fruit. The authors conclude that MCP and irradiation result in comparable inhibition of volatiles in the apples.

11.10.3 NUTRITIONAL QUALITY This aspect of food irradiation has been thoroughly investigated [15, 16]. Generally, low doses do not bring about significant losses of vitamins. The natural antioxidants, vitamins A, C, and E, are relatively labile, as are thiamine and vitamin B12. Irradiation is known to oxidize a portion of the total ascorbic acid (vitamin C) to the dehydro form [134]. While both the forms of this vitamin are biologically active, suggesting minimal nutritional impact [135], it is uncommon to see values for the dehydro form co-reported in analyses of ascorbic acid concentration. The impact of irradiation on vitamin C therefore tends to be overstated. Irradiated alfalfa sprouts showed (1) no

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239

significant decline in total ascorbic acid concentration and (2) increased antioxidant power compared to nonirradiated controls [136].

11.10.4 INFLUENCE

OF

VARIETY/CULTIVAR

An important factor in the irradiation of fruits is consideration of variety- or cultivar-specific responses. Reviews of produce radiation sensitivity often combine different varieties under a single heading, e.g., “leafy vegetables” or “grape” [15, 44], a practice that does not recognize evidence of varietal differences. Plant variety or cultivar can influence the sensory response to irradiation, as well as the sensitivity of associated bacteria. Apple varieties show different degrees of internal browning and changes in firmness [120]. Blueberry varieties have cultivar-specific sensitivity, resulting in variable fruit quality [130–132]. In examining studies of irradiated vegetables, two potato varieties showed differences in the postirradiation (0.20 kGy) viscosity of their starches, whether isolated from irradiated tubers or irradiated in vitro [82]. Differences were seen in the respiration rates, loss of firmness, and in sensitivities associated with aerobic bacteria on iceberg lettuce [137] vs. romaine lettuce [96]. In a direct-comparison study, the radiation sensitivity of E. coli O157:H7 varied significantly when surface inoculated on iceberg or boston lettuce vs. green leaf or red leaf lettuce (B.A. Niemira, unpublished data).

11.10.5 QUALITY CONTROL Because of the possible effects of irradiation on fruit quality at higher doses, it is essential to develop an extensive postirradiation quality control procedure. Basic quality criteria should be measured immediately after treatment and also after a few days of postirradiation storage for delayed effects. Surface color changes and internal discoloration are good indicators of injury. Respiratory rates, ethylene production, ripening, and senescence parameters, e.g., texture and color, give indications on the physiological effects of the treatment. Nutritional effects can be measured through determination of sugars, organic acids, pH, critical vitamins, or carotenoids. Enzyme assays, e.g., as related to enzymatic browning, may monitor biochemical effects. A rigorous sensory evaluation is an important element of treatment optimization and routine quality testing.

11.11 IRRADIATION OF FRUIT JUICES AND PULPS The majority of fruit juices sold in the U.S. receive a conventional heat pasteurization treatment. The pasteurization process results in the loss of essential oils and other juice components, changing the flavor of the resulting juice. Fresh (i.e., nonthermally pasteurized) juices are valued for premium flavor and aroma; however, these products have also been responsible for outbreaks of salmonellosis, enterohemorrhagic E. coli infection, and hemolytic uremic syndrome [138]. Several strains of E. coli O157:H7 were found to survive in fruit pulps during extended refrigerated storage (up to 30 d in the case of grape and up to 20 d in the case of passion fruit pulp [139]). The U.S. FDA has implemented a policy [140] requiring 5-log reductions in human pathogen load in fresh juices, with these regulations to be in full effect for all processors by 2004. A variety of nonthermal means of reducing the microbial load of fresh juices were recently discussed [141]. These include pulsed electric fields, minimal thermal processing, high-pressure processing, and ultraviolet radiation. Regulatory approval for the use of ionizing radiation is currently being sought [91].

11.11.1 MICROBIOLOGY

OF IRRADIATED JUICES AND

PULPS

Preservation and shelf-life extension have been the historical focus of research on irradiation of juices. The key organisms of interest were yeasts and molds, which tend to have a higher D10 value than bacterial pathogens [49]. Juices and pulps, because of their high water content, represent an area that should provide the maximum opportunity for generation and mobility of radical molecules.

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However, the same high water content means that the bulk density of these products is higher than what might be seen in packaged fruits. This must be taken into consideration with regard to the penetrability and max/min ratios attained during the irradiation process. Phytopathogenic fungi were evaluated in irradiated mango pulp, with D10 values ranging from 0.39 to 1.11 kGy [58]. A complete discussion of this study is presented later in this chapter (see “Mango”). Niemira [56] reported a D10 value for Salmonella enteritidis of 0.35 kGy when irradiated in reconstituted orange juice. Buchanan et al. [54] showed that irradiation (1 kGy) effectively inactivated E. coli O157:H7 in inoculated commercial apple juices, with D10 values of 0.12, 0.16 and 0.21 kGy for the three isolates tested. Niemira et al. [57] found that the resistance of four Salmonella isolates irradiated in orange juice also varied, with D10 values ranging from 0.35 kGy to 0.71 kGy. For the most resistant isolate (Salmonella anatum), 3.5 kGy was indicated as the dose required to achieve a 5-log reduction. Similarly, Salmonella enterica serotype Hartford was reduced to undetectable levels in irradiated (3 kGy) orange juice [142]. In that study, the pathogen was able to be recovered through selective enrichment. The antimicrobial efficacy of irradiation is influenced by several factors. Native variation in pathogen resistance among different isolates has been noted [54, 57]. Buchanan et al. [54] showed that the radiation sensitivity of three strains of E. coli O157:H7 irradiated in apple juice was reduced by 54 to 67% by previous growth of the cultures in acid environments. Also, the sensitivity of one test strain decreased in more turbid juices. This difference was ascribed to the antioxidant power of the suspended solids, although data for the antioxidant power of the juices was not presented. L. monocytogenes irradiated in vitro was increasingly protected in solutions of increasing antioxidant power [43]. However, S. enteritidis suspended in commercial citrus juices of varying composition (orange vs. orange/tangerine, extra pulp vs. pulp free, regular vs. calcium enriched, etc.), and varying antioxidant power did not differ in D10 value following gamma irradiation [56]. The influence of natural and artificial antioxidants on radiation sensitivity is therefore an area to be addressed more fully.

11.11.2 SENSORY PROPERTIES

OF IRRADIATED JUICES AND

PULPS

A frequent added benefit of the application of ionizing radiation to fruits is the increase in juice yield during processing due to the softening of internal tissues. As in whole fruits, irradiation oxidizes a portion of juice ascorbic acid to the dehydro form; as in fruits, this conversion is expected to have little nutritional impact [134, 135]. Fetter et al. [143] irradiated a variety of commercially pasteurized juices to a maximum dose of 5 kGy and used taste panels to evaluate juice quality. The irradiated juices of orange, guava, tomato, red currant, black currant, apricot, peach, pear, and apple showed no reduction of flavor quality; the quality of similarly irradiated grape juice was reduced [143]. Chachin and Ogata [144] treated grape, apple, and orange juices with sterilizing (2 to 80 kGy) doses of gamma radiation. Loss of grape juice anthocyanin and orange juice betacarotene was evident after 10 kGy, and was dose dependent up to 80 kGy. A dose of 5 kGy caused browning in apple juice after 5 kGy, and ascorbic acid was reduced in apple and orange juices. Addition of 0.01% propyl gallate reduced the negative effects of high-dose (10 kGy) irradiation on the quality of orange juice, but did not protect apple juice [144]. A later study of high-dose irradiation (10 kGy) of orange juice resulted in a similarly unacceptable degree of flavor degradation and browning; however, the addition of 0.1% sorbic acid before irradiation effectively eliminated loss of flavor and reduced browning [145]. Fan and Thayer [146] determined that irradiation of apple juice caused an initial reduction of the juice’s brownness (A420) and increased the antioxidant power of the juice. These effects were dose dependant up to 8.9 kGy. During subsequent storage of the juice at 5∞C, the rate of browning of the irradiated juice was greater than that of nonirradiated juice, but after 16 d in storage, the irradiated juice was lighter in color. The impact of irradiation was influenced by processing temperature, but not by exclusion of oxygen from the juice or by the level of suspended matter in the juice [146]. Unpasteurized

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0.15

a

a

A a

D b

0.10

0.15

a ab

ab

c

c

0.00 0.15

B a

b

c

0.00 0.15

E

0.10 a

d e

0.05 0.00 0.15

a

C

b

c

d

0.00

0.05 0.00 0.15

F

0.10

0.10 0.05

0.05

Neral (ppm)

Geranial (ppm)

0.05

0.10

0.10

bc

ab 0

a ab

ab

b

0.89 2.24 4.23 8.71 Dose (kGy)

a 0

a

a

a

a

0.89 2.24 4.23 8.71 Dose (kGy)

0.05 0.00

FIGURE 11.5 Concentration of geranial (A, B, and C) and neral (D, E, and F) in irradiated and nonirradiated orange juice. Juices were irradiated with 0.0, 0.89, 2.24, 4.23, or 8.71 kGy at 5∞C, and stored at 7∞C for up to 21 d. Volatile compounds were measured with gas chromatography after 1 d (A and D), 7 d (B and E), or 21 d (C and F). Bars with the same letter are not significantly different (LSD, P < 0.05). Comparisons were made within the same storage duration. (From Fan, X. and Gates, R.A., Degradation of monoterpenes in orange juice by gamma radiation, J. Agric. Food Chem., 49(5), 2422–2426, 2001. With permission.)

apple cider irradiated to 3 kGy was identifiable, but not unacceptable, to untrained sensory panelists evaluating aroma; similar results were obtained with reconstituted and fresh orange juices (B.A. Niemira and X. Fan, unpublished data). Niemira et al. [57] found no evident changes in the appearance or aroma of reconstituted orange juice irradiated to 2.5 kGy. Miller and McDonald [128] reported that the juice from irradiated grapefruit was acceptable after 0.3 kGy, but declined in quality at 0.6 kGy. Pickett et al. [142] reported increasing off-flavor in unpasteurized orange juice irradiated to 3 kGy, rendering the juice unpalatable. Spoto et al. [147] irradiated orange juice concentrate and determined the effect of storage time and temperature on juice quality and acceptability. In that study, the highest dose (up to 5 kGy) combined with 25∞C storage resulted in loss of “orange” flavor and increase in “bitter,” “medicinal,” and “cooked” ratings by sensory panelists. Lower dose (2.5 kGy) and cooler storage (0 or 5∞C) were proposed as an acceptable processing regimen. Fan and Gates [136] found that some aroma-related acyclic monoterpenes in orange juice were reduced immediately after irradiation; after refrigerated (7∞C) storage for 21 d, there was no significant difference from the nonirradiated controls (Figure 11.5). A consumer test determined that irradiated apple cider was as acceptable or more acceptable than pasteurized apple cider [149]. The factors that may lead to undesirable flavors, aromas, or changes in appearance of irradiated juices are complex. While extensive studies into the effect of the variety and maturity of the source fruit on the radiation sensitivity of the resulting juice are lacking, it is known that juice composition is variety dependent [150]. In light of the evidence for varietal specificity in irradiated whole fruits and vegetables, the possibility of varietal specificity in juices with regard to negative irradiation effects cannot be discounted. As with whole fresh produce, temperature control during the irradiation process is important. The irradiation process can lead to generation of ozone in the headspace

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Ripening is delayed in

Be ne f

ic ia l

Senescence is delayed in

N

fic ne e B ot

l ia

Lack of tolerance in

Ripening is accelerated in

Sweet cherries Apricots Papayas

Tomatoes Strawberries Figs

Storage decay is controlled in Pears Avocados Lemons Grapefruit Oranges

Bananas Mangoes Papayas

Tangerines Cucumbers Summer Squash Bell peppers Olives Peaches

Irradiation tolerance only in

Plums Apples Table grapes Cantaloupes

Nectarines

Pineapples Lychees Honeydew melons (?)

FIGURE 11.6 The response of 27 fruits to irradiation. (From Akamine, E.K. and Moy, J.H., Delay in postharvest ripening and senescence of fruits, in Preservation of Food by Ionizing Radiation, Vol. 3, Josephson, S.E. and Peterson, M.S. Eds., CRC Press, Boca Raton, FL, 1983, pp. 129–158. With permission.)

gas of sealed containers; consideration should be given to the possibility of secondary sensory effects from the ozone, rather than from the radiation directly. Another possible source of off-odors or flavors is the plastic used in the processing tests. A bag made of a plastic not suited for irradiation may cause unpalatable migration of radiolysis products. Another example is a fruit juice irradiated in a glass test tube with an unsuitable plastic cap or even a cap liner that is the source of a disagreeable odor or flavor. Any of these situations could lead to a mistaken conclusion regarding the suitability of irradiation for a particular product.

11.12 FRUITS CURRENTLY BEING IRRADIATED A very large volume of information, occasionally contradictory, exists on specific applications of gamma irradiation to fruits [16, 17, 118, 151]. Figure 11.6 shows the response of a number of fruits to irradiation, as summarized by Akamine and Moy [151]. The results are often difficult to compare because of differences in conditions of treatment. A few representative examples are discussed below.

11.12.1 STRAWBERRY Among fruits, strawberry is one of the most studied with respect to application of gamma irradiation. Market tests and commercial applications have been carried out in a number of countries. A dose of 2 kGy seems to be the optimal dose of irradiation in air for precooled, ripe strawberries. Success depends on cultivar: the firmer fruits of “Tioga” tolerated radiation better than the softer fruits of “Brighton” [152]. In 1992, commercial irradiation of strawberries was initiated in Florida. The fruits were irradiated at 0.3 to 1 kGy on pallets in air containing 10% carbon dioxide [153] at 50∞F. The plastic wrapping was removed after 3 d. After 18 d of storage, the strawberries irradiated under

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modified atmosphere were sweet, firm, slightly dark, free of mold, and still saleable, in contrast with control fruits held in air and fruits held under modified atmosphere for 3 d before unwrapping. According to several reports, marketing of the irradiated strawberries was quite successful [154].

11.12.2 MANGO The D10 of 14 phytopathogenic fungi were evaluated in irradiated (0.5 to 1.5 kGy) mango pulp [58]. These ranged from 0.39 for Aspergillus sydowii and Aspergillus ustus to 1.11 for Penicillium oxalicum, confirming the relatively high radiation resistance of fungi. The D10 values were also determined in saline solution, and demonstrated some variance with the data obtained in mango pulp. The more complex medium caused a decrease in D10 for some fungi, e.g., A. ustus (55%), Scopulariopsis brevicaulis (55%), and A. sydowii (76%), and an increased D10 for others, e.g., Pencillium brevicompactium (147%), Aspergillus flavus (159%), and P. oxicalum (179%). The highest dose (1.5 kGy) reduced total aerobic bacteria count by ~1.8 logs, a difference that persisted throughout the study (50 d). Shelf life in irradiated (1 kGy) fruits was extended from 25 to 50 d in refrigerated storage. While the interiors of the fruit used in this study were sampled and found to be free of contamination, the variable effect of fruit pulp on the radiation resistance of resident microorganisms is clearly demonstrated. Pulp represents a combination of plant cell contents with extracellular fluid, found in the intercellular spaces; nevertheless, this observation should be considered in light of evidence of internalization of pathogenic bacteria into intercellular spaces [33]. Numerous reports indicate that mango preservation would greatly benefit from treatment with ionizing radiation. The summary by Akamine and Moy [151] shows the optimal dose to be 0.75 kGy for three-quarter ripe fruits at room temperature. Combination with mild heat treatment by hot water dip or vapor for 5 min at 50 to 55∞C yields even better results [152]. A limiting factor is surface scalding [151]. The effects of irradiation depend on the degree of maturity. Scalding occurred at 0.25 kGy on the mature-green fruits, and tolerance increased with maturity. Refrigeration at 13∞C reduced tolerance to scalding. The susceptibility to radiation injury varied greatly with the origin of the fruits, although, in general, mangoes are considered highly tolerant [15]. These various factors have to be balanced to give maximal shelf life based on delay of ripening and control of disease. A market test for irradiated mangoes in 1986 in Florida was highly successful. Mangos irradiated to 1.5 kGy were as acceptable to a sensory panel as nonirradiated controls immediately after treatment based on odor, color, and overall appearance; irradiated fruit remained acceptable after 50 d of refrigerated storage, while nonirradiated controls became unacceptable after less than 30 d [58]. Mango is one of several fruits being researched as part of an IAEA cooperative research project on the application of irradiation to fruits, vegetables, and vegetablederived products [35].

11.12.3 PAPAYA Extensive research was carried out on gamma irradiation of papaya in Hawaii to overcome the strict quarantine regulations of the mainland against insects, particularly fruit flies. Papaya is considered relatively tolerant of radiation. An effective treatment is 0.75 kGy, in combination with a hot water dip at 48.9∞C for 20 min [151, 152]. The combined treatment delayed ripening, as assessed visually and by measurement of firmness and decay [151]. Hot water dip alone accelerated ripening, while irradiation alone provided only slight control of decay. Little difference was reported between irradiated and fumigated papayas. Irradiated fruits were firmer and had a better flavor than ethylene dibromide-treated fruits. Hot water dip was preferable to vapor-heat treatment. Extension of shelf life was obtained even without refrigeration. The authors concluded in favor of the feasibility of gamma irradiation of papayas. Papayas are one of the fruits currently being irradiated commercially at an x-ray facility in Hilo, HI. These products receive a minimum of 0.25 kGy. Whereas fruit that is steam-treated must be picked green, the irradiated fruit is allowed to tree-ripen longer,

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yielding a higher-quality fruit [88]. Irradiated papayas have quality and nutrition similar to heattreated papayas [155].

11.13 COMBINATION TREATMENTS As has been indicated earlier, achieving a 5-log reduction in bacterial population on fresh fruits and juices would require between 1.5 and 3.5 kGy [54–57]. The higher doses may be more than those many fruits would withstand from a quality standpoint. It is not surprising, then, that much interest has been expressed in combining lower doses of ionizing radiation with other processes to achieve a total reduction of 5 logs. In part, the goal of this research is to determine the extent to which two or more treatments applied together will deliver a greater kill than the sum of each individual treatments, i.e., synergistic effect vs. purely additive effect. The actual physical and chemical mechanisms behind antimicrobial synergy are complex, and vary depending on the treatments being examined, but they generally follow the same underlying rationale. An appreciable percentage of the bacterial population that survives the first treatment is injured or weakened, rendering the surviving population more susceptible to the second treatment than a normal population; thus, the second treatment is more effective than the first. Depending on the treatments being applied, the sequence of treatments may be an important factor in obtaining synergistic kill vs. purely additive. In a study of irradiated Y. entericolitica, Sommers and Bhaduri [48] showed that the plasmid-encoded virulence genes in the surviving population were damaged, rendering the surviving population less virulent. Damage to plasmid-encoded genes is therefore one possible mechanism by which irradiation may predispose a population to greater susceptibility to other antimicrobial treatments.

11.13.1 MILD HEAT The combination of irradiation and mild heat (45 to 55∞C) has been used successfully with papaya and mangoes for delayed ripening and control of anthracnose, even without refrigeration [151]. Heat could be supplied as a hot water dip or as water vapor. Storage under modified atmosphere brought additional benefits. Combination of a 12-min steam treatment and gamma irradiation (1 to 2 kGy) increased the shelf life of mango pulp to 270 d vs. 90 d for irradiated or nonsteamed, or 15 d for untreated mango pulp (nonsteamed and nonirradiated) [156]. The combination treatment had no undesirable effect on chemical or sensory properties of the mango pulp. Fan et al. [157] determined that a combination of a warm water dip (47∞C, 2 min) and lowdose irradiation (0.5 or 1 kGy) could effectively enhance the safety and quality of cut lettuce. Caution may be warranted, however, as the population dynamics of pathogens in storage, posttreatment, are not fully understood. Lettuce was treated with a combination of warm (47∞C) water and chlorine (100 mg/ml), and inoculated either before or after these treatments with L. monocytogenes and E. coli O157:H7. After an initial reduction, these pathogens rebounded and grew extensively on lettuce stored at abusive temperatures (10∞C). Growth was not observed in lettuce treated with cold (1∞C) water plus chlorine [158]. Though this study did not include irradiation as a treatment, it illustrates the importance of proper controls over every processing step, particularly when the effect of interacting treatments is not fully understood.

11.13.2 MODIFIED ATMOSPHERE PACKAGING (MAP) Low-dose irradiation combined with modified atmosphere is increasingly considered for control of microorganisms and delayed ripening. As stated above, reduced oxygen content in the atmosphere limits the efficiency of irradiation because less activated oxygen is available. An interesting alternative is irradiation in air enriched with carbon dioxide. Carbon dioxide is fungistatic at levels above 12%, a concentration beyond the tolerance of most plant tissues (strawberries are an excep-

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tion). Couture and Willemot [159] showed the synergistic action of the combined stresses of gamma radiation and high carbon dioxide for control of mold development on strawberries. Irradiation at the low dose of 0.3 kGy, combined with storage in air containing 10% carbon dioxide, delayed ripening, as assessed by anthocyanin content and mold development. The method has since been adapted commercially in Florida [153]. MAP of fruits involves equilibrating the respiratory carbon dioxide production and oxygen depletion by the plant material with the film permeability in order to maintain an adequate composition of the atmosphere (oxygen and carbon dioxide concentrations) within the limits of tolerance of the fruits inside the package. With irradiated fruits, the temporary increase in the rate of respiration due to the initial wound response has to be taken into account. The technology can be applied in several ways, e.g., irradiation of a wrapped pallet, of small individual containers, or of small perforated containers under a common wrapping on a pallet. Accumulation of carbon dioxide and depletion of oxygen may lead to fermentation and development of off-odors and off-flavors. Perforated, microporous, or semipermeable films may resolve this problem. Preirradiation flushing of the package with a low-oxygen gas mixture would reduce the undesirable effects of irradiation on the produce, e.g., loss of vitamin C, and on the packaging material, e.g., development of undesirable flavor and odor; however, this reduces the efficacy of the process. Sealed packaging acts as a water barrier and reduces weight loss. The integrity of the seal should resist the irradiation treatment in order to maintain the internal modified atmosphere and to prevent microbial recontamination. A study of lettuce and cabbage packed in a high-nitrogen atmosphere showed an extension of shelf life but no inhibitory effect on resident microflora, prompting the authors to caution against the potential for growth of pathogens on vegetables so packaged [160]. Irradiation of this type of packaged product would seem to be a valid means of addressing this concern. Combination of irradiation with MAP has shown success with irradiated packaged meats [59], and is an area of active research on fruits and vegetables [35, X. Fan, personal communication). Couture and Willemot [159] and Brecht et al. [161] showed beneficial effects of low-dose irradiation and storage under modified atmosphere for strawberries. Brecht et al. [161] obtained reduced decay incidence of Rhizopus stolonifer and Botrytis cinerea on strawberries packaged under 7% oxygen and 20% carbon dioxide and irradiated at 1 kGy. Grandison [99] reported a reduced rate of respiration and a delay of up to 3 d in ripening for bananas exposed to 0.1 to 0.3 kGy in an electron accelerator. Higher doses caused browning of the skin, splitting of the fruits, and loss of peel texture and vitamin C. Combination of irradiation with MAP under a 25-µm low density polyethylene (LDPE) film extended the shelf life, although not synergistically. MAP did not alleviate the disorders caused by electron beam irradiation to avocados. After irradiation under MAP, total aerobic counts were reduced by ~3 logs on iceberg lettuce [137] and by ~1.5 logs on romaine lettuce [96] by doses of 0.19 kGy and 0.35 kGy, respectively. Rate of spoilage and shelf life was reported to be unchanged for iceberg lettuce, but the shelf life of romaine lettuce was extended from 14 to 18 d to the length of the study, 22 d. MAP reduced total aerobic plate counts by ~2 logs on shredded carrots irradiated to 0.45 kGy, a difference in microbial population that persisted throughout a 9-d storage evaluation [162]. Many minimally processed fruits and vegetables, such as prepared salad mixes, incorporate MAP as a means to control spoilage and extend shelf life. The precise gas permeabilities of the plastics used in the packages are of critical importance for these products. Especially with regard to fruits and vegetables, which respire during storage, studies of MAP plus irradiation must take into account the effects of irradiation on the material used to hold the product.

11.14 IRRADIATION OF PACKAGING MATERIAL Like other foods treated by irradiation, fruits are prepackaged to facilitate handling and to prevent recontamination. Irradiation of fruits involves constraints on the types of packaging materials

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suitable for this technology. The combination of irradiation and MAP brings additional constraints on the suitability of the packaging material. Irradiation may affect the characteristics of materials and packaging performance. It is therefore important to know these effects as they will influence the choice of a packaging compatible with the requirements of the agri-food industry. The functional properties of the material should be preserved. The toxicological and sensory aspects should also be addressed. Although the effects of irradiation are generally minimal, i.e., below 10 kGy, volatiles may be generated during the treatment. They may be transferred to the food and may produce unpleasant flavors. Migration of other compounds through contact while packaging food must be monitored to insure that the food is chemically harmless. Selection criteria for packaging materials to be used with irradiation include sealability, resistance to cracking and delamination, protection against recontamination, barrier to oxygen and humidity, and chemical inertness. Traditional packaging materials such as metal, glass, and paper are relatively resistant to ionizing radiation. Metal and glass are essentially impervious to the effects of irradiation, although with high doses, glass can become somewhat browned. Paper and paper products such as pressboard, cardboard, etc., can lose mechanical strength due to the breakdown in cellulose at the microscopic level. However, the low water content of these products greatly reduces the sensitivity of the cellulose in paper vs. that in fruit or vegetables. Paper and cardboard are used mainly for support packaging. Plastics are the type of packaging that most frequently comes into contact with the food treated with ionizing radiation. Plastics have historically been basic resin polymers to which additives (e.g., stabilizers, antistatic and antislip agents, and plasticizers) have been added. Radiation acts on both the polymers and additives. Increasingly, the plastic films used for packaging are laminates, with two or more layers of dissimilar plastics combined to provide the desired mixture of strength, durability, gas permeability, ability to take inks, etc. Determining the effects on the packaging material is thus made more difficult as each plastic component of the laminate has a unique response to irradiation, and this response may or may not be the same for the plastic as part of a laminate as it would be for the plastic standing alone. Also, some specialty laminates can incorporate a metal foil layer, adding another layer of complexity. A relatively recent development in packaging technology has been the incorporation of preservatives and antimicrobial compounds within the plastic, designed to suppress the growth of spoilage organisms or pathogens [163]. These are of increasing interest to food processors and food safety professionals. For use in food products to be sold in the U.S., these compounds must be cleared by the U.S. FDA.

11.14.1 TYPES

OF

PLASTICS USED

IN

FRUIT PACKAGING

Fresh fruits are usually packaged in plastics of high or intermediate permeability (Table 11.2). The most commonly used materials are LDPE, polypropylene (PP), polyvinyl chloride (PVC), and ethylene vinylacetate (EVA). Seasonal trade requires punnets made of woven chip or thermoformed polystyrene (PS) with LDPE or PVC cling wrap. Small polyethylene terephthalate PET trays are also becoming popular. Juice-absorbing pads can be used to soak residual liquid from the fruit or prepackaging rinses. Properties of PET are well preserved during irradiation [164]. In the U.S., PS films have been approved for irradiation; however, the PS foam trays have to be PE-coated, which increases packaging cost. Single-layer plastic films commonly used for fruit packaging, including LDPE, PP, PVC, polyvinylidene cholride (PVDC), and PET, released odors after irradiation at the low doses used for fruits [165]. At high doses, cellulosic materials, such as paper and board, lose mechanical strength. At the doses typically used with fruits, this effect is minimal and may be offset by using PE or foils as a laminate for support [166]. Adhesives used in absorbing pads are not permitted for irradiation in the U.S. and Canada. Pads with a mechanical link between paper and polyethylene (PE) layers would be a solution. PP and PVC are not recommended for use with irradiated fruits for the reasons discussed earlier. Several suitable materials have not yet received

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TABLE 11.2 Suitability of Plastic Materials for Fruit Irradiation Polymer

Use

Polyethylene (PE) Polypropylene (PP) Ethylene vinyl alcohol (EVA) Polyvinyl chloride (PVC) Polyethylene tetraphtalate (PET) Polystyrene (PS) Polyvinylidene chloride (PVDC) Nylon-8 Nylon-11 Cellulose

Pallet shrink packaging, bags, lids coatings, wrap films Bags Coatings, MAP, frozen fruits, sealing MAP, overwrapping Bags Trays, films Barrier layer Bags Bags Bags, overwrapping

Suitability Good Mediocre Good Poor Good Excellent Good Excellent Good Mediocre

Source: Willemot, C.M., Marcotte, M., and Deschenes, L., Ionizing radiation for preservation of fruits. in Processing Fruits: Science and Technology, Vol 1: Biology, Principles and Applications, Somogyi, L.P. and Ramaswamy, H.P., Eds., Technomic Publishing Lancaster, PA, 1996, pp. 221–260, chap. 9.

clearance for irradiation. Because of the complexity of the problem, the applications have to be evaluated individually. Dried fruits irradiated for insect disinfestation are stored for long periods. The durability of the irradiated material has to be tested. Barrier films used with dried fruits are typically multilayers of polyolefins, EVA, PVDC, or nylon. Oxygen barrier films reduce radiation-induced vitamin C loss [166]; however, films laminated with adhesives have not yet been cleared for irradiation. At the present time, no packaging material can be considered suitable for irradiation of fruits unless appropriate migration tests have been performed. Even with approved plastic films, off-odors may be generated; however, as off-odors are typically the result of radiation-induced rancidity of fats, this risk is low in most fruits because of the low fat content [2]. The main parameters for the choice of a packaging material before fruit irradiation are the FDA list and the specific requirements of the fruits.

11.14.2 EFFECTS

OF IRRADIATION ON

PACKAGING PLASTICS

The effects of the ionizing radiation on polymers are mainly cross-linking and cleavage. The two effects can occur simultaneously, and the relative importance of each depends on the material and on the conditions under which the treatment is carried out [167]. Cross-linkage, the formation of bonds between adjacent chains, generates a very resistant three-dimensional network. The extent to which cross-linkage occurs between the layers of plastic laminates will depend on the plastics involved. Cleavage refers to reduction in chain length and, thus, to a loss of mechanical strength and an increase in porosity of the material. In general, the low doses applied to fruits cause only insignificant modifications of the physical properties of plastics; however, some materials are more susceptible to ionizing radiation and are not recommended for irradiation of prepackaged food. Mass transfer from packaging material to food after irradiation may involve migration, absorption, and permeation. Research has demonstrated that these three phenomena can be affected by irradiation. Migration refers to the transfer of low molecular weight molecules to the food and is of importance for toxicology and organoleptic quality. The plastics used for fruit packaging are mostly very high molecular weight thermoplastics derived from ethylene. They are chemically inert and have a low solubility in water and oil; therefore, migration concerns mainly low molecular weight additives and their products generated by irradiation. Migration rates have been shown to be affected

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by radiation [164]. The effect is more important with gamma rays than with accelerated electrons. Irradiation of plasticized PVC film causes migration of HCl and tainting of food by other lowmolecular-weight molecules [166]. Kilcast also showed migration from irradiated PET and high-impact polystyrene (HPS) trays. He suggested the use of expanded PS. No taint was observed from PE-based films, while PP is known to be very susceptible to irradiation [168]. Bourges et al. [168] showed that oxidation products of antioxidant additives produced at doses below 10 kGy could migrate into food-simulating solvents. A study (gamma and e-beam) of two semirigid copolyesters showed that 5 kGy had no effect, and that 25 or 50 kGy had no significant effect on migration of nonvolatiles into food-simulating solvents [169]. A later study (gamma) of an amorphous nylon polymer using the same doses (5, 25, and 50 kGy) showed the generation of volatiles but not of nonvolatile compounds [170]. No new compounds were detected in that study. Comparatively little is known at the present time about toxicity of degradation products of many other additives. The extent to which the concentration, efficacy, or migration of these antimicrobial compounds (or any of their radiolytic products) are affected by irradiation is essentially unexplored [163]. Permeation, the transfer of molecules through the packaging material, is only slightly affected by irradiation below 10 kGy [165]. This aspect should, however, be considered in the context of MAP [171]. Absorption of volatiles and other constituents of food by packaging material can be important. Scalping of terpenes that contribute to the aroma of citrus fruits by packaging material [172] is a serious problem for the fruit juice industry. Absorption has been only slightly studied in the context of food irradiation.

11.14.3 REGULATION PRODUCTS

OF

PACKAGING MATERIALS

FOR

USE

WITH IRRADIATED

The Codex General Standard for Irradiated Foods [173] established worldwide standards for packaging of irradiated food. These guidelines are very general and offer little precision with respect to the types of materials and additives suitable for prepackaging of food to be irradiated. The U.S. is the only country with a list of packaging materials approved for irradiation (Table 11.3). Some of these plastics may be amended with various adjuvants, including preservatives, etc. (Table 11.4). The FDA list draws on data from research carried out by the U.S. Army on irradiation-sterilized food packs, as well as data from more recent research. The regulation covers only plain films or laminates, presently used in food packaging. Juice-absorbing pads in film-covered trays often contain adhesives between plastic and paperboard layers. These pads and trays such as the popular uncoated polystyrene foam trays are not approved for irrradiation, although polystyrene films may be irradiated. In Canada, no such list of packaging materials approved for irradiation has been established. The standards are intended to serve only as guidelines. Whatever may be the material or the process, it must not constitute a health hazard. Until August 1988, there were no specific approvals for packaging materials. Since then, permission has been granted for Cryovac films and for bags used in the shrink- and vacuum-packaging of refrigerated products [174]. In Canada and the U.S., a clearance petition should be presented to the authorities for plastic materials that are not mentioned in the FDA list and that come in contact with the food during irradiation. Several materials that are good candidates for packaging of irradiated fruits cannot currently be used. These materials await a concerted effort of industry groups to petition the appropriate governmental regulatory bodies for approval.

11.15 COST-BENEFIT OF FRUIT IRRADIATION The beneficial effects of irradiation for fruit disinfestation, reduction of spoilage, and extension of shelf life offer potential for significant cost savings to the food industry. One of the major concerns

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TABLE 11.3 U.S. Code of Federal Regulations 21CFR179.45: Packaging Materials Approved for Irradiated Foods Material

Maximum Dose

Nitrocellulose-coated or vinylidene chloride copolymer-coated cellophane Glassine paper Wax-coated paperboard Films of polyolefin or polyethylene terephthalate. These may contain: 1. Sodium citrate, sodium lauryl sulfate, polyvinyl chloridea 2. Coatings comprising a vinylidene chloride copolymer containing a minimum of 85% vinylidene chloride with one or more of the following comonomers: Acrylic acid, acrylonitrile, itaconic acid, methyl acrylate, and methyl methacrylate Kraft paper (only as a container for flour) Polystyrene film Rubber hydrochloride film Vinylidene chloride–vinyl chloride copolymer film Nylon 11 Ethylene–vinyl acetate copolymers Vegetable parchments Polyethylene filma Polyethylene terephthalate filma Nylon 6 filmsa Vinyl chloride–vinyl acetate copolymer filma Acrylonitrile copolymersa

10 kGy 10 kGy 10 kGy 10 kGy

a

0.5 kGy 10 kGy 10 kGy 10 kGy 10 kGy 30 kGy 60 kGy 60 kGy 60 kGy 60 kGy 60 kGy 60 kGy

This material may be amended with additional materials, listed in Table 11.4.

TABLE 11.4 U.S. Code of Federal Regulations 21CFR179.45: Adjuvants and Amendments Approved for Incorporation into Certain Packaging Materials Approved for Irradiated Foods Adjuvant/Amendment

Limit (By Weight of Polymer)

Amides of erucic, linoleic, oleic, palmitic, and stearic acid BHA (butylated hydroxyanisole) BHT (butylated hydroxytoluene) Calcium and sodium propionates Petroleum wax Mineral oil Stearates of aluminum, calcium, magnesium, potassium, and sodium Triethylene glycol Polypropylene, noncrystalline

1% 1% 1% 1% 1% 1% 1% 1% 2%

of any potential user is whether the process is economically viable. Although irradiation plant capacity and processor commitment to the technology are growing at perhaps the fastest rate since irradiation was first developed over 40 years ago, the economic feasibility of irradiation has to be examined for each individual application. Also, the cost of this innovative technology has to be compared with other existing processing technologies [175].

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The cost of gamma irradiation is similar to irradiation using an electron accelerator. The latter source is not likely to be used with fruits because of the low penetration capacity of the electron beam; however, electron accelerator-generated x rays, with significant improvement in the x-ray yield, may become an interesting alternative for bulk irradiation of fruits. Irradiators are costly to install, and a substantial capital investment is needed [131]. The main factors contributing to the initial cost of any irradiation facility are the source (radionucleotides or electron accelerator equipment), the shielding, and the product conveyer system. This important capital investment has delayed the development of food irradiation. Lack of food irradiation capacity means that irradiated food cannot be in the marketplace, and it has no opportunity to gain market share. Without market share, there is insufficient economic incentive for the capital outlay of a new irradiation plant. The main operating costs of a gamma irradiator include depreciation and replacement of the source, while maintenance of the electronics and the electricity consumption are more significant factors for e-beam and x-ray irradiators. Urbain [13] concluded that the operating costs for food irradiation were in the range of commercial feasibility. The cost of irradiating foods was estimated between U.S. $0.02 and $0.04 per kg [176]. Giddings [177] estimated the processing cost at $0.05 for a throughput of 10 million kg a year. A study by Fundacion Chili in 1985 evaluated the cost of disinfesting fruits by gamma irradiation at $0.025 per kilogram for $1 million capital investment in a batch-type cobalt-60 facility [178]. Cost estimates are dependent on the throughput capacity of the irradiation plant, the dose delivered, the distance the produce must travel to and from the irradiator, the distance between the plant and the markets being served, etc. Ancillary market benefits of irradiation (reduction of storage losses, premium prices commanded by “specialty” markets, etc.), may be offset by ancillary market drawbacks (necessity for increased public education or outreach spending, potential for increased regulatory oversight, etc.). These complex factors make an accurate cost-benefit analysis of food irradiation difficult. There are few examples of large-scale commercialization from which to gain insight, but the recent increase in irradiation of ground meats and Hawaiian fruits is expected to allow for real-world data to be applied to these calculations. Kunstadt and Steeves [179] discussed in detail the economics of fruit disinfestation by irradiation and the effects of various parameters on the unit-processing cost. They presented a detailed analysis of the costs incurred for different types of cobalt-60 irradiators; this analysis remains instructive as x-ray treatment of fruits becomes more prominent in the marketplace. The influence of dose, packing density, and volume of operation was calculated. As expected, unit-processing costs decrease rapidly with increased throughput. Therefore, an economically successful irradiator has to operate at a level exceeding the minimum economic volume of production. It has to be designed for a realistic throughput. The unit-processing cost increases linearly with increasing dose of irradiation; it also depends on processing time. The linearly increasing cost with decreasing packing density is related to the efficiency of utilization of the source. Similar trends were observed with different types of irradiators. According to these calculations, the unit-processing cost of an irradiator designed for throughput of 60 million kg/year at a density of 0.4 g/cm3 and a dose of 0.15 kGy would vary by less than $0.01 for a throughput of 250,000 t/year to $0.107 per kg for 10,000 t.

11.16 THE FUTURE OF IRRADIATION PROCESSING OF FRUITS Treatment of fruits by irradiation is increasingly being recognized as an effective method for reducing postharvest losses, ensuring hygienic quality of produce and facilitating international trade of specific commodities of tropical origin; however, worldwide, more than 80% of food irradiation is applied for the disinfection of spices, herbs, and dehydrated seasonings. The industry has shown

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widespread interest in this technology, has provided quality control through the necessary permissions that were granted, the quality of the treated produce was high enough to offset the price increase, the cost per unit was acceptable to the consumer, and no mandatory negative labeling would discourage purchasing. Loaharanu [154] reported on several successful market trials, mainly for fruits, in the 1980s. In 1986, 2 t of mangos were irradiated at 1 kGy in Puerto Rico and flown to Miami. In 1987, Hawaiian papayas were shipped to Los Angeles and irradiated at a low dose. In both trials, the fruits were well received by consumers. In Lyon, France, in 1988, strawberries irradiated at 2 kGy sold at a slightly higher price than nonirradiated fruits. The consumers preferred the irradiated strawberries for their better quality. Since the opening of the $6.8 million Vindicator irradiator at Tampa, FL, in January 1992, various fruits have been irradiated and sold successfully in Florida and Chicago. The Hawaiian fruits now being treated with x-rays are reportedly selling well in their target markets. These fruits include papaya, avocado, rambutan, lychee, and star fruit. The increasing capacity of food irradiation facilities in the mainland U.S. suggests that as the processors seek to maintain their plants at or near full capacity, the range of products being treated may expand from ground and frozen meats to include more fruits and vegetables. Ongoing research and progress concerning the technical aspects of irradiation of fruits should further develop the use of this technology. Choice of cultivars, harvesting conditions, degree of maturity, combination treatments, suitable packaging, and minimum doses required for the desired benefits are among the parameters that will lead toward better quality and longer shelf life of irradiated fruits. The phasing out of several fumigants used for disinfestation is likely to encourage the use of irradiation. Following the U.S. ban on ethylene dibromide, many producers turned to hot water dips to disinfest mangoes and papayas; however, this treatment may cause physiological disorders to the fruits, particularly discoloration and internal browning. Irradiation is an alternative process of considerable interest because it controls microbial development and sterilizes or kills insect pests without affecting fruit “freshness.” It may become a major technique for meeting the quarantine requirements of several countries [180], including the U.S., with respect to disinfestation of fruits against insects such as the Mediterranean fruit fly. Developing countries will particularly benefit from the technology. Their often warm and humid climates accelerate ripening and decay of produce, often causing extensive losses. Most produce of tropical origin, e.g., mango and papaya, are chill sensitive. Irradiation is an alternative to low-temperature storage for preservation of some of these products. Developing countries are at great distances from their main industrialized markets, and irradiation would help maintain quality and safety of produce during transportation. The increase in consumption of fruits of tropical origin from developing countries, combined with the demand for safe, nutritious, and convenient food in industrialized countries will likely contribute to the expansion of the use of this technology. This can only serve to increase the trade in tropical fruits, thus benefiting the exporting and importing nations. Food-borne illness resulting from contaminated produce and juices has encouraged research into the use of irradiation of fruits for the purpose of improving the safety of these products. Sanitization, a goal that has a resonance with consumers, has joined the traditional goals of disinfestation, delay of ripening, and shelf-life extension as stated aims of produce irradiation. Ultimately, the consumer will benefit from the presence in the marketplace of a greater variety of fruits, including tropical fruits, year-round, that are fresher, longer-lived, healthier, and safer than before.

ACKNOWLEDGMENTS The authors would like to acknowledge the thoughtful reviews of J.L. Alford and J.S. Novak, and express grateful appreciation to K. Lonczynski for technical assistance in the preparation of this manuscript.

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REFERENCES 1. CAST, Ionizing Energy in Food Processing and Pest Control. Wholesomeness of Food Treated with Ionizing Energy, in Council for Agriculture Science and Technology, Washington, D.C., Report No. 109, 1986. 2. CDC, Frequently Asked Questions about Food Irradiation (http://www.cdc.gov/ncidod/ dbmd/diseaseinfo/foodirradiation.htm), 1999. 3. WHO, Wholesomeness of Irradiated Food. Report of a joint FAO/IAEA/WHO expert committee, in WHO Technical Report Series 659, Geneva, 1981. 4. WHO, Safety and Nutritional Adequacy of Irradiated Food, Geneva, 1994. 5. WHO, High-Dose Irradiation: Wholesomeness of Food Irradiated with Doses above 10 kGy, Joint FAO/IAEA/WHO Study Group on High-Dose Irradiation, Geneva, 15–20 September 1997, in WHO Technical Report Series 890, 1999. 6. FDA, FDA regulations concerning food irradiation. Fed. Reg., 51, 13376–13399, 1986. 7. FDA, Food Irradiation: A Safe Measure (http://www.fda.gov/opacom/catalog/ irradbro.html), 2000. 8. IFT, Radiation preservation of foods: expert panel on food safety and nutrition. Food Technol., 37(2), 55, 1983. 9. Moy, J.H., Radurization and radicidation: fruits and vegetables. In Preservation of Food by Ionizing Radiation, Vol. 3 Josephson, E.S. and Peterson, M.S., Eds., CRC Press, Boca Raton, FL,1983, pp. 83–108. 10. Diehl, J.F., Safety of Irradiated Foods. Marcel Dekker, New York, 1990. 11. Josephson, E.S. and Peterson, M.S., Eds., Preservation of Food by Ionizing Radiation, Vol. 3, CRC Press, Boca Raton, FL, 1983. 12. Thorne, S., Food Irradiation, Elsevier Applied Science, New York, 1991. 13. Urbain, W.M., Food Irradiation, Academic Press, New York, 1986. 14. Vasseur, J.P., Ionisation des Produits Alimentaires. Tech & Doc-Lavosier, Paris, 1991. 15. Kader, A.A., Potential application of ionizing radiation in post harvest handling of fresh fruit and vegetables. Food Technol., 40(6), 117–121, 1986. 16. Thayer, D.W. and Rajkowski, K.T., Developments in irradiation of fresh fruits and vegetables, Food Technol., 53(11), 62–65, 1999. 17. Niemira, B.A., Irradiation of fresh and minimally processed fruits, vegetables, and juices. In The Microbial Safety of Minimally Processed Foods, Novak, J.S., Sapers, G.M., and Juneja, V.K., Eds. CRC Press Boca Raton, FL, 2002, chap. 19. 18. Bord, R., Consumer acceptance of irradiated foods in the United States. In Food Irradiation, Thorne, S., Ed., Elsevier Applied Science, New York, 1991, pp. 61–86. 19. Feenstra, M.H. and Scholten, A.H., Consumer acceptance of irradiated foods, in Food Irradiation, Thorne, S., Ed., Elsevier Applied Science, New York, 1991, pp. 97–128. 20. Sumner, S.S. and Peters, D.L., Microbiology of vegetables, in Processing Vegetables: Science and Technology, Smith, D.S., Cash, J.N., Nip, W-K., and Hui, Y.H., Eds., Technomic Publishing, Lancaster, PA, 1997, chap. 4. 21. Bruhn, C.M., Consumer expectations regarding irradiated food, in Proc. Food Irradiation, April 3–5, Arlington, VA, Intertech, Portland, ME, 2000. 22. Frenzen, P.D., DeBess, E.E,. Hechemy, K.E., Kassenborg, H., Kennedy, M., McCombs, K., McNees, A., and the FoodNet Working Group. Consumer Acceptance of Irradiated Meat and Poultry in the United States. J. Food Prot., 64(12), 2020–2026, 2001. 23. Bricher, J.L., Editorial, Food Safety Magazine 7(6), 6, 2001. 24. Beuchat, L.R., Pathogenic microorganisms associated with fresh produce, J. Food Prot., 59(2), 204–216, 1996. 25. Tauxe, R., Kruse, H., Hedberg, C., Potter, M., Madden, J., and Wachsmuth, K., Microbial hazards and emerging issues associated with produce: a preliminary report to the National Advisory Committee on Microbiological Criteria for Foods, J. Food Prot., 60(11), 1400–1408, 1997. 26. Diehl, J.F., Safety of Irradiated Foods, 2nd ed. Marcel Dekker, New York, 1995. 27. Howard, L.R. and Buescher, R.W., Cell wall characteristics of gamma-radiated refrigerated cucumber pickles, J. Food Sci. 54(5), 1266–1268, 1989.

Ionizing Radiation Processing of Fruits and Fruit Products

253

28. Prakash, A., Inthajak, P., Huibregtse, H., Caporaso, F., and Foley, D.M., Effects of low-dose gamma irradiation and conventional treatments on shelf life and quality characteristics of diced celery, J. Food Sci. 65(6), 1070–1075, 2000. 29. Maxie, E.C. and Abdel-Kader, A.S., Food irradiation-physiology of fruits as related to the feasibility of the technology, Adv. Food Res., 15, 105–145, 1966. 30. Yu, L., Reitmeier, C.A., and Love, M.H., Strawberry texture and pectin content as affected by electron beam irradiation, J. Food Sci., 61(4), 844–846, 1996. 31. Osterholm, M.T. and Potter, M.E., Irradiation pasteurization of solid foods: taking food safety to the next level, in Emerging Infectious Disease, Vol. 3, no. 4. http://www.cdc.gov/ncidod/eid/vol3no4/osterhol.htm, 1997. 32. Anon., Food Irradiation: Available Research Indicates that Benefits Outweigh Risks. U.S. General Accounting Office, GAO/RCED-00–217, 2000. 33. Takeuchi, K. and Frank, J.F., Penetration of Escherichia coli O157:H7 into lettuce tissues as affected by inoculum size and temperature and the effect of chlorine treatment on cell viability, J. Food Prot., 63(4), 434–440, 2000. 34. Burnett, S.L. and Beuchat, L.R., Differentiation of viable and dead Escherichia coli O157:H7 cells on and in apple structures and tissues following chlorine treatment. J. Food Prot., 65(2), 251–259, 2002. 35. IAEA, Report of the first FAO/IAEA Research Coordination Meeting: Use of irradiation to ensure hygienic quality of fresh, pre-cut fruits and vegetables and other minimally processed food of plant origin, November 5–9, Rio de Janeiro, Brazil, 2001. 36. Adams, P., X-ray pasteurization — is it “the” answer? In Proc. Food Irradiation 2000, April 3–5, Arlington, VA, 2000. 37. Thayer, D.W. and Boyd, G., Radiation sensitivity of Listeria monocytogenes on beef as affected by temperature, J. Food Sci., 60, 237–240, 1995. 38. Nieto-Sandoval, J.M., Almela, L., Fernandez-Lopez, J.A., and Muñoz, J.A., Effect of electron beam irradiation on color and microbial burden of red paprika, J. Food Prot., 63(5), 633–637, 2000. 39. Hayashi, T., Comparative effectiveness of gamma-rays and electron beams in food irradiation, in Food Irradiation, Thorne, S., Ed., Elsevier Applied Science, New York, 1991, pp. 169–206. 40. Moy, J.H., Reyes, M.E.Q., Taladriz, L., Bian, X., Chinnasri, O., Boylston, T.D., Reitmeier, C.A., and Mosher, G.A., The effects of gamma-rays vs. x-rays on the qualities of papayas (cv. Rainbow). Abstr. 59–1, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002. 41. Chung, CS., Ko, Y-T., and Kim, W-S., Survival of Pseudomonas fluorescens and Salmonella typhimurium after electron beam and gamma irradiation of refrigerated beef. J. Food Prot., 63(2), 162–166, 2000. 42. Diehl, J.F., Radiolytic effects in foods, in Preservation of Food by Ionizing Radiation, Vol. 1, Josephson, B.S. and Peterson, M.S., Eds., CRC Press, Boca Raton, FL, 1991, pp. 279–357. 43. Sommers, C.H., Handel, A.P., and Niemira, B.A., Radiation resistance of Listeria monocytogenes on the presence or absence of sodium erythorbate, J. Food Sci., 67, 2267–2270, 2002. 44. Dupont, J.L., Narang, K., Thayer, D.W., Kehayia, J.J., Roberts, S.B., Reeds, P.J., and Russell, R.M., Nuclear Technology in Agriculture and Nutrition, Nuclear News, 40(7), 58–62, 1997. 45. Cao, G., Sofic, E., and Prior, R.L., Antioxidant capacity of tea and common vegetables, J. Agric. Food Chem., 44, 3426–3431, 1996. 46. Wang, H., Cao, G., and Prior, R.L., Total antioxidant capacity of fruits, J. Agric. Food Chem., 44, 701–705, 1996. 47. Snyder, O.P. and Poland, D.M., Food Irradiation Today. Hospitality Institute of Technology and Management, St. Paul, MN. (http://www.hi-tm.com/documents.irrad.html), 1995. 48. Sommers, C.H. and Bhaduri, S., Loss of crystal violet binding in Yersinia enterocolitica following gamma irradiation, Food Microbiol., 18, 367–374, 2001. 49. Monk, J.D., Beuchat, L.R., and Doyle, M.P. Irradiation inactivation of food-borne microorganisms, J. Food Prot., 58(2), 197–208, 1994. 50. O’Connor, R.E., and Mitchell, G.E., Effect of irradiation on microorganisms in strawberries, Int. J. Food. Microbiol., 12, 247–256, 1991. 51. Lovell, R.T. and Flick, G.J., Irradiation of Gulf Coast area strawberries, Food Technol., 29, 99–102, 1966.

254

Processing Fruits: Science and Technology, Second Edition

52. Lescano, G., Narvaiz, P., Kairiyama, E., and Kaupert, N., Effect of chicken breast irradiation on microbiological, chemical and organoleptic quality, Lebensm. Wiss. Technol., 24, 130–134, 1991. 53. Narvaiz, P., Lescano, G. and Kairiyama, E., Irradiation of almonds and cashew nuts. Lebensm. Wiss. Technol., 25, 232–235, 1992. 54. Buchanan, R.L., Edelson, S.G., Snipes, K., and Boyd, G., Inactivation of Escherichia coli O157:H7 in apple juice by irradiation, Appl. Envir. Microbiol., 64(11), 4533–4535, 1998. 55. Rajkowski, K.T. and Thayer, D.W., Reduction of Salmonella spp. and strains of Escherichia coli O157:H7 by gamma radiation of inoculated sprouts, J. Food Prot., 63(7), 871–875, 2000. 56. Niemira, B.A., Citrus juice composition does not influence radiation sensitivity of Salmonella enteritidis, J. Food Prot., 64(6), 869–872, 2001. 57. Niemira, B.A., Sommers, C.H., and Boyd, G., Irradiation inactivation of four Salmonella species in orange juices with varying turbidity, J. Food Prot., 64(5), 614–617, 2001. 58. El-Samahay, S.K., Youssef, B.M., Askar, A.A., and Swailam, H.M.M., Microbiological and chemical properties of irradiated mango, J. Food Safety, 20, 139–156, 2000. 59. Thayer, D.W. and Boyd, G., Reduction of normal flora by irradiation and its effect on the ability of Listeria monocytogenes to multiply on ground turkey stored at 7∞C when packaged under a modified atmosphere, J Food Prot., 63(12), 1702–1706, 2000. 60. Licciardello, J.J., Nickerson, J.T.R., and Goldblith, S.A., Inactivation of Salmonella in poultry with gamma radiation, Poult. Sci., 49, 663–675, 1970. 61. Tiwari, N.P. and Maxcy, R.B., Comparative growth of Salmonellae, Coliforms, and other members of the microflora of raw and radurized ground beef, J. Milk Food Technol., 35: 455–460, 1972. 62. Matches, J.R. and Liston, J., Growth of Salmonellae on irradiated and non-irradiated seafoods, J. Food Sci., 33, 406–410, 1968. 63. Carlin, F., Nguyen-The, C., and Morris, C.E., Influence of background microflora on Listeria monocytogenes on minimally processed fresh broad-leaved endive (Chicorium endiva var. latifolia), J. Food Prot., 59(7), 698–703, 1996. 64. Moreia, R.G., Brescia, G., and Castell-Perez, M.E., Low Energy Electron Irradiation of an Apple. Abstr. 91C–3, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002. 65. Mclaughlin, W.L., Jarret, R.D., and Olejnik, T.A., Dosimetry, in Preservation of Food by Ionizing Radiation, Vol. 1, Josephson, E.S. and Peterson, M.S., Eds., CRC Press, Boca Raton, FL, 1982. 66. Delincee, H., Detection methods for irradiated foods, in Proceedings of a Meeting on Irradiation for the Food Sector, Saint-Hyacinthe, QC, Canada, May 13, Publitech, FRDC, Agriculture Canada, SaintHyacinthe, QC, Canada, 1992, pp. 24–60. 67. Raffi, J., Les methodes de detection des aliments ionses, in Ionisation des Produits Ailmenidires, Vasseur, J.R., Ed., Tech & Doc-Lavoisier, Paris, 1991. pp. 219–233. 68. IAEA, Analytical Detection Methods for Irradiated Foods, IAEA-TECDOC-587, Vienna, 1991. 69. Den Drijver, L., Holzapiel, C.W., and Vander Linde, H.J., High performance liquid chromatographic determination of D-arabino-hexos-2 ulose (D-glucosone) in irradiated sugar solutions: application of the method to irradiated mangos, J. Agric. Food Chem., 34, 758–762, 1986. 70. Kovacs, E. and Keresztes, E., The effect of irradiation on starch content in Golden Delicious apples, in Food Microstruct., 8, 67–74, 1989. 71. Sjoberg, A.M., The effects of gamma irradiation on the structure of apple pectin, Food Hydrocolloid., 1, 271–276, 1987. 72. Kawamura, Y., Uchiyama, S., and Salto, Y., A half-embryo test for identification of gamma-irradiated grapefruit, J. Food Sci., 54, 379–382, 1989. 73. Rajkowski, K.T. and Thayer, D.W., Alfalfa seed germination and yield ratio and alfalfa sprout microbial keeping quality following irradiation of seeds and sprouts, J. Food Prot., 64(12), 1988–1995, 2001. 74. Raffi, J.J., Agnel, J.P.L., Buscarlet, L.A., and Martin, C.C., Electron spin resonance identification of irradiated strawberries, J. Chem. Soc. Faraday Trans., 1, 3359–3362, 1988. 75. Saint-Lebe, L. and Raffi, J.J., A point of view about identification of irradiated foods by electron spin resonance. In Health Impact, Identifications and Dosimetry of Irradiated Foods, Bogl, K.W., Regulla, D.F., and Suess, M.J., Eds., Neuerberg, ISH-Heft, 125:139–154, 1988. 76. Lee, H-J., Byun, W-W., and Kim, K-S., Detection of radiation-induced hydrocarbons and 2-alklycyclobutanones in irradiated perilla seeds, J. Food Prot., 63(11), 1563–1569, 2000.

Ionizing Radiation Processing of Fruits and Fruit Products

255

77. Rahman, R., Rigney, C.J., and Bush-Peterson, E., Irradiation as quarantine treatment against Ceratistis capitata (Diptera: Tephritidiae): Anatomical and cytognetic changes in mature larvae following irradiation, J. Econ. Entomol., 83, 1449–1454, 1990. 78. Delincee, H., Ehlermann, D.A.E., and Bogl, K.W., The feasibility of an identification of radiation processed food, in Health Impact, Identification and Dosimetry of Irradiated Foods, Bogl, K.W., Regulla, D.F., and Suess, M.J., Eds., Neuerberg, ISH-Heft, 125, 58–138, 1988. 79. Heide, L. and Bogl, K.W., Detection methods for irradiated food — luminescence and viscosity measurements, Int. J. Radiat. Biol., 57, 201–209, 1990. 80. Heide, L., Guggenberg, R., and Bogl, K.W., Application of thermoluminescence measurements to detect irradiated strawberries, J. Agric. Food Chem., 38, 2160–2163, 1990. 81. Schreiber, G.A., Ziegelwann, B., Guitzch, G., Helle, N., and Bögel, K.W., Luminescence techniques to identify the treatment of food by ionizing radiation, Food Struct., 12, 385–396, 1993. 82. Al-Kahtani, H.A., Abu-Tarboush, H.M., Abou-Arab, A.A., Bajaber, A.S., Ahmed, M.A., and ElMojahadidi, M.A., Irradiation and storage effects on some properties of potato starch and use of thermoluminescence for identification of irradiated tubers, Amer. J. Potato Res., 77, 245–259, 2000. 83. Hart, R.I., White, I.A., and Reid, W.I., Technical note: Occurrence of o-tyrosine in non-irradiated foods. Int. J. Food Sci. Technol., 23, 643–647, 1988. 84. Willemot, C., Chayer, D., Gesselin, C. Couture, R., Drouin, S., and Castalgne, F., La methode de detection de L= irradiation par la presence d-orthotyrosine dans les proteines n=est pas aplicable aux fraises, Can. Inst. Food Sci. Technol., 23, 350–352, 1989. 85. Grootveld, M. and Jain, R., Methods for the detection of irradiated foodstuffs: aromatic hydroxylation and degradation of polyunsaturated fatty acids, Radiat. Phys. Chem., 34, 336–338, 1989. 86. SureBeam, SureBeam Brazil joint venture to support state of Rio de Janeiro’s Center of Excellence. http://www.surebeam.com/pressreleases/2001/071301_surebeam.html, 2001. 87. Medieros, J.F.B. and Ross, R.T., Irradiation comes of age: the Brazilian experience, in Proc. Food Irradiation 2000, April 3–5, 2000, Arlington, VA, 2000, chap. 5. 88. Mermelstein, N.H., E-beam irradiated beef reaches the market, papaya and gamma-irradiated beef to follow, Food Technol., 54(7), 88–92, 2000. 89. ICGFI, Inventory of product clearances and governing regulations. http://www.iaea.or.at/icgfi/, 2002. 90. Anon., Irradiation in the production, processing, and handling of food, in Code of Federal Regulations, 21CFR179, 2000. 91. NFPA, National Food Processors Association — Petition to amend 21CFR179 (Irradiation in the production, processing, and handling of food). http://www.nfpa-food.org/petition/petition.pdf, 2000. 92. FAO/IAEA/WHO, High-Dose Irradiation: Wholesomeness of Food Irradiated with Doses above 10 kGy, Joint FAO/IAEA/WHO Study Group on High-Dose Irradiation, Geneva, 15–20 September 1997, in WHO Technical Report Series 890, 1997. 93. Maxie, E.C., Sommer, N.F., and Mitchell, F.G., Infeasibility of irradiating fresh fruit and vegetables, Hortic. Sci., 6, 202–204, 1971. 94. Willemot, C.M., Marcotte, M., and Deschenes, L., Ionizing radiation for preservation of fruits, in Processing Fruits: Science and Technology, Vol 1: Biology, Principles and Applications, Somogyi, L.P. and Ramaswamy, H.P., Eds., Technomic Publishing Lancaster, PA, 1996, pp. 221–260, chap. 9. 95. Salisbury, F.B. and Ross, C.W., Plant Physiology, 3rd ed. Wadsworth, Belmont, CA, 1984. 96. Prakash, A., Guner, A.R., Caporaso, F., and Foley, D.M., Effects of low-dose gamma irradiation on the shelf life and quality characteristics of cut Romaine lettuce packaged under modified atmosphere, J. Food Sci., 65(3), 549–553, 2000a. 97. Dubery, I.A., van Rensburg, L.J., and Schabort, J.C., Malie enzyme activity and related biochemical aspects during ripening of g-irradiated mango fruit, Phytochemistry, 23, 1383–1386, 1984. 98. Fan, X., Argenta, L., and Mattheis, J., Impacts of ionizing radiation on volatile production by ripening Gala apple fruit, J. Agric. Food Chem., 49(1), 254–262, 2001. 99. Grandison, A.S., Combination treatments including electron beam irradiation for extending the shelflife of fresh fruits, Communication at the 2nd FAO/IAEA Coordination Meeting on Irradiation in Combination with Other Processes for Improving Food Quality, June 28 to July 2, 1993, FRDC, SaintHyacinthe, QC, Canada 1993.

256

Processing Fruits: Science and Technology, Second Edition

100. Larrigaudiere, C., Latche, A., Pech, J.C., and Triantaphylides, C., Relationship between stress ethylene production induced by gamma irradiation and ripening of cherry tomatoes, J. Am. Soc. Hortic. Sci., 116, 1000–1003, 1991. 101. Mercier, J. and Lindow, S.E., Role of leaf surface sugars in colonization of plants by bacterial epiphytes, Appl. Environ. Microbiol., 66(1), 369–374, 2000. 102. Anon., Potential for infiltration, survival and growth of human pathogens within fruits and vegetables. U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition. (http://vm.cfsan.fda.gov/~comm/juicback.html), 1999. 103. Buchanan, R.L., Edelson, S.G., Miller, R.L., and Sapers, G.M., Contamination of intact apples after immersion in an aqueous environment containing Eschericia coli O157:H7, J. Food Prot., 62(5), 444–450, 1999. 104. Fisher, T.L. and Dolden, D.A., Fate of Escherichia coli O157:H7 in ground apples used in cider production, J. Food Prot., 61(10), 1372–1374, 1998. 105. Riordan, D.C.R., Sapers, G.M., and Annous, B.A., The survival of Escherichia coli O157:H7 in the presence of Penicillium expansum and Glomerella cingulata in wounds on apple surfaces, J. Food Prot., 63(12), 1637–1642, 2000. 106. Morris, C.E., Monier, J., and Jaques, M., Methods for observing microbial biofilms directly on leaf surfaces and recovering them for isolation of culturable microorganisms, Appl. Environ. Microbiol., 63(4), 1570–1576, 1997. 107. Carmichael, I., Harper, I.S., Conventry, M.J., Taylor, P.W.J., Wan, J., and Hickey, M.W., Bacterial colonization and biofilm development on minimally processed vegetables, J. Appl. Microbiol., Symposium Suppl., 85, 45S–51S, 1999. 108. Fett, W.F., Naturally occurring biofilms on alfalfa and other types of sprouts, J. Food Prot., 63(5), 625–632, 2000. 109. Dewanti, R. and Wong, A.C.L., Influence of culture conditions on biofilm formation by Escherichia coli O157:H7, Int. J. Food Microbiol., 26,147–164, 1995. 110. Korber, D.R., Choi, A., Wolfaardt, G.M., Ingham, S.C., and Caldwell, D.E., Substratum topography influences susceptibility of Salmonella enteritidis biofilms to trisodium phosphate, Appl. Environ. Microbiol., 63, 3352–3358, 1997. 111. Liao, C-H. and Sapers, G.M., Attachment and growth of Salmonella Chester on apple fruits and in vivo response of attached bacteria to sanitizer treatments, J. Food Prot., 63(7), 876–883, 2000. 112. Draughon, A., Evans, A., Hulbert, G., and Mount, J., Lethality of 5 MeV e-beam to Staphylococcus, Salmonella, and Listeria in sliced cantaloupe and tomato. Abstract T47. Proceedings of the 88th Annual Meeting of International Association Food Protection, Minneapolis, MN, Aug 5–8, 2001. 113. Farkas, J., Saray, T., Mohacsi-Farkas, Horti, C.K., and Andrassy, E., Effects of low-dose gamma radiation on shelf-life and microbiological safety of pre-cut/prepared vegetables, Adv. Food Sci., 19(3, 4), 111–119, 1997. 114. Couture, R., Makhlouf, J., Cheour, F., and Willemot, C., Production of CO2 and C2H4 after gamma irradiation of strawberry fruit, J. Food Qual., 13, 385–393, 1990. 115. Larrigaudiere, C., Latche, A., Pech, J.C. and Triantaphylides, C., Short-term effects of g-irradiation on 1-aminocyclopropane-1-carboxylic acid metabolism in early climacteric cherry tomatoes, Plant Physiol., 92, 577–581, 1990. 116. Dubery, I.A., Elicitation of enhanced phenylpropanoid metabolism in citrus flavedo by gammaradiation, Phytochemistry, 31–2659–2662, 1992. 117. Fryling, L., Dubery, I.A., and Schabort, J.C., Biochemical changes involved in stress response and ripening behavior of g-irradiated mango fruit, Phytochemistry, 26, 681–686, 1987. 118. Kader, A.A. and Heintz, C.M., Gamma Irradiation of Fresh Fruit and Vegetables: An Indexed Reference List (1965–1982), Postharvest horticulture series No. 4, University of California, Davis, CA, 1983, p.55. 119. Thomas, P., Radiation preservation of foods of plant origin. III. Tropical fruits: bananas, mangoes, and papayas, CRC Crit. Rev. Food Sci. Nutr., 23, 147–205, 1986. 120. Fan, X. and Mattheis, J.P., 1-Methylcyclopropene and storage temperature influence responses of Gala apple fruit to gamma irradiation, Postharvest Biol. Technol., 23 143–151, 2001. 121. Strydom, G.J., Van Staden, J., and Smith, M.T., The effect of gamma radiation on the ultrastructure of the peel of banana fruits, Environ. Exp. Bot., 31, 43–49, 1991.

Ionizing Radiation Processing of Fruits and Fruit Products

257

122. Bramlage, W.J. and Couey, H.M., Gamma radiation of fruits to extend market life. Marketing Res. Rep. No. 717. ARS-USDA, Washington, D.C., 1965. 123. Al-Bachir, M., Effect of gamma irradiation on storability of apples (Malus domestica L.). Plant Foods for Human Nutri. 54, 1–11, 1999. 124. d’Amour, J., Gosselin, C., Arul, J., Castaigne, F., and Willemot, C., Gamma- radiation affects cell wall composition of strawberries, J. Food Sci., 58,182–185, 1993. 125. Voisine, R., Vezina, L.P., and Willemot, C., Modification of phospholipid catabolism in microsomal membranes of g-irradiated cauliflower (Brassica oleracea L.), Plant Physiol., 102, 213–218, 1993b. 126. Magee, R.L., Caporaso F., and Prakash, A., Inhibiting irradiation induced softening in diced tomatoes using a calcium treatment. Abstr. 30G–35, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002. 127. Voisine, R., Hombourger, C., Willemot, C., Castaigne, F., and Makhlouf, J., Effect of high carbon dioxide storage and gamma irradiation on membrane deterioration in cauliflower florets, Postharvest Biol. Technol., 2, 279–289, 1993. 128. Miller, W.R. and McDonald, R.E., Postharvest quality of GA-treated Florida grapefruit after gamma irradiation with TBZ and storage, Postharvest Biol. Technol., 7, 253–260, 1996. 129. Boylston, T.D., Reitmeier, C.A., Mosher, G.A., Moy, J.H., and Taladriz, L., Sensory and nutrient quality of three tropical fruits irradiated by x-radiation. Abstr. 30B-14, IFT Annual Meeting, New Orleans, LA, June 23–27, 2001. 130. Miller, W.R. and McDonald, R.E., Quality of Brightwell and Tifblue blueberries after gamma irradiation for quarantine treatment, HortSci. 31(7), 1234, 1996a. 131. Miller, W.R., McDonald, R.E., and Smittle, B.J., Quality of Sharpblue blueberries after electron beam irradiation. HortSci. 30(2), 306–308, 1995. 132. Miller, W.R., McDonald, R.E., McCollum, T.G., and Smittle, B.J., Quality of Climax blueberries after low dosage electron beam irradiation, J. Food Safety, 17(1), 71–79, 1994. 133. Horrick, J.M., Foley, D., Caporaso, F., and Prakash, A. 2002. Effects of low-dose gamma irradiation on the shelf-life and quality characteristics of fresh cut cantaloupe. Abstr. 76C–33, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002 134. Romani, R.J., Van Kooy, J., Lim, L., and Bowers, B., Radiation physiology of fruit-ascorbic acid, sulfhydryl, and soluble nitrogen content of irradiated citrus, Radiat. Bot., 3, 58, 1963. 135. Thayer, D.W., Wholesomeness of irradiated foods, Food Technol., 48(5), 132–136, 1994. 136. Fan, X. and Gates, R.A., Degradation of monoterpenes in orange juice by gamma radiation, J. Agric. Food Chem., 49(5), 2422–2426, 2001. 137. Hagenmaier R.D. and Baker, R.A., Low-dose irradiation of cut iceberg lettuce in modified atmosphere packaging, J. Agric. Food Chem., 45, 2864, 2868, 1997. 138. Luedtke, A. and Powell, D., Fact Sheet: A Timeline of Fresh Juice Outbreaks. Food Safety Risk Management and Communications Project Fact Sheet, University of Guelph, Guelph, ON, Canada. (http://www.plant.uoguelph.ca/safefood/micro-haz/juice-outbreaks.htm), 2000. 139. Marques, P.A.H.F., Worcman-barninka, D., Lannes, S.C.S., and Landgraf, M., 2001. Acid tolerance and survival of Escherichia coli O157:H7 in fruit pulps stored under refrigeration., J. Food Prot., 64(11), 1674–1678 140. Anon., FDA publishes final rule to increase safety of fruit and vegetable juices. P01–03 (http://www.fda.gov/bbs/topics/NEWS/2001/NEW00749.html), 2001. 141. Sizer, C.E. and Balasubramaniam, V.M., New intervention processes for minimally processed juices, Food Technol., 53(10), 64–67, 1999. 142. Pickett, K.T., Foley, D., Caporaso, F., Vasconcellos, A., and Prakash, A., Effects of low-dose irradiation on the microbial counts, sensory attributes and ascorbic acid content of unpasteurized orange juice. Abstr. 86H–10, IFT Annual Meeting. http://ift.confex.com/ift/2000/techprogram/paper_3867.htm, 2000. 143. Fetter, F., Stehlik, G., Kovacs, J., and Weiss, S., Das flavourverhalten einger gamma-bestrahlter fruchtsaefte. Mitteilungen: -rube, -wein, -obstbau und fruechteverwertung. 19, 140–151, 1969. 144. Chachin, K. and Ogata, K., Changes of chemical constituents and quality in some juice irradiated with the sterilizing dose level of gamma rays, Food Irradiat., 4(1), 85–90, 1969. 145. Thakur B.R. and Arya Singh, S., Effect of sorbic acid on irradiation-induced sensory and chemical changes in sweetened orange juice and mango pulp, Int. J. Food Sci. Technol., 28(4), 371–376, 1993. 146. Fan, X. and Thayer, D.W., Gamma-radiation influences browing, antioxidant activity and malonaldehyde level of apple juice. J. Agric. Food Chem., 50(4), 710–715, 2002.

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147. Spoto, M.H.F., Domarco, R.E., Walder, J.M.M., Scarminio, I.S., and Bruns, R.E., Sensory evaluation of orange juice concentrate as affected by irradiation and storage, J. Food Process. Preserv., 21(3), 179–191, 1997. 148. Fan, X. and Thayer, D.W., Quality of irradiated alfalfa sprouts, J. Food Prot., 64(10), 1574–1578, 2001. 149. Yulianti, F., Boylston, T.D., and Reitmeier, C.A., Consumer evaluation of irradiated and pasteurized apple cider. Abstr. 30G–10, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002. 150. Huggart, R.L., Rouse, A.H., and Moore, E.L., Effect of maturity, variety and processing on color, cloud, pectin and water-insoluble solids of orange juice, In Florida State Horticultural Society Proceedings, 88, 342, 1975. 151. Akamine, E.K. and Moy, J.H., Delay in postharvest ripening and senescence of fruits, in Preservation of Food by Ionizing Radiation, Vol. 3, Josephson, S.E. and Peterson, M.S. Eds., CRC Press, Boca Raton, FL, 1983 pp. 129–158. 152. Baccaunaud, M. 1991. Effets de Ionisation sur les fruits et legumes destines a la consommation en frais, In Ionisation des Produits Alimentaires, Tech & Doc-Lavosir, Paris. 153. Marcotte, M., Irradiated strawberries enter the U.S. market, Food Technol., 46(5) 80–86, 1992. 154. Loaharanu, P., International trade in irradiated foods: regional status and outlook, Food Technol., 43(7), 77–83, 1989. 155. Moy, J.H., Bian, X., Pang, W.Q., and Kawabata., O., The effects of gamma-radiation vs. heat on the quality of papayas. Abstr. 96–1, IFT Annual Meeting, New Orleans, LA, June 23–27, 2001. 156. Youssef, B.M., Asker, A.A., El-Samahay, S.K., and Swailam, H.M., Food Res. Int., 35(1), 1–13, 2002. 157. Fan, X., Toivonen, P.M., and Sokorai, K.B., Combination of ionizing radiation and heat shock on quality of fresh-cut iceberg lettuce. Abstr. 30G–34, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002. 158. Delaquis, P.J., Toivonen, P.M. and Stewart, S., Growth of Listeria monocytogenes and Escherichia coli 157:H7 is enhanced in ready-to-eat lettuce washed in warm water. Abstr. T44. IAFP Annual Meeting. Minneapolis, MN. Aug 5–8, 2001. 159. Couture, R. and Willemot, C., Combinaison d’une faible dose d’irradiation avec l’atmosphere controlee pour ralentir le murissement des fraises, in Proceedings of the International Conference on Technological Innovation in Freezing and Refrigeration of Fruits and Vegetables, Reid, D.S., Ed., University of California, Davis, CA, 1989, pp.40–45. 160. Koseki, S. and Itoh, K., Effect of nitrogen gas packaging on the quality and microbial growth of freshcut vegetables under low temperatures, J. Food Prot., 65(2)326–332, 2002. 161. Brecht, J.K., Sargent, S.A., Bartz, J.A., Chau, K.V., and Edmond, J.P., Irradiation plus modified atmosphere for storage of strawberries, Proceedings of the Florida State Horticultural Society, 105, 97–100, 1992. 162. Hagenmaier, R.D. and Baker, R.A., Microbial population of shredded carrot in modified atmosphere packaging as related to irradiation treatment, J. Food Sci., 63(1), 162–164, 1998. 163. Bussey, R., Troubleshooting migration issues in packaging, Food Safety Magazine, 7(6), 58–60, 2001. 164. Kiloran, J.J., Chemical and physical changes in food packaging materials exposed to ionizing radiation, Rad. Res. Rev., 3, 369–388, 1972. 165. Varsanyl, I., Investigation of polymer membranes of food packaging quality to gases and water vapor after radiation treatment with radurizing doses, Acta. Alim., 4, 251–269, 1975. 166. Kilcast, D., Irradiation of packaged food, in Food Irradiation and the Chemist, Johnston, D.E. and Stevenson, M.H., Eds., The Royal Society of Chemistry, Cambridge, U.K. 1990, pp. 140–152. 167. Wilson, J.E., Irradiation of polymers: crosslinking versus scission, in Radiation Chemistry of Monomers, Polymers, and Plastics, Marcel Dekker, New York, 1974, chap. 7. 168. Bourges, F., Bureau, G., Dumonceasu, J., and Pascat, B., Effects of electron beam irradiation on antioxidants in commercial polyolefins: determination and quantification of products formed. Packag. Technol. Sci., 5, 205–209, 1992. 169. Komolprasert, V., Truong, P., Begley, T.H., and McNeal, T.P., The suitability of semi-rigid amorphous copolyesters for prepackaged irradiated foods: migration study. Abstr. 73D-15, IFT Annual Meeting, New Orleans, LA, June 23–27, 2001. 170. Komolprasert, V., McNeal, T.P., Adhikari, C., and Begley, T.H., The chemical changes in an morphous nylon polymer upon irradiation. Abstr. 100B-10, IFT Annual Meeting, Anaheim, CA, June 15–19, 2002.

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171. Deschenes, L., Stablia des embaliages plastiques et traitement ionisant, in Proceedings of a Meeting on Irradiation for the Food Sector, Saint-Hyacinthe, QC, Canada, May 13, 1992, Publitech, FRDC, Agriculture Canada, Saint-Hyacinthe, QC, Canada, pp. 9–23, 1992. 172. Sadler, G.D. and Braddock, R.J., Absorption of citrus volatiles by low density polyethylene, J. Food Sci., 56, 35–37,54, 1991. 173. Codex Alimentarius Comission, Codex General Standard for Irradiated Foods and Recommended International Code of Practice for Operation of Radiation Facilities Used for the Treatment of Foods, Vol. 15, WHO, Geneva, 1984. 174. Chuaqui-Offermans, N., Food packaging materials and radiation processing of food: a brief review, Radial. Phy. Chem., 34, 1005–1007, 1989. 175. CAST, Ionizing Energy in Food Processing and Pest Control. II. Applications Report No. 115. Ames, IA, 1989. 176. WHO, Food Irradiation: A Technique for Preserving and Improving the Safety of Food. WHO, Geneva, 1988. 177. Giddings, G.G., Radiation processing of fishery products, Food Technol., 38(4), 61–65, 1984. 178. Mills, S., Discussion paper. Issues in food irradiation. Science Council of Canada, Ottawa, 1987. 179. Kunstadt, P. and Steeves, C., Economics of food irradiation. Proceedings of the International Symposium on Cost/Benefit Aspectsof Food Irradiation Processing, Aix-en-Provence, France, March 1993, IAEA/FAO/WHO, pp. 395–416. 180. FAO/IAEA/WHO, International Symposium on Cost/Benefit Aspects of Food Irradiation Processing, Aix-en-Provence, France, March 1993, Report of the Working Group.

12 Microbiology of Fruit Products Randy W. Worobo and Don F. Splittstoesser CONTENTS 12.1 12.2

Important Groups of Microorganisms ...............................................................................262 Yeasts..................................................................................................................................263 12.2.1 General Properties ...............................................................................................263 12.2.2 Detection and Enumeration .................................................................................264 12.2.3 Identification ........................................................................................................264 12.2.4 Incidence ..............................................................................................................264 12.2.5 Spoilage................................................................................................................265 12.3 Molds..................................................................................................................................265 12.3.1 General Properties ...............................................................................................265 12.3.2 Mycotoxins ..........................................................................................................265 12.3.3 Detection and Enumeration .................................................................................266 12.3.4 Microscopic Methods ..........................................................................................267 12.3.5 Identification ........................................................................................................267 12.3.6 Distribution ..........................................................................................................267 12.3.7 Spoilage................................................................................................................268 12.4 Lactic Acid Bacteria...........................................................................................................268 12.4.1 General Properties ...............................................................................................268 12.4.2 Detection and Enumeration .................................................................................268 12.4.3 Incidence ..............................................................................................................268 12.4.4 Spoilage................................................................................................................269 12.5 Acetic Acid Bacteria ..........................................................................................................269 12.5.1 General Properties ...............................................................................................269 12.5.2 Detection and Enumeration .................................................................................269 12.5.3 Incidence ..............................................................................................................269 12.5.4 Spoilage................................................................................................................269 12.6 Coliforms............................................................................................................................270 12.7 Spore-Forming Bacteria .....................................................................................................270 12.8 Pathogenic Bacteria............................................................................................................271 12.9 Preservation ........................................................................................................................271 12.9.1 Pasteurization .......................................................................................................271 12.9.2 Hot Fill.................................................................................................................272 12.9.3 Aseptic Processes ................................................................................................272 12.9.4 Canning ................................................................................................................272 12.9.5 Microbiology........................................................................................................272 12.9.6 Heat-Resistant Molds ..........................................................................................272 12.9.7 Composition of the Fruit Product .......................................................................274 12.9.8 Frozen Fruit .........................................................................................................275 12.10 Low Water Activity ............................................................................................................275 12.10.1 Fruit Juice Concentrates ......................................................................................275 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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12.10.2 Preserves ..............................................................................................................275 12.10.3 Dried Fruits..........................................................................................................276 12.10.4 Osmotolerant Fungi .............................................................................................276 12.11 Refrigeration.......................................................................................................................276 12.12 Antimicrobials ....................................................................................................................277 12.12.1 Sulfur Dioxide .....................................................................................................277 12.12.2 Benzoic and Sorbic Acids ...................................................................................277 12.12.3 Dimethyl Dicarbonate and Diethyl Dicarbonate.................................................278 References ......................................................................................................................................278

12.1 IMPORTANT GROUPS OF MICROORGANISMS Many fruits and fruit products are high-acid foods that have a pH under 4 (Table 12.1). Although the organic acids will differ with the kind of fruit, the predominant acids are often citric, malic, and tartaric. The amount of acid and the pH is influenced by the cultivar, fruit maturity, climate, and makeup of the soils. Fruits produced in the regions having a cool climate usually are more acidic than those grown in warmer regions. Grapes harvested after a cool, cloudy growing season, for example, may have a pH under 3 and a titratable acidity of over 1%, while the same vines may yield fruit with a pH of 3.4 or higher and a titratable acidity of under 0.7% when the summer is hot and sunny. The low pH of many fruits is the major factor that influences the composition of their microflora. In general, most yeasts and molds grow well under acid conditions, and, thus, fungi are often the predominant microorganisms in fruit products. Only a few bacteria are sufficiently aciduric to be important.

TABLE 12.1 Principal Acids and pH of Various Fruits Fruit

pH

Nonvolatile Acids

Apple Banana Blackberry Cantaloupe Cherry Cranberry Grapefruit Grape Guava Lemon Mango Orange Papaya Passion fruit Peach Pear Pineapple Plum Watermelon

3.3–4.1 4.5–5.2 3.0–4.2 6.2–6.5 3.2–4.7 2.5–2.7 2.9–3.4 3.0–4.5 3.0–3.2 2.2–2.4 3.3–3.7 3.0–4.0 4.5–6.0 2.6–3.3 3.1–4.2 3.4–4.7 3.2–4.0 2.8–4.6 5.8–6.0

Malic, citric, lactic Citric, malic, tartaric Malic, citric, isocitric Citric, malic Malic, citric, isocitric Citric, malic, quinic Citric Tartaric, malic Citric, malic, lactic Citric Citric, tartaric Citric, malic Citric, malic ketoglutaric Citric, malic Malic, citric Malic, citric Citric, malic Malic, quinic Citric, malic

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TABLE 12.2 Some Yeast Genera Associated with Fruit Products Genus

Sexual Spores

Fermentative

Other Properties

Brettanomyces Candida Citeromyces Clavispora Cryptoccus Debaromyces Dekkera Endomyces Endomycopsella Filobasidiella Geotrichum Hanseniaspora Hansenula Issatchenkia Leucosporidium Lodderomyces Metschnikowia Nadsonia Pichia Rhodosporidium Rhodotorula Saccharomyces Saccharomycopsis Schizosaccharomycs Sporidioabolus Sporobolomyces Torulaspora Trichosporon Wickerhamiella Williopsis Yarrowia Zygosaccharomyces

— — + + — + + + + + — + + + + + + + + + — + + + + — + — + + + +

+ ± + + — ± + ± — ± ± + ± + ± + + + ± — — + ± + — — + — + + — +

Acetic acid produced Pseudo and septate hyphae Warty ascospores Clavate ascospores Capsules, sometimes hyphae Warty ascospores Ogival, acetic acid produced Blasidiospores Clavate conidia Basodiospores Septate hyphae, arthrospores Lemon-shaped cells Hat-shaped ascospores Pseudohyphae Teliospores, hyphae Pseudohyphae Club-shaped ascospores Bipolar budding Hat/saturn-shaped ascospores Carotenoid pigments Carotenoid pigments Strong fermenters Septate hyphae Cell fission, no budding Dark teliospores Cartenoid pigments No hyphae Arthroconidia One rough ascospore Ascospore saturn shaped Septate hyphae Preservative-resistant

Source: Barnett, J.A., Payne, R.W., and Yarrow, D. 1983. Yeasts: Characteristics and Identification. Cambridge University Press, Cambridge, U.K.; Deak, T. and Beuchat, L.R. 1987. Identification of foodborne yeasts. J. Food Prot., 50: 243–264.

12.2 YEASTS 12.2.1 GENERAL PROPERTIES Yeasts are fungi whose usual growth form is unicellular; many reproduce by budding. They do not represent a homogeneous taxonomic group. At least 215 species are important in foods (Deak and Beuchat, 1987), and it is estimated that 32 genera are associated with fruits and fruit products (Table 12.2). Only a few species of yeasts are pathogenic for man and other animals (Pfaff et al., 1978). None of the pathogenic species are common contaminants of fruits and fruit products.

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AND

ENUMERATION

Fruit samples are homogenized in a blender or StomacherTM, then further diluted in a protective solution such as 0.1% peptone water. The desired decimal dilutions are surface cultured on agars that suppress the growth of bacteria. Recommended media are potato dextrose agar, which is acidified to pH 3.5 with 10% tartaric acid, or plate count agar to which has been added to 100 mg/l chloramphenicol (Mislivec et al., 1992). The antibiotic-containing agar often will yield higher recoveries of yeasts than the acidified agar (Koburger, 1970), although the differences are minimal when high-acid foods such as many fruit products are being cultured (King, 1992). The plates are incubated at 20 to 25∞C for 5 d before counting. When culturing low water activity foods such as fruit juice concentrates, special plating techniques may be required (Tilbury, 1980; Corry, 1987). To avoid osmotic shock, dilutions are often performed in 0.1% peptone water containing 20% w/w sucrose. Appropriate dilutions are then inoculated onto a suitable yeast agar that contains 50% w/w glucose, fructose, or sucrose. The plates are incubated at 25 to 30∞C and counted at 5 d. Species of osmotolerant yeasts include Zygosaccharomyces rouxii, Torulaspora delbrueckii, and Debaromyces hansenii (Jermini and Schmidt-Lorenz, 1987b). A number of more rapid methods have been developed for the detection of yeasts in different fruit products. The application of gas chromatography to measure trace amounts of ethanol under 0.2% has long been used to monitor yeast growth in bulk-stored grape juice. Another procedure is impedimetric detection, which depends upon changes in resistance to the flow of an alternating current when the food is cultured in a medium selective for yeasts. The hours required to produce a measurable change in impedance (growth to 106 to 107 yeasts/ml) can be correlated with the initial yeast population in the fruit product (Schaertel et al., 1987). A very rapid and sensitive method is a bioluminescent procedure that measures the quantity of adenosine triphosphate (ATP) in living yeasts. The luciferin–luciferase–ATP reaction permits the detection of as few as 10 yeast cells in only 3 to 5 min. The procedure has considerable application for beverages that contain fruit juice (Little and LaRocco, 1986).

12.2.3 IDENTIFICATION Traditional methods for the identification of different species of yeasts rely on a very large number of tests and observations, which include cell morphology, spore formation, fermentation of sugars, assimilation of other compounds as a carbon source, utilization of nitrate as a nitrogen source, requirements for various growth factors, and resistance to cycloheximide (Lodder, 1970; Barnett et al., 1983). In addition to being laborious, some tests can be very difficult. For example, although ascospore formation is an important criterion, some yeast strains may lose their ability to produce spores or form so few that detection is very difficult. Various schemes have been proposed to simplify the identification of yeasts that are important in foods. Beech et al. (1968) devised criteria that did not utilize spore formation. Their procedure was based on colony and cell morphology, pigmentation, various sugar fermentations, and assimilation tests. More recently, Deak and Beuchat (1987) published a procedure for the identification of 215 species of yeasts from foods that involved only 10 to 15 tests that were largely assimilation and fermentation reactions. Several commercial kits have been developed for the identification of yeasts of clinical importance. When tested against 166 yeasts isolated from fruit juices, the API 20C kit (Analytab Products, Plainview, NY) correctly identified 86% of the cultures, while the ID 32C kit (BioMeriux Vitek Inc., Hazelwood, MO) correctly identified 76% (Deak and Beuchat, 1993).

12.2.4 INCIDENCE Yeasts are widely distributed in the environment. In the plant world, they can be isolated from leaves, flowers, and tree exudates, as well as from the surface of fruits (Pfaff et al., 1978). Fruits, as received

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at the processing plant, often are contaminated with large numbers of yeasts. Sound apples, for example, may yield viable populations of 103 to 104 per gram of fruit (Marshall and Walkley, 1951; Swanson et al., 1985). Grapes, which possess a greater surface area, are more fragile in texture than apples and have contained as many as 107 yeasts per gram (Splittstoesser and Mattick, 1981). In the latter study that involved 135 samples of grapes, 43% yielded counts of 106 per gram or higher. Fruit that has been damaged by birds, insects, or pathogenic fungi may contain very high yeast populations. The yeasts are introduced into the exposed tissue, often via insects, and are able to use the sugars and other nutrients to support their growth. Other sources of contamination include the surfaces of mechanical harvesters (Moyer et al., 1969) and the bins and lug boxes used in the transport of fruit to the processing plant. In the factory, many of the yeasts are removed by early processing steps that may include washing and peeling of the fruit. Recontamination can subsequently occur at points where fruit solubles collect and when there is opportunity for yeast growth. Conveyor belts, equipment that slices or dices fruit, presses, and the ports of filling machines are all potential sources of contamination (Berry, 1979).

12.2.5 SPOILAGE Manifestations of yeast growth will depend upon the nature of the fruit product and the strain of yeast. Growth of a strongly fermentative type such as certain strains of Saccharomyces cerevisiae may produce sufficient CO2 to burst the container, 90 lb/in.2 or more. Growth of some species in a clear fruit juice may produce only slight haze and sediment. Yeast populations of 105 per milliliter or less usually can be detected with the naked eye. While CO2 and ethanol are usually the predominant metabolic products of yeasts, glycerol, acetaldehyde, pyruvic acid, and a-ketoglutaric acid are also formed (Rankine, 1968). Certain oxidative yeasts such as species of Brettanomyces will generate acetic acid in wines and other fruit products. Although yeasts may possess hydrolytic enzymes that degrade pectins, starch, and certain proteins (Pfaff et al., 1978; Biely and Slavikova, 1990), enzymatic activity is usually much less than that exhibited by other aciduric microorganisms, molds in particular.

12.3 MOLDS 12.3.1 GENERAL PROPERTIES Filamentous fungi represent an important group within the microflora of fruit products for a number of reasons: 1. Some species are xerophilic and, thus, are potential spoilage agents of foods of low water activity such as dried fruits and fruit juice concentrates. 2. Growth of molds on processing equipment such as wooden tanks can result in the generation of off-flavors in wines, juices, and other fruit products. 3. The ascospores of some species are very heat-resistant and will survive the commercial pasteurization treatments that are given to most fruit products. 4. Mold-infected raw fruit may become soft after processing because pectinases were not inactivated by the thermal treatment (Ogawa et al., 1974). 5. The metabolic products of many molds are toxic to various animals and presumably man (Liewen and Bullerman, 1992).

12.3.2 MYCOTOXINS Over 100 different toxic compounds are produced by some 200 mold species (Krogh, 1989). Many are associated with foods other than fruits and fruit products. For example, aflatoxins, the

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TABLE 12.3 Mycotoxins Associated with Fruit Products Fungus Aspergillus flavus Byssochlamys

Eurotium Fusarium moniliforme Neosartorya

Penicillium expansum Talaromyces flavus

Mycotoxin

Fruit

Aflatoxins Patulin Byssochlamic acid Byssotoxin Malformins Patulin Physicon Trichothecenes Zearalenone Fumitremorgans Mevinolins Terrein Trypacidin Tryptoquivalins Verruculogen Patulin Dehydroaltenusin Mitorubrins Vermicelline Vermistatine Wortmannin Wortmannolone

Figs Canned juices, etc.

Reference Sharman et al., 1991 Davis and Diener, 1987 Frisvad and Samson, 1991

Preserves Banana

Frisvad and Samson, 1991 Chakrabarti and Ghosal, 1986

Canned juices, etc.

Frisvad and Samson, 1991

Apple juice Canned juices, etc.

Liewen and Bullerman, 1992 Frisvad and Samson, 1991

most thoroughly studied mycotoxins, are found primarily on cereals, including corn, cottonseed, and peanuts. Patulin appears to be the most common mycotoxin in processed fruits (Table 12.3). It is produced by a number of mold species, included Penicillium expansum, the most usual cause of rots of the apple (Harwig et al., 1973). Patulin is resistant to thermal destruction between pH 3.5 and 5.5 (Lovett and Peeler, 1973), and, thus, traces of the compound are commonly present in canned apple juice and other heated apple products. Recently, the U.S. Food and Drug Administration (FDA) implemented finished product patulin levels of less than 50 ppb for apple juice (Food and Drug Administration, 2001). Aflatoxins have been found in dried figs and fig paste (Sharman et al., 1991). About 11% of the fig paste and 9% of the whole dried figs yielded detectable levels, with 40 mg/kg being the highest concentration. The heat-resistant molds, Byssochlamys, Eurotium, Neosartorya, and Talaromyces, produce a number of mycotoxins when growth occurs in a thermally processed fruit product (Table 12.3). Growth by these species usually would not present a serious public health problem, however, because the spoiled food would be avoided by most consumers.

12.3.3 DETECTION

AND

ENUMERATION

Many of the plating procedures that were described previously for yeasts are also applicable for molds. Interpretation of the results may be difficult, however, because the number of colonyforming units will be influenced by the presence of fruiting structures that contain large numbers of spores, and also if the mycelial mass is broken into many viable fragments by the homogenization treatment.

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Certain specialized media have been recommended for the enumeration of filamentous fungi. Dichloran rose bengal chloramphenicol agar (DRBC) is useful when the fruit product contains molds whose colonies may spread over the entire surface of the plate (King et al., 1979). Dichloran 18% glycerol agar (DG18) permits the enumeration of xerophilic molds that might be present in dried fruits, fruit juice concentrates, and other foods of low water activity such as fruit cakes (Hocking and Pitt, 1980). A number of more rapid methods for the detection of molds in foods have been studied (Williams, 1989; Mislivec et al., 1992). One chemical procedure has been to quantitatively measure chitin, which is a component of the fungal cell wall (Lin and Cousin, 1985). The analysis is somewhat tedious and, thus, has not been widely adopted by the food industry. Immunoassays represent another improved method for detecting fungi (Lin et al., 1986; Notermans et al., 1986). An immunoassay specific for Aspergillus and Penicillium is based on the detection of a heat stable extracellular polysaccharide (Kamphuis and Notermans, 1992). Another approach is to analyze for metabolic products of molds. For example, the degree of moldiness in mechanically harvested grapes may be related to glycerol content of the must (Ravji et al., 1988). A difficulty with this method is that the procedure is only semiquantitative because different species and strains of fungi produce varying amounts of glycerol.

12.3.4 MICROSCOPIC METHODS Several microscopic procedures are used for the enumeration of molds (Splittstoesser, 1992). Perhaps the oldest is the Howard mold count that originally was developed to monitor the soundness of tomatoes used in the production of catsup (Howard, 1911). In the Howard method, the incidence of molds is determined by examining a number of fields under the microscope using a special counting chamber and carefully prescribed techniques. A positive field contains one or more mold filaments that exceed a certain length. The FDA has established mold count criteria (Defect Action Levels) for a number of fruit products, including apple butter; apricot, peach, and pear nectars and purees; frozen and canned berries; canned citrus juices; cranberry sauce; and pineapple products. A Geotrichum count has been developed to assess the cleanliness of equipment used in the processing of fruits for canning and freezing (Cichowicz and Eisenberg, 1974). The mold Geotrichum candidum forms a characteristic brush-like morphology when growing on the surfaces of processing equipment. Typical filaments are counted using a rot fragment slide and a stereoscopic microscope at 30 to 45 ¥ power. An advantage to the method is that it permits the evaluation of canned fruits as dead filaments can be counted. With frozen foods, a disadvantage is that Geotrichum counts do not always correlate well with viable counts when the two procedures are compared (Splittstoesser et al., 1977).

12.3.5 IDENTIFICATION Identification of filamentous fungi to the species level is based largely on colonial and microscopic morphology (Pitt and Hocking, 1985; Samson and van Reenen-Hoekstra, 1988). Unknown isolates are cultured on a number of different media to assure that mycelial morphology and sporulation are typical. Important characteristics include colony pigmentation and physical appearance, the morphology of fruiting structures, whether mycelia are septate, and the presence of ascospores. Successful identification depends upon the ability to recognize the different specialized structures.

12.3.6 DISTRIBUTION Molds are widely distributed in the environment. They are present in soils, in the air, on plants, and in water. They play a major role in the mineralization of organic materials in the soil. Growth occurs wherever conditions are favorable, which includes the walls and ceilings of fruit processing factories when a high relative humidity persists. Conidia and other mold spores, which are released

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TABLE 12.4 Major Lactic Acid Bacteria That Spoil Fruit Products Genus

Morphology

Fermentation

Species L. plantarum L. brevis L. collinoides L. fermentum L. fructivorans L. hilgardii L. mesenteroides L. paramesenteroides L. oenos P. pentosaceus

Lactobacillus Lactobacillus

Rods Rods

Homofermentative Heterofermentative

Leuconostoc

Cocci

Heterofermentative

Pediococcus

Cocci-tetrad

Homofermentative

in high numbers, survive desiccation and, thus, are common inhabitants of the air, including the atmospheres of food processing factories.

12.3.7 SPOILAGE Although molds are aerobic microorganisms, many are very efficient scavengers of oxygen, and, thus, processed fruits, including those hermetically sealed in cans or glass, are susceptible to spoilage. When the amount of vegetative growth is very limited, evidence of spoilage may be the changes produced by fungal enzymes such as the breakdown of starch or pectins. When more growth is permitted, colonies may develop in the headspace or as strands throughout a beverage or similar product.

12.4 LACTIC ACID BACTERIA 12.4.1 GENERAL PROPERTIES The lactic acid bacteria are gram positive, catalase negative organisms that grow very well under anaerobic conditions. The lactobacilli are rod-shaped, while the pediococci and leuconostocs are spherical (Table 12.4). The homofermentative species produce mainly lactic acid from hexose sugars; the heterofermenters produce one molecule of lactic acid, one molecule of carbon dioxide, and one two-carbon compound. The latter is usually acetic acid or ethanol or a combination of the two. The streptococci (Lactococcus and Enterococcus species) are not included in Table 12.4 because they rarely have been involved in the spoilage of fruit products (Carr, 1958). Perhaps this is because they are less tolerant to high acidity and a low pH.

12.4.2 DETECTION

AND

ENUMERATION

The lactic acid bacteria are very fastidious with respect to their nutritional requirements. MRS agar (deMan et al., 1960) is a rich medium that supports growth of most lactic acid bacteria. It can be made selective for this group with the addition of 0.02% sodium azide (Mundt et al., 1967). Another medium that is useful for lactic acid bacteria is plate count agar supplemented with 10% tomato juice (Stamer et al., 1964).

12.4.3 INCIDENCE Lactic acid bacteria can be isolated from the plant world, including grains, fruits, and vegetables (Splittstoesser and Gadjo, 1966). The numbers on fruit as received from orchards and vineyards is usually very low. Buildup occurs on processing equipment where fruit solubles can accumulate

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and during the fermentation of juices. Lactic acid bacteria are common contaminants of ciders, wines, and citrus juices (Luthi, 1959; Hatcher et al., 1992).

12.4.4 SPOILAGE Growth of lactic acid bacteria in juices and other fruit products may result in the formation of haze, gas, acid, and a number of other changes. Certain heterofermentative lactobacilli have been responsible for slime in cider (Carr, 1970). The lactobacilli and leuconostocs that are present in citrus juices generate acetylmethylcarbinol and diacetyl, compounds that give the juices an undesirable, buttermilk-like flavor (Parish and Higgins, 1988; Hatcher et al., 1992). Some strains are extremely tolerant of ethanol and, thus, can grow in wines. Lactobacillus fructivorans (formerly L. trichodes) has grown in appetizer and dessert wines containing as much as 20% ethanol (Fornachon et al., 1949). Growth has resulted in the formation of filaments, and in California, spoilage has been referred to as Fresno mold (Vaughn et al., 1949). Most lactic acid bacteria have the ability to decarboxylate malic acid to lactic acid. This malolactic fermentation is often desirable in high-acid wines because the acidity is reduced, and desirable flavors are produced (Wibowo et al., 1985). Oenococcus oenos (formerly Leyconostoc oenos) appears to be the most acid and alcohol-tolerant species and often is isolated from wines that are undergoing a malo-lactic fermentation. Cultures of O. oenos are available commercially and are used by many winemakers to inoculate their wines during or after the ethanol fermentation but before bottling. (Because of haze and gas, growth of malo-lactic bacteria in a wine after bottling would be considered a type of spoilage.)

12.5 ACETIC ACID BACTERIA 12.5.1 GENERAL PROPERTIES The acetic acid bacteria are gram negative, aerobic rods that are separated into two genera, Acetobacter and Gluconobacter (DeLey et al., 1984; DeLey and Swings, 1984). While both oxidize ethanol to acetic acid under acid condition, Acetobacter species can oxidize acetic acid to carbon dioxide, and, thus, the genus is termed an overoxidizer. Utilization of lactate by the acetobacters is another test for separating the two genera (Carr, 1968).

12.5.2 DETECTION

AND

ENUMERATION

Various media have been used for the isolation of acetic acid bacteria from fruits and fruit products (Blackwood, 1969; Carr and Passmore, 1979; Drysdale and Fleet, 1988). A typical agar might contain 2% yeast extract, 2% ethanol, 2% agar plus brom cresol green to signal acid formation. The addition of 50 to 100 mg/l cycloheximide is recommended to suppress growth of yeasts when they outnumber the acetic acid bacteria (Drysdale and Fleet, 1988; Passmore and Carr, 1975).

12.5.3 INCIDENCE Damaged fruit often undergo an alcoholic fermentation by yeasts, followed by growth of acetic acid bacteria. Grapes infected by Botrytis cinerea may exhibit a sour rot that results in the buildup of over 106 acetobacters per gram; Acetobacter aceti and A. pasteurianus have been the predominant species (Drysdale and Fleet, 1988). In studies on sound grapes, about 50% of the samples yielded acetic acid bacteria, with the populations ranging from 10 to 104 per gram. All of the isolates were strains of Gluconobacter oxydans (Splittstoesser and Churey, 1992).

12.5.4 SPOILAGE Because the bacteria are obligate aerobes, juices, wines, and cider are most susceptible to spoilage while held in tanks prior to bottling (Joyeux et al., 1984). Relatively small amounts of acetic acid

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have a marked effect on wine quality. In the U.S., the Bureau of Alcohol, Tobacco, and Firearms limits the amount of volatile acid to 0.12 g of acetic acid per 100 ml in white wines and 0.14 g per 100 ml in red wines. Growth in juices that do not contain ethanol results in hexose sugars being oxidized to gluconic and ketogluconic acids (Drysdale and Fleet, 1988). Other products of carbohydrate metabolism are dihydroxyacetone, 2,3-butanediol, and acetoin. Some strains of Acetobacter pasteurianus and Gluconobacter oxydans produce microfibrils composed of cellulose, which results in the formation of flocs in different fruit juice beverages (Juven and Shomer, 1985).

12.6 COLIFORMS Coliforms are often found in juices and other fruit products. They usually are species other than Escherichia coli. They can be recovered from the surface of fruit (Vaughn et al., 1952; Goverd et al., 1979), and their presence usually does not indicate direct fecal contamination. Because of the low pH of juices, most coliforms die off rapidly in single-strength products (Hahn and Appleman, 1952); however, in frozen juices, such as concentrated orange juice, coliforms may survive for months or longer (Wolford, 1956; Fuentes et al., 1985).

12.7 SPORE-FORMING BACTERIA Low numbers of viable bacterial spores can be recovered from many fruit products. The spores are widely distributed in the environment, and they are not destroyed by preservation processes such as fermentation, canning, or freezing. Most bacterial spores are of little concern to the processor of fruits because the low pH of fruit products prevents spore germination or outgrowth (Blocher and Busta, 1983). There are, however, a few reports that indicate that some relatively rare bacterial spore-forming species are sufficiently aciduric to be a potential problem. Townsend (1939) studied anaerobes responsible for the spoilage of canned fruits. He found that a vegetative cell inoculum of Clostridium pasteurianum could initiate growth in pear and apricot juices having a pH of 3.6. Bacillus macerans and B. polymyxa are gas-producing species that have been isolated from acidified vegetables and from canned peaches and pears that had an original pH of 3.8 to 4 (Vaughn and Stadtman, 1946; Vaughn et al., 1952). Several reports indicate that certain spore-forming bacilli can grow in wine. Gini and Vaughn (1962) isolated strains of Bacillus coagulans, B. subtilis, B. macerans, B. pumilis, B. sphaericus, and B. pantothenticus from dessert wines that contained as much as 20% ethanol by volume. Growth was obtained when the isolates were inoculated into a Grillo wine having a pH of 3.6 and 19% ethanol. More recently, aciduric spore formers have been obtained from wine corks (Daly, 1982; Lee et al., 1983). The cultures, identified as strains of B. coagulans, B. badius, and B. sphaericus, initiated limited growth when inoculated into red and white wines having an initial pH of 3.5 and 3.3, respectively. Spore-forming bacilli that actually prefer a low pH have been responsible for spoilage of apple juice (Cerny et al., 1984) and a blend of fruit juices (Haglund, 1993). The organisms have a pH optimum between 2.5 and 5.5 and, based on their cyclohexane fatty acids and hoppanoid content, are believed to be similar to Bacillus acidocaldarius (Cerny et al., 1984). However, B. acidocaldarius, which was originally isolated from acidic hot springs (Darland and Borck, 1971), grows at 75 to 80∞C, while the apple juice isolate had a maximum growth temperature of 50∞C and an optimum of 40 to 42∞C (Cerny et al., 1984). Subsequently in 1992, these organisms were reclassified to a new genus, Alicyclobacillus, based on their different comparative rDNA sequence analyses. The genus has expanded to include several species such as Alicyclobacillus acidocaldarius, A. acidoterrestris, A. cycloheptanicus, and A. hesperidium (Wisotzkey et al., 1992). Since the original

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observation, numerous reports of beverage and acid products due to Alicyclobacillus spp. has been identified (Splittstoesser et al., 1994; Walls, 1994; Wisse et al., 1998; Yamazaki et al., 1996). A report has found the spores of similar cultures in commercial pasteurized apple, grape, and cherry juices (Splittstoesser et al., 1994, unpublished data). These findings agree with Cerny et al. (1984), in that the organism does not grow in the usual rich bacteriological media such as trypticase soy agar, plate count agar, and brain–heart infusion agar. Good growth and sporulation, however, have been obtained in potato dextrose agar, either at pH 3.5 or 5.6, as well as K-media. Studies on heat resistance showed D-values for the spores in apple juice to be 23 min at 90∞C, with a zvalue of 7.7∞C. Thus, the spores are sufficiently heat resistant to survive the thermal process applied to many fruit products.

12.8 PATHOGENIC BACTERIA In general, the different bacteria that cause disease in humans are not associated with fruit products. Most originate from animal reservoirs, and many do not tolerate the low pH of fruits. There are, however, a few exceptions. Fruit such as melons may contain enteric pathogens on their rind, which then may be introduced onto the flesh when the fruit is cut. The original source of the contamination can be polluted irrigation water. Precut wrapped watermelon was responsible for 18 foodborne illnesses involving Salmonella oranienbert (Anon., 1979). Growth of Shigella sonnei, S. flexneri, S. dysenteriae, Salmonella derby, and S. typhi occurred when jicama (pH 5.97), papaya (pH 5.67), and watermelon (pH 6.81) were held at room temperature (Escartin et al., 1989). Storage of melons and cantaloupe at 5∞C prevented growth of salmonellae during a 24-h incubation although the viable populations did not decrease (Golden et al., 1993). Thus, refrigeration may improve the safety of melons when the incidence of pathogens is low. In New Jersey, 296 people became ill after consuming nonpasteurized cider that had been contaminated with Salmonella enterica serovar Typhimurium (Anon., 1975). The orchard had been fertilized with animal manure, which was thought to be the source of contamination because cider is commonly made from fruit collected from the orchard floor. Subsequent studies showed that S. enterica serovar Typhimurium could survive in apple juice of pH 3.6 for at least 30 d (Goverd et al., 1979). Another pathogen associated with fruit products is Escherichia coli O157:H7, an organism that produces bloody diarrhea and hemolytic uremic syndrome. Contaminated apple cider has been the cause of several outbreaks, including one in Massachusetts in which 23 people became ill (Besser et al., 1993). The bacteria will survive in cider held at 8∞C for 20 d or longer but often will die off in 3 d when the cider contains 0.1% sodium benzoate (Zhao et al., 1993). Although most producers of cider do not wish to pasteurize, our unpublished data show the organism to be quite heat sensitive. Thus, D-values in pure cider were 12 min at 52∞C and only 0.37 min when the cider contained 0.1% sodium benzoate.

12.9 PRESERVATION Various methods are used for extending the shelf life of fruit products. As will be shown in this section, the microorganisms of importance will vary with the preservation methods.

12.9.1 PASTEURIZATION The common thermal processes applied to fruit products include hot fill, aseptic systems, and traditional canning in which the food is heated after being filled and closed into a container (Lopez, 1987). The physical properties of the food and the nature of the container often dictate the process to be used.

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12.9.2 HOT FILL A fruit juice or similar product is heated to 88 to 95∞C, then filled into a glass or metal container. After closing, the container may be inverted for several minutes before cooling to kill any spoilage microorganisms that might be present on the cap. Applesauce that is packed in metal containers may require a higher closing temperature, 93∞C or above, to prevent detinning. Certain plastic containers become distorted by high fill temperatures. This can be circumvented by heating the juice to 90 to 95∞C, then cooling to a temperature of about 70∞C for filling. The lower temperature is still lethal for most aciduric spoilage microorganisms.

12.9.3 ASEPTIC PROCESSES Aseptic technology is widely used for the preservation of fruit juices and juice beverages in consumer-size packages. The juice and package are sterilized separately in a sterile chamber. The juice is often flashed to 93∞C or higher temperature in a heat exchanger, held for 30 sec and then cooled before filling into a sterile carton. The carton, often a paper–plastic–aluminum laminate, is presterilized by exposure to 30% hydrogen peroxide at an elevated temperature. Aseptic systems are also used for the preservation of large volumes of fruit products that will later be reprocessed. The method is applicable to 50-gal drums, as well as large tanks that hold thousands of gallons.

12.9.4 CANNING This process is used to preserve a large variety of fruit products, including berries, whole fruits, and fruit sections. After filling the fruit into cans or jars, a syrup is often added, and then air is removed by subjecting the container to a steam exhaust treatment. Following closing, the containers are held in a boiling water bath until fruit in the slowest heating region in the can or jar reaches a temperature of 88 to 91∞C.

12.9.5 MICROBIOLOGY The thermal processes cited above are much less severe than those used for low-acid foods such as vegetables and meats. The milder processes are usually adequate because the most heat-resistant microorganisms, bacterial spores, do not grow in a high-acid fruit medium, and most acid-tolerant microorganisms possess little heat resistance (Table 12.5). The data in Table 12.5 illustrate a number of principles regarding canning of fruits. First of all, of the three groups of microorganisms, only ascospores of certain molds have the potential to survive commercial pasteurization treatments. Although many molds produce asexual spores such as conidia, as illustrated with Aspergillus flavus, these structures possess little heat resistance. The ascospores of yeasts are relatively sensitive to heat although they are considerably more resistant than are the vegetative cells. Non-spore-forming bacteria are also very heat sensitive. The spoilage of pasteurized fruits by yeasts and bacteria, therefore, usually results either from gross underprocessing or because contamination occurred following the heat treatment. Cooling water, which usually is recycled, may permit the buildup of bacteria and yeasts and, thus, increase the opportunities for contamination when small volumes of water are introduced into the container during cooling.

12.9.6 HEAT-RESISTANT MOLDS Most fruit products are not given a thermal process that is sufficient to destroy the ascospores of Byssochlamys, Neosartorya, and Talaromyces. The reason that spoilage is not more common is not completely understood. Although the counts are usually low on sound fruit, less than one per gram,

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TABLE 12.5 Heat Resistance of Various Aciduric Microorganisms Microorganism

Medium

Aspergillus flavus conidia Byssochlamys fulva ascospores Eurotium herbariorum ascospores Neosartorya fischeri ascospores

Molds PO4 Glucose-tartrate Grape juice Apple juice

D-value

D55 D90 D70 D85

= = = =

3.1 10 2.5 18

Reference

Doyle and Marth, 1975 Bayne and Michener, 1979 Splittstoesser et al., 1989 Splittstoesser and Churey, 1989

Yeasts Saccharomyces cerevisiae ascospores vegetative Kluyveromyces bulgaricus ascospores vegetative cells

Escherichia coli O157:H7 Oenococcus oeni Lactobacillus plantarum Lactobacillus fructivorans

Apple juice

D55 = 106 D55 = 0.90

Splittstoesser et al., 1978

PO4-citrate

D60 = 40 D60 = 0.20

Put and DeJong, 1982

D52 D43 D45 D60 D60

McLellan et al., 1995 Splittstoesser, unpublished data

Bacteria Apple juice Wine Wine Wine Grape juice

= = = = =

12 0.33 0.35 1.7 10

ascospores can be recovered from a variety of fruits, including grapes, strawberries, and cherries, when larger samples of 100 g or more are cultured (King et al., 1969; Splittstoesser et al., 1971, 1974a). Fruit in contact with soil, such as strawberries and pineapple, often are the most heavily contaminated because soil seems to be the normal spore habitat. There is little data to indicate that growth of the molds on processing equipment is a significant source of ascospores. Various processing treatments such as washing, pressing, and filtration will remove many of the ascospores before pasteurization. The author has recovered ascospores from grape pomace after pressing and from diatomaceous earth used in the filtration of cherry juice. Thus, low initial contamination coupled with the removal of spores during processing may explain why more spoilage does not occur. A procedure for detecting ascospores is to heat the sample for 30 to 60 min at 70∞C and then to culture the homogenate or juice on a selective medium such as acidified potato dextrose agar at pH 3.5 or plate count agar containing 100 mg/l chloramphenicol. If the sample is a clear juice, a large volume may be cultured by passing the material through a membrane filter having a pore size that will retain the spores. The purpose of the heat treatment is to destroy heat-sensitive yeasts and molds and to activate dormant ascospores. Although activation greatly improves spore recovery, we have observed that only a fraction may develop into visible colonies even when incubations as long as 30 d have been used (Splittstoesser et al., 1993). Byssochlamys fulva and B. nivea appear to be the most important spoilers of pasteurized fruit products. Their anamorphs (no ascospores produced or observed) are Paecilomyces fulva and P. niveus. The colonies of B. fulva range from yellow-brown to olive-brown, while those of B. nivea are usually white to tan. The colonies of neither species are ever green. The smooth-walled ascospores of B. fulva are somewhat larger than those of B. nivea, while the latter also produces chlamydospores. The asci, which contain eight ascospores, are not formed in an ascocarp (Samson and van Reenen-Hoekstra, 1988).

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Neosartorya fischeri (anamorph: Aspergillus fischeri) is likely the most important food spoilage species within this genus. The asci are produced in white cleistothecia that give the cream to white colonies a granular appearance. The ascospores, which commonly are present in a free state, are readily recognized by the presence of two equatorial crests. In Talaromyces flavus (anamorph: Penicillium dangeardii), both the mycelia and gymnothecia are usually pigmented yellow, although reddish hues may also develop. The elliptical ascospores have spiny walls. Conidia are borne on penicillia-like structures.

12.9.7 COMPOSITION

OF THE

FRUIT PRODUCT

The composition of the food can have a marked effect on thermal resistance. This is illustrated by Lactobacillus fructivorans in which D-values were much lower in wine than in grape juice (Table 12.5). Other factors that have an effect are the concentration of sugars, the pH and types of organic acids, and the presence of certain antimicrobials. Numerous studies have shown that microorganisms become more heat-resistant as the concentration of sugars is increased (Beuchat, 1988a; Corry, 1976; King and Whitehand, 1990; Splittstoesser et al., 1974b). The ascospores of Zygosaccharomyces bailii, for example, gave a D-value of 9.4 min when heated at 58∞C in a medium containing 30% glucose (aw = 0.963), while in 60% glucose (aw = 0.858) the D-value was increased to 59 min (Jermini and Schmidt-Lorenz, 1987a). Similar responses have been observed for the spores of Byssochlamys (Beuchat, 1981a; Splittstoesser et al., 1974b) and Talaromyces flavus (Beuchat, 1988a; King and Whitehand, 1990). Some organic acids seem to sensitize microorganisms to heat while others afford protection. The response appears to differ depending upon the microbial species. Citric acid increased the resistance of Candida lambica, Saccharomyces chevalieri, and Torulopsis magnoliae when these yeasts were heated in orange juice concentrate (Juven et al., 1978). The ascospores of Byssochlamys fulva survived better in 0.05 M, pH 3 solutions of tartaric, malic, and citric acid than they did in distilled water (Splittstoesser et al., 1974b). The ascospores of Aspergillus (Neosartorya), however, were unaffected (Splittstoesser and Splittstoesser, 1977). Fumaric acid, and to a lesser extent, acetic and lactic acids sensitized the ascospores of Byssochlamys fulva to heat (Splittstoesser et al., 1974b). Fumaric acid also decreased the heat resistance of Talaromyces flavus ascospores (Beuchat, 1988b) while citric, lactic, malic, and tartaric acids had no effect (King and Whitehand, 1990). Gillespy (1940) was the first to observe that sulfur dioxide sensitized the ascospores of Byssochlamys fulva to heat. Thus, spores heated 10 min at 85∞C in pH 3 citrate buffer yielded 19% survival in the absence of SO2 and less than 0.5% survival in the presence of 1.0%. King et al. (1969) showed that 90 ppm SO2 reduced D-values of B. fulva and B. nivea about 50% when added to grape juice. The spores of Neosartorya fischeri were also greatly sensitized to heat: in 0.05 M, pH 3.3 tartaric acid solution, D80 = 123 min compared to 6.1 min when the solution contained 100 mg/l SO2 (Splittstoesser and Churey, 1991). Various studies have shown that sorbic and benzoic acids reduce the heat resistance of a variety of fungi. Thus, the presence of 500 and 1000 mg/l sorbic or benzoic acid reduced the heat resistance of conidia of Aspergillus flavus and Penicillium puberulum, ascospores of Byssochlamys nivea, and vegetative cells of Geotrichum candidum (Beuchat, 1981b). Research on 12 yeast genera indicated that benzoic and sorbic acids not only sensitized the yeasts to heat but also had a marked effect on the viable recovery of cells that had been injured by heat. Potassium sorbate was found to be more effective than sodium benzoate (Beuchat, 1981a). Our unpublished studies with Escherichia coli O157:H7 gave the opposite results, in that the average D50 in apple juice containing 600 mg/l benzoic acid was 5.9 min compared to 12 min in the same concentration of sorbic acid. Other evidence that the response to heat and preservative differs with the microorganism has been obtained with the ascospores of Neosartorya fischeri. With this organism, D85-values were about the same when the concentration of sorbic acid ranged from 0 to 1000 mg/l. Concentrations over 60 mg/l

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in the plating medium, however, produced a marked reduction in the recovery of heat-injured spores (Splittstoesser and Churey, 1989).

12.9.8 FROZEN FRUIT Numerous fruits and fruit products are preserved by freezing. These include whole berries and cherries, fruit juice concentrates and purees having a Brix of 45∞ or higher, and various sliced fruits. The products may be packed in retail-size cartons or in large containers for reprocessing, such as 50-gal and 30-lb tins. Dry sugar or syrup is often added to the fruit prior to freezing. The predominant microorganisms are usually yeasts and molds although lactic acid bacteria can be numerous in certain products such as orange juice concentrate. The numbers will vary, depending upon sanitary conditions and the type of process. Juice concentrates and pasteurized products will usually yield low counts because most of the microorganisms will have been killed by the heat treatment. Treating fruit products with sulfur dioxide will also destroy many of the contaminating microorganisms. Often, the counts in the frozen product will reflect the sanitary conditions of belts and other processing equipment. Many of the contaminating microorganisms, especially vegetative cells, are killed by the freezing process. A significant percentage, however, will survive freezing, and, thus, frozen fruit products can yield relatively high counts. During storage in the frozen state, additional death of microorganisms occurs, but the rate is slow. Considerable variation in the yeast and mold counts on fruits or pieces of fruit is a common phenomenon. As the fruit is transported across the various unit operations, there is an opportunity for each piece to contact a different contaminated surface and thus pick up a different load of microorganisms. It is important, therefore, that a representative sample be cultured. It has been my experience, for example, that a sample of frozen apple slices should weight at least 100 g.

12.10 LOW WATER ACTIVITY A variety of fruit products depend upon a low water activity for their preservation (Tilbury, 1980; Troller and Christian, 1978). These include fruit juice concentrates, jams and jellies, and dried fruits. The water activity (aw) of these foods will vary, depending upon the concentration of sugars and other water binding compounds and, in the case of dried fruits, upon the final water content. The fruit product usually will not spoil when the aw is below 0.65, even when high populations of contaminating microorganisms are present (Corry, 1987). At water activities above 0.70, spoilage may occur although at the lower aw values, factors such as numbers and kinds of microorganisms, storage temperature, pH, and the presence of preservatives all play a role. Often, growth of spoilage organisms is slow, and, thus, a desired shelf life can be achieved.

12.10.1 FRUIT JUICE CONCENTRATES Those of 70∞ Brix or higher are usually biologically stable. Water activities for three concentrates are: apricot 40∞ Brix = 0.93, grape 30∞ = 0.962, and orange 72∞ = 0.735 (Chen, 1987). Retail packs of juice concentrates such as orange and apple are generally at 45∞ Brix and have an aw of about 0.90. Because this is not sufficiently low to inhibit many yeasts and molds, these foods are marketed as frozen concentrates.

12.10.2 PRESERVES Jams, jellies, and related products have a soluble solids content of 65%, mainly sugar, and a water activity of 0.75 to 0.86. After cooking the fruit-sugar-pectin mixture to form a gel, the product is put into containers at a temperature of 88∞C or higher. The containers are then tipped to sterilize the cap or the top is sprayed with hot water.

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Most osmotrophic yeasts and molds possess little heat resistance, and, thus, the preserves are commercially sterile until the seal is broken. After opening, growth of chance contaminants is usually prevented by storing the jam or jelly at a refrigeration temperature. Dietetic products have higher water activities and thus permit growth of a wider range of microorganisms. Their spoilage is prevented by including a preservative such as benzoic acid or sorbic acid.

12.10.3 DRIED FRUITS In the U.S., the more widely dried fruits are raisins, prunes, dates, apples, figs, apricots, and peaches. Many are sun-dried to yield a product with less than 25% water. Dehydrated fruits, which often are utilized for reprocessing, have received a heat process that reduces the final water content to 1 to 5%. The aw of most sun-dried fruits, including raisins, is less than 0.65, and thus they do not support the growth of microorganisms. It is a common practice to treat fruits with 1000 to 4000 mg/l sulfur dioxide prior to drying to inhibit browning (Somogyi and Luh, 1986). This treatment also destroys many of the microorganisms that were originally present on the surface of the fruit. High-moisture dried prunes, which typically contain 35% water and have an aw of 0.94, would be classified as an intermediate moisture food. Growth of yeasts and molds in these foods is prevented by the addition of up to 0.1% sodium benzoate or potassium sorbate (Schade et al., 1973).

12.10.4 OSMOTOLERANT FUNGI Yeasts and molds are usually responsible for the spoilage of low aw fruit products. Although some bacteria can grow at high osmotic pressures, they are not acid-tolerant and thus are more likely to spoil more neutral foods such as salted fish. Zygosaccharomyces rouxii is perhaps the most osmotolerant yeast. Some strains can grow at an aw as low as 0.62 (Corry, 1987). It is not a true osmophile, in that most strains grow best at water activities above 0.90. Other species of Zygosaccharomyces, Torulopsis, and Hansenula are also osmotrophic (Tilbury, 1980). Some of the more osmotolerant molds are Xeromyces bisporus (minimum aw = 0.605), Chrysoporium xerophilum (aw = 0.686), and members of the Aspergillus glaucus group (aw = 0.71–0.77). Dried prunes were a source of some of these isolates (Pitt and Christian, 1968). Eurotium herbariorum (teleomorph: Aspergillus glaucus) is a potentially important mold in fruit preservation because, in addition to its osmotic tolerance, it produces ascospores that have some degree of heat resistance (Stadler, 1985). A culture isolated from spoiled grape jelly yielded optimal growth in media containing 40 to 60% sucrose, and its ascospores gave a D-value of 5.2 min when heated at 70∞C in 65∞ Brix Concord grape juice (Splittstoesser et al., 1989). Because the ascospores would have been killed by the hot-fill process, it was believed that contaminated lids were the cause of the spoilage outbreak. At the time of the trouble, the hot water sprays used to sterilize the lids were not functioning properly.

12.11 REFRIGERATION Although many species of yeasts and molds will grow at low temperatures, refrigerated storage is a practical means for extending the shelf life of fruit juice beverages, melon sections, and mixtures of fruit that may include citrus sections, berries, pineapple, and pieces of melon. Low temperatures lengthen the microbial lag phase, the dormant period before cell multiplication is initiated, and also retard the growth rate. Generally, the length of the lag phase is inversely related to the size of the microbial population. Refrigerated apple cider that has an initial yeast count of 104 to 105/ml may have a shelf life of only several weeks, compared to several months for a previously pasteurized, low-count orange juice that had been repackaged under very clean

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conditions. Thus, refrigeration is most successful when the initial population of spoilage organisms is low. The shelf life of refrigerated juices can be extended by the addition of preservatives, usually benzoic or sorbic acids. This will be discussed in the next section.

12.12 ANTIMICROBIALS Various compounds are used to extend the shelf life of fruit products. As will be discussed in this section, many are used in conjunction with other preservation procedures.

12.12.1 SULFUR DIOXIDE One of our earliest preservatives, SO2, is used in fruit products to prevent enzymatic and nonenzymatic browning reactions, to destroy microorganisms, and as a bleach for specialties such as maraschino cherries (Joslyn and Braverman, 1954; Roberts and McWeeney, 1972; Taylor et al., 1986). It is its undissociated, molecular form that is germicidal. With a pK of 1.81, this means that very little is active in fruit products having a pH above 3.5. Yeasts are believed to be more resistant to SO2 than most species of molds and bacteria. Some reports indicate that certain yeasts can grow in the presence of 1000 mg/l sulfur dioxide although it has not always been clear in the publications as to the concentration of the free SO2 and the amount that was in the undissociated form. Carr et al. (1976) observed that as little as 1.5 mg/l undissociated SO2 inhibited the growth of lactic acid bacteria in fermented cider. High levels of SO2 have been used for the preservation of grape juice at ambient temperature (Potter, 1979). The “brimstone process” involves the addition of 1200 to 2000 mg/l SO2. When the juice is to be fermented following storage, most of the SO2 is removed by flash heating and spraying with nitrogen. The effluent SO2 is trapped in a 10% milk of lime solution. It is claimed that the process yields a high-quality grape juice that can be fermented into wine. Sweet cherries and other fruits are preserved in solutions containing 7,500 to 15,000 mg/l SO2 (Payne et al., 1969). When they are to be made into maraschino cherries, calcium salts are added to promote firmness and to prevent cracking. There is a synergism between SO2 and low-temperature storage of grape juice. Studies on 135 juices showed that as little as 200 mg/l SO2 prevented spoilage of juice stored at 5∞C, providing the initial yeast population did not exceed 106/ml (Splittstoesser and Mattick, 1981). At the end of the 30-week storage period, the juices that had not fermented were completely free of viable yeasts.

12.12.2 BENZOIC

AND

SORBIC ACIDS

These compounds are widely used for the preservation of a variety of fruit products, including juices and juice beverages, cider, fresh fruit cocktails, dried fruit, jams and jellies, and wines. The two preservatives are discussed together because their mode of action appears to be similar, and thus they can substitute for each other in food systems. It is believed that their inhibitory mechanism involves cell membranes and the transport of substrates. Depletion of ATP appears to be responsible for the inhibition of Zygosaccharomyces cerevisiae (Warth, 1991). Both compounds are lipophilic acids with the undissociated molecule being the active form. At pH 3.5, a common value for fruit products, 83% of the benzoic acid is undissociated, compared to 95% undissociated sorbic acid. At pH 5.5, only 4.7% of the benzoic acid is active, compared to 15% of the sorbic acid. These differences are not always appreciated when the effectiveness of the preservatives is compared. In addition to pH, other properties of a food may influence the effectiveness of these preservatives. The percentage of ethanol in a table wine affects the concentration of sorbic acid needed to prevent refermentation by yeasts. A wine containing 10% ethanol required over 250 mg/l sorbic

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acid for stability, compared to only 100 mg/l when the ethanol content was 12% (Splittstoesser et al., 1978). In general, fungi are more sensitive to these preservations than are bacteria. Bell et al. (1959) observed that 66 species of fungi were inhibited by 0.1% sorbate in a pH 4.5 broth, while 20 out of 39 lactic acid bacteria grew under these conditions. Gluconobacter oxydans is another resistant bacterium. Some strains can grow in the presence of 1000 mg/l sorbic acid in a medium of pH 3.5 to 3.8 (Eyles and Warth, 1989; Splittstoesser and Churey, 1992). Some of the most resistant yeasts (strains that can grow in the presence of over 1000 mg/l sorbic acid) are Zygosaccharomyces bailli, Candida parapsilosis, and Pichia membranaefaciens (Deak et al., 1992). Yeasts and gluconobacters that possess resistance to sorbic acid are not uncommon on grapes (Splittstoesser and Churey, 1992). About 50% of 111 grape samples yielded counts that ranged from 10 to 104/g. Although there is little published information, it is likely that these microorganisms are also common contaminants of other fruits. Some of the microorganisms that spoil fruit products can degrade these preservatives. Lactic acid bacteria reduce sorbic acid to form 2-ethoxyhexa-3,5 diene, a compound that imports a geranium-like off-odor to wines (Crowell and Guymon, 1975; Edinger and Splittstoesser, 1986). Certain Penicillium species degrade potassium sorbate (Marth et al., 1966), while Aspergillus niger metabolizes benzoate (Shailubhai et al., 1982).

12.12.3 DIMETHYL DICARBONATE

AND

DIETHYL DICARBONATE

These compounds have been proposed as sterilants for wines, fruit juices, and related fruit products (Ough, 1993); however, because diethyl dicarbonate reacts with ammonia to form ethyl carbamate, a carcinogen, its use in foods was banned in 1972. Currently, 200 mg/l dimethyl dicarbonate (DMDC) can be added to wines and fruit juices to prevent their spoilage by bacteria and yeasts. The compound has a potential for use in fruit juices and other beverages. Vegetative cells and mold conidia are very sensitive to the dicarbonates. On the other hand, heat-resistant structures such as the ascospores of Byssochlamys fulva are quite resistant and, thus, are likely to survive an exposure to 200 mg/l DMDC (Splittstoesser and Wilkinson, 1973; Van der Riet et al., 1989). When added to a beverage, the dicarbonates rapidly hydrolyze to the corresponding alcohol and carbon dioxide. The challenge is to destroy the contaminating microorganisms before hydrolysis is completed. Effective metering and mixing equipment is required, and the numbers of contaminating microorganisms must be low.

REFERENCES Anon. 1975. Salmonella typhimurium outbreak traced to commercial apple cider. Morbidity and Mortality Weekly Report, 24: 87. Anon. 1979. Salmonella oranienburg gastroenteritis associated with consumption of precut watermelons. Morbidity and Mortality Weekly Report, 28: 522–523. Barnett, J.A., Payne, R.W., and Yarrow, D. 1983. Yeasts: Characteristics and Identification. Cambridge University Press, Cambridge, U.K. Bayne, H.G. and Michener, H.D. 1979. Heat resistance of Byssochlamys ascospores. Appl. Environ. Microbiol., 37: 449–453. Beech, F.W., Davenport, R.R., Goswell, R.W., and Burnett, J.K. 1968. Two simplified schemes for identifying yeast cultures. In Identification Methods for Microbiologists, Part B, B.M. Gibbs and D.A. Shapton (Eds.), Academic press, London, pp. 151–175. Bell, T.A., Etchells, J.L., and Borg, A.F. 1959. Influence of sorbic acid on the growth of certain species of bacteria, yeasts, and filamentous fungi. J. Bacteriol., 77: 573–580.

Microbiology of Fruit Products

279

Berry, J.M. 1979. Yeast problems in the food and beverage industry. In Food Mycology, M.E. Rhodes (Ed.), G.K. Hall, Boston, pp. 82–90. Besser, R.E., Lett, S.M., Weber, J.T., Doyle, M.P., Barrett, T.J., Wells, J.G., and Griffin, P.M. 1993. An outbreak of diarrhea and hemolytic uremic syndrome from Escherichia coli O157:H7 in fresh-pressed apple cider. JAMA, 269: 2217–2220. Beuchat, L.R. 1981a. Combined effects of solutes and food preservatives on rates of inactivation of and colony formation by heated spores and vegetative cells of molds. Appl. Environ. Microbiol., 41: 472–477. Beuchat, L.R. 1981b. Synergistic effects of potassium sorbate and sodium benzoate on thermal inactivation of yeasts. J. Food Sci., 46: 771–777. Beuchat, L.R. 1988a. Thermal tolerance of Talaromyces flavus ascospores as affected by growth medium and temperature, age, and sugar content in the inactivation medium. Trans. Br. Mycol. Soc., 90: 359–364. Beuchat, L.R. 1988b. Influence of organic acids on heat resistance characteristics of Talaromyces flavus ascospores. Intl. J. Food Microbiol., 6: 97–105. Biely, P. and Slavikova, E. 1990. Distribution of pectin-degrading activities among yeasts. Zentralblatt f. Mikrobiologie, 145: 345–351. Blackwood, M. 1969. Etude des bacteries acetiques isolees de raisins et de vins. I. Identification de souches isolees. Connaiss. Vigne. Vin, 3: 227–241. Blocher, J.C. and Busta, F.F. 1983. Bacterial spore resistance to acid. Food Technol., 37: 87–99. Carr, J.G. 1958. Lactic acid bacteria as spoilage organisms of fruit juice products. J. Appl. Bacteriol., 21: 267–271. Carr, J.G. 1968. Methods for identifying acetic acid bacteria. In Identification Methods for Microbiologists. B.M. Gibbs and D.A. Shapton (Eds.), Academic Press, London, pp. 1–8. Carr, J.G. 1970. Tetrad-forming cocci in ciders. J. Appl. Bacteriol., 33: 371–379. Carr, J.G., Davies, P.A., and Sparks, A.H. 1976. The toxicity of sulfur dioxide towards certain lactic acid bacteria from fermented apple juice. J. Appl. Bacteriol., 40: 201–212. Carr, J.G. and Passmore, S.M. 1979. Methods for identifying acetic acid bacteria. In Identification Methods for Microbiologists, 2nd ed., F.A. Skinner and D.W. Lovelock (Eds.), Society for Applied Bacteriology, Tech Series 14, Academic press, London, pp. 33–47. Cerny, G., Hennlich, W., and Poralla, K. 1984. Spoilage of fruit juice by bacilli: isolation and characterization of the spoilage organism. Z. Lebensm. Unters. For., 179: 224–227. Chakrabarti, D.K. and Ghosal, S. 1986. Occurrence of free and conjugated 12, 13-epoxytrichothecenes and zearalenone in banana fruits infected with Fusarium moniliforme. Appl. Environ. Microbiol., 51: 217–219. Chen, C.S. 1987. Calculation of water activity and activity coefficient of sugar solutions and some liquid foods. Lebensm. Wiss. Technol., 20: 64–67. Cichowicz, S.M. and Eisenberg, W.V. 1974. Collaborative study of the determination of Geotrichum mold in selected canned fruits and vegetables. J. Assoc. Offic. Anal. Chem., 57: 957–960. Corry, J.E.L. 1976. The effects of sugars and polyols on the heat resistance and morphology of osmophilic yeasts. J. Appl. Bacteriol., 40: 269–276. Corry, J.E.L. 1987. Relationships of water activity to fungal growth. In Food and Beverage Mycology, 2nd ed., L.R. Beuchat (Ed.), Van Nostrand Reinhold, New York, pp. 51–99. Crowell, E.A. and Guymon, J.F. 1975. Wine constituents arising from sorbic acid addition, and identification of 2-ethoxyhexa-3, 5-diene as source of geranium-like off-odor. Am. J. Enol. Vitic., 26: 97–102. Daly, N.M. 1982. The microbiology of wine corks. B.B. thesis, University New South Wales, Kensington, NSW. Darland, G. and Brock, T.D. 1971. Bacillus acidocaldarius sp. Nov., an acidophilic, thermophilic sporeforming bacterium. J. Gen Microbiol., 67: 9–15. Davis, N.D. and Diener, U.L. 1987. Mycotoxins. In Food and Beverage Mycology, 2nd ed., L.R. Beuchat (Ed.), Van Nostrand Reinhold, New York, pp. 517–570. Deak, T. and Beuchat, L.R. 1987. Identification of foodborne yeasts. J. Food Prot., 50: 243–264. Deak, T. and Beuchat, L.R. 1993. Comparison of the SIM, API 20C and ID 32C systems for identification of yeasts isolated from fruit juice concentrates and beverages. J. Food Prot., 56: 585–592. Deak, T., Reichart, O., Szakmar, K., and Peter, G. 1992. Spoilage yeasts of unusual sorbate resistance. In Modern Methods in Food Mycology, R.A. Samson, A.D. Hocking, J.I. Pitt, and A.D. King (Eds.), Elsevier, Amsterdam, pp. 55–59.

280

Processing Fruits: Science and Technology, Second Edition

DeLey, J. and Swings, J. 1984. Genus II. Gluconobacter asai 1935, 689 emend. mut. Char. Alsai, lizuka, and Komegata 1964. In Bergeys Manual of Systematic Bacteriology, Vol. 1 (9th Ed.), J.G. Holt (Ed.), Williams and Wilkins, Baltimore, MD, pp. 275–284. DeLey, J., Seings, J., and Golssele, F. 1984. Genus I. Acetobacter Beijerinck 1898. In Bergeys Manual of Systematic Bacteriology, Vol. 1 (9th Ed.), J.G. Holt (Ed.), Williams and Wilkins, Baltimore, MD, pp. 268–274. deMan, J.C., Rogosa, M., and Sharpe, M.E. 1960. A medium for the cultivation of lactobacilli. J. Appl. Bacteriol., 23: 130–135. Doyle, M.P. and Marth, E.H. 1975. Thermal inactivation of conidia from Aspergillus flavus and Aspergillus parasiticus. J. Milk Food Technol., 38: 678–682. Drysdale, G.S. and Fleet, G.H. 1988. Acetic acid bacteria in winemaking: A review. Am. J. Enol. Vitic., 39: 143–154. Edinger, W.D. and Splittstoesser, D.F. 1986. Production by lactic acid bacteria of sorbic alcohol, the precursor of the geranium odor compound. Am. J. Enol. Vitic., 37: 34–38. Escartin, E.F., Ayala, A.C., and Lozano, J.S. 1989. Survival and growth of Salmonella and Shigella on sliced fresh fruit. J. Food Prot., 52: 471–472. Eyles, M.J. and Warth, A.D. 1989. The response of Gluconobacter oxydans to sorbic and benzoic acids. J. Food Microbiol., 8: 335–342. Food and Drug Administration. 2001. Hazard Analysis and Critical Control Point (HAACP); Procedures for the Safe and Sanitary Processing and Importing of Juice. Final Rule. Fed. Register. 66: 6137–6202. Fornachon, J.C., Douglas, H.C., and Vaughn, R.H. 1949. Lactobacillus trichodes nov. spec., a bacterium causing spoilage in appetizer and desert wines. Hilgardia, 19: 129–132. Frisvad, J.C. and Samson, R.A. 1991. Filamentous fungi in foods and feeds: Ecology, spoilage, and mycotoxin production. In Handbook of Applied Mycology, Foods and Feeds. Vol. 3., D.K. Arora, K.G. Mukerji and E.H. Marth (Eds.), Marcel Dekker, New York, pp. 31–68. Fuentes, F.A., Hazen, T.C., Lopez-Torres, A.J., and Rechani, P. 1985. Klebsiella pneumoniae in orange juice concentrate. Appl. Environ. Microbiol., 49: 1527–1529. Genth, H. 1964. On the action of diethylpyrocarbonate on microorganisms. In Microbial Inhibitors in Food, N. Molin (Ed.), Almqvist and Wiksell, Stockholm, pp. 77–85. Gillespy, T.G. 1940. Studies on the mould Byssochlamys fulva. III. Annual Report of the Fruit and Vegetable Preservation Research Station, Campden, U.K., pp. 54–61. Gini, B. and Vaughn, R.H. 1962. Characteristics of some bacteria associated with the spoilage of California dessert wines. Am. J. Enol. Vitic., 13: 20–31. Golden, D.A., Rhodehamel, E.J., and Kautter, D.A. 1993. Growth of Salmonella spp. In cantaloupe, watermelon, and honeydew melons. J. Food Prot., 56: 194–196. Goverd, K.A., Beech, F.W., Hobbs, E.P., and Shannon, R. 1979. The occurrence and survival of coliforms and salmonellas in apple juice and cider. J. Appl. Bacteriol., 46: 521–530. Haglund, J.R. 1993. Personal communication. Hahn, S.S. and Appleman, M.D. 1952. Microbiology of frozen orange concentrate. I. Survival of enteric organisms in frozen orange concentrate. Food Technol., 6: 156–158. Harwig, J., Chen, Y.-K., Kennedy, B.P.C., and Scott, P.M. 1973. Occurrence of patulin and patulin producing strains of Penicillium expansum in natural rots of apple in Canada. Can. Inst. Food Sci. Technol. J., 6: 22–25. Hatcher, W.S., Weihe, J.L., Splittstoesser, D.F., Hill, E.C., and Parish, M.E. 1992. Fruit beverages. In Compendium of Methods for the Microbiological Examination of Foods, C. Vanderzant and D.F. Splittstoesser (Eds.), American Public Health Association Washington, D.C., pp. 953–960. Hocking, A.D. and Pitt, J.I. 1980. Dichloran-glycerol medium for enumeration of xerophilic fungi from low moisture foods. Appl. Environ. Microbiol., 39: 488–492. Howard, B.J. 1911. Tomato Ketchup under the Microscope with Practical Suggestions to Insure a Cleanly Product. U.S. Dept. of Agriculture, Bureau of Chemistry, Circular 68. Jermini, M.F.G. and Schmidt-Lorenz, W. 1987a. Heat resistance of vegetative cells and asci of two Zygosaccharomyces yeasts in broths at different water activity values. J. Food Prot., 50: 835–841. Jermini, M.F.G. and Schmidt-Lorenz, W. 1987b. Detection, isolation, and identification of osmotolerant yeasts from high sugar products. J. Food Prot., 50: 468–472.

Microbiology of Fruit Products

281

Joslyn, M.A. and Braverman, J.B.S. 1954. The chemistry and technology of the pretreatment and preservation of fruit and vegetable products with sulfur dioxide and sulfites. Advan. Food Res., 5: 97–160. Joyeux, A., Lafon-Lafourcade, S., and Ribereau-Gayon, P. 1984. Evolution of acetic acid bacteria during fermentation and storage of wine. Appl. Environ. Microbiol., 48: 153–156. Juven, B.J., Kanner, J., and Weisslowicz, H. 1978. Influence of orange juice composition on the thermal resistance of spoilage yeast. J. Food Sci., 43: 1074–1080. Juven, B.J. and Shomer, I. 1985. Spoilage of soft drinks caused by bacterial flocculation. J. Food Prot., 48: 52–53. Kamphuis, H.J. and Notermans, S. 1992. Development of a technique for the immunological detection of fungi. In Modern Methods in Food Mycology, R.A. Samson, A.D. Hocking, J.I. Pitt, and A.D. King (Eds.), Elsevier, Amsterdam, pp. 197–203. King, A.D. 1992. Methodology for routine mycological examination of food — A collaborative study. In Modern Methods of Food Mycology, R.A. Samson, A.D. Hocking, J.I. Pitt, and A.D. King (Eds.), Elsevier, Amsterdam, pp. 11–20. King, A.D., Hocking, A.D., and Pitt, J.I. 1979. Dichloran-rose bengal medium for enumeration and isolation of molds from foods. Appl. Environ. Microbiol., 37: 959–964. King, A.D., Michener, H.D., and Ito, K.A. 1969. Control of Byssochlamys and related heat-resistant fungi in grape products. Appl. Microbiol., 18: 166–173. King, A.D. and Whitehand, L.C. 1990. Alteration of Talaromyces flavus heat resistance by growth conditions and heating medium composition. J. Food Sci., 55: 830–832. Koburger, J. A. 1970. Fungi in foods. I. Effect of inhibitor and incubation temperature on enumeration. J. Milk Food Technol., 33: 433–434. Krogh, P. 1989. The role of mycotoxins in disease of animals and man. In J. Appl. Bacteriol. Symp. Suppl. M. O. Moss, B. Jarvis, and F.A. Skinner (Eds.), Blackwell Science London, 18: 99S–104S. Lee, T.H., Simpson, R.F., Van der Peer, J.M., Fleet, G.H., Davis, C.R., Daly, N.M., and Yap, A.S.J. 1983. Microbiology of wine corks. In Advan. Viticulture and Oenology for Economic Gain, T.H. Lee and T.C. Somers (eds.), Proceedings of the 5th Australian Wine Industry Technical Conference, Perth, pp. 435–450. Liewen, M.B. and Bullerman, L.B. 1992. Toxigenic fungi and fungal toxins. In Compendium of Methods for the Microbiological Examination of Foods, C. Vanderzant and D.F. Splittstoesser (Eds.), American Public Health Association, Washington, D.C., pp. 811–819. Lin, H.H. and Cousin, M.A. 1985. Detection of mold in processed foods by high performance liquid chromatography. J. Food Prot., 48: 671–678. Lin, H.H., Lister, R.M., and Cousin, M.A. 1986. Enzymes-linked immunosorbent assay for detection of mold in tomato puree. J. Food Sci., 51: 180–192. Little, K.J. and LaRocco, K.A. 1986. ATP screening method for presumptive detection of microbiologically contaminated carbonated beverages. J. Food Sci., 51: 474–476. Lodder, J. 1970. The Yeasts. North-Holland Publishing, Amsterdam. Lopez, A. 1987. A Complete Course in Canning. III. Processing Procedures for Canned Food Products. The Canning Trade Baltimore, MD. Lovett, J. and Peeler, J.T. 1973. Effect of pH on the thermal destruction kinetics of patulin in aqueous solution. J. Food Sci., 38: 1094–1095. Luthi, H. 1959. Microorganisms in noncitrus juices. Advan. Food Res., 9: 221–284. Marshall, C.R. and Walkley, V.T. 1951. Some aspects of microbiology applicable to commercial apple juice production. I. Distribution of microorganisms on fruit. Food Res., 16: 448–456. Marth, E.H., Capp, C.M., Hazenzahl, L., Jackson, H.W., and Hussong, R.V. 1966. Degradation of potassium sorbate by Penicillium species. J. Dairy Sci., 49: 1197–1205. Mislivec, P.B., Beuchat, L.R., and Cousin, M.A. 1992. Yeasts and molds. In Compendium of Methods for the Microbiological Examination of Foods, C. Vanderzant and D.F. Splittstoesser (Eds.), American Public Health Association, Washington, D.C., pp. 239–249. Moyer, M.C., Splittstoesser, D.F., and Bourne, M.C. 1969. The quality of mechanically harvested grapes. In Fruit and Vegetable Harvest Mechanization, Report 16, B.F. Cargill and G.E. Rossmiller (Eds.), Michigan State University, East Lansing, pp. 587–590. Mundt, J.O., Graham, W.F., and McCarty, I.E. 1967. Spherical lactic acid-producing bacteria of Southerngrown raw and processed vegetables. Appl. Microbiol., 15: 1303–1308.

282

Processing Fruits: Science and Technology, Second Edition

Notermans, S., Heuvelman, C.J., Van Egmond, H.P., Paulsch, W.E., and Besling, J.R. 1986. Detection of mold in food by enzyme-lined immunosorbent assay. J. Food Prot., 49: 786–791. Ogawa, J.M., Rumsey, J., Manji, B.T., Tate, G., Toyoda, J., Bose, E., and Dugger, L. 1974. Implications and chemical testing of two rhizopus fungi in softening of canned apricots. Calif. Agric., 28(7): 6–7. Ough, C.S. 1993. Dimethyl dicarbonate and diethyl dicarbonate. In Antimicrobials in Foods, P.M. Davidson and A.L. Branen (Eds.), Marcel Dekker, New York, pp. 343–368. Parish, M. and Higgins, D. 1988. Isolation and identification of lactic bacteria from samples of citrus molasses and unpasteurized orange juice. J. Food Sci., 53: 645–646. Passmore, S.M. and Carr, J.G. 1975. The ecology of the acetic acid bacteria with particular reference to cider manufacture. J. Appl. Bacteriol., 38: 151–158. Payne, C.H., Beavers, D.V., and Cain, R.F. 1969. The Chemical and Preservative Properties of Sulfur Dioxide Solution for Brining Fruit. Circular 629, Oregon State University, Corvallis, OR. Pfaff, H.J., Miller, M.W., and Mrak, E.M. 1978. The Life of Yeasts. 2nd ed., Harvard University Press, Cambridge, MA. Pitt, J.I. and Christian, J.H.B. 1968. Water relations of xerophilic fungi isolated from prunes, Appl. Microbiol., 16: 1853–1858. Pitt, J.I. and Hocking, A.D. 1985. Fungi and Food Spoilage. Academic Press, New York. Potter, R. 1979. The brimstone process of disulphiting stored grape juice. Food Technol. Aust., (March): 113–115. Put, H.M.C. and DeJong, J. 1982. Heat resistance studies of yeasts: vegetative cells versus ascospores: Erythromycin inhibition of sporulation in Kluyveromyces and Saccharomyces species. J. Appl. Bacteriol., 53: 73–79. Rankine, B.C. 1968. The importance of yeasts in determining the composition and quality of wines. Vitis, 7(1): 22–49. Ravji, R.G., Rodriguez, S.B., and Thornton, R.J. 1988. Glycerol production by four common grape molds. Am. J. Enol. Vitic., 39: 77–82. Roberts, A.C. and McWeeny, D.J. 1972. The uses of sulphur dioxide in the food industry: a review. J. Food Technol., 7: 221–238. Samson, R.A. and van Reenen-Hoekstra, E.S. 1988. Introduction to Food-Borne Fungi. Centraal Bureau voor Schimmel Cultures, Baarn, the Netherlands, 299. Schade, J.E., Stafford, A.E., and King, A.D. 1973. Preservation of high moisture dried prunes with sodium benzoate instead of potassium sorbate. J. Sci. Food Agric., 24: 905–911. Schaertel, B.J., Tsang, N., and Firstenberg-Eden, R. 1987. Impedimetric detection of yeast and mold. Food Microbiol., 4: 155–163. Scott, V.N. and Bernard, D.T. 1987. Heat resistance of Talaromyces flavus and Neosartorya fischeri isolated from commercial fruit juices. J. Food Prot., 50: 18–20. Shailubhai, K., Rao, N.N., and Modi, V.V. 1982. Degradation of benzoate and salicylate by Aspergillus niger. Indian J. Exp. Biol., 20: 166–171. Sharman, M., Patey, A.L., Bloomfield, D.A., and Gilbert, J. 1991. Surveillance and control of aflatoxin contamination of dried figs and fig paste imported into the United Kingdom. Food Addit. Contam., 8: 299–304. Somogyi, L.P. and Luh, B.S. 1986. Dehydration of fruits. In Commercial Fruit Processing, J.G. Woodruff and B.S. Luh (Eds.), AVI Publishing, Westport, CT, pp. 353–405. Splittstoesser, D.F. 1992. Direct microscopic count. In Compendium of Methods for the Microbiological Examination of Foods, C. Vanderzant and D.F. Splittstoesser (Eds.), American Public Health Association, Washington, D.C., pp. 97–104. Splittstoesser, D.F. and Churey, J.J. 1989. Effect of low concentrations of sorbic acid on the heat resistance and viable recovery of Neosartorya fischeri ascospores. J. Food Prot., 52: 821–822. Splittstoesser, D.F. and Churey, J.J. 1991. Reduction of heat resistance of Neosartorya fischeri ascospores by sulfur dioxide. J. Food Sci., 56: 876–877. Splittstoesser, D.F. and Churey, J.J. 1992. The incidence of sorbic acid-resistant gluconobacters and yeasts on grapes grown in New York State. Am. J. Enol. Vitic., 43 :290–293. Splittstoesser, D.F., McLellan M.R., and Churey, J.J. 1996. Heat resistance of Escherichia coli O157:H7 in apple juice. J. Food Prot., 59: 226–229.

Microbiology of Fruit Products

283

Splittstoesser, D.F., Churey, J.J., and Lee, C.Y. 1994. Growth characteristics of aciduric sporeforming bacilli isolated from fruit juices. J. Food Prot., 57: 1080. Splittstoesser, D.F., Downing, D.L., Roger, N.F., and Murdock, D.I. 1974a. Influence of the harvest method on contamination of fruit by Byssochlamys ascospores. J. Milk Food Technol., 37: 445–447. Splittstoesser, D.F., Einset, A., Wilkison, M., and Prezoise, J. 1974b. Effect of food ingredients on the heat resistance of Byssochlamys fulva ascospores. Proceedings of the 4th International Congress on Food Science and Technology, 3: 79–85. Splittstoesser, D.F. and Gadjo, I. 1966. The groups of microorganisms composing the total count population in frozen vegetables. J. Food Sci., 31: 234–239. Splittstoesser, D.F., Groll, M., Downing, D.L., and Kaminski, J. 1977. Viable counts versus the incidence of machinery mold (Geotrichum) on processed fruits and vegetables. J. Food Prot., 40: 402–405. Splittstoesser, D.F., Kuss, F.R., Harrison, W., and Prest, D.B. 1971. Incidence of heat resistant molds in eastern orchards and vineyards. Appl. Microbiol., 21: 335–337. Splittstoesser, D.F., Lammers, J.M., Downing, D.L., and Churey, J.J. 1989. Heat resistance of Eurotium herbariorum, a xerophilic mold. J. Food Sci., 54: 683–685. Splittstoesser, D.F. and Mattick, L.R. 1981. The storage life of refrigerated grape juice containing various levels of sulfur dioxide. Am. J. Enol. Vitic., 32: 171–173. Splittstoesser, D.F., Nielsen, P.V., and Churey, J.J. 1993. Detection of viable ascospores of Neosartorya. J. Food Prot., 56: 599–603. Splittstoesser, D.F., Queale, D.T., and Mattick, L.R. 1978. Growth of Saccharomyces naprous var. bisporus, a yeast resistant to sorbic acid. Am. J. Enol. Vitic., 29: 272–276. Splittstoesser. D.F. and Splittstoesser, C.M. 1977. Ascospores of Byssochlamys fulva compared with those of a heat resistant Aspergillus. J. Food Sci., 42: 685–688. Splittstoesser, D.F. and Wilkinson, M. 1973. Some factors affecting the activity of diethylpyrocarbonate as a sterilant. Appl. Microbiol., 25: 853–857. Stadler, D. 1985. Hitzresistenz von Schimmelpilz-Fructifikationsorganen. Doctoral thesis. Eidgenossischen Technischen Hochschule, Zurich. Stamer, J.R., Albury, M.N., and Pederson, C.S. 1964. Substitution of Manganese for tomato juice in the cultivation of lactic acid bacteria. Appl. Microbiol., 165–168. Swanson, K.M.J., Leasor, S.B., and Downing, D.L. 1985. Aciduric and heat resistant microorganisms in apple juice and cider processing operations. J. Food Sci., 50: 334–339. Taylor, S.L., Higley, N.A., and Bush, R.K. 1986. Sulfites in foods: Uses, analytical methods, residues, fate, exposure assessment, metabolism, toxicity, and hypersensitivity, Advan. Food Res., 30: 2–76. Tilbury, S.H. 1980. Xerotolerant (osmophilic) yeasts. In Biology and Activities of Yeasts, F.A. Skinner, S.M. Passmore, and R.R. Davenport (Eds.), Academic Press, London, pp. 153–179. Townsend, C.T. 1939. Sporeforming anaerobes causing spoilage in acid canned foods. Food Res., 4: 231–237. Troller, J.A. and Christian, J.H.B. 1978. Water Activity and Food. Academic Press, New York. Van der Riet, W.V., Botha, A., and Pinches, S.E. 1989. The effect of dimethylcarbonate on vegetative growth and ascospores of Byssochlamys fulva suspended in apple juice and strawberry nectar. Int. J. Food Microbiol., 8: 95–102. Vaughn, R.H., Douglas, C., and Fornachon, J.C.M. 1949. The taxonomy of Lactobacillus hilgardii and related heterofermentative lactobacilli. Hilgardia, 19: 133–139. Vaughn, R.H., Kreulevitch, I.M., and Mercer, W.A. 1952. Spoilage of canned foods caused by the Bacillus macerans-polymyxa group of bacteria. Food Res., 17: 560–170. Vaughn, R.H. and Stadtman, T.C. 1946. A note on pH tolerance of Aerobacter aerogenes genes and Aerobacillus macerans as related to natural ecology and decomposition of acid food products. J. Bacteriol., 51: 263. Walls, I. 1994. Sporeformers that can grow in acid and acidified foods. Report of Scientific and Technical Regulatory Activities, NFPA, Washington, D.C. Warth, A.D. 1991. Mechanism of action of benzoic acid on Zygosaccharomyces bacilii. Effects of glycolytic metabolite levels, energy production, and intracellular pH. Appl. Environ. Microbiol., 57: 3410–3414. Wibowo, D., Eschenbruch, R., David, C.R., Fleet, G.H., and Lee, T.H. 1985. Occurrence and growth of lactic acid bacteria in wine: a review. Am. J. Enol. Vitic., 36: 302–313. Williams, A.P. 1989. Methodological developments in food mycology. J. Appl. Bacteriol., Suppl., 18: 61s–67s.

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Wisotzkey, J.D., Jurtshuk, P., Fox, G.E., Deinhard, G., and Poralla, K. 1992. Comparative sequence analyses on the 16S rRNA (rDNA) of Bacillus acidocaldarius, Bacillus acidoterrestris and Bacillus cycloheptanicus and proposal for creation of a new genus, Alicyclobacillus gen. nov. Int. J. Syst. Bacteriol., 42: 263. Wisse, C.A. and Parish, M.E. 1998. Isolation and enumeration of thermo-acidophilic rod shaped bacteria from citrus processing environments. Dairy Food Environ. Sanit., 18: 504. Wolford, E.R. 1956. Certain aspects of the microbiology of frozen concentrate orange juice. Am. J. Public Health, 46: 708–715. Yamazaki, K., Teduka, H., and Shinano, H. 1996. Isolation and identification of Alicyclobacillus acidoterrestris from acidic beverages. Biosci. Biotech. Biochem., 60: 543. Zhao, T., Doyle, M.P., and Besser, R.E. 1993. Fate of enterohemorrhagic Escherichia coli O157:H7 in apple juice with and without preservatives. Appl. Environ. Microbiol., 59: 2526–2530.

Food Additives in Fruit 13 Direct Processing Laszlo P. Somogyi CONTENTS 13.1 13.2 13.3 13.4

13.5

13.6 13.7 13.8 13.9

13.10

13.11

13.12

13.13

Introduction ........................................................................................................................286 13.1.1 Definition and Functions .....................................................................................286 Food Additive Categories...................................................................................................287 Industry Segments..............................................................................................................287 Functions of Direct Food Additives ..................................................................................288 13.4.1 Preservation..........................................................................................................288 13.4.2 Processing ............................................................................................................288 13.4.3 Appeal and Convenience .....................................................................................288 13.4.4 Nutrition...............................................................................................................289 Government Regulations....................................................................................................289 13.5.1 U.S. ......................................................................................................................289 13.5.2 Europe ..................................................................................................................290 13.5.3 Japan ....................................................................................................................291 Trends, Issues .....................................................................................................................291 Food Additive Industry Structure ......................................................................................292 Utilization of Food Additives ............................................................................................292 Sweeteners..........................................................................................................................293 13.9.1 Functions..............................................................................................................293 13.9.2 Products, Applications, and Regulatory Status ...................................................294 13.9.3 High-Intensity Sweeteners...................................................................................294 13.9.4 Polyols .................................................................................................................298 13.9.5 Trends...................................................................................................................300 Acidulants...........................................................................................................................300 13.10.1 Functions..............................................................................................................300 13.10.2 Products and Applications ...................................................................................301 13.10.3 Regulatory Status.................................................................................................304 Thickeners and Stabilizers .................................................................................................305 13.11.1 Functions..............................................................................................................305 13.11.2 Products................................................................................................................305 13.11.3 Regulatory Status.................................................................................................311 13.11.4 Application of Gums in Fruit Products...............................................................311 Emulsifiers..........................................................................................................................313 13.12.1 Functions..............................................................................................................313 13.12.2 Products and Applications ...................................................................................313 13.12.3 Regulatory Status.................................................................................................314 Flavors ................................................................................................................................315 13.13.1 Products and Function .........................................................................................315

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285

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13.13.2 Technology of Manufacturing .............................................................................315 13.13.3 Regulatory Status.................................................................................................317 13.13.4 Use of Flavors in Processed Fruits .....................................................................318 13.14 Colors .................................................................................................................................318 13.14.1 Functions..............................................................................................................318 13.14.2 Products................................................................................................................318 13.14.3 Use of Color Additives in Fruit Products ...........................................................321 13.14.4 Trends and Regulatory Status..............................................................................321 13.15 Vitamins .............................................................................................................................323 13.15.1 Functions..............................................................................................................323 13.15.2 Products................................................................................................................324 13.15.3 Applications .........................................................................................................325 13.15.4 Regulatory Status.................................................................................................326 13.16 Preservatives.......................................................................................................................326 13.16.1 Functions..............................................................................................................326 13.16.2 Products................................................................................................................327 13.16.3 Regulatory Status.................................................................................................328 13.16.4 Trends and Issues ................................................................................................328 13.17 Antioxidants .......................................................................................................................329 13.17.1 Functions..............................................................................................................329 13.17.2 Products and Applications ...................................................................................329 13.17.3 Technology and Manufacture ..............................................................................333 13.17.4 Regulatory Status.................................................................................................333 13.17.5 Trends and Issues ................................................................................................334 13.18 Enzymes .............................................................................................................................334 13.18.1 Products and Functions .......................................................................................334 13.18.2 Applications in Fruit Processing .........................................................................335 13.18.3 Technology and Manufacture ..............................................................................336 13.18.4 Trends, Issues, and Developments ......................................................................337 References ......................................................................................................................................337

13.1 INTRODUCTION 13.1.1 DEFINITION

AND

FUNCTIONS

The broadest practical definition of food additive is any substance that becomes part of a food product either directly or indirectly during some phase of processing, storage, or packaging. Direct food additives that are discussed in this chapter are those that have been intentionally added to food for a functional purpose in controlled amounts, usually at low levels (from parts per million to 1 to 2% by weight). Indirect additives, on the other hand, are those entering into food products in small quantities as a result of growing, processing, or packaging. Examples of these are lubricating oils from processing equipment or components of packaging material that migrate into food before consumption. Direct food additives are used in foods for six main reasons: • • • •

To To To To

ensure microbial safety (e.g., against botulism and listeria) maintain product consistency improve or maintain nutritional value extend shelf life (e.g., retard the onset of rancidity)

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• •

287

To facilitate food processing (e.g., provide leavening or control acidity) To enhance flavor or texture, or impart desirable color

The differences between food ingredients and additives are mainly that of quantity used in any given formulation. Food ingredients can be consumed alone as food (e.g., sucrose and butter), whereas food additives are used in small quantities relative to the total food consumption but which nonetheless play a large part in the production of desirable and safe food products.

13.2 FOOD ADDITIVE CATEGORIES In the U.S., over 2500 chemicals, including more than 1500 aroma chemicals, are permitted for direct use. When intentionally added to food at very low levels, additives perform specialized functions that favorably impact the economics of processing, formulating, and distributing food products. Included in the overall direct food additive category are: • • • •

Inorganic chemicals — phosphates, sulfites, salt, etc. Synthetic chemicals — dyes, silicones, benzoates, vitamin A, etc. Extraction products from natural sources — gums, essential oils, vitamin E, etc. Fermentation-derived products — enzymes, yeast, citric acid, biogums, etc.

Fruits are important raw material sources for the manufacture of many food additives. Examples include citrus oil and essence as flavor; color from red grape skin; tartaric acid from grape juice; pectin from apple and citrus pulp; and enzymes from pineapple, papaya, and figs. Most food additives have a long history of use; others are the result of recent research and development to fill particular requirements of modern food processing. Some are common chemicals of industry that are upgraded in terms of purity to allow their use in food. There is much discussion about whether a food additive is natural or synthetic. The fact is that this classification, in many instances, has become somewhat arbitrary. Many food additives synthesized in chemical laboratories are also found occurring naturally in normal food. Vitamin C (ascorbic acid) produced by a chemical process is the same chemical that is found in oranges. Similarly, citric acid that is today produced commercially by enzymatic fermentation of sugars is the same chemically as the naturally occurring chemical that has been found to make lemons and limes tart. The practice of adding chemicals (e.g., spices, herbs, vinegar, and smoke) to food dates back many centuries. In recent years, however, the ubiquitous presence of chemical additives in processed foods has attracted much attention and public concern over the long-term safety of additives to man. Although the safety issue is far from subsiding, there is scientific consensus that food additives are indispensable in the production, processing, and marketing of many fruit products. The judicious use of chemical additives — typically in the range of a few parts per million to less than 2% by weight of the finished product — contributes to the abundance, variety, stability, microbiological safety, flavor, and appearance of the food supply. While direct food additives offer a major contribution to the palatability and appeal of a wide variety of foods, their level of use is relatively insignificant in the total human diet.

13.3 INDUSTRY SEGMENTS The food additive business comprises more than 20 segments. Ten major product categories of food additives and their functions are presented in Table 13.1 (Somogyi, et al., 1996). Compounds in these food additive categories that are utilized in fruit processing will be covered in this chapter.

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TABLE 13.1 Food Additives and Their Functions in Processed Fruit Products Food Additive

Nutritional Value

Preservation, Protection

Production Improvement

Appeal Modification

X X

X

X

X X

X X

X X X X X X

Acidulants Antioxidants Colors Emulsifiers Enzymes Flavors Nutrients Preservatives Sweeteners Thickeners/stabilizers

X X

X X X

X X

Within the same category, products may belong to several chemical classes and offer specialized functionality (e.g., water- and oil-soluble antioxidants that include ascorbic acid and hindered phenols; water-soluble azo-dyes, and water-dispersible carotenoid as food colors).

13.4 FUNCTIONS OF DIRECT FOOD ADDITIVES Direct food additives serve several major functions in foods. Many additives, in fact, are multifunctional. The basic functions are preservation, processing, appeal and convenience, and nutrition.

13.4.1 PRESERVATION Food preservation techniques have advanced in the past 100 years and now include thermal processing, concentration and drying, refrigeration and freezing, modified atmosphere, and irradiation. However, the use of chemical preservatives frequently augment these basic preservation techniques and represent the most economical way for food manufacturers to ensure a reasonable shelf life for the product. Antioxidants and antimicrobial agents perform some of these functions as well.

13.4.2 PROCESSING Food processors are increasingly using food additives to insure the integrity and appeal of finished products. Emulsifiers maintain mixtures and improve texture in breads, dressings, and other foods. They are used in ice cream when smoothness is desired, in breads to increase shelf life and volume and to distribute the shortening, and in cake mixes to achieve batter consistency. Stabilizers and thickeners assist in presenting an appealing, consistently-textured product. Sorbitol, a humectant and sweetener, is used to retain moisture and enhance flavor. With the removal of sugar from many foods for dietetic reasons, a bulking agent substitute such as polydextrose is growing in importance.

13.4.3 APPEAL

AND

CONVENIENCE

The changing eating habits of consumers, partly brought about by the large increase in the percentage of women who work outside the home, is creating a growing need for convenience foods. In many of these types of foods, it is essential that a variety of additives be used to provide the taste, color, texture, body, and general acceptability that are required. This need for convenience, while maintaining aesthetic appeal and taste, is becoming extremely important. Most food additives

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such as gums, flavoring agents, colorants, and sweeteners are included by food processors because consumers in the developed countries demand that food look and taste good as well as being easy and safe to serve.

13.4.4 NUTRITION There have been tremendous advances in knowledge of human nutrition, and consumers are increasingly aware of the value of good nutrition. Vitamins, antioxidants, proteins, and minerals are added to foods and beverages as supplements in an attempt to ensure proper nutrition for those who do not eat a well-balanced diet. In addition, additives such as antioxidants are often used to prevent deterioration of natural nutrients during processing. Recently there is more importance attributed to disease prevention through proper nutrition, as well as to increasing performance through sport nutrition products. On the other hand the medically based desire for good nutrition through a balanced diet may adversely affect consumer demand for some food additives such as fat substitutes.

13.5 GOVERNMENT REGULATIONS The application of food additives is highly regulated worldwide although regulatory philosophy, the approval of specific products, and the level of enforcement differ from country to country.

13.5.1 U.S. In the U.S., the Food and Drug Administration (FDA) has the primary jurisdiction over food additives, although clearance for use of food additives in certain products must be obtained from other government agencies as well. For example, pesticides used on raw agricultural products are covered by the Federal Insecticide, Fungicide, and Rodenticide Act that was issued in 1972 and amended in 1988. In the U.S. (Code of Federal Regulations, 2002), the only additives that can be marketed for food are: • •

Included in an approved list and comply with existing specifications for food grade material Received pre-marketing clearance by the FDA

For regulatory purposes, all food additives fall into one of three categories: • • •

Prior sanctioned substances Generally Recognized As Safe, known as GRAS substances Regulated direct and indirect additives

Prior sanctioned substances (approximately 1400) are products that already were in use in foods prior to the 1958 Food Additives Amendment of the Federal Food, Drug and Cosmetic Act and therefore are considered exempt from the approval process. Some prior sanctioned substances also appear in the GRAS list. This is the “grandfather clause” of the amendment. GRAS substances (approximately 700 compounds) are a group of additives regarded by qualified experts as safe because their extensive use in the past has not shown any harmful effects. The FDA is involved in an ongoing review of GRAS and prior-sanctioned lists to ensure that these substances are tested by the latest scientific methods. Likewise, the FDA also reviews substances that are not currently included on the GRAS list to determine whether they should be added.

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All other additives are regulated, meaning that a specific food additive petition must be filed with the FDA, requesting approval for use of the additive in any application not previously approved. A food or color additive petition must provide convincing evidence that the proposed additive performs as it is intended. Animal studies using large doses of the additive for long periods are necessary to show that the substance would not cause harmful effects at expected levels of human consumption. In deciding whether an additive should be approved, the agency considers the composition and properties of the substance, the amount likely to be consumed, its probable long-term effects, and various other safety factors. Long delays (>5 years) and the high cost of regulatory clearance limit new introductions to only a handful of products that have been successfully petitioned in the past decade. In addition, the FDA operates the Adverse Additive Monitoring System to help as an ongoing safety check for all additives. The system monitors and investigates all complaints by individuals or their physicians that are believed to be related to specific foods, food additives, or dietary supplements. All natural and synthetic color additives are subject to the Color Additive Amendment of 1960 and are not included in the food additive regulations. Flavor substances are regulated somewhat differently, and rules are less restrictive. In the U.S., there is a comprehensive list of flavor substances that are deemed GRAS, based on history of their use, review of available toxicology, and the opinion of experts. These GRAS lists (through GRAS list 21 by Smith et al., 2003) have been compiled since 1970 by the expert panel of Flavor Extracts Manufacturers Association of the U.S. (FEMA). Over the years, over 1800 compounds were deemed GRAS by the FEMA panel. Founded in 1909, FEMA is an industry association that originally started pursuing voluntary self-regulation and later was granted quasi-official status on regulatory matters regarding flavor chemicals by FDA. The FEMA expert panel was formed in 1960. This independent panel, composed of eminently qualified experts recruited from outside the flavor industry, has expertise in human nutrition, physiology, metabolism, toxicology, and chemical structure–activity relationships.

13.5.2 EUROPE Food additives are regulated by the European Parliament and Council Directive 89/107/EEC of December 21, 1988. The Directive requires that all permitted food additives be assessed by the European Scientific Committee for Food (SCF) for their safety against the criteria that are listed in the Annex of the Directive (Goodburn, 2001). The Directive provides for the adoption of specific directives outlining positive lists of authorized food additives and conditions for their use. The three specific directives were adopted in 1994 and 1995. Since then all regulations relating to the use of additives have been the same in all 15 EU member states. Previously, each country had widely different rules. The three specific directives are: • • •

Sweeteners: Directive 94/35/EC of June 1994 Food colors: Directive 94/36EC of June 1994 Food additives other than colors and sweeteners: Directive 95/2/EC of February 1995

Since 1988, several amendments to, and adaptations of, Directive 89/107/EEC have been introduced or proposed. These efforts have been toward a uniform registration process so that a registration obtained in one country would be valid in all EU member countries. The new EU food additive law, however, will not prevent individual countries from asking for additional or country-specific requirements

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for new product registrations. At the EU level, several institutions and groups are involved in the development of food additives law, including the Scientific Committee for Food (SCF), one of the institutions of the European Commission (the Union’s executive body) which deals with safety issues; representatives from different national professional organizations, representatives from the food industry, and retailers, etc. The Standing Committee on Foodstuffs ensures close cooperation between the Commission and the member states. The EU rules for the evaluation, marketing, and labeling of novel food such as genetically modified foods are also being developed. Labels need to be carried by foods containing genetically modified living organisms such as yogurt containing an altered culture and any food whose modification might raise moral or health worries to consumers. A tomato, for example, containing a strawberry protein would have to carry a label to alert consumers allergic to strawberries. However, novel foods that are identical to conventional foods (e.g., sugar from beet genetically modified to resist disease) would not have to be labeled.

13.5.3 JAPAN In Japan, the Food Chemistry Division of the Ministry of Health and Welfare (MHW) has jurisdiction over food additives through the Food Sanitation Law that was enacted in January 1948, with several amendments adopted since then. Amendments to the regulations as well as additions or deletions to Kohetisho (the Japanese Codex of Food Additives) were mostly influenced by two major objectives: (1) protection of food sanitation and customer safety, and (2) harmonization with international regulatory requirements (Anon., 1994a). Most discussions on regulating food additives in Japan have been related to defining what food additives should be under legal restriction, and on labeling requirements. Very often in these discussions, differentiating “synthetic” and “natural” food additives had been at issue. In Japan, those two generally used terms have often misled customers into a blind belief in natural food additives. However, regulatory bodies as well as the food additive industry no longer distinguish additives with these terms. On the other hand, the problem of environmental pollution, particularly by chemical companies, was brought into public consciousness and criticism of chemical substances became severe. A negative image of chemical substances spread over the public, including chemicals used in food. Because the term chemical caused a negative reaction, industry began to use “non-chemical” or “natural” for substances that exist in natural products. The public also preferred “natural” as an opposite to “synthetic,” believing that “natural” meant “better.” Industry put great effort into development of natural substances instead of new chemicals because there was little legal restriction on natural food additives while there were very strict legal requirements on chemical additives. Since May 1996 natural food additives have been regulated under the Food Sanitation Law (Somogyi et al., 1996). However, a large number of “natural” food additives that are listed as “existing food additives” remain outside of legislation. The list of existing “nonchemically synthesized” food additives that was finally confirmed by the MHW on April 16, 1996 includes 489 “existing natural” food additives. Today both synthetic and natural food additives need to be declared on the labeled.

13.6 TRENDS, ISSUES While there are many differences in food tastes and preferences among the different regions of the world, the major trends driving the food additive utilization appear to be very similar in all regions: • • •

Concern over safety of processed food Health consciousness; desire for low-calorie foods Desire for convenience

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• • • • •

Desire by consumers, as well as food manufacturers, for value-added products Increasing regulatory constraints Shift from synthetic to natural- or semi-natural-based products Increasing costs of new products in terms of R&D and product commercialization Expansion of food service products (fast-food, airline meals, etc.)

New and improved technologies for the most part aim at processing and preserving fruits with minimal use of chemical additives. Aseptic packaging, controlled atmosphere packaging, irradiation, and membrane separation/microfiltration processes are typical examples of this trend. Conversely, microwave heating for both industrial and home use and extrusion represent technologies that are likely to stimulate expanded use of additives such as less volatile flavors more compatible with microwave heating and specialized blends of gums/emulsifiers for the reformulation of innovative extruded food products. A shift away from commodity to more processed, higher value food products will favor increased use of food additives for processing. Increasingly, sales of ingredient-and-additive blends will dominate in the future. The synergistic effects that enhance the functionality of these materials while reducing the quantity needed will play an ever more significant role in formulated foods. Information on these blends will be scarce because they will be developed in-house by food additive suppliers as well as food manufacturers wishing to maintain confidentiality in order to optimize exclusive commercial benefit. China’s increasing ability to produce and export acceptable-quality food additives pushed prices and profitability downward for some previously attractive additives like vitamin C, sodium erythorbate, and benzoates. The Republic of Korea and India may prove to be factors in the near future as well. Other issues affecting the food additive industry include increasing government regulatory activity, increasing R&D and legal expenses, and the great length of time needed to perfect, gain approval, and market new food additive products.

13.7 FOOD ADDITIVE INDUSTRY STRUCTURE Food additive suppliers are an important part of the food manufacturing system, marketing products to both commodity processors and food processors. Figure 13.1 depicts the food additive industry structure and flow of products. Some 60 to 70% of food additives are used in the manufacturing of food; 20 to 30% are used in commodity processing operations such as fruit juice processing, vegetable packing, oil seed crushing, flour milling, meat packing, etc.; and the remaining 5 to 10% in other uses such as pharmaceutical and personal care products.

13.8 UTILIZATION OF FOOD ADDITIVES The technical need and level of utilization of food additives vary within each food manufacturing sector served, depending on the functional requirements of the application (e.g., microbial preservation, preventing oxidation, improving color, flavor, nutrition qualities, etc.) and the strength and performance characteristics of the additive (e.g., solubility, heat and light stability, chemical compatibility). Fruit processing operations that include processing, compounding, packaging, and preservation require the use of a variety of food additives. In general, the level of direct food additive use in processed fruits is small in comparison with other food industry sectors. In particular, formulation of dairy products, baked goods, snack foods, or confections require greater use and a wider variety of food additives. In Table 13.2 the extent of food additive utilization in various fruit processing operations is illustrated.

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COMMODITY PROCESSOR • Sugar refining • Grain milling • Slaughtering • Oil seed processing

20–30%

FOOD ADDITIVE MANUFACTURERS

5–10%

• Acidulants • Emulsiltors • Flavors, colors, etc. 60–70%

DISTRIBUTORS BLENDERS

FOOD MANUFACTURERS

REFINED INGREDIENTS • Flour • Sugar • Fats & oils • Fruit juice, etc.

• Canned food • Snack food • Dairy products • Bread, cookies, etc.

30%

70% RETAIL FOOD STORES • Produce • Processed food

OTHER USES • Pharmaceutical • Cosmetics • Animal feed, etc.

FOOD SERVICE • Fast food meals • Hospital food • Airline meals, etc.

FIGURE 13.1 Food additives: pattern of use in food and other applications.

TABLE 13.2 Unit Operations in Fruit Processing and the Need for Food Additives Unit Operation

Need for Additives

Preparation of Fruits for Processing Crushing Low Shelling Low Washing/cleaning Low Extraction/refining Medium Clarification Medium Concentration Medium Cutting/sizing Low Drying Low Controlled atmospheric storage Low Manufacturing Fruit-Based Compounding/formulating Extruding Canning/aseptic pack Freezing Dehydrating Fermentation Irradiation

Products High High Low Medium Medium Medium–high Low

13.9 SWEETENERS 13.9.1 FUNCTIONS Sweeteners are used in fruit processing for many functional reasons, as well as to impart sweetness. They render certain fruits palatable, and add flavor, body, bulk, and texture. They change the freezing

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point, control viscosity (which contributes to texture), and prevent spoilage. Certain sweeteners bind moisture that is required by detrimental microorganisms in fruits. Alternatively, some sweeteners can serve as food for fermenting organisms that produce acids that preserve the fruit. These auxiliary functions must be kept in mind when considering applications for artificial sweeteners. Sweeteners may be classified in a variety of ways: • • • •

Nutritive or nonnutritive: Materials either are metabolized and provide calories or are not metabolized and thus are noncaloric. Natural or synthetic: Commercial products that are modifications of a natural product, e.g., honey or crystalline fructose, are considered natural. Regular or low-calorie/dietetic/high-intensity: Although two sweeteners may have the same number of calories per gram, one may be considered low-calorie or high-intensity if less material is used for equivalent sweetness. As foods: Fruit juice concentrates or low moisture fruit powders, for example, can impart substantial sweetness.

13.9.2 PRODUCTS, APPLICATIONS,

AND

REGULATORY STATUS

Sucrose, commonly known as table sugar (or refined sugar) is the standard against which all sweeteners are measured in terms of quality taste and taste profile. It is consumed in the greatest volume of all sweeteners; however, sucrose, high fructose corn syrup, dextrose, and other natural sweeteners such as molasses, honey, maple syrup, and lactose sweeteners are normal ingredients (e.g., they can be consumed alone as food) and are not considered additives (Wodicka, 1980) and, therefore, will not be covered in this chapter. The discussion that follows concentrates on the highintensity sweeteners (e.g., saccharin, aspartame, acesulfame K, etc.) and polyol alternative sweeteners (e.g., xylitol, sorbitol, etc.).

13.9.3 HIGH-INTENSITY SWEETENERS High-intensity sweeteners are manifoldly sweeter than sucrose (30 times greater or more) and closely mimic its sweetness profile (Anon., 1992). Because of the very low use levels, however, high-intensity sweeteners cannot perform other auxiliary functions in food and often must be used in conjunction with other additives such as bulking agents. High-intensity sweeteners, once used mainly for dietetic purposes, are now used as food additives in a wide variety of fruit products such as fruit drinks, dry fruit beverage bases, jams, jellies, and other fruit desserts (O’Brien Nabors and Galardi, 1991). With the obesity epidemic and the public attention it currently receives, sweeteners that allow for consumer-appealing labeling are of particular interest. These sweeteners when used alone or in combination may permit such labeling as “low-calorie,” “reduced calorie,” “light,” “sugar-free,” and “does not promote tooth decay” (O’Brien Nabors, 2002). Approval of high-intensity sweeteners differs from country to country. At present, four highintensity sweeteners are in use in the U.S.: aspartame, acesulfameK, sucralose, and saccharin. In addition, tagatose received approval by the FDA in 2002. Cyclamates are also permitted in Canada. Inside the European Community, six sweeteners have been generally approved: acesulfameK, aspartame, cyclamate, neohesperidin, saccharin, and thaumatin. In Japan, saccharin, aspartame, and the plant extracts glycyrrhizine and stevioside are widely used certified food additives. The relative sweetness and regulatory status potency of several important, current and proposed sweeteners are given in Table 13.3. 13.9.3.1 Alitame Alitame is approved in Australia, New Zealand, Chile, Colombia, China, Indonesia and Mexico for use in food, beverages, and as table top sweeteners. Approval is pending in the U.S., Japan, Brazil, and Canada.

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TABLE 13.3 Sweeteners and Regulatory Status of Sweeteners in Selected Regions Regulatory Statusa

Sweetener

Sweetnessb Relative to Sucrose

U.S.

Canada

European Community

Japan

Sugar (sucrose) High-fructose corn syrup Crystalline fructose Dextrose Xylitol Mannitol Sorbitol Lactitol Sodium Cyclamate Aspartame Acesulfame K Saccharin Sucralosec Alitamed Thaumatine Neohesperidine Stevioside Glycyrrhizine Neotame Tagatose Erythritol Isomalt Lactitol Maltitol Mannitol Sorbitol Xylitol

1 1–1.5 1.2–1.7 0.75–0.85 1 0.7 0.5–0.7 0.3–0.4 30 200 200 300 600 2000 3000 2000 300 50 8000 0.9 0.6–0.7 0.4 0.3–0.4 0.9 0.5 0.6 1

A A A A A A A P P A A A A P A N N N P A A A A A A A A

A A A A A A A P A A A N A P A N N N N N A — A A A A A

A A A A A A A A A A A A A P A A N N N N P A A A A A A

A A A A A A A A N A P A A P A N A A N N A A A A A A A

A = approved; P = petition filed; N = not approved. Sweeteness is dependent on several factors, including the concentration of sweetener, temperature, pH, and type of medium used. c Sucralose is approved in Australia and Russia. d Alitame is approved in Australia, Mexico, New Zealand, China, Indonesia, and Mexico. e Glycyrrhizin, Thaumatin, and Neohesperidin are approved as flavorant and flavor enhancer but not as a sweetener in the U.S. a

b

Source: Sugar and Sweetener Situation and Outlook Report, U.S. Department of Agriculture, Economic Research Service, December 1991, and updated by the author.

13.9.3.2 Acesulfame K (ace-K) In 1998, the U.S. FDA approved the use of Hoechst AG’s acesulfame K (SunetteTM) in nonalcoholic beverages, following a string of earlier approvals for use in: • •

Chewing gum, dry beverage mixes, instant coffee and tea, gelatins, puddings, nondairy creamers, and table sweeteners (1988) Candies and hard and soft caramels (1992)

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• •

Bakery goods, dairy products, syrups, and bulk sweeteners (1994) Alcoholic beverages (1995)

Presently, it is approved for use in foods and beverages in 34 countries. Acesulfame K has a rapidly perceptible sweet taste 200 times as potent as sucrose. It has a good shelf life and is relatively stable across temperatures and pH ranges associated with preparation and processing of foods. One major advantage of ace-K is its synergy with other sweeteners, including nutritive and nonnutritive types. It is being used in canned fruits and sugar-reduced jams and some dry beverage bases. 13.9.3.3 Aspartame Aspartame was approved in the U.S. in 1981 for use in prepared foods, dry beverage mixes, as a table top sweetener, and, in 1983, in liquid soft drinks. Presently, it is approved for use in 75 countries as well. Aspartame is about 200 times sweeter than a 4% sugar solution, and despite the shortcomings of its short shelf life (in beverages), it is the most successful high-intensity sweetener currently used. Worldwide, there are more than 4200 products containing aspartame in 46 product categories. Its role as a food ingredient that enhances fruit flavors makes it highly suitable for soft drinks and yogurt. Aspartame is also used in chilled and frozen desserts, fruit juice beverages, and sugar-free jelly and whips. Two major disadvantages of aspartame are its instability in acidic conditions and its loss of sweetness during prolonged heating. 13.9.3.4 Cyclamates Cyclamates were discovered in the U.S. in 1937, and they started being used as a sweetener in the mid-1950s. In 1970, they were banned from use in all foods and beverages in the U.S., the U.K., and many other countries. But on the basis of recent data, cyclamates have been reapproved in over 50 countries including Canada and most European countries. In the U.S., a petition for reapproval is still pending. Cyclamates are 30 times sweeter than sucrose and have been particularly useful in fruit products because they enhance fruit flavors, even in low concentrations, and can mask the natural tartness of some citrus fruits. Moreover, the cyclamate solutions used for canned fruits have a lower specific gravity and osmotic pressure than sucrose syrups and, hence, do not draw water out of the fruit. Thus, fruit packed in cyclamate solutions tend to have a greater drained weight than those packed in sucrose. Thickening and consistency represent the major problems with jams and jellies sweetened with cyclamate. Low-methoxy pectin is usually employed as a gelling agent in jams and jellies because it does not require sugar for gel formation. Due to the lower concentrations of osmotically active compounds, jams and jellies containing cyclamates may require a preservative to extend their shelf life. 13.9.3.5 Glycyrrhizin The FDA had granted GRAS status to ammoniated glycyrrhizin for use as a flavor enhancer and a natural flavoring agent. Applications include use as a flavorant, flavor modifier, and foaming agent. In Japan, glycyrrhizin is most commonly used as a sweetener. 13.9.3.6 Neohesperidin DC Currently used in Belgium, Holland, and Germany, neohesperidin DC is extracted from the peel of bitter Spanish orange (Citrus aurantium). It offers an outstanding sweetness, being thousands of times sweeter than sucrose; however, its taste is quite different from that of sugar, but neohesperidin works well in low concentrations, coupled with other sweeteners such as aspartame and acesulfame K. Its somewhat limited applications hinge mainly on beverages, confectionery, and desserts. It is especially recommended for fruit juices and nectars.

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13.9.3.7 Saccharin Saccharin has been used as a food additive since the early 1900s and is the most widely used nonnutritive sweetener worldwide. Saccharin is an organic petroleum compound. It is 200 to 700 times sweeter than sucrose in its various forms. It is both inexpensive and very stable in processing and storage but, due to its bitter aftertaste, has limited potential. Because of its less-than-perfect sweetness profile, saccharin tends to be regarded as only a partial replacement for sugars or is used in combination with other high-intensity sweeteners that mask the aftertaste. The market for saccharin in diet drinks has grown alongside the rise to fame of aspartame, but this is now diminishing as manufacturers reformulate drinks to include other blends of high-intensity sweeteners. Other uses include pickles, jams, jellies, stewed fruits, and canned fruits. The FDA took saccharin off the GRAS list in the early 1970s as a result of a study suggesting it caused cancer in rats. A ban on saccharin use in the U.S. was proposed, but stayed by Congress in 1977. In 1991, the FDA withdrew the proposed ban on saccharin. It was reapproved for tabletop, personal care, and pharmaceutical use in Canada in 1992 and a petition has been filed for reapproval in beverages and foods. However, saccharin is sold only in pharmacies in Canada. Saccharin is approved for use in more than 100 countries. 13.9.3.8 Sucralose Sucralose is a highly stable compound derived by the selective chlorination of sucrose and is 600 times sweeter than sugar. In April 1998, sucralose received the broadest initial FDA approval ever given a low-calorie sweetener. Sucralose was approved for use in 15 food and beverage categories including baked goods and mixes, beverages, frozen dairy desserts, jams and jellies, and processed fruits and fruit juices. In August 1999 approval for the use of sucralose as a general-purpose sweetener was granted. Petitions for approval to use it in multiple food and beverage categories are also in file for the EU and Japan. Sucralose is now approved in 34 countries and has been in use since 1991. Marketed under the brand name SplendaTM, it is used in a wide range of products, including jams, jellies, and canned fruits. 13.9.3.9 Stevioside An extract from the South American plant Stevia rebaudiana is approved for sweetening purposes in Japan, the Republic of Korea, and Brazil. It can be used in dietary supplements in the U.S. Pure stevioside is about 300 times as sweet as a 0.4% sucrose solution. This high-intensity sweetener has been increasingly popular in Japan because of its taste and the perception that, as a “natural” product, it is healthier than synthetic sweeteners. Stevioside is commonly added to Japanese-style vegetables, dried seafood, soy sauce, and miso products, but it has only limited use for sweetening fruit products. 13.9.3.10 Thaumatin Thaumatin is approved in the U.S., Canada, Mexico, Taiwan, Japan, the European Union, Australia, New Zealand, South Africa, Switzerland, Israel, the Republic of Korea, and Vietnam. Although it is approved for use as a sweetener in many countries, thaumatin is used primarily as a flavor enhancer. In the U.S., thaumatin was granted GRAS status as a flavor enhancer by the FDA. 13.9.3.11 Tagatose Tagatose occurs naturally in dairy products, but the commercial product is made via a patented process. It has the bulk of sugar, is almost as sweet, but provides only 1.5 kcal/g. It has the potential for use in many products where sucrose is currently used. Tagatose was declared as GRAS by the manufacturer’s self-determination process, and can be used in the U.S. food supply.

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13.9.4 POLYOLS Polyols (sugar alcohols or polyalcohols) are chemically reduced carbohydrates. These compounds are important sugar substitutes that are utilized when their different sensory, special dietary, and functional properties make them desirable (Giese, 1993). Yet, because polyols are absorbed more slowly from the digestive tract than is sucrose, they are useful in certain special diets. When consumed in large quantities (25 to 50 g/d), however, they have a laxative effect, apparently because of the comparatively slow intestinal absorption. In most European countries, sorbitol, xylitol, mannitol, and lactitol are utilized in low-calorie food formulations because the European Community Nutritional Labeling Directive of 1991 has assigned an average caloric value of 2.4 kcal/g to all polyols (Giese, 1993). In the U.S., polyols are currently used mainly in sugar-free confections and in foods for diabetics. The recent approval of low calorie claims, however, should make them attractive for new applications. The caloric values of polyols for labeling purposes are as follows: • • • • • • • •

Erythritol, 0.2 kcal/g Lactitol, 2.0 kcal/g Maltitol, 3.0 kcal/g Mannitol, 1.6 kcal/g Sorbitol, 2.6 kcal/g Xylitol, 2.4 kcal/g Isomalt, 3.0 kcal/g Hydrogenated starch hydrolysates, 3.0 kcal/g

Food products sweetened with polyols and containing no sucrose can be labeled as “sugarless,” “sugar-free,” or “no sugar” but must also bear the statement “Not a reduced calorie food,” “Not a low calorie food,” or “Useful only in not promoting tooth decay.” FDA recently approved a health claim for polyols as reducing or not promoting the risk of tooth decay. 13.9.4.1 Sorbitol Sorbitol occurs naturally in many edible fruits and berries, including pears, apples, cherries, prunes, and peaches. Its nontoxic nature has long been recognized, and in 1974, the FDA named sorbitol as one of the first four chemicals on its revised list of substances, of which that sanctioned for use in foods is GRAS. Sorbitol is only 60% as sweet as sucrose; however, it has many functional properties desirable in a sweetener (Dwivedi, 1991). The useful functional properties of sorbitol include high viscosity (contributing to body and texture), hygroscopicity (resulting in its humectant, as well as its softening, nature), sweetness or cool taste, sequestering ability, crystallization and modification (retardation), and bulking agent ability. Because sorbitol can be digested without insulin and is also noncariogenic (not promoting tooth decay), in Europe, it is used as a sugar substitute in diabetic food products (marmalades, jams, and fruit juices, etc.). In general, sorbitol is used in fruits either to aid in the retention of the original quality of the products on aging and shipment to the end user or to provide a texture or other product quality that would otherwise not be present in the original formula. Application of sorbitol in fruit products includes its use as humectant and/or bodying agent in shredded coconut, glazed, and dried fruits, and gelatin products. In these applications (some of which are suggested and may not be in actual use), smaller concentrations of sorbitol are required than in its use in sugarless gums and candies. 13.9.4.2 Mannitol Mannitol is only about 70% as sweet as sucrose and is also noncariogenic. It occurs in fungi and is produced by bacterial reduction of fructose in some fermentation reactions. Applications include

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as a dusting agent for chewing gum and as a bulking agent in powdered food. Mannitol has a more serious laxative effect than sorbitol, and a warning label is required against consumption exceeding 20 g/d. Amounts consumed as a fruit additive sweetener are believed to be minor. 13.9.4.3 Xylitol Xylitol is a five-carbon polyol with a sweetness similar to sucrose. It is found in small amounts in a variety of fruits and vegetables and is formed as a normal intermediate in the human body during glucose metabolism. Xylitol has good solubility, blends well with foods, and has a lower melting point than sucrose, an advantage in the manufacture of confectionery products. There is also evidence that xylitol is not only noncariogenic but reduces dental cavities when used as a replacement for sucrose. In the U.S., it is mainly used in compressed candies, chewing gum, and dental care products. Xylitol is very expensive; therefore, it is usually used in small amounts in combination with other sweeteners. In a blend with aspartame, the two compounds have an excellent synergistic effect. Also, xylitol is blended with other polyols to minimize undesirable properties, such as hygroscopicity or the laxative effect of sorbitol, or to improve the solubility of mannitol. 13.9.4.4 Lactitol Lactitol monohydrate, a sugar alcohol, has physicochemical properties different from those of sugars. It is derived from milk sugar and is used as a sweetener in Japan, Israel, and Switzerland. In the U.S., lactitol may be used in bakery products, chocolate, ice cream, chewing gum, and other confections. Lactitol has a sweetness value approximately one third that of sucrose and is therefore suitable where bulking with low sweetness is required. A special grade lactitol DC is used for direct compression in mints, tabletop sweeteners, and other sugar-free compressed products. 13.9.4.5 Erythritol Erythritol is about 70% as sweet as sucrose. Like other polyols it does not promote tooth decay and is safe for diabetics. However, it is distinctive for its caloric content (the lowest of the polyols: 0.2 cal/g) and its high digestive tolerance. Studies have shown that due to its small molecular size and structure, more than 90% of ingested erythritol is absorbed and excreted unchanged through the kidneys within 24 h, so that laxative effects sometimes associated with polyol consumption are unlikely. In 1997, the FDA accepted for filing a GRAS affirmation petition allowing manufacturers to produce and sell erythritol-containing foods in the U.S. According to the petition, erythritol is intended for use as a flavor enhancer, formulation aid, humectant, nutritive sweetener, stabilizer and thickener, sequesterant, and texturizer. 13.9.4.6 Maltitol Maltitol, approximately 0.9 times a sweet as sucrose with similar sweetness and body, has application in products such as chewing gum, dry bakery mixes, and chocolate. Because of its heat stability and good handling properties, it is suitable for use in sugar-free baked products. 13.9.4.7 Isomalt Approximately 0.45 to 0.65 times sweet as sugar, isomalt can be used in candies, gums, ice creams, jams and jellies, fillings and frostings, beverages, and baked products. As a sweetener/bulking agent, it has no off-flavors and works well in combination with other sweeteners. Isomalt has been available in the U.S. since 1990 and is used in more than 40 countries worldwide.

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13.9.4.8 Hydrogenated Starch Hydrolysates (HSH) A mixture of sorbitol, maltitol, and hydrogenated oligosaccharides, HSH is also known as maltitol syrup and hydrogenated glucose syrup. Depending on the sorbitol and maltitol content, the sweetness of HSH can vary from 0.25 to 0.5 times that of sucrose. HSH serve a number of functional roles including use as bulk sweeteners, viscosity or bodying agents, humectants, crystallization modifiers, and rehydration aids. They also can serve as sugar-free carriers for flavors, colors, and enzymes. HSH has been used by the food industry for many years, especially in confectionery products.

13.9.5 TRENDS The rapid growth of aspartame since its 1981 approval, plus the ban on cyclamate and the partial ban on saccharin, spurred extensive research and development on low-calorie and no-calorie sweeteners. The FDA’s approvals of acesulphame-K and sucralose, its acceptance of GRAS petition for erythritol, and the changes sanctioned in the caloric value labeling of polyols promise significant change in the future sweetener use. Price, safety, taste, solubility, and stability are expected to be the significant factors determining which of the approved and available dietetic sweeteners will be used.

13.10 ACIDULANTS 13.10.1 FUNCTIONS Acidulants are acids that either occur naturally in fruits and vegetables or are used as additives in food processing. Acidulants serve many functions; the following compounds are utilized in fruit processing: • • • • • • • •

Acetic acid Adipic acid Citric acid Fumaric acid Lactic acid Malic acid Phosphoric acid Tartaric acid

These compounds perform a variety of functions in fruit processing. The primary functions of acidulants in fruit processing include: • • • • • • • •

Acidifier pH regulator Preservative and curing agent Flavoring agent Chelating agent Buffer Gelling/coagulating agent Antioxidant synergist

Acidulants are used to add a desired tartness to many food products. Because of the low pH created when used, food acids can also prevent growth of microorganisms that might cause food to spoil. Through chelation of trace metal ions and low pH, acidulants also prevent rancidity and discoloration of foods by functioning as synergists to antioxidants such as butylated hydroxyanisole

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TABLE 13.4 Typical Food Uses of Acidulants in Fruit Processing

Acid

Carbonated and Noncarbonated Soft Drinks

Citric acid Phosphoric acid Malic acid Fumaric acid Adipic acid Lactic acid Tartaric acid

X X X X X X X

Dry Beverage Bases X

Wine

Jams, Jellies, Preserves

Gelatin Desserts

Fruit Snacks

Frozen Foods

Canning

X

X

X

X

X

X

X X X

X X X

X

X X

X

X X

Pickles and Olives

X

X X

(BHA), butylated hydroxytoluene (BHT), and ascorbic acid. They are also used as buffers during various stages of food processing, as well as in finished products. The versatility of acidulants will certainly be a factor in their continued and increased use as food additives in the future (Table 13.4). At equal concentrations, the acidulants vary in their ability to depress pH and in the degree of acidic taste, or intensity of tartness, produced. The following replacement percentages using anhydrous citric acid as equal to 100% tartness are approximate guides.

13.10.2 PRODUCTS

AND

Acid

% Required to Replace Anhydrous Citric Acid

Citric, anhydrous Fumaric Tartaric Malic Adipic Phosphoric (85% soln.)

100 67–72 80–85 78–83 110–115 55–60

APPLICATIONS

13.10.2.1 Citric Acid Citric acid is the most versatile and widely used food acidulant. It has been used in foods for more than 100 years (Gardner, 1980). As a result, it is often employed as the standard for comparison in evaluating the effects of other acidulants. Citric acid is found in numerous natural products, and it is one of the important acids involved in plant respiration. Several fresh fruits such as lemons and limes owe their tangy taste to the presence of citrate ions. Its useful characteristics include excellent solubility, extremely low toxicity, chelating ability, and pleasantly sour taste. The traditional method of obtaining citric acid at the beginning of the 20th century was extraction from the juice of citrus fruits, especially lemons and limes, and later from pineapple wastes. These processes became practically nonexistent, however, with the development of fermentation technology. Today, most citric acid is produced commercially by mold fermentation of sugar solutions (most commonly, dextrose derived from enzyme-treated cornstarch and beet molasses) using strains of Aspergillus niger. Fermentation can be carried out either in shallow pans (surface fermentation) or in deep tanks (submerged fermentation). The technology and commercial processes to produce citric acid by candida yeast fermentation on paraffins also exists. Citric acid is used as a flavor enhancer, as a preservative (e.g., in beverages and syrups), as an antioxidant synergist with ascorbic or erythorbic acid (e.g., for fresh and frozen fruits and

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vegetables), and as a pH regulator (e.g., in gelatin desserts and jellies). Sodium and potassium citrates are widely used as buffers, often in combination with the parent acid in fruit butters, fruit jellies, and preserves. The FDA classifies citric acid and its sodium, potassium, and calcium salts as GRAS for sequestering, miscellaneous, and/or general purpose uses in foods. Beverages are the major food use for citric acid, accounting for an estimated 65% of citric acid’s total food acidulant consumption. Citric acid and its sodium salt are used extensively in carbonated beverages. It is also used as a flavor enhancer and preservative. Sodium citrate often is used in carbonated beverages as a buffer to regulate tartness if the acid level is high. Aseptically packaged or canned fruit drinks (fruit-juice-added drinks and sodas) are large markets for citric acid. Wine coolers, cocktail mixers, ice tea mixes, thirst quenchers, and other beverages are additional citric acid beverage markets. In candy, citric acid is used primarily to enhance the flavor of fruits and berries, but also to invert the sucrose and prevent crystallization of the sugar and to prevent oxidation of the ingredients. Citric acid imparts the desired degree of tartness that is especially important in dried-fruit snacks. Citric acid is used in jams, jellies, preserves, and gelatin desserts to adjust the pH for maximum gel formation and to enhance the flavor. In jams and jellies, citric acid is added in amounts sufficient to compensate for a deficiency of fruit acidity. In the processing of frozen foods, citric acid is used for several reasons. Through its chelating and pH adjustment properties, citric acid is able to optimize the stability of frozen food products by enhancing the activity of antioxidants and inactivating enzymes. In processing frozen fruits, citric acid solutions are used to neutralize the residual lye from lye peeling operations. Citric acid is also used in canning fruits to: • • •

Lower pH, thereby reducing heat-processing requirements Optimize flavor in canned fruits such as prunes and grapefruit Enhance activity of antioxidants to prevent color and flavor degradation (by chelating trace metal ions that might otherwise catalyze enzymatic oxidation)

Citric acid and its salts are also used in a variety of other applications: to prevent crystallization in honey, to stabilize spices, and to act as a synergist for antioxidants used in retarding rancidity in foods containing fats and oils. Other uses of this versatile acid and its salts include their incorporation in enzyme preparations for clarifying fruit juices. 13.10.2.2 Malic Acid Malic acid is similar to citric acid both in its acidifying character and taste effects but does not exhibit the “burst” effect of citric. It is found widely in fruits and is strongly associated with apples, although it predominates in several fruits and vegetables. It is the second major acid next to citric in citrus fruits and is present in most berries, beans, and tomatoes. Malic acid is one of the miscellaneous or general purpose food additives in the FDA list of GRAS substances. It is an optional ingredient in the FDA standards for fruit butters, preserves, jams, and jellies. Malic acid is prepared by hydrolyzing maleic anhydride to maleic acid and, at elevated temperatures and pressures, forming an equilibrium mixture of maleic acid, fumaric acid, and malic acid. Malic acid is isolated from the other two acids, which are recycled if not wanted. Malic acid is used in a variety of products, but mostly in fruit-flavored sodas such as those with apple and berry flavor. Malic acid is the preferred acidulant in low-calorie drinks, and in cider and apple drinks, it enhances flavor and stabilizes the color of carbonated and noncarbonated fruitflavored drinks and cream sodas. In sugar-free drinks, malic acid masks the off-taste produced by sugar substitutes. Recent and projected future good growth is chiefly the result of use in dietetic

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fruit-flavored sodas containing aspartame because of its synergism and ideal blending properties. Blends of malic acid and citric acid are now available that exhibit some better taste characteristics than either separate acidulant. 13.10.2.3 Phosphoric Acid Phosphoric acid has a characteristic flavor and tartness and is used almost entirely in cola-flavored carbonated beverages. (A small quantity is also used in some root beer brands.) It is the only inorganic acid extensively employed as a food acidulant. Occasionally, phosphoric acid is utilized as a buffering agent in jams and jellies to adjust acidity for maximum gel formation. It is the least costly of all the food-grade acidulants; it is also the strongest, giving the lowest attainable pH. Phosphoric acid is produced primarily by the furnace process. Elemental phosphorus (P4) is burned at about 1650 to 2750∞C, yielding phosphorus pentoxide (P2O5), which reacts with water to produce phosphoric acid (H3PO4). Although the product is relatively pure, food-grade material requires additional purification to meet standards. Food-grade phosphoric acid is supplied mostly as 75% aqueous solutions, but 80% and 85% solutions are also available. 13.10.2.4 Fumaric Acid Fumaric acid is principally used in fruit juices and gelatin desserts, pie fillings, maraschino cherries, and wines. Fumaric acid increases the strength of gelatin gels and acts as a calcium liberator when incorporated in alginate preparations. In many of these applications, fumaric acid competes with other acidulants such as citric acid, tartaric acid, and malic acid. Although it is less costly than some alternatives, its relatively strong acid taste and low solubility make it less appropriate for certain food uses. It is currently manufactured in the U.S. by acid catalyzed isomerization of maleic acid. To overcome the slow solubility rate of fumaric acid in water at low temperatures, the acid is mixed with 0.3% dioctyl sodium sulfosuccinate (DOSS) and 0.5% calcium carbonate. This is sold as a modified acid for use in dry beverage powders, frozen fruit concentrates, and similar other applications. Rapid rates of solution are claimed for the mixture, even in ice water, and the fumaric acid remains as a homogeneous suspension in frozen concentrates for long periods of time. Furthermore, fumaric acid and its salts have a tendency to stabilize the suspended matter in both flash-pasteurized and frozen concentrates and to inhibit the development of off-odors and darkening. 13.10.2.5 Adipic Acid In fruit products, it is principally used as an acidulant and gel-inducing agent in bottled beverages and as powdered concentrates for fruit-flavored beverages, imitation jellies, jams, and gelatin desserts. In grape-flavored products, it adds a lingering supplementary flavor and provides an excellent set to food powders containing gelatin. It is often used in dry food products because it is nonhygroscopic and offers longer storage life under humid conditions. Adipic acid is currently produced commercially in the U.S. by nitric acid oxidation by cyclohexanol. 13.10.2.6 Lactic Acid Lactic acid is one of the most widely distributed acids in nature and one of the earliest used in foods. Because of its mild taste relative to other food acids, it is also used to enhance flavor in fruit drinks and confectionery. In addition, lactic acid is used as a preservative in brine-preserved foods such as pickles and Spanish-style olives where it insures clarity of the brine by inhibiting spoilage and further fermentation. It is also used in certain frozen desserts (to provide a mildly tart flavor without masking that of the natural fruit) and in some jams and jellies. Lactic acid derivatives

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are consumed by the food industry; for example, ethyl lactate is used in flavorings, and sodium lactate is used as a preservative and seasoning. Calcium lactate serves as a firming salt for fruits and vegetables and is also used as a gelling agent for demethylated pectins. Unlike the other acids, it is a viscous, nonvolatile liquid, food-grade lactic acid sold as 88% and 50% aqueous solutions, both of which are colorless and odorless. Until 1963, lactic acid was produced solely by fermentation of carbohydrates such as corn, potato, or rice starch; whey, cane, or beet sugar; or beet molasses. Now it is manufactured also by synthetic means in the U.S. as a by-product of acrylonitrile manufacture. Lactonitrile is produced from acetaldehyde and hydrogen cyanide. It is then esterified with methanol and hydrolyzed to produce lactic acid. 13.10.2.7 Tartaric Acid Tartaric acid has a strong, tart taste and augments natural and synthetic fruit flavors, especially grape and cranberry. Tartaric acid is a natural component of numerous fruits including the currant, raspberry, cranberry, and grape. As a food acidulant, tartaric acid is widely used in cranberry and grape-flavored foods and beverages and in candies (in conjunction with citric acid) to produce the sour apple, wild cherry, and other especially tart flavors. For similar reasons, it is selected as the acidulant for grapeflavored and for tart-tasting jams and jellies. Both tartaric acid and its mono potassium salt (cream of tartar) are used in baking powders and leavening systems. Specialized uses, high prices, and limited availability inhibit tartaric acid from widespread use as a food acidulant. Food grade tartaric acid is obtained from waste products of the wine industry such as press cakes, unfermented grape juice, or argols. In the manufacture of the acid, the by-products are first extracted with hot water and treated with hydrochloric acid and then lime. The calcium tartarate is then crystallized and decomposed with sulfuric acid to obtain the acid. 13.10.2.8 Vinegar In the U.S., the use of the term vinegar without qualifying adjectives implies only cider vinegar. There are several popular types of vinegar produced commercially for the use as an additive in food products. Although a 4 to 8% solution of pure acetic acid would have the same taste characteristics as cider vinegar, it could not qualify as vinegar. In the U.K., malt vinegar is supplied by trade agreement. In Europe, wine vinegar is the most common variety. Vinegar is produced from apple cider, grapes, or wine, sucrose, glucose, or malt by successive alcoholic and acetous fermentation. The resulting vinegar is then pasteurized and filtered and often concentrated for convenience of handling and storage. 13.10.2.9 Acetic Acid Very little pure acetic acid, as such, is used in fruit products. But it is the principal component of vinegar, one of the first food acidulants. It is found in unprocessed figs. It has a pungent odor and is utilized in pickled fruits. Pure acetic acid is produced synthetically by the oxidation of acetaldehyde and butane and as the reaction product of methanol and carbon monoxide. Smaller quantities are obtained from the pyroligneous acid liquors acquired in the destructive distillation of hard wood.

13.10.3 REGULATORY STATUS In the U.S., the major food acidulants are listed by the FDA as GRAS when used in accordance with good manufacturing practice. Fumaric acid is also on the FEMA GRAS list as a flavor component and is a food additive in FDA regulations. In Western Europe, the major acidulants are widely approved; the new EC guidelines, however, indicate tartaric acid should not be used for sparkling water. In Japan, major food-use acidulants

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are considered conventional (i.e., chemically synthesized) food additives; the exception is phosphoric, which is not recognized as an acidulant in Japan.

13.11 THICKENERS AND STABILIZERS 13.11.1 FUNCTIONS Hydrocolloids are the main products used as thickeners and stabilizers in fruit processing. Thickening and stabilization agents provide a number of useful effects to fruit products (Table 13.5). The technical base for these effects results from the ability of these materials to modify the physical properties of water. Most fruit products are composed largely of water. Hydrocolloids (also called gums, water-soluble materials) are long-chain polymers that function as (Dziezak, 1991): • • • • •

Rheology modifiers, affecting the flow and feel (mouth) of food and beverage products Suspension agents for food products containing particulate matter Stabilizers for oil/water mixtures Binders in dry and semidry food products Agents able to create both hard and soft gels in food products that require this physical form

Hydrocolloids are obtained from a variety of sources. Most hydrocolloids are derived from plant materials such as seaweed, seeds, and tree exudates; others are products of microbial fermentation, and still others are produced by chemical modification of natural polysaccharides (Glicksman, 1982). They can be categorized as: • • •

Natural gums — extracted from plant- and animal-based products Modified or semi-synthetic gums — chemically derivatized from natural organic materials such as cellulose and starch or produced via microbial fermentation of natural materials Synthetic gums — obtained by polymerization of monomers synthesized from petroleum or natural gas precursor (This group, which includes such hydrocolloids as polyacrylic acid, polyvinylpyrolidone [PVP], associative thickeners, etc., will not be discussed here because they are not utilized in fruit products.)

13.11.2 PRODUCTS 13.11.2.1 Unmodified Starches Unmodified starches, produced by the wet milling of field corn, supply the major amount of thickening material for the American food and beverage market. Unmodified cornstarch is produced by the wet milling of field corn, which involves treatment with a mild acid, grinding it to a fine paste, and removing the starch by centrifuging. Other natural starches of significance include potato and wheat starches used in Europe, and tapioca, arrowroot, and sago used in other areas. Unmodified cornstarch, commonly called pearl starch, is used in the fruit processing industry in the preparation of pie fillings. It is the base ingredient in many formulated thickening products because of its modest price. In addition to its thickening function, it is also a significant source of nutritional value. It is modestly priced in comparison to modified starch but even more so compared with such exotic materials as cellulose ethers, guar, xanthan, and alginates. But in recent years, unmodified starch consumption has suffered a decline in popularity because of the shift to lowcalorie foods, the shift to prepared foods, and the availability of alternative modified and

X X

X X X

X X X X X X X X X X

X

X

X

X

X X

Gelation

X X

X

X

Crystalization Control

X

X X

X

X X

Water Binding

X

X X X

X

X

Mouthfeel

X

X

X

X X

Foam Stabillization

X X

Flavor Fixation

X

X

X

X X X X

Protective Film Forming

Source: Newman H. Giragosian, Outlook for Food Thickeness, paper presented at the Chemical Marketing Research Association Meeting, Houston, TX, February 1983.

X

X X

X X X X X X

X X

Unmodified starches Modified starches Casein Gelatin Carboxy methylcellulose Methylcellulose Guar gum Alginates Xanthan gum Pectin Locust bean gum Gum arabic Carrageenan Agar X X

X X

Thickening

Suspending Properties

Hydrocolloid

Emulsion Stabilization

TABLE 13.5 Major Food Thickeners and Stabilizers and Their Principal Functions

X

X X X

X

Synergistic Effect

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semi-synthetic thickeners. Unmodified starches are used mainly for their thickening, gelling, adhesive, and film-forming properties. 13.11.2.2 Modified Starch Modified starch is starch that has undergone one of a variety of treatments to alter its physical properties and/or functionality. Possible treatments include roasting; treatment with acids, alkalis, or enzymes, bleaching, esterification, and oxidation. Modified starches used in food products or food processing have been modified for several reasons to: • • • • • •

Extend the bodying or gelling effect of normal starches Improve resistance to acid or heat degradability and to low temperature and freeze-thaw (eliminating aggregation) Improve texture Modify gelling tendencies as desired Increase viscosities at high temperature without gelling on cooling Improve instant solubility and gelling in cold water

Modified or derivatized starches are generally designed for more selective food applications than is unmodified starch. They are used in a wide variety of fruit products including baby foods, purees, candy (e.g., bonbons), jellies, cake mixes, doughs, puddings, pie fillings, batter mixes, dairy desserts, snack foods, and canned fruits. 13.11.2.3 Casein Casein is mostly imported from New Zealand and Ireland, although some is obtained domestically. It is a protein occurring naturally in and obtained from milk; it is the main ingredient in cheese. Casein is usually precipitated from skim milk by an acid or by rennin. The commercial sodium caseinate is obtained by evaporating a solution of casein in sodium hydroxide. Casein is marketed as sodium, calcium, potassium, or magnesium caseinate and is used in confections, puddings, bakery fillings, and frostings. Its use in fruit products depends mainly on its excellent cohesive and adhesive dried film properties. Researchers are working with a casein derivative to develop an edible film that keeps cut fruits and other produce fresh in the refrigerated case (McHugh and Krochta, 1994). In 1970, only five protein products were available from milk. In the last 15 years, numerous advances have been made in the methods to isolate and modify proteins. As a result, we now have hundreds of specialty milk proteins available. Most suppliers offer multitudes of specialty casein products. 13.11.2.4 Gelatin Derived from collagen, gelatin is the only food hydrocolloid extracted from animal source. Gelatin is an insoluble fibrous protein that occurs in vertebrates and is the principal constituent of the connective tissues and bones. It is obtained from pork skin and bones (Type A) or beef skin and bones (Type B). Gelatin is about 97% protein and can absorb up to ten times its weight in water; it can be chemically modified to render it insoluble. Type A is used mostly for confectionery products and Type B for dairy applications. The largest food applications for gelatin are dairy products such as ice cream and yogurts, confectionery products such as the gummy animal chewable, and gelatin fruit desserts. Gelatin is not heat stable.

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13.11.2.5 Guar Gum Guar gum is one of the most economical and widely used gums in the U.S., with extensive use in a variety of industrial and food applications. Along with natural starch and gelatin, guar gum is close to being a commodity product. Major uses for guar gum include ice cream, imitation bakery jellies and dry mix bakery formulations, beverages, and water-based frozen desserts. Guar gum is derived from a soy-like legume native to the semi-arid regions of the Asian subcontinent (India and Pakistan). It has been used in these regions as a vegetable, as well as livestock fodder since prehistory. In spite of several attempts by major domestic guar concerns to encourage domestic production of guar in Texas and other arid agricultural areas of the American Southwest, most of the guar gum consumed in the U.S. is derived from imported degummed guar beans (splits) sourced primarily from India. Because of changing supplies due to weather and harvest conditions, as well as the demand for guar gum for industrial applications, the price of guar gum tends to shift dramatically and therefore is one of the most difficult to forecast. 13.11.2.6 Locust Bean Gum Locust bean gum is obtained from the carob tree, and the major source of locust bean gum is the Mediterranean countries. The size and quality of the crop is directly related to climatic conditions; therefore, there are periodic shortages and great fluctuations of supplies. Chemically, locust bean gum is similar to guar gum. Anionic, cationic, and hydroxyalkyl derivatives are also produced commercially. Locust bean gum swells in cold water, but heating is necessary for maximum solubility. Locust bean gum is widely used in frozen dairy products, in conjunction with guar gum and carrageenan, and is used for preventing syneresis in cream cheese. In addition, locust bean gum is used in many nonemulsified sauces and dressings as a thickener and, in bakery products, as a moisture retention aid. 13.11.2.7 Agar Agar is produced in Portugal, Spain, and Morocco; a significant amount of material is produced in Japan as well. Methods of harvesting seaweed for agar production are similar to those used for carrageenan production. Other collection methods include the use of skin-divers to harvest seaweed from the sea floor. The product is usually sun dried locally than shipped for the location of the final production sites. Agar is used primarily in baked foods (icings, toppings, and meringues) and in confectionery products. Because agar is the most expensive of the seaweed extracts, there have been continual efforts to substitute it with other gums such as carrageenan. 13.11.2.8 Gum Arabic Gum arabic is the most water-soluble of the natural gums (up to 50%), and its solutions are of relatively low viscosity. Other advantages of gum arabic as a food additive include nontoxicity and its lack of odor, color, and taste. Gum arabic is obtained by hand-picking dried exudate from various trees of the genus Acacia, primarily from Acacia senegal. It is also known as gum acacia, gum senegal, mimosa, kordofan, verek, and gum egyptian. The Sudan accounts for about 85% of the output, the remainder coming principally from West Africa. Some upgrading may be undertaken by the use of air flotation or other mechanical means for removal of foreign matter. A premium grade of gum arabic is produced by dissolving, filtering, and spray drying the exudate. Gum arabic is used as an emulsifier in beverages for citrus oil and flavors, as a crystallization retarder and emulsifier in confectioneries, and as a stabilizer in dairy and bakery products. As the source of supply has sometimes been unreliable because of political and social events in the Middle East, many U.S. users have turned to substitutes including starch derivatives.

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13.11.2.9 Carrageenan Carrageenan is readily soluble in water to form an inelastic gel and is commonly used in combination with other gums. Its most unique property is a high degree of reactivity with certain proteins such as casein. The largest application of carrageenan food use is in dairy products (frozen desserts, flavored milk powder, and nondairy creamers). For example, cocoa can be suspended in milk with the use of about 0.025% carrageenan. Carrageenan is extracted from Irish moss, a common name applied chiefly to various Chondrus and Gigartina species, which are harvested off the Atlantic shores of New England, the Canadian Maritime Provinces, and several Western European countries. The Philippines supply a carrageenan type called “Philippine carrageenan” that does not correspond to the grade used in Western Europe for food applications and should only be used for pet food. 13.11.2.10 Pectin Pectin is a fruit extract obtained mainly from apple, orange, and lemon wastes. Whereas pectin obtained primarily from citrus fruits is used in the U.S., pectin from apples represents a major part of Western European pectin production. The main commercial types used in foods are pectin itself and potassium pectinate, sodium pectinate, and amidated pectin. Pectic substances are extracted with hot water that has been acidified (pH 1.5 to 3), usually with hydrochloric or nitric acid. Some de-esterification normally occurs in this step. Following separation from the residue, the extract may be concentrated and marketed as liquid pectin. Solid pectin may be produced by spray drying or precipitation with alcohol, or it may be produced as an insoluble salt. The supply of pectin is limited due to the availability of raw material for processing. In the U.S., pectin processing has been discontinued and production facilities were relocated to Mexico because of constraints in the regulatory approvals required by the EPA to operate a plant with the large volume of waste generated in processing. Pectin is the most frequently used hydrocolloid in processed fruits. Traditional uses of pectins are in jellies and jams. Newer applications include gummy candies, fruit snacks, and fruit-flavored juices and carbonated drinks. In gummy candies and jellies, it is continuing to replace starch for the fruit flavor aspect. It is also easier to work with, and the texture is better for cuttables and chewables. In fruit-flavored drinks, it stabilizes the constituents and makes the product more appealing. Pectin is well known and perceived by consumers as a natural product. Because of this, it is unlikely to suffer the same negative antiadditive trend as some other food additives. 13.11.2.11 Alginates The alginate category includes the various salts of alginic acid and propylene glycol alginate (PGA). The term algin is usually applied to sodium alginate. The gum is extracted principally from Macrocystis pyrifera, a giant brown algae or kelp of the Pacific Ocean that is commercially harvested, mainly off the coasts of Southern California and Australia. A smaller brown kelp comprising various Laminaria species is harvested from the North Atlantic, principally off the coasts of Maine, Nova Scotia, and the British Isles. The gum is commonly extracted at about 80∞C at pH 5 to 6 for 8 to 9 h. Isolation is based on disintegration of the plants in sodium carbonate, filtering, and precipitation of alginic acid with mineral acid. A unique feature of the usual isolation process for agar involves a freeze–melt extraction type of purification of the gellied extract prior to final drying. Alginic acid may be marketed as such or converted to sodium alginate. Sodium alginate is used primarily as a binder in frozen desserts, instant pudding mixes, and fabricated puddings. Sodium alginate has been used in a cold-processed lemon pie filling to provide freeze-thaw stability.

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Major suppliers are devoting substantial R&D efforts to developing novel applications, such as production of structured “imitation” fruit pieces that do not break down during baking. The nonsodium light metal salts of alginic acid are used as sodium alginate alternatives in low-sodium and dietary food specialties. 13.11.2.12 Xanthan Gum The FDA approved xanthan gum for food use in 1969. Since then, its scope of approved applications has greatly increased. Xanthan gum is produced by fermentation of carbohydrates using cultures of Xanthomonas campestris. The fermentation broth may be concentrated and sold as such for technical uses, or the polysaccharide may be recovered and purified by precipitation with alcohols. Xanthan gum is used in a wide range of relishes, syrups, sauces, bakery fillings, prepared puddings, glazes and toppings, beverage mixes, and fruit and carbonated beverages. A significant amount of food use is in dairy industry products, where it prevents the separation of the contained whey from the rest of the food product. Xanthan gum is showing some interesting synergies with other natural water-soluble polymers. Physical synergies can be observed between xanthan and guar gum, and xanthan and locust bean gum. Combinations of xanthan and locust bean gum can form gels whereas mixtures of guar and xanthan have a higher viscosity than each of these products alone (whereas xanthan is higher than either of the galactomannanes). 13.11.2.13 Gellan Gum Gellan gum is the latest hydrocolloid approved for food use. The product received FDA approval in 1990 for limited use in icings, frostings, bakery fillings, and low-solids jams and jellies, and in 1993, it was approved for use in all processed foods (Duxbury, 1993). It is also approved for food use in Japan, but it is not yet approved in European countries. Gellan gum is produced with a fermentation process (such as that used for the fermentation of xanthan gum), by the organism Auromonas elodea. There are two forms of gellan gum. The first is a high-acetyl gum that is partly acetylated and provides thermo-reversible gels. The second is a low-acetyl gum forming a firmer and more brittle gel. It forms gels at use levels as low as 0.05% and is stable in a pH range of 3.5 to 8. 13.11.2.14 Cellulose Ethers Cellulose ethers include the carboxymethyl, hydroxyalkyl, methyl, and ethyl derivaties. These ethers are produced by reacting alkali cellulose with the appropriate halide, sulfate, or epoxide. 13.11.2.14.1 Carboxymethylcellulose (CMC) CMC is the primary cellulose ether consumed in food and beverage applications. In fruit-related products, CMC is used in the preparation of frozen dairy products, beverages, bakers’ goods, dry drink mixes, syrups, glazes, and icings and toppings. The current search for microwavecompatible food additives makes CMC a candidate for this rapidly growing formulated food market area. CMC, a nonnutritive substance, is also popular in diet food formulations requiring thickeners and stabilizers. 13.11.2.14.2 Other Cellulose Ethers Methylcellulose (MC) and hydroxypropylcellulose (HPC) are also used in specialized food and beverage applications, but their relatively high market prices preclude them from volume applications in processed fruit.

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13.11.2.15 Other Hydrocolloids A number of other thickening agents have been used by the U.S. food industry. They are currently used infrequently commercially and represent a very minor portion of the U.S. food additives market. Most are higher priced, in erratic supply, and increasingly face competition from the principally used thickeners. Such other thickening and stabilizing agents and their principal uses include the following: • • • • •

Ghatti gum is an exudate from the tree Anogeissus latifolia, obtained from India and Sri Lanka; no functional properties other than thickening and emulsion stabilization exist. Tragacanth gum is the exudate from several species of Astragalus, obtained from the Middle East and used in salad dressings and sauces. Karaya gum is the exudate of Sterculia urens, a tree native in India and used for extreme thickening to pastelike gels. Tara gum is permitted for use in the U.S. It originates from the tara bush, indigenous in South America. This gum is structurally similar to guar and locust bean gum. Konjac flour, obtained by grinding the root of the Amorphophallus konjac plant, konjac is GRAS and is widely used in Asia in noodles and similar products. Clinical studies have shown that konjac flour added to some foods can cut serum cholesterol by 10% and blood glucose levels by 75%.

13.11.3 REGULATORY STATUS In the U.S., all hydrocolloids listed here are approved and classified as either food additives or GRAS compounds by the FDA (Code of Federal Regulations, Title 21, Parts 172.580–172.874 and Parts 182.1480–184.1724, respectively). Worldwide, most hydrocolloids are approved as having established food grade specifications in the Food Chemical Codex. Their safety has been confirmed by the Food Agriculture Organization/World Health Organization Joint Expert Committee on Food Additives and have European Economic Community (EEC) specifications as well. In general, natural hydrocolloids face increasing environmental regulation due to the large volumes of aqueous effluents and solids generated in processing.

13.11.4 APPLICATION

OF

GUMS

IN

FRUIT PRODUCTS

13.11.4.1 Water Dessert Gels In the U.S., gelatin is by far the most commonly used hydrocolloid for the preparation of water dessert gels. Solutions containing 1 to 2% gelatin form thermally reversible, elastic, clear gels when prepared with boiling water and cooled below room temperature. Dessert gels based on seaweed extracts are also used, primarily because of their ability to set up gels without refrigeration at room temperature. In countries where refrigeration is not common, particularly in tropical countries, this is an important advantage. The soluble salts of algin have the ability to enter into controlled chemical reactions with calcium or other polyvalent ions to form edible gels. This important and unique property has been utilized in formulating dry powdered mixes that are cold-water-soluble and set up gels at room temperature. The gels formed are chemically set, nonthermally reversible, and therefore will not melt at room temperature. Algin concentrations of 0.4 to 1% of the finished dessert are used. Carrageenan water solutions of 0.5 to 1% concentration form gels on heating and cooling, which set up at room temperature.

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13.11.4.2 Fruit Drinks, Juices, and Nectars CMC is used at approximately 0.5% to thicken fruit juices and to prevent floating or settling of fruit during preparation, as well as impart a clearer, brighter appearance, produce a desirable gel texture, and reduce syneresis. Alginates, particularly propylene glycol alginate is used at 0.1 to 0.2% concentration to suspend pulp in fruit drinks. Guar is used as a thickening and viscosity control agent in fruit nectars. Gum Arabic is used as an emulsifier in fruit flavors. Gelatin is used extensively for clarification or fining of cider, fruit juices, and wine. Among the products available for clarification, when applied correctly, gelatin does not modify the organoleptic quality of the finished product. Amount of gelatin used is in the range of 50 to 300 g/1000 l juice to be treated. 13.11.4.3 Pie Fillings and Jellies Starch is used as a primary thickener in most pie fillings; however, all-starch fillings have a firm starchy appearance that is not always desirable and may be subject to syneresis. By the selection of the proper starch, usually a modified one, and the proper gum, a variety of textures ranging from a semifluid to firm consistency can be prepared. In lemon pie fillings containing about 4% starch, the addition of 0.5% high-viscosity CMC gives firm body and prevents cracking, shrinkage, and syneresis. In canned or frozen fruit pie fillings, gums are used as a partial or complete replacement for starch because starches tend to retrograde in liquid media. The use of CMC has been recommended in canned berry, frozen peach, or cherry pie fillings at 0.2 to 0.5% alone or in combination with modified waxy maize starch to give a less opaque appearance on storage. MC is used in pie fillings to reduce absorption of water by the crust during baking, to protect flavor during baking, and to stabilize the gel after baking. The incorporation of 0.1% algin in bakery jellies reduces syneresis and improves spreading qualities and storage life. Algin, at approximately 0.3% reduces boil-off of juices during the preparation of fruit pies and controls weeping and spreading after preparation. Locust bean gum has been used as a stabilizer for canned berry pie fillings and in frozen pie fillings in combination with starch. Guar-starch combinations also have been used in frozen fruit pie fillings to prevent dehydration, shrinking, and cracking, as have tragacanth–starch combinations to provide thickening, clarity, and brilliance. 13.11.4.4 Jams, Jellies, and Preserves Most jellies, jams, and preserves produced in the U.S. utilize pectin as the gelling agent; however, at one time or another, all the hydrocolloids have been used or proposed for use in these products. Marmalade, jams, and jellies are one of the important uses of furcellaran gum, where it has the advantage of not requiring the prolonged boiling necessary with pectin, thus retaining many of the volatile flavor compounds in the fruit, giving a fresh flavor product. 13.11.4.5 Flavor Fixation Gum arabic is almost exclusively used as the fixative in spray-dried fruit flavors, which are extensively used in dry packaged mixes such as beverage powders, puddings, gelatin desserts, cake mixes, etc., where flavor stability and long shelf life are important. These spray-dried powdered flavors are produced by preparing an oil-in-water emulsion of citrus oils or other fruit flavors, with gum arabic as the emulsifier. The emulsion is then dried by spraying a fine mist of the liquid into a current of hot air. The flavor material is encapsulated in the gum arabic, resulting in a dry,

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free-flowing powder, and preventing evaporation of volatile constituents or oxidative deterioration of the flavor. 13.11.4.6 Dehydrated Fruits The rehydration properties of dehydrated fruit juices can be improved by the addition of gum prior to drying. CMC has been used for the preparation of dehydrated fruit juices to improve the resuspension and reconstitution properties. The use of MC as a drying aid in the preparation of spray-dried grapefruit and orange juice at 0.1 to 4 solids basis can facilitate the production of free-flowing powders. 13.11.4.7 Low-Calorie Food The market for low-calorie foods has had a rapid growth in recent years. In most of these products, sucrose and other sugars are replaced by high-intensity sweeteners such as saccharin or aspartame. The role of gums in these products is primarily to replace the viscosity and “mouth feel” lost when sugar or oil is removed. In still other products, such as puddings and pie fillings, gums may be used to replace all or part of the starch normally used (Hegenbart, 1993). The gum may also function as a bulking agent, a flavor-blending media, a masking agent for the aftertaste caused by artificial sweeteners, or to provide a feeling of fullness or satiety. The use of CMC has been reported to improve stability and inhibit syneresis in dietetic jams and jellies. A combination of 0.4% highviscosity CMC and 0.8% low-methoxy pectin has been recommended.

13.12 EMULSIFIERS 13.12.1 FUNCTIONS Emulsifiers are additives that allow normally immiscible liquids, such as oil and water, to form a stable mixture. Emulsifiers possess both hydrophilic and lipophilic groups within the same molecule; the ratio of hydrophilic to lipophilic groups, known as the HLB value, is a characteristic indicator for emulsifiers, which allows assessment of their action and performance. Emulsifiers are widely used in foods in order to perform one or more of the following functions (Stauffer, 1996): • • •

Increase stability and prevent phase separation in food emulsions (e.g., mayonnaise, salad dressings, etc.) Improve the shelf life of flavors and retard the onset of rancidity in fats and oils containing food emulsions Improve texture, reduce crumb firmness, and complex with starches (baked goods)

Recent interest in this category of food additives has been encouraged by consumers’ health consciousness. As most emulsifiers can decrease the amount of fat or oil in the product or add bulkiness or soften texture by increasing the amount of air suspended throughout the product, emulsifiers can serve to some degree as substitutes for fat and can decrease calories (MentzerMorrison, 1992). In the case of lecithin, the growth of its market is supported by some functions of lecithin itself, such as improvement in and/or prevention of adult diseases such as hypertension, arteriosclerosis, and hyperlipidemia.

13.12.2 PRODUCTS

AND

APPLICATIONS

The most common and commercially important emulsifiers are (Somogyi et al., 1996):

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• • • • • •

Monoglycerides and diglycerides of fatty acids and their esters (e.g., glyceryl monostearate) Lactylated esters (e.g., sodium stearoyl lactylate) Propylene glycol esters (e.g., propylene glycol monostearate) Lecithin Sorbitan esters (e.g., sorbitan monostearate) Polysorbates (e.g., polyoxyethylene sorbitan monolaurate)

A great number of other emulsifiers are also used by the food industry, but in smaller volumes. With the possible exception of lecithin, few emulsifiers are used as a single additive. Mostly, emulsifiers are offered as blends of emulsifiers, emulsifiers and water, emulsifiers and fats, or as blends with other functional additives (e.g., colors, flavors, gums for specialized applications, i.e., ice cream, baked goods, etc.). These products are formulated for specific applications or specific customers so that the combination provides both enhanced performance and ease of use. Emulsifiers may be divided into natural and synthetic. This division is significant because of consumers’ concerns related to use of artificial food additives. Within the natural emulsifiers, the lecithins are by far the main products. Raw lecithin is produced mainly from soybeans. Lecithin is the most widely used chocolate emulsifier. Other important applications include baked goods and dairy products. The more prominent food uses of synthetic emulsifiers are baking, shortenings, ice cream, imitation dairy products, prepared mixes, margarine, salad dressings, and miscellaneous prepared foods (e.g., confectionery, pasta, potatoes, and peanut butter). Application of emulsifiers in processed fruit is in fruit drinks, flavor emulsions, pie fillings, and fruit snacks. The addition of dill oil to processed pickle products requires an emulsifier to solubilize the oil in the brine. A hydrophilic emulsifier is used at a level of 2 parts to 80 parts of oil to produce a clear solution of the spice oil in the brine. Similarly, orange oil used to flavor orange drink or soda is solubilized with a hydrophilic emulsifier. The manufacturing method for glycerin fatty acid esters is either esterification of glycerin and fatty acid or ester exchange between fat and glycerin. The latter method is mainly used, yielding a mixture of monoglycerides, diglycerides, triglycerides, free fatty acids, and free glycerin. Monoglyceride is obtained by distillation, with a yield of at least 90%. About 80% of the total consumption of glyceride is distilled glycerides.

13.12.3 REGULATORY STATUS Classes of permitted emulsifiers, worldwide, include: • • • • • • • • • • • •

Mono- and diglycerides Acetylated monoglycerides Stearyl-2-lactilates Lactilated esters of fatty acids Diacetyl tartaric acid esters (DATEM) Polyglycerol esters and ethoxylates Sorbitan esters Citric acid esters Succinylated monoglycerides Lecithin Propylene glycol esters Sucrose fatty acid esters

In the U.S., lecithin, mono- and diglycerides, DATEM, and triethyl citrate are classified as GRAS by the FDA. The other emulsifiers are approved under multipurpose additive regulations that permit their use in specific products at set levels.

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In Europe, food emulsifiers are classified under EEC Council Directive reference numbers. Examples are mono- and diglyceride of fatty acids–E471; acetic, citric lactic, and phosphoric esters of mono- and diglycerides–E472 a–f; and polyglycerol esters of fatty acids–E475. Sorbitan esters are not approved for food use in Western Europe (only in cosmetics and pharmaceuticals). Propylene glycol esters are not approved in Germany. In Japan, glycerin fatty acid esters, sorbitan fatty acid esters, propylene glycol fatty acid esters, sucrose fatty acid esters, and lecithin are listed in the positive list of food additives. Also, saponins, phytosterol, and other natural products are permitted.

13.13 FLAVORS 13.13.1 PRODUCTS

AND

FUNCTION

Flavors are concentrated preparations used to impart a specific aroma to a food or beverage. This group of additives comprises more than 3000 individual products. Fruits are frequently utilized as sources of raw material for the preparation of many flavor compounds (e.g., lemon oil and berry concentrates, etc.). In the U.S., natural fruit flavors represent over one third of the flavor market. Annual consumption of natural flavors are estimated at $400 million in 2000 (Somogyi and Kishi, 2001). Flavors may be added to food products for the following reasons (Giese, 1994): • • • • •

To create a totally new taste (e.g., Coca-ColaTM) To enhance, extend, round out, or increase the potency of flavors already present To supplement or replace flavors to compensate for losses during processing To simulate other more expensive flavors or replace unavailable flavors To mask less desirable flavors, i.e., to cover harsh or undesirable tastes naturally present in some food (rather than to hide spoilage)

Some of the large specific products are as follows (Somogyi and Kishi, 2001): •

• •

Essential oils and natural extracts: aromatic raw materials isolated or concentrated from botanical or animal sources by processes such as distillation, expression, solvent extraction, and maceration. Included are vanilla, cocoa, cola, and spice oleoresins, etc. They are used to compound flavors and fragrances to impart flavor, color, and aroma to products. Aroma chemicals: aromatic chemically defined structures prepared by synthesis or isolated from natural sources (e.g., menthol from cornmint oil). Included are anethole, vanillin, citronellol, and esters, geraniol/nerol and esters, Linalool and esters, etc. Compounded flavors: complex blends/mixtures of various components containing from few to 100 or more constituents. Compounded flavors may contain aroma chemicals, natural extracts, essential oils, solvents, and in some cases other functional ingredients (e.g., antioxidants and emulsifiers, etc.). They may be concentrated, diluted with solvents, or bound to carriers (e.g., free-flowing powder). These are the flavors of commerce utilized by the food, tobacco, cosmetics, and pharmaceutical industry. They are usually customized for a specific application and/or specific customers.

13.13.2 TECHNOLOGY

OF

MANUFACTURING

A flowsheet depicting the preparation of flavors from starting materials to the end product is presented in Figure 13.2. 13.13.2.1 Artificial Flavors Artificial flavors are substances that are chemically synthesized for flavor applications. They originate from other than natural sources. They are available in liquid and dry form. The production

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PETROCHEMICAL RAW MATERIALS

PLANTS, ANIMALS

Agriculture

• Steam Distillation • Extraction • Expression Synthetic Chemistry

Chemical Industry

Separation Distillation Modification AROMA CHEMICALS Flavor and Fragrance Industry

Also: Specialized Blenders for Semimanufactured Flavors

ESSENTIAL OILS, EXUDATES, SECRETIONS • Processing • Concentrating • Dissolving NATURAL RAW MATERIALS

• Compounding • Mixing • Blending

Essential Oil and Extract Industry

Flavor and Fragrance Industry

FLAVOR COMPOSITIONS

CUSTOMERS: MANUFACTURERS OF: FOOD AND BEVERAGES

FIGURE 13.2 The flavor industry: flow of manufacturing processes for flavors.

process of artificial flavors consists of compounding or mixing together the various aroma chemicals specified in the formula. In the case of dried flavors, a spray-drying process is added after mixing. Compounds having characteristic functional groups are incorporated to artificial flavor compositions. For example, esters give the characteristic fruity aroma, a-undecalactone is usually included in a typical peach flavor composition, and ethyl methyl phenyl glycidate imparts strawberry flavor character. For the production of synthetic aroma chemicals, a chemical plant is needed, consisting primarily of multipurpose reactors and some specialized equipment for special reactions (nitration), as well as crystallization devices and drying chambers for solid products. The most important equipment is for efficient distillation (solvent stripping, purification of raw products) and fractional distillations (separation of products from mixtures). These purification procedures determine the quality of the aroma chemical. Most artificial flavors are produced on short, customized, batch runs. 13.13.2.2 Natural Flavors Natural flavors include essential oil, oleoresin, essence or extractive, distillate, or any product of roasting, heating, or enzymolysis that contains the flavoring constituents derived from a spice; fruit or fruit juice; vegetable or vegetable juice; edible yeast, herb, bark, bud, root, leaf or similar plant material; meat, seafood, poultry, egg, dairy products; or fermentation product thereof, whose significant function in food is flavoring rather than nutritional. In practice, natural flavors are essential oils, oleoresins, and true fruit extracts. The other items are raw materials for compounding flavors. The following aromatic compounds are responsible for the distinct flavor of fruits (Reineccius, 1994): • • • •

Alcohols in the homologous series C1 to C12 series Carbonyls (benzaldehyde, furfural, etc.) Volatile acids (formic and acetic) Esters

Direct Food Additives in Fruit Processing

• •

317

Lactones Phenols

Essential oils and essences are aromatic volatile fractions obtained from plant material through various methods. In processing essential oils and natural extracts, distillation (stream, vacuum, and fractional, etc.), solvent extraction, expression (or pressing), and enzymolysis are the major methods (Fischetti, 1980). Purification of these oils and extracts is achieved mainly by various types of distillation (normal, steam, vacuum, molecular, and hydrodiffusion). Natural flavors can be compounded from a wide range of ingredients to produce flavor types in concentrations sufficiently high to satisfy the requirements of processed foods. Compounding is a technically simple operation. In a kind of open pot (usually stainless steel, capacity from 4 l to 10,000 l) equipped with a mixing device and possibly a heating coil, the ingredients are added and thoroughly mixed, and then packed into appropriate containers. In a compounding plant system, pipes deliver solvents and large-volume liquid ingredients from a tank farm. Small-volume constituents are weighed in and added manually. In view of this and the large number of different composition produced in a given plant, the possibilities for automation are limited. A special type of natural flavor is fruit flavor concentrate. This type is defined as a fruit concentrate or puree where some of the water is removed and natural flavors are added, as well as possibly gums, starches, and colors. Fruit flavor concentrates are used in yogurt, baked goods, snacks, and beverages. The most common fruit-flavor concentrates include apple, berry, grape, and citrus products (Giese, 1994). Compositions prepared by compounding are always liquid. For some flavor applications, liquid flavors are encapsulated, primarily by spray-drying an emulsion containing the liquid flavor and a carrier (gum arabic, starch, etc.). Powdered flavors usually contain 10 to 25% of the liquid flavor composition.

13.13.3 REGULATORY STATUS Safe use of flavoring materials was outlined by the 1958 Food Additives Amendment. The law permits that an expert panel, qualified to determine the safety of flavoring materials, provides GRAS flavor status to a material that could then be used without further testing. An expert panel sponsored by the Flavor and Extract Manufacturers Association (FEMA) issues lists of GRAS flavor materials. All of the FEMA GRAS lists have been published by Food Technology; the most recent list, the 21st, was published in 2003 (Smith et al., 2003). The flavor-labeling regulations (Federal Code of Regulations, Title 21 and 101.22) describe how flavors must be declared in food labels. Spices and natural and artificial flavors may be declared as such in the ingredient listing of the product without naming specific flavoring ingredients. This inclusive labeling of flavors may be done as long as no flavor representation is made on the package of food. If a flavor representation is made on the package of the food (e.g., “lemon drink”), the name of the food must indicate the nature of the flavor (e.g., “artificially flavored”). U.S. labeling laws distinguish as natural only those flavor compositions containing chemicals extracted from natural sources produced by fermentation or made enzymatically from naturally occurring ingredients. Western European legislation considers as natural those flavors compounded with natural and/or nature-identical ingredients. The narrower definition curtails the latitude of food processors to label the food natural in the U.S. and serves to stimulate the U.S. demand at premium prices for natural ingredients (Somogyi and Kishi, 2001). In addition, federal “Standards of Identity” regulations (CFR Title 21: Part 145 for canned fruit and fruit juice products, Part 150 for fruit butters, jellies, preserves, and related products) covering many canned and frozen fruit products and jams allow only the inclusion of natural flavoring substances in these products.

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Following the lead of the U.S., inclusion in a positive list (compiled by a federal agency) that spells out which chemicals are permitted for food use has become the prevalent basis for legislation for regulating flavor chemicals worldwide. Most industrial countries have their own regulations and control over flavoring use, and legislation is frequently changing Bauer (1994). Salzer and Jones (1998) compiled an update on worldwide flavor legislation.

13.13.4 USE

OF

FLAVORS

IN

PROCESSED FRUITS

Fruit flavors, both natural and synthetic products, represent the largest product category of the flavor industry, encompassing about 48% of the sales value. Flavors are utilized in a wide range of products. Fruit beverages (including alcoholic beverages) are the largest user of fruit flavors, followed by desserts, dairy products, confectioneries, baked goods, snacks, jellies, and jams. The most frequent usage of fruit flavors is the citrus varieties by the beverage industry.

13.14 COLORS 13.14.1 FUNCTIONS Colors are used in foods to improve appearance and thereby influence the perception of texture and taste. For centuries, natural color materials have been mixed with foods in an attempt to improve appearance. When synthetic colors were introduced in the late 19th century, they were immediately adopted by the food industry. These were certified FD&C color additives that were allowed for use in foods according to the Federal Food, Drug and Cosmetic Act of 1938 and its amendments, and food color additives (mostly natural materials) that were exempt from FD&C certification. In order to market their products, U.S. producers of certified colors must submit product samples from each batch of material to the FDA. The U.S. Food and Drug Administration analyses these materials to see that they meet specific purity classifications. In other parts of the world, only selfcertification exists. Food colors may be added to food for the following reasons (Francis, 1998): • • • • • • •

To restore the original appearance of the food where the natural colors have been destroyed by heat processing (only occasionally in canned fruits) and with subsequent storage To ensure uniformity of color due to natural variations in color intensity, e.g., fruits obtained at different times during the season, thereby assuring uniformity in appearance and acceptability To intensify colors naturally occurring in foods where the color is weaker than that which the consumer associates with a food of that type of flavor (e.g., fruit yogurts, sauces, soft drinks) To help protect flavor and light-sensitive vitamins during shelf storage by a sunscreen effect To give attractive appearance to foods that would otherwise look unattractive or unappetizing (e.g., colorless gelatin-based jelly) and thus enhance enjoyment To help preserve the identity or character by which foods are recognized, i.e., product identification To serve as visual indication of quality — thus, in addition to enhancing the acceptability of foods, colors aid in food manufacture, storage, and quality control

13.14.2 PRODUCTS 13.14.2.1 Certified Food Colors Certified food colors can be divided into dyes and lakes. Of the eight colors certified for use in food in the U.S. (Table 13.6), eight can be used as dyes (one, Citrus Red No. 2, is limited to use

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TABLE 13.6 FD&C Certified Dye and Lake Colors Approved for Use in the U.S. Color

FD&C Dye

FD&C Lake

Blue No. 1 Blue No. 2 Red No. 3a Red No. 40 Yellow No. 5 Yellow No. 6 Green No. 3 Citrus Red No. 2

X X X X X X X X

X X X X

Notes: The color additive Orange B (which is listed for use in casings of Frankfurters and sausages) has not been manufactured in recent years. A proposal by FDA to ban the permanently listed uses of Red No. 3 is still pending. a

Source: Report of Certification, U.S. Food and Drug Administration.

in coloring orange skins) and five as lakes. Dyes are water-soluble compounds that impart color to a substance through dissolution. Color regulators specify a minimum of 85% pure dye for primary colors, but most dye lots contain from 90 to 93% pure dye. Certified dyes fall into several chemical classes: azo-dyes (Yellow No. 5, Yellow No. 6, Red No. 40, Citrus No. 2), triphenyl-methane dyes (Blue No. 1, Green No. 3), xanthine type (Red No. 3), and sulfonated indigo (Blue No. 2). Table 13.7 shows the physical properties of these eight certified food colorants. Certified food colorants, both primary and blends, are produced in a variety of forms including powder, liquid, granules, paste, and dispersion. FD&C dyes are also used in the production of lakes, which are pigments prepared by combining a certified dye with an insoluble alumina hydrate substratum. Lakes are both water- and oil-insoluble and impart color through dispersion in food. Thus, they are suitable for coloring foods that cannot tolerate water and products in which the presence of water is undesirable, such as dry beverage bases and dessert powders. The FD&C lakes do not have a legal specified minimum dye content; manufacturers use formulations of from 11% (standard) to 42% pure dye (concentrated). Certified food colors are used in most products in the 50 to 300 ppm ranges. 13.14.2.2 Noncertified Food Colors Noncertified colors can be from either natural origins (primary sources), such as vegetables (carrot oil, red beet juice, paprika, etc.) and fruits (grape skin, cranberry juice concentrate), or produced synthetically (e.g., ferrous gluconate, apo-carotenal, riboflavin, etc.) (Linden and Lorient, 1999). Food color additives exempt from certification in the U.S., their colors are listed in Table 13.8. In general, the traditional markets for noncertified food colors have been the lipid-based, highfat food systems such as butter, margarine, shortening, popcorn oil, processed cheeses and spreads, salad dressing, and snack foods (Somogyi et al, 1996). Water-soluble forms are often also available, and other applications include baked foods, fruit juice drinks, confections, and dairy products. Certain noncertified food colors are important in the beverage industry. For example, caramel is used in beverages, primarily in cola drinks, and synthetic carotenoids are important in orange drinks (Borenstein and Bunnel, 1967). No doubt technological advances will lead to new applications for the noncertified colors, as well as wider use in existing markets.

N/A Fair

Citrus Red Fast Green FCF

a

N/A Poor

Poor

Poor

Fair Fair Fair Fair

Stability to Oxidation

N/A Good

Good (unstable in alkali) Poor

Poor Good Good Good

Stability to pH Change

N/A Good

Very poor

Good

Poor Very good Good Good

Compatibility with Food Components

Food use restricted to coloring skins of oranges at a level not to exceed 2 ppm by weight (not used in processing).

N/A = not applicable.

Citrus Red No. 2 Green No. 3

Very poor

Sodium Indigo Disulfonates a

Blue No. 2

Fair

Brilliant Blue FCF

Blue No. 1

Fair Very good Fair Good

Erythrosine Allura Red AC Sunset Yellow FCF Tartrazine

Common Name

Red No. 3 Red No. 40 Yellow No. 6 Yellow No. 5

FD&C Name

Stability to Light

TABLE 13.7 Physical and Chemical Properties of Certified U.S. Colorants

Good Excellent

Poor

Excellent

Very good Very good Good Good

Tinctorial Strength

Orange–red Blue

Deep–blue

Blue Yellow Red Lemon yellow Green-blue

Hue

Insoluble 20

1.6

20

9 25 19 20

Water Solubility

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TABLE 13.8 Food Color Additives Exempt from Certification Color Additive

Color

Source

Annatto extract Beet juice Dehydrated beets (beet powder) Canthaxanthina Caramel Apocarotenalb Beta-carotene Carrot oil Cochineal extract (carmine) Cooked cottonseed flour Ferrous gluconatec Fruit juice (grape and cranberry) Grape skin extractd (enocianina) Paprika Paprika oleoresin Riboflavin Saffron Titanium dioxidee Turmeric Turmeric oleoresin Vegetable juice

Yellow Red Purple Red Brown Orange Yellow Yellow Red Brown Yellow Red Red Red Red Yellow Yellow White Yellow Yellow Red

Vegetable Vegetable Vegetable Synthetic Semisynthetic Synthetic Synthetic Vegetable Insect Semisynthetic Synthetic Fruit Fruit Vegetable Vegetable Synthetic Vegetable Synthetic Vegetable Vegetable Beet and red cabbage juice

Notes: Under the Code of Federal Regulations, Title 21. No color additive may be used in foods for which standards of identity have been promulgated under Section 401 of the Federal Food, Drug & Cosmetic Act, unless the use of added color is authorized by such standards. a b c d e

May not exceed 66 mg/kg of solid, or 1 pint of liquid, food. May not exceed 33 mg/kg of solid, or 1 pint of liquid, food. Coloring ripe olives only. Used only in beverages. May not exceed 1% by weight of the food.

Although a great number of noncertified food colorants are used by the U.S. food processing industry, demand is concentrated in a few products, namely paprika, caramel (principal use is in cola beverages), annatto, and synthetic carotenoids.

13.14.3 USE

OF

COLOR ADDITIVES

IN

FRUIT PRODUCTS

In the U.S., most canned fruit products, jams, and pure juice (canned, frozen) are regulated by Standard of Identity that prohibits the use of food colorants. Fruit products that are colored by synthetic or natural colorants are fruit drinks, fruit beverage powders, pie fillings, fruit desserts, maraschino cherries, apple rings, crab apples, flavored apple sauces, and certain jellies.

13.14.4 TRENDS

AND

REGULATORY STATUS

As a general guideline, in the U.S., FD&C dyes and lakes can be used in any food product unless otherwise prohibited by special regulations such as the Standard of Identity.

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TABLE 13.9 Major Western European Synthetic Dyes for Food Applications Color Tarazine Yellow Orange S Amaranth Cochineal Red A Erythrosine Patent Blue Indigotin

EC No. E E E E E E E

102 110 123 124 127 131 132

Application Desserts, lemonades, pasta, ice cream, syrup Fruit juices, fruit preserves Candies, ice cream, jams, fruit preserves Candies, ice cream, jams, beverages, desserts Fruits, maraschino cherries, ice cream Candies, beverages Candies, beverages

In the years since 1970, food colors, especially the synthetic organic colors, have received tremendous publicity — nearly all of it bad. Food colors have been the prime target of consumer activists. As a result of this, the food industry prefers the use of natural colorants in deference to synthetic colorants. Due to inadequate heat stability of most natural colorants, however, the FD&C certified synthetic colorants are expected to continue to be used by the U.S. food industry. A number of formerly certified FD&C colors have been “delisted” under the provisions of the Delaney Clause of the Food, Drug and Cosmetic Act, either because they were found to be carcinogenic or because there was no assurance that they could be made free from carcinogenic impurities. These actions have steadily reduced the number of certified colors available to the U.S. food industry (Table 13.6). The technical, safety, and legal issues of whether some synthetic colorants are carcinogenic is expected to continue. Presently, all eight certified colors are permanently listed as color additives by the FDA regulations, while all lake colors except Red No. 40 are provisionally listed. In 1990, the FDA banned FD&C Red No. 3 lake colors and imposed restrictions on the use of Red No. 3 dye. Some replacement colors have been suggested such as Red No. 40, carmine, cochineal extract, and beet concentrates, but no equivalent viable alternative colors are yet available for many food products. Opportunities for inventions of new color additives are highly limited because of the strict regulations for colors required for FDA approval. New product possibilities include colors produced by fermentation. As an example, an alga that has high beta-carotene content is being developed by biotechnology. Food application approval of a synthetic azo-dye carmazine (Yellow No. 10) has been petitioned, and presently, the compound is undergoing animal testing. Carmazine would be more stable and similar in color to FD&C Yellow No. 5. The Nutrition Education and Labeling Act of 1990 requires that all certified colors be specifically declared by name. Prior to this regulation, food processors were required only to put “food color” on the label with the exception of Yellow No. 5 (tartazine), which has been shown to cause an allergic reaction in 2% of the population. There is some increasing consumer concern about the use of colorants in food preparations in Western Europe. In line with this, a distinct trend toward the use of natural colors can be observed. The principal synthetic colorings in Western Europe are shown in Table 13.9. As for the natural colorings, caramel color accounts for over 90% of all coloring used in food. Natural colors approved in Western Europe include annatto, cochineal, carmine, curcumin (extracted from turmeric), chlorophyll derivatives, spray-dried beet root juice, or spices with a secondary coloring effect. In Japan, color additives are under regulation by the Food Sanitation Law. The legal definition is somewhat different from the general notion about synthetic (Table 13.10) and natural food colors (Table 13.11). In general, the term natural color additives refers to a substance that satisfies one of the conditions in the following list:

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TABLE 13.10 Synthetic Colors Approved for Food Use in Japan

• • • • • • •

Color

Common Name

Dye

Food Red No. 2 Food Red No. 3 Food Red No. 40 Food Red No. 102 Food Red No. 104 Food Red No. 105 Food Red No. 106 Food Yellow No. 5 (U.S. FD&C No. 6) Food Yellow No. 4 (U.S. FD&C No. 5) Food Blue No. 1 Food Blue No. 2

Amaranth Erythrosine Allura Red AC New Coccine Phloxine Rose Bengal Acid Red Sunset Yellow FCF Tartrazine Brilliant Blue FCF Indigo Carmine

X X X X X X X X X X X

Lake

X

X X X X

Color agents obtained from living organisms Chemically modified color agents originally derived from natural resources Chemically synthesized color substances that are exactly the same structure as naturally existing color agents Colors obtained by crushing dry plants or animals The squeezed juice of plants or animals that contain color agents Colors isolated from a microbial medium Colors obtained by enzymatic or microbial treatment of a natural substance

Since 1987, certificates from the government for dye colors are no longer required in Japan, having been replaced by self-imposed inspection.

13.15 VITAMINS 13.15.1 FUNCTIONS Vitamins are nutritive substances required for normal growth and maintenance of life. They play an essential role in regulating metabolism, converting fat and carbohydrates into energy, and forming tissues and bones. A total of 13 vitamins are recognized as essential for human health, and deficiency diseases occur if any one is lacking. Because the human body cannot synthesize most vitamins, they must be added to the diet. Most vitamins are currently consumed as pharmaceutical preparations or over-the-counter vitamin supplements. Some, like vitamins A, C, D, and E and folic acid, however, are directly added to food products. They are added for several related reasons to: • • • • •

Restore vital nutrients lost during processing — important with canned fruits, fruit juice, and refined and processed foods Standardize nutrient levels in foods when these fluctuate due to seasonal variations, soil differences, and methods of preparation Fortify fabricated foods that are low in nutrients and promoted as substitutes for traditional products, including complete fruit drinks and imitation products Fortify a major staple, such as bread, with a nutrient known to be in short supply Prepare functional foods (nutraceuticals) containing vitamins that are shown to be useful in preventing chronic diseases

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TABLE 13.11 Japanese Natural Color Additives for Food Processing Name

Color

Alkanet color Annatto extract Cacao color Carob germ color Cowberry color Elderberry color Gooseberry color Japanese persimmon Kusagi color Licorice color Madder color Orange color Red cabbage color Turmeric oleoresin/curcumin Uguisukagura color

Red–red purple Yellow–orange Brown Yellow Red–blue Red–blue Red–blue Dark red Blue Yellow Yellow–red purple Yellow Red–purple red Yellow Red–blue

Colorant Alkanin (naphthoquinone) Bixin, norbixin (carotenoid) Flavonoid Flavonoid Anthocyanin Anthocyanin Anthocyanin Flavonoid Trichotomin Isoliqueritigenin (flavonoid) Rubrtorin (anthraquinone) Xantophil (carotenoid), carotene Anthocyanin Curcumin Anthocyanin

Japanese Foodstuffs Also Used as Nonsynthetic Color Additives Fruit Juice Uguisukagura juice Elderberry juice Orange juice Cowberry juice Gooseberry juice Salmonberry juice Thimbleberry juice Strawberry juice Dark sweet cherry juice Cherry juice Dewberry juice Huckleberry juice Grape Juice

Blueberry juice Blackberry juice Black current juice Plum juice Berry juice Boysenberry juice Whortleberry juice Mulberry juice Morello cherry juice Raspberry juice Red currant juice Loganberry juice

Vegetable Juice Red cabbage juice Beet red juice Beefsteak plant juice Onion juice Tomato juice Carrot juice Rennet casein

Others Paprika Saffron

Source: Food Chemistry Division, Japanese Ministry of Health and Welfare, 1996.

While the addition of vitamins is practiced primarily for nutritional purposes, vitamins may also be used as functional ingredients in foods. The antioxidant vitamins C and E frequently are utilized to prevent undesirable color changes and to retard the development of rancid flavor. Riboflavin (vitamin B2: provides a greenish lemon-yellow color) and beta-carotene (pro-vitamin A: provides an orange color) are utilized as natural coloring substances.

13.15.2 PRODUCTS Vitamins are usually divided into two groups: fat-soluble and water-soluble. The fat-soluble group consists of vitamins A, D, E, and K. The water-soluble group consists of vitamin C (ascorbic acid), as well as the B vitamins. Eight nutritive B complex vitamins are required for health reasons by humans. These are niacin, riboflavin, pantothenic acid, pyridoxine, folic acid, thiamin, biotin, and vitamin B12.

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TABLE 13.12 Major Vitamins and Their Marketed Forms Consumed as Food Additives Principal Synonyms

Vitamin Retinol

Vitamin A Vitamin B Niacin

Vitamin B3

Thiamine Riboflavin Pantothenic acid Pyridoxine Cyanocobalamin Biotin Folic acid Vitamin C

Vitamin B1 Vitamin B2 Vitamin B5 Vitamin B6 Vitamin B12 Vitamin H or B8 Vitamin Bc or Bg Ascorbic acid

Vitamin D D2 D3 Vitamin E Tocopherols

DL-alpha tocopherol

Vitamin K K1 K3

Phytonadione Menadione

Major Market Forms Vitamin A acetate Vitamin A palmitate Nicotinic acid Niacinamide Thiamine hydrochloride Riboflavin Calcium pantothenate Pyridoxine hydrochloride

Folic acid Ascorbic acid Sodium ascorbate Calcium ascorbate Ascorbyl palmitate

Ergocalciferol Cholecalciferol DL-alpha tocopherol acetate DL-alpha tocopherol D-alpha tocopherol Phylloquinone

Menadione sodium bisulfite

The most commonly marketed forms of the major vitamins consumed in the U.S. as food additives are listed in Table 13.12. Vitamin C (ascorbic acid) is the most commercially important vitamin used as a food additive in terms of volume. Approximately 30 to 40% of total U.S. demand for this product is as a nutritional additive for foods and beverages. Vitamin C occurs naturally in citrus fruits, berries, and most other fruits. It is commercially produced exclusively from glucose through hydrogenation to sorbitol, followed by fermentation of sorbitol to L-sorbose, and several steps of chemical synthesis and purification procedures.

13.15.3 APPLICATIONS The most important applications for vitamin C include fruit juices, fruit-flavored drinks, juiceadded sodas, and dry cocktail or beverage powder mixes. As an antioxidant, this vitamin is frequently added to fruit juice to preserve and protect against color change of the fruit ingredients. By doing so, these products are also promoted as the high vitamin sources. In fruit processing operations, primarily following blanching, thermal processing, and freezing, significant losses in ascorbic acid and other nutrients were observed. Further losses amounting to 26 to 47% of ascorbic acid occurred in canned fruits during two-years’ of storage at 80∞F. Therefore, an excess of all vitamins must be added to insure that the food contains the amount declared on

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the label after the intended storage period (Borenstein, 1975). It is necessary to add as much as 30% higher an amount over the nutrition claim. Besides vitamin C, most essential vitamins are largely added to milk, breakfast cereal, and meal replacement products; however, beta-carotene is utilized in orange flavor fruit drinks and fruit snacks, also called “energy bar” products aimed at health-conscious customers and often fortified with several essential vitamins.

13.15.4 REGULATORY STATUS In the U.S., vitamins are included in the GRAS list of the FDA under dietary supplements (21 CFR 182, Subpart F) or under nutrients (subpart I). The exception is calcium pantothenate, which is cleared as a food additive for special dietary uses and requires a label statement of its appropriate concentration, expressed as pantothenate. In Western Europe, vitamins are not regarded as food additives unless they are fulfilling such additional functions as: • •

Antioxidants: ascorbic acid (vitamin C), tocopherols (vitamin E) Colors: riboflavin-5-phosphate (vitamin B2)

In Japan, vitamins used as food additives are classified as dietary supplements under Japanese regulations. No maximum-use or labeling obligations are legislated for vitamins. Vitamin E (D g tocopherol) is not included in this category because its use is approved only as an antioxidant. Vitamin C is used as an antioxidant (see the antioxidant section). Riboflavin and its derivatives are used as vitamin supplements, as well as color additives.

13.16 PRESERVATIVES 13.16.1 FUNCTIONS In the broadest sense, a chemical preservative may be defined as any additive substance that tends to prevent or retard deterioration when added to foods. It may prevent or retard changes in odor, flavor, nutritive value, or appearance; therefore, chemical preservatives fall into three main categories: antimicrobial agents, antioxidants, and sequesterants. In this section, only the antimicrobial agents utilized in fruit preservation are discussed. Economic loss can be considerable without adequate control of undesirable microorganisms in production, transit, storage, and distribution of fruits. Antimicrobial agents inhibit the contamination of foods by microorganisms such as yeasts, bacteria, molds, or fungi. The principal mechanisms are reduced water availability and increased acidity. They may also preserve, to some degree, other important food characteristics such as flavor, color, texture, and nutritional value. The number of antimicrobial preservatives approved for use in food is remarkably limited. The primary food additives used in the U.S. for this function are: • • • •

Sorbic acid and its potassium salt Calcium and sodium propionates Sodium benzoate Parabens

Only sorbates, benzoates, propionates, and sulfites are used broadly in fruit processing. Although these preservatives are sometimes applied alone, usually they are used in conjunction with other methods of preservation such as refrigeration, freezing, and dehydration to obtain more thorough control of deleterious microorganisms. Also, several organic acids such as citric, malic,

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TABLE 13.13 Factors Influencing the Effectiveness of Antimicrobial Preservatives in Processed Fruits Food Composition Moisture content Presence of other inhibitors Salt Sugar Spices Hydrogen ion concentration (pH) Contamination Sanitary condition of ingredients and equipment Processing methods and control Temperature (time vs. temperature) Filtration Radiation Types of microorganisms present Handling of Processed Product Packaging Length of storage Storage environment (temperature, humidity, etc.)

lactic, ascorbic, erythorbic acid, and tartaric function as preservatives, but because of their greater use as acidulants or antioxidants, they are covered elsewhere in this chapter. While sulfites have an antimicrobial effect as well and are frequently utilized in dried fruits, usually their primary role is to function as an antioxidant (sulfites are covered in the antioxidant section). Chemical preservatives play a very important role in the fruit processing industries, from manufacture through distribution to the ultimate consumer. The choice of a preservative takes into consideration the product to be preserved, the type of spoilage organism endemic to it, the pH of the product, period of shelf life, ease of application, and so on (Table 13.13). No one preservative can be used in every product to control all organisms, and therefore, combinations are often used. In certain foods, specific preservatives have very little competition. In the concentrations used in practice, none of the preservatives discussed here is lethal to microorganisms in foods. They therefore will not reduce existing contamination but, instead, retard further growth of organisms already present, provided the degree of contamination is not too high; therefore, products that have already spoiled or fruits prepared under poor sanitary conditions will not benefit from the use of these preservatives.

13.16.2 PRODUCTS 13.16.2.1 Benzoic Acid Benzoic acid, usually in the form of the potassium or sodium benzoate salt, is most effective in the pH range of 2.5 to 4. The food processors, because of the low solubility of the free acid prefer the salt forms. Important uses of benzoates are in fruit juices and drinks, jams and jellies, pie fillings, fresh fruit cocktails, pickles, and condiments. In carbonated drinks, 0.03 to 0.05% is used;

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in noncarbonated drinks, up to 0.1% is used. The potassium benzoate salt form is utilized preferentially over sodium benzoate in many food products. The potassium salt form was developed specifically for use in reduced or low-sodium food products. Benzoates are more active against yeasts and molds and less effective against bacteria. It is not recommended for bacterial control because the use level is restricted to 0.1%, and the activity is poor above pH 4 where bacteria is the greatest problem. It is of interest that benzoic acid occurs naturally in some fruits such as cranberries, plums, and olives. 13.16.2.2 Sorbic Acid Sorbic acid is an unsaturated organic acid. Since 1955, it has been used as a food preservative. The free acid and its potassium salt forms, collectively referred to as sorbates, are used in food systems. They exhibit a broad spectrum of antimicrobial activity; they are more effective against yeast and mold than against bacteria. Sorbates function in an inhibitory microbial activity to a maximum pH of 6.5, which is sufficient range for most fruit products. With its neutral taste character, it is a choice preservative. Potassium sorbate salt is used where high water solubility is desired. Sorbates are frequently used in dried fruits, fruit salads, and carbonated and noncarbonated beverages. Usage rates of sorbates in fruits are low, being 0.025 to 0.075% in fruit drinks, 0.1% in beverage syrups, 0.05 to 0.1% in pie fillings, and 0.02 to 0.05% in dried fruits. 13.16.2.3 Propionic Acid Propionic acid and its sodium and calcium salts are highly effective mold inhibitors but have essentially no effect against yeast and bacteria. Because of its corrosive nature, propionic acid, a liquid, is rarely used in the food industry. The sodium and calcium salts are used in its place, yielding the free acid within the food at low pH. Their effectiveness against yeast extends up to pH 6; therefore, the main market for propionate salts is in bakery products, chiefly because these salts do not inhibit yeast action. In fruit product applications of 0.2 to 0.4%, propionates are recommended to retard mold growth on blanched apple slices, figs, cherries, blackberries, and dried plums.

13.16.3 REGULATORY STATUS Sorbates, benzonates, and propionates are included in the FDA GRAS list. Sodium and potassium benzoates are restricted to a maximum level of 0.1% in foods that are not covered by Standards of Identity. There is presently no upper limit imposed on the use of sorbates and propionates in foods. Labels of foods in which these preservatives are used must bear a statement of the common name of the compound and its purpose, such as “added to retard mold growth” or “added as preservative.” These preservatives are also approved in the western European countries and in Japan.

13.16.4 TRENDS

AND ISSUES

The chemical synthesis methods of producing chemical preservatives are well established. No new novel processes for these compounds have been commercialized in recent years. In general, increased demand by consumers for processed and prepared foods, as people tend to do less cooking at home, has stimulated use of antimicrobial preservatives over the past several years. At the same time, however, media and consumer reaction to chemical preservatives has stymied or limited the growth of several preservatives in favor of “all-natural” and “no preservatives added” food products. The irradiation of foods to prevent microbial growth, instead of the use of chemical preservative additives, has been under discussion for a number of years. However, it does not appear that this

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approach to preventing growth of molds and bacteria will have a significant detrimental impact on demand for chemical preservatives in the U.S. in the near future. Most chemical preservatives in use today have specialized uses and established niche markets in the food industry. A great amount of interchangeability does not exist because of specific inhibitory actions toward bacteria or mold or yeast. Blends of antioxidants and preservatives (some natural and some synthetic chemicals) can provide multiple functions for fruit products. One such combination of ingredients is a blend of erythorbic acid, citric acid, and potassium sorbate as an antioxidant and antimicrobial substitute for sulfites on fresh-cut fruits. With the high cost of developing a new preservative (or other food additive) and the time required to obtain FDA approval, it is unlikely that new preservative chemicals will be developed in the future. Rather, blends of existing preservatives and other available inhibiting food additive products will be used.

13.17 ANTIOXIDANTS 13.17.1 FUNCTIONS Antioxidants are food additives that retard atmospheric oxidation and its degrading effects, thus extending the shelf life of foods. Examples of food oxidative degradation include products that contain fats and oils in which oxidation would produce objectionable rancid odors and flavors, some of which might even be harmful. Antioxidants are also used to scavenge oxygen and prevent the discoloration of cut or bruised fruits and vegetables. Oxidative browning, which is caused by the action of polyphenol oxidase with catechol tannins, is a very important problem in handling most fruits, especially peaches, nectarines, apples, and cherries. Discoloration is encountered in all methods of processing, but mainly with methods that do not involve cooking, such as freezing, drying, and making juices. Changes in flavor accompany changes in color. Browning may take place before peeling, due to bruising, but is accelerated once the skin is broken or tissue cells are ruptured. One of the important methods of controlling browning of fruits is by using water-soluble antioxidants. To improve the performance of antioxidants, two other types of food additives, sequesterants (e.g., EDTA, citric acid) and synergists (e.g., mixtures of antioxidants and lecithin), are frequently used with them. Antioxidants may also be added to food packaging, but such use is not covered in this chapter. Food antioxidants are effective in very low concentrations (0.01% or less of the fat content of foods) and not only retard rancidity, but also protect the nutritional value of the food by minimizing the breakdown of vitamins and essential fatty acids. The maximum concentration of food antioxidants approved by the FDA is 0.02%. At one time, safety questions were raised about several synthetic antioxidants. These have largely been resolved, although the impact on the market for synthetic antioxidants still exists.

13.17.2 PRODUCTS

AND

APPLICATIONS

Food antioxidants are generally classified as natural or synthetic. The most commonly used natural antioxidants are ascorbic acid (vitamin C) and erythorbic acid and their sodium salts, plus the mixed a- and g-tocopherols. Vitamin C finds more major use as a nutritive supplement or in pharmaceutical preparations. Small amounts, however, are intentionally used for antioxidant purposes. Citric acid and tartaric acid are also natural antioxidants (and antioxidant synergists) but most frequently are added to foods as acidulants and are covered in the acidulant section of this chapter. Synthetic antioxidants used as direct food antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butyl hydroquinone (TBHQ), and propyl gallate (PG). Most food-grade antioxidants are sold as either solids or solutions.

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TABLE 13.14 Food Antioxidants Oil-Soluble Products: Butylated hydroxyanisol (BHA)a Butylated hydroxytoluene (BHT)a tert-Butyl hydroquinone (TBHQ)a Propyl gallate (PG)a Tocopherolsa Thiodipropionic acid Dilauryl thiodipropionate Ascorbyl palmitate Ethoxyquin Water-Soluble Products: Ascorbic acida Erythorbic acida Glucose oxidase/catalase Gum guaiac Sulfitesa a

Major products.

Also, antioxidants may be divided into water-soluble compounds and oil-soluble ones (Table 13.14). The major use of oil-soluble antioxidants is in edible fats and oils. They are also used to prevent rancidity of shelled nuts used as snacks and as an ingredient in bakery products, citrus oils and other fruit flavor emulsions, confectioneries, and so on. Water-soluble antioxidants are widely used in fresh prepared (peeled, cut), frozen, dehydrated and thermally processed fruits (especially those packaged in nonmetallic containers), fruit juices and drinks, juice concentrates, flavoring compositions, and fruit snacks. The increased use of lightly processed foods containing fruits and fruit juice drinks is utilizing large quantities of water-soluble antioxidants, mainly ascorbic acid. 13.17.2.1 BHA BHA was introduced commercially in foods in 1948. Commercial preparations of BHA contain two isomers: 3-tertiary-butyl-4-hydroxyanisole and 2-tertiary-butyl-4-hydroxyanisole. The major applications for BHA are in edible rendered fats, edible frying fats and oils, salad oils, and shortenings. It has also been found to be an effective antioxidant for nut meats, orange-flavored fruit drinks, and processed fruits containing carotene pigment. While BHA can and is used alone as an antioxidant, it has also been used in combination with other antioxidants and synergists. BHA is frequently blended with BHT, TBHQ, or PG to optimize performance. 13.17.2.2 BHT BHT, a hindered phenolic-type compound was approved for use as a food antioxidant in 1954. BHT is used in blends with BHA or BHA/propyl gallate in vegetable oils and in edible animal fats. The major use of BHT, however, is as an antioxidant for waxes used in the manufacture of food packages and wrappers. Like BHA, BHT is effective at low concentrations. Concerns over the safety of chemical additives in food caused a decrease in demand for BHT. Also, currently, the availability of oxygen absorbent products has reduced the necessity for antioxidants such as BHT and BHA.

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TABLE 13.15 European Community Usage Levels for Ascorbic Acid in Processed Fruits and Beverages Application

Quantities Used

Prevention of Browning in: Frozen fruit 300–1000 mg/kg Fruit juices 120–200 mg/kg Nectars 300–500 mg/kg Canned fruit 1000–3000 mg/kg

Maximum Levels Recommended

2000 200 500 2000

mg/kg mg/kg mg/kg mg/kg

Prevention of Discoloration in: Olives 100–500 mg/kg Flavor Protection in: Citrus beverages 150–200 mg/l Wine 25–100 mg/l

200 mg/kg 200 mg/kg

13.17.2.3 TBHQ TBHQ was first introduced for food applications in 1972. TBHQ shows an exceptional activity in protecting unsaturated vegetable oils and animal fats from rancidity. One of its largest applications is in soybean oils. Although mostly used by itself, TBHQ can be used in combination with BHT and BHA. TBHQ is often used as a replacement product for propyl gallate. 13.17.2.4 Propyl Gallate (PG) PG has been used as a food antioxidant since the 1950s. Its current primary use is more as a synergist in combination with BHA and BHT. This antioxidant is a highly effective product for animal fats. 13.17.2.5 Ascorbic Acid Ascorbic acid (vitamin C) and sodium ascorbate is widely used as antioxidants and vitamin supplements. As an antioxidant, ascorbic acid is used primarily in prepared foods such as frozen fruits, fruit drinks, canned fruits, and applesauce. Its use as a direct food additive has increased in the recent past. 13.17.2.6 Erythorbic Acid Erythorbic acid (iso-ascorbic acid–a stereo isomer of ascorbic acid) and sodium erythorbate are used primarily as antioxidants in cured meats (e.g., bacon, sausages) and in salad bars as an oxygen scavenger. They are also used in canned and frozen fruits, fruit juices and nectars, olives, and refrigerated guacamole products to retard discoloration and off-flavor development (Table 13.15). The principal difference between ascorbic and erythorbic acid is that ascorbic acid has vitamin C activity while erythorbic acid does not. Vitamin activity is not necessary for antioxidant application, and in some instances, regulatory constraint makes it undesirable to add a nutrient to processed fruit product. Replacement of ascorbic acid with less expensive erythorbic acid provides comparable results in respect to antioxidant activity on an equal weight basis, or 1.23 kg of sodium erythorbate can replace 1 kg of ascorbic acid in various frozen or canned fruit products.

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Erythorbic acid (and salt) has benefited significantly from the FDA’s 1986 ban on the use of sulfites for fresh or uncooked vegetables in salads. 13.17.2.7 Tocopherols Tocopherols are naturally occurring antioxidants found in seed oils, fruits, vegetables, and animal tissues and are receiving considerable attention as a possible replacement for synthetic antioxidants. Tocopherols are of considerable interest both as an antioxidant and as a nutrient. Vitamin E is atocopherol. Although all isometric forms (a, g, and d) of tocopherol show antioxidant activity, the 80% gamma and 20% delta mixture of natural tocopherols has the best antioxidant activity (DLalpha-tocopherol, which is actually a D,L-racemic mix, is primarily noted as a source of nutritional vitamin E). The mixed natural tocopherol products can be used to protect a variety of fruit products, including dehydrated and processed fruits and citrus fruit drinks. 13.17.2.8 Sulfites Sulfur dioxide, sodium sulfite, sodium bisulfite, potassium bisulfite, sodium metabisulfite, and potassium metabisulfite are very effective to retard the browning of fruits in which the enzymes have not been inactivated by sufficiently high heat (Sapers, 1993). In addition to preventing enzymatic browning, sulfur compounds will reduce destruction of carotene and ascorbic acid, which are the important nutrients of fruits. Sun-dried fruits (e.g., apricots, peaches, and pears) are usually exposed to the fumes of burning elemental sulfur before being put out to dry in the sun. Apples are often treated with solutions of sulfite before dehydration. Solutions range from 0.2 to 0.5% (as SO2) and are made up of sodium sulfite and sodium bisulfite in about equal portion. Sulfite solutions are less suitable than burning sulfur because the solutions penetrate the fruit poorly and leach out natural sugar, acid, and flavor components. Treatment of fruits with sulfites is the most effective means available today to control browning; however, because sulfites have been banned in certain categories of products and their regulatory status for other categories is in question, the food industry is looking for alternative means of controlling browning. For years, restaurants and other food service outlets had prevented the browning of fresh produce by the use of sulfites. But because of allergic reactions of some consumers (especially asthmatics) to sulfites, regulations were issued and alternatives sought (Taylor and Bush, 1986). One of the most promising antioxidant/preservative alternatives is a blend of erythorbic and citric acids. The ascorbic acid derivatives, ascorbic acid 2-phosphate and ascorbic acid-6-fatty acid esters, are also reportedly effective. Another suggested substitute (which functions in water but not with fats and oils) is the sequesterant and chelating agent ethylene diamine tetraacetic acid (EDTA), which has been widely used in processed potatoes, salad dressings, sauces, and beverages. Cyclodextrin is another sulfite alternative that can be used to prevent browning. Finding a complete substitute for sulfites, however, has not yet been realized. This is because the sulfites not only acted as antioxidants to prevent browning, but also performed preservative functions in preventing microbial spoilage. 13.17.2.9 Gum Guaiac Gum guaiac is a resinous secretion of a tropical evergreen, Guaiacum officinalis. This resin contains complex phenolic compounds chemically related to guaiacol, guiaretic acid, and guaiaconic acid. Like other phenolic compounds, gum guaiac is more effective in animal fats than vegetable oils. Gum guaiac is an approved antioxidant for natural flavoring substances and other natural substances used in conjunction with fruit flavors. It is also approved for addition to food packaging materials.

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13.17.2.10 Spice Extracts Rosemary-based antioxidants have been of continuing interest to some food processing segments because they appear to offer superior antioxidant performance in certain applications and are considered label-friendly. Rosemary extracts is used primarily in processed meats and poultry, as well as in snack foods, dehydrated products, beverages, and flavoring oils. While enjoying GRAS status as a natural flavoring, spice extracts do not have specific FDA approval for the use as antioxidants and thus cannot be promoted as such.

13.17.3 TECHNOLOGY

AND

MANUFACTURE

Natural antioxidants are extracted from certain plants and plant seeds; however, the most widely used natural-based antioxidants are currently produced commercially by fermentation processes of natural raw materials followed by synthetic chemical operations (e.g., ascorbic acid production from sorbitol). Tocopherols can either be manufactured from vegetable oil sources or produced by fermentation from the intermediate phytol. Erythorbic acid is obtained as a coproduct in the manufacture of ascorbic acid, or by chemical synthesis.

13.17.4 REGULATORY STATUS Past developments have had a detrimental effect on the demand for BHA. In June 1982, the Japanese government reported that, according to feeding studies done in Japan with BHA at 2% of the entire diet, BHA was found to be carcinogenic. Consequently, BHA would not be allowed in food products sold in Japan after July 1, 1982. Various governments, including those of the U.S., Canada, and the U.K., requested a delay in the implementation date until further studies could be done. The date was then deferred to February 1, 1983, and the ban was never implemented. Subsequent to the initial deadline, the World Health Organization’s Food and Agriculture Organization studies showed that BHA dosage levels would have to be high (e.g., about 2% of the oil or fat content of the food) before any carcinogenic effects would become apparent. (Normal BHA content level is 200 ppm of the fat or oil content of the food.) However, due to Japan’s announcement of its initial study, BHA was removed from some of the food and food packaging sold in the U.S. and Japan. The findings of the Japanese study relative to BHA were surprising because several other studies conducted worldwide had found BHA to be anticarcinogenic. The original Japanese researcher has now agreed that BHA is not carcinogenic; however, irreparable damage to BHA use as a food antioxidant has no doubt already occurred (Anon., 1994b). Although BHT was never removed from the FDA’s GRAS list, demand for BHT as a direct food additive dropped significantly in the 1970s, largely due to an FDA proposal to restrict the use of BHT as a food additive throughout the 1980s. That proposal, as well as the general trend toward the use of all-natural ingredients in foods, has negatively impacted BHT use in foods. In 1986, six sulfiting agents — sulfur dioxide, sodium sulfite, sodium bisulfite, potassium bisulfite, sodium metabisulfite, and potassium metabisulfite — were banned by the FDA for use in raw vegetables and fruits in salad bars. In 1987, the FDA ruled that all packaged foods that contain 10 ppm or more of sulfur dioxide equivalents disclose on the label that sulfiting agents are present. GRAS status is not recognized for use of sulfates in food recognized as a source of vitamin B1 and of fruits and vegetables intended to be served raw to customers or sold raw to consumers or to be presented to the customer as fresh. Also in 1988, FDA established limitations of the maximum residual sulfur dioxide equivalent levels in many food categories. Those limits, applying to fruit products are as follows: jams and jellies, 30 ppm; nut products, 25 ppm; dried fruits, 2000 ppm; fruit juice concentrates, 1000 ppm; single-strength fruit juice, 300 ppm; glace fruits, 150 ppm; maraschino cherries, 150 ppm; wine, 275 ppm; and vinegar, 75 ppm.

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13.17.5 TRENDS

Processing Fruits: Science and Technology, Second Edition AND ISSUES

Opportunities exist for manufacturers of food antioxidants and other preservatives to increase the shelf life of fresh and processed foods. (The sulfite ban has created one such opportunity.) As people incorporate larger amounts of polyunsaturated fats in their diets and decrease the perceived unhealthy saturated fats, the need for food antioxidants also increases. This is because many of the unsaturated fats, such as that found in olives and nuts, are relatively unstable and become rancid more quickly. With total American consumption of fats and oils increasing due to snack, convenience, and processed foods, more antioxidants are needed. Common table salt and sugar are known to be natural antioxidants and preservatives; use of these products is declining also. With the trend toward lowered sodium and more dietetic foods and beverages, a higher level of protection is needed from substitute or replacement chemicals. In addition, shelf life is a major consideration in the development and marketing of snack foods that are proliferating in the U.S. In these products, consumer acceptance depends on maintaining the original taste, appearance, freshness, flavor, odor, and texture. Antioxidants preserve many of these qualities. The food industry has a preference for natural antioxidants. In this industry, it is desirable to have label designations on food products identifying the food antioxidants as naturally occurring or derived from naturally occurring materials. Once the natural-based antioxidants can be made as cost-effective as the synthetic types, their impact on this market will be greater. The use of synthetic antioxidants is tightly regulated in western Europe. TBHQ is not approved in EC countries. The existence of different legislation for synthetic antioxidants (e.g., no BHA is allowed in Germany; no BHT is allowed in France) means that food companies and suppliers of antioxidants must be aware of the differences and adapt their strategies accordingly. Because synthetic antioxidants are paid unfavorable attention, research on and development of effective natural antioxidants is actively carried out. Some examples of development are the extract of Glycyrrhiza glabra, polyphenol extracted from tea leaves, lignans from sesame, and extracts of spices such as rosemary, cinnamon, clove, and allspice. In the case of rosemary (Rosmarinus officinalis, L.), a standardized extract of its oleoresin has the active antioxidant components (Six, 1994). Rosemary extracts at very low, about 200 to 400 ppm, concentrations have particularly excellent antioxidant properties. Specific application include pickling brines and snack food nuts. Because of the disadvantage of conferring the particular rosemary taste and some color to food, its use is limited to food types where this is not a problem. Another development that could impact the use of current food and beverage antioxidants is the use of specialty silicate powders that are capable of filtering oxygen from liquids. Currently, research is in progress to commercially develop an insoluble organic powder that could be used to remove oxygen from beverage products such as fruit juices, wines, and carbonated soft drinks before packaging. The antioxidant material could also be coated or implanted in individual product packages. Such a product could have broad commercial use in the food industry.

13.18 ENZYMES 13.18.1 PRODUCTS

AND

FUNCTIONS

Enzymes are catalysts used during food processing to make chemical changes to the food. They are biological catalysts that make possible or greatly speed up chemical reaction by combining with the reacting chemicals and bringing them into the proper configuration for the reaction to take place. Enzymes are not affected by the reaction themselves. All enzymes are proteins and become inactive at temperatures above around 40∞C or in unfavorable conditions of acidity or alkalinity. Enzymes are important in food technology because of the roles they play in the composition, processing, and spoilage of foods. Sometimes, the presence of natural enzymes is advantageous; for example, cellulases and pectic enzymes assist in the softening of fruits during ripening to give

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desirable texture. In other cases, natural enzymes may produce undesirable reactions such as the browning reactions of cut fruits due to polyphenol oxidases. Recognition of the function of enzymes and their usefulness in bringing about desirable changes has led to their large-scale use as modifiers of food ingredients (Blenford, 1993). Enzymes are extremely specific and can act only on a single class of chemicals such as proteins, carbohydrates, or fats. Some of the specific functions food enzymes perform are to: • • • • • • • •

Speed up reactions Reduce viscosity Improve extractions Carry out bioconversions Enhance separations Develop functionality Create/intensify flavor Synthesize chemicals

Today, many fruit processes utilize enzymes. The food-grade enzymes encompass a wide variety of commercial products that are employed in the production, conversion, and modification of foods because of their highly efficient and selective catalytic function.

13.18.2 APPLICATIONS

IN

FRUIT PROCESSING

Many of the advances in fruit juice technology have been closely linked to developments in enzyme technology. The introduction of pectic enzymes made clarification of fruit juices possible, and the reduction in viscosity by enzymes permitted the preparation of concentrates. Treatment of fruit pulp with pectic enzymes made it possible to press soft fruits such as strawberries, pears, and soft cold-storage apples. A more recent development is enzymatic liquefaction where combinations of pectinases and cellulases can liquefy fruit with the resulting advantages of increased yields (Wrolstad et al., 1994). 13.18.2.1 Pectic Enzymes Pectic enzymes are used to catalyze the hydrolysis of pectinaceous materials. Complete hydrolysis leads to the production of pectic acids. Pectic substances are found widely in plant tissues, particularly in fruits. While pectic enzymes occur in many fruits, the commercial enzyme products are derived from strains of the fungus Aspergillus niger. The commercial enzymes are mixtures of several of the pectic enzymes, particularly pectin methylesterases and polygalacturonases. Differences of the commercial preparations are due to variations in the kinds and amounts of particular pectic enzymes present. Pectic enzymes have many uses in fruit processing. The importance of enzymes in fruit juice and wine processing is emphasized by their separate coverage in Chapter 4 of this book. Briefly, through the uses of these enzymes, juices of clarity and high yield may be produced. Also, pectic enzymes are indispensable for making fruit juice concentrates or purees. Fruit juices for jelly manufacturing are usually depectinized completely by pectic enzymes in order to obtain clear juices and make uniform jelly possible by adding back a standard amount of pectins when the jelly is made. Pectic enzymes are also used for the recovery and stabilization of citrus oils from lemon and orange peels. 13.18.2.2 Amylases These enzymes act on starch and starch-containing plant materials. Of all the commercial enzymes, amylases have the most numerous applications, primarily in the hydrolysis of starch to sugars. The use of amylases in fruit processing is minor, compared to industrial starch processing, but important.

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Fungal a-amylases produced from Aspergillus oryzae by fermentation are used for the starch removal from fruit juices and extracts, from flavoring extracts, and in the preparation of starch-free pectin. For the latter use, special amylase preparations completely free from pectinase are requisite. 13.18.2.3 Cellulases Cellulose, the most abundant of all naturally occurring organic compounds are present in all fruit tissues. Multienzyme cellulase complexes are utilized commercially for conversion of cellulose to glucose. The commercial cellulases are derived from Aspergillus niger, or other microorganisms, such as Trichoderma viride by controlled fermentation. Commercial applications for cellulases in fruit processing is limited. It includes usage in fruit juice manufacture to aid extraction and clarification of citrus juice and extraction of flavoring material from citrus pulp. Cellulase offers improved production of fruit juices by destroying the cellulosic components of the plant and allowing more free flow of the juice. An application in coffee manufacture results in the hydrolysis of coffee gums to prevent liquid coffee concentrates to gel. Crude cellulase preparations are used to treat organic wastes from fruit processing operations. 13.18.2.4 Glucose Oxidase and Catalase Commercial glucose oxidase preparation, usually combined with catalase, is designed to keep oxygen out of products that might be sensitive to it by catalyzing the reaction of glucose to gluconic acid and absorbing oxygen in the process. The presence of catalase in the enzyme preparation is needed to provide oxygen for the reaction of glucose to gluconic acid. The commercial glucose oxidase preparations are derived from Aspergillus niger. Oxygen is responsible for a wide range of deterioration in processed fruits, most importantly those of undesirable flavor and color changes. Light, particularly sunlight and fluorescent light, results in a deterioration of citrus beverages in a manner that is referred to as forming “sunstruck flavor.” Oxygen is necessary for the formation of peroxides and the resultant unwanted changes in flavor and color of citrus oils. By enzymatic removal of oxygen, successful applications for glucose oxidase were reported with citrus concentrates and drinks in preventing the development of offflavor (Scott, 1975) and the prevention of enzymatic browning of fresh frozen fruits. Also, glucose oxidase is effective in protecting cans of beverages against oxidative corrosion, and it has been recommended for the prevention of iron pickup in canned fruit drinks. 13.18.2.5 Bromelain, Ficin, and Papain Enzyme systems for the tenderization of muscle protein are usually made up from proteases extracted from fruit tissues. Thus, certain fruits are utilized to produce protease enzymes, which are important functional food ingredients as they are capable of hydrolyzing both plant and animal proteins to peptides and amino acids. One of the potent proteases is extracted from the latex of a tropical fig tree. Papain is derived from the latex of papaya (Carica papaya); and bromelain from pineapple. These enzymes can hydrolyze a variety of proteins.

13.18.3 TECHNOLOGY

AND

MANUFACTURE

The majority of today’s food processing (and other industrial) enzymes are produced by submerged fermentation using bacteria, yeast, and fungi (Ashie, 2003). In the vast majority of cases, the fermentation culture is batch processed using a carbon-containing feedstock such as glucose, molasses, or starch hydrolysates on which the yeast, fungi, or bacterial microbe can grow. Other components of the broth include a nitrogen source for the proteinaceous enzyme’s growth, plus mineral salts and yeast extracts. Fermentation normally takes 2 to 3 days with aerobic stirring (to

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provide the oxygen for the microbe’s growth). Most exudate (enzyme) is extracellular. Recent upgrades of enzyme production, however, include intracellular production of the enzyme followed by rupture of the cell wall and collection of the product enzyme. Animal glands and plant tissue also provide enzymes, but to a much smaller extent today. As compared to animal- or plant-derived enzymes, microbial enzymes offer several advantages such as purity, economy, consistency, and availability. The microbial enzymes can be modified to suit the needs of a reaction. For example, properties such as the thermostability or pH tolerance of an enzyme can be enhanced by routine mutagenesis of more sophisticated techniques such as protein engineering.

13.18.4 TRENDS, ISSUES,

AND

DEVELOPMENTS

Current research and development work continues to look for more applications and more specialized functioning enzymes (some genetically engineered), but the commercial use of enzymes in food processing and food modification has not advanced tremendously since the development of an enzymatic process for high-fructose corn syrup in the 1960s. New commercial applications that could significantly expand the current market for food-grade enzymes are slow to appear on the horizon, and few appear to be of economic value. Enzyme infusion into intact plant tissues has received recent attention to achieve specific changes in fruits. To date, this technique has focused primarily on manipulation of cell wall components (McArdle and Culver, 1994). For example, enzymes can be applied to intact citrus products to selectively alter albedo structure and facilitate peeling and segmentation. In another promising work, pectin methyl esterase was infused into blanched peach halves. After thermal processing, infused fruit were significantly firmer than the untreated fruits, presumably because the enzyme-induced removal of polyuronide methoxyl groups increased calcium binding of the cellwall tissues. Although many food and agricultural applications are amenable to treatment via enzyme technology, the projected returns often do not defray the high cost of developing new enzymes for small-volume applications. The impetus to use enzymes in these situations will depend on a price differential sufficiently large and stable to justify the development costs. Much of the scientific basis appears to be available now and ready to be refined.

REFERENCES Anon. Sweetness with calorie reduction. Food Ingr. Process. Int., (3): 15–19, 1992. Anon. Japanese Standards for Food Additives. 6th ed., English version, Japan’s Ministry of Health & Welfare, Tokyo, 1994a. Anon. FASEB panel: BHA is not a cancer risk. Inform, 5(10): 1167, 1994b. Ashie, I. N. A. Bioprocess engineering of enzymes. Food Technol., 57(1): 44–51, 2003. Bauer, K. International flavor legislation, in Source Book of Flavors, G. Reineccius, Ed., Chapman & Hall, New York, 1994. pp. 876–901. Blenford, D. Enzymes–food engineering’s catalytic converters. Food Ingr. Proc. Int., (1): 12–13, 1993. Borenstein, B. Vitamin fortification technology, in Technology of Fortification of Foods, National Academy of Sciences, Washington, D.C., 1975. pp. 1–7. Borenstein, B. and Bunnel, R. H. Carotenoids: Properties, occurrence, and utilization in foods. Adv. Food Res., 15: 195–276, 1967. Code of Federal Regulations, Food and Drugs. Title 21, Subchapter B — Food for Human Consumption. Parts 100–199. U.S. Government Printing Office, Washington, D.C., 2002. Duxbury, D. D. Multi-functional gum gets final approval. Food Process., 54(2): 62, 1993. Dwivedi, B. K. Sorbitol and mannitol, in Alternative Sweeteners, 2nd ed., O’Brien Nabors, L., and Geraldi, R. C., Eds., Marcel Dekker, New York, 1991, chap. 18. Dziezak, J. D. Gellan gum receives FDA approval. Food Technol., 44(11): 88–90, 1990.

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Dziezak, J. D. A focus on gums. Food Technol., 45(3): 117–132, 1991. Fischetti, F., Jr. Natural and artificial flavors, in Handbook of Food Additives, 2nd ed., Furia, T. E. Ed., CRC Press, Boca Raton, FL, 1980, chap. 11. Francis, F. J. Colorants. American Association of Cereal Chemists. St. Paul, MN, 1998. Gardner, W. H. Acidulants in food processing, in Handbook of Food Additives, 2nd ed., Furia, T.E. Ed., CRC Press, Boca Raton, FL, 1980, chap. 5. Giese, J. H. Alternative sweeteners and bulking agents. Food Technol., 47(1): 114–126, 1993. Giese, J. H. Modern alchemy: use of flavors in food. Food Technol., 48(2): 106–116, 1994. Glicksman, M. Origins and classification of hydrocolloids, in Food Hydrocolloids, Vol. 1, Glicksman, M. Ed., CRC Press, Boca Raton, FL, 1982, pp. 4–17. Goodburn, K. EU Food Law. CRC Press, Boca Raton, FL, 2001. Hegenbart, S. Bind for glory: Designing foods using gums. Food Prod. Des., 2(10): 21–42, 1993. Linden, G. and Lorient, D. Pigments and aromas, in New Ingredients in Food Processing, CRC Press, Boca Raton, FL, 1999, chap. 15. McArdle, R. N. and Culver, C. A. Enzyme infusion: a developing technology. Food Technol., 48(11): 85–89, 1994. McHugh, T. H. and Krochta, J. M. Milk-protein-based edible films and coatings. Food Technol., 48(1): 97–103, 1994. Mentzer-Morrison, R. Fat replacers. Inform, 3(12): 1270–1282, 1992. O’Brien Nabors, L. Sweet choices: Sugar replacements for food and beverages. Food Technol., 56(7): 28–34, 2002. O’Brien Nabors, L. and Gelardi, R. C. 1991. Alternative sweeteners: an overview. In Alternative Sweeteners, 2nd ed., O’Brien Nabors, L. and Gelardi, R. G., Eds., Marcel Dekker, New York, 1991, chap. 1. Reineccius, G. Source Book of Flavors. Chapman & Hall, New York, 1994. Salzer, U. J. and Jones, K. Legislation, Toxicology, in Flavourings, Ziegler, E. and Ziegler, H. Eds., WileyVCH, New York, 1998, chap. 7. Sapers, G. M. Browning of foods: control by sulfites, antioxidants and other means. Food Technol., 47(10): 75–84, 1993. Scott, D. Applications of glucose oxidase, in Enzymes in Food Processing, 2nd ed., Reed G. Ed., Academic Press, New York, 1975, pp. 519–549. Six, P. Current research in natural food antioxidants. Inform, 5(6): 679–688, 1994. Smith, R. L. et al. GRAS flavoring substances. Food Technol., 57(5): 46–59, 2003 Somogyi, L. P., et al. Food Additives. Specialty Chemicals Strategy for Success. SRI Consulting, Menlo Park, CA, 1996. Somogyi, L. P. and Kishi, A. Aroma chemicals and the flavor and fragrance industry. Chemical Economics Handbook, SRI Consulting, Menlo Park, CA, 2001. Stauffer, C.E. Emulsifiers for the food industry, in Y.H. Hui, Ed., Bailey’s Industrial Oil & Fat Products, 5th ed., Vol. 3, Wiley-Interscience, New York, 1996. Taylor, S. L. and Bush, R. K. Sulfites as food ingredients. Food Technol., 40(6): 47–50, 1986. Wodicka, V. O. Legal considerations on food additives, in Handbook of Food Additives, 2nd ed., Furia, T.E. Ed., CRC Press, Boca Raton, FL, 1980, chap. 1. Wrolstad, R. E., Wightman, J. D., and Durst, R. W. Glycosidase activity of enzyme preparations used in fruit juice processing. Food Technol., 48(11): 90–98, 1994.

Assurance, 14 Quality Quality Control, Inspection, and Sanitation Conrad O. Perera and Anne D. Perera CONTENTS 14.1 14.2 14.3 14.4

Introduction ........................................................................................................................339 Quality Control (QC) .........................................................................................................340 Quality Assurance ..............................................................................................................341 Quality Systems .................................................................................................................342 14.4.1 Hazard Analysis Critical Control Points (HACCP)............................................342 14.4.1.1 HACCP Principles .............................................................................343 14.4.1.2 Prerequisite Programs for HACCP....................................................344 14.5 Quality Control, Inspection, and Testing...........................................................................344 14.5.1 Moisture Content .................................................................................................345 14.5.2 pH.........................................................................................................................345 14.5.3 Respiration of Fruits............................................................................................345 14.5.4 Time of Harvesting..............................................................................................346 14.5.5 Postharvest Handling ...........................................................................................346 14.5.6 General.................................................................................................................346 14.6 Inspection and Test Procedures .........................................................................................347 14.6.1 Sampling ..............................................................................................................348 14.6.2 Sensory Evaluation ..............................................................................................348 16.6.3 Textural Characteristics .......................................................................................348 14.6.4 Determination of Drained Weight .......................................................................348 14.6.5 Frozen and Canned Fruits ...................................................................................349 14.7 Sanitation............................................................................................................................349 14.7.1 Microbiological Sampling Plans .........................................................................349 14.7.2 Sanitation Procedures ..........................................................................................351 14.8 Summary ............................................................................................................................352 References ......................................................................................................................................352

14.1 INTRODUCTION The need to address both food safety and food quality concerns has become increasingly important to the food industry in general. There have been several contributing factors: increasing awareness of consumers to the importance of nutrition in prevention of certain diseases, health risks associated with residues in certain foods, the availability of a wide variety of foods to consumers and the resulting level of competitiveness in the food industry, stringent government regulations relating 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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to the composition of food products and food additives, nutritional labeling requirements, international competitive factors relating to regional trade agreements, and environmental issues. Additionally, there is a danger of losing customer confidence and future demand for products, thus affecting the success of businesses, if quality and safety of their products are compromised. As a result, food manufacturers must now address concerns that traditionally were only peripheral to the scope of their operations, in addition to their traditional issues. This applies almost universally to all sectors of the food processing and manufacturing industry. The importance of the fruit sector of the fresh produce food industry is based on consumer dependence on fruit as a source of vitamins, minerals, carbohydrates, and dietary fiber and on their use as beverages, as specialty products, and as ingredients in many processed products. Consequently, fruits find use as freshly harvested produce, as well as processed products. In general, fresh fruits are high-moisture products, with moisture levels ranging from 50% up to 96% and are thus extremely susceptible to spoilage and microbial contamination. In this respect, the preservation of their quality and safety is a particular challenge to the food industry. Increasingly, the food industry is becoming dependent on the use of formal quality assurance (QA) programs for ensuring the safety and quality of foods. These quality assurance programs have evolved from their status as inspection-oriented programs to encompass a much broader scope involving all areas of management and production personnel. A food quality assurance program can be viewed broadly as a program designed to ensure the achievement of both safety and quality criteria or specifications of a product during all stages of handling, processing, preparation, packaging, storage, and distribution. It is critical, therefore, to establish specifications or at least certain criteria at all stages of these operations. Because of the wide variety of fruits and their uses and applications, the criteria for evaluating safety and quality vary considerably. For sale as fresh produce, it is of major importance that fruit should possess sensory attributes such as firmness, surface color, flavor, and spoilage-free appearance. Of greater importance for fruit intended for processing are attributes such as wholeness, good drained weight, and the absence of insects, residues, and microorganisms. Consequently, the safety and quality requirements vary, and there is the accompanying diversity in activities directed at achieving these specified requirements. Traditionally, achievement of safety and quality requirements was heavily dependent on inspections, by which materials and products were screened for defects and lack of conformance. With the growing trend in competitiveness and shrinking profit margins, food industries have had to take a much more proactive and preventative approach to ensure that their products meet the current stringent market demands for safety and quality. This chapter covers the principles of total quality assurance as it relates to the fruit industry, along with specific details on sanitation and inspection procedures necessary to achieve the requirements of safety and quality desired in this industry.

14.2 QUALITY CONTROL (QC) The fruit processing industry knows what good quality food production is, but often does little to maintain standards unless motivated by economics, regulations, and adverse publicity because of the lack of effective management, lack of apparent increased profits, high cost of quality control programs, or due to lack of knowledge of effective control programs. Fruit processors need QA and QC systems because of (1) liability claims — there are about 500 deaths per year due to Listeria outbreaks alone in the U.S. (ERS-USDA 2001), (2) consumer demand and loss of sales due to poor quality, (3) investigative costs for testing, experts, and future QC work; (4) cleanup costs and plant closings; and (5) food handler illnesses that affect efficiency. The benefits of good quality programs are (1) increased yield or productivity, (2) reduced distribution costs, (3) reduced cost of consumer relations and services, (4) more effective sales, (5) reduced recall costs, (6) decreased operating costs, (7) decreased marketing costs, (8) pride of workforce, (9) job security, and (10) profits.

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QC involves (1) conforming to standards and specifications of a product, (2) issuing manufacturing instructions, (3) setting in-process specifications, (4) sample collection and testing, (5) record keeping and reporting, (6) inspections, and (7) managing consumer complaints. A company’s objective should be to make a profit. Its objective in manufacturing a product is to satisfy consumer needs and expectations. Consumer complaints fall into many different categories. They may be concerned with the flavor, odor, color, texture, appearance, net volume or weight, and deterioration, etc. A procedure for handling consumer complaints should be developed and instituted.

14.3 QUALITY ASSURANCE QA oversees and evaluates quality control, wholesomeness, and integrity. It involves (1) reviewing quality control sampling protocols, (2) developing recall protocols, (3) developing managementreporting systems, (4) implementing HACCP and food safety programs, (5) helping to develop QC methods, and (6) development and improvement of laboratory methodologies. Functions of QA will include some of the following: 1. Managing Good Manufacturing Practices (GMP) including: • Documenting and developing processing procedures, equipment cleaning and sanitation schedules, a preventative maintenance program for equipment, and a pest and rodent control program • Providing employee training on GMP, sanitation, and proper product handling procedures • Conducting monthly audits for adherence to GMP and quality assurance practices • Inspecting plant and equipment sanitation daily prior to production 2. Managing Good Laboratory Practices (GLP) including: • Ensuring adherence to GLP • Documenting all testing methods, record keeping, and calibration processes • Evaluating data generated by the laboratory • Conducting monthly internal GLP audits 3. Managing a safety program including: • Ensuring employee safety • Adhering to Occupational Health and Safety Act (OSHA) or similar and other regulatory agency requirements 4. Assuring the quality of raw materials by: • Implementing the Vendor Certification Program to evaluate the microbiological, chemical, and biological quality of raw materials • Assuring appropriate raw material storage practices • Documenting procedures for out-of-specification raw materials 5. Assuring the quality and traceability of finished products by: • Initiating a microbiological and chemical testing program for finished products • Assuring appropriate finished product storage practices • Implementing systems for lot-identification and tracking all stages of distribution • Documenting procedures for out-of-specification finished products • Developing a product recall program 6. Evaluating the plant environment by: • Conducting an environmental microbiological sampling program • Documenting the environmental sampling program • Evaluating microbiological data and suggesting corrective actions if necessary 7. Developing an HACCP plan including: • Documenting of process flow charts

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• Identifying of key process control points and limits • Preparing of detailed process control procedures and corrective actions 8. Managing employee training including: • GMP training • HACCP training • Job training QA is based on a deliberate, conscious planning of all safety and quality-related activities, with a continuous monitoring and evaluation of all programs and processes related to the production and use of a given product. The success of this type of approach depends on the involvement and commitment of all personnel, including management personnel who are in any way responsible for the product at any given stage of processing. QC is viewed as the assessment of the quality and safety of raw materials in process products and finished products by means of an examination or inspection in relation to the established criteria or specification; this activity covers only one aspect of activities within the scope of quality assurance. An effective QA program can be developed around QC activities with a preventative approach to ensure finished product quality and safety.

14.4 QUALITY SYSTEMS A quality system may be defined as “The organization, structure, responsibilities, procedures, processes, and resources for implementing quality management” (Burrows, 2001). During the past decade, quality experts have proposed approaches, concepts, and systems for addressing quality during manufacturing. Total Quality Management (TQM) and ISO 9000 have become firmly established in large manufacturing and service industries worldwide. Many food manufacturing organizations have developed and implemented TQM or ISO 9000 programs in order to enhance their quality objectives. Details on applications of these quality programs in the food industry have been reviewed recently (Golomski, 1993; Battaglia, 1993; Surak, 1992) and are beyond the scope of this chapter.

14.4.1 HAZARD ANALYSIS CRITICAL CONTROL POINTS (HACCP) HACCP is a management tool developed in the late 1960s to ensure the safety of foods in space flight programs (Ropkins and Beck, 2000). This technique for ensuring food safety has become widely accepted in the food industry, both by government regulatory agencies and food manufacturers, as well as world organizations such as WHO and FAO (Ropkins and Beck, 2000; Perera and De Silva, 1999). It has been implemented in North America, the European Union, Australia, New Zealand, Singapore, and many other countries today. HACCP is now considered to be an effective means for ensuring the safety of foods and food products with respect to known hazards and has become the basis for controlling and ensuring food safety in food manufacturing processes; it has become part of total quality assurance programs in the food industry and can be incorporated within an ISO 9000 quality system (BSI Quality Assurance, 1991; Ropkins and Beck, 2000; Perera and De Silva, 1999). In the fruit industry, the implementation of an HACCP program can lead to the identification of known hazards that affect the safety of the final product, with the view of eliminating these hazards at the earliest possible stages and at any subsequent stage of the manufacturing or production system (Alli, 1993; Perera and De Silva, 1999). A hazard analysis determines the significance of a hazard associated with a product for the safety of the consumer of the product with which the hazard is associated. This results in the identification of critical control points, which are points in a process (location, step operation, and raw material) that, if not effectively controlled, may cause, allow, or contribute to the presence of the hazard in the final product. There is a wide variety of known hazards in the fruit industry. Depending on the type of product, whether fresh, frozen, or canned fruit, the detriments associated with the product can vary in significance. These hazards

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are grouped into the following three broad categories: biological, chemical, and physical. In the biological category, bacteria such as Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, Salmonella, and Clostridium botulinum lead the list of hazards in terms of their widespread occurrence and the dangers that they pose (Stauffer, 1988; Mazzotta, 2001). Certain molds (e.g., Aspergillus flavus, Aspergillus ochraceous, Penicillium articae) are also included in this category because they produce mycotoxins like aflatoxins, ochratoxins, and patulins that can be very poisonous (Wogan and Marletta, 1985; Anklam and Battaglia, 2001). Insects and their fragments in fruits and fruit products also fall in this category. Chemical hazards include compounds that present a threat to the consumer’s health; they include agricultural residues (e.g., insecticides and fungicides used on farms and during storage), heavy metals, and regulated chemicals, as well as food additives. Physical hazards include the presence of extraneous material such as metal fragments, wooden splinters, and stones, introduced primarily during harvesting or processing. In certain processes, where the size of fruit is critical for adequate thermal processing, such as in the case of canned fruits, lack of uniformity in the size of the raw fruits could also act as a physical hazard, and this needs to be identified and controlled. 14.4.1.1 HACCP Principles HACCP is based on seven accepted principles; the successful implementation of an HACCP program demands that each of these principles be well understood and strictly adhered to. In addition, a successful HACCP program requires that certain prerequisite programs be present to support the HACCP program (Perera and De Silva, 1999). In the fruit industry, an effective program would demand an initial detailed knowledge of all fruits and raw materials coming in, including packaging materials and all finished products, as well as their intended use, shelf life stability, and conditions for storage and distribution, such as temperature and labeling instructions. A detailed step-by-step flow diagram of the process is also required. This should cover all processes from the point of entry of raw materials, through manufacturing, packaging, and all postmanufacturing steps, including storage distribution, retailing, and consumer handling. Once the developed flowchart has been verified as accurately reflecting the actual operations used for preparation of the product on the factory floor, the following seven principles of the HACCP program can be applied: 1. Hazard assessment: All steps in the manufacturing process, including the use of raw materials and ingredients, processing, distribution, and marketing, through to use by the consumer should be evaluated for identification of known biological, chemical, or physical hazards. 2. Critical control point determination: This is the process to determine which of the identified hazards require control such that loss of control results in an unacceptable safety risk. The identification of the critical control points requires a thorough familiarity with the food process, including each step in the operation from the receiving of fresh fruits and other raw materials to the shipping of the finished product. 3. Establishment of specification: Appropriate target levels and associated tolerances need to be established so that they can be used as criteria for determining whether the steps that are identified as critical points are actually in control. 4. Development of monitoring and testing procedures: Testing procedures for monitoring each critical control point will have to be developed to ensure that each critical control point is consistently monitored. 5. Establishment of corrective action: This involves the development of procedures that must be followed whenever the monitoring process demonstrates that an identified critical point is out of control on the basis of the established target levels and tolerances, including the obligatory corrective action to be taken in the event that this deviation from control occurs.

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6. Record keeping: A system of documentation needs to be developed that will describe all aspects of the HACCP program and give evidence of its functioning based on all data obtained from testing and analysis, any deviation, and corrective actions. 7. Verification procedures: Procedures need to be established as part of the HACCP program to verify that the operating HACCP system consistently conforms to the established procedures. A successfully implemented HACCP program, when properly monitored, will ensure effective control of hazards and public health risks and will lead to improved food safety. As the HACCP program takes care of all known hazards in any production line, it improves product efficiency, reduces waste, and reduces the need for rework. This should cut down on cost, improve sales performance, and enhance consumer confidence in the product. The quality and safety of processed fruits are directly related to the state of the incoming fresh fruits used in production. Microbial contaminations, impurities, and the general appearance of the incoming fruits must therefore meet certain minimum specifications. Producers of processed fruits such as canned, frozen, and modified atmosphere packaged (MAP) fruit products can demand that suppliers follow an HACCP program to ensure that all raw materials coming in meet the desired specifications. This will not only cut down costs but will also ensure a safer and better quality product. 14.4.1.2 Prerequisite Programs for HACCP The effectiveness of an HACCP program in any fruit processing establishment is dependent upon the existence of adequate procedures that control the operational conditions within the establishment, so that the environmental conditions are favorable to the production of a safe product. Good Manufacturing Practices (GMP) are a prerequisite for effective implementation of an HACCP program. This means control of the premises and facilities, sanitation, receiving and warehousing of fruits and any raw materials, handling and storage of products, maintenance of equipment, training of personnel, and developing a recall program. These are required to be in operation before a successful HACCP program can be implemented.

14.5 QUALITY CONTROL, INSPECTION, AND TESTING In order to address the quality characteristics of fruits, the entire production chain from planting through harvesting, postharvest handling, storage, and at every stage of fruit processing until the final product is obtained requires control. QC should be exercised during the production of the crop, with a view to the intended use of the produce obtained at harvesting (Kramer, 1973). The quality of harvested fruits is dependent on the quality of seeds, use of fertilizers and pesticides, and general field management practices. For example, seeds must be tested for extraneous matter and weed seeds, in addition to percentage of germination. Certain micronutrients may have a deleterious effect on the end product; for example, lime applied to oranges whose peel is to be used for marmalade should not contain magnesium, which will cause the marmalade to be lumpy (Kramer, 1973). Pesticides must be free of any substances not approved for use, and the concentration of the active ingredient must conform to specifications. Chemicals applied to growing crops may exert profound effects on the appearance of fresh and processed food products, and these must be monitored. For example, copper sprays have been found to reduce the size of cherries (Hartz and Lawver, 1965), certain herbicides cause lemon and other citrus fruits to turn yellow and then to brown (Johnson, 1945), and other chemicals have been found to cause off-flavors in both fresh and processed products (Mahoney, 1962). Chemicals used during crop production must therefore be chosen to protect or enhance product quality and must be used at appropriate concentrations and times so that harvested fruits are free of pests and contain pesticide residues below the approved limit. Prior to harvesting, fruits intended for the fresh market should be supplied with more water, especially toward the end

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of the growing season in order to maximize total yield and improve appearance. If the crop is intended for use as a concentrated or dehydrated product, the later irrigations should be reduced or entirely suspended to cut down on dehydration costs (Kramer, 1973). Control of postharvest biochemical and physiological processes presents a special challenge. These processes increase the susceptibility of fruits to spoilage after harvesting and affect their quality and shelf life. The shelf life of fruits after harvesting is affected by various intrinsic and extrinsic properties of the harvested fruit. The intrinsic properties of fruits that affect their shelf life and quality include the moisture content of the fruit, pH, rate of respiration, biological structure, and ethylene production, whereas extrinsic factors include time and method of harvesting, handling, temperature, humidity, and general hygiene, as well as the methods used in packaging (Haard and Chism, 1996; Day, 1993). These are considered below.

14.5.1 MOISTURE CONTENT The high moisture content of most fresh fruits, between 50 and 96% (Kramer, 1973), make them generally susceptible to spoilage and they act as media for the growth of food poisoning microorganisms. Thus, much precaution needs to be taken after harvesting to limit the exposure of the harvested fruits to any of these food spoilage microorganisms so as to ensure safety and preserve the quality of the fruits. Water loss in fruits often results in wilting and weight loss, which could be desirable or undesirable depending on how the fruits are to be used. For example, water loss in apples reduces succulence and textural quality, which is undesirable. Loss of water in dates to be sold as dried dates is, however, very desirable. The surrounding atmosphere of harvested fruits should therefore be carefully controlled to prevent any undesirable loss in quality.

14.5.2

PH

The pH of harvested fruits is an important parameter in determining whether potential food spoilage microorganisms are liable to grow during postharvest handling and storage. Most fruits such as lemons, oranges, pineapples, apples, and peaches are high-acid products, having pH values below 4.5 (Holdsworth, 1983), which do not favor the growth of C. botulinum. They may, however, support growth of some acid-tolerant yeasts and molds (Kramer, 1973). Fruits that have pH values above 4.5 are, however, susceptible to the growth of C. botulinum, and a rigid temperature control must be maintained (below 3∞C) (Day, 1993) to safeguard against the growth of these microorganisms. It is also important to control the acidity of fruits after harvesting because changes in the pH of the fruit affect both the flavor and ultimate wholesomeness of the product, whether intended for the fresh market or for further processing. As most fruits mature, the acidity decreases, and the sugar content increases; thus, a relatively high sugar content and low acidity indicate full maturity. If pH of the fruit is low, it is an indication that the fruit is somewhat immature (Khattak, 1973). For example, the pH of citrus varies from 2.9 to 3.1 for immature valencia oranges and from 4.1 to 4.4 for very mature fruit of the same kind (Harding et al., 1945).

14.5.3 RESPIRATION

OF

FRUITS

Most fleshy fruits show a characteristic rise in respiratory rate coincident with the obvious changes in color, flavor, and texture that typify ripening (Haard and Chism, 1996). The rate of respiration is indicative of the rapidity with which compositional changes are taking place within the plant material and, hence, gives an indication of the potential shelf life of the fruit (Day, 1988). Respiration rates of fruits are markedly influenced by temperature, maturity, type of fruit tissue, size, and variety. For example, the respiration rate of apples at 25∞C is 10 times that at 5∞C (Haard and Chism, 1996). Fruits of the same variety with a larger surface area have higher respiration rates than smaller ones (Day, 1993). Bruised or cut fruits tend to have increased respiration rates and,

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therefore, have a lower shelf life. In the handling of fruits after harvesting, it is essential to avoid cutting and bruising in order to keep the respiration rate at an optimum.

14.5.4 TIME

OF

HARVESTING

The quality and subsequent shelf life of harvested fruit is dependent on the maturity of fruits at the time of harvesting. It is therefore critical that fruits be harvested at optimal maturity (Zomorodi, 1990). Most fruits are firmer prior to their peak maturity, and when harvested at this stage, are less susceptible to mechanical damage during handling and processing. Harvesting, when done too early, often gives poor quality fruits and erratic ripening, whereas late harvesting may increase susceptibility to decay and premature softening (Day, 1993), which subsequently leads to shorter shelf life of most fruits, increasing their risk of contamination by dangerous microorganisms. It is important that the timing of harvesting be carefully controlled to reduce these potential setbacks after harvesting.

14.5.5 POSTHARVEST HANDLING Handling of fruits during harvesting and after, if not done with care, causes damage to the plant cellular structure, destroying the natural protective tissue and textural integrity of the fruits. Such injuries increase moisture loss, enzymatic activities (e.g., browning of bruised fruits, which is catalyzed by the enzyme polyphenoloxidase) (Haard and Chism, 1996), susceptibility to microbial spoilage, discoloration, and respiration. These adverse effects can be minimized by eliminating bruising at all stages of harvesting and postharvesting. It is important that fruits be stored at adequate temperatures and humidity after harvesting to ensure their safety and quality. Respiration rates of fruits usually increase with increasing temperature. Moreover, other tissue attributes that indirectly influence biochemical transformations, such as membrane integrity, are influenced by temperature. Lowering of temperature tends to retard undesirable physiological reactions, as well as spoilage caused by growth of microorganisms (Haard and Chism, 1996). Fruits that have relatively fast respiratory rates respond best to lower temperatures, and with such fruits, precooling prior to shipping or storage can contribute greatly to an extended shelf life (Haard and Chism, 1996). Certain fruits, particularly of tropical origins, are susceptible to chilling injury by exposure to temperatures below 5 to 15∞C. Visible effects of chilling injury include localized tissue necrosis (e.g., apple), failure to ripen (e.g., banana), development of areas of tissue that do not soften on cooking, surface pitting (e.g., grapefruit), and wooliness in texture (e.g., peach) (Haard and Chism, 1996). Susceptible produce such as bananas, papayas, avocados, melons, and pineapples should be stored at recommended temperatures so as to eliminate such temperature-induced injury. (Because most fruits are 80 to 95% water, they lose moisture rapidly whenever the relative humidity is less than 80 to 95 % below saturation. Moisture losses of 3 to 6% are usually enough to cause marked deterioration of quality, and, thus, a balance between temperature and relative humidity must be maintained to preserve the quality of fruits after harvesting. It is generally recommended that fruits be stored at relative humidities sufficiently high to minimize water loss and to maintain cell turgor, but not so high as to cause condensation and accompanying microbial growth (Haard and Chism, 1996).

14.5.6 GENERAL Lack of adequate hygienic practices at all stages of harvesting, postharvesting, and processing, drastically increases the risk of contamination with food poisoning bacteria. This jeopardizes the safety and quality of the fruit for consumption. All equipment used in the handling of harvested fruits must be cleaned regularly and personnel should be adequately trained to observe personal hygiene, good sanitation, and good manufacturing practices. The quality characteristics of processed fruit products can be adversely affected by processing. Most processed fruit products are thermally processed (e.g., jams, syrups), dehydrated (e.g., raisins), or frozen. These processes affect the texture, flavor, and color, as well as the nutritive value of the

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product, and must be carefully controlled if a high-quality product is desired. Processing variables that may influence chemical changes during processing include time, temperature, mode of heat transfer, type of container used, water quality, and pH (Haard and Chism, 1996). Proper preparation and pretreatment of fruits prior to processing is vital to achieving the desired quality. Overheating during thermal processing may result in excessive softening of fruits, browning, off-flavors, and loss of nutritive value, while insufficient heating may result in inadequate inactivation of enzymes and destruction of microorganisms. Frozen products must be kept at temperatures well below the freezing point ( -18∞C or below) (Haard and Chism, 1996) to prevent off-flavors and color changes from developing. The texture of dried fruits is generally dependent on the method of moisture removal, and precautions must be taken during dehydration to minimize oxidation, browning reactions, case hardening, and nutrient loss. It is quite clear therefore that the way in which fruits are handled after harvesting will generally determine their shelf life, safety, and quality at the time of consumption. The implementation of effective and efficient quality control techniques must, therefore, follow immediately after harvesting to maintain the standards specified for each product. Monitoring of the desired quality characteristics by use of appropriate inspection and test procedures becomes necessary to ensure that effective control is exercised.

14.6 INSPECTION AND TEST PROCEDURES An effective quality control system includes an organized receiving department where all incoming merchandise is thoroughly checked for conformance to desired specifications. Inspection and test procedures are used routinely to confirm that raw materials and products have the desired quality characteristics. In this section, the quality characteristics of fruits and the inspection and test methods are discussed. All fruits intended for the fresh market must be inspected for bruises, size, color, ripeness, or level of maturity. Fruits should be handled carefully in order to reduce bruising and decomposition. Where washing is required, wash water must be potable and under enough pressure to remove all adhering dirt. Sieving, sifting, or hand-picking must be done immediately to sort out all decomposed or unclean fruits, as well as under- or oversized fruits. Unfit products should be set aside and disposed of in such a place and manner that they cannot contaminate food and water supplies (FAO, 1984). Selected fruits for the fresh market must be stored at the appropriate temperature and humidity. During storage, care must be taken to prevent any form of infestation from rodents and insects and contamination by bacteria that affect the safety and shelf life of the fruits. Fruits received for further processing into products such as canned fruit syrups, concentrated fruit juices, and jams must be handled with equal care. All arrivals of delivery must be anticipated and adequate freezer and cooler space made available prior to arrival of merchandise. Trucks supplying fresh fruits and other raw materials must be inspected for cleanliness, freedom from foreign odors, and other contamination. All packaging should be inspected for imperfections. If the interior temperature of the truck does not conform to that specified for storage of the incoming fruits, they should be rejected or special precautions must be taken in their handling and a note made of these. Incoming fruits should be checked for soundness, ripeness, fungal rot, bacterial rot, fly eggs, and maggots before being accepted for processing. If the fruits are acceptable, unloading must be accomplished quickly to minimize exposure to exterior temperatures (Thorner and Manning, 1983). Proper records must be kept of the date and time goods were received, descriptions of goods and conditions under which they were received for future referencing, or whether they need to be traced in the event of a recall. During processing, written procedures for every stage of production must be critically followed and continuously monitored. Any deviations must be noted and recorded. Temperatures, product consistencies, flavors, textures, and so on must be continuously inspected during processing because they can have a great effect on the safety and quality of the finished product.

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Fruits that require washing before further processing should be properly cleaned with a stream of water under enough pressure to remove all adhering dirt. Conveyors and containers for transporting raw materials should be of a material and construction that will permit thorough cleaning. Extreme care must be taken in transporting perishable products to prevent bruising and product contamination (FAO, 1984). Although inspection procedures during processing will vary from one processed fruit to the other, routine quality control inspection procedures for the fruit industry will involve proper sampling techniques; sensory evaluation (appearance of product-uniformity of size and color, texture, and flavor characteristics such as taste and aroma); determination of syrup density; sugar content; fill container and maximum headspace; drained weight; viscosity and flow properties; presence of insect filth (maggots, rodent hair, and mites); and condition of container, cartons, cases, vacuum cans, plastic films, or any other product used for a product’s wrapping (Thorner and Manning, 1983).

14.6.1 SAMPLING Sampling is the selection of a certain portion or number of units from a particular lot of the same product for testing. It is extremely important that the sample selected be as representative as possible of the entire batch from which it is sampled. The sample size should be large enough to allow repeat analysis, and the condition of the sample analyzed at the laboratory should reflect conditions at the time of sampling (FAO, 1984). A sampling plan must be followed in sample collection during processing to allow the data collected to be easily analyzed statistically.

14.6.2 SENSORY EVALUATION Sensory evaluation of processed fruit products should be done by well-trained personnel who can make fine distinctions between the senses of taste, smell, touch, and sight. It is sometimes preferable in certain instances that sensory evaluation be done by untrained personnel, and this decision must be made by the quality control department. Odor, taste, and appearance play an almost indispensable role in the acceptability of the finished product. Specifications should be set up as part of the quality control program by which all samples taken during processing and at the end of production will be compared with each other. Samples should be observed for color, density, viscosity, and visual evidence of spoilage. A taste panel should be used to test the product for its aroma, taste, “mouth feel,” and aftertaste, in accordance with predetermined procedures and the results compared with desired expectations to determine if the product is acceptable for sale or not.

16.6.3 TEXTURAL CHARACTERISTICS Textural characteristics of the product such as density and viscosity measurements can be measured by any standard procedure, and equipment designed for these measurements can be purchased in the market. It is important that results be reliable and reproducible. Sugar content of syrups that affect mouth feel of canned fruit syrups and juices can be measured using standard hydrometers.

14.6.4 DETERMINATION

OF

DRAINED WEIGHT

For most fruits packed in liquid, the drained weight of the solid product is more important than the total net weight. A standard drained weight must be specified for the finished product on the basis of which inspections and tests will be done. An example of a test procedure for drained weight (Thorner and Manning, 1983) involves emptying contents of the container onto a previously weighed specified screen, allowing it to drain for 2 min, and reweighing the product on the screen. The weight of the screen is subtracted, and the difference is the drained weight of the product. This is a fast and easy procedure that can be routinely used to test for the drained weight of finished produce.

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AND

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CANNED FRUITS

Freezing for fruit preservation utilizes low temperatures to prevent the multiplication of microorganisms. The quality of raw materials and sanitary conditions of equipment are therefore critical. Materials should be handled and prepared as quickly as possible under the best sanitary conditions to minimize the number of bacteria present at the time of freezing (FAO, 1984). The time and temperature combination for heating and the pH of fruits must be carefully controlled in all canned produce manufacture to effectively control microbial spoilage. A heat treatment process (time and temperature combination) that has maximum destructive effect on microorganisms and minimum effect on the quality of the food must be used. These process parameters must be properly documented and consistently monitored and inspected for compliance. The pH of the fruit produce must be inspected at all stages of production using a reliable pH meter or by titration. The pH of the final product should not deviate significantly from the desired pH.

14.7 SANITATION Even though the health benefits associated with regular consumption of fresh fruits have been clearly demonstrated, an increasing proportion of reported outbreaks of foodborne illnesses are traced to fresh produce (Zepp et al., 1998). This was further highlighted in the recent outbreaks of foodborne illnesses due to consumption of minimally processed fruits and fruit juices (Anon., 2001). Salmonella grows extremely well on any low acid fruit or vegetable with sugar in it at temperatures above 15∞C. The realities of postprocess distribution and consumer handling make it highly likely that temperature abuse may occur. Ingestion of as few as 10 live cells of the most aggressive strains can be fully infectious to susceptible individuals. These strains often harbor genetic elements that give them multiple resistance to medical antibiotics and therefore infections are very difficult to treat (IFT Expert Report, 2002). Therefore, improved surface decontamination methods are needed to increase our confidence in the microbial safety of fresh-cut and non-heatprocessed juice products or minimally processed produce.

14.7.1 MICROBIOLOGICAL SAMPLING PLANS Sampling plans are statements describing the collection, methodology, and acceptance criteria of testing. Microbiological methods, no matter how accurate and reproducible, are inadequate to appraise the microbiological quality of food without a satisfactory sampling plan. The criteria one uses to control microbiological hazards are: • • •

Education and training Inspection of facilities and operations Testing

Microbiological standard is a criterion that is part of a law or regulation. It is mandatory to follow this standard. Microbiological specification is a criterion that is applied as a condition of acceptance. It is also mandatory if the produce is to be accepted by the buyer. Microbiological guideline is an advisory criterion that is used to monitor food processes or systems. Selection of criteria used to control microbiological hazards are based on the following: • • • • •

Evidence of hazard to health Microbiology of the raw material Effect of the process Likelihood of contamination Consequence of contamination

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• • • • • • • • •

Category of risk to the consumer Cost to benefit ratio Type of food Contaminants Methods Size of sample Limits Distribution of organisms Practicality

Knowledge about possible contaminants is useful in selecting suitable sampling plans. For example, S. aureus is an environment contaminant that can be passed on by improper or unhygienic handling practices. L. monocytogenes can occur in minimally processed or any ready-to-eat fruit products. Effective criteria and standards are required for control of contamination. There is little relationship between Aerobic Plate Counts (APC) and safety. However, APC and quality attributes are related. High APC means poor hygienic practices in the process and, consequently, poor product quality. APC should be used to determine control of the process. In general, coliform and E. coli counts should be used to indicate process control and food contamination. They indicate bad hygienic practices. There is little relationship between yeast and mold (Y/M) and safety. However, Y/M and quality attributes are related. As in the case of APC, Y/M should be used to determine control of the process. Lactobacilli could also be used as a guide to indicate process control and food contamination. Recognizing the need to minimize the microbial food safety hazards in fresh fruits and vegetables including fresh-cut products, the FDA, USDA, and the Center for Disease Control and Prevention have come up with a guide for handling of such products (FDA, 1998). It covers aspects of the control of potential microbial hazards all the way from the farm to the fork. The safety precautions and procedures needed in dealing with water, manure and municipal biosolids, worker health and hygiene, sanitary facilities, field sanitation, pack house sanitation, transportation, and traceability of contaminated products are discussed extensively (FDA, 1998). Plant sanitation and cleanliness of equipment are of the utmost importance in the success of any quality control program. The plant premises and areas surrounding it should be kept completely free of all decomposing materials and pools of stagnant water that generate foul odors and are ideal for the breeding of flies and other insects. Entrances into the plant and areas surrounding it should be well paved to reduce the amount of mud and dirt brought in by workers and clients (Khattak, 1973). These pavements should be regularly cleaned to prevent the accumulation of mud and dirt. Entrances for the handling of raw materials should be kept particularly clean and, if possible, separated from the general entrance to the plant. All floors in the factory should be cleaned regularly and must be properly constructed with good drainage. Water should not be allowed to remain stagnant on the factory floor because this breeds unwanted insects and flies. Doors and windows of the plants should be screened tightly to prevent the entrance of flies and other insects. All equipment must be rinsed and sanitized on a daily basis and, in some instances, whenever there is a change in shift, if this is appropriate. If possible, equipment should be rinsed and sanitized each time a product change is made that will affect the incoming product with contamination of the previous product run. Lighting in all areas of the plant should be sufficient for adequate performance of all assigned duties. Areas where cleaning, sorting, picking, and product inspection are done should be provided with sufficient nonglaring light to enhance work performance. The plant should be designed in such a way as to ensure sufficient ventilation of all factory floors. Adequate ventilation is extremely important to prevent buildup of odors and recontamination of product. The design of the plant should also ensure that temperatures maintained during

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processing are optimal for the product being manufactured. Too high or low temperatures on the factory floor could seriously affect the quality of the final product and also affect the comfort and productivity of workers. Air quality must be controlled to prevent buildup of condensate and bacteria. Water quality must be checked and controlled. Steam facilities must be evaluated for adequacy for all plant operations, especially where steam is used for sterilization or thermal processing. Waste disposal must be properly controlled to prevent the possibility of recontamination. Sufficient attention should be paid to the waste and garbage disposal system to ensure that it is large enough to carry peak loads and does not contribute to contamination of the finished product. All waste and garbage from the factory floor should be properly disposed of immediately and not left open or stored. Health habits of personnel involved in production should reflect a number of basic rules of hygiene, including washing hands in hot water; using plenty of soap; drying hands on a clean cloth or paper towel; keeping fingernails short and clean; using a hairnet or cap; remaining away from food when a person has an infected cut, boil, or other infection of the exposed skin; using sanitized tools and equipment; covering sneezes and coughs by means of a tissue or handkerchief; reporting all cases of diarrhea and fever, etc., and having periodic checkups (Thorner and Manning, 1983).

14.7.2 SANITATION PROCEDURES Sanitation consists of two parts: (1) cleaning and (2) sanitizing. Cleaning means the removal of residue of food, dirt, dust, foreign material, or other soiling ingredients or materials. Sanitizing means the effective bactericidal treatment of clean surfaces of equipment, utensils, and even surfaces of fruits (Thorner and Manning, 1983; Annous et al., 2001). Some of the sanitizers used in the food industry — chlorine, chlorine dioxide, ozone, and peroxides — have a number of limitations in terms of removing some pathogenic bacteria, especially listeria (Kim et al., 1999; Simons and Sanguansri 1997; Anon., 2000). Besides, some of the above sanitizers have adverse effects on nutritional quality (Ozen et al., 2001). The use of steam (Pao and Davis, 2001; Li et al., 2001; Mazzotta, 2001; Suslow and Cantwell, 2001) and hot water (Fleischman et al., 2001) for surface sanitation of fruits and vegetables intended for minimal processing has received considerable interest recently. Acidified electrolyzed water could be used effectively for washing of fruits to rid them of microbes as well as viruses. There have also been a number of reports in the literature on the effect of acidified electrolyzed water as an antimicrobial and antiviral agent (Izumi, 1999; Kim et al., 2000; Koseki and Itoh, 2001; Koseki et al., 2001). Frequent cleaning of plant equipment should be maintained at all times, and all parts should be continually washed by well-located continuous water sprays. This must be done especially on equipment such as inspection belts and conveyor belts used to convey fruits and other products. All utensils used must be absolutely clean. Buckets, knives, and drain pans should be cleaned and rinsed whenever empty or not in use (Khattak, 1973). A sanitation schedule should be designed that includes every area, machine, and all preparation equipment within the premises. Employees should be assigned specific tasks in sanitizing equipment and factory floors, and this should be inspected and rigorously monitored. An employee should be assigned the task of caring for all cleaning equipment such as rags, sponges, mops, and brushes. When not in use, these should be stored in a utility closet and a sink must be provided. Cleaning equipment should be rinsed and cleaned before storage. Cleaning and sanitizing operations must be carried out by properly trained personnel. The choice of cleaning agents for each piece of equipment should be made based on the type of soil and dirt encountered in processing. Soaping alone might not be enough. Detergents and sanitizers must be applied to equipment regularly after initial rinsing and left on long enough before rinsing again to ensure adequate sanitizing of equipment. During all shutdown periods and at the end of each production line, equipment such as inspection belts, packing tables, belt conveyors, filling machines, mills, choppers, cutters, washers, and similar equipment should be thoroughly washed with hot water, steam hosed, and rinsed off

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with clean cold water (Khattak, 1973). At each washing, all debris should be collected and removed to an appropriate disposal container prior to starting the actual rinsing and soaping of the processing structures. All drain gates must be removed and thoroughly washed with soap or detergent on both sides. Drains must be thoroughly washed with soap or detergent and thoroughly rinsed. The factory floor must be rinsed thoroughly and drain gates replaced. After all equipment and the floors have been washed and rinsed, sanitizers should be applied, which should remain on structures to eliminate residual bacteria in the air. The sanitizers should be applied on all the structures where the product comes in contact with the equipment such as all belts, the inside of tanks, centrifuge baskets, stainless steel hoppers, and all other equipment where the final product will have direct contact. All sanitation personnel should wear cloth gloves, thick rubber gloves, rubber boots, rain gear, safety glasses, hairnets, and bump hats prior to starting their daily cleaning duties. This equipment should be kept clean to ensure against recontamination of sanitized equipment.

14.8 SUMMARY Current trends in both the food manufacturing and food service industries indicate that the use of generic-type programs (such as TQM, ISO 9000, HACCP, industry standards) to address both quality and safety requirements of products has become common practice. These programs attempt to cover the entire scope of food manufacturing, from the acquisition of raw materials to the distribution of the finished product, and contain the critical elements required to address the quality and safety concerns of the fruit sector of the food industry. The concepts and tools advocated in these programs are applicable equally for both ensuring quality and safety of fresh fruits and processed fruits.

REFERENCES Alli, I. 1993. Quality control of MAP products. In Principles and Applications of Modified Atmosphere Packaging of Foods. R.T. Parry (Ed.), Blackie Academic & Professional, New York, pp. 101–113. Anklam, E. and Battaglia, R. 2001. Food analysis and consumer protection. Trends Food Sci. Technol., 12: 197–2002. Annous, B.A., Sapers, G.M., Mattrazzo, A.M., and Riodan, D.C.R. 2001. Efficacy of washing with a commercial flatbed brush washer using conventional and experimental washing agents in reducing populations of Escherichia coli on artificially inoculated apple. J. Food Prot., 64(2): 159–163. Anon. 2000. Food Safety and Hygiene. Food Science Australia. August 2000. Anon. 2001. Information bulletin — Public Advisory on food safety measures for cantaloupe. Canadian Food Inspection Service. June 1, 2001. Battaglia, R. 1993. Quality management in food trade with a view toward 1993 in Europe. Food Res. Int., 26: 69–74. BSI Quality Assurance. 1991. Guidance notes for the application of BS 5750 Part 2/ISO 9002/EN 29002 for the food and drink industry BSI, London, pp. 1–12. Burrows, G. 2001. Quality management system and hazard analysis critical control points. In Fruit Processing: Nutrition, Products, and Quality Management. 2nd ed. D. Arthey and P.R. Ashurst (Eds.), Aspen Publishers, Gaithersburg, MD, pp. 249–280. Day, B.P.F. 1988. Optimization of parameters for modified atmosphere packaging of fresh fruits and vegetables. CAP ’88, Schotland Business Research, Princeton, NJ, pp. 147–170. Day, B.P.F. 1993. Fruits and vegetables In Principles and Applications of Modified Atmosphere Packaging of Foods. R. T. Parry (Ed.), Blackie Academic & Professional, New York, pp. 114–133. ERS-USDA, 2001. Product liability and microbial foodborne illnesses, AER-799. 3-8. www.ers.usda.gov/publications/aer799/aer799c.pdf FAO. 1984. Manuals of food quality control 5. Food Inspection (FAO Food and Nutrition Paper) FAO, Rome, Italy, pp. 9–96.

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FDA Guide to minimize food safety hazards for fresh fruits and vegetables. 1998. Food and Drug Administration U.S. Department of Agriculture, Centers for Disease Control and Prevention. October 26, 1998 Fleischman, G.J., Bator, C., Merker, R. 2001. Hot water immersion to eliminate Escherichia coli 0157:H7 on the surface of whole apples: Thermal effects and efficiency. J. Food Prot. 64(4): 451–455. Golomski, W.A. 1993 Total quality management and the food industry. Why is it important? Food Technol., 47(5): 74–79. Haard, N.R. and Chism, G.W. 1996. Characteristics of edible plant tissues. In Food Chemistry. 3rd ed. O.R. Fennema (Ed.), Marcel Dekker, New York, pp. 943–1011. Harding, P.L., Winstoo, L.R., and Fisher, D.F. 1945. Seasonal changes in Florida Oranges. USDA Tech. Bull., 753. Hartz, R.E. and Lawver, K.E. 1965. The effect of sprays on quality factors of canned red tart cherries. Food Technol., 19(3): 103–105. Holdsworth, S.D. 1983. The preservation of fruit and vegetable food products. In Science in Horticultural Series, L. Broadbent (Ed.), Macmillan, London, pp. 61–98. IFT Expert Report on Emerging Microbiological Food safety Issues: Implications for Control in the 21st Century. 2002. Released at IFT International Food Safety and Quality Conference and Expo, Atlanta, GA, Feb. 20, 2002. Izumi, H. 1999. Electrolyzed water as a disinfectant for fresh-cut vegetables. J. Food Sci., 64: 536–539. Johnson, E. 1945. Effect of hormone weed killers on citrus trees. California Citrograph, 30: 305. Kim, J.M., Huang, T.S., Marshall, M.R., and Wei, C.I. 1999. Chlorine dioxide treatment of seafoods reduce bacterial loads. J. Food Sci., 64(6): 1089–1093. Kim. C., Hung, Y.C., and Brackett, R.E. 2000. Role of oxidation-reduction potential in electrolyzed oxidizing and chemically modified water for the inactivation of food related pathogens. J. Food Prot., 63: 19–24. Khattak, L.H. 1973. A Production and Quality Control Manual for the Frozen Fruit and Vegetable Processing Industry of Prince Edward Island. Department of Industry and Commerce, Charlottetown, Prince Edward Island, pp. 2–89. Koseki, A. and Itoh, K. 2001. Prediction of microbial growth in fresh-cut vegetables treated with acidic electrolyzed water during storage under various temperature conditions. J. Food Prot., 64(12): 1935–1942. Koseki, A., Yshida, K., Isobe, S., and Itoh, K. 2001. Decontamination of lettuce using acidic electrolyzed water. J. Food Prot., 64(5): 652–658. Kramer, A. 1973. Fruits and Vegetables. In Quality Control for the Food Industry. 3rd ed. Vol. 2, A. Kramer and B. A. Twigg (Eds.), AVI Publishing, Westport, CT, pp. 157–228. Li, Y., Brackett, R.E., Chen, J., and Beuchat, L.R. 2001. Survival and growth of Escherichia coli 0157:H7 inoculated onto cut lettuce before and after heating in chlorinated water, followed by storage at 5 or 15∞C. J. Food Prot., 64(3): 305–309. Mahoney, C.H. 1962. Flavor and quality changes in fruit and vegetables in the United States caused by application of pesticide chemicals. Residue Rev., 1: 11–23. Mazzotta, A.S. 2001. Heat resistence of Listeria monocytogenes in vegetables: evaluation of blanching process. J. Food Prot., 64(3): 385–387. Ozen, B.F., Han, Y., Floros, J.D., and Nelson, P.E. 2001. Quality characteristics of green peppers treated with Ozone (O3) and chlorine dioxide (ClO2) gases. Abstract of paper presented at IFT Annual Meeting, June 22–27, 2001, New Orleans, LA, 88E-6. Pao, S. and Davis, C.L. 2001. Microscopic observation and processing validation of fruit sanitizing treatments for the enhanced microbiological safety of fresh orange juice. J. Food Prot., 64(3): 310–314. Perera, A.D. and De Silva, T. 1999. Hazard analysis and critical control point (HACCP). In Handbook of Food Preservation. M.S. Shafiur (Ed.), Marcel Dekker, New York, pp. 735–768. Ropkins, K. and Beck, A.J. 2000. Evaluation of worldwide approaches to the use of HACCP to control food safety. Trends Food Sci. Tech., 11: 10–21. Simons, L.K. and Sanguansri, P. 1997. Advances in the washing of minimally processed vegetables. Food Aust., 49(2): 75–80. Stauffer, L.E. 1988. Hazard analysis critical control points. In Quality Assurance of Food, Ingredients, Processing, and Distribution. Food & Nutrition Press, Westport, CT, pp. 19–40. Surak, L.G. 1992. The ISO 9000 standards. Establishing a foundation for quality. Food Technol. (November): 74–80.

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Suslow, T. and Cantwell, M. 2001. Recent findings on fresh-cut cantaloupe and honeydew melon. Fresh Cut (April): 18–20, 32. Thorner, M.E. and Manning, P.B. 1983. Quality Control in Food Service. AVI Publishing, Westport, CT, p. 232. Wogan, G.N. and Marletta, M.A. 1985 Undesirable or potentially undesirable constituents of foods. In Food Chemistry., 2nd ed. O.R. Fennema (Ed.), Marcel Dekker, New York, pp. 689–724. Zepp, G., Kuchler, F., and Lucier, G. 1998. Food safety and fresh fruits and vegetables: is there a difference between imported and domestically produced products? Vegetables and Specialties, Situation and Outlook Report, ERS/USDA, VGS-274: 23–28, April, 1998. Zomorodi, B. 1990. The technology of processed/prepackaged produce. Preparing the product for modified atmosphere packaging (MAP). CAP ’90, Schotland Business Research, Princeton, NJ, pp. 301–311.

of Fruits and 15 Packaging Vegetables James P. Smith, Devon Zagory, and Hosahalli S. Ramaswamy CONTENTS 15.1 15.2

Introduction ........................................................................................................................356 Growth of Packaging of Fruits and Vegetables.................................................................357 15.2.1 Developments in New Polymeric Barrier Packaging Materials .........................357 15.2.2 Increased Urbanization ........................................................................................357 15.2.3 Market Needs and Consumer Demands for Convenience..................................357 15.2.4 Increasing Energy Costs ......................................................................................358 15.3 Definition and Functions of Packaging .............................................................................358 15.3.1 Container..............................................................................................................358 15.3.2 Protection .............................................................................................................358 15.3.3 Medium of Communication ................................................................................359 15.3.4 Means of Minimizing Costs ................................................................................359 15.3.5 Means of Selling Product ....................................................................................359 15.4 Types of Packaging Materials............................................................................................360 15.4.1 Wood and Textiles ...............................................................................................360 15.4.2 Paper and Board ..................................................................................................361 15.4.3 Glass.....................................................................................................................362 15.4.3.1 Types of Glass Containers .................................................................363 15.4.4 Metal ....................................................................................................................364 15.4.4.1 Steel Cans ..........................................................................................364 15.4.4.2 Aluminum Cans .................................................................................365 15.4.4.3 Aluminum Foil...................................................................................367 15.4.4.4 Composite Cans .................................................................................367 15.4.5 Plastics .................................................................................................................368 15.4.5.1 Flexible Single Films.........................................................................369 15.4.5.2 Coated Films ......................................................................................371 15.4.5.3 Laminated Films ................................................................................371 15.5 Barrier Properties of Packaging Materials ........................................................................372 15.6 Selection of Proper Packaging...........................................................................................372 15.7 Packaging Requirements of Fruits and Vegetables ...........................................................373 15.8 Bulk Packaging of Fresh Fruits and Vegetables................................................................376 15.9 Packaging of Fresh Fruits and Vegetables at the Store Level...........................................378 15.10 Retail Packages ..................................................................................................................378 15.10.1 Plastic Films ........................................................................................................378 15.10.2 Backings...............................................................................................................379 15.10.3 Boxes....................................................................................................................379 15.10.4 Bags......................................................................................................................379

0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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15.11 Choice of Package .............................................................................................................379 15.12 Packaging of Frozen Fruits and Vegetables ......................................................................380 15.12.1 The Plastic Pouch ................................................................................................381 15.12.2 Laminated Cartons...............................................................................................382 15.12.3 Composite Can ....................................................................................................382 15.13 Modified Atmosphere Packaging.......................................................................................383 15.13.1 Active and Passive Modification .........................................................................383 15.13.2 Equilibrium MAP ................................................................................................384 15.13.3 Microperforated and Microporous Films ............................................................387 15.13.4 Fresh-Cut Fruit Packaging...................................................................................389 15.14 Other Methods of MAP .....................................................................................................389 15.14.1 Individual Seal Packaging ...................................................................................389 15.14.2 Ethylene Absorbents ............................................................................................390 15.14.3 Edible Films.........................................................................................................390 15.15 Public Health Concerns about MAP Fruits and Vegetables..............................................390 15.16 Thermally Processed Fruits and Vegetables ......................................................................391 15.16.1 Canning of Fruits and Vegetables .......................................................................391 15.16.2 Aseptic/Ultra-High-Temperature (UHT) Packaging ...........................................391 15.17 Packaging of Dry Fruit and Vegetable Products ...............................................................393 15.18 Conclusion..........................................................................................................................393 References ......................................................................................................................................394

15.1 INTRODUCTION Fruits and vegetables are an important part of a healthful, balanced diet. They provide us with essential vitamins, are a rich source of protein and fiber, and are also aesthetically pleasing to the eye and the olfactory sense. However, unlike most other food commodities, fruits and vegetables continue to be living organisms even after harvest. As a result of their biological nature, they are subject to physical, chemical, and microbiological deterioration from the time they are harvested until consumption. Physical deterioration includes bruising, softening, and moisture loss, the latter resulting in shriveling of products. Fruits and vegetables also continue to respire after harvest. These biochemical changes result in breakdown of carbohydrates and a buildup of CO2 and C2H4, changes in composition, and changes in the color and odor of many fruits and vegetables. In addition, enzymatic activity, e.g., pectinases, can result in softening of fruit. Microbiological deterioration due to the growth of molds, yeasts, and bacteria results in changes in the color, odor, and texture of products. All of these changes can occur alone or in conjunction with one another. For example, physical bruising will enhance the activity of polyphenoloxidase (PPO) enzymes and cause browning of tissue. Bruising will also facilitate the entry of spoilage microorganisms into the underlying tissues, thereby facilitating spoilage and the survival of human pathogens, should they be present (Wells and Butterfield, 1977, 1999). While good manufacturing practices and proper temperature or humidity control can reduce physical, chemical, and microbiological deterioration, proper packaging can also play an important role in maintaining the quality and shelf life of fresh produce throughout the distribution chain. Many fruits and vegetables undergo further processing, (e.g., thermal processing, freezing, or drying) to inhibit or delay enzymatic and microbial deterioration and to extend product shelf life. Here again, the success or failure of any processing operation is dependent, in part, on the correct choice of packaging container. For example, if dried fruits are packaged in a material that has too low a moisture barrier, the product will pick up moisture from the external atmosphere to a level conducive to mold growth. In other instances, the packaging container is an integral part of the

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food process operation and ensures the quality and safety of the processed products, e.g., canned fruits and vegetables. There is a multitude of packaging materials in today’s marketplace, each incorporating specific properties. The correct choice of packaging is not only dependent on a knowledge of the physical, chemical, and microbiological characteristics of fruits and vegetables, but also on the functional properties of the packaging materials available for a particular product or preservation technology. This chapter will give a brief overview of the properties of the materials most commonly used for packaging of fresh and processed fruits and vegetables and the packaging technologies that can be applied for shelf-life extension of products.

15.2 GROWTH OF PACKAGING OF FRUITS AND VEGETABLES In the past three decades, there has been a tremendous growth in prepackaged fruits and vegetables on supermarket shelves and new food processing or packaging technologies, such as aseptic processing and controlled or modified atmosphere packaging, for shelf-life extension of these products. The growth of packaging, for both short- and long-term preservation of fruits and vegetables, is due to a number of interrelated factors.

15.2.1 DEVELOPMENTS

IN

NEW POLYMERIC BARRIER PACKAGING MATERIALS

The success of any packaging technology as a means of extending the shelf life of food is dependent on the permeability characteristics of the packaging materials surrounding a product. Developments in polymer chemistry have resulted in the production of packaging films such as low-density polyethylene (LDPE), polystyrene (PS), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) commonly known as Saran, and ethylene vinyl alcohol (EVOH). These films have a range of water vapor, gas barrier, and heat-sealing characteristics that enable them to be used alone or together in laminated and coextruded structures with the desired permeability characteristics for shelf-life extension of products. In addition, developments in high-speed continuous and thermoforming packaging equipment, compatible with the machinability characteristics of these films, have also promoted the growth of packaging of fruit and vegetable products.

15.2.2 INCREASED URBANIZATION In the past, production of food was a major task for the majority of the population. With the industrial revolution, rural populations decreased as people moved into cities to be closer to their workplace. Most urbanized nations are dependent on a food supply chain that extends all the way from the farm gate, which may be thousands of miles away, to the urban meal table. The food processing and packaging industry provides an essential chain in this long link by ensuring that consumers have a constant supply of a variety of fresh and processed fruit and vegetables that are nutritious and safe to eat.

15.2.3 MARKET NEEDS

AND

CONSUMER DEMANDS

FOR

CONVENIENCE

Over the past 20 to 30 years, there have been many changes in consumers’ lifestyles. The fundamental change is in relation to the traditional roles of women, one of which was meal preparation. With over 60% of the women in industrialized countries in the workplace, the time previously available for shopping and food preparation has decreased substantially. The result is that, today, with the need for convenience, many consumers are prepared to pay to have food products that require a minimum of preparation, e.g., prepared salads, frozen vegetables, instant mashed potatoes, and fruit cocktail. In fact, the consumer is indirectly asking the food industry to take over a part of, or in some cases all of, the more time-consuming steps associated with food preparation. In

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many cases, packaging serves as a convenience tool for easy preparation, handling, opening and recycling, portion control, or even as a serving utensil (TV dinners).

15.2.4 INCREASING ENERGY COSTS Increasing energy costs associated with traditional methods of food preservation and storage, such as freezing, have resulted in the growth of less energy-intensive and more economical methods of short- and long-term preservation, e.g., modified atmosphere packaging (MAP). It has been estimated that MAP is 18 to 20% less energy-intensive than freezing for shelf-life extension of bakery products (Aboagye et al., 1986). As a result of these interrelated factors, food packaging technology has gone through a tremendous transformation. Packaging of fruits and vegetables now provides increased consumer information and is used very effectively as a marketing tool. It has clearly evolved from its primary, and previously singular, role of protection to be a more multifaceted tool.

15.3 DEFINITION AND FUNCTIONS OF PACKAGING There are several definitions of the term packaging. Robertson (1992) defined packaging as “the enclosure of products, items, or packages in a wrapped pouch, bag, box, cup, tray, can, tube, bottle, or other container to perform the following functions: containment; protection; or preservation; communication; and utility or performance.” If the device or container performs one or more of these functions, it is considered a package. This definition implies that packaging serves more than one function; i.e., it is polyfunctional. Brown (1992) stated that the functions of a package are “to preserve the quality and freshness of food, to add appeal to consumers, and to facilitate its storage and distribution.” The basic functions required of a package can be grouped under five major categories.

15.3.1 CONTAINER A primary function of any package is to contain and facilitate handling, storage, and distribution all the way from the manufacturer to the ultimate user. However, there are usually various levels of packaging. A primary package is one that comes into direct contact with the contained product, e.g., metal cans, glass jars, and plastic pouches. By law, a primary package must not yield any substance that may be injurious to the health of the consumer. A logical development to facilitate handling even further is to bundle a series of these primary packages together, and this leads us to the concept of secondary packages. Examples of secondary packages are tins of apple juice inside a corrugated box. As methods of handling and transportation have become more sophisticated, these secondary packages are often palletized and secured by strapping with metal or, more commonly, by shrink- or stretch-wrapped film to give yet another level of packaging, tertiary packaging. In turn, these pallet loads may be packed into large metal containers, i.e., quaternary packaging for transportation over long distances by air, land, or sea.

15.3.2 PROTECTION The most important function of any container is to protect the product contained against any form of loss, damage, deterioration, spoilage, or contamination that might be encountered throughout the distribution chain. Packaging can prevent physical damage, e.g., bruising caused by vibrational shocks during transportation or stacking in a warehouse. Proper packaging will also prevent material loss, e.g., potatoes from a weak sack or juice from a leaky can. Packaging can also protect products against moisture loss or gain, dust, and light, especially UV light, which causes deterioration of some light-sensitive products. It can also protect the package contents against temperature

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fluctuations in the transit of chilled and frozen foods. Packaging can also be used to control the availability of oxygen to fruits and vegetables and to protect against loss of flavor or fragrance and help products retain their nutritional value. Proper packaging may also protect against microbial spoilage by bacteria, yeasts, and molds. It can also protect against macrobiological spoilage of stored products due to rodents and insects.

15.3.3 MEDIUM

OF

COMMUNICATION

An important function of any food package is to identify the product and its origin; to inform the consumer how to use the contents; to provide any other information needed or required; and very importantly, to attract the user and encourage purchase of the product. The information a package can convey to the consumer may include the following: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Product manufacturing and best buy dates Proper storage conditions Cooking instructions Size and number of servings or portions per pack Nutritional information per serving Manufacturer’s name and address Cost Suggested recipes Country of origin

15.3.4 MEANS

OF

MINIMIZING COSTS

An important fact often overlooked is that packaging actually reduces costs for the consumer. Packaging reduces food costs by reducing the cost of processing. Foods can be processed where they are grown, waste is treated at the processing plant, and shipping weights are reduced, thereby lowering the cost of transportation. The handling of packages in quantity is important for the economics of bulk storage, warehousing, transport, and distribution. Proper packaging facilitates efficient and mechanized handling, distribution, and marketing of products, thus reducing the high labor costs that would have to be absorbed into the price of the product.

15.3.5 MEANS

OF

SELLING PRODUCT

Another important function of packaging is to help sell the product it contains. Packaging is often referred to as the “silent salesman.” Robertson (1992) concisely summarized the multifunctions of packaging when he stated that “a package must protect what it sells and sell what it protects.” According to Jelen (1985), primary packages should have the following characteristics to facilitate the sale of products: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Sanitary Nontoxic Transparent Lightweight Tamper evident Easy to pick up and handle Easy to fit into cupboards, shelves, refrigerators, etc. Easy to open and dispense from Easy to reclose Returnable, recyclable, or reusable Safe and presents no hazards in the way of broken glass or sharp jagged metal edges

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TABLE 15.1 Functional Requirements of Packaging Materials Functional Property

Specific Factors

Gas permeability Protection against environmental factors Mechanical properties

O2, CO2, N2, H2O vapor Light, odor, microorganisms, moisture Weight, elasticity, heat-sealability, mechanical sealability, strength (tensile, tear, impact, bursting) Grease, acid, water, color Attractiveness, printability, cost Disposability, repeated use, resealability, secondary use

Reactivity with food Marketing-related properties Convenience

Source: Jelen, P. 1985. Food packaging technology. In Introduction to Food Processing, Reston Publishing, Reston, VA, pp. 249–266.

TABLE 15.2 Commonly Used Packaging Materials Materials

Examples of Use

Wood Cloth Paper Glass Metal Plastics Laminates

Crates, pallets Sacks Bags, boxes, cartons Bottles, jars Cans, aluminum foil Overwraps, bags, cups, bottles Multilayered plastics, cartons

Source: Jelen, P. 1985. Food packaging technology. In Introduction to Food Processing, Reston Publishing, Reston, VA, pp. 249–266.

The desirable polyfunctional properties of packaging materials are summarized in Table 15.1.

15.4 TYPES OF PACKAGING MATERIALS A variety of packaging materials, each with specific functional properties, is commercially available for packaging fruits and vegetables. These include wood, cloth, paper, glass, metal, and plastic (Table 15.2). Each of these materials will be briefly reviewed.

15.4.1 WOOD

AND

TEXTILES

Wooden containers were traditionally used for the bulk transportation of fruits and vegetables to the marketplace. Wood offers good mechanical protection, good stacking characteristics, and a high weight-to-strength ratio. However, it is not an effective moisture or gas barrier, and it can also be a source of microbial contamination, particularly by molds. With the advent of plastics, wooden containers are gradually being replaced by polystyrene, polypropylene, and polyethylene containers, which are lighter and have lower transportation costs. Textile containers, e.g., jute sacks, are also used sparingly for the bulk transportation of fruits and vegetables to the marketplace. While jute sacks are durable and have a high tear resistance,

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TABLE 15.3 Types of Paper Commonly Used as Packaging Material Product

Characteristics

Kraft paper

Brown, unbleached paper. Good strength and resistant to bursting White paper, may be glossy. Less strength than unbleached paper Translucent paper treated with H2SO4 to gelatinize surface layers High-density paper, very smooth surface

Bleached paper Parchment paper Greaseproof paper Glassine

High-density, greaseproof paper. Transparent, brittle Lightweight paper produced from most pulps

Tissue Paperboard or cardboard Corrugated cardboard

Compacted paper pulp Paperboard sheets interspersed with paper corrugations

Example Heavy duty bags and sacks White bags, wrapping paper Butter and margarine wrap Wrapping paper requiring high resistance to grease Overwraps on candy Lightweight and used to protect soft products from dust and bruising Cartons, boxes, trays, separators Secondary boxes of many kinds

Source: Adapted from Jelen, P. 1985. Food packaging technology. In Introduction to Food Processing, Reston Publishing, Reston, VA, pp. 249–266; Brown, W. E. 1992. Properties of plastics used in food packaging. In Plastics in Food PackagingProperties, Design and Fabrication, Marcel Dekker, New York, pp. 103–139.

they have a low extensibility, are very poor barriers to moisture and gases, and are subject to mold growth. They are also being replaced by multiwall paper sacks and plastic net sacks.

15.4.2 PAPER

AND

BOARD

Paper and board are still very popular packaging materials in North America. Kelsey (1989) estimated that paper and paperboard packaging comprise about 31% of the approximately 70 million t of paper products produced annually, with a market value of $16 billion. Paper pulp is produced from wood chips by acid or alkaline hydrolysis. The pulp is suspended in water and beaten with rotating impellers and knives to split the cellulose fibers longitudinally. The fibers are then refined and passed through heated rollers to reduce the moisture content and then through finishing rollers to give the final surface properties to the paper. Alkaline hydrolysis produces sulphate pulp and acid hydrolysis produces sulfite pulp. The various types of paper and paperboard containers used as food packaging are shown in Table 15.3. Kraft paper is made from at least 80% sulphate wood pulp. It is a very strong paper, which is used to make grocery bags, multiwall bags, shipping sacks, and specialty bags that require both economy and strength for bulk packaging of powders, flour, sugar, fruits, and vegetables. Bleached papers are more expensive and weaker than unbleached ones, and they have excellent printability. Vegetable parchment is produced from sulphate pulp, which is passed through a bath of sulphuric acid. It has a more intact surface than kraft paper and therefore has greater grease resistance and wet strength properties than kraft paper. Because of its high grease resistance and wet strength, it is used for packaging butter and shortening (Fellows, 1988; Brown, 1992). Sulfite paper is lighter and weaker than sulphate papers. Greaseproof paper is made from sulfite pulp in which the fibers are more thoroughly beaten to produce a closer structure. It is resistant to oils and fats when dry, but these properties are lost when the paper becomes wet. Packaging applications for greaseproof papers include margarine wraps, french-fry bags, inner liners for

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multiwall sacks, and a liner in composite cans for packaging frozen juices. Glassine is a greaseproof sulfite paper that is given a high-gloss finish by the finishing rollers. It is used as wrapping material for candy products and certain bakery products. Tissue paper is a soft, nonresilient paper used to protect fruits against dust and bruising (Brown, 1992). A major disadvantage of paper as a packaging material is its poor barrier properties against moisture, gases, grease, and odors. Furthermore, it cannot be heat-sealed. To improve its barrier and heat-sealing properties, paper is often combined with wax, plastic film, metal foil, or a combination of foil and plastic film. Paperboard is made in a similar way to paper but is thicker in order to protect foods from mechanical damage. The main characteristics of board are thickness, stiffness, the ability to crease without cracking, the degree of whiteness, surface properties, and suitability for printing (Brown, 1992). White board is suitable for contact with food and is often coated with polyethylene, polyvinyl chloride, or wax for heat-sealability. It is commonly used to prevent freezer burn in stored frozen products. Pulp containers are made from paper pulp compressed in molds to remove moisture. Pulp containers are used for egg cartons, low-cost food trays, and cushioning of food products. Corrugated board is the most comnmon form of secondary food packaging and is used by virtually every industry. According to Kelsey (1989), 280 billion ft2 of corrugated board, with a market value of $11.8 billion, was produced in 1986. Corrugated board has an outer and inner lining of kraft paper with a central corrugating (or fluting) material. This is made by softening kraft paperboard with steam and passing it over corrugating rollers. The liners are then applied to each side using a suitable adhesive. The board is formed into cutouts, which are then assembled into cases at the filling line. There are four different flute sizes — A, B, C, and E flutes. These vary in height and the number of flutes per unit length of board. They can be used alone or in combination with one another to give single-face, single-wall, double-wall, and triple-wall corrugated board constructions, as shown in Figure 15.1. Corrugated board has good impact abrasion and compression strength and is mainly used as secondary packaging containers. The most standard type of secondary packaging material is single wall C flute. A high storage humidity may cause delamination of the corrugated material. This is prevented by lining with polyethylene or greaseproof paper or coating with microcrystalline wax and polyethylene (Brown, 1992).

15.4.3 GLASS Glass is one of the most important packaging materials because of its high barrier characteristics. Over 100 billion glass containers, with a market value of approximately $5 billion, are used annually by the food and beverage industry (Kelsey, 1989). Glass is an inorganic substance formed from a mixture of sand (73%), sodium oxide (13%), and calcium oxide (12%), with a proportion of broken glass or culler (15 to 30% of total weight). When heated to a high temperature (2700∞F), the raw materials liquefy. Specific amounts of molten glass or gobs are shaped in a parison mold by the blow-and-blow process or the press-and-blow process. The glass is then annealed at ~1000∞F to remove stresses and cooled under carefully controlled conditions to prevent distortion or fracturing. According to Fellows (1988), glass containers have several characteristics that make them ideal for food and beverage packaging: 1. 2. 3. 4. 5. 6.

They They They They They They

are impervious to moisture, gases, odors, and microorganisms. are inert and do not react with or migrate into food products. have filling speeds comparable to those of cans. are suitable for heat processing when hermetically sealed. are transparent to microwaves. are reusable and recyclable.

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(1)

(2)

(3)

(4)

(5)

FIGURE 15.1 Various types of corrugated board construction: (1) A flute, single wall; (2) B flute, single wall; (3) C flute, single wall; (4) C and C, double wall; (5) A, B, and C, triple wall (courtesy of MacMillan Bathurst, Montreal).

7. 8. 9. 10. 11. 12.

They They They They They They

are resealable. are transparent and display the contents. can be molded into a variety of shapes and colors. are perceived by the customer to add value to the product. are rigid and allow stacking without container damage. can be printed on directly or by using paper labels.

The main disadvantages of glass as a packaging container are: 1. Higher weight and hence higher transportation costs than other types of packaging containers 2. Lower resistance than other materials to fractures, scratches, and thermal shock 3. More variable dimensions than other containers 4. Potentially serious hazards from glass splinters or fragments in foods 5. Permeability to UV light This latter problem can be overcome by incorporating various oxides, sulphides, or selenides to color glass and block out the incident UV radiation (Fellows, 1988; Brown, 1992). 15.4.3.1 Types of Glass Containers Commonly used glass containers are bottles, jars, tumblers and jugs, carboys, vials, and ampoules (Kelsey, 1989). Bottles account for the bulk of glass containers. They are made in various shapes and sizes (from 4 oz to 1 gal) and are characterized by a round neck that is much narrower than the body. This facilitates pouring of contents and allows attachment of suitable closures such as screw-type or snap-on caps or cork plugs. Fruit juices and drinks are often packaged in bottles.

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Jars are widemouthed bottles with no neck, and this affords easy access to the product. They are used for liquid, viscous, solid, and semisolid products such as fruit pieces, sauces, and tomato pastes. They are closed in a similar manner to bottles, but with larger closures. Tumblers are similar to jars but without a neck and a “finish” for the end closure. They are shaped like a drinking glass and are used for such products as jams and jellies. Jugs are large sized bottles with carrying handles. They are used to package wine and institutional, industrial, and household products. Carboys are large globular wicker-covered glass bottles for holding acids or other corrosive liquids. Vials and ampoules are small, thin-walled glass containers. They are mainly used in the pharmaceutical industry for drugs and in the food industry for small quantities of very expensive ingredients, such as flavors (Kelsey, 1989).

15.4.4 METAL Metal cans, made from steel or aluminum, are widely used by the food industry to process a wide range of foods. It is estimated that over 100 billion cans, with a market value of ~$15 billion, are produced annually for the food and beverage industry (Kelsey, 1989). Metal cans have a number of advantages over other types of containers, including the following: 1. 2. 3. 4.

They have a high strength-to-weight ratio. They can be heat processed. They have excellent barrier and protective properties. They produce shelf-stable products that are safe and nutritious to eat and can be stored at ambient temperature. 5. They are tamperproof. However, the high cost of metal and relatively high manufacturing costs make cans expensive. Furthermore, they are heavier than other materials, except glass, resulting in increased transportation costs for the finished product. 15.4.4.1 Steel Cans 15.4.4.1.1 Three-Piece Cans One of the most commonly used primary packaging containers for a wide variety of processed fruits and vegetables is the three-piece can or sanitary can. It is made from steel that is electrolytically coated on both sides with either a thin layer of tin (tin-plated steel) or a layer of chromium–chromium dioxide (tin-free steel). Two main types of base steel are commonly used in can manufacturing: type L and type MR. Type L is very corrosion-resistant and is used in canning of very corrosive products, e.g., apple juice, berries, prunes, and pickles. Type MR steel is more suitable for canning moderately to mildly corrosive products, e.g., grapefruit, peaches, peas, and corn. Plain, uncoated tin plate or tin-free plate can be used to make cans when the interactions between the food and the container are not significant or when the quality of the food is better in an uncoated can. However, to further improve the tin plate or tin-free plate for use with certain classes of fruit and vegetable products, it is coated with a lacquer or enamel. There are certain desirable qualities enamels should possess before being applied to food cans (Ellis, 1979). They should: 1. 2. 3. 4. 5.

Be nontoxic Not affect the flavor or color of the food Provide a good barrier between the food and the container Be easy to apply to the tin plate Not peel off during sterilization or storage of canned product

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TABLE 15.4 General Types of Coatings Used in Canned Fruits and Vegetables Coating Fruit enamel C-enamel Citrus enamel Beverage can enamel

Typical Uses Dark colored berries and other fruits requiring protection from metallic salts Corn, peas, and other sulphur-bearing products Citrus fruits and concentrates Vegetable juices; red fruit juices; highly corrosive fruits; noncarbonated beverages

Type Oleo-resinous Oleo-resinous suspended zinc oxide pigment Polybutadienes Two-coat systems with various base coats and vinyl or acrylic top coats

Source: Adapted from Ellis, R. F. 1979. Rigid metal containers. In Fundamentals of Food Canning Technology, J. M. Jackson and B. M. Shin (Eds.), AVI Publishing, Westport, CT, pp. 95–123.

6. Have mechanical resistance to can manufacturing 7. Be economical Examples of the common types of enamels used by the food industry are shown in Table 15.4. Oleoresinous linings are the most common enamels used in the food industry. They include R fruit enamel and C corn enamel. These are formulated to give a good barrier between the can and acid products. The R enamel is used to protect the natural pigment of highly colored fruits such as berries, cherries, and beets. The C enamel is used to prevent black discoloration in foods such as corn and peas. The C oleoresin enamel contains about 15% zinc oxide, which reacts with sulphides evolved during heat processing to form white or essentially colorless products. Epoxy linings are characterized by their heat stability. They do not impart flavor to the food and can be modified with phenolics for use with fruit products. Vinyl linings are used as double coatings in combination with oleo-resinous and phenolic enamels for highly corrosive products, e.g., fruit juices. They do not impart flavor to the food but have poor resistance to high temperatures. They are well suited for acidic products that do not need to be heat-sterilized and can be processed at temperatures below 100∞C. Outer coatings can also be applied to the outer can surfaces to prevent corrosion. Outside coatings of acrylics, phenolics, oleoresins, and vinyls are usually pigmented. They must be able to survive the heat-processing treatment and be receptive to decorative coatings and inks (Ellis, 1979). Three-piece cans are fabricated as shown in Figure 15.2. Sheets of tin plate or tin-free plate, with or without enamel coating, are cut into pieces to form the body of the can. Each body blank is hooked at the corners, flattened, and then seamed by soldering, cementing, or welding (Figure 15.2). The body blank is flanged, and the can bottom (manufacturer’s) is double seamed onto the body. The can top is seamed on at the production line after the can is filled with product. Three-piece cans come in a variety of shapes and sizes. Examples of common can sizes used by the food industry are summarized in Table 15.5 (Jelen, 1985). Two-piece drawn and ironed and two-piece drawn and redrawn cans are also made. The advantage of two-piece steel cans is that it is possible to eliminate the side seam and bottom sealing steps, resulting in improved structure and elimination of leakage problems; however, their production rate is slower than for three-piece cans. 15.4.4.2 Aluminum Cans Two-piece cans made of aluminum are commonly used for packaging fruit drinks and beverages. The advantages of aluminum cans compared to three-piece tin or tinless cans are:

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FIGURE 15.2 Construction of three-piece (sanitary) tin can (courtesy of Ball Packaging, Montreal).

TABLE 15.5 Types of Cans Used for Fruit and Vegetable Products Trade Name

Can Size

#1 Picnic 8Z Short 8Z Tall 8Z Mushroom #300 #1 Tall #303 #2 #2Cyl #10

211 211 211 300 300 301 303 307 307 603

¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥

400 300 304 400 407 411 406 409 512 700

Capacity (ml)

Typical Use

310 225 247 420 433 474 480 584 750 3109

Vegetables, soups Fruit, vegetables Fruit, vegetables Mushrooms Fruit juices Fruit, olives Fruit, soups, vegetables Fruit juices, vegetables Juices, soups Various products

Source: Jelen, P. 1985. Food packaging technology. In Introduction to Food Processing, Reston Publishing, Reston, VA, pp. 249–266.

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367

Lighter weight Lower transportation costs Easier to recover or recycle More resistant to corrosion Easier to open (pull tab)

The main disadvantages of aluminum cans compared to tin cans are less strength and higher production costs. Two-piece aluminum cans are made by the draw-and-wall-iron (DWI) or the draw-and-redrawn (DRD) process (Brown, 1992). The DWI process results in cans with thinner walls than the DRD process and is used to produce cans for carbonated drinks where the gas pressure supports the container. Thicker-walled DRD cans are able to withstand the head-space vacuum required in heat sterilization. Lacquers are applied internally to prevent interactions between the metal and the product. The type of lacquer used depends on the type of product packed. Epoxy-phenolic or vinyl-based lacquers are commonly used. 15.4.4.3 Aluminum Foil Aluminum foil is sheet metal of a very thin gauge. It is produced by the cold reduction process through which pure aluminum is pressed to reduce its thickness to less than 0.152 mm and annealed to give folding properties. The advantages of foil as a packaging material are: 1. 2. 3. 4. 5.

Good appearance Excellent dead-folding properties Ability to reflect radiant energy Excellent barrier to moisture, gases, and odors Nonabsorbent and nontoxic

Foil (more than 0.015 mm thick) is totally impermeable to moist gases, light, and microorganisms. It is widely used for wraps (0.009 r bottle caps [0.05 mm]), and trays for frozen and ready meals (0.0^ mm). Foil trays are coated with vinyl epoxy compounds to make them suitable for microwave heating without damage to the magnetron. The disadvantages of aluminum foil are: 1. Low strength due to its thin gauges 2. Readily attacked by high-acid products 3. Not heat-sealable To overcome these problems, the foil is often laminated on the outside paperboard (to increase its strength) and with low-density polyethylene on the inside to impart resistance to high-acid products and to develop heat-sealant characteristics. Aluminum is also used to metallize flexible films. 15.4.4.4 Composite Cans With increased costs of steel and aluminum, cans made from a combination of paperboard, foil, and plastic are now making inroads into areas previously dominated by metal cans. Kraft paper is the main component in the can body. The inside of the can is plastic (low-density polyethylene, polypropylene, or ionomer) often backed by foil for added barrier properties. End closures can be made of metal, plastic, paper, or a combination of these materials. Composite cans are manufactured by a spin convolute wound method, with spiral cans dominating the market due to their superior barrier characteristics. Composite cans are widely used to package shortening, powdered and dehydrated baby foods, aseptically packaged single-strength fruit juices, and frozen dough.

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TABLE 15.6 Examples of Basic Plastics Used as Packaging Material Materials

Structural Unit

Cellulose

Glucose

Polyethylene

Ethylene

Polyester Polyamide

Ethylene glycol + terephthalic acid Diamine + various acids

Polypropylene

Propylene

Polystyrene Poly vinyl-chloride

Styrene Vinyl chloride

Polyvinylidene-chloride Ethylene vinyl acetate

Vinyl alcohol + Vinylidene chloride Vinyl acetate + ethylene

Ethylene vinyl alcohol

Vinyl alcohol + ethylene

Ionomer

Methacrylic acid + ethylene

Important Properties Good strength, poor H2O and gas barrier, good printability, no heatsealability Good strength, flexible, extensible, high H2O barrier, poor gas barrier, low melting point, good heat-sealability Stiff, strong, inert, excellent mechanical properties, poor heatsealability, moderate H2O and gas barrier Stiff, strong, inert, clear excellent machinability, heat-sealable, poor H2O barrier, high gas barrier when dry Tough, inert, clear, low melting point, high H2O barrier, poor gas barrier Stiff, strong, brittle, low H2O and gas barrier Soft, inert, clear, extensible good H2O barrier, and moderate gas barrier Inert, clear, not very strong, high melting point, heat-sealable at high temperature, excellent H2O and gas barrier Tough, clear, inert, highly extensible, low melting point, heatsealable, intermediate H2O barrier, poor gas barrier Strong, stiff, inert, heat-sealable at low temperature, low H 2O barrier, high gas barrier Tough, inert, clear, heat-sealable at low temperature, intermediate H2O barrier and low gas barrier

Source: Adapted from Jelen, P. 1985. Food packaging technology. In Introduction to Food Processing, Reston Publishing, Reston, VA, pp. 249–266; Brown, W. E. 1992. Properties of plastics used in food packaging. In Plastics in Food PackagingProperties, Design and Fabrication, Marcel Dekker, New York, pp. 103–139.

15.4.5 PLASTICS Over the past 30 years, there has been a tremendous increase in the use of plastics replacing traditional packaging materials such as glass, metal, and paper. The raw materials for plastics are petroleum, natural gas, and coal. They are formed by a polymerization method that creates linkages between many small repeating chemical units (monomers) to form large molecules or polymers. Examples of some of the common plastic materials and their monomer building blocks are summarized in Table 15.6. Many plastics contain very small amounts of additives such as plasticizers, antioxidants, lubricants, antistatic agents, heat stabilizers, and UV stabilizers. These are added to facilitate processing of plastics or to impart some desirable properties to the plastics. For example, plasticizers are added to soften plastics, thus making them more flexible and less brittle for use in cold climates or with frozen stored products. According to Fellows (1988), the advantages of plastics as packing materials are as follows: 1. 2. 3. 4. 5. 6. 7.

Relatively low cost Good barrier properties against moisture and gases Heat-sealable to prevent leakage of contents Suitable for high-speed filling Wet and dry strength Suitable for printing Easy to handle and convenient for the manufacturer, retailer, and consumer

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8. Add little weight to the product 9. Fit closely to the shape of the food, thereby wasting little space during storage and distribution Plastics may be made as flexible films or as semirigid and rigid containers to meet the varied packaging and processing requirements of food. Plastic films are made with a wide range of mechanical, optical, heat-seal, and barrier properties. Furthermore, they can be coated with another polymer or metallized to give a laminated structure with superior properties. Examples of some of the common flexible films and their properties will be briefly reviewed. 15.4.5.1 Flexible Single Films 15.4.5.1.1 Cellulose Cellophane is produced from wood pulp, treated chemically, and cast into a film on heated rollers. Glycerol is added as a softener, and the film is dried on heated rollers. Higher quantities of softener produce more flexible films. Plain, uncoated cellulose is a glossy, transparent film that is odorless, tasteless, and biodegradable. It is tough and puncture-resistant although it tears easily. However, it is not heat-sealable, and it has poor water and gas vapor barrier characteristics. Plain cellophane is used for foods that do not require a complete moisture or gas barrier. Most cellophane is sold coated either with nitrocellulose on one or both sides or with polyvinylidene chloride (Saran). These coatings improve the gas barrier and heat-seal characteristics of cellophane (Fellows, 1988; Brown, 1992). 15.4.5.1.2 Polyethylene Polyethylene is the most commonly used polymer in food packaging applications. It is produced by polymerization of ethylene. Three main types are produced: low-density polyethylene (LDPE), high-density polyethylene (HDPE), and linear low-density polyethylene (LLDPE). The main characteristics of low-density polyethylene are it is heat-sealable at low temperatures (~80∞C, 175∞F) and it is chemically inert, odor-free, and shrinks when heated. It is a good moisture barrier but a poor gas barrier. This selective permeability makes it a good choice of packaging material for such products as fresh meat, fruits, and vegetables. It is less expensive than most films and is therefore widely used for many packaging applications, including applications in shrink- or stretch-wrapping of products. High-density polyethylene is less branched and more crystalline in structure than LDPE. It is therefore stronger, thicker, less flexible, less transparent, and more brittle and has lower permeability to gases and moisture than low-density polyethylene. It also has a higher softening temperature (121∞C) and can therefore be heat-sterilized. It is commonly used in the production of bags, as liners, and as an overwrap. Linear low-density polyethylene has a highly linear arrangement of molecules and combines the clarity of LDPE and the strength of HDPE. It is used where strength is required in packaging, e.g., sacks and bags (Fellows, 1988; Brown, 1992). 15.4.5.1.3 Polyester Polyester is a polycondensation product of ethylene glycol and terephthalic acid. The major polyester in the market place is polyethylene terephthalate (PET), marketed under the trade name Mylar. The main characteristics of PET are its strength and toughness, its clarity, its good barrier properties to moisture and gases, and its high melting point. These characteristics make it an ideal packaging material for carbonated soft drink bottles and as a component of boil-in-bag food packages and retortable pouches. A more crystalline PET (CPET) is used to make “dual-ovenable” food trays that enable precooked food and entrees to be heated in a radiant oven or microwave without deformation of the packaging tray (Fellows, 1988; Brown, 1992).

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15.4.5.1.4 Polyamide Polyamide is made by the condensation polymerization of an organic acid and an amine and marketed under the trade name nylon. Nylon is a clear, tough film with good mechanical properties over a wide temperature range (from –60∞C to 200∞C). It provides good gas and aroma barriers, but has poor moisture barrier properties. However, the films are expensive to produce, and they require high temperatures to form a heat seal. Nylon is commonly used as the outer layer of laminated structures to add strength to the laminated structure. The barrier and mechanical properties of nylon can be enhanced through biaxial orientation to give biaxially oriented nylon (BON) (Fellows, 1988; Brown, 1992). 15.4.5.1.5 Polypropylene Polypropylene is made by the polymerization of the monomer propylene. Two main types are made: nonoriented or cast polypropylene (PP) and oriented polypropylene (OPP). Polypropylene is one of the lightest of all plastics. Compared to LDPE and HDPE, PP is stiffer, tougher, and more transparent. It also has superior gas and moisture barrier properties and higher heat resistance than LDPE and HDPE, which make it suitable for boil-in-bag and retortable products. It stretches, although less than polyethylene, and it has low friction, which minimizes static buildup and makes it suitable for high-speed filling equipment. However, it is more brittle than polyethylene at low temperatures. PP can also be used to package soft bakery products and fresh produce because it is flexible enough to fit around irregular shapes. Biaxially oriented polypropylene (OPP) is superior to PP in terms of clarity, impact strength, and barrier properties to water vapor and gases (Fellows, 1988; Brown, 1992). 15.4.5.1.6 Polystyrene Polystyrene is made by the polymerization of styrene via the double bond in the ethylene group attached to the benzene ring of the monomer unit. Polystyrene is a brittle, clear, and sparkling lowstrength film with a low melting point and poor impact resistance. PS films are used as windows in paperboard boxes due to their excellent clarity. Polystyrene may be oriented to improve its gas barrier properties. Polystyrene foam or expanded polystyrene (EPS) is made by adding hexane during polymerization. EPS is a rigid, low-density material that is widely used to make meat trays, egg cartons, cups, and containers. However, EPS is a poor gas barrier to oxygen and moisture vapor. EPS trays are commonly used with PE or PP overwraps that provide the necessary barrier properties to moisture (Fellows, 1988; Brown, 1992). 15.4.5.1.7 Polyvinyl Chloride Polyvinyl chloride is made by the low-pressure polymerization of vinyl chloride. It is an extremely brittle film, which requires large amounts of plasticizers to soften the film. Plasticized PVC films are tough, clear, and glossy with excellent moisture resistance and low gas permeability, and they can be processed to give films with good shrink properties. PVC is widely used to make clear plastic bottles and as an overwrap with EPS trays for meat and fresh produce (Fellows, 1988; Brown, 1992). 15.4.5.1.8 Polyvinylidene Chloride Polyvinylidene chloride (PVDC) is made by copolymerizing the monomers vinylidene chloride and vinyl chloride. Commonly known as Saran, PVDC is a clear, very strong film with excellent cling properties and is commonly used for packing cheese. It has very low gas and water vapor permeabilities and is used alone or in combination with other films when high barrier characteristics are required, e.g., gas packaging. Furthermore, it is heat-shrinkable and heat-sealable. However, because it is expensive to produce, it is commonly used in very thin gauges and laminated to other films for mechanical support and strength (Fellows, 1988; Brown, 1992).

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15.4.5.1.9 Ethylene Vinyl Acetate Ethylene vinyl acetate (EVA) is comprised of low-density polyethylene copolymerized with vinyl acetate. EVA is a clear, tough film (especially at low storage temperatures), and it has moderate moisture and poor gas barrier properties. It is heat-sealable at low temperatures and is commonly used as a heat-sealant layer in many laminated structures. EVA is used as an overwrap for fresh meat and poultry and as a tie layer when two films of dissimilar properties are laminated together (Fellows, 1988; Brown, 1992). 15.4.5.1.10 Ethylene Vinyl Alcohol Ethylene vinyl alcohol (EVOH) is a hydrolyzed copolymer of vinyl alcohol and ethylene. EVOH films are strong, have good clarity, are heat-sealable, and have excellent odor, gas, and moisture barrier characteristics. The major disadvantage of EVOH films is that they are hydrophilic and hygroscopic. When they absorb moisture at high relative humidity, the absorbed moisture acts as a plasticizer and the gas barrier properties of the film decrease. This can be overcome by increasing the ethylene content of the film, by laminating it between two films that protect it against moisture, or by adding a desiccant to the tie layer. EVOH is commonly used in laminated structures where high gas and moisture barrier characteristics are desired, e.g., modified atmosphere packaging applications (Fellows, 1988; Brown, 1992). 15.4.5.1.11 Ionomer lonomers are copolymers of methacrylic acid and ethylene and have some of the hydrogen atoms of the carboxyl groups replaced by either zinc or sodium atoms. Commercially known as surlyn, ionomers have exceptional toughness and clarity and heat-seal characteristics. They are commonly used to give a strong seal in laminated films used for packaging products with a high fat content, e.g., meat products. They are also used in skin packaging applications where clarity and toughness are required in the package. Ionomers also form strong bonds with aluminum foil (Fellows, 1988; Brown, 1992). A summary of the important characteristics of these films is shown in Table 15.6. 15.4.5.2 Coated Films Individual films are often coated with other polymers or aluminum to improve the barrier properties or to impart heat-sealability. For example, nitrocellulose is coated on one side with cellulose film to provide a moisture barrier while retaining oxygen permeability. A nitrocellulose coating on both sides of the film improves the barrier to oxygen, moisture, and odors and enables the film to be heat-sealed when broad seals are used. A polyvinylidene chloride coating is applied to cellulose, using either an aqueous dispersion (MXXT/A cellulose) or an organic solvent (MXXT/S cellulose). In each case, the film becomes heat-sealable, and the barrier properties are improved. A coating of vinyl chloride or vinyl acetate gives a stiffer film that has intermediate permeability. Sleeves of this material are tough, stretchable, and permeable to air and moisture. A thin coating of aluminum (termed metallization) produces a very good barrier to oils, gases, moisture, odors, and light. Metallized film is less expensive and more flexible than foil laminates that have similar barrier properties, and it is therefore suitable for high-speed filling on form-fillseal equipment. Cellulose, polypropylene, or polyester are metallized by depositing vaporized aluminum onto the surface of the film under vacuum. Metallized polyester has higher barrier properties than metallized polypropylene, but polypropylene is finding more widespread use as it is currently less expensive (Fellows, 1988; Brown, 1992). 15.4.5.3 Laminated Films Lamination of two or more films improves the appearance, barrier properties, or mechanical strength of a package. Materials that can be laminated to each other include plastic to plastic, paper to

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plastic, paper to aluminum foil, and paper to aluminum foil and then to plastic. Several methods can be used to laminate materials, including dry and wet bonding thermal extrusion and coextrusion. Laminated materials are used when high gas and moisture characteristics are required for a long shelf life. Laminated structures usually consist of an outer protective tougher layer, e.g., nylon or polypropylene, a middle high gas barrier layer, e.g., EVOH or PVDC, and an inner heatsealant layer. LDPE is commonly used as a heat-sealant layer because of its low melting temperature; however, it sometimes does not give a good seal with starchy or greasy food products. The choice of sealant layer for these food products is either EVA or Surlyn (Fellows, 1988; Brown, 1992).

15.5 BARRIER PROPERTIES OF PACKAGING MATERIALS Many materials can be selected for packaging fruit and vegetable products. When choosing the appropriate packaging material, the following factors should be considered: 1. 2. 3. 4. 5. 6. 7.

Gas barrier properties Moisture barrier properties Antifog properties Machinability Mechanical strength Sealability Performance vs. cost

One of the most important characteristics is the barrier properties to both oxygen and moisture vapor, which varies greatly from material to material. Because tin plate and glass are excellent gas barriers, examples of the barrier properties of various flexible films only are shown in Table 15.7 and Table 15.8 respectively. Examples of laminated structures and their barrier properties are shown in Table 15.9. High-barrier materials usually provide high barriers to both moisture and oxygen, e.g., glass, tin plate, and aluminum foil; however, the barrier properties to oxygen and moisture may be different and may also vary as a function of the relative humidity and temperature of the storage conditions. A good example is EVOH, a hygroscopic film that is an excellent oxygen barrier at low relative humidity. At higher relative humidity, it absorbs moisture that has a plasticizing effect and reduces the barrier characteristics to oxygen. Some films have mixed barrier properties, i.e., low oxygen barrier characteristics and high-moisture vapor barriers. A good example is LDPE, which explains why this film is selected for packaging fresh meat and produce and for frozen stored products to prevent freezer burn.

15.6 SELECTION OF PROPER PACKAGING The food processor has a variety of packaging materials to choose from for packaging fruits and vegetables, specifically, paper, glass, metal, and plastics. According to Fellows (1988), the choice of the proper packaging material will be made by the food processor based on the following criteria: 1. 2. 3. 4. 5. 6.

Composition of the food (solid or liquid) Physical, chemical, and microbiological and deteriorative reactions that might occur Storage conditions and time of storage Socioeconomic situation of the anticipated customer or market Desired package attractiveness Cost of the packaging material

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TABLE 15.7 Oxygen Transmission Rate (OTR) of Selected Packaging Materials cc/m2/da High Barrier Materials Glass Aluminum EVOH (0% RH) PVDC Medium Barrier Materials Oriented nylon Oriented PET Nonoriented nylon Nonoriented PET Rigid PVC EVOH (100% RH) Low Barrier Materials Polystyrene HDPE PP Polycarbonate Surlyn LDPE

0 0.1 0.2 2.5

28 36 78 109 150–205 160–280

1500 1705 2320 3500 5500 7500

Note: EVOH = ethylene vinyl alcohol; PVDC = polyvinylidene chloride; PET = polyester; PVC = polyvinyl chloride; HDPE = high-density polyethylene; PP = polypropylene; LDPE = low density polyethylene. a

Measured @23∞C and 0% RH.

7. Packaging technology selected 8. Specific functional properties of the packaging material Brown (1992) listed several reasons for selecting or rejecting a particular packaging material over another, as summarized in Table 15.10.

15.7 PACKAGING REQUIREMENTS OF FRUITS AND VEGETABLES The shelf life of packaged fruits and vegetables is controlled by the properties of the product (including water activity, pH, susceptibility to enzymic or microbiological deterioration, mechanism of spoilage, and the requirement for or sensitivity to oxygen, light, carbon dioxide, and moisture) and the properties of the package. Moisture loss or uptake is one of the most important factors that controls the shelf life of fruits and vegetables. Fruits and vegetables are high-moisture products, with moisture contents ranging from 75 to 95%. Loss of moisture under normal storage conditions causes wilting and shriveling of product. Proper packaging is able to extend storage life of fresh products by keeping moisture loss during storage to 5% or less, thereby preventing wilting. The rate of moisture loss depends on the vapor pressure deficit between the saturated intercellular spaces of the fruit or vegetable and

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TABLE 15.8 Moisture Vapor Transmission Rate (MVTR) of Selected Packaging Materials cc/mil/m2/da High Barrier Materials Glass Aluminum HDPE PVDC PP LDPE Oriented PET

0 0 3.8 4.0 6–10 18–23 19

Medium Barrier Materials EVOH Surlyn Rigid PVC Nonoriented PET

22–59 27 39–48 46

Low Barrier Materials Polystyrene Oriented nylon Polycarbonate Nonoriented nylon

78–132 1523 217 340

Note: 1 mil = 25 µ = 0.001 in. a

Measured @37.8∞C and 100% RH.

the atmosphere surrounding them. Packaging in plastic films greatly reduces the vapor pressure deficit and the rate of moisture loss. This is one of the chief benefits of packaging fresh-cut products in plastic films. The use of small perforations in some films to ensure a constant supply of oxygen has little appreciable effect on moisture loss. Fruits and vegetables are living organisms and, even after harvest, they continue to respire and transpire. Respiration involves the uptake of oxygen and breakdown of organic matter into water, carbon dioxide, heat, and metabolic energy. If there is not enough oxygen, fermentation occurs, and small amounts of alcohol, acetaldehyde, and other volatile compounds are produced. This results in off-flavors and off-odors and spoilage of the commodity; therefore, packaging materials for fruits and vegetables should not create too high a barrier to oxygen. The thermal properties of the packaging material should also be taken into consideration to minimize temperature fluctuations. Ripening and senescence can be slowed down by storage at refrigeration temperatures because this reduces the respiration rate and the synthesis of ethylene, which accelerates these processes. However, too low a temperature may cause chilling damage to products of tropical or subtropical origin. Therefore, proper packaging can also ensure temperature distribution within the package and prevent chilling injury. Some packages are required to withstand processing conditions (for example, hot filling, heat-sterilization, boil-in-the-bag). Packaging should retain desirable odors (e.g., strawberries), or prevent odor absorption by dried products. There should also be negligible odor absoprtion from the plasticizers, printing inks, adhesives, or solvents used in the manufacture of the packaging material. Packaging protects food from mechanical damage caused by transportation or handling, e.g., vibration, impact, and compression damage. Compression damage during storage may arise owing

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TABLE 15.9 Examples of Laminated Packaging Materials High Barrier Materials (Flexible) Nylon/Saran/EVA Nylon/Saran/Surlyn Nylon/EVOH/Surlyn PET/Saran/EVA PET/Saran/Surlyn Nylon/Saran/LDPE PET/Foil/LDPE High Barrier Materials (Semirigid) PVC/EVOH/EVA coextrusion PVC/EVOH/Surlyn coextrusion Medium Barrier Materials (Flexible) Nylon/EVA Nylon/Surlyn Polyester/EVA Polyester/Surlyn Low Barrier Materials (Flexible) OPP/EVA OPP/Surlyn PP/EVA PP/Surlyn Note: OPP = oriented polypropylene; EVA = ethylene vinyl acetate.

TABLE 15.10 Reasons for Selection and Rejection of Specific Packaging Materials Paperboard

Glass

Steel

Plastics

Easily machined and folded Easy to bond Composites well Printability

Selection Product visibility Impervious, inert Image of high quality Dual-ovenable

Strong, stiff Malleable Retortable Permanence

Fabricability Variety of forms Tough, lightweight Wide range of properties

Likes water Penetrable Image Tears, punctures

Rejection Shatters High weight-to-strength ratio Limited shapes Large sizes

Corrodes Limits shapes Appearance Flavor distortion

Thermal limit Permeable Absorbs flavors Distortion and creep

Source: Brown, W. E. 1992. Properties of plastics used in food packaging. In Plastics in Food PackagingProperties, Design and Fabrication, Marcel Dekker, New York, pp. 103–139.

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to overstacking. Metals, wood, or fiberboard shipping cases prevent mechanical damage, and products are held tightly within retail containers by molded trays to prevent their movement. Groups of retail containers are similarly immobilized by shrink- or stretch-wrapping. Molds and bacteria are a major cause of deterioration and loss of fruits and vegetables. They can either grow on the surface of the products or may spread inside the product due to surface bruising or cuts and cause internal decay. Careful handling and proper packaging can minimize physical damage and delay microbial spoilage of fruits and vegetables. However, too high a moisture barrier may cause a high internal relative humidity in the package. If the package is exposed to temperature fluctuations, moisture can condense and this is conducive to microbial growth. Packaging cannot prevent fresh fruits and vegetables from spoilage and decay. On the other hand, incorrect packaging can accelerate spoilage. Packaging can serve to protect against contamination, physical damage, and excessive moisture loss.

15.8 BULK PACKAGING OF FRESH FRUITS AND VEGETABLES The primary functions of bulk packaging of fruits and vegetables are to provide a means of shipping a suitable quantity of product in one marketing unit and to protect the products during loading, transport, unloading, and marketing from physical injury and spoilage. The susceptibility of various fruits and vegetables to physical damage is summarized in Table 15.11 (Wills et al., 1989). To meet these requirements, bulk packaging containers should be able to:

TABLE 15.11 Susceptibility of Produce to Types of Mechanical Damage Type of Injury Produce

Compression

Impact

Vibration

Apple Apricot Banana, green Banana, ripe Cantaloupe Grape Nectarine Peach Pear Plum Strawberry Summer squash Tomato, green Tomato, pink

S I I S S R I S R R S I S S

S I I S I I I S I R I S I S

I S S S I S S S S S R S I I

Note: S = susceptible; R = resistant; I = intermediate. Source: Wills, R. B. H., McGlasson, W. B., Graham, D., Lee, T. H., and Hall, E. G. 1989. Handling, packaging, and distribution. In Postharvest — An Introduction to the Physiolology and Handling of Fruits and Vegetables, New South Wales University Press, NSW, Australia, pp. 132–144.

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1. 2. 3. 4. 5. 6.

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Protect products from physical injury (impact, compression, vibration, and abrasion bruising) Facilitate adequate temperature control throughout distribution and storage Protect from water loss to prevent shriveling or wilting Facilitate certain treatments, e.g., fumigation, ethylene treatment to enhance ripening Be compatible with handling systems, e.g., palletization Be adaptable to handling and storage requirements, e.g., high relative humidity, ice packing, and controlled atmosphere storage

Containers used for bulk distribution of fruits and vegetables include nailed wooden boxes, wire bound veneer crates, and, most commonly, fiberboard containers. Returnable plastic containers (RPCs) are becoming another important form of packaging because they are preferred, and even required, by some of the largest retailers. Wooden boxes and RPCs provide rigidity and strength, allow rapid cooling of products, and do not lose their shape, appearance, and strength under high-humidity storage conditions. Wirebound veneer crates also lend themselves to mechanized filling and closing, allow good ventilation and cooling during storage, and have good stacking strength. However, they lack the rigidity of wooden boxes and are more prone to deformation during prolonged storage and shipment. Some buyers still prefer wirebound crates for green beans, asparagus, and other products due to their old fashioned look. Fiberboard containers are usually less expensive than wooden boxes or veneer crates. Other important advantages include printability, adaptability to mechanical filling and sealing, lower shipping weight, freedom from damaging nails and wood splinters, and recyclability. Fiberboard containers can be constructed to suit the packaging requirements of specific commodities. For example, enhanced temperature management can be achieved through venting to allow products to cool down or through insulation to prevent chilling injury in products stored under refrigerated conditions. Some commodities, e.g., grapes, may be packed with sulphur dioxide emitting pads as a fumigant and require liners to restrict ventilation. Other commodities must be protected from ethylene. This can be achieved through in-package ethylene scrubbers, which perform best in containers with limited venting. While physical injury can be minimized by not overfilling containers or packing products too tightly, fiberboard containers can also be improved to prevent physical damage of fresh produce. For example, the addition of bottom cushion pads can minimize impact bruises during unloading of boxes, the use of inner liners of corrugated board or full telescope covers can increase stacking strength and the incorporation of fiberboard dividers or molded PS or paper pulp trays can separate and protect the more injury-sensitive and expensive products from abrasion and vibration damage. The principal disadvantage of fiberboard containers is their tendency to absorb moisture resulting in a substantial loss of rigidity and stacking strength. However, this can be overcome by the use of PE liners or curtains with small perforations to allow gas exchange or the addition of certain waxes to impart water resistance to the containers (Wills et al., 1989). A recent trend in carton packaging has been the adoption by many foodservice distributors and retailers of an industry standardized corrugated footprint that is merchandise friendly and open on the top. The purpose of standardizing cartons and trays is to permit containers to meet the standard of stacking evenly in any combination to allow display in the corrugated container and fit pallets for the 48 in. ¥ 40 in. GMA and the 1200 mm ¥ 1000 mm Euro pallets. The Fibre Box Association (FBA) Modularity Standard Task Force, with support from the American Forest & Paper Association (AF&PA), have developed a Corrugated Produce Container Modularity Standard (CPCMS) and are currently working with produce growers and shippers, repackers, wholesalers, retailers, and the trucking industry in evaluating this standardized packaging footprint. The standard provides for two container footprint systems, with dimensions of 24 in. ¥ 16 in. and 16 in. ¥ 12 in. (the 5 to 10 Down System) and 20 ¥ 16 and 16 ¥ 10 (the 6 to 12 Down System), specified stacking tab and

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tab receptacle sizes and locations, and an open top specification. The latter may also be achieved by either providing a carton or tray with an open area on the top or an easily and quickly removable cover or lid. In addition, nondisplay shipping containers, which work with the display containers, are covered by the standard if they meet the required size and receptacle requirements. Other container design criteria, including height, style, materials, colors, cooling or venting features, graphics, etc., remain a matter for commercial negotiation between the container manufacturer and the customer. Many retailers and food service companies now require their produce suppliers to use the standard footprint containers.

15.9 PACKAGING OF FRESH FRUITS AND VEGETABLES AT THE STORE LEVEL Bulk packaging of fruits and vegetables provides a measure by weight, count, or volume of products delivered to retail stores. In some cases, many fruits and vegetables may be displayed on store shelves in their opened bulk shipper case. While this may convey the impression of a more natural and fresh product to consumers, it is not too efficient in terms of quality preservation. To maintain product quality and freshness and to facilitate handling and transportation by customers, most fruits and vegetables are either prepackaged by the store or packaged by the customer in the store in small convenient retail units. The packaging requirements of fruits and vegetables at the retail store level are: 1. To minimize moisture loss and wilting 2. To keep products clean and free from external contamination 3. To facilitate transportation of products from the store Prepackaging at the retail level has many advantages. These are: 1. 2. 3. 4. 5. 6.

Good control of inventory and consumers’ choices and preferences Better convenience for consumers in terms of choice, unit weight, and cost Pricing of products when packaged More effective quality control because products are inspected and trimmed prior to sale Less chance of spoiled merchandise being placed on supermarket shelves Less opportunity for individual fruits, such as grapes, to fall on the floor and create a hazard that may lead to tripping and falling

Some of the disadvantages of retail packaging are: 1. 2. 3. 4.

Additional investment required for equipment, packaging materials, and space Small-scale operations justifying this initial investment Higher costs to the consumer Personnel need to be trained to pack properly and in the correct material

15.10 RETAIL PACKAGES Packaging materials at the retail level include plastic films, backing, and boxes. Bands and closures can help close packages and keep produce fresh.

15.10.1 PLASTIC FILMS Films are available in the form of sheets, rolls, bags, and sleeves and may be shrinkable or nonshrinkable. Plastic films used for fruits and vegetables should be high-moisture barriers and poor gas barriers. This latter characteristic is important to prevent fermentation that can occur at

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< 3% oxygen in the package headspace, depending on the temperature and commodity. If a film with high gas barrier characteristics is used, the package may be “vented” by making small holes in the package to prevent anaerobic conditions from occurring. Cellophane is a nonshrink film with poor gas and water vapor barrier characteristics. It is usually coated with nitrocellulose or PVDC to improve both its gas and water vapor barrier characteristics and heat-sealability. However, the coated film is not very permeable to oxygen, and therefore, the film is usually perforated by passing it over a roller studded with sharp needles. PVC stretch films have excellent clarity and are tough and nonfogging. Some films have intermediate water and gas vapor characteristics. They are commonly used as overwraps for fruits and vegetables. Low-density polyethylene is the most widely used nonshrink film for fruits and vegetables. It is strong, tear-resistant, easily heat-sealed, and inexpensive, and has good water and gas vapor characteristics. It is commonly used to bag heavier products such as oranges, carrots, onions, and potatoes. For products that have a high respiration rate, e.g., asparagus or broccoli, it must be vented. LDPE sleeves are commonly used for celery, and small bags with perforations are used to package lettuce. Polystyrene is a clear film with a high permeability to gases. It is used for lettuce and tomatoes, and is also available as a heat-shrinkable film. Polypropylene is a tough film and is used for fragile products such as lettuce, mesclun mixes, cabbage, and cauliflower. It is sometimes perforated because it is an intermediate barrier to oxygen. More recently, individual shrink film packaging has been used for fresh fruits and vegetables. Shrink packaging results in films with superior strength and barrier properties to moisture, resulting in greater protection of products. Common materials used in shrink packaging are LDPE and OPP.

15.10.2 BACKINGS Backings consist of a tray that holds the produce with a sleeve of film or film overwrap. They are made from paperboard, polystyrene, polypropylene, or laminations of plastics and aluminum foil. Each type is designed for a particular application, e.g., round products such as apples or tomatoes or long products such as corn or cucumbers. Many softer products are protected by placing them in PS trays or PP baskets that are overwrapped with PVC film. Both PS and PVC prevent moisture loss. PVC also has excellent clarity, gloss, and stretch wrap properties that enhance package appearance.

15.10.3 BOXES Small paperboard and plastic boxes (tills) are used for small fruits like berries and cherry tomatoes. Injection molded plastic tills are most commonly used. However, these are rapidly being replaced by plastic clamshells.

15.10.4 BAGS This kind of package includes all bags other than plastic film bags. Bags in this category include fiber net bags, plastic net bags, paper mesh, and plastic mesh bags. Mesh and net bags made from HDPE are commonly used to pack potatoes, onions, oranges, and grapefruits. Field packing of grapes in plastic mesh netting makes the fruit easily visible and it can be washed and stored in the bag. Bags can be closed by twist ties or by cellophane adhesive and stapling.

15.11 CHOICE OF PACKAGE As fruits and vegetables vary in their physical attributes (shape, size, and toughness), as well as their moisture contents and respiration rates, a specific type of packaging has to be selected to minimize the perishability of the product.

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Spoilage and rotting are the major problems of soft fruits because they bruise and squash easily. Typical soft fruits include grapes, blueberries, strawberries, raspberries, and plums. Usually, these are packaged in semirigid plastic containers, e.g., HDPE or PP, with a plastic overwrap of PS, which is vented to prevent fogging. Hard fruits such as avocados, peaches, pears, bananas, citrus fruits, and tomatoes have lower respiration rates and are less sensitive to handling. Commonly used packaging systems are open PS or paperboard trays with a plastic overwrap of LDPE or PVC. Hard fruits, e.g., apples, may be bag-hard fruits or they may cut the packages, e.g., pineapples. In this case, heat-shrink PVC is used. This film is tear-resistant and can withstand the sharp contours of the fruit. Stem products, e.g., rhubarb and asparagus, are highly perishable because they lose moisture rapidly. They are usually packaged in high-barrier moisture films such as LDPE with ventilation. They can also be banded or sleeved with shrink films. Root vegetables usually have a long shelf life. Typical root vegetables include carrots, radish, onions, beets, and potatoes. These are usually packaged in LDPE bags to prevent moisture loss during prolonged storage at ambient temperature. In the case of potatoes that are light-sensitive, printing the film or tinting it an amber color prevents greening. Green vegetables such as brussels sprouts, cabbages, lettuce, broccoli, and cauliflower tend to lose moisture rapidly, resulting in wilting of product. Furthermore, they have high respiration rates, and so anaerobic conditions must be prevented within the packaged product. Packaging materials that prevent moisture loss and are also low barriers to oxygen include LDPE and PVC. PP is also commonly used because it is a high-moisture barrier; however, it must be perforated because it is also a high oxygen barrier. Ready prepared cut vegetables and salads have a high surface area; therefore, they will lose moisture rapidly and respire faster. The packaging film of choice is again LDPE or LDPE/EVA or OPP, which will ensure the desired shelf life of these products.

15.12 PACKAGING OF FROZEN FRUITS AND VEGETABLES The preservation of fruits and vegetables allows for the year-round supply and consumption of these seasonal commodities. Many fruits and vegetables are sold in the frozen section of the supermarket to ensure a long shelf life (approximately 6 months at –20∞C). While freezing inhibits the growth of microorganisms, biochemical and enzymatic spoilage may still occur. Chemical reactions in frozen fruits and vegetables, although slow, are of significance over time. Important chemical reactions in frozen fruits and vegetables include oxidation of vitamin C (ascorbic acid), as well as beta-carotene and other pigments such as anthocyanins, chlorophylls, and flavonoids, resulting in loss of flavor, color, and nutritive value of the frozen product. While many of these reactions are catalyzed enzymatically, most fruits and vegetables are blanched prior to freezing to inactivate these deleterious enzymes. However, chemical spoilage can take place in the absence of enzymes if oxygen and light are present. Although frozen fruits and vegetables are stable microbiologically, enzymatically, and chemically to a certain extent, they require adequate packaging to maintain maximum quality in their subzero environment. Physical spoilage due to freezer burn is a major concern in frozen, stored products. The frozen chamber has a very low relative humidity because water vapor condenses on evaporator coils. Such an environment promotes the loss of moisture (by sublimation from the frozen product into the surrounding atmosphere) resulting in desiccation of the product, which is called freezer burn. Packaging materials or technologies should be used to minimize these deteriorative changes in the frozen fruit and vegetable products throughout storage. The specific packaging requirements for frozen fruit and vegetable products should include the following considerations:

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1. The packaging material should be impervious to moisture and, if possible, to oil and light. 2. Flexible materials should be used to fit the contours of the frozen product and minimize free air space. 3. The packaging material should not become brittle or deteriorate during prolonged frozen storage. 4. The packaging material should be puncture-resistant, leakproof, and waterproof. 5. The packaging material should not contribute any flavor or odor changes to the product. 6. The packaging material should be aesthetically pleasing and have a low cost. In today’s market, three main types of packages are used for long-term frozen storage of fruits and vegetables and their juices: (1) the plastic pouch, (2) the laminated carton, and (3) the composite can.

15.12.1 THE PLASTIC POUCH Vegetables such as cauliflower, broccoli, peas, corn, rhubarb, beans, and brussels sprouts, as well as fruits such as cherries, strawberries, and others, are commonly packaged in plastic pouches of various sizes. The most common film used for such pouches is low-density polyethylene. LDPE is a hydrophobic, partially crystalline polymer film. The hydrophobic nature of LDPE gives it the high water barrier characteristics required for frozen products to prevent freezer burn. In addition, it prevents water from being absorbed and ice crystals from being formed on the inner surface of the film. Furthermore, LDPE is waterproof and resistant to water released during product thawing and resistant to acids, thus making it an ideal packaging container for acid fruit products. The amorphous nature of the film enables it to be flexible at low temperatures and allows it to form-fit the product, thus eliminating most of the air within the package that could promote in-package desiccation. This flexibility also ensures that the film does not become brittle and does not tear during storage. Because of its low density, LDPE has good clarity and forms excellent seals at low temperatures. Furthermore, LDPE contributes no off-odors or off-flavors, is nontoxic, and is one of the least expensive polymers available. The disadvantages of LDPE as a packaging material for frozen fruits and vegetables are its high transmission of light and high oxygen permeability, both of which can enhance light-catalyzed oxidative reactions. The light barrier properties can be enhanced through graphics or by incorporating UV absorbers into the film itself. While LDPE is a poor gas barrier, this is not a problem at frozen storage temperatures where permeability is greatly reduced. Because of its flexibility, LDPE does not give good protection to its contents from mechanical impact, and the pouches have a tendency to slip off each other, making the product difficult to stack. Other packaging systems available for frozen fruits and vegetables include: 1. 2. 3. 4.

PET bags inside a paperboard box Coextrusions of nylon/PET/HDPE Coextrusions of HDPE/EVA Coextrusions of EVA/LLDPE/EVA

Another packaging option for consumers is boil-in-the-bag vegetables. These can be removed from their paperboard box and simply heated in a pot of boiling water. Because the bags must be able to withstand high temperatures, PET bags or laminates of nylon/PET/HDPE, which have excellent high-temperature stability, are the materials of choice. Frozen fruits and vegetables can also be distributed in bulk packages, e.g., to large retail outlets or restaurants. Packaging materials with a high tensile strength are required for bulk storage of such frozen products. The main packaging systems that can be used for bulk packaging of frozen

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products are coextrusions of EVA and HDPE or LLDPE. These provide additional strength as well as excellent barrier characteristics to moisture vapor. EVA provides excellent seal characteristics at low melting temperatures.

15.12.2 LAMINATED CARTONS Paperboard remains an economic, versatile, strong, and easily converted packaging material. Because paperboard is very hygroscopic and readily absorbs moisture, it must be laminated. Examples of laminated cartons are: 1. Wax-coated paperboard box 2. Polyethylene-coated paperboard box 3. Microwavable paperboard/polyester/aluminum foil trays inside a polyethylene/paperboard box All of these packaging systems offer product protection throughout 6 months’ storage at –20∞C. The coatings also prevent the paperboard trays from absorbing moisture and delaminating. Wax-coated paperboard or corrugated board was the packaging choice many years ago because it offered good protection against loss of moisture and flavor volatiles and oxidation problems. A major problem with this type of packaging was flaking of wax during storage, which was unsightly and decreased the barrier properties of the packaging material. These cartons have nearly all been replaced by paperboard coated on both sides with polyethylene, which offers better moisture protection and heat-sealing properties. The outer layer of LDPE also protects the package from soiling and scuffing. The laminated carton has most of the advantages of the LDPE pouch (with the exception of flexibility and transparency). However, the laminated paperboard carton has the added advantage of crush protection, easy stackability, and protection against light. Because of the high-quality printing that can be accomplished on paperboard, such cartons have a more luxurious appearance than their plastic bag counterparts. With the increase in ownership of microwave ovens, microwavable packaging materials are being demanded by the North American consumer. The innermost tray, consisting of paperboard/polyester/aluminum foil, which acts as a susceptor film, is filled with the product and placed inside the outer PE/paperboard tray. The consumer simply opens the packet, takes out the inside container and microwaves the product according to the instructions on the lid of the microwavable tray. This technology enables a crisper and browner product to be cooked in a shorter cooking time than deep frying and offers the consumer the ultimate in convenience. As an added measure of protection, individual, two, or three packs of the above-mentioned packaged products can be placed inside an outer sleeve of low-density polyethylene and shrinkwrapped. The shrink-wrapped packages not only offer greater barrier protection, but this method of packaging is also aesthetically pleasing to the consumer.

15.12.3 COMPOSITE CAN The composite can is a common packaging material for frozen fruit juice concentrates, purees, and pieces of fruit in light syrup. Beause fruit juice concentrates are generally prepared and packaged at the location of fruit picking, e.g., Florida, and transported long distances to the consumer, the packaging materials for these products must be lightweight to reduce transportation costs. They must be strong enough to ensure package integrity during transportation in order to avoid leakage problems as the contents become mostly liquid upon thawing. The composite can provides such characteristics because it is made of several plies of lightweight materials such as plastic, plastic film, and sometimes aluminum foil. The can ends are made of lacquered metal (lacquer must be acid-resistant) or plastic. Both spirally wound (cylindrical) and convolute (square) composite cans

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are used in the frozen fruit juice industry, with the spirally wound composite can being the most common. The cans are usually constructed of (1) an innermost layer of LDPE, which acts as a moisture and acid barrier while not disintegrating into the product at frozen storage temperature; (2) aluminum foil, which acts as a gas and light barrier to prevent the pigments and vitamins in fruit and vegetable juice from oxidative destruction; and (3) kraft paper, which is also a good barrier to light and gives the can structural rigidity and strength. A paper or foil pre-printed label with attractive illustrations is applied to the outer paper layer. This is protected from scuffing and soiling by a protective coating of LDPE. This outer coating of LDPE also protects the kraft paper from softening due to ice formation on the outer surfaces during handling and storage. When higher barrier characteristics and rigidity are required, PP is incorporated into the structure. Convoluted composite cans are made of several plies of kraft paper. The inner layer of the body may be sprayed with a water barrier coating. The disadvantages of convoluted cans are their slow rate of manufacture and low to medium barrier characteristic.

15.13 MODIFIED ATMOSPHERE PACKAGING Modified Atmosphere Packaging (MAP) can be defined as “the enclosure of food products in a film in which the gaseous environment has been changed or modified to slow respiration rates, reduce microbiological growth, and retard enzymatic spoilage with the intent of extending shelf life” (Young et al., 1988). MAP is becoming an increasingly popular method of shelf life extension of food products when an extended shelf life at refrigerated temperatures is required. Two methods are used to modify the atmosphere within the packaged products. These are (1) passive modification and (2) active modification. Fruits and vegetables are still living and respiring, even after they are cut and packaged. Therefore, their physiological requirements must be met or they will rapidly deteriorate. Furthermore, through the process of respiration, cut fruits and vegetables are constantly consuming oxygen and producing carbon dioxide, heat, and water. The rates at which these processes proceed can be greatly slowed by holding the fruit at low temperatures, as near 0∞C (32∞F) as possible. In this way, fresh-cut fruits and vegetables will modify the package environment and their physiology will, in turn, be modified. The changes in gas composition of the package environment are referred to as Modified Atmosphere Packaging (MAP) and this technology is central to maintaining the quality and shelf life of fresh-cut produce. Creation of a package environment that supports modification beneficial to the product is the goal. An inappropriate package environment can rob the product of quality and reduce shelf life. The effects of Modified Atmosphere Packaging are based on the often-observed slowing of plant respiration in low O2 environments. There is about 21% O2 in air. As the concentration of O2 inside the package falls below ~10%, respiration starts to slow. This suppression of respiration continues until O2 reaches ~2 to 4% for most produce. If O2 becomes lower than 2 to 4% (depending on product and temperature), fermentative metabolism begins to replace normal aerobic metabolism and off-flavors, off-odors, and other undesirable volatiles are produced. Slowing the respiration rate of produce reduces the rate at which internal reserves of sugars, starch, and organic acids are used up. Because these are all integral to the flavor and aroma features of fruits and vegetables, their preservation leads directly to increased quality and shelf life.

15.13.1 ACTIVE

AND

PASSIVE MODIFICATION

In passive modification, the atmosphere is modified as a result of a commodity’s respiration, i.e., O2 consumption and CO2 generation. In active modification, the package headspace is flushed with a known concentration of O2, CO2, and N2 (Smith et al., 1990). Products differ in their tolerance

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TABLE 15.12 Recommended MA Conditions for Fruits Storage Temperature

Atmosphere

Commodity

(∞C)

O2

CO2

Apple Apricot Avocado Banana Cherry (sweet) Grapefruit Kiwifruit Mango Papaya Peach Pear Pineapple Strawberry

0–5 0–5 5–13 12–15 0–5 10–15 0–5 10–15 10–15 0–5 0–5 10–15 0–5

2–3 2–3 2–5 2–5 3–10 3–10 2 5 5 1–2 2–3 5 10

1–2 1–2 3–10 2–5 10–12 5–10 5 5 10 5 0–1 10 15–20

Source: Kader, A. A. 1986. Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol., 49: 99–104.

to O2 and CO2. The recommended MA conditions for selected fruits and vegetables are shown in Table 15.12 and Table 15.13, respectively. Modifying the atmosphere surrounding fresh produce may be effected by vacuum packaging or by introducing a desired gas mixture into a package, through the use of O2 and CO2 absorbers, or passively through the respiration of the plant material. It is important to recognize that by adding gas mixtures to the bag (active MAP) or by drawing a vacuum before sealing (vacuum packaging), the equilibrium atmosphere is not affected. These measures allow the desired atmosphere to evolve more rapidly and this may be desirable in some cases. But the atmosphere that exists inside a MAP is a function of the film and the product. For this reason, it is essential to know the respiratory requirements of the product (how much O2 the product will consume under specified conditions) and the permeability properties of the film that will be used. Exposure of fresh produce to levels of O2 or CO2 outside the limits of tolerance can increase fermentative metabolism and the development of off-flavors, as well as cause other physiological disorders. It is crucial, therefore, that the appropriate atmosphere, as well as the proper temperature, be maintained for a given commodity to realize the optimum quality and postharvest life.

15.13.2 EQUILIBRIUM MAP Gases move across films through a process called permeation. Gas permeation across a film is determined by the film’s structure, thickness, surface area, gas concentration gradient, temperature, and differences in gas pressure. Relative humidity may affect permeation characteristics of some films. Permeability ranges for some common films are shown in Table 15.14. Permeation is typically slow compared to the normal movements of the gases in air, so the films act as a partial barrier to gas movement. When the permeation of gases is so slow that it is close to zero, the film is called a barrier film. These films are useful for maintaining atmospheres nearly free of oxygen, such as for snack foods and bakery goods. Films with intermediate gas diffusion rates are more applicable for packaging respiring commodities, those that require some supply of oxygen to maintain a marketable condition. Some films have perforated holes,

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TABLE 15.13 Recommended MA Conditions for Vegetables Atmosphere

Commodity

Storage Temperature (∞C)

O2

CO2

Asparagus Beans, snap Broccoli Brussels sprouts Cabbage Cantaloupe Cauliflower Corn, sweet Cucumber Honeydew Lettuce Mushrooms Bell peppers Spinach Tomatoes (mature) Tomatoes (partly ripe)

0–5 5–10 0–5 0–5 0–5 3–7 0–5 0–5 8–12 10–12 0–5 0–5 8–12 0–5 12–20 8–12

20 2–3 1–2 1–2 3–5 3–5 2–5 2–4 3–5 3–5 2–5 Air 3–5 Air 3–5 3–5

5–10 5–10 5–10 5–7 5–7 10–15 2–5 10–20 0 0 0 10–15 0 10–20 0 0

Source: Kader, A. A. 1986. Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol., 49: 99–104.

TABLE 15.14 Permeabilities of Packaging Films Used in MAP of Fruit and Vegetables Permeabilitya to Film Type

Carbon Dioxide

Polyethylene, low density (LDPE) Polyvinyl chloride (PVC) Polypropylene (PP) Polystyrene (PS) Saran (PVDC) Polyester (PET)

7,700–77,000 4,263–8,138 7,700–21,000 10,000–26,000 52–150 180–390

a

Oxygen 3,900–13,000 620–2,248 1,300–6,400 2,600–7,700 8–26 52–130

Permeability expressed as cc/m2/mil/d/atm.

Source: Zagory, D. and Kader, A. A. 1988. Modified atmosphere packaging of fresh produce. Food Technol., 42: 70–77.

microporous membranes, or pores that allow gases to move across the film more rapidly than would be possible by permeation alone. These films are appropriate for packaging very high respiration rate commodities where oxygen depletion would otherwise be likely (Figure 15.4). Flexible films vary in their rates of gas transmission, commonly known as the oxygen transmission rate (OTR) or CO2 transmission rate (CO2TR). The relationship of film OTR with product respiration and package properties can be summarized by the following equation:

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OTR = RRO2 • t • W/[A • (O2atm – O2pkg)] where OTR* RR t W A O2pkg

= = = = = =

film O2 transmission rate (OTR) respiration rate (O2 consumption in ml/kg-d) film thickness (mils) product weight (kg) film surface area (100 in.2) desired O2 concentration in the pkg (%)

The determinant of the relative proportions of CO2 and O2 in the package is the ratio of the film’s permeability to CO2 and O2. This ratio is referred to as b (CO2TR/OTR), and is one of the most useful descriptive parameters of a plastic film. Films with a high b value will allow CO2 to escape the package rapidly, relative to the rate at which O2 enters, resulting in an atmosphere with a low concentration of CO2. Films with lower b values will allow greater buildup of CO2 in the package as a result of CO2 leaving the package relatively slowly compared to the rate at which O2 enters the package. For most low density polyethylene films b~2 to 4. It will be different for other film structures. The permeability ratio (b) determines the possible combinations of O2 and CO2 concentrations inside the package. Gas flushing, vacuum packing, changing the size of the bag, or changing the amount of the product in the bag will not affect these possible gas concentrations.

Effect of β Value on Material Choices

Cherry

15

Concentration (%)

Carbon Dioxide

12

9

β=1

6 β=3 3 β=6 0 1

3

5

7

9

11

13

15

17

19

21

Oxygen Concentration (%)

FIGURE 15.3 b lines define the combinations of O2 and CO2 in a package. A film with b = 1 would be appropriate for cherries.

* OTR, according to the American Society for Testing and Materials (ASTM) standard, has the units cc/100 in. 2-d. However, in the produce industry, permeability is referred to as OTR, is used in that sense here, has the units cc-mil/100 in. 2-atm-d.

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TABLE 15.15 Shelf Life of Selected Fruits and Vegetables Stored under MAP Conditions MA Environment Packaging Film

%O2

% CO2

Shelf Life

Sealed PE bags Sealed PE film tubes PE pallet covers Cryovac PE film Sealed PE bags Sealed PE bags PVC overwraps PVC overwraps PVC overwraps Sealed PE bags Cellophane film PVC film PE bags PE bags PE bags PE bags + 4.5% EVA PE bags/liners PE bags PVC overwrap PVC overwrap

10–15 2–5 1–2 10–15 3–5 N/A 3 2–3 2–3 5 0.5 14 6–11 2–5 3–4 1–2 5 17 2 2

0.5–2.5 5–7 3–5 15–25 7–9 3–4 3 3–4 3–4 10 –30 3 4–6 5–10 3–6 8 9 3 10–12 10

6 months 4 months ~6 weeks N/A 8–10 d 6 months 15 d 2–3 weeks 2–3 weeks –12 d 7d N/A N/A N/A 8 weeks 3 weeks 5 weeks 15 months 5d 6–7 d

Commodity Apples and pears Apples Blueberries Peaches Avocados Kiwifruit Banana Cabbage Brussels sprouts Lettuce Beans Peppers Sweet corn Artichokes Broccoli Celery Carrots Mushrooms Mixed salad greens

Note: N/A = not applicable. Source: Adapted from Prince, T. 1989. Modified atmosphere packaging of horticultural products. In Controlled/Modified Atmosphere/Vacuum Packaging of Foods, A. L. Brody (Ed.), Food and Nutrition Press, Trumbell, CT, pp. 67–100.

Because fruits and vegetables vary in their tolerance to CO2 and in their ability to benefit from high CO2, the b value of a film is very important as a predictor of the relative amounts of O2 and CO2 that will accumulate in the package. The absolute amounts of O2 and CO2 will be determined by the possible gas concentrations in a MAP and the factors in the equation above. Figure 15.3 illustrates the possible combinations of O2 and CO2 that could evolve in packages made from films with b values of 1, 3, or 6. Films with b values of 3 or 6 will provide atmospheres with low O2 and low CO2 due to their allowing CO2 to exit the package 3¥ or 6¥ faster than O2 enters the package. A film with a b = 1, which is characteristic of microperforated and microporous films, will allow atmospheres to evolve with low O2 and high CO2, which would be suitable for cherries (or other commodities that tolerate high CO2, such as broccoli, strawberries, and others). Examples of commodities packaged under modified atmospheres and their possible extension in shelf life are summarized in Table 15.15.

15.13.3 MICROPERFORATED

AND

MICROPOROUS FILMS

Alternative approaches to providing high OTRs, especially in applications where there is limited package surface area for gas exchange, have included films with mechanically or electrically perforated holes, microporous membranes, or pores. Most cut fruit is packaged in rigid, gas impermeable trays with a permeable film lid stock sealed to the tray. Due to the tray being

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O2

O2

O2

CO2 CO 2

CO2

CO2 > 20% CO2; 10 to 12%) will also suppress the growth of many of the spoilage microorganisms, but not the pathogenic bacteria. Plant respiration, which consumes O2 and produces CO2, increases dramatically with increasing temperature. Therefore, MAP products that are exposed to warm temperatures will rapidly deplete the O2 in the package and replace it with CO2. Such conditions can suppress the growth of many spoilage bacteria while potentially allowing growth of pathogenic bacteria, should they be present. The suppression by MAP of spoilage bacteria prevents the development of off-odors that would signal an unacceptable loss of quality. At the same time, the absence of a vigorous population of spoilage bacteria could allow pathogenic bacteria to grow in the absence of competition, should pathogens be present. Thus MAP, under temperature abuse conditions, could encourage the growth of pathogenic bacteria at the expense of benign ones. For this reason, there has been concern within the fresh-cut produce industry. This concern has translated into increased attention to sanitation, Good Manufacturing Practices, and HACCP programs. The best tools to prevent potentially

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hazardous situations from arising are to prevent pathogens from contaminating the product to start with, and then keep the product constantly cold to prevent growth of pathogens, should they be present. Effective prevention strategies include HACCP plans, which are becoming de facto standards within the fresh-cut industry (Zagory and Hurst, 1996). There has been a great deal of public health concern about foods packaged under MAP atmospheres due to the possible outgrowth of Clostridium botulinum (Lambert et al., 1991). Although there have never been commercial problems from Clostridium in fresh-cut products, Kauter et al. (1978) detected botulism toxin after 2 to 3 d at 20∞C in mushrooms overwrapped in PVC film. They recommended that films be perforated to maintain oxygen levels at > 2% in the package headspace (Kauter et al., 1978). Dodds (1989) also detected toxin in vacuum packaged potatoes after 7 to 14 d at ambient temperatures. MAP products should be stored under strict temperature control to prevent outgrowth of and toxin production by C. botulinum. A comprehensive review of botulism and its control in fruits and vegetables is provided by Notermans (1993). Because package performance can only be specified within a particular temperature range, it is important that packages be used within that range. If the temperature gets outside of that range, produce quality will suffer and one of the hurdles to pathogen growth, the presence of O2, may be lost.

15.16 THERMALLY PROCESSED FRUITS AND VEGETABLES 15.16.1 CANNING

OF

FRUITS

AND

VEGETABLES

The thermal processes used for canning of fruits and vegetables depend on the pH of the product. High-acid products (pH < 4.5) require only mild heat treatment, e.g., pasteurization, while lowacid products (pH > 4.5) require heat sterilization at a predetermined time and temperature (usually 1 to 2 h at 121∞C). Thermally processed fruits and vegetables are either packaged in tin plate or glass. Examples of can sizes and types of lacquers used for specific fruits and vegetables are summarized in Table 15.4 and Table 15.5, respectively. A more comprehensive review of thermal processing of fruits and vegetables can be found in Chapter 9 of this book.

15.16.2 ASEPTIC/ULTRA-HIGH-TEMPERATURE (UHT) PACKAGING Heat sterilization is the unit operation in which foods are heated at a sufficiently high temperature for a sufficiently long time to destroy microbial and enzymatic activity. As a result, sterilized foods have a shelf life of 6 to 12 months or more. The most common method of sterilizing solid and viscous food products is in-container sterilization, e.g., canning; however, the main disadvantages associated with in-sterilization of food products are (1) the low rate of heat penetration to the thermal center of the container, (2) long processing times to achieve the desired sterility, (3) damage to the nutritional and sensory characteristics of the product, (4) low productivity, and (5) high energy costs. To overcome the limitations of in-container sterilization, products can be sterilized at higher temperatures for a shorter time prior to filling into presterilized containers under sterile conditions. This forms the basis of ultra-high-temperature (UHT) processing or aseptic packaging, which has been defined as “the independent sterilization of food and packaging and assembly under sterile conditions” (Smith et al., 1990). Aseptic processing was developed in the early 1940s and has been used successfully in Europe and Japan for over 2 decades. It has been rapidly gaining in popularity as a thermal processing technique in North America since the use of hydrogen peroxide was approved for sterilizing of packaging materials in 1981 (Merson and Wolcott, 1985). The process has long been successfully employed to sterilize a wide range of thin liquid products, e.g., milk, fruit juices and concentrates, cream, yogurt, salad dressing, egg, and ice cream. Recent developments in high-barrier plastic packaging materials and aseptic processing and filling technology have resulted in the process being expanded to sterilize both acid-low and acid-viscous, and semisolid foods that contain

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6–LAYER CONSTRUCTION 1. POLYETHYLENE 2. SURLYN 3. FOIL 4. POLYETHYLENE 5. PAPERBOARD 6. POLYETHYLENE

FIGURE 15.5 Construction of a Tetra-Pak container used in aseptic processing.

discrete particles, such as cottage cheese, baby foods, tomato pastes, fruits and vegetables, soups, and rice desserts. In the aseptic process, a feed pump continuously passes the fluid food through a scraped or swept surface heat exchanger (SSHE) or perhaps a simpler shell-and-tube or plate heat exchanger for nonviscous liquid products, which quickly raises the temperature to the sterilizing levels. The product is then passed through a hold tube, where the sterilization process is completed, and then through a second heat exchanger, usually another SSHE setup, to cool the product quickly. The cooled product is then transferred to a sterile tank that serves as a feed tank for aseptic filling. Presterilized containers, usually laminated cartons (Tetra-Pak), are aseptically filled with the cold product, and the containers are sealed within the aseptic chamber. A package suitable for aseptic processing or packaging must be able to be sterilized, must lend itself to aseptic filling, and should permit application and maintenance of hermetic sealing, as well as maintenance of commercial sterility of the product during storage and handling. Rigid, semirigid, and flexible packaging containers have all been used for aseptic packaging, the sterilization and sealing techniques being somewhat different in each situation. Basic packaging materials used in aseptic packaging include metal sheet and foil, glass, plastic films, and paper. Metal has been used from the onset of aseptic packaging because it offers the best choice in terms of barrier properties, hermetic sealing, sterilizability, and durability; however, metal containers are also relatively more expensive, have poor adaptability to shapes other than cylindrical, and cannot be heated in microwave ovens. Glass containers offer advantages similar to metal containers but suffer from additional disadvantages of fragility and density. Plastic materials offer versatility but cannot be used by themselves because they do not offer all the required properties and must be laminated to other materials to give a package with the desired package requirements. A currently popular packaging material used for aseptic packaging is a paper-board/foil/plastic laminate or Tetra-Pak. This laminated structure (Figure 15.5) consists of as many as six layers of materials: polypropylene, surlyn, foil, polyethylene, paperboard, and polyethylene as the innermost layer. Other variations that can also be used for thermal sterilization include laminations consisting of saran, ethyl vinyl alcohol (EVOH), polyethylene, and polystyrene or metallized polyester consisting of vinyl ethyl acetate, nylon, foil, and polyethylene. Aluminum foil is the most commonly used barrier material, with polypropylene or polyolefin (polyethylene) being the common heatsealing and food contact surfaces. When foil is used, it needs to be protected against mechanical damage, which is usually provided by paperboard. All of these composite packages yield the desirable moisture, oxygen, light and microbial barrier properties, strength, and heat-sealability needed for a successful aseptic package (Smith et al., 1990). Whatever the choice of packaging material used, it must be presterilized prior to filling. Methods commonly used for sterilization of packaging materials or packages include steam (saturated or superheated), hot air, hydrogen peroxide, ultraviolet light, irradiation, or the heat generated during the coextrusion of certain films. Steam is the most widely used technique for sterilization of metal

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and glass containers. Superheated steam, although desirable because of larger temperature potential and smaller effluent condensate, may be less effective against microbial spores than saturated steam. Hot air (dry) has similar disadvantages. Hydrogen peroxide, together with heat or ultraviolet radiation, is one of the common agents used for sterilization of paper-based packaging materials. Although the exact mechanism of action of hydrogen peroxide is not known, its effect is generally attributed to the liberation of nascent oxygen by the effect of heat or the formation of hydroxyl radicals by ultraviolet radiation. There are certain restrictions on the concentration of hydrogen peroxide that can be used for application, as well as the residual hydrogen peroxide in the product. The heat employed for melting and coextrusion of plastic resins used for fabrication of thermoformed containers is usually high enough to sterilize the packaging material. The sterility is maintained by the application of a thin superficial film that is aseptically removed prior to fabrication (Lopez, 1987). Finally, the use of gamma rays is also an effective way of presterilizing packaging materials, although relatively long exposure times are required.

15.17 PACKAGING OF DRY FRUIT AND VEGETABLE PRODUCTS Dehydration is another method to extend the shelf life and maintain the quality of perishable fruit and vegetable products. Methods of drying include through-flow drying, cross-flow drying, spraydrying, freeze drying, puff drying, and microwave drying, again described in Chapter 7 of this book. The method of dehydration will have an effect on the choice of packaging material. For example, potato chips dried by the hot oil immersion technique will have different packaging requirements from sun-dried or oven-dried potatoes. Cut dried fruits and vegetables are often treated with sulphur dioxide to maintain their light natural color during extended storage. Oxygen is detrimental as it oxidizes SO2 and accelerates darkening of product. Light is also detrimental to dehydrated fruits and vegetables. Beta-carotene in fruits and vegetables is light-sensitive and therefore reduced in the presence of light. Light also increases the rate and amount of SO2 loss and hence causes darkening of product. Moisture ingress should be avoided to prevent caking problems in powdered products and enzymatic activity and mold growth in dried, cut fruit and vegetable products. Selection of appropriate packaging materials should be based on the relevant factors limiting the shelf life of dehydrated fruit and vegetable products. Materials used include laminated structures with good barrier properties to light, oxygen, moisture, sulphur dioxide, and flavor volatiles. Common materials used include foil-laminated pouches and cellophane/PE/foil/PE and paperboard/PE/foil/PE laminates. For products that are highly susceptible to oxidation, products may be vacuum packaged or gas flushed with 100% N2 or packaged with an oxygen scavenger.

15.18 CONCLUSION Food packaging is an essential technique for preserving food quality, minimizing food wastage, and reducing the use of chemicals, additives, and stabilizers. The food package serves the important functions of containing the food, providing protection against chemical and physical damage, providing convenience in using the product, and conveying consumer information to the consumers. It protects the food by acting as a barrier to oxygen, moisture, chemical compounds, and microorganisms that are detrimental to the quality of the food product. It provides consumers with convenient features such as microwavability, resealability, and ease of opening. It conveys useful information such as a description of food contents, weight-to-volume ratio, manufacturer’s name and address, directions for preparing foods, and nutrition values. It serves as an effective marketing tool for promoting product identification and selling the product. In conclusion, some exciting possibilities and challenges face the growth of packaging technology. Ground has been successfully broken in bulk storage of fruits and vegetables under

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controlled atmospheres, and new opportunities exist at the retail level for their distribution in small retail units packaged under modified atmospheres. This domain is even more complex because it consists of a living and respiring system undergoing physiological, microbial, and chemical deterioration. The challenge lies in finding a packaging system that allows in sufficient O2 to maintain respiratory activities while dissipating end products of metabolism such as CO2 and ethylene in order to delay ripening and senescence. This is a complex task requiring control of significantly more variables than other products. The use of edible films and active packaging materials also warrants further investigation for shelf-life extension of fruit and vegetable products.

REFERENCES Aboagye, N. Y., Ooraikul, B., Lawrence, R. A., and Jackson, E. D. 1986. Energy costs in modified atmosphere packaging and freezing processes as applied to a baked product. In Food Engineering and Process Applications, Vol. 2, Unit Operations, A. LeMaguer and P. Jelen (Eds.), Elsevier Applied Science, New York, pp. 417–427. Ben-Yehoshua, S. 1989. Individual seal packaging of fruit and vegetable in plastic film. In Controlled/Modified Atmosphere/Vacuum Packaging of Foods, A. L. Brody (Ed.), Food and Nutrition Press, Trumbell, CT, pp. 101–118. Brown, W. E. 1992. Properties of plastics used in food packaging. In Plastics in Food Packaging-Properties, Design, and Fabrication, Marcel Dekker, New York, pp. 103–139. Day, B. F. P. 1993. Fruit and vegetables. In Principles and Applications of Modified Atmosphere Packaging of Food, R. T. Parry (Ed.), Blackie Academic and Professional, Bishopbriggs, Glasgow, U.K., pp. 114–132. Dodds, K. L. 1989. Combined effect of water activity and pH on the inhibition of toxin production by Clostridium botulinum in cooked, vacuum-packed potatoes. Appl. Environ. Microbiol., 55: 656–660. Ellis, R. F. 1979. Rigid metal containers. In Fundamentals of Food Canning Technology, J. M. Jackson and B. M. Shin (Eds.), AVI Publishing, Westport, CT, pp. 95–123. Fellows, P. 1988. Packaging. In Food Processing Technology, Ellis Norwood, Chichester, U.K., pp. 421–447. Jelen, P. 1985. Food packaging technology. In Introduction to Food Processing, Reston Publishing, Reston, VA, pp. 249–266. Kader, A. A. 1986. Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol., 49: 99–104. Kauter, D. A., Lilly, T., Jr., and Lynt, R. 1978. Evaluation of the botulism hazard in fresh mushrooms wrapped in commercial polyvinyl-chloride film. J. Food Prot., 41: 120–121. Kelsey, R. J. 1989. Packaging in Todays Society. Technomic Publishing, Lancaster, PA. Kester, J. J. and Fennema, O. R. 1986. Edible films and coatings: a review. Food Technol., 40: 47–59. Lambert, A., Smith, J. P., and Dodds, K. L. 1991. Combined effect of modified atmosphere packaging and low dose irradiation on toxin production by Clostridium botulinum in fresh pork. J. Food Prot., 54: 97–104. Lopez, A. A. 1987. A Complete Course in Canning and Related Processing. 12th ed., The Canning Trade, Baltimore, MD. Merson, R. L. and Wolcott, T. K. 1985. Recent developments in thermal process design. Proc. IV Int. Cong. Eng. Food, Elsevier Applied Science, New York. Notermans, S. W. H. 1993. Control in fruits and vegetables. In Clostridium botulinum — Ecology and Control in Foods, H. W. Hauschild and K. L. Dodds (Eds.), Marcel Dekker, New York, pp. 233–260. Prince, T. 1989. Modified atmosphere packaging of horticultural products. In Controlled/Modified Atmosphere/Vacuum Packaging of Foods, A. L. Brody (Ed.), Food and Nutrition Press, Trumbell, CT, pp. 67–100. Robertson, G. L. 1992. Food Packaging — Principles and Practice. Marcel Dekker, New York. Smith, J. P., Ramaswamy, H. S., and Simpson, B. K. 1990. Developments in food packaging technology. Part n: storage aspects. Trends Food Sci. Technol., 1: 112–119. Wells, J. M. and Butterfield, J. E. 1997. Salmonella contamination associated with bacterial soft rot of fresh fruits and vegetables in the marketplace. Plant Dis., 81: 867–872.

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Wells, J. M. and Butterfield, J. E. 1999. Incidence of Salmonella on fresh fruits and vegetables affected by fungal rots or physical injury. Plant Dis., 83: 722–726. Wills, R. B. H., McGlasson, W. B., Graham, D., Lee, T. H., and Hall, E. G. 1989. Handling, packaging, and distribution. In Postharvest — An Introduction to the Physiolology and Handling of Fruits and Vegetables, New South Wales University Press, NSW, Australia, pp. 132–144. Young, L. L., Reviere, R. D., and Cole, A. B. 1988. Fresh red meats: a place to apply modified atmospheres. Food Technol., 42: 65–69. Zagory, D. 1995. Principles and practice of modified atmosphere packaging of horticultural commodities, In Principles of Modified-Atmosphere and Sous Vide Product Packaging, J. M. Farber and K. L. Dodds (Eds.), Technomic Publishing, Lancaster, PA, pp. 175–206. Zagory, D. and Hurst, W.R. (Eds.). 1996. Food Safety Guidelines For The Fresh-cut Produce Industry. International Fresh-cut Produce Association. Zagory, D. and Kader, A. A. 1988. Modified atmosphere packaging of fresh produce. Food Technol., 42: 70–77.

Standards, and 16 Grades, Food Labeling Y.H. Hui and Charles Huxsoll CONTENTS 16.1 16.2

Background ........................................................................................................................398 U.S. Food and Drug Administration..................................................................................398 16.2.1 Standards of Identity ...........................................................................................398 16.2.1.1 What Is a Standard of Identity for a Food Product? ........................398 16.2.2 Standards of Quality............................................................................................399 16.2.2.1 What Are the Standards of Quality? .................................................399 16.2.3 Standards of Fill of Containers ...........................................................................399 16.2.3.1 What Are the Standards of Fill of Containers?.................................399 16.2.4 Miscellaneous Standards of Quality....................................................................400 16.2.5 Food Packaging and Labeling .............................................................................400 16.2.5.1 Categories of Information..................................................................401 16.2.5.2 Table of Contents for Regulations.....................................................401 16.2.5.3 The New Food Label .........................................................................401 16.2.6 Labeling and Fruit Juices ....................................................................................409 16.2.7 Standards for Processed Fruit and Fruit Products ..............................................410 16.3 U.S. Department of Agriculture.........................................................................................413 16.4 Subsistence Specifications and Commercial Item Description.........................................415 16.4.1 Federal Specification ...........................................................................................415 16.4.2 Commercial Item Description .............................................................................416 16.4.2.1 Definition............................................................................................416 16.4.3 CID: Oranges, Canned (Mandarin) .....................................................................417 16.4.3.1 Salient Characteristics........................................................................417 16.4.3.2 Style....................................................................................................417 16.4.3.3 Sizes (Whole Segment Only) ............................................................417 16.4.3.4 Packing Media ...................................................................................417 16.4.3.5 Contractor’s Certification...................................................................418 16.4.3.6 Regulatory Requirements...................................................................418 16.4.3.7 Quality Assurance ..............................................................................418 16.4.3.8 Preservation, Packaging, Packing, Labeling, and Marking ..............418 16.4.3.9 Notes ..................................................................................................418 16.5 Country of Origin Labeling ...............................................................................................418 16.6 National Organic Program (NOP) .....................................................................................420 References ......................................................................................................................................421

0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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16.1 BACKGROUND All over the U.S., if you buy canned apple juice, you can be sure you are getting essentially the same product. The various kinds of food grades and standards set by the federal government make this possible. Standards of identity set by the U.S. Food and Drug Administration (FDA) define what certain food products are, and U.S. Department of Agriculture (USDA) grade standards define levels of quality for various foods. Standards of identity are mandatory. They set requirements that products must meet if they move in interstate commerce. They protect against deception because they define what a food product must consist of to be legally sold by its common or usual name — apple juice, for example. USDA grade standards for food are voluntary. Federal law does not require that a food processor or distributor use the grade standards. The standards are widely used, however, as an aid in wholesale trading, because the quality of a product affects its price. The grade (quality level) also is often shown on food products in retail stores, so consumers can choose the grade that best fits their needs. Under authority of the Agricultural Marketing Act of 1946 and related statutes, USDA has issued grade standards for many processed fruits. USDA provides official grading services, often in cooperation with state departments of agriculture, for a fee to packers, processors, distributors, or others who wish official certification of the grade of a product. The grade standards also are often used by packers and processors as a quality control tool. The specific content and labeling requirements help assure consumers that they are getting what the label says they are getting. Standards, however, do not keep different companies from making distinctive recipes.

16.2 U.S. FOOD AND DRUG ADMINISTRATION Section 401 of the Federal Food, Drug, and Cosmetic Act (21 U.S. 341), which is enforced by the FDA, states: Whenever in the judgment of the Secretary such action will promote honesty and fair dealing in the interest of consumers, he shall promulgate regulations fixing and establishing for any food, under its common or usual name so far as practicable, a reasonable definition and standard of identity, a reasonable standard of quality, and/or reasonable standards of fill of containers. However, no definition and standard of identity or standard of quality shall be established for fresh or dried fruits, fresh or dried vegetables, or butter, except that definitions and standards of identity may be established for avocados, cantaloupes, citrus fruits, and melons.

The “Secretary” refers to the Secretary of Health and Human Service who has delegated authority to the FDA. Thus, to “promote honesty and fair dealing in the interest of consumers,” the FDA has issued the three kinds of standards prescribed in the preceding quotation (viz., identity, quality, and fill of containers) for a number of processed fruit product. The premises and criteria for each of the standards are explained briefly in the following sections.

16.2.1 STANDARDS

OF IDENTITY

16.2.1.1 What Is a Standard of Identity for a Food Product? 1. It establishes or defines what a given food product is; for example, it specifically establishes which ingredients the food must contain. 2. It specifies the correct name of the food and other required label information. 3. It limits the amount of water permitted. 4. It sets the required amounts of expensive ingredients and limits the inexpensive ones. 5. It defines the kind and amount of certain vitamins and minerals that must be present in foods labeled “enriched.”

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6. In prescribing a standard of identity for a food in which optional ingredients are permitted, the FDA must designate those to be named on the label. 7. If a proposed amendment or alteration of an existing food standard involves a food additive, all federal regulations relating to the use of a food additive in food must be complied with. 8. Temporary permits may be issued for interstate shipment of experimental packs of food varying from the requirements of definitions and standards of identity; assuming all established procedures have been followed. 9. A food standard is based on the assumption that the food is properly prepared from clean and sound materials. It usually does not relate to such factors as harmful impurities, filth, decomposition, and bacteria. However, there are exceptions, such as standards for products containing whole egg, egg white or yolk, and milk and milk products shipped interstate, requiring that they be pasteurized or otherwise treated to destroy all viable microorganisms such as Salmonella. 10. A food represented as or purporting to be a food for which a standard of identity has been promulgated must comply with the specifications of the standard or definition in every respect. It is in noncompliance if it fits the following description: • Contains an ingredient not prescribed in the standard. (However, under certain circumstances, if such an ingredient is an incidental additive, it may be permitted). • Does not contain one or more of the prescribed ingredients in the standard. • Contains an ingredient or component that deviates from the prescribed quantity. 11. Any standard of identity prescribed for avocados, cantaloupes, citrus fruits, or melons is related only to maturity and the effects of freezing. Thus, the standards of identity for processed fruit and fruit products are some of the basic food standards issued by the FDA. They protect consumers from being cheated by inferior products or confused by misleading labels. At the time of this writing, they number more than 40, some of which are supplemented by standards of quality and fill of containers.

16.2.2 STANDARDS

OF

QUALITY

16.2.2.1 What Are the Standards of Quality? 1. They are minimum standards for canned fruits and establish specifications for quality requirements or factors such as ripeness, tenderness, color, and freedom from defects. 2. They protect the buyer from unknowingly receiving such products as mushy apples, grapefruit sections with excess seeds or pectins, bruised peaches, etc. 3. If a food for which a standard of quality has been promulgated falls below such standard, it must bear a special labeling: “Below Standard in Quality,” followed by either “Good Food — Not High Grade” or a statement showing the kind of defect, such as “Excessive Peel” or “Excessively Broken.” The statements must be in prescribed size and style of type. 4. Standards of quality must not be confused with the “standards for grades” issued by the USDA for agricultural products. Standards of quality are minimum standards only, whereas standards for grades may classify the products from average to excellent in quality. Standards for grades are not required to be stated on the label; however, if they are stated, the product must comply with the specifications of the declared grade.

16.2.3 STANDARDS

OF

FILL

OF

CONTAINERS

16.2.3.1 What Are the Standards of Fill of Containers? 1. They specify how full the container must be to avoid deception of the consumer and charges of “slack-filling.”

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2. They are especially needed for products composed of a number of units or pieces packed in a liquid or products that shake down after filling. 3. Formal standards of fill of containers have been issued for some fruits. 4. If a food for which a standard of fill of container has been promulgated falls below such standard, it must bear a general statement of substandard: “Below Standard in Fill.” Such a statement must follow a certain prescribed size and style of type. 5. In prescribing any standard of fill of container, due consideration must be given to the natural shrinkage in storage and in transit of fresh natural food and to the need for the necessary packing and protective material. The technical details and requirements that explain how the above three types of standards for food products are established can be located in 21 CFR (Code of Federal Regulations) 130, “Food Standards: General.”

16.2.4 MISCELLANEOUS STANDARDS

OF

QUALITY

The quality of a food depends upon numerous characteristics including, but not limited to, the levels of microorganisms and such physical factors as turbidity, color, flavor, and odor. Such characteristics are indicative of the quality of the raw materials and ingredients; the degree of quality control used in manufacture, processing, and packing; and the conditions of distribution and storage. The diversity of raw materials, food processing, and distribution practices, as well as the variation in quality factors important to consumers, require that individual standards of quality be established for different types of food. The label of a food that fails to state the requirements of an applicable standard of quality promulgated shall bear the general statements of substandard, e.g., “Below Standard in Quality” and “Good Food — Not High Grade,” as discussed earlier. However, in lieu of such a general statement of substandard quality, the label may bear the alternative statement, “Below Standard in Quality _____,” the blank to be filled in with whichever of the following are applicable: 1. 2. 3. 4.

“Contains Excessive Bacteria” “Excessively Turbid” “Abnormal Color” The phrase specified in the applicable standard of quality to describe any other quality deviation

16.2.5 FOOD PACKAGING

AND

LABELING

The two federal statutes that regulate food packaging and labeling are the Federal Food, Drug, and Cosmetic Act and the Fair Packaging and Labeling Act (15 U.S.C. 1453, et seq.). The relevant provisions of the latter are explained in detail in the FDA publication, “Food Packaging and Labeling Act Requirements.” The relevant regulations promulgated by the FDA include the following: 1. 21 CFR 1. General regulations for the enforcement of the Federal Food, Drug, and Cosmetic Act and the Fair Packaging and Labeling Act (Subpart A, General Provisions, and B, General Labeling Requirements) 2. 21 CFR 101. Food labeling 3. 21 CFR 102. Common or usual name for nonstandardized foods For illustration, two definitions are described. A label is the display of written, printed, or graphic matter on the immediate container or package of a consumer commodity; it may be affixed to the package. “Labeling” generally refers to all associated printed or graphic material such as

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TABLE 16.1 Categories of Information on a Food Label Mandatory information Statement of identity Language requirement Net quantity of food contents Name and address of business or manufacturer Ingredients Manufacturing code Prominence of required statements Mandatory wordings of certain information if it is included on the food Iabel Nutrition labeling Grades Labeling for special dietary use Saving representations Warning statement Optional information Universal Product Code Dates Symbols Others

would appear on a display case, sign, placard, or similar notice, near or adjacent to the immediate container at any time while such article is in interstate commerce or held for sale after shipment or delivery in interstate commerce. The principal display panel of a food package is the surface of the package that, either by design or through general use, is customarily displayed to the consumer. The “information panel” means that part of the label immediately contiguous and to the right of the principal display panel as observed by an individual facing the principal display panel. There are many exceptions to this definition. 16.2.5.1 Categories of Information The information on the label or other location of a food package can be categorized as in Table 16.1. 16.2.5.2 Table of Contents for Regulations Table 16.2 describes the table of contents for 21 CFR 101, Food Labeling. 16.2.5.3 The New Food Label 16.2.5.3.1 Nutrition Labeling and Education Act The Nutrition Labeling and Education Act of 1990 (NLEA) requires, among other things, nutrition labeling for most foods (except meat and poultry) and authorizes the use of nutrient content claims and appropriate FDA-approved health claims. Manufacturers are required to comply with most of the new labeling requirements. Among key changes in the new labeling regulations established by the FDA, which implements the NLFA, are • •

Nutrition labeling for almost all foods Information on the amount per serving of saturated fat, cholesterol, dietary fiber, and other nutrients that are major health concerns to today’s consumers

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TABLE 16.2 Contents of 21 CFR 10I 21 CFR 101 Section

101.1 101.2 101.3 101.4 101.5 101.6 101.8 101.9 101 10 101.11 101.12 101.13 101.14 101.15 101.17 101.18

101.22 101.25 101.29 101.30 101.33 101.35

Subject Matter Subpart A: General Provisions Principal display panel of package form food Information panel of package form food Identity labeling of food in packaged form Food: designation of ingredients Food: name and place of business of manufacturer, packer, or distributor Label designation of ingredients for standardized foods Labeling of food with number of servings (revised May 8, 1994) Nutrition labeling of food Nutrition labeling of restaurant food Saccharin and its salts; retail establishments notice Reference amounts customarily consumed per eating occasion Sodium labeling Health claims: general requirements Food: prominence of required statements Food labeling warning and notice statements Misbranding of food Subpart B: Specific Food Labeling Requirements Foods: labeling of spices, flavorings, colorings, and chemical preservatives Labeling of foods in relation to fat and fatty acid and cholesterol content Labeling of kosher and kosher-style foods Percentage juice declaration for foods purporting to be beverages that contain fruit or vegetable juice Labeling declaration of D-erythro-ascorbic acid when it is an ingredient of a fabricated food Notice to manufacturers and users of monosodium glutamate and other hydrolyzed vegetable protein products

101.44 101.45

Subpart C: Specific Nutrition Labeling Requirements and Guidelines Nutrition labeling of raw fruit, vegetables, and fish Substantial compliance of food retailers with the guidelines for the voluntary nutrition labeling of raw fruit, vegetables, and fish Identification of the 20 most frequently consumed raw fruit, vegetables, and fish in the U.S. Guidelines for the voluntary nutrition labeling of raw fruit, vegetables, and fish

101.54 101.56 101.60 101.61 101.62 101.65 101.67 101.69

Subpart D: Specific Requirements for Nutrient Content Claims Nutrient content claims for “good source,” “high,” a n d “m o r e ” Nutrient content claims for “light” or “lite” Nutrient content claims for the calorie content of foods Nutrient content claims for the sodium content of foods Nutrient content claims for fat, fatty acids, and cholesterol content of foods Implied nutrient content claims and related label statements Use of nutrient content claims for butter Petitions for nutrient content claims

101.70 101.71 101.72 101.73

Subpart E: Specific Requirements for Health Claims Petitions for health claims Health claims: claims not authorized Health claims: calcium and osteoporosis Health claims: dietary lipids and cancer

101.42 101.43

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TABLE 16.2 (Continued) Contents of 21 CFR 10I 21 CFR 101 Section 101.74 101.75 101 76 101.77

Subject Matter

101.78

Health claims: sodium and hypertension Health claims: dietary saturated fat and cholesterol and risk of coronary heart disease Health claims: fiber-containing grain products, fruits, and vegetables and cancer Health claims: fruits, vegetables, and grain products that contain fiber, particularly soluble fi b e r, a n d r i s k o f coronary heart disease Health claims: fruits and vegetables and cancer

101 100 101.103 101.105 101.108

Subpart F: Exemptions from Food Labeling Requirements Food: exemptions from labeling Petitions requesting exemptions from or special requirements for label declaration of ingredients Declaration of net quantity of contents when exempt Temporary exemptions for purposes of conducting authorized food labeling experiments Appendix B to Part 101: Graphic Enhancements Used by the FDA

• •

• • • •

Nutrient reference values, expressed as percent of daily values, that can help consumers see how a food fits into an overall daily diet. Uniform definitions for terms that describe a food’s nutrient content — such as “light,” “low-fat,” and “high fiber” — to ensure that such terms mean the same for any product on which they appear. These descriptors will be particularly helpful for consumers trying to moderate their intake of calories or fat and other nutrients or for those trying to increase their intake of certain nutrients such as fiber. Claims about the relationship between a nutrient and a disease, such as calcium and osteoporosis, and fat and cancer. These will be helpful for people who are concerned about eating foods that may help keep them healthier longer. Standardized serving sizes that make nutritional comparisons of similar products easier. Declaration of total percentage of juice in juice drinks. This will enable consumers to know exactly how much juice is in a product. Voluntary nutrition information for many raw foods.

The new regulations will require nutrition labeling on most foods. In addition, nutrition information currently is voluntary for many raw foods. Although voluntary, the programs for raw produce and raw meat, fish, and poultry carry strong incentives for retailers to participate. The NLEA states that if voluntary compliance is insufficient, nutrition information for such raw foods will become mandatory. Also, packages with less than 12 in.2 available for labeling do not have to carry nutrition information; however, they must provide an address or telephone number for consumers to obtain the required nutrition information. 16.2.5.3.2 Nutrition Panel: Content The new food label will feature a revamped nutrition panel. It will be headed with a new title, “Nutrition Facts,” which replaces “Nutrition Information per Serving.” The new name will signal to consumers that the product label meets the new regulations. There will be a new set of dietary components on the nutrition panel. The mandatory (underlined) and voluntary components and the order in which they must appear are as follows:

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• • • • • • • • • • • • • • • • • • • • • • •

Total calories Calories from fat Calories from saturated fat Total fat Saturated fat Polyunsaturated fat Monounsaturated fat Cholesterol Sodium Potassium Total carbohydrate Dietary fiber Soluble fiber Insoluble fiber Sugars Sugar alcohol (for example, the sugar substitutes xylitol, mannitol, and sorbitol) Other carbohydrate (the difference between total carbohydrate and the sum of dietary fiber, sugars, and sugar alcohol, if declared) Protein Vitamin A Vitamin C Calcium Iron Other essential vitamins and minerals

If a claim is made about any of the optional components or if a food is fortified or enriched with any of them, nutrition information for these components then becomes mandatory. These mandatory and voluntary components are the only ones allowed on the nutrition panel. The listing of single amino acids, maltodextrin, calories from polyunsaturated fat, and calories from carbohydrates, for example, may not appear as part of the nutrition facts of the label. The required nutrients were selected because they address today’s health concerns. The order in which they must appear reflects the priority of current dietary recommendations. Thiamin, riboflavin, and niacin will no longer be required in nutrition labeling because deficiencies of each are no longer considered of public health significance; however, they may be listed voluntarily. 16.2.5.3.3 Nutrition Panel Format The format for declaring nutrient content per serving also has been revised. Now, all nutrients must be declared as a percent of their daily value — the new label reference values. The amount, in grams, of macronutrients (such as fat, cholesterol, sodium, carbohydrates, and protein) still must be listed to the immediate right of each of the names of each of these nutrients. But, for the first time, a column headed “% Daily Value” will appear, as will a footnote to help consumers place their individual nutrient needs with respect to the daily values used on the label. Requiring nutrients to be declared as a percent of the daily value is intended to prevent misinterpretations that arise with qualitative values. For example, a food with 140 mg of sodium could be mistaken for a high-sodium food because 140 is a relatively large number. In actuality, however, that amount represents less than 6% of the daily value for sodium, which is 2400 mg. On the other hand, a food with 5 g of saturated fat could be construed as being low in that nutrient. But, in fact, that food would provide one fourth the total daily value because 20 g is the daily value for saturated fat based on a 2000 cal diet. Small- and medium-sized packages will be granted certain exceptions to make the nutrition labeling practical in the smaller space.

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16.2.5.3.4 Serving Sizes Whatever the format, the serving size remains the basis for reporting each food’s nutrient content. However, unlike in the past, when the serving size was up to the discretion of the food manufacturer, serving sizes now will be more uniform and will reflect the amounts that people actually eat. They also must be expressed in both common household and metric measures. NLEA defines serving size as the amount of food customarily eaten at one time. The serving sizes that appear on food labels will be based on FDA-established lists of “Reference Amounts Customarily Consumed per Eating Occasion.” These reference amounts, which are part of the new regulations, are broken down into 139 FDA-regulated food product categories, including eleven groups of foods specially formulated or processed for infants or children under four. They list the amounts of food customarily consumed per eating occasion for each category, based primarily on national food consumption surveys. FDA’s list also gives the suggested label statement for serving size declaration. For example, the category “dried fruits” has a reference amount of 40 g, and the appropriate label statement for dried fruits is “__ piece(s) (__ g)” for large pieces (e.g., dates, figs, prunes); “__ cup(s) (__ g)” for small pieces (e.g., raisins). Certain rules apply to food products that are packaged and sold individually, If such an individual package is less than 200% of the applicable reference amount, the item qualifies as one serving. Thus, a 360-ml (12-fluid-oz) can of soda is one serving because the reference amount for carbonated beverages is 240 ml (8 oz). However, if the product has a reference amount of 100 g or 100 ml and or more and the package contains more than 150% but less than 200% of the reference amount, manufacturers have the option of deciding whether the product can be one or two servings. 16.2.5.3.5 Daily Value — DRVs The new label reference value, daily value (DV), comprises two new sets of dietary standards: daily reference values (DRVs) and reference daily intakes (RDIs). Only the daily value term will appear on the label, though, to make label reading less confusing. As part of new regulations, DRVs are being introduced for macronutrients that are sources of energy — fat, carbohydrate (including fiber), and protein — and for cholesterol, sodium, and potassium, which do not contribute calories. DRVs for the energy-producing nutrients are based on the number of calories consumed per day. A daily intake of 2000 calories has been established as the reference. This level was chosen because it has the greatest public health benefit for the nation. DRVs for the energy-producing nutrients are calculated as follows: • • • • •

Fat based on 30% of calories Saturated fat based on 10% of calories Carbohydrate based on 60% of calories Protein based on 10% of calories (The DRV for protein applies only to adults and children over four. RDIs for protein for special groups have been established.) Fiber based on 11.5 g of fiber per 1000 cal

Because of current public health recommendations, DRVs for some nutrients represent the uppermost limit that is considered desirable. The DRVs for fats and sodium are: • • • •

Total fat: less than 65 g Saturated fat: less than 20 g Cholesterol: less than 300 mg Sodium: less than 2400 mg

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16.2.5.3.6 Daily Value — RDIs The RDI replaces the term “U.S. RDA,” which was introduced in 1973, as a label reference value for vitamins, minerals, and protein in voluntary nutrition labeling. The name change was sought because of confusion that existed over “U.S. RDAs,” the values determined by FDA and used on food labels, and “RDAs” (recommended dietary allowances), the values determined by the National Academy of Sciences for various population groups and used by the FDA to figure the U.S. RDA; however, the values for the new RDIs will remain the same as the old U.S. RDAs for the time being. Under the provisions of the Dietary Supplement Act of 1992, FDA plans to propose new values for the RDIs. 16.2.5.3.7 Nutrient Content Descriptors The new regulations also spell out what terms may be used to describe the level of a nutrient in a food and how they can be used. Following are the core terms. Free: This term means that a product contains no amount of, or only trivial or “physiologically inconsequential” amounts of, one or more of these components: fat, saturated fat, cholesterol, sodium, sugars, and calories. For example, “calorie-free” means fewer than 5 calories per serving and “sugar-free” and “fat-free” both mean less than 0.5 per serving. Synonyms for free include without, no, and zero. Low: This term could be used on foods that could be eaten frequently without exceeding dietary guidelines for one or more of these components: fat, saturated fat, cholesterol, sodium, and calories. Thus, descriptors would be defined as follows: • • • • • •

Low fat: 3 g or less per serving Low saturated fat: 1 g or less per serving Low sodium: less than 140 mg per serving Very low sodium: less than 35 mg per serving Low cholesterol: less than 20 mg per serving Low calorie: 40 cal or less per serving

Synonyms for low include little, few, and low source of. Lean and extra lean: These terms can be used to describe the fat content of meat, poultry, seafood, and game meat. • •

Lean: less than 10 g fat, less than 4 g saturated fat, and less than 95 mg cholesterol per serving and per 100 g Extra lean: less than 5 g fat, less than 2 g saturated fat, and less than 95 mg cholesterol per serving and per 100 g

High: This term can be used if the food contains 20% or more of the daily value for a particular nutrient in a serving. Good source: This term means that one serving of a food contains 10 to 19% of the daily value for a particular nutrient. Reduced: This term means that a nutritionally altered product contains 25% less of a nutrient or of calories than the regular, or reference, product; however, a reduced claim cannot be made on a product if its reference food already meets the requirement for a “low” claim. Less: This term means that a food, whether altered or not, contains 25% less of a nutrient or of calories than the reference food. For example, pretzels that have 25% less fat than potato chips could carry a “less” claim. Fewer is an acceptable synonym. Light: This descriptor can mean two things:

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1. That a nutritionally altered product contains one-third fewer calories or half the fat of the reference food. If the food derives 50% or more of its calories from fat, the reduction must be 50% of the fat. 2. That the sodium content of a low-calorie, low-fat food has been reduced by 50%. In addition, “light in sodium” may be used on food in which the sodium content has been reduced by at least 50%. The term light still can be used to describe such properties as texture and color, as long as the label explains the intent, for example, “light brown sugar” and “light and fluffy.” More: This term means that a serving of food, whether altered or not, contains a nutrient that is at least 10% of the daily value more than the reference food. The 10% of daily value also would apply to “fortified,” “enriched,” and “added” claims, but in those cases, the food must be altered. 16.2.5.3.8 Other Definitions The regulations also address other claims, including the following: Percent fat-free: A product bearing this claim must be a low-fat or a fat-free product. In addition, the claim must accurately reflect the amount of fat present in 100 g of the food. Thus, if a food contains 2.5 g fat per 50 g, the claims must be “95% fat free.” Implied: These types of claims are prohibited when they wrongfully imply that a food contains or does not contain a meaningful level of a nutrient. For example, a product claiming to be made with an ingredient known to be a source of fiber (such as “made with oat bran”) is not allowed unless the product contains enough of that ingredient (for example, oat bran) to meet the definition for “good source” of fiber. Meals and main dishes: Claims that a meal or main dish is “free” of a nutrient, such as sodium or cholesterol, must meet the same requirement as those for individual foods. Other claims can be used under special circumstances. For example, low calorie means the meal or main dish contains 120 cal or less per 100 g. Low sodium means the food has 140 mg or less per 100 g. Low cholesterol means the food contains 20 mg cholesterol or less per 100 g and no more than 2 g saturated fat. Light means the meal or main dish is low fat or low calorie. Standardized foods: Any nutrient content claim, such as “reduced fat,” “low calorie,” and “light” may be used in conjunction with a standardized term if the new product has been specifically formulated to meet the FDA’s criteria for that claim, if the product is now nutritionally inferior to the transitional standardized food, and if the new product complies with certain compositional requirements set by the FDA. A new product bearing a claim also must have performance characteristics similar to the referenced traditional standardized food. If the product does not and the differences materially limit the product’s use, its label must state the differences (for example, not recommended for baking) to inform consumers. Healthy: The term healthy may used on the label if it complies with the following: 1. It meets the definition of “low” for fat and saturated fat. 2. It contains no more than 480 mg sodium and no more than 60 mg cholesterol per serving. Fresh: Although not mandated by NLEA, FDA also issued a regulation for the term fresh. The agency took this step because of concern over the term’s possible misuse on some food labels. The regulation defines the term fresh when it is used to suggest that a food is raw or unprocessed. In this context, fresh can be used only on a food that is raw, has never been frozen or treated, and contains no preservatives. (Irradiation at low levels is allowed.) Fresh frozen, frozen fresh, and freshly frozen can be used for foods that are quickly frozen while still fresh. Blanching (brief scalding before freezing to prevent nutrient breakdown) is also allowed.

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16.2.5.3.9 Health Claims Claims for seven relationships between a nutrient or a food and the risk of a disease or healthrelated condition will be allowed for the first time. They can be made in several ways: through third-party references, such as the National Cancer Institute; statements; symbols, such as a heart; and vignettes or descriptions. Whatever the case, the claim must meet the requirements for authorized health claims. For example, they cannot state the degree of risk reduction, and they can only use may or might in discussing the nutrient or food–disease relationship. And they must state that other factors play a role in that disease. They also must be phrased so that the consumer can understand the relationship between the nutrient and the disease and the nutrient’s importance in relationship to a daily diet. An example of an appropriate claim is: “While many factors affect heart disease, diets low in saturated fat and cholesterol may reduce the risk of this disease.” The allowed nutrient–disease relationship claims, the headings under which they are presented, and rules for their use are as follows: Calcium and osteoporosis: To claim a product relationship to this category, a food must contain 20% or more of the DV for calcium (200 mg) per serving, have a calcium content that equals or exceeds the food’s content of phosphorus, and contain a form of calcium that can be readily absorbed and used by the body. The claim must name the target group most in need of adequate calcium intakes (that is, teens and young adult White and Asian women) and state the need for exercise and a healthy diet. A product that contains 40% or more of the DV for calcium must state on the label that a total dietary intake greater than 200% of the ITV for calcium (i.e., 2000 mg or more) has no further known benefit. Fat and cancer: To present claims in this category, a food must meet the descriptor requirements for “low-fat” or, for fish and game meats, for “extra lean.” Saturated fat and cholesterol and coronary heart disease (CHD): This category may be used if the food meets the definitions for the descriptors “low saturated fat,” “low cholesterol,” and “low-fat” or, for fish and game meats, for “extra lean.” It may mention the link between reduced risk of CHD and lower saturated fat and cholesterol intakes to lower blood cholesterol levels. Fiber-containing grain products, fruits, and vegetables and cancer: To make product claims in this category, a food must be or must contain a grain product, fruit, or vegetable and meet the descriptor requirements for “low-fat” and, without fortification, be a “good source” of dietary fiber. Fruits, vegetables, and grain products that contain fiber and risk of CHD: To make claims in this category, a food must be or must contain fruits, vegetables, and grain products. It also must meet the descriptor requirements for “low saturated fat,” “low cholesterol;” and “low fat” and contain, without fortification, at least 0.6 g soluble fiber per serving. Sodium and hypertension (high blood pressure): To make claims in this category, a food must meet the descriptor requirements for “low sodium.” Fruits and vegetables and cancer: Claims in this category may be made for fruits and vegetables that meet the descriptor requirements for “low fat” and that, without fortification, for “good source” of at least one of the following: dietary fiber or vitamins A or C. This claim relates diets low in fat and rich in fruits and vegetables (and thus vitamins A and C and dietary fiber) to reduced cancer risk. The FDA authorized this claim in place of an antioxidant vitamin and cancer claim. 16.2.5.3.10 Folic Acid The FDA is denying the use of a health claim for folic acid and neural tube defects. In September 1992, the U.S. Public Health Service (PHS) recommended that all women of childbearing age consume 0.4 mg of folic acid daily to reduce their risk of having a pregnancy affected with a neural tube defect. PHS identified several issues that remain to be resolved before the FDA can take appropriate action to implement the recommendation and to decide whether to authorize a claim. The issues include the appropriate level of folic acid in food, safety concerns regarding increased intakes of folic acid, and specific options for implementation.

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16.2.5.3.11 Ingredient Labeling As part of the new rules, the list of ingredients will undergo some changes, too. Chief among them is a new regulation that requires full ingredient labeling on “standardized foods” that previously were exempt. Ingredient declaration will now have to be on all foods that have more than one ingredient. Also, the ingredient list will include, when appropriate: • • •

FDA-certified color additives, such as FD&C Blue No. 1, by name Sources of protein hydrolysates, which are used in many foods as flavors and flavor enhancers Declaration of caseinate as a milk derivative in the ingredient list of foods that claim to be nondairy, such as coffee whiteners

The main reason for these new requirements is that some people may be allergic to such additives and will now be better able to avoid them. As required by NLEA, beverages that claim to contain juice now must declare the total percentage of juice on the information panel. In addition, the FDA’s regulation establishes criteria for naming juice beverages. For example, when the label of a multi-juice beverage states one or more — but not all — of the juices present, and the predominantly named juice is present in minor amounts, the product’s name must state that the beverage is flavored with that juice or declare the amount of the juice in a 5% range, for example, “raspberry flavored juice blend” or “juice blend, 2 to 7% raspberry juice.”

16.2.6 LABELING

AND

FRUIT JUICES

The new labeling regulations require a percent juice declaration for any food that purports or claims to be a beverage that contains fruit juice. The beverage may be carbonated or noncarbonated, concentrated, full-strength, diluted, or contain no juice. The criterion is, of course, the requirement “purport” or “claim.” A soft drink (soda) that does not represent or suggest by its physical characteristics, name, labeling, ingredient statement, or advertising that it contains fruit juice is not required a percent juice declaration. If the beverage contains fruit juice, the percentage will be declared by the words, “Contains __ percent (or %) __ juice” or “__ percent (or %) juice” or a similar phrase, with the first blank filled in with the percentage expressed as a whole number not greater than the actual percentage of the juice and the second blank (if used) filled in with the name of the particular fruit (e.g., “Contains 50% apple juice” or “50% juice”). If the beverage contains less that 1% juice, the total percentage juice will be declared as “less than 1% juice” or “less than 1% __ juice,” with the blank filled in with the name of the particular fruit: For example, assume a beverage satisfies the following criteria: 1. The beverage contains 100% juice. 2. The beverage also contains nonjuice ingredients that do not result in a diminution of the juice soluble solids or in the case of expressed juice, in a change in the volume. 3. When the 100% juice declaration appears on a panel of the label, it does not also bear the ingredient statement. If this is the case, then, the beverage must be accompanied by the phrase “with added __,” the blank filled in with a term such as “ingredient(s),” “preservative,” or “sweetener,” as appropriate (e.g., “100% juice with added sweetener”). However, when the presence of the nonjuice

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ingredient(s) is declared as a part of the statement of identity of the product, this phrase need not accompany the 100% juice declaration. As another example, assume a beverage contains minor amounts of juice for flavoring and is labeled with a flavor description using tests such as flavor, flavored, or flavoring with a fruit name and does not bear three specific descriptives such as: 1. The term juice on the label other than in the ingredient statement 2. An explicit vignette depicting the fruit from which the flavor derives, such as juice exuding from a fruit 3. Suggest a specific physical resemblance to a juice or distinctive juice characteristic such as pulp Then, in this case, total percentage juice declaration is not required. If this beverage deviates from the above three criteria, then the label shall declare “contains zero (0) percent (or %) juice.” Alternatively, the label may declare “Containing (or contains) no __ juice,” “no __ juice,” or “does not contain __ juice,” the blank to be filled in with the name of the fruits represented, suggested, or implied; but if there is a general suggestion that the product contains fruit juice, such as the presence of fruit pulp, the blank shall be filled in with the word “fruit” as applicable (e.g., “contains no fruit juice” or “does not contain fruit juice”). For more details on juice labeling, consult the original regulations. In enforcing these regulations, the FDA will calculate the labeled percentage of juice from concentrate found in a juice can/juice beverage using the minimum Brix levels in Table 16.3 where single-strength (100%) juice has at least the specified minimum Brix listed. If there is no Brix level specified, the labeled percentage of that juice from concentrate in a beverage will be calculated on the basis of the soluble solids content of the single-strength (unconcentrated) juice used to produce such concentrated juice. Juice directly expressed from a fruit (i.e., not concentrated and reconstituted) is considered to be 100% juice and will be declared as “100% juice.” Calculations of the percentage of juice in a juice blend or a diluted juice product made directly from expressed juice (i.e., not from concentrate) is based on the percentage of the expressed juice in the product computed on a volume/volume basis. If the product is a beverage that contains a juice whose color, taste, or other organoleptic properties have been modified to the extent that the original juice is no longer recognizable at the time processing is complete, or if its nutrient profile has been diminished to a level below the normal nutrient range for the juice, then that juice to which such a major modification has been made should not be included in the total percentage juice declaration. A beverage required to have a percentage juice declaration on its label is not permitted to bear any other percentage declaration that describes the juice content of the beverage in its label or in its labeling (e.g., “100% natural” or “100% pure”). However, the label or labeling may bear percentage statements clearly unrelated to juice content (e.g., “provides 100% of U.S. RDA of vitamin C”).

16.2.7 STANDARDS

FOR

PROCESSED FRUIT

AND

FRUIT PRODUCTS

At the time of this writing, the FDA has established standards for four categories of processed fruit and fruit products: • • • •

21 21 21 21

CFR CFR CFR CFR

146 345 150 152

— — — —

Canned fruit juices Canned fruit Fruit butters, jellies, preserves, and related products Fruit pies

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TABLE 16.3 Juice and Brix Juice Acerola Apple Apricot Banana Blackberry Blueberry Boysenberry Cantaloupe melon Carambola Carrot Casaba melon Cashew (caju) Celery Cherry, dark, sweet Cherry, red, sour Crabapple Cranberry Currant (black) Currant (red) Date Dewberry Elderberry Fig Gooseberry Grape Grapefruit Guanabana (soursop) Guava Honeydew melon Kiwi Lemon Lime Loganberry Mango Nectarine Orange Papaya Passion fruit Peach Pear Pineapple Plum Pomegranate Prune Quince Raspberry (black) Raspberry (red) Rhubarb Strawberry

100% Juicea 6.0 11.5 11.7 22.0 10.0 10.0 10.0 9.6 7.8 8.0 7.5 12.0 3.1 20.0 14.0 15.4 7.5 11.0 10.5 18.5 10.0 11.0 18.2 8.3 16.0 10.0 16.0 7.7 9.6 15.4 4.5b 4.5b 10.5 13.0 11.8 11.8 11.5 14.0 10.5 12.0 12.8 14.3 16.0 18.5 13.3 11.1 9.2 5.7 8.0

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TABLE 16.3 (Continued) Juice and Brix Juice Tangerine Tomato Watermelon Youngberry a b

100% Juicea 11.8 5.0 7.8 10.0

Indicates Brix value unless other value specified. Indicates anhydrous citrus acid percent by weight.

The specific standards for the first three categories are listed in Table 16.4, Table 16.5, and Table 16.6, respectively. The fourth one discusses fruit pies only. Refer to Chapter 19 on apricots and peaches in Part II of this series. It provides examples of standards of identity for canned products.

TABLE 16.4 Table of Contents for 21 CFR 146, Standards for Canned Fruit Juices 21 CRF 146 Section

3

Subject Matter Subpart A: General Provisions Definitions

Subpart B: Requirements for Specific Standardized Canned Fruit Juices and Beverages 113 Canned fruit nectars 114 Lemon juice 120 Frozen concentrate for lemonade 121 Frozen concentrate for artificially sweetened lemonade 126 Frozen concentrate for colored lemonade 132 Grapefruit juice 135 Orange juice 137 Frozen orange juice 140 Pasteurized orange juice 141 Canned orange juice 145 Orange juice from concentrate 146 Frozen concentrated orange juice 148 Reduced acid frozen concentrated orange juice 150 Canned concentrated orange juice 151 Orange juice for manufacturing 152 Orange juice with preservative 153 Concentrated orange juice for manufacturing 154 Concentrated orange juice with preservative 185 Canned pineapple juice 187 Canned prune juice

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TABLE 16.5 Table of Contents for 21 CFR 145, Standards for Canned Fruits 21 CRF 145 Section 3 110 115 116 118 120 125 126 128 130 131 134 135 136 140 145 170 171 175 176 178 180 181 185 190

Subject Matter Definitions Canned applesauce Canned apricots Artificially sweetened canned Canned apricots with rum Canned berries Canned cherries Artificially sweetened canned Canned cherries with rum Canned figs Artificially sweetened canned Canned preserved figs Canned fruit cocktail Artificially sweetened canned Canned seedless grapes Canned grapefruit Canned peaches Artificially sweetened canned Canned pears Artificially sweetened canned Canned pears with rum Canned pineapple Artificially sweetened canned Canned plums Canned prunes

apricots

cherries

figs

fruit cocktail

peaches pears

pineapple

TABLE 16.6 Table of Contents for 21 CFR 150, Standards for Fruit Butters, Jellies, Preserves, and Related Products 21 CRF 150 Section

Subject Matter

110 140 141 160 161

Fruit butter Fruit jelly Artificially sweetened fruit jelly Fruit preserves and jams Artificially sweetened fruit preserves and jams

16.3 U.S. DEPARTMENT OF AGRICULTURE As discussed at the beginning of this chapter, the U.S. Department of Agriculture (USDA) issues standards for grades for processed fruit and fruit products. An alphabetical listing of such standards is provided in Table 16.7.

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TABLE 16.7 USDA Standards for Grades for Processed Fruit and Fruit Products 7 CRF 52 Section

Product

2161 2481 2341 361 2801 301 6321 331 2641 3871 5761 5521 6241 5881 551 581 611 771 801 821 3161 951 6281 1001 2821 1021 1051 1081 1111 3831 1341 2451 1141 1171 1251 6121 1221 3021 1281 1311 3481 4021 5481 3951 1421 2521 5361 1551

Apples, canned Apples, dried Apples, dehydrated (love moisture) Apples, frozen Apple butter, canned Apple juice, canned Apple juice, concentrated, frozen Applesauce, canned Apricots, canned Apricots, dehydrated (low moisture) Apricots, dried Apricots, frozen Apricots, solid-pack, canned Berries, frozen Blackberries, and other similar berries, canned Blueberries, canned Blueberries, frozen Cherries, red tart pitted, canned Cherries, red tart pitted, frozen Cherries, sweet canned Cherries, sweet frozen Cranberry sauce, canned Cranberries, frozen Dates Figs, Kadota, canned Figs, dried Fruit cocktail, canned Fruit jelly Fruit preserves (or jams) Fruits for salad, canned Grape juice, canned Grape juice, concentrate, sweetened, frozen Grapefruit, canned Grapefruit, frozen Grapefruit and orange for salad, canned Grapefruit juice Grapefruit juice, concentrated, frozen Grapefruit juice, dehydrated Grapefruit juice and orange juice, canned Grapefruit juice and orange juice, concentrated, blended, frozen Grapefruit juice for manufacturing, concentrated Grapes, canned Lemon juice, canned Lemon juice, concentrated, for manufacturing Lemonade, concentrate, frozen Limeade, concentrate, frozen Melon balls Orange juice

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TABLE 16.7 (Continued) USDA Standards for Grades for Processed Fruit and Fruit Products 7 CRF 52 Section 1451 3911 5801 3551 2561 2601 1611 5841 1711 1741 1761 1781 2911 3231 5601 3181 2741 1841 3311 1871 1981 2071 2931

Product Orange marmalade Peaches, dehydrated (low moisture) Peaches, dried Peaches, frozen Peaches, clingstone, canned Peaches, freestone, canned Pears, canned Pears, dried Pineapple, canned Pineapple, frozen Pineapple juice, canned Plums, canned Plums, frozen Prunes, dehydrated (low moisture) Prunes, dried, canned Prunes, dried Pumpkin (squash), canned Raisins, processed Raspberries, canned Raspberries, frozen Strawberries, frozen Tangerine juice, canned Tangerine juice, concentrated for manufacturing, canned

16.4 SUBSISTENCE SPECIFICATIONS AND COMMERCIAL ITEM DESCRIPTION 16.4.1 FEDERAL SPECIFICATION A federal specification is a document used by federal agencies in procurement that: 1. Describes the essential and technical requirements for items, materials, or services of general application and use by federal agencies 2. Includes procedures for determining compliance with the requirements A federal specification provides all federal user agencies a document for the procurement of essential goods and services on a competitive basis. In the case of processed fruits and vegetables, this is achieved by clearly and realistically defining the product and its requirements as needed in order to minimize misunderstanding between buyer and vendor. Specifications reflect the capabilities of the supplying industries and, thus, promote efficient and competitive buying of those goods that are available from multiple sources. Because the essential and technical requirements of the product are clearly defined, the procuring agency is assured of the safety and quality of the product specified. The use of specifications is important when ordering foods, regardless of the size of the order. The specific details will vary for each type of food purchased. All specifications should include at least the following information:

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1. 2. 3. 4. 5.

Name of product Grade or quality designation Size of container or package Number of purchase units Any other pertinent information

16.4.2 COMMERCIAL ITEM DESCRIPTION 16.4.2.1 Definition A commercial item description (CID) is a simplified description of a commercially available product that contains only those essential quality factors that are important to the buyer and seller and that determine the commercial value of the product. The CID represents the current state of the art of commercially available finished products and accepted commercial practice. (See Table 16.8.) Commercial item descriptions (CIDs) are for use by all federal agencies in lieu of federal specifications to purchase commercial off-the-shelf products of good commercial quality when such products adequately serve the government’s requirements, provided such products have an established commercial acceptability. ClDs are a new series of federal specifications and usually contain the same basic components; however, there are optional components, depending on the category of products. The basic format for a CID contains the following components: 1. Title: A simple statement naming the product to be purchased, e.g., “jelly, fruit” 2. Code: A code number, e.g., A-A-20078 for “jelly, fruit” 3. Salient characteristics: classification of the product as found on the market with respect to style, form, type, variety, and container size; descriptive terms defining acceptable quality criteria 4. Regulatory requirement: where applicable, compliance with all applicable federal and state mandatory requirements and regulations to the preparation, packaging, and so on 5. Quality assurance: If available, USDA grade standards are referenced, as well as the requirements for inspection and certification. If no U.S. grade standards are available, producers’ product specifications may be referenced

TABLE 16.8 Fruit and Fruit Products with Commercial Item Descriptions FSCa

Document Name

Document Number

Date of Issue

8915 8915 8915 8915 8915 8915 8915 8915 8915 8925 8930 8930

Cherries, maraschino, pitted, red Crabapples, spiced, canned Cranberry juice cocktail, canned Juice, lemon, frozen concentrated Juice, lemon, reconstituted Juice, lime, frozen, and frozen concentrate for limeade Juice, prune, canned Nectars, fruit, canned (apricot, pear, and peach) Oranges, canned (mandarin) Coconut, dried, prepared Jelly, fruit Preserves (or jams), fruit

A-A-20121A A-A-20157 A-A-20121 A-A-20102 A-A-20144A A-A-20122A A-A-20117A A-A-20118A A-A-20119A A-A-20174 A-A-20078 A-A-20079

05/12/93 06/18/87 06/24/85 03/30/90 02/01/93 05/12/93 09/09/93 09/09/93 05/12/93 06/29/89 04/16/84 04/16/84

a

FSC = Federal subsistence classification.

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16.4.3 CID: ORANGES, CANNED (MANDARIN) The commercial item description (CID) covers mandarin oranges processed in cans or other approved commercially acceptable containers and suitable for use by the federal government. 16.4.3.1 Salient Characteristics The product shall be prepared from sound, mature oranges of the variety Citrus reticulata Blanco and packed in water or other suitable liquid packing medium. The fruit shall be properly washed and peeled, and the membrane and seeds shall have been substantially removed from the segments. The color of the segments shall be a rich yellow to orange, typical color of properly prepared and properly processed fruit. The liquid packing medium shall be reasonably clean. The product shall have a normal flavor and odor, free from any foreign flavors or odors. The texture shall be reasonably firm and characteristic for the product. The product shall be reasonably free from defects within the limits specified by the Codex Standard for Canned Mandarin Oranges. The container shall be filled with not less than 90% of the water capacity of the container. The drained weight shall be not less than 55% of the water capacity of the can for whole segments and 58% of the water capacity of the can for broken segments or pieces. The canned mandarin oranges shall conform to each of those options, specified in the following list, which shall be specified as required in the contract or purchase order. 16.4.3.2 Style 1. Whole: consists of practically intact fruit segments 2. Broken: consists of portions or segments that retain at least half the original size 3. Pieces: consists of small portions of segments large enough to remain on a wire mesh screen of 2-mm diameter openings 16.4.3.3 Sizes (Whole Segment Only) 1. 2. 3. 4.

Large — 20 or less whole segments per 100 g fruit Medium — 21 to 35 whole segments per 100 g fruit Small — 36 or more whole segments per 100 g fruit Mixed — a mixture of two or more single sizes

16.4.3.4 Packing Media 1. 2. 3. 4.

Water Citrus juice: mandarin orange juice or other citrus juice Mixed citrus juices: two or more citrus juices, including mandarin orange juice Water and citrus juice(s): water and mandarin orange juice or water and any other citrus juice 5. Syrup: prepared with any of the following sweeteners: sucrose, invert sugar syrup, dextrose, fructose, fructose syrup, dried glucose syrup, glucose syrup, or invert sugar The following designations shall be used for labeling purposes: 1. 2. 3. 4.

Lightly sweetened (name of fruit) juice: not less than 14∞ Brix Heavily sweetened (name of fruit) juice: not less that 18∞ Brix Light syrup: not less than 14∞ Brix Heavy syrup: not less than 18∞ Brix

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16.4.3.5 Contractor’s Certification By submitting an offer, the contractor certifies that the product offered meets the specified salient characteristics of this CID; conforms to the producer’s own specifications and standards, including product characteristics, manufacturing procedures, quality control procedures, and storage and handling practices; has a national regional distribution from storage facilities located within the U.S., its territories, or possessions; and is sold on the commercial market. The government reserves the right to determine proof of such conformance prior to the first delivery from point of origin and any time thereafter, up to and including delivery at final destination, as may be necessary to determine conformance with the provisions of the contract. 16.4.3.6 Regulatory Requirements The delivered product shall comply with all applicable federal and state mandatory requirements and regulations relating to the preparation, packaging, labeling, storage, distribution, and sales of the product in the commercial marketplace. All deliveries shall conform in every respect to the provisions of the Federal Food, Drug, and Cosmetic Act and regulations promulgated thereunder. 16.4.3.7 Quality Assurance When required in the contract or purchase order, the mandarin oranges shall be product inspected and certified as to the salient characteristics and processing and also condition of containers by the Processed Products Branch, Fruit and Vegetable Division, Agricultural Marketing Service, U.S. Department of Agriculture. The inspection and certification of the mandarin oranges and the condition of container evaluations shall be made in accordance with applicable provisions contained in the Regulations Governing Inspection and Certification of Processed Fruits and Vegetables and Related Products and the U.S. Standards for Condition of Food Containers in effect on the date of the soliciation and in accordance with the specified criteria contained in this commercial item description. 16.4.3.8 Preservation, Packaging, Packing, Labeling, and Marking The mandarin oranges shall be preserved, packaged, packed, and case marked in accordance with good commercial practice. Commercial labeling and packaging, as may be augmented by the contract or purchase order, shall be acceptable. Shipping containers shall comply with National Motor Freight Classification or Uniform Freight Classification, as applicable. 16.4.3.9 Notes Purchases should specify in the contract: • • • •

product style, size, and packing media desired can size and net weight country of origin (optional) labeling requirements, if different from commercial practice

16.5 COUNTRY OF ORIGIN LABELING The Farm Security and Rural Investment Act of 2002 (Public Law 107-171) amended the Agricultural Marketing Act of 1946 to require retailers to inform consumers of the country of origin for covered commodities. The Act required the Agricultural Marketing Service of the U.S. Department of Agriculture (AMS) to issue Country of Origin Labeling (COOL) guidelines for voluntary use by retailers who wish to inform their customers of country of origin of their covered commodities. In addition,

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Public Law 107-171 required the Secretary of Agriculture to “promulgate a regulation” for mandatory country of origin labeling of covered commodities, to be effective September 30, 2004. The voluntary guidelines were published and became effective October 11, 2002 in the Federal Register, Vol. 67, No. 198, and development of the mandatory regulation commenced in April 2003. It is anticipated that the voluntary guidelines will form the basis for the mandatory regulation that will be developed through standard rule-making procedures. The covered commodities are: beef (including veal), lamb, pork, fish, perishable agricultural commodities, and peanuts. The terms “retailers” and “perishable commodities” are defined in Public Law 170-171 as in the Perishable Agricultural Commodities Act of 1930 (PACA) (7 U.S.C. 499a(b)). Under PACA a “retailer” is one who buys or sells perishable agricultural commodities solely for sale at retail with a cumulative invoice in any calendar year of more than $230,000. Fresh and frozen fruits and vegetables are defined as perishable agricultural commodities. Current federal law, the Tariff Act of 1930 as amended (19 U.S.C. 1304), and other legislation requires most imports to bear labels that inform the “ultimate purchaser” of their country of origin. The “ultimate purchaser” is defined as the last U. S. person to receive the imported article in its imported form. For example, a grocer receiving a box of apples from New Zealand would be informed that the apples are a product of New Zealand, and the box would be so labeled. The grocer could remove the apples from the box and offer them for sale in a bin or other bulk container as individual apples, without being required to identify them to the consumer as a product of New Zealand. When the mandatory country of origin regulation becomes effective on September 30, 2004, the bin or the individual apples would be required to be marked or labeled with their country of origin. On the other hand, under current law, for example, grapes from Chile in a box or carton containing individual retail-sized, consumer-ready, bags of grapes would be required to have a label on each individual bag, identifying it as a product of Chile, because the “ultimate purchaser” is the individual U. S. consumer. Therefore, the mandatory COOL regulations will not alter the situation in the grape example because country of origin labeling is already required. To convey country of origin information to consumers, P.L. 170-171 allows retailers to use “a label, stamp, mark, placard, or other clear and visible sign on the covered commodity or on the package, display, holding unit, or bin containing the commodity at the final point of consumption.” Food service establishments, restaurants, bars, food stands, and similar facilities are exempt from this labeling requirement. The law also excludes foods from COOL when a covered commodity is included as an “ingredient in a processed food item.” Public Law 170-171 does not define a “processed food item”; therefore, AMS must develop a definition of “processed food item” for each covered commodity. The National Organic Program (NOP) defines processing very broadly. Using this definition would eliminate many foods that Congress intended to be covered in P. L.170-171. For example, the NOP defines the mere cutting of a product as processing, so that slicing apples and selling them in sliced form would eliminate them from the labeling requirement, using this definition. Therefore, AMS has decided to define “processed food item” in two ways. A processed food item could be a combination of ingredients in which the final product is not identified as a covered commodity. Apples in apple pies are cited as an example. Also, a processed food item could be a product where the final form substantially differs from the covered commodity. Fruit juices are cited as examples of the latter definition of a processed food item. Blended products, such as a blend of frozen fruits, are not excluded from COOL if the individual pieces maintain their identity. For example, a blend of frozen strawberries, melons, and peaches would not be excluded if the individual fruit pieces could be identified. Similarly, if a frozen fruit is packaged with only preservatives, seasonings, sweeteners, or other minor ingredients, it is not excluded from COOL. To use the label “U.S. Country of Origin” the perishable commodity must be “exclusively produced, packed, and, if applicable, processed in the U.S.” For products that may be produced, packed, or processed in several countries, AMS must determine how they should be labeled. For

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example, a label may read: “produced in county X, packed in country Y, and processed in the U.S.” For a blended product in which individual items originated in different countries, the labeling would specify the country of origin for each component in the order of prominence by weight. A label might read: component A from country X, component B from country Y, and component C from country Z. Retailers and their suppliers will be required to maintain an “audit trail,” to substantiate COOL claims. The guidelines require 2-year records retention. AMS chose this timeframe to be consistent with current PACA requirements.

16.6 NATIONAL ORGANIC PROGRAM (NOP) Congress passed the Organic Food Production Act of 1990 (OFPA) that provided the basis for marketing foods labeled with the term “organic.” Details of the regulations for marketing “organic” foods are contained in U.S.C. 7 CFR Part 205. The program is called the National Organic Program (NOP). The Agricultural Marketing Service (AMS) of the USDA administers the NOP. The NOP provides a set of standards that must be met for growing and handling foods to be designated as “organic.” The standards include restrictions on the use of various fertilizers and pesticides, as well as requirements for various agronomic and other production practices that ensure a sustainable agriculture. The program utilizes approved agents called certifying agents who work with those interested in producing organic foods to ensure that the production is in accordance with the NOP standards. The certifying agent certifies that the food is organic, and the name of the certifying agent must appear on the product label. There are four defined categories for organic products: “100% Organic,” “Organic,” “Made with Organic Ingredients,” and the fourth merely informs the consumer that the product contains some organic ingredients. To be marketed as “100% organic,” products must contain organic ingredients only. If the product contains more than one ingredient, the label must contain an ingredient statement. The label may identify the product with a term such as “100% organic fruit salad.” A product labeled “organic” must contain at least 95% by weight organic ingredients, not counting added water or salt. It may contain up to 5% nonorganically produced ingredients that are not commercially available in organic form, or other substances allowed by 7 CFR 205.605. It must not contain added sulfites. The label must show an ingredient statement and identify the organic ingredients. The label may identify the product using the term “organic,” such as “organic peach pie.” It may also give the percentage of organic content such as “X% organic” or “X% organic ingredients.” When a product is labeled as “Made with Organic Ingredients,” it must contain at least 70% organic ingredients, not counting added water or salt. It must not contain added sulfites, but wine may contain added sulfur dioxide in accordance with 7 CFR 205.605. The product may contain up to 30% nonorganically produced agricultural ingredients and/or other substances, including yeast, allowed by 7 CFR 205.605. An ingredient statement must be shown with the organic ingredients identified. The label may use the term “Made with Organic” followed by the name of the specified ingredients, such as “Made with Organic Apples” or “Made with Organic Fruit.” To merely claim that a product contains some organic ingredients, the product may contain less than 70% organic ingredients, not counting added water or salt. It may contain over 30% nonorganically produced agricultural ingredients and/or other substances, without being limited to those in 7 CFR 205.605. The label must show an ingredient statement when the word organic is used, and the organic ingredients must be identified when “% organic” is displayed. If water and salt are listed on the ingredient statement they must not be labeled as organic. A statement such as “X% organic ingredients” may appear on the label.

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Products labeled as “100% Organic,” “Organic,” or “Made with Organic Ingredients,” must include a statement, under the name of the handler (processor, packer, manufacturer, bottler, etc.), the statement: “Certified organic by__”, or similar statement, followed by the name of the certifying agent. Seals of certifying agent will not meet this requirement; however, the seals may be added to the label in addition to the name of the agent. The USDA Organic seal may also be added to the label of “100% Organic” and “Organic” products. Products that only contain some organic ingredients must not contain the certifying agent or the USDA Organic seals on the label.

REFERENCES Hui, Y. H. 1986. United States Regulations for Processed Fruits and Vegetables. John Wiley & Sons, New York. US. Code of Federal Regulations, Titles 7 and 21, 1994.

Management in Fruit 17 Residual Processing Plants Jatal D. Mannapperuma CONTENTS 17.1 17.2 17.3 17.4

17.5

17.6 17.7

17.8 17.9

Introduction ........................................................................................................................424 17.1.1 Fruit Processing Operations ................................................................................424 17.1.2 Environmental Impact..........................................................................................424 Residual Hierarchy.............................................................................................................426 Residue Assessment and Water Accounting......................................................................427 17.3.1 A Case Study (Mannapperuma et al., 1994b) ....................................................427 Water Conservation ............................................................................................................429 17.4.1 Water Quality Preservation..................................................................................429 17.4.2 Membrane Technology ........................................................................................431 17.4.3 Other Separation Technologies............................................................................432 17.4.4 Hydraulic Transport .............................................................................................432 17.4.5 Heating Operations ..............................................................................................432 17.4.6 Direct Contact Cooling Operations.....................................................................433 17.4.7 Indirect Contact Cooling Operations ..................................................................434 17.4.8 Evaporator Condensate ........................................................................................434 17.4.9 Brining and Curing..............................................................................................434 17.4.10 Peeling Operations...............................................................................................435 By-Product Recovery for Human Consumption ...............................................................435 17.5.1 Dietary Fiber and Pectins from Pomace .............................................................436 17.5.2 Grape Sugars from Grape and Raisin Residues .................................................436 17.5.3 Fruit Juice and Syrup Recovery from Fruit Process Waters ..............................436 17.5.4 Candied Fruit Peels and Reformed Fruit Pieces.................................................437 17.5.5 Oils from Seeds and Stones ................................................................................437 17.5.6 Functional Food Compounds ..............................................................................437 By-Product Recovery for Animal Feed .............................................................................437 Recovery of Energy ...........................................................................................................440 17.7.1 Ethanol from Wet Residues.................................................................................440 17.7.2 Methane Gas from Solid Residues......................................................................441 17.7.3 Catalytic Gasification of Wet Residues...............................................................441 17.7.4 Olive and Peach Pits as a Boiler Fuel ................................................................441 17.7.5 Charcoal Briquettes from Peach, Apricot, and Olive Pits ..................................442 Land Application as Fertilizers and Soil Conditioners .....................................................442 Wastewater Treatment ........................................................................................................443 17.9.1 Regulatory Background .......................................................................................443 17.9.2 Cost of Wastewater Disposal...............................................................................445 17.9.3 Pretreatment and Primary Treatment ..................................................................445

0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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17.9.4 Secondary Treatment ...........................................................................................447 17.9.5 Tertiary Treatment ...............................................................................................448 17.9.6 Anaerobic Treatment ...........................................................................................449 17.10 Landfills, Impoundments, Incineration, and Hazardous Waste Disposal .........................449 17.11 Food Process Residual Management in the Future...........................................................449 References ......................................................................................................................................450

17.1 INTRODUCTION Residuals are a necessary consequence of processing agricultural raw materials. These residuals, dumped at local landfills and discharged to natural streams, have now become a major environmental concern. Food plants that were built in rural areas a few decades ago are now surrounded by residential neighborhoods. Rising standards of living and education have made the population more sensitive to the environmental issues related to food processing residuals. Environmental protection standards are becoming more stringent, causing disposal costs to escalate. Meeting these challenges of environmental protection and economic competitiveness requires a fresh look at the management of fruit processing residuals. Fruit processing waste is extremely diverse due to the wide variety of fruits, the broad range of processes, and the multiplicity of products that characterize the industry. Residual management in the fruit processing industry reflects this diversity; however, basic principles involved in assessment, characterization, and categorization of residuals, and in management strategies can be applied across the industry.

17.1.1 FRUIT PROCESSING OPERATIONS Fruits are processed for marketing as fresh produce; in refrigerated, frozen, dehydrated, canned, and pureed forms; as juices, jams and jellies, and more. The unit operations involved in these processes include washing, sorting, trimming, stemming, peeling, coring, pitting, halving, slicing, dicing, pressing, cooking, blending, blanching, canning, sterilizing, retorting, cooling, refrigerating, freezing, thawing, pulping, finishing, concentrating, bleaching, curing, brining, drying, and so on. A comprehensive listing of unit operations involved in processing apples, apricots, caneberries, cherries, citrus products, cranberries, grape juice, olives, peaches, pears, pineapples, plums, raisins, strawberries, and dried fruits, complete with flow diagrams, was presented by Somogyi and Kyle (1978).

17.1.2 ENVIRONMENTAL IMPACT Fruit processing plants are major water users and residue generators. Water use and residual generation in fruit processing are extremely variable because they are affected by numerous factors. These factors include raw commodity and processed products, plant size and age, processing rate and plant capacity utilization, raw material quality and preparation equipment, water reuse frequency, and housekeeping practices. The statistics presented in Table 17.1 can be used as a guide to the amount of residue generated in the fruit industry for various products. The wastewater parameters that are of major pollutional significance are biochemical oxygen demand (BOD), total suspended solids (TSS), oils and grease (O&G), and pH. Other parameters that are of occasional importance are temperature, nitrogen, phosphorus, chemical oxygen demand (COD), and total dissolved solids (TDS). Some of these parameters are defined below: Biochemical oxygen demand (BOD): the quantity of oxygen used in the biochemical oxidation of organic matter in a specified time under specified conditions; usually specified as milligrams of oxygen used per liter of effluent at 20∞C in a 5-day period

1,050 120 190 7,800 1,100 410 900 460 12,030

Item Apples Apricots Cherries Citrus Peaches Pears Pineapples Others Total 2,100 8,700 2,400 1,800 5,600 3,000 500 8,000

(Gallons per ton) 2,200 1,000 400 19,000 6,200 1,200 500 3,700 34,200

(Million gallons) 36 71 26 16 62 42 20 20

(Pounds per ton) 38 9 5 125 68 17 18 9 289

(Million pounds)

Biochemical Oxygen Demand (BOD)

6 16 5 3 13 12 28 10

(Pounds per ton) 6 2 1 23 14 5 7 5 63

(Million pounds)

Suspended Solids (TSS)

580 240 280 790 530 660 890 152

290 16 26 3,080 290 140 400 70 4,312

(Thousand pounds)

Solid Residues (Pounds per ton)

Source: Somogyi, L. P. and Kyle, P. E. 1978. Overview of the Environmental Control Measures and Problems in the Food Processing Industries. Appendix 8. Overview of Fresh, Canned, Frozen, and Dehydrated Fruit and Vegetable Industries. Report prepared for U.S. Environmental Protection Agency, Cincinnati, OH. (Grant No. R804642–01).

Raw Product (Thousand tons)

Wastewater

TABLE 17.1 Total Residue Generation in Canned and Frozen Fruit Industries

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Chemical oxygen demand (COD): the quantity of oxygen used in the chemical oxidation of organic and inorganic matter; invariably higher than BOD Total suspended solids (TSS): organic and inorganic particles that exist in suspension in the effluent; can be partially separated by gravity settling and completely separated by filtration Total dissolved solids (TDS): organic and inorganic matter that is dissolved in the effluent; cannot be separated by filtration It is recognized that hazardous pollutants such as heavy metals and pesticides are not present in food process residues in significant quantities (Somogyi and Kyle, 1978); however, fruit processing uses a variety of substances such as antioxidants, juice clarifying aids, preservatives, reducing agents, firming agents, buffering agents, sequesterants, pH controllers, anticaking agents, thickening agents, acidulants, nutrients, and flavor enhancers. Listings of substances used in different processes were presented by Somogyi and Kyle (1978).

17.2 RESIDUAL HIERARCHY A major portion of fruit processing plant residuals can be used for different purposes because of their nontoxic nature. The residual hierarchy concept introduced by Shober (1989) is an excellent approach that emphasizes the uses of food processing residuals. Figure 17.1 is a graphical presentation of this concept. Reduction of both material losses and waste generation is the most effective residual management strategy and is at the top of the residual hierarchy. Product recovery for human uses, byproduct recovery for animal feed, land application as fertilizer, landfill disposal, and disposal at hazardous waste facilities are the levels that follow down the hierarchy. Higher levels in this hierarchy result in greater benefits. At lower levels, costs and liabilities of residual management are higher; therefore, greater emphasis should be laid on employing management strategies at higher levels of the hierarchy.

Source reduction and Water conservation

Human Food Animal Food Fertilizer and soil conditioners Landfill, impoundment and incineration Hazardous waste facility

FIGURE 17.1 Food processing residual management hierarchy.

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17.3 RESIDUE ASSESSMENT AND WATER ACCOUNTING Reduction of waste at the source is the most effective form of residual management because it results in a reduction in raw materials input or an increase in product output; however, source reduction and water conservation require a thorough knowledge of product, residue, and water flow in the plant, in contrast to “end of the pipe” waste management. The first step in this direction is to conduct an overview of the plant flow that identifies the major systems in the plant where residuals are generated. This is followed by a more detailed survey, during which residuals and water flows are measured and sampled in the identified systems. A flow diagram of the plant, indicating the flow of product, residue, and water, is an important tool that should be developed during this stage. The next step is to quantify the solid residues and process water flows. Solid residues are relatively easy to quantify. Weighing and counting residue containers is the most frequently used method. Sometimes, volumetric measurements are more convenient. The determination of density and moisture content of solid materials is important to convert all measurements to a common basis such as “weight of dry matter per day.” Water flows are somewhat more difficult to measure. Most food plants have flowmeters permanently installed on the main inlet. Some plants have flowmeters on several major distribution lines. Propeller-type flowmeters with counters have been the most common type. Magnetic flowmeters are finding increasing use in remote readout and automatic flow control situations. Frequently, the main outlet from the plant is a drain. Flumes and weirs are permanent features of outlet drains. Evaluation of water conservation strategy requires placing more emphasis on process water flows in individual unit operations. Installation of permanent water flowmeters is not practical in these situations. Simpler methods such as container and timer for flows out of open-ended spouts and float and timer for open drains are the most widely used under these conditions. The portable ultrasonic flowmeter is a versatile method for measurement of flow in pipes. This nonintrusive technique is fast and reasonably accurate. Characterization of process water requires adequate sampling and numerous analytical tests. Grab samples taken several times a day and composite samples consisting of measured portions collected at regular intervals are the most common sampling methods. Representative sampling is extremely important irrespective of the sampling method used. Sample analysis is done to obtain characteristics required to evaluate water conservation strategies. When reuse of water in another operation is the objective, some characteristics of interest are: temperature, pH, acidity, alkalinity, residual chlorine, dissolved solids, and suspended solids. When product or by-product recovery is the objective, it is important to conduct compositional analysis of the process water streams.

17.3.1 A CASE STUDY (MANNAPPERUMA

ET AL.,

1994B)

A flow diagram of an orange processing plant is shown in Figure 17.2. Oranges are washed and sorted in the initial stages of the process. Rejected fruits and washwater are the discharges at this stage. Peeling and slicing processes produce a solid waste consisting of juice sacks and a transport water discharge stream. Peel processing involves multiple washing and transporting operations that result in three process water discharges. Juice finisher rejects are the third solid residual stream. All the solid waste residuals are collected and transported to a local farm and used for animal feed. Table 17.2 is a summary of a water account of the orange processing operation. This account identifies the dewatering operations after peeling and blanching as the major contributors to water and organic matter discharge. Recovery of orange juice for reconstitution into the final product seemed possible; therefore, the process water streams were analyzed for the presence of fruit sugars and organic acids. Table 17.3 is a summary of this study. This analysis indicated that over 10,000 lb of orange solids are

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Unloader

Fresh water

Sorter Washer

1

Sorter Solid waste

Peeling and Slicing

Finisher

Evaporator

Dewater 1

2 Solid waste

3

Blancher R1

Dewater 2

4

Pump Dewater 3

5

Package

FIGURE 17.2 Flow diagram of an orange processing plant.

TABLE 17.2 Water Account of the Orange Processing Line Stream

Flow Rate (g/m)

Flow Rate (g/d)

Total Solids (mg/l)

COD (mg/l)

COD (lb/d)

Suspended Solids (mg/l)

Dissolved Solids (Brix)

Washer Peeling Dewatering 1 Dewatering 2 Dewatering 3 Total

9.0 5.2 23.6 55.9 5.3 90.0

8,100 4,680 21,240 50,310 4,770 81,000

3,400 13,000 15,500 13,500 8,300

9,770 20,180 25,132 16,340 17,100

659 787 4,447 6,848 679 13,420

— 760 4,120 1,120 1,280

0.2 1.5 1.4 1.7 0.7

Note: COD = chemical oxygen demand. Source: Mannapperuma, J. D., Park, K. H., Merson, R. L., and Shoemaker, S. P. 1994a. Membrane Applications in Fruit and Vegetable Industry — Eight Inplant Demonstrations. A report submitted to the Electric Power Research Institute, Menlo Park, CA.

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TABLE 17.3 Constitution of Orange Process Water Component

Concentration (g/l)

Quantity (lb/d)

Fructose Glucose Sucrose Total sugars

Sugars (g/l) 3.25 3.52 2.86 9.63

2190 2375 1928 6493

Oxalic Citric Malic Succinic Formic Acetic Total acids

Acids g/l 0.01 4.69 1.47 0.30 0.74 0.15 7.36

7 3161 994 200 501 102 4966

Source: Mannapperuma, J. D., Park, K. H., Merson, R. L., and Shoemaker, S. P. 1994a. Membrane Applications in Fruit and Vegetable Industry — Eight Inplant Demonstrations. A report submitted to the Electric Power Research Institute, Menlo Park, CA.

discharged daily with process waters. This is equivalent to about 11,500 gal of orange juice. Recovery of these solids would require concentration using membranes and debittering using ion exchange.

17.4 WATER CONSERVATION Water is the most convenient transfer medium in a food processing plant. It functions as a momentum transport medium in cleaning and in hydraulic conveying applications. In blanching, retorting, and cooling operations, water acts as a heat transfer medium. In sanitizing and brining operations, it is used as a mass transfer medium. In some operations such as cooking and peeling, it serves as a momentum, heat and mass transfer medium. The nature and degree of contamination and regulatory constraints on reuse of water in food contact applications govern the water conservation prospects in these operations. The general principles that govern water use in food contact applications are that it be free of microorganisms, toxic chemicals, discoloration, and off-flavors that make it harmful to public health or adversely affect the quality of the processed food (Katsuyama, 1979). Good Manufacturing Practice guidelines state, “Water used for washing, rinsing or conveying of food products shall be safe and of adequate sanitary quality. Water may be reused for washing, rinsing, or conveying of food if it does not increase the level of contamination of food” (Anon., 1989).The industrywide practice is to use potable water for the final rinse, while reusing water after some degree of treatment in the upstream washing operations.

17.4.1 WATER QUALITY PRESERVATION Water conservation frequently involves the reuse of process water in either the same or another operation. Process water accumulates solids during use. Reuse of water involves treatments to remove accumulated solids. Gravity settling is used to remove particles that are significantly heavier

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than water. Screening is used to separate large particles. Both these methods remove solids that are suspended in water. Buildup of dissolved solids in process water is more difficult to control. Dissolved solids can be inorganic matter such as salts, acids, and alkali, or organic matter such as sugars and organic acids. Inorganic matter buildup affects mass transfer, hindering cleaning and washing operations. Organic matter buildup in process water leads to microbial growth, in addition to affecting mass transfer. Batch recirculation with periodic renewal is the simplest method of achieving dissolved solids control. Batch renewal intervals as low as 2 h have been observed in raisin washing, where dirt accumulation is very high (Mannapperuma et al., 1994b). When the operation involves relatively little contamination, as in hydraulic transport of frozen cranberries using hot water, a batch time of 24 h has been observed (Mannapperuma et al., 1994a). Semicontinuous systems that allow for overflow of recirculating water balanced by the introduction of fresh water is another approach used to control dissolved solids buildup. When the buildup is through ordinary diffusion driven by a concentration difference, the buildup itself serves as a self-limiting mechanism; however, overflow/makeup systems also need complete renewal at intervals due to other quality concerns. The microbial buildup is kept under control through the use of sanitizing agents. Gaseous chlorine is the most commonly used sanitizer due to its ease of application and low cost; however, its use requires evacuation plans to be in place for emergencies. Chlorine derivatives, such as sodium hypochlorite and calcium hypochlorite, overcome the need for evacuation plans but are costlier than gaseous chlorine (Shapton and Shapton, 1991). Chlorine dioxide is another sanitizer that is increasingly used in food processing plants. It is a greenish-orange-colored gas, generated at the site using sodium chlorate as the raw material. Chlorine dioxide has a stronger residual action and is effective in lower concentrations and over a wider pH range than chlorine; however, it has been approved only for use with uncut produce at present. Cut and peeled potatoes are exempted from this limitation. Safety concerns arising due to by-products of chlorination have created a need for alternative methods of microbial control. Ozonation is one avenue that is being actively evaluated. Ozone is an extremely powerful oxidizer and has been used in drinking water treatment since the turn of the century in Europe and Africa (Langlais et al., 1991). More recently, it has been introduced for sanitation of noncontact cooling water in cooling tower systems (Anon., 1992). Inorganic matter and organic matter in process water act as initiators, promoters, and inhibitors of ozonation reactions in water; therefore, the effectiveness of ozone in process water treatment is significantly lower compared to drinking water treatment. Ozonation of water in hydrocooling systems in cherry packing houses, which has been reported recently (Anon., 1994), is a direct food contact application. This development could lead to a wider application of this technology subject to regulatory concurrence. Ultraviolet (UV) radiation induces mutation of microorganisms. UV treatment uses low-energy electromagnetic radiation obtained from mercury vapor electric lamps equipped with a special formula glass. These lamps produce a discontinuous spectrum with most of the energy released at 253.7 nm. The major absorption band of nucleic acids is in the range of 260 nm, making them the primary targets of UV radiation. The penetrating power of UV radiation is limited. It penetrates about 4 m in air and 300 mm in clear water, but only 1 mm in fruit juice and 0.1 mm in milk (Shapton and Shapton, 1991). Therefore, its effectiveness in turbid waters is limited. A typical 30 W UV lamp emits about 7 W of power at 253.7 nm. About 10 to 30 sec of residence time is required for treatment to be effective at this power intensity (White, 1992). Excimer lamps use a mixture of gases and emit pulsed monochromatic radiation. Excimer lamps provide a source of noncoherent radiation, while excimer lasers provide a coherent source (Lagunas-Solar, 1994). These sources offer a selective energy transfer process to target molecules

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that enhance photochemically induced reactions through large concentrations of energy in ultrashort (nanosecond) pulses. In a turbulent flow of turbid waters, particles remain close to the tube walls for brief time intervals. A pulsed UV source can deliver a lethal concentration of energy to a target molecule during these short time periods, which makes this technology more effective in turbid waters compared to technology using conventional lamps. Pulsed UV has proven to be effective in simultaneous chemical and microbial decontamination of food processing waters in laboratory tests. Large-scale tests and economic assessments are being conducted on this promising technology. UV radiation does not have any residual effect. It is effective in killing unprotected microorganisms, but does not prevent their future growth in the treated waters; therefore, its use should be limited to situations where residual effect is not required.

17.4.2 MEMBRANE TECHNOLOGY Dissolved solids comprised of organic and inorganic matter of low molecular weight cannot be removed by simple methods such as gravity settling or screening. High-pressure membrane filtration is an alternative method that can remove dissolved solids from water under most conditions. This technology has found several applications in industrial water conservation. Cross flow membrane filtration is used to separate particles ranging in size from about 5 mm down to about 5 Å. The membrane filtration spectrum is divided into four narrower ranges based on the particle size. Figure 17.3 is an illustration of the filtration spectrum in relation to food process water contaminants. Microfiltration separates large suspended particles that are typically over 0.1 mm in diameter. The permeate recovered from microfiltration is clear and sterile under most conditions. Recovery of spent caustic from evaporator cleaning operations at a Sunkist orange juice plant in Tipton, CA (Pimental and Torres, 1994), has been reported recently. Microfiltration can be used to maintain sanitary conditions in recirculating water streams, but this is found to be uneconomical under present conditions. Reverse osmosis separates small dissolved molecules such as salts, sugars, and organic acids. It can be used to produce soft water for boiler feed and for other cleaning applications in food processing plants. Polishing of evaporator condensate by reverse osmosis has been reported recently (Grigas, 1994). Reverse osmosis can be used to separate salts from brining, curing, and grading operations, when disposal of salts is a concern. These applications are rarely economical from a water and salt recovery perspective under present conditions. Ultrafiltration

Reverse Osmosis

Microfiltration

Nanofiltration Atoms

Latex

Synthetic Dye

Metal Ions Salts

Viruses

Bacteria Yeast

Gelatin

Flavors and Fragrances

Pore size - Microns 0.001

Paint Pigment

Oil Emulsions

Detergents

Molecular Weight 100

Enzymes

1,000

Particulates

100,000 0.01

1,000,000 0.1

1

FIGURE 17.3 Membrane filtration spectrum in relation to food process water contaminents.

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Nanofiltration is a recently introduced term to identify the filtration range that separates sugars from water. These membranes operate at a lower pressure and produce higher permeate fluxes than does reverse osmosis and, hence, are more cost-effective. Therefore, nanofiltration systems can separate dissolved sugars from process water streams and enable reuse of water, and recovery or alternative disposal of sugar concentrates.

17.4.3 OTHER SEPARATION TECHNOLOGIES Electrodialysis is a well-established membrane process where a series of anion and cation exchange membranes are placed between two electrodes to selectively transport ions in an aqueous salt solution. A salt concentrate and salt-free water are the final products. This process is widely used in seawater desalination and desalting of whey. Conceptually, it is more advantageous to move the solute than the solvent through a membrane in the case of dilute solutions. This technology has the potential for water conservation and salt recovery applications in brining and curing, and for grading process waters that contain salts. Bipolar electrodialysis uses two selectively bipolar membranes to separate a salt solution into a concentrated acid, a concentrated base, and water. This process, presently being tested on bench scale (Farmer, 1994), is an attractive means of recovering process water and chemicals from caustic peeling, caustic cleaning, and acid cleaning applications. Capacitive deionization of water by a stack of carbon electrodes, where ions are electrostatically separated and held in electric double layers at the electrodes, has been demonstrated (Farmer, 1994). This process is somewhat similar to ion exchange but has the advantage that regeneration is done electrically and not chemically. This technology also has application potential in salty process waters. Pervaporation is a membrane process where a solute is separated from a solution by permeation through a selectively permeable membrane and evaporation into an inert gas. It is used for separation of water from azeotropic mixtures of ethanol–water, isopropanol–water, etc., on a commercial scale. Recovery of low-molecular-weight organic aroma compounds from evaporator condensates in tomato, orange, and other juice concentrators is possible, and some have been tested.

17.4.4 HYDRAULIC TRANSPORT Hydraulic transport of fruits in fluming systems is nearly always accompanied by water recirculation. A common practice is to combine washing and transport into a single operation. Countercurrent flow is extremely effective in reducing water use in these systems. Figure 17.4 is a flow diagram of a countercurrent washing and fluming system. Fruits move forward from the receiving end to the process lines through three fluming systems, while water moves backward from the process lines to the receiving end. Direct contact between food and water results in the transfer of food solids to water, which gradually increases the level of solids in recirculating water until equilibrium conditions are reached. Gravity settling and screening are commonly used to separate heavy and large particles, while makeup/overflow maintains dissolved solids at acceptable levels. Prolonged use of recirculating water gives rise to microbial growth and development of off-flavors. Sanitizing chemicals such as chlorine and chlorine dioxide are used to control these effects.

17.4.5 HEATING OPERATIONS When water is used as a heat transfer medium, it gets thermally contaminated. In heating operations such as blanching and pasteurization, hot water gets cooled. Spent water is heated by passing it through heat exchangers and recirculating it back, forming a closed circuit. In the case of blanching, water has to be periodically renewed or continuously diluted to maintain the quality of recirculating water.

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System without Reuse Product inlet

Dump Flume

Wash Flume

Distribution Flume

Peeler

First Rinse

Final Rinse

Wastewater outlet System with Reuse Fresh water inlet Product inlet

Dump Flume

Wash Flume

Distribution Flume

Solids Screen

Surge Tank

Surge Tank

Peeler

First Rinse

Final Rinse

Solids Screen

Wastewater outlet

FIGURE 17.4 Flow diagram of countercurrent washing and fluming systems.

Pasteurizers in fruit juice processing use hot water for pasteurizing the juice. This water can be reused indefinitely by recirculation through heat exchangers. Pasteurizers also use cooling water to cool already pasteurized juice when use of the downstream equipment is interrupted. Water use in this situation can be reduced by countercurrent juice-to-juice heat exchangers.

17.4.6 DIRECT CONTACT COOLING OPERATIONS Freezing plants use chilled water to cool fruits prior to freezing them. This is done in belt coolers where a large flow of chilled water trickles on the moving product, or in a flume that transports as well as cools the fruit. The temperature of water increases due to heat removed from the product during the cooling process. Water is recirculated through the chiller to remove this heat and is reused repeatedly. Prolonged recirculation results in buildup of product solids and microbes in the water, which is kept under acceptable limits by limited makeup/overflow and by the use of sanitizing chemicals. Public health and environmental concerns, as well as the rising cost of chemicals dictate a fresh look at these systems. The short contact time limits the temperature rise of water in the belt coolers. Reusing the cooling water after rechilling is economical under these conditions. A countercurrent cooler — where chilled water is reused several times — is a better alternative from an overall water, energy, and chemical management perspective. Cooling water leaves the countercurrent cooler well above room temperature, making it more economical to use fresh water in the chiller while the warm water is reused in washing processes upstream. Figure 17.5 illustrates the conventional cooling water system and the countercurrent system. Multiple use of water in one pass allows shorter residence time for the water in the system, which limits solids buildup and microbial growth, thus reducing or eliminating chemical use. The only disadvantage of the countercurrent cooler is that it requires a larger heat transfer area due to reduction in the effective temperature available for heat transfer. This is more than compensated for by optimizing the design process and by other savings.

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FIGURE 17.5 Direct contact cooling systems: conventional and countercurrent.

17.4.7 INDIRECT CONTACT COOLING OPERATIONS Cooling of cans and bottles after retorting or aseptic filling requires large quantities of water. This water is relatively uncontaminated. The practice in many plants is to discharge the spent cooling water to natural waters under a noncontact cooling water discharge permit. Temperature is the only permit limit of concern in this situation. In locations where water is in plentiful supply, use of excess water easily overcomes this difficulty. When fresh water is in short supply, this practice is not possible. When the quality of water is poor, the addition of chemicals is required to improve its quality, which may preclude discharge of spent cooling water to natural streams. Cooling towers that enable reuse of cooling water is a solution under these conditions.

17.4.8 EVAPORATOR CONDENSATE Fruit juice is concentrated in multiple effect evaporators. Steam from the last effect is condensed in a barometric leg rising from a wet well. This water absorbs the heat of the steam and the trace organic matter accompanying it. The temperature of water is maintained by the introduction of fresh makeup water. Overflow from the system is used for cleaning and for other applications that require moderate-quality water. Evaporator condensate is not used in many plants due to the risk of contamination with the product through occasional tube failures or due to the presence of volatile organic compounds. Detection-rejection circuits can be used to divert contaminants due to tube failures, while reverse osmosis systems can be used to separate most of the volatile compounds to enable reuse of the condensate. Reverse osmosis and pervaporation enable the recovery of useful aroma compounds from the condensate.

17.4.9 BRINING

AND

CURING

In brining and curing of fruits, water serves as the mass transfer medium. Brining is used for preservation of cherries and olives in off-season processing. Curing of olives involves lye

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treatment to remove bitterness-causing compounds, while cherry curing involves removal of color-causing chemicals. Sweet cherries are stored in calcium sulfite brines to bleach, firm, and preserve the fruit for subsequent processing into maraschino, candied, or glazed cherries. Disposal of spent brines is an acute problem. These brines have been reclaimed by filtration, activated carbon treatment, and by the addition of sulfur dioxide and lime (Panasuik et al., 1977). Reclaimed brines compared well with fresh brines in brining trials. The salient feature of brining and curing applications is the use of large amounts of chemicals. These include acids, alkalis, and salts, which dissolve completely in water and have very low molecular weights. Cured and brined fruits have to be washed to remove chemicals. Disposal of storage brines, curing chemicals, and washwaters are becoming increasingly troublesome due to environmental concerns. Storage and curing of black ripe olives use about 200 lb of chemicals (dry basis) and about 5000 gal of water per ton of olives. Processing Spanish olives requires a smaller quantity of chemicals and water, but at much higher concentrations. Black ripe olive storage brine and curing lye have been successfully treated by activated carbon treatment and reused on a pilot scale (Mercer et al., 1970). Spent Spanish olive storage brines are clarified by microfiltration and used for packing cured olives at a commercial level but on a small scale (Penna, 1994).

17.4.10 PEELING OPERATIONS Peeling of fruits is done by the use of hot caustic (for peaches, apricots, pears, and apples) or by mechanical means (for pears, pineapples, apples, and oranges). Caustic peeling is a leading source of wastewater in peach canning operations. Dry caustic peeling was introduced for peaches following its success with potatoes (Ralls, 1971). The process involved hot caustic application, followed by removal of loosened peel by the mechanical rubbing action of rubber rollers. Dry caustic peeling of peaches was done successfully on a commercial scale. The peeling effectiveness and peeling losses were similar to those for wet caustic peeling, while water use was drastically reduced; however, this method has become obsolete due to problems in disposal of dry peels and the benefits of caustic peeling washwater in balancing acidity of fruit processing effluents.

17.5 BY-PRODUCT RECOVERY FOR HUMAN CONSUMPTION By-products recovered for human consumption yield a high return to the producer; therefore they are placed at a high level in the residual management hierarchy. Investigation of alternative residual recovery strategies should begin at this level; however, it should be recognized that this category faces the highest regulatory scrutiny due to health and safety concerns. Handling and preservation of residues need special care when recovery for human consumption is intended. Often, it becomes necessary to separate the residuals and process water streams at the point source. Cleaning chemicals and low-grade residuals are common sources of contamination. A list of materials and their maximum limits in human and animal foods, above which the U.S. Food and Drug Administration (FDA) would take legal action to remove the product from the market, is published regularly by the FDA (FDA, 1992). Table 17.4 is a listing of these materials. Blending of a product containing a substance in excess of an action level with another product is not permitted, regardless of the level of the contaminant in the final mix. The action levels are revised by the FDA at intervals. Pesticide residues are a common source of most of the action level substances. Peels may contain a high concentration of pesticide residues; hence, peel recovery schemes should pay special attention to this concern.

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TABLE 17.4 FDA Action Level Substances, August 1992 Aflatoxin Aldrin, dieldrin Benzene hexachloride Cadmium Chlordane Cotalaria seeds Dicofol (Kelthane) DDT, DDE, TDE

Dimethyl nitrosamine Endrin Ethylene dibromide Heptachlor Heptachlor epoxide Lead Lindane Mercury

Methyl alcohol Mirex N-nitrosamine Paralytic shellfish Polychlorinated biphenyls (PCB) Toxaphene Toxin

Source: Brandt, R. C. and Martin K. S. 1994. The Food Processing Residual Management Manual, Pennsylvania Department of Environmental Resources, Harrisburg, PA.

17.5.1 DIETARY FIBER

AND

PECTINS

POMACE

FROM

Dietary fiber extracted from apple and pear pomace is marketed by several producers. The extraction process involves mechanical filtration, dehydration of filtered solids, and screening. The marketed apple product contains 56% fiber, and the pear product contains 77% fiber. The products have the consistency of wheat flour and a bland taste. They are used in breads, baked foods, cereals, granola products, laxatives, pharmaceuticals, and pet foods (Morris, 1985). Fiber extraction from apple pomace by solvents has been done on a laboratory scale (Walter et al., 1985). This process involved drying, grinding, sieving, and extraction with sodium hydroxide or other aqueous solvents. Pectin can be extracted from apple pomace, citrus peel and several other pulpy fruit wastes. It is used as a gelling agent in jams and fruit preserves. Apple pectin has superior gelling properties compared to citrus pectin. The process involves shredding the raw material, extraction with hot water, concentration by evaporation, precipitation by organic solvents (ethanol or hexane) and drying.

17.5.2 GRAPE SUGARS

FROM

GRAPE

AND

RAISIN RESIDUES

Manufacture of brandy and alcohol for wine fortification must be done with grape sugars because of regulatory requirements. This makes the recovery of sugars from grape and raisin processing waste a profitable exercise. Rejects from grape processing plants are routinely used as raw material in the manufacture of alcoholic beverages. Washwater from raisin packaging plants also contain grape sugars. Recovery of sugar from this water by clarification and concentration through membrane systems for fermentation was proposed (Mannapperuma et al., 1994b). National Raisin Company in Fowler, CA, installed a tubular reverse osmosis system to process about 70,000 gal/d of raisin washwater (Anon., 2002a). The plant has reduced its sewer bill and created a new source of income.

17.5.3 FRUIT JUICE

AND

SYRUP RECOVERY

FROM

FRUIT PROCESS WATERS

Peels and cores from apple and pear processing plants are used in fruit juice manufacture. Pectinase enzymes are used to facilitate juice recovery by depectinization. This juice is frequently used as a filler in cans in place of syrup. Pomace left after extraction of cranberry juice is used in the manufacture of cranberry sauce. It is possible to recover fruit juice from peach pitter water by clarification and concentration using membrane systems (Mannapperuma et al., 1994b).The recovered juice can be used as a filler in peach cans.

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Orange peels are sliced and blanched in the marmalade process. Blanching water contains sugar and other orange solids including bitterness-causing compounds. Recovery of orange juice from this water requires debittering. The process involves concentration and clarification using membranes and debittering using ion exchange process (Mannapperuma et al., 1994b). Dyeing of maraschino cherries is done using a solution of dye and sugar. The water used to wash the dyed cherries contains residual dye and from about 2 to 3% sugar. This water is concentrated using reverse osmosis, and both sugar and dye are recovered. The payback on this membrane system was reported to be only 4 months (Spatz, 1973).

17.5.4 CANDIED FRUIT PEELS

AND

REFORMED FRUIT PIECES

Candied citrus fruit (orange, lemon, grapefruit) peels can be used in baked goods or as snack food. The candying process involves slicing the peels, boiling them in 20% sugar syrup, gradually increasing the sugar concentration up to between 65 and 70% over 4 to 5 d, then rinsing and drying. The marmalade process for orange peels is similar to candying with the omission of drying. Pulp extracted from sound fruit processing residue can be reformed into artificial fruit pieces. The process involves concentrating the pulp, adding sugar and the gelling agent sodium alginate, and mixing with calcium chloride to solidify the structure.

17.5.5 OILS

FROM

SEEDS

AND

STONES

Seeds of some fruits have significant quantities of oils that may have specialized markets. These include grape, papaya, passion fruit, mango, apricot, and peach. The process involves drying, grinding and pressing, or solvent extraction. Mango oil is considered to have a fatty acid profile very similar to cocoa butter. Grape seed oil has been proposed as an alternative to olive oil.

17.5.6 FUNCTIONAL FOOD COMPOUNDS Fruit processing residuals are a promising source of functional food compounds that have favorable nutritional properties. The beneficial compounds include carotenoids, polyphenolics, tocopherols, and ascorbic acid. Dietary supplements and food fortification are possible ways to incorporate these compounds into diets (Schieber et al., 2001). Apple and grape pomace are good sources of polyphenolics that are localized mostly in the peels and extracted partially into juice. Grape seeds are also a rich source of polyphenols. Drying pomace at high temperatures reduces the extractable polyphenols and also affects the antioxidant activity. Extraction by organic solvents has been reported. Enzyme treatment enhances release of phenolic compounds. Peels and seeds of citrus and mangos also have high antioxidant activity.

17.6 BY-PRODUCT RECOVERY FOR ANIMAL FEED Solid waste from fruit processing plants is a valuable feed source for the animal farms in the area. Some of these by-products have good nutritional value, while others are only of limited value. Byproduct feeds are classified as concentrates and roughages based on energy and fiber contents, while concentrates are subdivided further into energy feeds and protein feeds (Bath, 1981). The nutrient content of some fruit processing byproducts used in animal feeds is listed in Table 17.5. The composition is based on a dry matter basis while the dry matter content on an “as fed” basis is also listed. Ruminant animals are capable of fermenting fibrous materials such as cellulose to metabolic compounds. This ability makes it possible for a ruminant to utilize fibrous by-products that are of little value to humans. The ruminant converts this waste material into milk and meat, which are valuable human foods.

89 90 91 65 87 21 89 94

Item

Apple Pomace Citrus pulp Grape pomace Citrus molasses Pineapple bran Pineapple presscake Soybean meal Citrus sludge 1590 1760 740 1760 1630 1650 1890 —

Maintenance 970 1160 0 1160 1060 990 1260 —

Gain 1570 1760 610 1760 1670 1630 1860 —

Lactation

Net Energy (Mcal/kg)

69 77 30 77 73 71 81 —

Total Digestible Nutrients (%) 4.9 6.9 12.7 10.9 4.6 5.3 51.8 38.6

Crude Protein (%) 17 14 30 — 20 26 5 13

Crude Fat (%) 26 23 54 — 28 34 — —

Acid Detergent Fiber (%)

0.13 2.07 — 2.01 0.24 0.28 0.36 1.49

Calcium (%)

0.12 0.13 — 0.14 0.12 0.08 0.75 1.59

Phosphorus (%)

Source: Bath, D. L. 1981. Feed byproducts and their utilization by ruminants, in Upgrading Residues and Byproducts for Animals, Huber, J. T., Ed., CRC Press, Boca Raton, FL.

Dry Matter as Fed (%)

TABLE 17.5 Nutrient Content of Some Fruit Processing By-product Feeds

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The most important by-product of the fruit industry is the presscake left over in fruit juice plants by apple, citrus, pineapple, grape, and cranberry juice processes, for example. Peels, cores, and process rejects also contribute to animal feed. Cull fruits and surplus from the fields are also used in animal feed. Pomace is the presscake left after the extraction of juice from apples, grapes, cranberries, and so forth. It is used as animal feed: fresh, dried, or ensiled. Most juice extraction processes use rice hulls — as an aid in a squeezing operation — that end up in the pomace as a contaminant. Rice hulls have very little nutritional value and hence reduce the feeding value of the pomace. Pesticide contamination has been a concern when feeding the pomace to animals. More stringent control of pesticide application has helped alleviate this concern. Apple pomace is a medium-energy, low-protein, very palatable feed. It has been used — with proper supplementation — up to about 33% in dairy cattle concentrates and from 15 to 20% in complete feed lot rations. Grape pomace consists mainly of seeds, stems, and skins. It is very low in energy and protein. In concentrates, it serves only as a filler. Citrus pulp has both energy and digestible fiber and, hence, is of value as a concentrate and as roughage. It has to be introduced gradually into a ration to get cattle accustomed to its distinct taste and smell. Once cattle get accustomed, up to 40% in concentrate and from 15 to 20% in the total ration are acceptable. Citrus pulp is usually fed in the dried form but can also be used in fresh or ensiled form. Feeding level of fresh pulp is about 25 to 30 lb per cow per day. Transportation costs make it uneconomical for use in farms far from processing plants. Sludge from activated sludge treatment of concentrated citrus waste has been further processed and successfully used as an ingredient in poultry feed. The sludge was thickened by gravity settling, dewatered by centrifuging, and dried in a kiln. The dried sludge contained 38.6% crude protein and high levels of crude fiber, calcium, and phosphorus. The dried sludge was incorporated up to a 7.5% level in poultry feed without any detrimental effects on meat or egg quality (Jones et al., 1975). Pineapple bran consists of the outer shell of the fruit and other waste from the pineapple canning operation. It has a high fiber level, but the digestibility of the fiber is relatively low. It is fed up to 15 lb per cow per day in Hawaii during the season. Pineapple juice presscake is the high moisture residue from the juice pressing process. It has been fed fresh up to 30 lb per cow per day in Hawaii. Pineapple presscake is a high-acid product; hence, animals should become accustomed to it gradually. Due to its high acidity, it does not undergo a normal fermentation process during ensiling; however, it keeps well, for up to 2 weeks when held in stacks. Addition of nonprotein nitrogen increases the value of low-quality roughages. Urea is the most widely used nitrogen source, while ammonia is the most economical. Ammoniation of citrus pulp increases the protein level, prevents spoilage, inhibits energy losses due to respiration, and increases digestibility. The publications of the Association of American Feed Control Officials, Inc. (AAFCC) are the best source of information on regulatory authorization for use of fruit processing by-products in animal feed. The official publication of the AAFCC, published yearly, contains the definitions of acceptable feed ingredients. The association conducts investigations and makes recommendations on materials within several categories of feed stuffs. Citrus residuals is one of these categories. Materials are authorized for use as animal feed through three categories: generally recognized as safe (GRAS), regulated food additives, and approval by individual merit. Each of the 50 states of the U.S. has a feed law or equivalent legislation to regulate animal feeds. AAFCC also publishes a uniform state feed bill that recommends the acceptance of feed ingredients in different states. Materials not listed in those three categories of authorization may have to be authorized appropriately. Most states accept the definitions of the official publication of the AAFCC. Suggestions for definition of new materials are made to the investigators of the AAFCC.

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17.7 RECOVERY OF ENERGY The extent and methods of energy recovery from food residuals depend on its moisture content. Solid residuals such as peach pits and olive pits are burned directly in boiler furnaces or converted to charcoal briquettes. Wet residues and process water containing fruit sugars are potential raw materials for ethanol fermentation and methane production.

17.7.1 ETHANOL

FROM

WET RESIDUES

Fruit processing residues, including sorting rejects and liquid waste, contain sugars that can be converted to ethanol through fermentation. Ethanol is a very versatile source of energy because it can be used as a transportation fuel; however, its production is an expensive process involving fermentation and distillation. The economics of ethanol production can compete with other waste handling alternatives only in a few situations. In many instances, residuals with high potential are already being used in ethanol production or have been evaluated and found uneconomical (Badger and Broder, 1989). The U.S. Gasohol Corporation converted a winery in Lockeford, CA, to a 10 million gal per year ethanol plant to turn food processing waste into alcohol. About 70,000 gal were produced in 1982, using cherry, apricot, plum, grape, peach, and orange processing residues. Although the canneries were enthusiastic about the project, unfavorable markets for fuel alcohol made the operation uneconomical, and the project was abandoned (Badger and Broder, 1989). Apple pomace fermentation was found to be uneconomical due to the high cost of the enzyme (Sargent et al., 1982). Another study concluded that grape pomace fermentation for ethanol production was not economical even when cellulose was converted (Kranzler and Davis, 1983). Apple, cantaloupe, grape, honeydew, orange, peach, pear, and plum culls were fermented, and the resulting beers were distilled on a laboratory scale (Hills and Roberts, 1984) to obtain alcohol yields ranging from 12.5 to 82.4 l/t (Table 17.6). The resulting stillages were then digested anaerobically to produce methane. In most cases, the heat content of the methane (estimated at 22.5 MJ/m3) was adequate to provide the energy for distillation (estimated at 10 MJ/l). Ethanol has a heating value of 22.3 MJ/l. Based on these values, the combined process has a net energy surplus. Jacquin Distillers, Winter Haven, FL, and Florida Distillers, Lake Alfred, FL, produce about 4 million gal of ethanol per year from citrus waste. The manufacture of alcohol, using production rejects and market returns of fruit juice and other liquids, a profitable commercial venture at Parallel Products in Rancho Cucamonga, CA (Williams and Eastman, 1992). Alcohol is sold for blending

TABLE 17.6 Ethanol Recovery from Fruit Culls and Methane Recovery from Stillage Item

Sugar Content (%)

Ethanol Yield (l/t)

Ethanol Recovery Efficiency (%)

Methane Yield (m3/t)

Apples Cantaloupe Grapes Honeydew Orange Peach Pear Plum

12.1 7.1 14.5 4.0 4.0 6.2 12.0 11.0

58.6 19.0 82.4 12.5 22.5 26.1 64.5 54.2

84.3 46.6 98.2 54.4 98.2 73.4 93.5 85.7

22.7 37.8 12.9 30.2 10.7 22.7 15.6 28.7

Source: Hills, D. J. and Roberts, D. W. 1984. Energy form cull fruit, Trans. ASAE, 27(4): 1240.

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with gasoline in the production of unleaded high octane gasoline. Other by-products include dry brewers yeast for pet foods and liquid protein supplement for animal feed. Liquid waste from fruit processing plants contains significant quantities of sugars but in a very dilute form. Using membrane systems to concentrate the process waters in recovering reusable permeate and as a concentrate in ethanol fermentation has added benefits. These include savings in water supply costs, wastewater disposal costs, and BOD disposal costs; however, the volume of concentrate available at individual plants is not adequate to justify fermentation/distillation operations. The feasibility of collecting the concentrate at a centrally located plant for ethanol production is presently being evaluated (Kettani, 1994).

17.7.2 METHANE GAS

FROM

SOLID RESIDUES

Wet fruit processing residues can be digested anaerobically to produce methane gas, which can be used as a boiler fuel. A laboratory-scale study to digest tomato, peach, and honeydew solids was reported by Hills and Roberts (1982). Carbon:nitrogen:phosphorus ratio was satisfactory for tomato and honeydew residues, but peach residue was deficient in nitrogen. Under standard conditions, about 0.31 m3 of methane gas with a heating value of 7.0 MJ was obtained per kg of COD digested. Anaerobic digestion is more often used with the primary objective of wastewater treatment, energy extraction being a secondary benefit. Different digester designs used for anaerobic treatment include anaerobic lagoons, anaerobic contact reactors, anaerobic filters, upflow anaerobic sludge blankets, anaerobic fluidized beds, and hybrids (Trotzke, 1988). Anaerobic fermentation of liquid waste is more efficient at high-organic-matter concentrations. Therefore, it could be used to digest the concentrate stream from a membrane system recovering process water for reuse.

17.7.3 CATALYTIC GASIFICATION

OF

WET RESIDUES

Food processing liquid waste has been gasified using a nickel metal as a catalyst. The process, carried out on a laboratory scale at 360∞C and 3000 psi, converts 99% of the COD in a 10% lactose solution during a residence time of less than 5 min. The product of the process is a fuel gas containing methane, carbon monoxide, and hydrogen, which can be burned in a furnace (Elliot et al., 1993). Like anaerobic fermentation, this process is also better suited for high-strength waste; hence, it will fit in well with process water recovery systems using membranes. This process is presently being tested on a pilot scale.

17.7.4 OLIVE

AND

PEACH PITS

AS A

BOILER FUEL

Olive pits were burned in fluidized bed burners to produce steam at Lindsay Olive Growers, Lindsay, CA (Anon., 1977). Olive pits contained 50% moisture and provided 25% of the steam energy requirements of the plant. A cogeneration plant burning peach pits was installed at S&W Fine Foods in Modesto, CA (Anon., 1981). The plant generates 50,000 to 60,000 lb of steam per h and 3.5 MW of electricity. The cost of the installation was $3 million, and the anticipated period of payback was 3.1 years. Crushed peach pits are fired with 20% supplementary conventional fuel in a boiler at Tri Valley Growers in Modesto, CA. Scrubbers and other air pollution control equipment are used to maintain air quality standards (Anon., 1979). Several biomass-fueled cogeneration plants were begun as a result of the Public Utilities Regulatory Policies Act (PURPA) of 1978, which required public utilities to purchase power from qualifying facilities at the avoided cost. Changing economic conditions have forced many of these facilities out of operation in recent years (Simons et al., 1994).

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17.7.5 CHARCOAL BRIQUETTES

FROM

PEACH, APRICOT,

AND

OLIVE PITS

Peach, apricot, and olive pits are converted to charcoal briquettes by C. B. Hobbs Corp., in Elk Grove, CA (Anon., 1971). In the early 1970s, this plant used 150,000 t of fruit pits and nut shells to generate 35,000 t of high-quality charcoal briquettes. The process involves controlled burning under low oxygen conditions in a series of four hearths. The charred product is quenched by water as it is discharged, ground, screened, mixed with 5% starch binder, and formed into briquettes. Flue gasses are preheated with natural gas and burned completely.

17.8 LAND APPLICATION AS FERTILIZERS AND SOIL CONDITIONERS Fertilizers supply growing plants with essential nutrients, while soil conditioners produce physical and chemical changes that enhance plant growth. Food process residues can serve as fertilizers and soil conditioners when used in well-designed and properly managed land application systems. Characteristics of food processing residues that influence these applications are listed in Table 17.7. The BOD loading rates vary among soil types from about 30 to about 100 lb/acre/d. When BOD loading is too high, the soil becomes anaerobic and odors arise. Calcium carbonate equivalent is a measure of the liming characteristics of soil conditioners. Carbon–nitrogen ratio is an indicator of inorganic nitrogen availability from organic matter. Nitrogen, phosphorus, and potassium are essential nutrients for plant growth. Soluble salts primarily include calcium, magnesium, sodium, potassium, chlorides, sulfates, bicarbonates, and nitrates. Electrical conductivity (EC) of process water is a good indicator of the total amount of soluble salts. TDS in mg/l is estimated by multiplying the EC reading in milliSiemens by 700. Excessive salt content increases the osmotic pressure, which makes it difficult for roots to extract water and reduces plant growth. High levels of sodium ions compared to calcium and magnesium ions alter the soil structure and reduce soil permeability. In addition to these effects, certain specific ions can have toxic effects on plants. Land application of food processing residuals in solid or liquid form requires a nutrient management program designed to suit the fields. This program should consider the field fertility, nutrient requirements of the planted crop, and nutrient application history to sustain the quality of

TABLE 17.7 Characteristics of Food Processing Residues That Influence Land Applications Biochemical oxygen demand (BOD) Calcium carbonate equivalent (CCE) Carbon nitrogen ratio Fats and oils Foreign materials Heavy metals and PCBs Nutrients

Odors Organic matter Pathogens pH Solids content Soluble salts Toxicity

Note: PCB = polychlorinated biphenyl. Source: Brandt, R. C. and Martin K. S. 1994. The Food Processing Residual Management Manual, Pennsylvania Department of Environmental Resources, Harrisburg, PA.

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the soil. The California League of Food Processors has published a manual to provide guidance in land application of food process water (Anon. 2002b).

17.9 WASTEWATER TREATMENT Natural water streams possess the ability to purify themselves. Water becomes aerated as it flows, and dissolved oxygen enables it to sustain a variety of microorganisms. These microorganisms are primarily responsible for the stream’s self-purification ability. Food process effluent is harmless compared to effluents from other major industries, but it is rich in organic matter. This organic matter creates an additional demand for dissolved oxygen that can far exceed the aeration capacity of the stream. The suspended matter in food process effluent, if discharged to natural streams, increases the turbidity of water, which reduces sunlight’s ability to penetrate into water. Light suspended matter forms scum on the water surface, and heavy suspended matter settles to the bottom as sludge. Creation of foul odors, color, and attraction of insects are secondary effects of suspended matter. For these reasons, food process wastewater treatment involves removal of dissolved and suspended solids.

17.9.1 REGULATORY BACKGROUND The growth of population and industry has increased the pollution of waterways. The attempts to legislate a solution resulted in a series of federal laws. The Water Pollution Control Act of 1948 authorized the federal government to intervene in pollution control programs when the state governments fail, after a prolonged period, to take corrective action. This act was considered a compromise law, an aid in the preparation of a more permanent law. The 1961 amendments to the 1948 act gave the federal government enforcement authority over navigable waters. It provided for federal grants, more research programs, and federal water research programs. It directed the Secretary of the then Department of Health, Education, and Welfare to develop practical technology for treating municipal and industrial wastewaters. The Water Quality Act of 1965 created the Federal Water Pollution Control Administration. It increased grants for the construction of wastewater treatment plants and provided federal grants to formulate strong water quality standards. This act, its predecessors, and the subsequent legislation in 1966 and I970 did not result in prevention of water pollution or reversal of pollution trends to any significant degree; however, a basis for water quality protection was laid down, and a national environmental agency was established. The U.S. Environmental Protection Agency was established by executive order of the president in 1970 to combine the environment-quality-related functions of the U.S. Departments of the Interior, Health Education and Welfare, and Agriculture. This was followed by the passage of the Federal Water Pollution Control Act Amendments of 1972 (FWPCA or Public Law 92–500). The FWPCA was one of the most comprehensive and complex measures enacted by Congress. Its ambitious ultimate goal was to “restore and maintain the chemical, physical and biological integrity of the nation’s waters.” Achieving water quality suitable for recreational contact and for propagation of fish and wildlife was an intermediate goal. The FWPCA required limiting the discharges of pollutants from industries and municipalities (known as “point sources”) into any body of water through a technology-based effluent control system. Responsibility for the environment was shifted from society to the individual, accountability being enforced by this law. The FWPCA mandated that industrial point sources should meet increasingly stringent effluent standards by 1977 and 1983 deadlines. By 1977, all industrial point source discharges to natural streams were required to meet the limitations based on Best Practicable Control Technology Currently Available (BPT). This level of technology was defined as the average of the best current waste treatment performance within an

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industry category or subcategory. The fruit processing industry occupied several subcategories within the category of fruit and vegetable processing. By 1983, the effluents were to meet the standards based on Best Available Technology Economically Achievable (BAT). This involves best treatment measures, including in-plant modifications and process changes that have been or are capable of being developed within an industry category or subcategory. The Clean Water Act of 1977, or Public Law 95-217, maintained the basic structure of FWPCA but revised the controls over “conventional pollutants,” added more controls over “toxic pollutants,” and established a new class of “nonconventional pollutants.” Most of the fruit processing industry effluents were conventional pollutants, partially defined to include BOD, TSS, pH, and fecal coliform bacteria. The technology applicable to conventional pollutants is the Best Conventional Pollutant Control Technology (BCT). The limitations guidelines are similar to the BPT set forth in the FWPCA, but they had to be met only by 1984. BCT defines a twofold criterion for cost-effectiveness. First, there should be a reasonable relationship between cost and effluent reduction benefits. Second, the cost and effluent reduction should compare with those from publicly owned treatment works (POTW). These guidelines indicate that BCT may result in limitations somewhere between BPT and BAT in stringency. The passage of the FWPCA in 1972 created a widespread interest in fruit processing waste reduction and water treatment. Federal funding that became available with this act was responsible for some of the most comprehensive research projects conducted in fruit processing waste management. The passage of the clean water act terminated this activity by classifying most of the fruit processing industry as BCT, which was equivalent to BPT. The primary objective of the FWPCA of 1972 was the restoration and maintenance of the integrity of the nation’s waters. Reduction and final elimination of pollutants entering the waters was the means of achieving this objective. The mechanism for reduction of pollutant discharge to natural waters is the National Pollutant Discharge Elimination System (NPDES). Under this program, industrial, municipal, and other point sources must obtain permits that specify limitations on the pollutants discharged to the nation’s waters. The NPDES permit contains effluent limits, compliance schedules, and monitoring requirements in a basic format. The permit issuance and administration is done by USEPA and approved state agencies. Nationally consistent application deadlines, treatment standards, effluent limits, and enforcement are assured through federal regulations and guidelines. The NPDES permit also contains detailed self-monitoring requirements that include the parameters to be monitored, sample type (grab or composite), sample frequency, flow measurement frequency, analytical methods, and report frequency. The FWPCA requires compliance monitoring to ascertain the NPDES permit holders’ compliance status. This includes compliance reviews, where reports and other documents submitted by the permit holder are viewed, and compliance inspections, which involve inspection, sampling, and sensing at sites. Food processors sometimes have the option of discharging wastewater to a POTW that treats and discharges it to a natural stream in compliance with an NPDES permit. The FWPCA of 1972 required the POTWs to upgrade their facilities. A substantial portion (75%) of the upgrading cost was provided through federal financial assistance. The POTW bills its clients for capital cost recovery and for operation and maintenance. Both these charges are prorated among its clients, based on the extent of use by each client of three components: wastewater flow, TSS, and BOD. The capital cost recovery charge, which is also known as industrial cost recovery, is aimed at recovering the federal grant portion without interest over a 30-year period; however, some POTWs do not include the federal portion of the capital and interest in this charge. This charge is prorated among the POTW’s clients based on total annual discharges or peak discharges.

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The “user charges” recover the expenses of operation and maintenance of the POTW facility. Two schemes are used to ascertain these charges. Under the more common quantity/quality scheme, the rate structure is proportional to the throughput of the three effluent components mentioned earlier. In the surcharge scheme, only the rate for wastewater flow applies in general. Surcharges for BOD and TSS are imposed only when they exceed preset limits. Certain aspects of the POTW charge system influence the water management strategy of individual customers. Some POTW–client agreements include a reserved capacity commitment, which is equivalent to the client purchasing its own treatment plant. This guarantees the client’s right to expand in the future, but allows for little or no incentive to reduce discharge when it is below the reserved limits. POTWs have to recover all costs from their client base. When an individual client is responsible for a disproportionately high share of POTW’s throughput, any reductions in the quality and quantity of plant effluent will result in increases in the rates charged by the POTW. The management of fruit processing plants should be well informed of the changing regulatory requirements concerning the treatment and discharge of wastewater. Participating in the initial design and subsequent upgradation of the POTW through public hearings, and taking part in the establishment of the scheme for distributing the charge among the POTW’s customers are two important activities for the management. The choices made at these stages will affect the treatment charges over a long period of time. Once the rate structure has been implemented, the in-plant water management activities should be taken into account. The same investment in reduction of flow, BOD, and TSS will result in different returns from each, depending on the rate structure. The choice of strategy should be based on the return. The rate structure is not fixed by any means; there is a great deal of flexibility in making changes. A final note is that the FWPCA does not require a client to continue to pay the POTW if it stops using the POTW, leaving open the option of breaking with the POTW completely.

17.9.2 COST

OF

WASTEWATER DISPOSAL

Wastewater disposal by POTWs is becoming an increasingly costly affair. A recent survey conducted in California (Mannapperuma et al., 1993) highlighted the differences in rates and in rate structures within the POTWs of a single state. Table 17.8 is a listing of some statistics extracted from this survey. Most POTWs have only a loading rate (user charge). This usually has three components: flow, BOD, and TSS. Some POTWs base their rates solely on flow, while others have rate components for ammonia, fats, grease, etc. A number of POTWs have an annual capacity charge (industrial cost recovery). The basis for this is usually, but not always, the capacity utilization during the maximum month. The differences in rate structures make it difficult to compare the rates; therefore, the total charge for disposing of 100 million gal of wastewater containing 800,000 lb of BOD and 600,000 lb of TSS was computed for rate comparison. The discharge was assumed to take place equally during 100 d, for computation of capacity charges. The last column of Table 17.8 shows this comparison. The total cost of water for this median plant varied from $121,641 to $827,240. Such large differences could have a significant impact on the competitiveness of the plant.

17.9.3 PRETREATMENT

AND

PRIMARY TREATMENT

Pretreatment and primary treatment are intended for physical separation of solids from water, usually through size and density difference. They also remove BOD associated with the separated solids. The bulk of the BOD in fruit processing plants is associated with dissolved organic matter, which is not removed in primary treatment.

1991 1991 1991 1991 1991 1991 1991 1991 1991 1991 1991 1991 1991 1991 1991 1991 1994

Stockton, CA Watsonville, CA Turlock, CA Hollister, CA Modesto, CA Sacramento, CA Visalia, CA Oroville, CA Atwater, CA Fullerton, CA San Jose, CA Merced, CA Monterey, CA Santa Cruz, CA Orland, CA Santa Rosa, CA San Diego, CA — — 160.00 784.00 433.30 — — 1,038.00 — — 1,925.00 — — 1,805.00 250.00 1,570.00 1,900.00 165.04 77.70 381.00 362.22 518.60 360.96 281.58 270.00 459.00 215.00 1,470.00 300.00 715.20 3,770.00 6,810.00 4,040.00 2,110.00

Flow ($ per million gallons) 13.56 27.41 21.80 45.96 61.34 96.72 71.00 146.00 142.00 121.00 74.00 166.97 148.70 — — 223.60 —

BOD ($ per thousand pounds per day) 55,545 77,064 10,261 — — — — — — — 30,678 40,300 — — — — — 9,400 7,800 1,200 — — — — — — — 8,376 10,680 — — — — —

BOD ($ per thousand pounds per day) 6,600 23,520 700 — — — — — — — 9,744 8,570 — — — — —

TSS ($ per thousand pounds per day)

Waste Water Capacity Charge Flow ($ per million gallons)

210,033 322,450 121,641 190,150 178,138 145,362 147,358 272,800 209,300 211,300 613,650 465,812 295,780 557,500 706,000 827,240 554,000

Total Cost Median Plant ($ per year)

Source: Mannapperuma, J. D., Yates, E. D., and Singh, R. P. 1993. Survey of water use in the California Food Processing Industry, Proceedings of the Food Processing Environmental Conference, Atlanta, GA.

20.56 20.28 43.40 64.60 56.46 53.15 104.00 42.00 83.00 155.00 98.00 208.46 175.50 — — 145.60 255.00

TSS ($ per thousand pounds per day)

Waste Water Loading Rates

Note: BOD = biochemical oxygen demand; TSS = total suspended solids.

Year

Community

Fresh Water ($ per million gallons)

TABLE 17.8 Cost of Water Supply and Disposal in Some Communities

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Grit chambers with grids and coarse screens are used to separate twigs, bottle caps, and other large objects. This is followed by separators with finer screens. Static tangential screens without moving parts are fast, trouble-free devices that work well with fibrous waste. Vibratory and rotary screens are used where separation of finer solids is desired. These devices typically have openings down to about 100 mm. Wedge wire filters and centrifugal screens are also used in this particle range. These devices are capable of accommodating screens down to a few microns in opening size, but in this range, the throughput becomes low and frequent plugging becomes a problem. Density separation is effective in removing heavier and lighter particles from water. The gravity sedimentation tank is the most elementary density separator and also the most widely used one. The settling rate of particles in water depends mainly on their diameter and density. More effective density separation requires an increase in particle size, a change in density, or an increase in gravity. Decanter centrifuges and basket centrifuges generate separation effects equivalent to up to 30,000 g (acceleration due to gravitational force). These are used frequently in sludge thickening but rarely in pretreatment. Hydrocyclones operate at a much lower range, typically about 1000 g but are more popular in pretreatment applications. Hydrocyclones have a definite advantage over centrifuges in not having high-speed moving parts. Both centrifuges and hydrocyclones are used as classifiers, where both denser and lighter fractions are separated from water. Coagulation and flocculation are used to remove colloidal particles that are larger than dissolved molecules but smaller than settleable particles. Colloidal particles are held in emulsion by electrostatic forces. Flocculation and coagulation involve destabilizing the emulsion by the introduction of coagulating agents. A variety of multivalent inorganic compounds, polyelectrolytes, and natural materials are used for this purpose. In electrocoagulation, polyvalent metallic ions are introduced directly into water by gradual decomposition of a metallic electrode through electrolysis (Bulley, 1975). Flotation is density separation in which contaminants lighter than water are allowed to rise to the surface and are removed as a scum layer. Unaided flotation is strongly rate-limited. Increasing gravity through centrifugation and cycloning is one method of aiding flotation action. Increasing buoyancy by introduction of fine air bubbles is another approach. Dissolved air flotation (DAF) involves attachment of fine air bubbles to suspended matter to increase buoyancy. Air bubbles are provided by dissolving air in a smaller volume of water at high pressure and releasing the pressurized water in a larger tank. The sudden lowering of pressure forces the dissolved air to come out of solution as fine bubbles. This technology finds applications in water with suspended oily waste such as olive waste. Coagulation and flocculation are used frequently to further aid dissolved air flotation. A disadvantage of the use of coagulants in food plant wastewater is that several chemicals used for this purpose prevent the separated solids from being used as animal feed. Electroflotation is a related technology where air bubbles are produced by electrolysis of water (Beck et al., 1974; Ramirez and Clemens, 1978), it has not found much applicability. Stabilization is an essential pretreatment when some effluent characteristics such as temperature and pH vary widely over time. Wastewater is allowed to accumulate in large stabilization tanks, which evens out the temporal fluctuations. Sometimes, this is not adequate to rectify the situation. Adjustment of pH by addition of acids or alkali is required under these conditions.

17.9.4 SECONDARY TREATMENT Secondary treatment of food process wastewater involves biochemical reactions to decompose dissolved organic matter. This is done by aerobic treatment in the presence of oxygen or by anaerobic treatment in the absence of oxygen. Aerobic treatment systems vary in sophistication from simple lagoons to high-rate activated sludge plants. Shallow ponds are the simplest aerobic treatment systems, but their BOD removal

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capacity is limited. A typical loading rate is 20 to 40 lb of BOD per acre per day. Addition of supplementary aeration enables ponds to be more efficient in BOD removal. Aerated lagoons are about 12 ft deep with typical loading rates of 600 to 800 lb of BOD per acre per day. Aeration and mixing require about 1.2 kWh/lb of BOD removal. The activated sludge process involves returning a portion of the clarified sludge to be mixed with the effluent-entering system and activating biodegradation, thus giving the process its name. The activated sludge process originated as a plug flow system, effluent and sludge enter at one end and move along the system, while aeration is supplied through its full length. Step aeration and complete mix systems were developed to overcome the problems due to high local oxygen demand in handling high-strength waste. A complete mix system is preferred for high-strength food processing waste. Instead of air, a high-purity oxygen system uses nearly pure oxygen generated at the site, thus accelerating the process. This system is used successfully in several food processing plants. The activated sludge process is the most common treatment system used with food processing wastewaters. A typical design for 1 million gal/d containing 1000 mg/l of BOD requires about 600 hp of installed electric motors and consumes about 1.4 kWh/lb of BOD removed. Oxidation ditch is a process developed in Europe for small municipalities. It is a compact, efficient activated sludge process designed as a closed-loop channel with brush-type aerators in the channel. This relatively trouble-free system has found several applications in the food industry (Parker and Skerry, 1973). A rotating biological contactor (RBC) has several large-diameter disks mounted on a horizontal shaft in a semicircular tank. The disks are rotated slowly with the lower half submerged in water. Microorganisms grow on the disk surfaces. The rotating action introduces oxygen into the water and helps shear excess cell mass from the disks. This system has not found many applications in food plants. A trickling filter is one of the oldest biological treatment systems, but it is not really a filter. A typical trickling filter is a 6-ft deep bed of 2.5- to 4-in. rocks arranged in a 25-ft diameter circle. Effluent is trickled over the bed, while atmospheric air moves within the voids in the rock bed, creating contact between water and air. The loading rates are about 5 to 20 lb of BOD per day per 1000 ft3 of filter volume for standard filters. Plastic or wooden media, and effluent recirculation are used in super-rate filter towers, where loading rates are about ten times those for the standard filters.

17.9.5 TERTIARY TREATMENT Tertiary treatment processes, also known as advanced water treatments (AWT), are used increasingly for reuse of water or to meet NPDES permit requirements for discharge to natural streams. Tertiary treatments include polishing lagoons, sand filters, activated carbon filters, ion exchangers, chlorinators, ozonators, electrodialysis, and cross-flow membrane filtration. Advanced water treatments are sometimes used as pretreatments with food process wastewaters. Lagoons and filters fall into this category. Tertiary treatments are also used in the food industry to remove color, odor, salt, and taste compounds that are not removed by biological treatment. Activated carbon, reverse osmosis, and chlorination fall into this category. Filters are used as tertiary treatment to remove suspended solids. Microstrainers, sand filters, and multimedia filters are used for this purpose. Microstrainers come with screen openings down to 1 mm. However, the practical range is about 20 to 100 mm. Sand filters and multimedia filters are equivalent to about 20- to 30-mm screens in rejection characteristics. Slow sand filters require periodic manual cleaning, and the filtration fluxes are about 60 to 90 gfd (gallons per square foot per day). Rapid sand filters employ graded sand and periodic backwashing. They are easier to maintain, and the filtration fluxes are about 3,000 to 10,000 gfd. Multimedia filters use graded media of different densities so that the optimum layered structure is self-maintained during backwashes.

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Activated carbon is an excellent adsorption medium (due to its large surface area) for removing dissolved organic matter that is leftover from biological treatments. Once limited to chemical industries, it is now finding increased use in other industries due to improved equipment and cost reductions.

17.9.6 ANAEROBIC TREATMENT In comparison to aerobic treatment, anaerobic technology requires a lower energy input. It can deliver a net energy surplus under most conditions. Methane gas produced in anaerobic treatment can itself be used to provide the heating energy required for the process, while the excess can be used to fuel boilers or internal combustion engines. Advances made in recent years in the development of this technology have reduced the retention times from weeks to hours. New applications have broadened the range of wastewaters amenable to anaerobic treatment (Jean and Strickney, 1988). Anaerobic treatment is better suited to high concentration waste, while aerobic treatment is better suited to low- concentration wastewaters. Concentration of process waters by reverse osmosis would enable the reuse of permeate in the process, while the concentrate can be treated by anaerobic digestion. This would result in more compact, more cost-effective water management systems. A major problem in anaerobic treatment of food process wastewaters is competition from sulfate-reducing bacteria. These anaerobes convert sulfates into a series of sulfur compounds — from thiosulfates to hydrogen sulfide. These compounds are responsible for the odors and corrosion frequently associated with anaerobic digestion. Several methods can remove hydrogen sulfide and the products of its combustion.

17.10 LANDFILLS, IMPOUNDMENTS, INCINERATION, AND HAZARDOUS WASTE DISPOSAL When industrial solid wastes are not recoverable for any beneficial use, they have to be disposed at a landfill. Landfills are classified into nonhazardous, designated, and hazardous, according to the environmental compatibility of the wastes they can accept. Generally, food processing residues can be disposed of at the least restrictive landfills. Similar restrictions apply to liquid residues that cannot be discharged to natural streams, or disposed of by land applications or treatment plants. Impoundments classified as hazardous or designated should be used for these liquids. Wastewater from brine curing and storage operations falls in the designated liquid waste category. These impoundments should be double lined with leachate collection and monitoring facilities. Substances that pose a threat to human health are classified as hazardous wastes. These include over 600 federally listed substances, and any other waste that is ignitable, corrosive, reactive, or toxic to humans. This mode of disposal is rarely required for food processing residuals.

17.11 FOOD PROCESS RESIDUAL MANAGEMENT IN THE FUTURE Planning and design of food plants in the past were based on the availability of good-quality fresh water at low cost, and easy disposal of solid waste and process waters. As these conditions change, it becomes necessary to reduce water use and residue generation; however, plant design limits the success of these efforts. Often, it appears that major changes in plant design are necessary to achieve substantial improvements. Water conservation and residue reduction should be important aspects of the design of food processing plants in the future. When plant modifications, new equipment additions, and process changes are planned, water use, chemical use, and residue generation should be considered as important evaluation criteria. Integrated evaluation procedures encompassing all aspects of the process change are necessary to prevent unpleasant surprises.

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The planned deregulation of electric utilities could make operation of biomass cogeneration plants uneconomical. Several million tons of fruit processing residue is burned in these plants annually. This convenient and beneficial means of disposal would be lost to the fruit processing plants if cogeneration plants cease to function. Air quality standards are restricting incineration, which is another avenue for disposal of solid waste. The Clean Air Act, amended in 1990, calls for reducing carbon monoxide emissions and ozoneforming compounds through changes in fuel formulation. Blending with ethanol is one means of achieving this objective. Use of food process residues in ethanol production may become attractive under these conditions. Anaerobic treatment is a very attractive alternative to the more popular aerobic treatment for treating high-strength organic waste from food processing plants. It involves lower capital investment, energy consumption and chemical use. Recent advances in membrane technology can supplement anaerobic technology through preconcentration of effluent while recovering clean water for reuse. One factor preventing wider use of anaerobic treatment is restrictive air emission standards. Food processing residues finally have to end up in one or more of the three disposal media: air, water, and land. All three are being subjected to increased scrutiny by environmental regulations, often with valid reasons. Decision making in residue disposal has become a sophisticated exercise requiring in-depth knowledge of a wide range of subjects. Source reduction is undoubtedly the best path to follow in directing future policy.

REFERENCES Anon. 1971. Turning waste material into profits, Canner Packer, July. Anon. 1977. Olive canner finds use for pits: Making boiler fuel, Canner Packer, April. Anon. 1979. Tri-Valley fires boiler with peach pits, Processed Prepared Foods, 33. Anon. 1981. Biomass fueled co-generation plant produces 60,000 lb/hr of steam, pays way in 3.1 years, Food Process., July: 64–65. Anon. 1989. Current good manufacturing practice in manufacturing, processing, packing, or holding human food. Part 110 — Sanitation, in The Almanac of the Canning, Freezing, Preserving Industries, Edward E. Judge & Sons, Inc. Westminster, MD. Anon. 1992. Ozonation of Cooling Tower Water, Electric Power Research Institute, 3412, Palo Alto, CA, 94303. Anon. 1994. Ozone as potential sanitizer, Packer/Shipper, 1994 (May/June): 28. Anon. 2002a. National raison cuts wastewater costs and protects environment, Pollut. Eng., September 2002: 42. Anon. 2002b. Manual of Good Practice for Land Application of Food Process/Rinse Water. California League of Food Processors, Sacramento, CA. Badger, P. C. and Broader, J. D. 1989. Ethanol production from food production wastes, HortScience, 24(2): 227. Bath, D. L. 1981. Feed byproducts and their utilization by ruminants, in Upgrading Residues and Byproducts for Animals, Huber, J. T., Ed., CRC Press, Boca Raton, FL. Beck, E. C., Giannini, A. P., and Ramirez, E. R. 1974. Electrocoagulation clarifies food wastewater, Food Technol., 28(2): 18. Brandt, R. C. and Martin K. S. 1994. The Food Processing Residual Management Manual, Pennsylvania Department of Environmental Resources, Harrisburg, PA. Bulley, N. R. 1975. Egg processing water recovery, Proceedings of the 6th National Symposium on Food Processing Waste, U.S. Environmental Protection Agency, Cincinnati, OH. (EPA12060–03/75). Elliot, D. C., Baker, E. G., Butner, R. S., and Sealock, L. J., Jr. 1993. Bench scale reactor tests of low temperature, catalytic gasification of wet industrial wastes, J. Sol. Energ.-Trans. ASME, 115: 53. Farmer, J. C. 1994. Personal Communication. Lawrence Livermore National Laboratory, Livermore, CA. FDA 1992. Action Levels for Poisonous and Deleterious Substances in Human and Animal Feed, Department of Health and Human Services, Public Health Service, Food and Drug Administration, Washington, D.C. Green, J. H. and Kramer, A. 1979. Food Processing Waste Management, AVI Publishing, Westport, CT.

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Grigas, M. 1994. Current technology in the polishing of evaporator condensate, Practical Short Course on Membrane Separations in Food Processing, Food Protein Research and Development Center, Texas A&M University, College Station Texas, 77843–2476. Hills, D. J. and Roberts, D. W., 1982. Conversion of tomato, peach, and honeydew solid waste into methane gas, Trans. ASAE, 25(3): 820. Hills, D. J. and Roberts, D. W. 1984. Energy from cull fruit, Trans. ASAE, 27(4): 1240. Huber, J. T. 1981. Upgrading Residues and Byproducts for Animals, CRC Press, Boca Raton, FL. Jean, R. E. St. and Strickney, A. R. 1988. Onsite anaerobic treatment of combined pea and potato processing wastewaters using a mobile microprocessor controlled, three reactor pilot facility, Proceedings of the 1988 Food Processing Waste Conference, Georgia Institute of Technology, Atlanta, GA. Jones, R. H., White, J. T., and Damron, B. L. 1975. Waste Citrus Activated Sludge as a Poultry Feed Ingredient. EPA-660/2–75–001. U.S. Environmental Protection Agency, Corvallis, OR. Katsuyama, A. M. 1979. A Guide for Waste Management in the Food Processing Industry, The Food Processors Institute, Washington, D.C. Kettani, M. S. 1994. Ethanol Production from Fruit and Vegetable Processing Waste, M. S. Thesis, Department of Agricultural Engineering, University of California, Davis, CA. Kranzler, G. A. and Davis, D. C. 1983. Utilization of pomace for fruit juice energy requirements, Paper number 83–6003, ASAE San Joseph, MI. Lagunas-Solar, M. 1994. Personal communication. Crocker Nuclear Laboratory, University of California, Davis, CA. Langlais, B., Reckhow, D. A., and Brink, D. R. 1991. Ozone in Water Treatment, Application and Engineering, Lewis Publishers, Chelsea, MI. Mannapperuma, J. D., Park, K. H., Merson, R. L., and Shoemaker, S. P. 1994a. Membrane Applications in Fruit and Vegetable Industry — Eight Inplant Demonstrations, A report submitted to the Electric Power Research Institute, Menlo Park, CA. Mannapperuma, J. D., Park, K. H., Kelly, P. A., and Shoemaker, S. P. 1994b. Membrane Applications in a Fruit Juice Processing Plant. A report submitted to the Electric Power Research Institute, Menlo Park, CA. Mannapperuma, J. D., Yates, E. D., and Singh, R. P. 1993. Survey of water use in the California Food Processing Industry, Proceedings of the Food Processing Environmental Conference, Atlanta, GA. Mercer, W. A., Maagdenberg, H. J., and Ralls, J. W. 1970. Reconditioning of olive processing brines, Proceedings of the 1st National Symposium on Food Processing Waste. U.S. Environmental Protection Agency, Cincinnati, OH. (EPA12060–04/70). Morris, C. E. 1985. Apple and pear fiber, Food Engineering, January: 72. Panasuik, O., Sapers, G. M., and Ross, L. R. 1977. Recycling bisulfite brines used in sweet cherry processing, J. Food Sci., 42(4): 953. Parker, C. D. and Skerry, G. P. 1973. Cannery Waste Treatment by Anaerobic Lagoons and Oxidation Ditch, U.S. Environmental Protection Agency, EPA R2-73–017. Penna, M. 1994. Private communication. M&CP Farms, Orland, CA. Pimental, K. and Torres, G., 1994. Ceramic Filtration Systems, Research Report, Rosmead, CA. Ralls, J. W. 1971. Dry caustic peeling of tree fruit to reduce liquid waste volume strength, Proceedings of the 2nd National Symposium on Food Processing Waste. U.S. Environmental Protection Agency, Cincinnati, OH. (EPA12060–03/71). Ramires, E. R. and Clemens, O. A. 1978. Physico-chemical treatment of rendering wastewater by electrocoagulation, Proceedings of the 9th National Symposium on Food Processing Waste. U.S. Environmental Protection Agency, Cincinnati, OH. (EPA12060–03/78). Sargent, S. A., Steffe, J. F., and Tennes, B. R. 1982. Apple pomace as fuel for food processors, Paper number 82–6032, ASAE, San Joseph, MI. Schieber, A., Stinzing, F. C., and Carle. 2001. By-products of plant food processing as a source of functional compounds — recent developments. Trends Food Sci. Technol., 12(11): 401–413. Shapton, D. A. and Shapton, N. F. 1991. Principles and Practices for the Safe Processing of Food, Butterworth Heinmann, Oxford, U.K. Shober, R. T. 1989. Water conservation/waste reduction in food processing facilities. Proceedings of Food Processing Waste Management and Water Conservation Conference, P. D. Robillard and H. A. Elliot, Eds., Hershey, PA, November 14–15, 1989, pp. 91–102.

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Simons, G., Johannis, M., and Miller, W. 1994. Institutional issues facing California’s biomass energy industry and the role of a collaborative, Bioenergy ’94, Proceedings of the 6th National Bioenergy Conference, Reno/Sparks, NV, p. 547. Somogyi, L. P. and Kyle, P. E. 1978. Overview of the environmental control measures and problems in the food processing industries. Appendix 8. Overview of fresh, canned, frozen, and dehydrated fruit and vegetable industries. Report prepared for U.S. Environmental Protection Agency, Cincinnati, OH. (Grant No. R804642–01). Spatz, D. D. 1973. Reclamation and reuse of waste products from food processing by membrane processes, Water-1972; AIChE Symposium Series, 69(129): 89. Trotzke, D. E. 1988. Anaerobic treatment technology — 1988 update, Proceedings of the 1988 Food Processing Waste Conference, Georgia Institute of Technology, Atlanta, GA. Walter, R. H., Rao, M. A., Sherman, R. M., and Cooley, H. J. 1985. Edible fibers from apple pomace, J. Food Sci., 50: 747–9. White, C. G. 1992. Handbook of Chlorination and Alternative Disinfectants, Van Nostrand Reinhold, New York, p. 1219. Williams, D. W. and Eastman, R. V. 1992. Commercial scale production of fuel ethanol from food processing and beverage industry wastes, Liquid Fuels from Renewable Resources. ASAE, San Joseph, MI.

Part II Major Processed Products

18 Apples and Apple Processing William H. Root and Diane M. Barrett CONTENTS 18.1 18.2 18.3

Introduction ........................................................................................................................455 U.S. and World Apple Production .....................................................................................456 Apple Cultivars ..................................................................................................................459 18.3.1 Origin of the Current Popular Cultivars .............................................................460 18.4 Handling of Apples for Processing....................................................................................461 18.4.1 Processed Apple Products ...................................................................................463 18.4.2 Apple Juice Processing........................................................................................463 18.5 Processed Apple Products..................................................................................................469 18.5.1 Apples for Processing..........................................................................................469 18.5.2 Applesauce...........................................................................................................472 18.5.3 Sliced Apples .......................................................................................................472 18.6 Dried Apple Products.........................................................................................................473 18.7 Specialty Apple Products ...................................................................................................474 18.8 Quality Control ..................................................................................................................475 18.9 Nutritional Value of Apples ...............................................................................................476 Acknowledgments ..........................................................................................................................476 References ......................................................................................................................................478

18.1 INTRODUCTION Apple has been grown by mankind since the dawn of history. This is mentioned in early legends, poems, and religious books. The “fruit” that the Bible says Adam and Eve ate in the Garden of Eden is believed by many to have been an apple. The ancient Greeks had a legend that a golden apple caused quarreling among the gods and brought about the destruction of Troy. The Greek writer Theophrastus mentions several cultivars grown in Greece in the fourth century B.C. Apple trees were grown and prized for their fruit by the people of ancient Rome. The apple species Malus pumila, from which the modern apple developed, had its origin in southwestern Asia in the area from the Caspian to the Black seas. The Stone Age lake dwellers of central Europe used apples extensively. Remains found in their habitations show that they stored apples fresh and also preserved them by cutting and drying in the sun. The apple was brought to America by early colonists from Europe. Some cultivars originating in Europe were grown by the colonists, but the main method of planting apples in the new land was by seed. As the pioneers migrated westward, they carried apple seeds with them and established plantings where they settled. Almost everyone is familiar with John Chapman, “Johnny Appleseed,” born in June 1774 in Leominster, MA, and the story of how he carried apples west like many of the early settlers. In these early times, most of the apple crop was home processed into cider. The common seedling trees were satisfactory for this cider production. Not many of the cultivars brought across 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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50° 35°

0°

30° 45°

FIGURE 18.1 Primary apple-growing latitudes of the world. Data from U.S. Apple Situation — Apple Commodities page, U.S. report, http://www.fas.usda.gov/htp/circular/2003/9-12-03%20Web%20Art%20 Updates/2003%20USApple%208-23-03.pdf.

the Atlantic by our ancestors adapted well to the North American climate. There was a need to develop American cultivars from the seedlings to improve the apple production and storage characteristics. Those selected cultivars were given local names (Upshall, 1970).

18.2 U.S. AND WORLD APPLE PRODUCTION The apple is more widely grown than any other fruit. Apple trees of one cultivar or another grow all around the world but are mainly concentrated in the Northern hemisphere. About 95% of all apples grown, with some exceptions due to isolated microclimates, are found between the 35∞N and 50∞N latitudes and between the 30∞S and 45∞S latitudes. These bands of primary apple growing areas around the globe are pictured in Figure 18.1. Annual world production of apples was about 45 million metric tons during 2002 to 2003. World apple production trends are given in Table 18.1. World production declined during the period mentioned for the second consecutive season due to lower production in both China and the U.S. These reduced production rates offset increased apple production by other major producers, including Turkey (USDA, 2003). Apple production in some selected countries for this season is illustrated in Table 18.2. Northern hemisphere countries, particularly China, the U.S., Turkey, Italy, and Poland dominate the world market. During the 10-year period from 1992 to 2002, apple production in China increased dramatically (Figure 18.2), from approximately 20% to over 45% of the world production (USDA, 2003). Production and storage facilities in China are expected to improve, and lower Chinese fruit prices will also boost fruit sales. However, if urban Chinese consumers have increasingly greater purchasing power, China may actually import more apples. Commercial apple production for the U.S. during the 2002/03 period was approximately 3.9 million U.S. tons. This was down by approximately 4% from the 5.2 millon tons produced in 2001/02 due to reduced supplies, higher domestic prices, and a strong U.S. dollar which reduced U.S. apple exports. Apple production in the U.S. is primarily in the states of Washington, New York, Michigan, California, Virginia, and Pennsylvania (Figure 18.3). These states produce over three quarters of the total U.S. production. The other regions — New England, eastern, central, and other western states — produce the remainder.

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TABLE 18.1 World Apple Production Trends, 1999 through 2003 Areas

1999–2000

2000–2001

2001–2002

2001–2003

North America Europe China Southern Hemisphere WORLD TOTAL

5.40 8.90 20.80 4.03 38.13

5.13 9.45 20.43 4.40 39.41

4.78 8.03 21.00 4.14 37.95

4.41 7.65 20.50 4.36 36.92

Note: Expressed in million metric tons. Source: United States Department of Agriculture, Foreign Agricultural Service. World Apple Situation: Acreage Continues to Decrease in Major Producing and Trading Countries, March 2003. http://www.fas.usda.gov/htp/circular/2003/3-7-03%20Web%20Art.%20Updates/ World%20Apple%20Situation%202002-03.pdf; FAO Production Yearbook, Vol. 45, Food and Agriculture Organization of the United Nations. 1991.

TABLE 18.2 Production of Apples in Specified Countries, 2002–2003 Northern Hemisphere Belgium–Luxembourg 351 Netherlands Canada 510 Poland China 20,500 Russia France 2,140 Spain Germany 1,429 Sweden Greece 220 Taiwan Hungary 470 Turkey Italy 2,210 U.K. Japan 912 U.S. Northern Hemisphere total

365 2,107 1,400 742 54 10 2,500 134 3,900 32,858

Southern Hemisphere Argentina 1,000 Chile Brazil 825 New Zealand Australia 328 South Africa Southern Hemisphere total

1,060 462 680 4,355

Note: Expressed in 100 t. Source: USDA, world statistics from USDA/FAS World Horticultural Trade and U.S. Export Opportunities, March 2003.

Apple production in the U.S. has declined due to continued reduction in apple acreage as a result of financial problems that have forced many growers out of business. Apple-bearing land in 2002–2003 in the U.S. is estimated at 430,000 acres as compared to 470,000 acres in 1989/99. According to the U.S. Department of Agriculture (USDA) (USDA, 2003), the apple industry faces low domestic prices, caused primarily by overproduction, stagnant domestic demand, and remarkably increased imports of lower price apple juice from China.

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World Production of Apples

Percent of World Production

50 40

China

30 USA

20

Turkey

10

02 20

01 20

00 20

99 19

98 19

97 19

96 19

95 19

94 19

93 19

19

92

0

Marketing Years

FIGURE 18.2 World production of apples (1992–2002). Data from World Apple Situation — Apple Commodities page, http://www.fas.usda.gov/htp/circular/2003/3-7-03%20Web%20Art.%20Updates/World%20 Apple%20Situation%202002-03.pdf. Percent of U.S. crop grown by state

Washington Michigan New York California Pennsylvania Virginia

FIGURE 18.3 Percentage of U.S. apple crop grown by major producing states. Data from World Apple Juice Situation — Apple Juice Commodities page, http://www.fas.usda.gov/htp/circular/2003/4-4-03%20Web%20 Art.%20Updates/Apple%20Juice%20Feature%2004-03.pdf.

In 2002, the percentage of apples marketed fresh was 63% of the total, and 37% was processed. Of the processed apples, 18% was utilized in juice and cider, 13% was canned, 3% was dried, 2% was frozen, and 1% was used in other miscellaneous products such as vinegar, wine, and jelly. Over the past 10 years (1992 to 2002), the utilization of the apple crop has changed to a higher percentage of fresh apples (from 55 to 63%) and a lower percentage of juice, canned, dried, and frozen apple products (USDA-NASS, 2003). World apple juice production is expected to remain strong, with record production in China, which in 2002–2003 accounted for 33% of the world production. This is more than double the market share of 15% that China held in 1998–1999. The U.S., on the other hand, now accounts for only 12% of world apple juice production, which is half its 1998–1999 level (Figure 18.4). In the U.S., some processors have had to import apple juice products, particularly in the forms of concentrate. This was to ensure an adequate supply of raw material for their manufacturing facilities to maintain consistent distribution from year to year. U.S. imports of apple juice increased from approximately 210,000 t in 1998–1999 to 260,000 t in 2002–2003.

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1998/99

China Poland USA Argentina

(a) 2002/03

China Poland USA Argentina

(b)

FIGURE 18.4 Share of world apple-juice production (1998/89 vs. 2002/03).

18.3 APPLE CULTIVARS There are hundreds of apple cultivars, many of them are shown with color plates in The Apples of New York by Beach et al. Only about 20 cultivars are now grown commercially in the U.S. More than 90% of the production is represented by 14 cultivars (Table 18.3). Of these, five — Red Delicious, Golden Delicious, Gala, Fuji, and Granny Smith — account for most of the world apple production. The recent trend in the U.S. is to plant newer apple cultivars. These newer cultivars, are now appearing in fruit markets. Gala, Fuji, Jonagold, Braeburn, and Lady Williams are relatively new varieties that the consumer has accepted as an alternative to traditional varieties. Gala and Fuji, in particular, have displace older varieties in terms of their market share. Most of the new commercial plantings are selected red strains of the primary cultivars. Some cultivars, like Gala, mature in 100 days or less while others, like Lady Williams, grown in Western Australia, require over 200 frostfree days to reach maturity. Some cultivars are very winter and frost hardy while others are very

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TABLE 18.3 Apple Production by Cultivar in the U.S., 2002 Cultivar

Primary Region

Production (000 42 lb units)

Delicious (all) Golden Delicious Gala Fuji McIntosh Rome Beauty Granny Smith Jonathan York Stayman Cortland Newtown Winesap Northern Spy Rhode Island Greening

West West West West East East West Central East East East West West East East

63,625 27,891 18,960 20,656 7,840 7,922 19,394 3,561 3,724 1,268 1,750 1,319 431 1,085 1,264

Gravenstein

West

134

Source: USDA 2003.

tender. Some cultivars like Delicious require long cold winters to break dormancy, others like Anna, a cultivar grown in Israel, can be grown in mild Mediterranean type climates. Washington State grows 54% of the apples produced annually in the U.S., over 116 million 42-lb units as compared to about 24 million 42-lb units in New York, the second largest producer. The pie chart in Figure 18.3 shows the apple production percentages by growing region. Consumers are requesting high quality apples with distinctive flavors. The trend in a Washington State tree survey shows continuation of the dominance of Delicious, Golden Delicious, and Granny Smith. California does not produce many Delicious apples, but acreage of Gala and Fuji are increasing. Future U.S. planting densities will increase when new plantings are made, therefore annual apple volume will continue to increase.

18.3.1 ORIGIN

OF THE

CURRENT POPULAR CULTIVARS

The original Red Delicious apple was discovered as a chance seedling in 1881 by Jesse Hiatt near Peru, Iowa. Stark Bros. Nurseries, Louisiana, Missouri, bought the rights to Red Delicious in 1894 and promoted it heavily. Presently over 100 strains of Red Delicious have been propagated by growers and nurserymen. Red Delicious is a sweet, mild apple for eating, not cooking. The trees are productive and adaptable to different growing conditions. Golden Delicious originated around 1900 in West Virginia but is not related to the Red Delicious apple except that it was also purchased, promoted, and named by Stark Bros. Nurseries. It is of interest that these two most popular apples are not the result of organized fruit breeding programs. “Goldens” have a sweet, delicate flavor and store well. The Golden Delicious is the parent of several modern varieties such as Jonagold, Spigold, Gala, and Mutsu. McIntosh is the dominant commercial apple in New England and eastern Canada. The first tree was a chance seedling, introduced around 1811, on John McIntosh’s farm in Matilda, Ontario. It is a thick-skinned, tender-fleshed, aromatic apple. McIntosh apples ripen early and were a commercial favorite of growers trying to deliver early to the fresh-apple-hungry metropolitan areas of New York and Chicago. McIntosh is a parent of Spartan, Empire, and other hardy modern cultivars.

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Granny Smith, the third most popular apple in the world, originated in the 1860s. It was a chance seedling in Marie Smith’s back yard near Sydney, Australia, thus, the name Granny Smith. The Granny Smith needs a long growing season and is grown commercially in the U.S. mainly on the West coast. It is a very firm, green, juicy, tart apple ideal for apple pie and contributes acidity when used in juice production. Jonathan was a seedling from Esopus Spitzenburg. Esopus Spitzenburg, although not a major cultivar today, originated in 18th century in Esopus, New York, and was claimed to be Thomas Jefferson’s favorite apple grown at Monticello. Jonathan was named after Jonathan Hasbrouck near Woodstock, New York. Jonathan is the primary variety grown in several areas outside the U.S., including Hungary. It is a very good flavored apple that is superior for eating and makes excellent applesauce and apple juice. Jonathan is a parent of many modern cultivars such as Jonagold. Cortland is a McIntosh/Ben Davis cross developed in 1915. Although larger than McIntosh, it is not as flavorful and ripens a week later. Cortland is another early ripening favorite to sell in the fresh market early. Newtown originated in Newtown, Long Island, in the early 1700s. It once was considered the most flavorful and best all-around quality dessert apples. Newtown became known for its superb flavor and keeping quality. It is a hard, crisp, juicy, and tart apple that grows best in temperate climates such as Virginia and Oregon. Newtown picked late in the harvest season ripens gradually in storage. Benjamin Franklin introduced the English to Newtown apples in 1759, thus beginning the U.S. apple export trade. Winesap is a small English cider apple that was brought to Virginia in colonial times. It is tart, crisp, flavorful, and an excellent storage apple. Winesap popularity spread across the U.S. and was one of the major cultivars grown in the early Yakima region of Washington. However, modern storage technology has reduced its popularity. Winesap grows best in temperate climates. It is the parent of the Stayman Winesap variety grown from a seed by Dr. J. Stayman in Kansas in 1866. Northern Spy was first grown near Rochester, New York, about 1800 and became well known after the Civil War. This hearty apple is a favorite for eating and cooking in the north. It is a lateblooming, biennial apple that has kept it from commercial popularity in modern times. These late apples still may be ripening on the trees into December. Rhode Island Greening first grew in 1748 in Newport, Rhode Island, at a tavern owned by Mr. Green. The smooth, waxy skinned fruit was juicy, tart, and distinctively flavored. Guests of the tavern took cuttings of the tree to adjoining states, making it one of the first cultivars propagated throughout the colonial Northeast. Gravenstein is an early season European or Russian cultivar that arrived in the U.S. by the 1820s. Plantings of this apple were developed in the Northern California coastal region. The fruit is large, juicy, and tart but only a fair keeper. The apple ripens slowly on the tree over several weeks. Gravenstein apples make excellent applesauce and pies. Detailed descriptions of these and many other of the world’s apple cultivars are available in books by Upshall (1970) and Bultitude (1983). The newer cultivars — Gala, Jonagold, and Fuji — came from breeding programs. Gala originated in New Zealand and is a cross between Golden Delicious and Kidds Orange. Jonagold, discussed previously, originated in New York State as a cross between Golden Delicious and Jonathan. Fuji came from Japan and is a cross between Delicious and Ralls Janet. Several red mutations of these cultivars have been selected and are now grown and available to the consumer.

18.4 HANDLING OF APPLES FOR PROCESSING All of the apple varieties grown commercially are used to some extent in processed products. Some varieties, such as York Imperial, are grown almost exclusively for processing. Only sound, ripe fruit should be used for processed products. Processing quality can be affected by decay, damage, maturity, firmness, color, soluble solids, acids, and other chemical compounds, such as tannins,

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contained in the fruit (Downing, 1989). In one study (Harper and Greene, 1993) of price discounts and premiums for three processing apple cultivars, it was determined that discounts were statistically significant for fruit size, bruising, bitter pit, decay, misshapen apples, and internal breakdown. Insect damage and apple scab did not result in significant price discounts. The cultivar used in processing will be dictated to some degree by the quality of the product to be produced. Many of the apples that have some imperfections, such as skin blemishes or off shapes rendering them undesirable for the fresh market, are utilized by processors. These are perfectly good quality apples and are in high demand. An average of about 20% of the Delicious and other fresh market apples are processed. Varieties such as Golden Delicious, Rome Beauty, Granny Smith, McIntosh, and others may have more than 20% of the volume diverted to processing. Delicious apples that are firm, sweet, and juicy yield a good volume of high quality juice. Although sauce can be produced using Delicious apples the product would not be of good quality, particularly in relation to texture and color. The applesauce yield is less with the Delicious apple due to the thicker skin that results in greater loss during peeling. Golden Delicious on the other hand not only makes a good quality juice but produces a high quality sauce and sliced processed product. Cultivars utilized in processed products are determined by availability of the raw product, quality of the product produced, and market demand from the region grown. Apples may be grown specifically for processing, a practice common among orchards in the Eastern U.S., but most apples sold to the processor are salvaged from fruit grown for the fresh market. Production costs for processing apples have been reported to be lower than costs for fresh market apples (Childers, 1983). This is not necessarily true. Because a premium price is paid for large, bruise-free apples delivered to the processor, growers must give full attention to the cultural management details similar to those given apples grown for the fresh market. Production practices for apples vary with the climate and soils in which they are grown. Space does not permit a detailed description of these practices. Interested readers are referred to several of the many books by Childers (1983), Tukey (1978), and Westwood (1978). Plus extension publications available: Heinicke (1975) Lord and Costante (1977), Forshey (1980 and 1981), Swales (1971), and U.S. Department of Agriculture Farmers Bulletin No. 1897 (1972). Apples for processing should be harvested at optimum maturity for good fresh market storage and handling. Only in a few instances are apples harvested with the processed product in mind. To date the majority of the apple crop is still harvested by hand. Large bins, about 4 ft by 4 ft by 21/2 ft high, holding 750 to 1000 lb of fruit have replaced the old traditional 42-lb wooden field box. Fruit is picked in canvas bags or lined buckets, placed in the large bins, and loaded with fork lifts on trucks, or stacked for transport by special straddle bin carriers to the packing house or processing plant. If there has been extensive fruit damage from hail or sunburn, apples that will not pack out to an acceptable grade will be harvested “field run” or “orchard run” and delivered directly to a processor. Mechanical harvesters designed to shake the tree and catch the falling fruit without bruising have not been perfected for apples. It is estimated that less than 10% of the apples in the world are harvested by mechanical methods (Downing, 1989). Processing of apples is mainly regarded as a salvage operation. The majority of the processing apples are sort-outs from the fresh market packing line. The volume available depends on fresh market demand and the quality of the current apple crop. As a result those apples to be processed are picked and stored in the same manner as fruit destined for the fresh market. Few if any processors can utilize all of the fruit as it is delivered to the plant during the harvest season. Early in the season some fruit to be processed will be stored directly in the bins in regular atmosphere storage without the benefit of refrigeration. This type of storage is short term and limited to the plant’s immediate processing capacity. Early in the season, continuing through January and early February, there are large quantities of fruit available from refrigerated storage. Refrigerated storage temperatures range from 1 to 4∞C, depending on the cultivar in question. After January or February, processing apples are available from controlled atmosphere storage. Controlled atmosphere (CA) storage normally consists of a modified atmosphere, 2 to 3% oxygen

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and 1 to 4% carbon dioxide in conjunction with reduced temperatures. Both the atmosphere and storage temperature must be adjusted for the cultivar in question. Controlled atmosphere apples are generally stored four to six months before removal from storage and distribution to market. Apples from controlled atmosphere storage are generally in very good condition. However, the apples should be allowed to “normalize” several days prior to being processed. There is some loss of apple flavor and acid during CA storage but not significant enough to make the apples undesirable for processing. These apples are capable of producing good quality processed apple products. Due to higher cost of controlled atmosphere storage, the maximum volumes of apples are marketed fresh and the desired quantities of apples for processing are not always available. Apples from both refrigerated and controlled atmosphere storage are capable of producing quality products (Drake et al., 1979). The product produced and the grade desired must be taken into consideration by the manufacturer when considering apples from not only the different types of storage but directly from the field as well. The processor may choose to hold the fruit at elevated temperature to allow for further maturation development (softening, color change, etc.). Some cultivars such as Delicious require additional press aid and filtration as they advance in maturity and become softer. Different grades of applesauce can be manufactured from the same cultivar depending on the type of storage, time of storage, and maturity when processed.

18.4.1 PROCESSED APPLE PRODUCTS Americans consume an average of 47 lb per capita of apples and apple products per year; 17 lb of this was as fresh apples in 2002. Over 27 lb of apples per capita are processed apple products. Apples are processed into a variety of products, although apple juice, averaging 19 lb of apples per capita, is the most popular processed apple product. Apples for processing should be sound, mature, reasonable size, and of uniform shape to be peeled. These peeled apples are processed into canned, frozen, and dehydrated apple slices and dices, plus several styles of applesauce. Applejuice is processed from apples that are unsuitable for use in other peeling operations. “Eliminator” apples, smaller than 21/4 in., are too small to peel, even for applesauce, and are diverted to juice.

18.4.2 APPLE JUICE PROCESSING By far the largest volume of processed apple products is in the form of juice, with approximately110,000 t (70/71∞ Brix equivalent) of apple juice produced in the U.S. in 2002/03. On the other hand, imports of apple juice into the U.S. from China, Argentina, Chile, and other countries continue to increase. Apple juice is processed and sold in many forms. Fresh apple juice or sweet cider is considered to be the product of sound, ripe fruit that has been pressed and bottled or packaged with no form of preservation being used, other than refrigeration. This type of fresh juice is normally sold at roadside stands or in the fresh section of stores not far from the producer. Worldwide, naturally fermented applejuice is called apple cider and is usually fermented to a specific gravity of 1 or less (National Association of Cider Makers, 1980). In the U.S. apple cider refers to “sweet cider” that is made from the unfermented applejuice pressed from early-season, tart apples. Shelf-stable apple juice is sweet cider that has been treated by some method for preservation. This processed apple juice can be in several styles: clarified juice, crushed apple juice, “natural” unfiltered juice, or apple juice concentrate, either frozen or high brix. Apple juice that has been clarified with some form of depectinization and filtration before pasteurization and bottling is the most popular apple juice product produced in the U.S. “Natural” juice is juice as it comes from the press with often about 2% ascorbic or erythorbic acid added to preserve color. It is then pasteurized and bottled. Some forms of natural apple juice are produced with the use of heat treatment only. This process results in a darker apple juice. Crushed apple juice is a product with a high pulp content. The crushed apple juice is produced, without the aid of a cider press, by

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passing coarsely ground apples through a pulper and desecrator before pasteurization. Frozen apple juice concentrate can be either natural or clarified juice concentrated to 42∞ Brix, packaged and quick frozen. Commercial apple juice concentrate is normally the clarified apple juice that has been concentrated to 70∞ Brix or higher, evaporating much of the water. Most commercial cultivars of apples will produce an acceptable juice, particularly when blended. The character of the apple juice is directly related to the cultivar and maturity of the apple used to make the product. Juices produced in the eastern U.S. are more acid than those juices produced in the west (Downing, 1989). This flavor difference is directly related to the cultivars predominantly grown in these areas. In both eastern and western juice manufacturing facilities, the juice product is usually a blend of the juice from two or more cultivars. This blending procedure allows for a more uniform product throughout the season and from season to season. Regardless of the cultivar, only sound, ripe fruit showing no decay should be used. “Wind falls” or apples picked up from the ground should not be used for juice due to the pronounced “musty” or “earthy” flavor the apples pick up. Immature apples produce a juice lacking in flavor and very “starchy” and astringent. Over-mature apples are very difficult to press, clarify, and filter. Figure 18.5 illustrates the process typically used for making apple juice and concentrate. Apples for juice are dumped, either by the bulk truck load or pallet bins, into a water filled receiving tank. In this tank the apples are soaked to remove soil and other foreign material. The raw fruit is then conveyed from the water to be inspected, and any damaged or decayed fruit removed or trimmed. In recent years there has been concern for the presence of over 50 ppb of patulin in the finished juice. Patulin is a micotoxin produced by the mold Penicillium expansium, found in “bulls eye” rot of apples. Although this micotoxin is easily destroyed by oxidation, the concern of patulin is an indicator to determine if the juice was produced from mistreated or spoiled apples. Some manufacturers rely on brush scrubbers to remove any decayed areas on the fruit to eliminate patulin producing mold spots from the apples. Sorting and trimming of apples to remove damage or decayed fruit is mandatory. If not removed, damaged or decayed fruit may also impart off-flavors to the finished product and increase the risk of microbiological contamination. Before pressing, whole apples are ground into a mash or pulp for extraction. This mashing process is accomplished with either a disintegrator, hammer, or grating mill. These mills crush or cut the apple to proper consistency, depending on the maturity of fruit. When milling firm fruit for juice, small particles are desired. As the season progresses and the apples become softer, pressing becomes more difficult, thus bigger particles of pulp are preferred for pressing. Equipment used to extract juice from apples is of several types and many variations (Nelson and Tresler, 1980). The pressing process can be batch or continuous, depending upon the type of press used. More common types of presses apply pressure via hydraulic, pneumatic, screw, basket, or traveling belt methods. The vertical hydraulic press is a batch type operation and very labor intensive but requires no press aid, and the juice has a low level of solids. Although the hydraulic press is one of the older types of juice extraction, it is still in widespread commercial use around the world. There are several other types of juice presses that are modern versions of the basic hydraulic press. These newer presses are automated, allowing a greater percentage of juice extraction from a given volume of apple pulp. However, these presses require press aids, as added 1 to 2% paper pulp and/or rice hulls, to reduce slippage and increase juice channels in the mash. Predrainers of different types, including rotating basket and traveling belt, have been used to extract free-run juice from the mash. This reduces the volume being mixed with the press aid for final pressing. The apple mash has many natural enzymes but at rather low concentrations. Enzymes are substrate specific, which means a given enzyme can catalyze only one particular reaction. Pectolytic enzyme products contain the primary types of pectinases: pectinmethylesterase (PME), polygalacturonase (PG), pectinlyase and pectin transeliminase (PTE). PME deesterifies the galacturonic acid, liberating methanol from the side chain, which then allows PG to hydrolyze the long pectin chains.

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Raw Apple Dump Pit ⇓ Washing & Sorting (to remove rot etc.) ⇓ Disintegrator (Chop or Milling) ⇓ Juice Extraction (Hydraulic or Enzymes) ⇓ 12° apple juice ⇓ Enzyme Treat Tanks Hot = 1 hr @ 50°C. Cold = 8 hr @ 21°C. ⇓ Filtration (Diatomacious Earth) or Polish Filter (Microfiltration) ⇓

Multiple Stage Evaporator 4 stage with aroma recovery ⇓ 71° concentrate ⇓ Standardizing Tanks (adjust AJC to 70° brix) ⇓ Drum Filling ⇓ Drum/Bulk Storage

FIGURE 18.5 Apple juice concentrate flowchart.

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Enzymatic mash treatment has been developed to improve the pressability of the mash and therefore the throughput and yield. The enzymes added at about 80 to 120 ml/t of apple mash break down the cell structure. High molecular weight constituents of cell walls, like protopectin, are insoluble, inhibit the extraction of the juice from the fruit, and keep solid particles suspended in the juice. Pectinase used in the apple process is extracted from the mold Aspergillus niger, a commonly occurring natural product. Pectinase developed for apple mash pretreatment acts mainly on the cell wall, breaking the structure and freeing the juice. Also, the viscosity of the juice is lowered, and it can emerge more easily from the mash. The high content of pectin esterase (PE) causes the formation of deesterified pectin fragments that have a low water-binding capacity and which reduce the slipperiness. These pectins consist of chains of galacturonic acid joined by alpha-glycoside linkage. Xylose is covalently bound as a monomer, and galactose and arabinose as polymers. These polymers form a link with the cellulose. The entire system forms a gel that retains the juice in the mash. Even if the pectins are partially broken down by the pectinase enzyme, more juice is released from the mash and pressing or extraction becomes easier. When used with mash predraining, the pomace acts like a pressing aid. By inexpensive pretreatment of mash with enzymes and heating to 50∞C, the press throughput can be increased about 30 to 40% and juice recovery increased over 20%. Mash pretreatment will also increase the flux rate of ultrafiltered apple juice up to 50%. When using enzymes for mash treatment, particularly in Europe, care must be taken to avoid over treatment, thus rendering the pressed apple pomace undesirable for commercial pectin extraction processing. Residual pectic enzymes in apple concentrate can cause set-up problems when the concentrate is used in making apple jelly. In recent years there has been development of juice extraction by “liquefaction” of the raw fruit by using enzymes. Apple mash contains pectins, starch, arabinose, hemicelluloses, and cellulose. The liquefaction procedure is facilitated by heating the mash and treating with an enzyme “system” to completely break down and free the juice from the mash. The commercial enzyme systems available contain up to 120 substrate specific enzyme components. The liquefied juice is extracted from the residual solids by the use of decanter centrifuges and rotary vacuum filters. Some processors have added additional cellulase enzyme to the mash to further break down the cellulose to soluable solids, increasing the juice brix nearly 5%. Mention should be made of the counter-current extraction method or diffusion extraction first developed in the 1970s in South Africa and refined in Europe (Brunische-Olsen, 1969). Europeans report a recovery of 75 to 80%, but this depends on water temperature, enzymes, and apple variety. The counter-current system recovery is best with hard apples and does not work well with soft dessert apples. In this system the mash is heated, predrained, and counter-washed with water and recycled hot juice. Capacity of the system is about 5 tons/h. A 90 to 95% recovery is obtained when the throughput is reduced to 3 tons/h. Due to dilution, the final juice brix drops from 11∞ Brix to between 6∞ and 8∞ Brix. Some industry regulators consider the extracted juice not true apple juice. Juice yields from the different types of extraction processes vary greatly from about 70 to 95%. Juice yield depends on many factors, including the variety and maturity of the fruit; type of extraction, equipment, and press aids; time; temperature; and the addition and concentration of commercial enzymes to the apple mash. After juice extraction, raw apple juice for clarified juice is enzyme-treated to remove suspended solid material (Smock, 1950). The soluable pectin in the juice has colloidal properties and inhibits the separation of the undissolved cloud particles from the clear juice. Pectinase enzyme hydrolyzes the pectin molecule so it can no longer hold juice. Treatment dosage of pectinase depends on the enzyme strength and varies from one manufacturer to another. A typical “3¥” enzyme dosage would be about 100 ml/1000 gal of raw juice. Depectinization is important for a viscosity reduction and the formation of galacturonic acid groups that help flocculating the suspended matter. This material, if not removed, blinds filters, reduces production, and can result in a haze in the final juice product. Two methods of enzyme treatment are commonly used: (1) hot treatment where the enzymes are added to 54∞C juice, mixed, and held for 1 to 2 h or (2) cold treatment where the enzymes are

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added to room temperature (20∞C) juice and held for 6 to 8 h. The complete breakdown of the pectin is monitored by means of an acidified alcohol test; 5 ml of juice is added to 15 ml of HClacidified ethyl alcohol. Pectin is present if a gel develops in 3 to 5 min after mixing the juice with the ethanol solution. When no gel formation is observed, the juice depectinization is complete. Other polymers such as starch and arabans may cause post-process clouding in a clear juice and can be treated with amylase and arabinase enzymes. The purpose of using enzymes is to bring about a partial or complete breakdown of these substances in the process. The fractured pectin chains and tannins are scavenged from apple juice by addition of about 1 to 1.5 lb of 200 bloom, type A or B gelatin per 1000 gal of juice. Best results are obtained when hydrating 1% gelatin in 60∞C water. Gelatin can be added in combination with the enzyme treatment or bentonite, or by adding midway through the enzyme treatment period. The positively charged gelatin will facilitate removal of the negatively charged suspended colloidal material from the juice. Bentonite, a clay fining agent commonly mined in Wyoming, has been successfully used in the wine industry. The legal limits for use are 8 lb/1000 gal of product. Common usage is about 3 to 5 lb of rehydrated bentonite per 1000 gal of juice to be fined. Bentonite can be added to increase efficiency of settling, for protein removal, and to prevent cloudiness caused by metal ions. An excellent reference on enzyme treatment of apple juice is presented by Kilara and Van Buren and can be found in Processed Apple Products (Downing, 1989). Enzymes are usually not used when producing cloudy or natural apple juice. After the enzyme treatment and the fining and settling process, the apple juice is pumped from the settled material (lees) and further clarified by filtration. Many types of juice filters are available and their capacity can accommodate any scale of production. These include pressure leaf, rotary vacuum, frame, belt, and millipore filters. To obtain the desired product color and clarity, most juice manufacturers use a filter medium or filter aid in the filtration process. The filter mediums include diatomaceous earth, paper pulp pads, cloth pads or socks, and ceramic membranes, to name a few. The filter aid helps prevent blinding of the filters and increases throughput. As the fruit matures, more filter aid will be required. Several types of filter aids are available, the most commonly used is diatomaceous earth or cellulose type materials. Additional juice can be recovered from the tank bottoms (“lees”) by centrifugation or filtration. This recovered juice can then be added back to the raw juice prior to filtration. Diatomacious earth (kieselguhr) is a form of hydrated silica. It has also been called fossil silica or infusorial earth. Diatomacious earth is made up of the skeletal remains of prehistoric diatoms that were single-cell plant life related to the algae that grow in lakes and oceans. Diatomacious earth filtration is a three-step operation. First a firm, thin, protective precoat layer of filter aid, usually a cellulose, is built up on the filter septum (which is usually a fine-wire screen, synthetic cloth, or felt) by recalculation. Second, the use of the correct amount of a diatomite body feed or admix (about 10 lb/100 ft2 of filter screen). Third is the separation of the spent filter cake from the septum prior to the next filter cycle. Before filtration, centrifugation may be used to remove a high molecular weight suspended solids. In some juice plants, high speed centrifugation is used instead of filtration. This centrifugation process produces a product not as clear as filtered juice; however, it allows more or less continuous production. Centrifugation used with filtration reduces the solids about 50%, thus reducing the amount of filter aid required. Pressure, vacuum, and membrane filter equipment are available, and all can produce an acceptable product. The type of filter used must match the capacity to maintain plant production. The filtration process is critical not only from production consideration, but quality of the end product. Both pressure and vacuum filters have been used with success in juice production (Nelson and Tresler, 1980). Membrane (ultrafiltration) filtration is a recent development. Ultrafiltration based on membrane separation has been used with good results to separate, clarify, and concentrate various food products. Ultrafiltration of apple juice cannot only clarify the product but, depending on the size of the membrane, can remove the yeast and mold microorganisms common in apple juice.

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Preservation of apple juice can be by refrigeration, pasteurization, concentration, chemical treatment, membrane filtration, or irradiation. By far the most common method is heat pasteurization based on temperature and time of exposure. The juice is heated to over 83∞C, held for 3 min, filled hot into the container (cans or bottles), and hermetically sealed. The apple juice is held 1 min, then cooled to less than 37∞C. When containers are closed when they are hot and then cooled, a vacuum develops, reducing the available oxygen that also aids in the prevention of microbial growth. After the heat treatment the juice product may also be stored in bulk containers, but aseptic conditions must be maintained to prevent microbial spoilage. Aseptic packaging is another common process where, after pasteurization, the juice is cooled and packaged in a closed, commercially sterile system under aseptic conditions. This process provides the shelf-stable juice in laminated, softsided consumer cartons, bag-in-box cartons, or aseptic bags in 55-gal drums. Apple juice concentration is another common method of preservation. The single-strength apple juice is concentrated by evaporation or freeze concentration, preferably 70 to 71∞ Brix. By an alternate method, the single-strength juice is preconcentrated by reverse osmosis to about 40∞ Brix, then further conventionally concentrated. This method of final concentration is energy-saving. The reduced water activity and natural acid make the final concentrated apple juice relatively shelf stable at room temperature. There are several evaporation systems used for apple juice, including rising film evaporators, falling film evaporators, multiple effect tubular and plate evaporators. Due to the heat sensitivity of the apple juice, the multiple effect evaporator with aroma recovery is most commonly used. The general method in a multiple effect evaporator is heating the juice in the second stage to about 90∞C and evaporate-capturing the volatile (aroma) by distillation. This is followed by reheating the 20 to 25∞ Brix juice concentrate in the first stage to about 100∞C and evaporating it to about 40 to 45∞ Brix; heating it again to about 45∞C and evaporating it in the third stage to about 50 to 60∞ Brix; then, final heating in the fourth stage to 45∞C and evaporating it to 71∞ Brix. The warm concentrate is chilled to 4 to 5∞C prior to standardizing to 70∞ Brix prior to barreling or bulk storage. Preservation by use of chemicals such as benzoic or sorbic acid and sulfur dioxide is not commonly practiced. If chemicals are used, it is only to reduce spoilage of unpasteurized juice either in bulk storage or as an aid in helping to preserve refrigerated products. There are several other methods of apple juice preservation that have not been adopted commercially but may be used in the future. These include, but are not limited to, irradiation and ultrasonics. Apple essence is recovered during the concentration of apple juice. The identification of volatile apple constituents, commonly known as essence or aroma, has been the subject of considerable research. Early progress was very slow due to two problems: first, the difficulty of recovering representative quantities of the volatiles, and, second, the analytical techniques that were labored and unsusceptible to trace components. The essence recovery problems were resolved in 1944 by H. P. Milleville and R. K. Eskew at the USDA, with the development of the essence recovery system during apple juice concentration. This system was the forerunner of the commercial concentration systems used throughout the world today (Milleville, 1944). The analytical problems were solved by the application of a combination of mass spectrometry and gas chromatographic instrumentation. In 1967 researchers at the USDA identified 56 separate compounds from apple essence. These compounds were further refined by organoleptic identification, using a trained panel of sensory specialists. These laboratory evaluations revealed 18 threshold compounds, identified as Delicious apple components consisting of alcohols, aldehydes, and esters. Three of the 18 compounds had “apple like aromas,” according to the taste panel. These were 1hexanal, trans-2-hexenal, and ethyl 2-methyl butyrate (Flath et al., 1967). It is generally agreed there are six components contributing the most to the quality of apple essence or aroma. These can be divided into three desirable and three undesirable components as tabulated in Table 18.4.

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TABLE 18.4 Six Important Apple Essence Components Desirable Apple Essence Components: Ethyl 2-methyl butyrate (E2MB) Ripe apple aroma 1-Hexanal Green apple aroma Trans-2-hexenal Green apple aroma Undesirable Apple Essence Components: Ethyl acetate Airplane glue aroma 1-Butanol Solvent or petroleum aroma c-3-Hexanol Green grass aroma

Until recently, the apple concentrate producers around the world have had little economic incentive to refine their essence recovery methods nor have they cared about the quality of the raw fruit used in essence production. The distillation process naturally recovers any lighter volatiles including ethyl alcohol. Care must be exercised so as not to recover and keep essence from fermented juice, spoiled, or over age fruit. Early evaporation systems with essence recovery were required to register with the Bureau of Alcohol, Tobacco, and Firearms (ATF). This requirement was discontinued in July 1982 (27 CFR Parts 18 to 240) as apple juice processors had demonstrated that their goal was to produce good quality concentrate, not alcohol. Through the previously mentioned component research it was also confirmed the desirable volatiles in apples decrease significantly in storage. Apple juice produced from late season cold storage or CA stored apples will not produce the typical “fresh” apple aroma.

18.5 PROCESSED APPLE PRODUCTS In 1992 approximately 45% of the total U.S. raw apple crop was utilized for processing. Canned applesauce and apple slices rank second to apple juice in importance among processed apple products. This represented 738,100 tons or 13.7% of the total U.S. apple crop (International Apple Institute Clinic, 1993). Of the processing apples, an average of 75% are used for applesauce, 12% for slices, and 12% for other canned products such as apple pie filling, whole baked apples, and spiced apple products. Of the processed apples, 7.2% are dried (USDA/AMS, 1992). The major geographical areas processing canned apple products in the U.S. are the Appalachian area (North Carolina, Virginia, and West Virginia) and Pennsylvania, followed by Michigan, New York, and the West. The Appalachian region produces about 40% of the processed canned product; New York, 20%; Michigan, 17%; and Washington, 11% (Marketing Northwest Apples, 1992).

18.5.1 APPLES

FOR

PROCESSING

Most all apple cultivars can be used for processing applesauce but only a few are considered ideal. Quality attributes in raw apples that produce a high quality finished product are described by LaBelle (1981). Desirable characteristics in apples for applesauce include high sugar solids, high acid, aromatic, bright golden or white flesh, variable grain or texture, and sufficient water-holding capacity. In the Appalachian region the most important sauce-type apples are York Imperial, Golden Delicious, Jonathan, Stayman, Rome, and Winesap. New York uses primarily Rhode Island Greening, Northern Spy, Twenty Ounce, Cortland, and to a lesser extent, Mutsu and Monroe. In the western states, particularly California, Gravenstein and Yellow Newtown are used, along with Granny Smith and Golden Delicious. McIntosh, though not considered an ideal sauce apple, is used in the Northeast because it is so plentiful. McIntosh is generally blended with three or four

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other cultivars, a common technique used by processors to maintain a uniform product in taste and texture. Processing operations for applesauce and frozen and canned apple slices are summarized in Figure 18.6. A typical apple blend for applesauce might be primarily York (more than 50%) with Golden Delicious and Rome, each contributing a lesser percentage. Nearly 100% of McIntosh sauce is made for the New England market. For many years Northern Spy was a favorite of processors in Michigan and the Northeast but production of this cultivar has decreased significantly in recent years because of poor productivity. Jonathan is more commonly used for processing applesauce in Michigan. Rome is a popular cultivar in a number of regions because the tree yields a heavy crop and because the apple’s shape is well suited to mechanical peeling. However, for applesauce, Rome is less desirable than most cultivars because of poor flesh color (Way and McLellan, 1989). Sauce made with a high percentage of Rome apples will have an off-color and weak, runny texture. Processors in the Appalachian region consider York the ideal processing apple. York has a very firm creamy yellow flesh producing a high quality sauce with grainy texture and good color. The fruit resists bruising and stores exceptionally well, characters favored by processors. York apples have a small core and thus yield a high percentage of processed product when peeled, cored, and trimmed (Rollins, 1989). Golden Delicious, a popular fresh-market apple, is also processed in large quantities. Its high soluble solids and resistance to oxidative browning of the flesh make it attractive for sauce and slicing. Applesauce produced from Goldens in the Pacific Northwest exhibits a runny consistency due to the higher moisture content of the apples grown with extensive irrigation programs. It is often necessary to sweeten this applesauce with dry sugar rather than syrup to improve the consistency. Apples for canned slices must be firm, maintain integrity of the flesh when sliced, and have good color. York, Stayman, Golden Delicious, Northern Spy, Rhode Island Greening, Yellow Newtown, and Jonathan are preferred for making slices. Sweetness is less important in making slices than in sauce. Regardless of whether apples are for sauce or slices the most important factors are fruit quality and maturity. Eastern sound and mature Delicious apples can produce a highquality processed apple slice (Childers, 1983). Huehn (1987) characterized the “ideal” processing apple as: a perfect sphere; 3 in. in diameter with a small core; thin, light-colored skin; firm flesh; pressure test of 89 N October 1st; 67 N about June 1st; from common storage; high, minimum 13∞ Brix; soluble solids; mildly acidic, 0.2 to 0.25%; and of pleasing taste, such as Northern Spy, with a long supple stem strongly attached to the fruiting spur until the day of harvest at which time it could be easily detached. If such an apple existed, processors would probably emphasize freedom from defects, size, and shape in their grade and pricing structure. Apples for applesauce, slices, and other canned products are received and handled by the processor similarly. When a load of fruit is received, a representative sample is taken for grading and testing. The standard tests include flesh firmness, soluble solids (brix), acid, defects, and decay. Processing apples are graded after peeling into categories based on the percentage trim waste and presence of major defects: U.S. No. 1 — less than 5% trim waste; No. 2 — 5 to 12% waste; cider — more than 12% trim waste; and Culls — less than 21/4 in. size and without major defects. Prices paid to the grower are based on grade and size; large fruit commands a premium price. Some processors also downgrade for bruises. Fresh bruises are generally not considered serious because they do not interfere with the finished product; however, if the fruit is stored, bruised tissue becomes corky and may appear as a defect in the finished product. Tests to predict the quality of finished product from raw-product indices have not been too successful (Wiley and Thompson, 1960; Wiley and Toldby, 1960). Apples for processing are dumped in water, blended at dumping, washed, size graded, peeled, cored, inspected for defects, and trimmed before delivery to the designated processing line. See the apple processing flowchart. Automatic peeling and coring machines have replaced the once

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Bin Dumping Fruit Washer Size Grader Peeler/Corer

Applesauce

Juice Eliminators

Apple Slices

Inspection Dicer/Chopper Screw Cooker Pulper/Finisher

[Canned]

[Frozen]

[IQF Slices]

Inspection

Vacuum Tank

Vacuum Tank

Vacuum Tank

Preheater

Blancher

Blancher

IQF Freezing

Piston Filler

Volume Filler

Net Wt Filler

Net Wt Filler

Seamer/Capper

Seamer/Capper

Carton Caser

Carton Caser

Holding/Cooker

Rotary Cooker

Palletizing

Palletizing

Cooler Box

Rotary Cooler

Blast Freezer

Palletizing

Label/Caseer

Palletizing

Palletizing

Storage Whse

FIGURE 18.6 Apple processing flowchart.

common labor intensive hand-fed peelers. Automatic peelers require more uniform sized, firm fruit because soft fruit tends to spin off these peelers. Some processors use sodium hydroxide (NaOH) or potassium hydroxide (KOH), chemical peelers that produce a reduced trim waste. Another method used by some processors relies on high-pressure steam for peeling. Labor shortages and higher production costs have encouraged apple processing plants to become highly automated. Electronics has enabled a number of hand labor tasks to be automated (Cogley, 1976) including defect elimination. The potential for using robotics in several facets of apple processing has been described (Yang and O’Connor, 1984).

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18.5.2 APPLESAUCE Apples previously selected and prepared for sauce are diced or chopped and fed to a stainless steel screw type cooker, either live steam injected or steam jacketed. Sugar, either liquid blend or dry, and other desired ingredients are added into the sauce just before cooking. Liquid sugar is preferred because it imparts a desirable “sheen” to the finished applesauce appearance. Cooking to a temperature of between 93 and 98∞C for about 4 to 5 min softens the fruit tissue and inactivates the polyphenoloxidase that is responsible for enzymatic browning. Time, temperature, and raw product input must be controlled to produce sauce of good texture, color, and consistency. After cooking, applesauce is passed through a pulper with a 0.065- to 0.125-in. “finishing” screen that removes defects and defines texture as smooth or grainy. Large screens produce a more grainy sauce. Baby food sauce is “finished” with fine, 0.033 in. screens to a very smooth texture. The hot applesauce is poured over a flat plastic sheet with back lighting and inspected for defects. Any defects such as specks, peel, blossoms, or stems are removed by hand, using a flexible vacuum tube (Cogley, 1976). The inspected applesauce is preheated to 90∞C and piston-filled into glass jars or metal cans immediately. Applesauce must be closed at a temperature of 88∞C in the seamer or capper. To insure a vacuum in the container, a jet of steam may be passed over the top of the container just prior to sealing. As the steam condenses, a vacuum is created in the container. This step is important in cans to prevent headspace detinning. The containers are held for 1 to 2 min prior to cooling to insure sterilization of the lids or caps. Water is cooled in draper belt, walking beam, or reel coolers to an average of 35 to 40∞C to prevent “stack cooking” in the warehouse. The finished product may be conveyed to the labeler and/or caser prior to palletizing. Alternately, the containers may be conveyed to a palletizing machine where they are “bright stacked,” unlabeled for completing future private label orders. Some processors pack aseptic individual molded plastic single-serving size containers. In addition to regular applesauce, many processors produce specialty products such as natural, no sugar added, “chunky,” cinnamon, or a mixture of applesauce and other fruit such as apricot, peach, or cherry.

18.5.3 SLICED APPLES The dumping, washing, grading, peeling, and coring steps for processing apple slices are similar to those used for sauce production with a few notable exceptions. Slice packs generally consist of a single cultivar, thus eliminating the need for blending. Apple slice texture is very important. Therefore, apples with firm flesh and high quality are desired. Consistency of slice size can be controlled by using fruit from within a preselected size range. The slicing operation is usually an integral part of the peeling and coring process where the apples are sliced into 12 to 16 pieces in the coring section. After slicing, the apples are inspected for defects such as blossom or calyx, carpel tissue, skin, and bruises and are conveyed over a shaker screen to remove small chips. The slices must be handled quickly at this point to avoid enzymatic browning. Apple slices contain about 25% occluded oxygen that is removed by vacuum treatment. The apple slices are placed in a vessel that is sealed and a 27- to 29-in. Hg vacuum drawn. The vacuum is broken by the injection of water, salt, ascorbic acid, and/or sugar. The apple slices are then steam blanched to soften and to allow specified container fill. Several automated systems to vacuum treat and blanch apple slices have been described (Ellett, 1968 and Keifer, 1963). From the blancher, the slices are filled hot, 77 to 82∞C, into cans (sizes 303, 21/2, or No. 10). The slices are normally over-filled into cans from premeasuring pockets. An automatic plunger gently pushes the over-fill volume into the can. The cans are closed with a steam-vacuum process after adding hot water or syrup to insure there is no entrapped air. Some processors use the steam flow closing method. A jet of live steam is passed over the top of the can immediately prior to applying the lid to insure a vacuum will be developed.

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The canned apples must be processed immediately after closing to a can center temperature of 82.2∞C; there are several types of sterilizers available, Batch retort vessels, and continuous rotary cookers that operate either at atmospheric or pressurized conditions. Immediately after sterilizing, the cans should be cooled at 37 to 40∞C to prevent “stack burn” or loss of product color in storage. Refrigerated, frozen, or dehydrofrozen apple slices, representing only about 15% of the apples processed, are prepared much like canned apple slices except they are not heat processed. To prevent enzymatic browning, sliced raw product is subjected to one of several available antibrowning treatments. Apple slices to be bulk frozen are vacuum treated and blanched in the same manner as canned apple slices. From the blancher, the apples are filled into 30-lb tins or poly-lined boxes by an automatic particulate net weight filler. The tins or boxes are then sealed, frozen and stored frozen at –17∞C or lower. Individually quick frozen (IQF) apple slices are usually treated with a sodium bisulfate bath after inspection. The slices are then filled into vacuum tanks where the vacuum is pulled and broken with a brine or ascorbic acid solution. From the vacuum tank, the apples pass through an IQF unit where the slices are individually frozen. Various freezing mediums can be used: the apple slices are subjected to nitrogen (N2) or carbon dioxide (CO2) on a metal draper type belt. The freezing air is forced upward through a perforated tray that fluidizes the product, plus acts as a freezing medium. From the freezing unit, the slices are filled into tins or poly-lined boxes and stored frozen at –17∞C or below. Dehydrofrozen apple slices are dehydrated and frozen to less than 50% of their original weight and volume. The dehydrofrozen slices are packed in cardboard containers or large metal cans with polyethylene liners and rapidly frozen in forced-air freezers before storing. Frozen slices are thawed then soaked in a combined solution of sugar, CaCl2, and ascorbic acid or SO2. The advantage in processed dehydrofrozen slices over regular frozen slices have been noted by Hall (Hall, 1989b). Fresh and refrigerated sliced apples are desired by many bakeries in the manufacture of their products. From the slicing and inspection operations, the apple slices are normally treated with 0.2 to 0.4% SO2 alone or in combination with 0.1 to 0.2% CaCl2. Ascorbic acid has been substituted for SO2 with good results (Ponting et al., 1972). Calcium-treated apples appear to resist enzymatic browning and microbial spoilage better than non-Ca treated slices (Hall, 1989b). Fresh slices, if blanched, will resist browning up to 48 h; however, blanching does result in loss of sugar, acid, and flavor that can produce a blander product. The treated slices are passed over a shaker screen and packed into 30-lb poly-lined boxes for shipment. This type of product is usually shipped and used in a very short period of time. Apple-pie filling is another preparation of apple slices. Varieties preferred in Michigan for good quality pie filling are Ida Red, Jonathon, Empire, Spy, and York of medium-firmness, with 12 to 16 pressure test. As given previously in this section, the apples are selected, peeled, cored, and sliced into 12 or 16 segments. The slices are vacuum treated in a brine solution to inhibit polyphenoloxidase enzyme activity that causes browning. The treated slices are filled into containers by volume. A precooked slurry mixture of water, corn syrup or sugar, starch, and spice is poured into the cans and rapidly occupies any air spaces in the container. Precooking of the slurry activates the starch, causing it to gel or set slightly as it cools. The container is closed and is conveyed to the retort cooker where it is cooked to render it commercially sterile, to tenderize the apples, and to set the starch slurry. The containers are cooled to about 37∞C to allow evaporation of the water from the container and to avoid any continued cooking. The cooled containers are either labeled and cased or “bright” stacked on pallets to be labeled later.

18.6 DRIED APPLE PRODUCTS Drying has been used for centuries to preserve food products. Dried apples are convenient to handle, store, and use (Somogyi and Luh, 1986). Under proper storage conditions they are almost immune to spoilage.

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Dried apple products are prepared from sound, properly ripened fruit that has been peeled, sorted, trimmed, and cut into the desired piece size prior to drying. Most good processing cultivars are acceptable for drying. A desirable characteristic in apples for drying is a high sugar–water ratio (Smock and Neubert, 1950). Delicious apples, either Golden or Red, are generally recognized by industry as superior for drying. Sulfur dioxide (SO2) is the primary agent used to control enzyme activity and preserve the color of dried apple tissue. A number of factors affect drying including size and geometry of pieces, temperature, humidity, air velocity and pressure within the drier, and wet-bulb depression. Hall (1989a) has presented a detailed description of equipment and methods for drying apples. Evaporated apples and dehydrated apples are the two types of dried apple products recognized under U.S. standards. Evaporated apples, also called regular moisture and dried, are cut to desired size and dried to average not more than 24% moisture by weight. Evaporated apples are either cut to rings, pie pieces, or dices prior to drying; “fresh cut”; or sliced to rings then dried to 24% moisture prior to cutting to the desired “dry cut” dimensions. Unsulfured evaporated apples should average not more than 20% moisture. Packaging is usually in fiber board boxes of 40 lb net weight. Evaporated apples can be stored for short periods of time, less than three months, at ambient, room temperatures in a dry atmosphere. For prolonged storage, 7∞C or less is required. Unsulfured evaporated apples require 4 to 5∞C cold storage. The end usage, process, size, and style of cut will dictate the correct reconstitution ratio. Evaporated apples will generally fully reconstitute with one part apple in five parts water by weight. Dehydrated apples, also called low moisture apples, are cut to desired sizes, pie pieces, dices, flakes, or granules prior to drying to not more than 31/2% moisture by weight. A variation of this is a flake powder prepared from pureed, sieved applesauce then dried to 31/2% moisture on a rotary drum drier. Only 300 ppm maximum SO2 is necessary to prevent color deterioration in apple-flake powders. To prevent caking, 0.5% maximum calcium stearate may be added. Packaging is generally in fiber board boxes with a net weight from 15 to 40 lb, depending upon the product density. Dehydrated apples should be stored in a cool, less than room temperature, dry atmosphere. The end usage, process, size, and style of cut will dictate the correct reconstitution ratio. Dehydrated apples will generally fully reconstitute with one part apple in six parts water by weight. Maximum allowable SO2 level in dried apple products in the U.S. is 1000 ppm; maximum 500 ppm is allowed in the European Community (EC). A unique method for producing dehydrated apples is “explosion puffing,” developed by the USDA, Agricultural Research Service. In this process, partially dehydrated apple pieces are heated in a closed rotating cylindrical container called a “gun” until the internal pressure has reached a predetermined value. At this point the gun is discharged instantly to atmospheric pressure producing a highly porous piece of apple tissue. For more details see Eisenhardt et al. (1964) and Sullivan et al. (1980). Evaporated and dehydrated apples are used in many baking, cereal, and snack applications. Low moisture apples also make an excellent substitute for other fruit and berries in dry products. The neutral flavored low moisture apple is color dyed then impregnated with the desired fruit or berry flavor. This apple product has gained wide acceptance in the breakfast cereal industry around the world.

18.7 SPECIALTY APPLE PRODUCTS Specialty apple products usually require more time and hand labor than applesauce or apple slice products. Less than 1% of the processed apple volume is in this specialty category. Examples of specialty apple products are whole baked or glazed apples, spiced apple rings, spiced crabapples, apple butter, and apple jelly. Baked and glazed apples require large, 23/4 to 3 in. firm, symmetrical fruit such as Rhode Island Greening, Rome, or Stayman. These apples are cored, partially peeled, and baked at 176.6∞C either

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by the short method in the can or by the long method before canning (Wiley and Binkley, 1989). A 40 to 50∞ Brix syrup is used as a cooking or filling media. The canned, sealed product should be processed to a center temperature of 87.7 to 90.5∞C, then cooled to about 37∞C. Spiced apple rings are used as a garnishment. The apples are cored and sliced on the apple peeler to about 3/8 in. thick, and then smaller rings and end pieces are sorted out, prior to blanching, to remove the air. The apple slices are filled into jars and covered with a hot, 40 to 42∞ Brix seasoned syrup and processed at 87.7∞C for 20 to 30 min, then cooled. A typical syrup formula for apple rings: to 80 gal of hot water in a steam kettle, add 7 to 8 oz of coloring (either FD&C 90% Lime Green Shade or FD&C Red 40); mix the color well and add 400 lb of sugar; add additional water to a volume of 100 gal and heat to 87.7∞C; and finally add either peppermint or cinnamon flavoring. The flavoring mix can be obtained from any of the spice or flavor manufacturers. Apple butter is processed much like applesauce except a slower batch cooking in swept-surface, steam-jacketed kettles is used to produce a thicker, carmelized, more stable product. Fresh, whole, small apples are usually used but “tailings” (peel waste) and lower quality fruit may also be used in making pulp for apple butter. More sugar is used in this process than in applesauce. A typical apple butter formula: to 100 gal of apple pulp, add 30 gal of 44∞ Brix apple concentrate, 150 lb of sugar and spice with 8 oz of ground cinnamon, 4 oz of ground cloves, and 4 oz of ground allspice. The final cooked down product should be about 45% solids. Fill into containers as with applesauce. Apple jelly is made from apple juice concentrate. Federal regulations dictate the amount of fruit solids required. When concentrate is used, it is necessary to use sufficient concentrate to provide the amount of apple solids normally obtained from single strength juice. Example: for 165 lb of a 65% soluable solids “45 to 55” apple jelly, the basic formula would be 15.6 lb of 70∞ Brix apple juice concentrate, about 8 gal or 65 lb of water, 12 oz of 150 grade citrus pectin (slow set), and 100 lb of sugar. Adjust the pH to between 3.0 and 3.2 with a citric acid solution. The pectin should be dispersed in water. The water may be adjusted so very little cooking is needed to reach the desired jelly soluable solids.

18.8 QUALITY CONTROL Good process quality programs are essential to provide assurance that a safe, sound, wholesome product is shipped to the consumer. These programs can provide both financial and other intangible benefits, such as improving operating efficiencies and reducing waste. Quality control is maintained throughout processing, beginning with information on growers’ pesticide programs and maturity of fruit, then blending as it relates to finished product specifications, on-line measurements such as trim and coring efficiency, filling volumes, and processing and cooling temperatures. Finally, there is the container condition for consideration, including closing, headspace, and labeling quality. Finished products are examined and tested to insure commercial sterility and buyers’ specifications, plus maintenance of the U.S. standards for grades of canned apple products, when applicable. The microbiology of apple products is generally restricted to yeasts, molds, and aciduric bacteria capable of growth at the low pH of apple products (Swanson, 1989). The previously mentioned micotoxin, patulin, can be avoided by using only whole, clean, sound fruit that has been carefully handled. The microorganisms found in apples are heat sensitive and usually destroyed when recommended processing times and temperatures are attained. Viability is further restricted by the reduced water activity of apple juice concentration and apple dehydration. To conform to FDA standards, products should be filled to not less than 90% of the overflow capacity of the container. One exception is glass containers with an over-flow capacity of 61/2 fluid ounces or less, where the fill is not less than 85%. For a more complete description of the FDA Standards of Identity, Quality and Fill or the USDA Grade Standards, refer to the Almanac of the Canning, Freezing, and Preserving Industries.

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There are several approaches to quality management available today. Statistical Quality Control (SQC), Total Quality Management (TQM), Hazard Analysis and Critical Control Points (HACCP), and, in international trade, International Organization for Standardization (ISO or ISO 9000).

18.9 NUTRITIONAL VALUE OF APPLES Almost everyone has heard the expression “An apple a day keeps the doctor away,” reflecting the notion that apples and apple products are nutritious. Fresh apples are considered moderate in energy value and low in protein, lipid, and vitamin content. There are about 242 cal/lb of apples as purchased. Carbohydrates are the principal nutrient of apples and apples are a good source of dietary fiber, while low in fat and sodium. It is evident from Table 18.5 that apples and apple products are sources of potassium, phosphorus, calcium, vitamin A, and ascorbic acid. Fructose, sucrose, and glucose are the most abundant sugars. The nutritive value of most processed apple products is similar to the fresh raw product. Dried or dehydrated apples have a higher energy value per gram tissue due to the concentration of sugars (Lee and Mattick, 1989). The USDA tabulated in Composition of Foods, Handbook No. 8, 1975, that apples are about 84.5% water, 1% fiber, 14.5% carbohydrates, 0.6% fat, and 0.2% protein. In recent years there has been growing interest in the presence of polyphenolic antioxidants in various fruit and vegetable crops. Apples are a rich source of these beneficial phytonutrients that epidemiological studies have found to be associated with protection against aging diseases and cancers. Two recent publications (van der Sluis et al., 2001; van der Sluis et al., 2002) have highlighted the effects of apple cultivar, harvest year, storage conditions and apple-juice processing methods on the concentration of polyphenolics. Four apple cultivars (Jonagold, Golden Delicious, Cox’s Orange, and Elstar) that can be utilized either fresh or as processed products were compared with regard to flavonol, catechins, phloridzin, and chlorogenic acid concentrations and antioxidant activity. Jonagold apples had both the highest polyphenolic concentration and antioxidant activity. There were no differences related to season in the 3-year study nor did long term storage under refrigerated air or controlled atmospheres affect either polyphenolic concentration or antioxidant activity. Juice produced from Jonagold apples by either pulping or straight pressing had a significantly lower level of both polyphenolics and antioxidant activity. Polyphenolic levels were reduced to between 50% (chlorogenic acid) and 3% (catechins) of the concentrations in fresh apples. Antioxidant activity was reduced to only 10 to 13% of that in fresh apples by the juice-making process. It was determined that most of the polyphenolic antioxidants were retained in the pomace or press cake and were not extracted into the juice. These results have ramifications for apple juice processors interested in producing juice with higher nutritional value. It may be of interest to either market cloudy apple juice as a superior product or at least to utilize the pomace as a source of polyphenolic antioxidants.

ACKNOWLEDGMENTS The authors wish to acknowledge the contribution of Max Williams, Stephen R. Drake, and Stephen Miller, USDA ARS, whose unpublished 1986 paper was the outline for this chapter. Also we would like to thank Robert Dennis, Tree Top (retired), for his technical assistance, suggestions, and constructive criticism.

Raw Fresh Applesaucea Unsweet Apple juice Frozen sliceda Applebuttera Dried, 24% Dehydrated 2% 0.8 0.9 0.9 0.5 0.9 1.8 4.5 6.4

Protein (grams) 2.5 0.5 0.9 0.1 0.5 3.6 7.3 9.1

Fat (grams) 60.5 108.0 49.0 54.0 110.2 212.3 325.7 417.8

Carbohydrate (grams) 29 18 18 27 23 64 141 181

Calcium (mgm) 42 23 23 41 27 163 236 299

Phosphorus (mgm) 1.3 2.3 2.3 2.7 2.3 3.2 7.3 9.1

Iron (mgm) 4 9 9 5 64 9 23 32

Sodium (mgm) 459 295 354 458 308 1143 2,581 3,311

Potassium (mgm) 380 180 180 — 80 0 — —

Vitamin A value (IU)

Source: USFDA, Composition of Foods, raw, processed, prepared, in Agriculture Handbook No. 8. U.S. Department of Agriculture. 1975.

With sugar.

242 413 186 213 422 844 1,247 1,601

Apples

a

Food Energy (calories)

TABLE 18.5 Nutrients in the Edible Portion of 1 lb of Fruit as Purchased

0.12 0.08 0.08 0.03 0.05 0.05 0.26 0.02

Thiamin (mgm)

0.08 0.05 0.05 0.07 0.14 0.09 0.53 0.26

Riboflavin (mgm)

0.3 0.2 0.2 0.4 1.0 0.7 2.3 2.9

Niacin (mgm)

16 5 5 4 33 9 48 47

Ascorbic acid (mgm)

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REFERENCES Anon., Cider. National Association of Cider Makers, NACM, Dorchester, Dorset, U.K. 1980. Anon., Establishing and Managing Young Apple Orchards. U.S. Deptartment Agriculture, Farmers’ Bulletin No. 1897. 1972. Beach, S. A. et al., The Apples of New York. Brunische-Olsen, H., A method of extraction by diffusion technique. Proceedings of the International Federation of Fruit Juice Products. 1969. Buckner, L. R., Apple & Pear Variety Production and Planting Trends in Washington State, Tree Top, 1994. Bultitude, J., Apples, A Guide to the Identification of International Varieties. University of Washington Press, Seattle, WA. 1983. Childers, N. F., Modern Fruit Science. Horticultural Publications, Gainesville, FL. 1983. Cogley, J. R., Method for Producing a Pome Fruit Sauce with Electronic Inspection of Diced Fruit. U.S. Patent 3,950,522. April 13, 1976. Composition of foods, raw, processed, prepared, in Agriculture Handbook No. 8. U.S. Department of Agriculture. 1975. Downing, D. L., Ed., Processed Apple Products. Van Nostrand Reinhold, New York. 1989. Drake, S. R., Nelson, J. W., and Rowers, J. R., The influence of controlled atmosphere storage and processing conditions on the quality of applesauce from ‘Golden Delicious’ apples. J. Am. Soc. Hortic. Sci., 104: 68–70. 1979. Eisenhardt, N. H., Eskew, R. K., and Cording, J., Jr., Explosion puffing applied to apples and blueberries. Food Eng., 36(6): 53–55. 1964. Ellett, A. S., Process and apparatus for continuous deaeration of fruits. Food Process. Rev., 21: 93–99. 1968. FAO Production Yearbook, Vol. 45, Food and Agriculture Organization of the United Nations. 1991. Flath, R. A., Black, D. R., Guadagni, D. G., McFadden, W. H., and Schultz, T. R., J. Agric. Food Chem., 1967. Food Institute Report. A weekly newsletter published by American Institute of Food Distribution. Forshey, C. G., Cultural Practices in the Bearing Apple Orchard. New York State Agricultural Experiment Station Information Bulletin No. 160. 1980. Forshey, C. G., Elfving, D. C., and Lawrence, R. T., The Planting and Early Care of the Apple Orchard. New York State Agricultural Experiment Station Information Bulletin No. 65. 1981. Hall, G. C., Dried apple products, in Processed Apple Products. Van Nostrand Reinhold, New York.1989a. pp. 257–278. Hall, G. C., Refrigerated, frozen, and dehydrofrozen apples, in Processed Apple Products. Van Nostrand Reinhold, New York. 1989b. Harper, J. K. and Greene, G. M., Fruit quality characteristics influence prices received for processing apples. HortScience, 28(11): 1125–1128. 1993. Heinicke, D. R., High-density apple orchards — planning, training, and pruning, in Handbook No. 458. U.S. Deptartment of Agriculture, 1975. Huehn, W. G., Processed Apple Industry Overview. Memo 9 p. National Fruit Products Co., Winchester, VA. 1987. Keifer, W. L., Process for the blanching of apples. Food Process. Rev., 21: 77–79. 1963. Lord, W. J. and Costante, J., Establishing and Management of Compact Apple Trees. Univ. Mass. Coop. Ext. Service. Pub. C-102. 1977. LaBelle, R. L., Apple quality characteristics as related to various processed products. Quality of Selected Fruits and Vegetables of North America, American Chemical Society, Washington, D.C. pp. 61–76. 1981. Lee, C. Y. and Mattick, L. R., Composition and nutritive value of apple products, in Processed Apple Products, Van Nostrand Reinhold, New York. 1989. Marketing Northwest Apples, 1992 Crop. USDA Agriculture Marketing Service, Fruit and Vegetable Division, Market News Branch. 1992. Miller, R. R., Competitive fruit situation. AppleNews 19(3): 5–8. 1988. Milleville, H. P. and Eskew, R. K., Recovery and Utilization of Natural Apple Flavors, Fruit Prod. J. Am. Food Manuf., pp. 48–51. 1944. Nelson, P. E. and Tresler, D. K., Fruit and Vegetable Juice Processing Technology. 3rd ed., AVI Publishing, Westport, CT. 1980.

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Ponting, J. D., Jackson, R., and Watters, G., Refrigerated apple slices. II. Preservative effects of ascorbic acid, calcium, and sulfites. J. Food Sci., 37: 434–436. 1972. Rollins, H. A., Jr., The “York Imperial” apple. Fruit Var. J., 43(1): 2–3. 1989. Smock, R. M. and Neubert, A. M., Apples and Apple Products. Interscience Publishers, New York. 1950. Somogyi, L. P. and Luh, B. S., Dehydration of fruits, in Commercial Fruit Processing, 2nd ed. AVI Publishing, Westport, CT. 1986. Sullivan, J. F., Craig, J. C., Jr., Konstance, R. P., Egoville, E. M., and Aceto, N. C., Continuous explosion puffing of apples. J. Food Sci., 45: 1550–55, 1558. 1980. Swales, J. E., Commercial Apple-Growing in British Columbia, Minister of Agriculture, Victoria, B.C. 1971. Swanson, K. M., J., Microbiology and preservation, in Processed Apple Products. Van Nostrand Reinhold, New York. 1989. Tukey, H. B., Dwarfed Fruit Trees. Macmillan, New York. 1964. United States Department of Agriculture, Foreign Agricultural Service. World Apple Situation: Acreage Continues to Decrease in Major Producing and Trading Countries, March 2003. http://www.fas.usda.gov/htp/circular/2003/3-7-03%20Web%20Art.%20Updates/World%20Apple%20 Situation%202002-03.pdf United States Department of Agriculture, National Agricultural Statistics Service. U.S. Apple Situation, March 2003. http://www.fas.usda.gov/htp/circular/2003/9-12-03%20Web%20Art%20Updates/2003%20US Apple%208-23-03.pdf Upshall, W. H., North American Apples: Varieties, Rootstocks, Outlook. Michigan State University Press, East Lansing, MI. 1970. van der Sluis, A. A., Dekker, M., de Jager, A., and Jongen, W. M. F., Activity and concentration of polyphenolic antioxidants in apple: effect of cultivar, harvest year, and storage conditions. J. Agric. Food Chem., 49: 3606–3613, 2001. van der Sluis, A. A., Dekker, M., Skrede, G., and Jongen, W. M. F., Activity and concentration of polyphenolic antioxidants in apple juice: 1. Effect of existing production methods. J. Agric. Food Chem., 50: 7211–7219, 2002. Way, R. D. and McLellan, M. R., Apple cultivars for processing, in Processed Apple Products. Van Nostrand Reinhold, New York. pp. 1–29. 1989. Westwood, M. N., Temperate-zone Pomology. W.H. Freeman, San Francisco. 1978. Wiley, R. C. and Binkley, C. R., Applesauce and other canned apple products, in Processed Apple Products. Van Nostrand Reinhold, New York. NY. 1989. Wiley, R. C. and Thompson, A. H., Influence of variety, storage, and maturity on the quality of canned apple slices. Proceedings American Society for Horticultural Science. 75: 61–84. 1960. Wiley, R. C. and Toldby, V., Factors affecting the quality of canned apple sauce. Proceedings American Society for Horticultural Science. 76: 112–123. 1960. World Horticultural Trade and U.S. Export Opportunities, United States Department of Agriculture, Foreign Agriculture Service, p. 17. November, 1994. Yang, J. and O’Connor, T., Possible application of robotics in apple processing, Proceedings of Processed Apples Institute Research Seminar, University of Maryland, College Park, MD. 1984.

19 Peach and Apricot Ralph Scorza CONTENTS 19.1 19.2 19.3

Introduction ........................................................................................................................481 Breeding .............................................................................................................................482 Breeding Goals...................................................................................................................484 19.3.1 Disease and Pest Resistance................................................................................484 19.3.2 Environmental Stress Tolerance ..........................................................................487 19.3.3 Growth Control ....................................................................................................487 19.3.4 Fruit Characteristics.............................................................................................488 19.3.5 New Breeding Techniques...................................................................................488 19.4 Horticulture ........................................................................................................................489 19.4.1 Propagation ..........................................................................................................489 19.4.2 Orchard Establishment ........................................................................................489 19.4.3 Pruning and Tree Training...................................................................................489 19.4.4 Flowering and Fruiting ........................................................................................490 19.4.5 Maturity Indices...................................................................................................491 19.5 Fruit Quality Factors ..........................................................................................................491 19.5.1 Flesh Texture........................................................................................................491 19.5.2 Flesh Adhesion ....................................................................................................492 19.5.3 Flesh Color ..........................................................................................................492 References ......................................................................................................................................493

19.1 INTRODUCTION The genus prunus within the family Rosaceae encompasses a large number of fruit tree species bearing “stone fruits” in which the seed is encased within a hard, lignified endocarp referred to as the “stone.” The edible portion of the stone fruit is the juicy mesocarp. The major commercial stone fruit species are P. persica (L.) Batsch (peach, nectarine), P. domestica L. (European or prune plum), P. salicina Lindl. (Japanese plum), P. cerasus L. (sour cherry), P. avium L. (sweet cherry), and P. armeniaca L. (apricot). The almond (P. amygdalus Batsch) is a prunus species cultivated for its edible seed, the mesocarp being dry, leathery, and inedible. All commercial peaches are P. persica. This species includes the nectarine, which differs from the peach in the absence of pubescence on the fruit surface, this character being controlled by a single gene or a group of tightly linked genes. The apricot grown for commerce in much of the world is P. armeniaca. Other closely related species that are cultivated for more limited production or local consumption include P. armeniaca var. ansu, P. armeniaca var. holosericea Batal., P. sibirica L., P. mandshurica (Maxim.) Koehne, P. mume (Sieb.) Sieb. et Zucc., and P. X dasycarpa Ehrh. Several of these species such as P. armeniaca var. ansu and P. mume are grown for processing into syrup, jam, pickles, or liquor (Mehlenbacher et al., 1990). The haploid number of chromosomes

0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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of both peach and apricot is eight. Peaches and apricots are not cross compatible but crosses between apricot varieties and species are compatible. Interspecific hybridization in Prunus, especially between cultivated and noncultivated species, is only variably successful. Hybrid progeny are usually sterile (Layne and Sherman, 1986). While no peach cultivars have been developed through interspecific hybridization, hybrids of apricot with plum that are called pluots (>50% plum) and apriums (> 50% apricot), have been produced and successfully marketed (Frecon, 1996). Peaches and apricots are both native to China, the peach having been cultivated for over 4000 years (Hesse, 1975). Both species followed westward human migration through trade routes and in the wake of conquering armies, and found their way into Greece. It appears that Alexander the Great brought the apricot from Armenia to Greece (Cullinan, 1937). According to Pliny, the peach was cultivated in Greece by 332 B.C. (Hedrick, 1917). From Greece these fruits spread throughout the Roman Empire. Mention of apricots and peaches can be found in the writings of Pliny, Dioscorides, and Virgil (Hedrick, 1917; Cullinan, 1937). From Europe, apricots, and more commonly, peaches, spread to North and South America on the ships of the explorers and settlers. The very fact that peaches and apricots, as well as all other stone fruit species, have their seed encased in a “stone” allowed for their distribution over long distances without the need for special storage conditions. Peach and apricot seeds will retain their viability for a year at room temperature and for several years if refrigerated. Peaches and apricots are now grown on all continents excluding Antarctica. Commercial production generally lies between latitudes 30∞ and 45∞ north and south. Extreme cold temperatures below –35∞C to –40∞C or the absence of a cold enough temperature to satisfy dormancy requirements are the major limiting factors for commercial production. Major producing countries are listed on Tables 19.1 and Table 19.2.

19.2 BREEDING The peach and apricot cultivars that are the bases for commercial production worldwide represent only a small fraction of the genetic diversity in these two species (Scorza et al., 1985; Scorza and Okie, 1990; Mehlenbacher et al., 1990; Scorza and Sherman, 1996). The loss of tree fruit germplasm is occurring at an alarming rate as urbanization, deforestation, and the worldwide need for firewood accelerates. The cost of maintaining large collections of trees and the long-term nature of tree fruitbreeding programs has discouraged germplasm collection and maintenance in many developing countries. Furthermore, Western stone fruit cultivars, which have a restricted germplasm base but high fruit quality, productivity, and generally superior handling characteristics, usually replace land race cultivars that are adapted to local biotic and abiotic stress factors. Most major producing countries have active peach and apricot breeding programs. The development of a new peach or apricot cultivar is generally a 15 to 20-year process. The breeding process includes: 1. Pollen collection from desired male parents. 2. Individual hand emasculation of flowers of female parents for self-compatible types. (The peach is a self-compatible species while apricots may be self-compatible or selfincompatible, depending on the particular genotype.) 3. Hand pollination. 4. Collection of seed from fruit that developed from hybridization. 5. Seed stratification and germination. 6. Greenhouse or nursery culture of seedlings. 7. Field planting of the seedlings. 8. Selection of superior phenotypes.

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TABLE 19.1 World Production of Peaches and Nectarines Peaches and Nectarines Production (Mt) World Afghanistan Albania Algeria Argentina Armenia Australia Austria Azerbaijan, Republic of Bolivia Bosnia and Herzegovina Brazil Bulgaria Cameroon Canada Chile China Cyprus Czech Republic Ecuador Egypt France Georgia Germany Greece Hungary India Iran, Islamic Republic of Iraq Israel Italy Japan Jordan Kazakhstan Korea, Democratic People’s Republic Korea, Republic of Kyrgyzstan Lebanon Libyan Arab Jamahiriya Macedonia (the former Yugoslavian Republic) Madagascar Mexico Moldova, Republic of Morocco New Zealand Pakistan Paraguay Peru

Year 2002 13,413,343 14,000 6,100 60,000 252,263 12,000 90,000 6,000 42,000 36,500 3,500 184,000 45,000 400 32,000 249,400 4,224,267 2,800 5,500 19,977 256,997 483,000 10,000 14,100 667,000 55,000 120,000 270,000 20,000 49,760 1,700,000 175,800 12,000 2,500 110,000 175,000 4,800 52,500 9,500 4,600 7,500 153,336 22,000 46,130 11,015 40,984 1,250 26,466

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TABLE 19.1 (Continued) World Production of Peaches and Nectarines Peaches and Nectarines Production (Mt) Portugal Romania Russian Federation Réunion Slovakia Slovenia South Africa Spain Switzerland Syrian Arab Republic Tajikistan Tunisia Turkey Turkmenistan Ukraine United States of America Uruguay Uzbekistan Venezuela Yemen Yugoslavia, Federal Republic of Zimbabwe

Year 2002 85,000 12,000 32,000 500 2,000 5,300 208,690 1,215,200 360 32,750 18,000 82,000 450,000 4,000 25,000 1,355,050 13,682 45,000 8,500 2,096 19,500 750

Source: United Nations Food and Agricultural Organization website.

The juvenility period in peach and apricot is from 3 to 5 years (Sherman and Lyrene, 1983). The large land areas necessary for growing segregating seedling populations and the labor involved in caring for these trees until fruiting represent a considerable investment. Methods used or proposed for reducing the time and space required for seedling evaluation include the use of high density fruiting nurseries, cultural manipulation (i.e., grafting seedlings onto mature rootstocks), girdling, growth regulator treatments, and breeding with dwarf germplasm (Hansche, 1990; Hansche and Beres, 1980; Sherman and Lyrene, 1983). While many commercial peach and apricot cultivars have been developed from a restricted germplasm base, they remain heterozygous for many loci, and cross-hybridization or selfing usually produces a phenotypically variable population of offspring. The effects of heterozygosity are most evident when cultivated and noncultivated forms are hybridized. In most cases the characteristics that are desirable for commercial cultivars, including large fruit size, high coloration of the fruit epidermis, and firmness of the flesh, are recessive (Bailey and French, 1949). Additional generations of hybridization and selection are required to produce commercial quality fruit with adaptive traits obtained from noncommercial genotypes.

19.3 BREEDING GOALS 19.3.1 DISEASE

AND

PEST RESISTANCE

Peaches and apricots are susceptible to numerous pathogens and pests (Bailey and Hough, 1975; Hesse, 1975; Mehlenbacher et al., 1990; Scorza and Okie, 1990; USDA, 1976). Although some

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TABLE 19.2 World Production of Apricots Apricots Production (Mt)

Year 2002

World Afghanistan Albania Algeria Argentina Armenia Australia Austria Azerbaijan, Republic of Bosnia and Herzegovina Bulgaria Cameroon Canada Chile China Cyprus Czech Republic Ecuador Egypt France Georgia Germany Greece Guadeloupe Hungary India Iran, Islamic Republic of Iraq Israel Italy Jordan Kazakhstan Kyrgyzstan Lebanon Libyan Arab Jamahiriya Macedonia (the former Yugoslavian Republic) Madagascar Mexico Moldova, Republic of Morocco New Zealand Pakistan Peru Portugal Romania Russian Federation Slovakia Slovenia

2,738,601 37,500 1,700 60,000 19,500 7,500 20,000 6,000 14,000 493 12,000 400 1,700 44,000 85,956 1,900 1,200 280 70,765 186,000 1,500 6,000 80,000 20 15,000 9,900 282,890 22,000 9,141 200,000 3,100 5,500 11,900 66,000 16,500 2,600 1,200 1,900 5,100 104,350 7,000 126,404 165 6,000 25,000 70,000 1,800 290

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TABLE 19.2 (Continued) World Production of Apricots Apricots Production (Mt) South Africa Spain Switzerland Syrian Arab Republic Tajikistan Tunisia Turkey Turkmenistan Ukraine United States of America Uzbekistan West Bank Yemen Yugoslavia, Fed Rep of Zimbabwe

Year 2002 61,488 121,800 7,100 86,015 20,000 25,000 580,000 13,000 50,000 81,370 17,000 700 6,648 15,496 30

Source: United Nations Food and Agricultural Organization website.

pathogens are common to both species, others are limited in their host range. Several book chapters and review articles have summarized the most important disease problems and have discussed breeding strategies and programs aimed at obtaining disease resistance (Bailey and Hough, 1975; Childers and Sherman, 1988; Hesse, 1975; Layne and Sherman, 1986; Okie et al., 1985; Scorza, 1991; Scorza and Sherman, 1996). Interest in breeding for disease and pest resistance has increased in recent years due to the concern over (1) the cost of agricultural chemicals, (2) the potential negative impact of chemicals upon humans and animals, and (3) genetic uniformity as the basis of vulnerability to epiphytotics. Peach genotypes have been screened for resistance or tolerance to ring nematode (Criconemella xenoplax) (Okie et al., 1987), a primary factor in peach tree short life syndrome (PTSL); Cytospora canker caused by Leucostoma spp. (Chang et al., 1989; Scorza and Pusey, 1984); and brown rot (Monilinia fructicola) (Gradziel and Wang, 1993). These studies have revealed somewhat low but potentially useful levels of disease resistance. Other studies examining the response of numerous peach and nectarine cultivars to Stigmina carpophila, Monilinia laxa, Sphaerotheca pannosa, Tranzschelia pruni-spinosae, Taphrina deformans and Xanthomonas campestris pv. pruni (Scorza, 1992; Simeone, 1985; Simeone and Corazza, 1987; Werner et al., 1986) found most cultivars susceptible to these pathogens. The existence of locally adapted, seed propagated apricot populations that have been selected in areas of disease pressure have produced high levels of resistance to certain diseases. Mehlenbacher et al. (1990) and Simeone (1982) present a comprehensive overview of resistance of apricot varieties to the major apricot diseases that include brown rot (Monilinia laxa [Aderh. and Ruhl.]) and M. fructicola (Aderh. and Ruhl.), bacterial spot (X. campestris pv. pruni), Pseudomonas canker (Pseudomonas syringae van Hall), plum pox virus, Cytospora canker (Leucostoma spp.), and shothole (Stigmina carpophila [Lev.] Ell.). Major pests of peach and apricot include insects that attack fruit, causing malformation, early fruit drop, or other injury rendering them unmarketable; insects that feed on foliage, weakening trees; insects that bore through twigs, stems, and trunk, debilitating trees; and virus vectors. Only

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a few cases of insect resistance in cultivated genotypes have been reported (Mehlenbacher et al., 1990; Scorza and Okie, 1990). Nematode feeding weakens trees and can cause severe decline syndromes such as “replant problem” and PTSL. Tomato ringspot virus, a serious disease in many peach and apricot growing regions, particularly the U.S. mid-Atlantic states, is spread by the dagger nematode (Xiphenema spp.). All peach rootstocks are susceptible to Xiphenema. Marianna (P.cerasifera x P. munsoniana), a rootstock that may be used for apricot but not peach, is resistant to the virus (Haldbrendt et al., 1991). Resistance to rootknot nematodes (Meloidogyne spp.) has been relatively easy to isolate in peach rootstocks. Apricots and some peaches appear to be tolerant to Pratylenchus spp., which are associated with replant problems (Scorza and Okie, 1990). Some tolerance of ring nematode, a primary factor in PTSL, has been reported in peach (Okie et al., 1987). There are a number of virus diseases that affect peaches and apricots. Some have little noticeable effect on production and fruit quality and some cause devastating losses (USDA, 1976). Plum pox or Sharka is considered to be the most serious stone fruit virus. Apricots are generally susceptible to this disease although resistance has been reported. Peaches are generally susceptible. Symptoms range from mild to severe (Polak et al., 1997; Badenes et al., 2003).

19.3.2 ENVIRONMENTAL STRESS TOLERANCE Peaches are widely adapted throughout their range and cultivars developed in one growing region can become commercially successful in many production regions. On the other hand, apricots, while grown in many parts of the world, are in general commercially successful only in limited areas due to their restricted range of adaptability. Two distinct ecological conditions appear to favor apricot production (Mehlenbacher et al., 1990). In Central Asia apricots are grown where winters are long, cold, and dry. Summers are hot and dry, and the change from winter to summer is rapid. Mediterranean conditions under which a distinct group of cultivars can be grown, are characterized by short, dry, mild winters, hot, dry summers, and a gradual change of seasons. Generally, cultivars adapted to one set of conditions perform poorly under other conditions. Early bloom under conditions of fluctuating spring temperatures is a major problem with apricot production in many nonadaptive areas. Disease problems plague apricots in areas with hot, humid summers. An important breeding objective for both apricot and peaches in northern regions is greater winter hardiness of both flower buds and whole trees (Bailey and Hough, 1975; Hesse, 1975; Mehlenbacher et al., 1990; Scorza and Okie, 1990). For peach, the most important factor limiting production in midcontinental and northern climates is the lack of flower bud hardiness. Flower bud hardiness in peach has been shown to be a polygenic trait and is inherited quantitatively (Mowry, 1964). Peach and apricot avoid low temperature injury through deep supercooling, a physical state that depresses the freezing point of cells. The degree of deep supercooling has been related to xylem and flower-bud cold hardiness in Prunus. Cultivated species generally supercool to a lesser degree than hardy wild species (Quamme et al., 1982).

19.3.3 GROWTH CONTROL A major cost of stone fruit production is pruning. The genetic control of tree growth habit could reduce the need for pruning and allow for the development of more productive, more easily managed high-density production systems (Scorza, 1984). Several loci have been shown to control peach tree size and canopy architecture producing compact (Mehlenbacher and Scorza, 1986), spur-type (Scorza, 1987), semidwarf (Fideghelli et al., 1979; Scorza, 1984), columnar (Scorza et al., 1989; 2002) dwarf (Hansche, 1988; Lammerts, 1945; Monet and Salesses, 1975) and weeping (Monet et al., 1988) trees. Apricots also demonstrate a range of growth habits. While much of this variation is evident between apricot species, variation is also found within P. armeniaca. Growth forms

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include a variety of dwarf types, spur-forming types, upright, and weeping (Mehlenbacher et al., 1990; Quarta et al., 1986; Scorza, 1982).

19.3.4 FRUIT CHARACTERISTICS Ultimately, it is consumer acceptance of fruit in the market that drives peach and apricot production. While breeding programs have produced cultivars of high quality, much of this quality depends upon the fruit reaching physiological maturity. When this stage of maturity is reached, the process of softening begins. Because shipping and handling in all but local markets precludes the harvesting of fully mature fruit, consumers generally receive fruit that were harvested at the early stages of maturity, which are somewhat low in flavor and aroma. A common consumer complaint is that fresh peaches and apricots are large and attractive, but lack flavor. Producers recognize that peach, if not apricot, consumption is decreasing. In the 1960s, average per capita consumption of peaches in the U.S. was 4.4 kg (Frecon, 1988). In the past 20 years the consumption level has remained flat at 2.0 kg (Cristoso, 2002). This low level of consumption in relation to other temperate fruits (apples 16 kg per year per capita) is due, at least in part, to the marketing of immature fruit (Frecon, 1988). Breeders have consistently selected for fruit firmness. This is apparent when comparing old and new cultivars. The gradual increase in firmness following generations of selection appears to be an additive gene effect. Major genes that dramatically effect fruit firmness in peach have also been identified. These include the stony-hard (Yoshida, 1976) and the slow-ripening (Ramming, 1991) genes. Several other peach fruit traits have been shown to be simply inherited. These include flesh color (white dominant to yellow), peach dominant to nectarine, melting flesh dominant to nonmelting flesh, soft melting flesh dominant to firm melting flesh, freestone dominant to clingstone, low malic acid dominant to the normal malic acid content, and saucer shape dominant to round. A more complete discussion of the inheritance of peach fruit quality traits can be found in Hesse (1975) and Scorza and Sherman (1996). Considerably less is known about the inheritance of fruit characteristics in apricot (Layne et al., 1996).

19.3.5 NEW BREEDING TECHNIQUES The long-term nature and expense of peach and apricot improvement programs has prompted research into molecular genetics to elucidate the genetic control of important tree and fruit traits, and the development of molecular technologies for peach and apricot improvement. Molecular markers have been developed for agronomic traits such as tree growth habit, leaf width and color, flower color, disease and nematode resistance, and for a number of fruit quality traits such as size and weight, shape, brix, epidermis color, and flesh adhesion (Abbott et al., 1990; Eldredge et al., 1992; Chaparro et al., 1994; Rajapakse et al., 1995; Verde et al., 2002; Etienne, et al., 2002; Guo et al., 2002; Hayashi and Yamamoto, 2002; Bergougnoux et al., 2002; Yamamoto et al., 2001; Salava et al., 2002; Scorza et al., 2002). Genes active during peach fruit ripening have been isolated and partially or fully sequenced, including genes for ACC oxidase (Callahan et al., 1992), endopolygalacturonase (Lee et al., 1990) and ethylene receptors (Bassett and Artlip 1999; Bonghi et al., 2002). The over-expression or repression of genes involved in fruit ripening and/or softening could allow for the direct manipulation of these processes through transfer of altered genes or gene promoters into peach and apricot cultivars or germplasm (Callahan et al., 1989; 1993). Genetic transformation of peach and apricot has been reported (Smigocki and Hammerschlag, 1991; Laimer da Machado et al., 1992) but reliable methods for routinely transferring genes into these species are currently lacking. Genetic markers, gene isolation, studies of gene expression, and genetic transformation will provide additional tools, along with the classical approach of hybridization and selection, for the development of improved peach and apricot cultivars (Scorza, 1991; 2001).

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19.4 HORTICULTURE In many respects the horticultural practices applied to peaches and apricots are similar if not identical; thus both species will be discussed as one. Where significant differences occur, they will be noted.

19.4.1 PROPAGATION Peach and apricot cultivars are propagated by grafting buds of the desired scion or fruit-producing cultivar onto rootstocks. Peach seeds for rootstock production were, in the past, generally collected from canneries. Currently, most rootstock seeds are collected by nurseries from trees specifically grown for rootstock seed production. In some areas particular soil or pathogen problems require the use of resistant rootstocks. In these cases seed are collected from rootstock cultivars developed to carry dominant genes for resistance or otherwise transmit the resistance character to the progeny. Such rootstocks include nematode resistant ‘Nemaguard,’ ‘Nemared,’ and ‘Flordaguard’ (Ramming and Tanner, 1983; Sherman et al., 1991), peach tree short life resistant ‘Guardian’ (Okie et al., 1994), cold hardy ‘Bailey’ ‘Siberian C,’ and ‘Harrow Blood’ for peach (Layne, 1987) and cold hardy ‘Haggith,’ wet soil tolerant Marianna G.F. 8-1, and high pH tolerant ‘Pollizo’ (P. instititia) for apricot (Crossa-Raynaud and Audergon, 1987). Bud-grafting the scion onto the selected rootstock is usually done during the first year of seedling rootstock growth in June or August–September, depending on the amount of growth of the rootstock. If grafted in June, growth of the bud begins immediately and trees can be ready for sale in the winter of the same year. Trees grafted later are ready for transplanting to the orchard following the next full season of growth. Such trees are larger and become established more readily than June-budded trees and command a higher price.

19.4.2 ORCHARD ESTABLISHMENT One of the most important factors affecting peach and apricot production is orchard site selection. Low areas subject to flooding and heavy soils that retain moisture lead to root and tree death due to oxygen depletion. Most soil-borne diseases florish in wet soils. Trees without supplemental irrigation grow poorly in dry soil conditions. Ideally, orchard soils should be deep, rich, and well drained. As with water drainage, air drainage is important, particularly during the winter. Cold air, which is heavier than warm air, sinks and can build up in low, poorly drained areas, causing damage to flower buds. In extreme cases, vegetative buds and wood can be killed. Trees damaged by cold are subject to infection by pathogens including Leucostoma and Pseudomonas.

19.4.3 PRUNING

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TREE TRAINING

Pruning is important for reliable production of high quality fruit. Pruning begins early in tree development as trees are trained to any one of a number of different systems including open vase, central leader, “Y” forms, spindle, and others (Loreti and Massai, 2002). Each system has its advantages and disadvantages. A particular system may dominate in one growing region or a mix of systems may be found within a region. Regardless of the system, in general, trees are pruned in the dormant season, late winter or early spring. Because peach flower buds are generally formed on current season growth, pruning is necessary to allow lateral buds to grow. This produces an abundance of new growth on which flower buds for the following season’s fruit production are formed. At the same time dormant season pruning reduces the potential for shading in the following growing season by removing excessive growth that occurred in the previous season. This shading would inhibit the growth of new flower-bud producing branches and would also inhibit coloration of current season fruit. Dormant-season pruning reduces the number of flowers that will be produced in the growing season following pruning because flower-bearing wood is removed. Overproduction

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of flowers and young developing fruit necessitates excessive fruit thinning in the growing season (which is expensive) and can cause a decrease in final size of the remaining fruit. In many areas, particularly where conditions favor vegetative growth, peach trees are also summer pruned. Summer pruning improves light penetration, which favors the development of external fruit color. It also improves air penetration, which reduces humidity in the canopy and improves disease control. Many apricot cultivars produce fruit on very short lateral branches or spurs. Spurs may be only a few centimeters in length. Spur type trees do not require the same amount of new branch growth as do nonspur trees because most fruit are not produced on the new vigorous growth but, rather, on existing spurs. Pruning to a central leader form is common for spur type trees. Central leader pruning encourages growth of a central major branch. This has a depressing effect on the vigor of the lower lateral branches. These laterals are thinned out to provide light and air penetration into the canopy. After central leader trees begin to bear, the leader may be gradually cut back. This process strengthens the lateral branches. Eventually, the central leader can be removed. A strongframed, spreading tree is thus developed, which provides light penetration to the fruit-bearing spurs. This pruning method for apricot is called the “Winters System” (Ryugo, 1988).

19.4.4 FLOWERING

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FRUITING

Fruit bud development occurs during the growing season and continues at a slow pace during the winter. Maximum flower bud hardiness is achieved in midwinter following a period of cold, nonlethal temperatures. Warm temperatures during this period can cause deacclimatization of flower buds and subsequent damage when cold temperatures return. With the onset of spring, cell multiplication and enlargement of the ovary and other flower parts continue (Barnard and Reed, 1933; Scorza et al., 1991). Under the same climatic conditions, apricots generally flower earlier than peaches, thus they are more subject to damage by early spring frosts. In fact, in many regions crop loss due to spring frost is the major limiting factor for apricot production. Peaches are also subject to spring frost damage but to a lesser extent. Peach and apricot fruit are drupes with a thin epidermis and a fleshy mesocarp that may or may not become free from the endocarp. Fruit growth of both species follows a characteristic double sigmoid pattern. First, growth is rapid, slows to a low rate, and then undergoes a second period of rapid growth that ends at maturity (Conners, 1920; Gage and Stutte, 1991; Harrold, 1935; Lilleland, 1933; Tukey, 1934). Following anthesis, the first period of rapid growth is nearly equal in duration for all peach cultivars studied (Tukey, 1934). The period of slow growth occurs when the peach fruit is approximately half its final size if thinned to normal crop load. The duration of the second and third phases of growth differs between cultivars. In early-maturing cultivars the period of reduced growth is barely noticeable, whereas in late ones it may extend for several weeks (Tukey, 1934). The endocarp is hardened before the final swell and does not enlarge during the final growth stage. Genotypic differences in peach fruit size have been shown to be mainly a function of cell number. Further, these differences are evident as early as 175 d prebloom (Scorza et al., 1991). The seeds of early peach cultivars are incompletely developed at the time of fruit maturity. In some early ripening cultivars such as ‘Springtime’ the embryo may still be microscopic in size at fruit maturity (Hesse, 1975). Slightly later cultivars have embryos that fill the testa but are still physiologically immature. They contain little stored reserves. All such seed are ungerminable unless special methods are used. An important factor in fruit grading is size. In order to produce a large crop of adequately sized fruit, thinning of fruit is necessary. Fruit thinning not only increases fruit size, it reduces limb breakage, maintains tree vigor, and prevents alternate bearing, which is a particular problem in apricot production. Thinning is best done as early as possible to reap the greatest benefits in terms of increased fruit size and flower bud formation for the following season. Thinning at 40 d postbloom in apricot and early to mid-season peaches and up to 70 d in late ripening peaches has been shown to be adequate (Teskey and Shoemaker, 1978). Earlier thinning increases the benefits. In this respect,

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thinning of open flowers has been promoted (Byers et al., 1990). Thinning is usually done manually. Peaches are thinned to leave a spacing of 15 to 20 cm between fruit.

19.4.5 MATURITY INDICES Peach and apricot cultivars have been developed by breeders to mature at various periods of time following bloom. These intervals range from approximately 55 to 160 d. While time of maturity can be affected by climate during bloom or during the growing season, generally cultivars retain their order of maturity under the same growing conditions. It is therefore useful to classify maturity based on certain standard cultivars. ‘Elberta’ was classically used as such a standard cultivar for peach. More recently ‘Redhaven’ and other more widely planted cultivars have been used as orderof-maturity standards because ‘Elberta’ has ceased to be an important commercial cultivar. The time of fruit harvest is dependent upon firmness, color, and the intended market. Fruit for fresh, local use can be harvested later than those that are marketed fresh following transport to distant markets. Fruit destined for the fresh and processed markets have specific standards of flavor, color, and firmness qualities that must be considered when deciding on harvest date. One of the most reliable indicators of maturity is the change from green to yellow undercolor or ground color. Development of the red blush in peaches is not a good indicator of maturity. In fact, some nectarine cultivars display a deep red blush by the beginning of the second growth stage, literally months before maturity. Fresh market apricots have traditionally been yellow-orange at maturity with little or no red blush. Maturity was evaluated in part by the change from green to yellow ground color. New apricot cultivars have been developed with significant red blush. As in peach, the appearance of red blush in apricot is not a good measure of maturity. As new cultivars are developed with higher percentages of the fruit surface covered by red blush (Layne and Hunter, 2003), the determination of apricot fruit maturity will become more problematic. Pressure testing can also be used to gauge maturity based on fruit firmness, and there are several simple devices marketed that can be used (Sistrunk and Moore, 1983). Teskey and Shoemaker (1978) list five commercial stages of peach harvesting maturity, with three to four days at 21∞C generally being required to pass from one maturity stage to the next. These stages are: Hard — the peach does not yield to moderate pressure. This is the stage when peaches are picked for transport to distant markets. Usually the ground color is just beginning to turn from green to yellow at this stage. If not carefully selected, fruit picked at this stage may be too immature and provide a poor quality product. Firm — the peach yields slightly to moderate pressure. Firm-ripe — fruit have a slight give under pressure and the yellow-green ground color is evident. Firm-ripe fruit are fairly palatable and will store for a reasonable period. Tree-ripe — the fruit yields readily to pressure and is in prime eating condition with a useful shelf life of 1 to 2 d. Even at 0∞C storage life is limited and fruit must be marketed to the consumer immediately after storage. Soft — soft fruit are of the highest eating quality but cannot be marketed other than direct from orchard to consumer.

19.5 FRUIT QUALITY FACTORS 19.5.1 FLESH TEXTURE Peaches can be classified as melting (MF) or nonmelting flesh (NMF) types. Traditionally, in the U.S., MF peaches have been marketed for fresh or dessert consumption, and NMF fruit have been used for canning. The NMF trait has been shown to be due, at least in part, to a mutation in the endopolygalacturonase gene (Lester et al., 1996). Although NMF types were generally not used

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for fresh consumption in the U.S., Brovelli et al. (1999b) showed that beyond the textural differences between MF and NMF genotypes, both types can produce fruit with acceptable flavor for the fresh market. The lack of endopolygalacturonase expression in selected NMF genotypes was shown not to affect ethylene production, the respiratory climacteric, or CO2 evolution (Brovelli et al., 1999a). In recent years there has been a progressive shift in the market, particularly in California, with newly developed NMF peaches marketed for fresh consumption. This trend is based on the ability of NMF fruits to resist handling damage and to retain firmness for longer periods of time pre- and postharvest. Thus, NMF fruit can be held on the tree and harvested at a more mature stage of ripening with higher levels of sugars and other “tree-ripe” related flavor attributes. Peaches used for processing as canned halves and slices are exclusively NMF types as this flesh type retains its integrity through processing. Fruit destined for processing are usually harvested by trunk- or branchshaking machinery. The “stony hard” flesh type (Yoshida, 1976) is distinct from NMF and MF. It is extremely firm and maintains firmness for long periods postharvest. The trait is associated with low acid sweet flavor. Cultivar development using the stony hard trait is in progress but stony hard fruit are not currently in the U.S. market (Liverani et al., 2002: Goffreda, 1999). While apricots are not classified as melting or nonmelting flesh types, there are significant differences in flesh firmness between cultivars, making some more suitable than others for fresh market shipping and handling. Most apricots are dried or canned. As with peaches, apricots destined for fresh market sales are hand harvested while processed fruit are mechanically harvested.

19.5.2 FLESH ADHESION Both peach and apricot fruit can be described as freestone or clingstone in reference to the adhesion of the mesocarp (“flesh”) to the endocarp (“stone”). In peaches this trait has been reported to be controlled by a single gene (Hesse, 1975). MF peaches can be either freestone or clingstone but NMF genotypes are, with rare exceptions, clingstone. It has been suggested that there is a tight genetic linkage between the genes controlling NMF and flesh adhesion (Scorza and Sherman, 1996). Further, NMF and MF early-maturing peaches are clingstone physiologically, if not genetically. This is apparently related to the short duration of the fruit development period. Thus, while midseason fresh-market dessert peaches have, until recently, been for the most part MF freestone types vs. NMF clingstone types, it has been rather easy to justify developing NMF clingstone early ripening peaches because they are clingstone regardless of flesh texture. Because NMF fruit can be harvested at a more mature stage, the flavor of early-ripening peaches, which are generally less flavorful than later season fruit, can be improved by using NMF cultivars.

19.5.3 FLESH COLOR Both white- and yellow-flesh peaches, nectarines, and apricots are marketed (Frecon, 1996). Whiteflesh fruit generally have higher soluble solids or are perceived as sweeter than yellow-flesh fruit due to low acid levels. Historically, white-flesh sub-acid fruit has been more popular in the eastern hemisphere (China, Japan, Korea) and yellow “acid” types more popular in the West although subacid white-flesh fruit are gaining in popularity in many Western countries (Frecon, 1996; Liverani, 2002). A number of U.S. stone fruit breeding programs are producing new white-flesh sub-acid cultivars (Frecon, 1996; Frecon et al., 2002; Goffreda, 1999). Red flesh color produced by anthocyanins can be found in peach. This character has been associated with antioxidant activity (Cevallos -Casals et al., 2002). Red-flesh fruit are not currently marketed commercially. Tree-ripened peaches and apricots are among the most delectable of fruits. It is unfortunate that only in local markets can tree ripe fruit be delivered to the consumer. Increasing competition from other fruit in the market increases the need for peach and apricot cultivars with greater firmness at the later stages of maturity that can be harvested, stored, shipped, and marketed when sweetness

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and flavors are optimal. Improved handling and storage systems are also critical. In turn, an expanded market for peaches and apricots can stimulate the research and development necessary for providing solutions to production and handling and marketing challenges.

REFERENCES Abbott, A., L. Eldredge, R. Ballard, W.V. Baird, A. Callahan, P. Morgens, and R. Scorza. 1990. Genetic mapping-implications for variety improvement and identification. Proceedings of the 1990 S.E./National Peach Convention, Georgia Peach Council, Athens, GA. pp. 23–32. Badenes, M.L., J. MartÌnez-Calvo, and G. Llacer. 2003. SEOPA-1 and GOLGI-2 apricot seedlings are resistant to plum pox virus. HortScience 38: 135–137. Bailey, J.S. and A.P. French. 1949. The inheritance of certain fruit and foliage characters in the peach. Mass. Ag. Expt. Sta. Bul. 452. Univ. Mass., May, 1949. Bailey, C.H. and L.F. Hough. 1975. Apricots. In J. Janick and J.N. Moore (Eds.). Advances in Fruit Breeding. Purdue University Press, West Lafayette, IN. pp. 367–383. Barnard, C. and F.M. Reed. 1933. Studies of growth and fruit bud formation. J. Dept. Agric. Victoria, Australia, April 1933. Bassett, C.L. and T.S. Artlip. 1999. Isolation of an ETR1 ethylene receptor homologue from peach (Prunus persica). HortScience 34: 542. Bergougnoux, V., M. Claverie, N. Bosselut, A.C. Lecouls, D. Esmenjaud, E. Dirlewanger, and G. Salesses. 2002. Marker-assisted selection of the Ma gene from Myrobalan plum for a complete-spectrum rootknot nematode (RKN) resistance in Prunus rootstocks. Acta Hortic. 592: 223–228. Bonghi, C. A. Rasori, F. Ziliotto, A. Ramina, and P. Tonutti. 2002. Characterization and expression of two genes encoding ethylene receptors in peach fruit. Acta Hortic. 592: 583–588. Brovelli, E.A., J.K. Brecht, W.B. Sherman, and C.A. Sims. 1999a. Nonmelting-flesh trait in peaches is not related to low ethylene production rates. HortScience 34: 313–315. Brovelli, E.A., J.K. Brecht, W.B. Sherman, C.A. Sims, and J.M. Harrison. 1999b. Sensory and compositional attributes of melting- and non-melting-flesh peaches for the fresh market. J. Sci. Food Agric. 79: 707–712. Byers, R.E., D.H. Carbaugh, and C.N. Presley. Government Printer, Melbourne. 1990. The influence of bloom thinning and GA3 sprays on flower bud numbers and distribution in peach trees. J. Hortic. Sci. 65: 143–150. Callahan, A.M., P.H. Morgens, and R.A. Cohen. 1993. Isolation and initial characterization of cDNAs for mRNAs regulated during peach fruit development. J. Amer. Soc. Hortic. Sci. 118: 531–537. Callahan, A.M., P. Morgens, and E. Walton. 1989. Isolation and in vitro translation of RNAs from developing peach fruit. HortScience 24: 356–358. Callahan, A.M., P.H. Morgens, P. Wright, and K.E. Nichols, Jr. 1992. Comparison of pch 313 (pTom13 homolog) RNA accumulation during fruit softening and wounding of two phenotypically different peach cultivars. Plant Physiol. 100: 482–488. Cervallos-Casals, B.A., D.H. Byrne, L. Cisneros-Zevallos, and W.R. Okie. 2002. Total phenolics and anthocyanin content in red-fleshed peaches and plums. Acta Hortic. 592: 589–592. Chang, L.S., A.F. lezzoni, G.C. Adams, and G.S. Howell. 1989. Leucostoma persoonii tolerance and cold hardiness among diverse peach genotypes. J. Amer. Soc. Hortic. Sci. 114: 482–485. Chaparro, J.X., D.J. Werner, D.O. O’Malley, and R.R. Sederoff. 1994. Targeted mapping and linkage analysis of morphological isozyme, and RAPD markers in peach. Theor. Appl. Genet. 87: 805–815. Childers, N.F. and W.B. Sherman (Eds.). 1988. The Peach. Horticultural Publications, Gainesville, FL. Connors, C.H. 1920. Some notes on the inheritance of unit characters in the peach. Proceedings of the American Society Horticultural Science 16: 24–36. Cristoso, C. 2002. How do we increase peach consumption? Acta Hortic. 592: 601–605. Crossa-Raynaud, P. and J.M. Audergon. 1987. Apricot rootstocks. pp. 295–320. In R.C. Rom and R.F. Carlson (Eds.). Rootstocks for Fruit Crops. John Wiley & Sons, New York. Cullinan, F.P. 1937. Improvement of stone fruits. In USDA Yearbook of Agriculture. Washington, D.C. pp. 605–702.

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Eldredge, L., R. Ballard, W.V. Baird, A. Abbott, P. Morgens, A. Callahan, R. Scorza, and R. Monet. 1992. Application of RFLP analysis to genetic linkage mapping in peaches. J. Amer. Soc. Hortic. Sci. 27: 160–163. Etienne, C., E. Dirlewanger, P. Cosson, L. Svanella-Dumas, R. Monet, A. Moing, C. Rothan, C. Bodénès, C. Plomion, and J. Kervella. 2002. QTLs and genes controlling peach fruit quality. Acta Hortic. 592: 253–258. Fideghelli, C., P. Rosati, G. Della Strada, and R. Quarta. 1979. Genetic semi-dwarf peach selections. Ann. 1st. Sperimentale Frutticoltura 10: 11–18. Frecon, J.L. 1988. Underconsumption or overproduction of fresh peaches. PeachTimes 33: 1–11. Frecon, J.L., R. Belding, and G. Lokaj. 2002. Evaluation of white-fleshed peach and nectarine varieties in New Jersey. Acta Hortic. 592: 467–477. Frecon, J.L. 1996. Plant and pest advisory. Fruit edition. July 9, 1996. Rutgers Coop. Ext., N.J. Ag. Expt. Sta. http://virtualorchard.net/plantpest/fr0709.pdf. Gage, J. and G. Stutte.1991. Developmental indices of peach: An anatomical framework. HortScience 26: 459–463. Gradziel, T.M. and D. Wang. 1993. Evaluation of brown rot resistance and its relation to enzymatic browning in clingstone peach germplasm. J. Amer. Soc. Hortic. Sci. 118: 675–679. Goffreda, J.C. 1999. White-fleshed peach and apricot breeding. Proceedings of the IDFTA Conference, February 20–24, 1999, Hamilton, Ontario, Canada. http://www.idfta.org/cft/1999/october/06_ Goffreda.html Guo, J., Q. Jiang, K. Zhang, J. Zhao, and Y. Yang. 2002. Screening for the molecular marker linked to saucer gene of peach. Acta Hortic. 592: 267–271. Halbrendt, J.M., E.V. Podleckis, A. Hadidi, R. Scorza, and R. Welliver. 1991. Evaluation of Prunus for resistance to natural infection of tomato ringspot virus. HortScience 26: 748. Hansche, P.E. 1988. Two genes induce brachytic dwarfism in peach. HortScience 23: 604–606. Hansche, P.E. 1990. Plant size and number affect genetic analysis and the improvement of fruit and nut tree cultivars. HortScience 25: 389–393. Hansche, P.E. and W. Beres. 1980. Genetic remodeling of fruit and nut trees to facilitate cultivar improvement. HortScience 15: 710–715. Harrold, T.J. 1935. Comparative study of developing and aborting fruits of Prunus persica. Bot. Gaz. 96: 505–520. Hayashi, T. and T. Yamamoto. 2002. Genome research on peach and pear. J. Plant Biotech. 4: 45–52. Hedrick, U.P. 1917. The peaches of New York. Rpt. New York Agr. Expt. Sta. 1916. Hesse, C.O. 1975. Peaches. In J. Janick and J.N. Moore (Eds.). Advances in Fruit Breeding. Purdue University Press, West Lafayette, IN. pp. 285–335. Laimer da Camara Machado M., A. da Camara Machado, V. Hanzer, H. Weiss, F. Regner, H. Steinkellner, D. Mattanovich, R. Plail, E. Knapp, B. Kalthoff, and H. Katinger. 1992. Regeneration of transgenic plants of Prunus armeniaca containing the coat protein gene of plum pox virus. Plant Cell Repts. 11: 25–29 Lammerts, W.E. 1945. The breeding of ornamental edible peaches for mild climates. 1.lnheritance of tree and flower characters. Amer. J. Bot. 32: 53–61. Layne, R.E.C. 1987. Peach rootstocks. In R.C. Rom and R.F. Carlson (Eds.). Rootstocks for Fruit Crops. John Wiley & Sons, New York. pp. 185–216. Layne, R.E.C. and D.M. Hunter. 2003. “AC Haroblush” apricot. HortScience 38: 142–143. Layne, R.E.C. and W.B. Sherman. 1986. Interspecific hybridization of Prunus. HortScience 21: 48–51. Lee, E., J. Speirs, J. Gray, and C.J. Brady. 1990. Homologies to the tomato endopolygalacturonase gene in the peach genome. Plant Cell Environ. 13: 513–521. Lester, D.R., W.B. Sherman, and B.J. Atwell. 1996. Endopolygalacturonase and the melting flesh (M) locus in peach. J. Amer. Soc. Hortic. Sci. 121: 231–235. Lilleland, O. 1933. Growth study of the peach fruit. Proc. Amer. Soc. Hortic. Sci. 29: 8–12. Liverani, A., D. Giovannini, and F. Brandi. 2002. Increasing fruit quality of peaches and nectarines: The main goals of ISF-FO (Italy). Acta Hortic. 592: 507–514. Loreti, F. and R. Massai. 2002. The high density peach planting system: Present status and perspectives. Acta Hortic. 592: 377–390. Mehlenbacher, S.A. and R. Scorza. 1986. Inheritance of growth habit in progenies of “Compact Redhaven” trees. HortScience 21: 124–126.

Peach and Apricot

495

Mehlenbacher, S.A., V. Cociu, and L.F. Hough. 1990. Apricots (Prunus). In J.N. Moore and J.R. Ballington, Jr. Genetic Resources of Temperate Fruit and Nut Crops. Acta Hortic. 290. pp. 65–107. Monet, R. and G. Salesses.1975. A new mutant for dwarfing in peach (in French). Ann. Amelior. Plantes 25: 353–359. Monet, R., Y. Bastard, and B. Gibault. 1988. Genetic study of the weeping habit in peach (in French). Agronomie 8: 127–132. Mowry, J.B. 1964. Seasonal variation in cold hardiness of flower buds on 91 peach varieties. Proceedings of the American Society Horticulture Science 85: 118–127. Okie, W.R., T.G. Beckman, A.P. Nyczepir, G.L. Reighard, W.C. Newall, Jr., and E.I. Zehr. 1994. BY520–9, a peach rootstock for the southeastern United States that increases scion longevity. HortScience 29: 705–706. Okie, W.R., A.P. Nyezepir, and C.C. Reilly. 1987. Screening of peach and other prunus species for resistance to ring nematode, in the greenhouse. J. Amer. Soc. Hortic. Sci. 112: 67–70. Okie, W.R., D.W. Ramming, and R. Scorza. 1985. Peach, nectarine, and other stone fruit breeding by the USDA in the last two decades. HortScience 20: 633–641. Polák, J., I. Oukropec, P. Komínek, B. Krˇska, and M. Bittóova. 1997. Detection and evaluation of resistance of apricots and peaches to plum pox virus. J. Plant Dis. and Protect. 104: 466–473. Quamme, H.A., R.E.C. Layne, and W.G. Ronald. 1982. Relationship of supercooling to cold hardiness and the northern distribution of several cultivated and native Prunus species and hybrids. Can. J. Plant Sci. 62: 137–148. Quarta, R., E. Arias, C. Fideghelli, M. Scortichini, and Y.U. Shin. 1986. Genetic dwarf apricots. Acta Hortic. 192: 329–335. Rajapakse, S., L.E. Belthoff, G. He, A.E. Estager, R. Scorza, I. Verde, R.E. Ballard, W.V. Baird, A. Callahan, R. Monet, and A.G. Abbott. 1995. Genetic linkage mapping in peach using morphological, RFLP, and RAPD markers. Theor. Appl. Genet. 90: 503–510. Ramming, D.W. 1991. Genetic control of a slow-ripening fruit trait in nectarine. Can. J. Plant Sci. 71: 601–603. Ramming, D.W. and O. Tanner. 1983. “Nemared” peach rootstock. HortScience 18: 376. Ryugo, K. 1988. Fruit Culture. Its Science and Art. John Wiley & Sons, New York. Salava, J., Y. Wang, B. Krˇska, J. Polák, P. Komínek, R.W. Miller, W.M. Dowler, G.L. Reighard, and A. Abbott. Molecular genetic mapping in apricot. Czech. J. Genet. Plant Breed. 38: 65–68. Scorza, R. 1982. Apricot breeding for the eastern United States of America. Acta Hortic. 121: 223–226. Scorza, R. 1984. Characterization of four distinct peach tree growth types. J. Amer. Soc. Hortic. Sci. 109: 455–457. Scorza, R. 1987. Identification and analysis of spur growth in peach (Prunus persica L. Batsch). J. Hortic. Sci. 62: 449–455. Scorza, R. 1991. Gene transfer for the genetic improvement of perennial fruit and nut crops. HortScience 26: 1033–1035. Scorza, R. 1992. Evaluation of foreign peach and nectarine introductions in the U.S. for resistance to leaf curl [Taphrina deformans (Berk.) Tul.]. Fruit Var. J. 46: 141–145. Scorza, R. 2001. Progress in tree fruit improvement through molecular genetics. HortScience 36: 855–858. Scorza, R., D. Bassi, and A. Liverani. 2002. Genetic interactions of pillar (columnar), compact, and dwarf peach tree genotypes. J. Amer. Soc. Hortic. Sci. 127: 254–261. Scorza, R., G.W. Lightner, and A. Liverani. 1989. The pillar peach tree and growth habit analysis of compact ¥ pillar progeny. J. Amer. Soc. Hortic. Sci.114 : 991–995. Scorza, R., L.G. May, B. Purnell, and B. Upchurch. 1991. Differences in number and area of mesocarp cells between small- and large-fruited peach cultivars. J. Amer. Soc. Hortic. Sci. 116: 861–864. Scorza, R., S.A. Mehlenbacher, and G.W. Lightner. 1985. Inbreeding and coancestry of freestone peach cultivars of the eastern United States and implications for peach germplasm improvement. J. Amer. Soc. Hortic. Sci. 110: 547–552. Scorza, R., L. Melnicenco, P. Dang, and A. Abbott. 2002. Testing a microsatellite marker for selection of columnar growth habit in peach [Prunus persica (L.) Batsch]. Acta Hortic. 592: 285–289. Scorza, R. and W.R. Okie. 1990. Peaches (Prunus) In J.N. Moore and J.R. Ballington, Jr. (Eds.).Genetic Resources of Temperate Fruit and Nut Crops, Vol. 1. Acta Hortic. 290. pp. 177–231. Scorza, R. and P.L. Pusey. 1984. A wound-freezing inoculation technique for evaluating resistance to Cytospora leucostoma in young peach trees. Phytopathology 74: 569–572.

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Scorza, R. and W.B. Sherman. 1996. Peaches. pp. 325–440. In J. Janick and J.N. Moore (Eds.). Fruit Breeding, Vol. 1: Tree and Tropical Fruits. John Wiley & Sons. Sherman, W.B. and P.M. Lyrene. 1983. Improvement of peach rootstocks resistant to root-knot nematodes. Proceedings of the Florida State Horticultural Society 96: 207–208. Sherman, W.B., P.M. Lyrene, and R.H. Sharpe. 1991. “Flordaguard” peach rootstock. HortScience 216(4): 427–428. Simeone, A.M. 1982. A study on the varietal susceptibility of apricot (Prunus armeniaca) to some of the principle diseases. (In Italian). Ann. 1st. Sperimentale Frutticoltura 13: 19–32. Simeone, A.M. 1985. Study on peach and nectarine cultivars susceptibility to the main fungus and bacteria. Acta Hortic. 173: 541–551. Simeone, A.M. and L. Corazza. 1987. Reaction of peach and nectarine cultivars to Taphrina deformans (Berk.) Tul. (in Italian). Inst. Sper. Frutt. (Rome) 18: 35–44. Ann. Sistrunk, W.A. and J.N. Moore. 1983. Quality. In J.N. Moore and J. Janick (Eds.). Methods in Fruit Breeding. Purdue University Press. West Lafayette, IN. pp. 274–293. Smigocki, A.C. and F.A. Hammerschlag. 1991. Regeneration of plants from peach embryo cells infected with shooty mutant strain of Agrobacterium. J. Amer. Soc. Hortic. Sci. 116: 1092–1097. Teksey, B.J.E. and J.S. Shoemaker. 1978. Tree Fruit Production. AVI Publishing, Westport, CT. Tukey, H.B. 1934. Growth of the peach embryo in relation to growth of fruit and season of ripening. Proceedings of the American Society Horticulture Science 30: 209–218. United States Department of Agriculture, Agricultural Research Service, 1976. Virus Diseases and Noninfectious Disorders of Stone Fruits in North America. Agricultural Handbook 437. U.S. Government Printing Office, Washington, D.C. United States Department of Agriculture. 1992. Agricultural Statistics 1992. U.S. Government Printing Office, Washington, D.C. Verde, I., R. Quarta, C. Cedrola, and M.T. Dettori. QTL analysis of agronomic traits in a BC1 peach population. Acta Hortic. 592: 291–297. Werner, D.J., D.F. Richie, D.W. Cain, and E.I. Zehr. 1986. Susceptibility of peaches and nectarines, plant introductions, and other Prunus species to bacterial pot. HortScience 21: 127–130. Yamamoto, T., T. Shimada, T. Imai, H. Yaegaki, T. Haji, N. Matsuta, M. Yamaguchi, and T. Hayashi. 2001. Characterization of morphological traits based on a genetic linkage map in peach. Breed. Sci. 51: 271–278. Yoshida, M. 1976. Genetical studies on the fruit quality of peach varieties. III. Texture and keeping quality (in Japanese). Bul. Hortic. Res. Stat. (Hiratsuka) A3: 1–16.

Cherry and Sour Cherry 20 Sweet Processing Mark R. McLellan and Olga I. Padilla-Zakour CONTENTS 20.1 20.2

Introduction ........................................................................................................................497 Fruit Quality.......................................................................................................................499 20.2.1 Modified Atmosphere Packaging (MAP) of Sweet Cherries .............................500 20.3 Canned Sweet and Sour Cherries ......................................................................................500 20.3.1 Soaking ................................................................................................................500 20.3.2 Pitting...................................................................................................................501 20.3.3 Filling...................................................................................................................502 20.3.4 Exhausting and Process .......................................................................................502 20.4 Preservation by Freezing....................................................................................................502 20.5 Brined Cherries ..................................................................................................................503 20.6 Cherry Juice Processing.....................................................................................................505 20.7 Dehydrated Cherries ..........................................................................................................506 20.8 Dessert Specialties and Other Products.............................................................................507 20.9 Waste Management ............................................................................................................508 References ......................................................................................................................................508

20.1 INTRODUCTION The sweet cherry (Prunus avium L.) is believed to have come from the region between the Caspian and Black Seas. Wild cherry trees, however, do inhabit all of continental Europe where it is likely that birds were responsible for significant spread of the seeds. Earliest records indicate that the cherry was first domesticated in Greece; it was documented in detail around 300 B.C. (Hedrick, 1914). Cherries were among the earliest fruit brought to and cultivated in the New World by settlers when they arrived from Europe. The French are credited with the first known plantings, located throughout the early settlements of the Canadian Maritimes. New England colonists had both sweet and tart cherries available from their orchards, and it appears that much of the propagation was by seed (Hedrick, 1914). Sweet cherries are mainly used for the fresh market although they are also processed into canned, frozen, brined, candied, and dried forms, juice, wine, jams, and jellies. Tart or sour cherry (Prunus cerasus L.) originated around the Caspian and Black Seas on the border of Europe and Asia (Webster, 1996). Tart cherry cultivars are classified in three major groups: Amarelles, Morellos, and Marasca. Montmorency, a clear-fleshed Amarelle cultivar, is a major tart cherry grown in the U.S. and is used extensively for the production of cherry pies. However, because of its low natural color, there is interest in identifying tart cherry cultivars with higher levels of anthocyanin pigments. Chandra et al. (1993) reported that Michigan State University (MSU) hybrid selections II 7(30), I 21(33), and II 9(11) had higher pigment levels than Montmorency. Balaton, 0-8493-1478-X/05/$0.00+$1.50 © 2005 by CRC Press LLC

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TABLE 20.1 U.S. Cherry Production (1998–2003) State

1999

2000

2001

2002

2003 (estimated)

California Idaho Michigan Montana New York Oregon Pennsylvania Utah Washington Total

Sweet Cherries — Production in Tons 79,500 47,000 55,200 55,500 1,900 3,000 1,400 1,700 27,000 19,500 23,000 2,700 720 1,100 2,300 2,220 1,050 900 1,100 350 50,000 37,000 40,000 31,000 800 500 580 355 1,150 2,400 700 400 67,000 95,000 106,000 86,000 229,120 206,500 230,380 180,225

50,000 2,500 9,000 1,800 500 40,000 340 2,100 95,000 211,340

Colorado Michigan New York Oregon Pennsylvania Utah Washington Wisconsin Total

Tart Cherries — Production in Tons 300 450 400 150 92,500 100,000 148,500 7,500 8,500 8,300 7,350 6,350 2650 2,200 1,200 1,600 3,600 3,050 1,950 1,900 7,250 16,500 6,000 1,500 8,250 8,750 12,750 10,250 5,000 5,000 6,500 2,000 128,050 144,250 184,650 31,250

250 75,000 3,500 950 1,800 12,000 10,000 5,500 109,000

Source: National Agricultural Statistics Service, USDA, Washington, D.C. 2003.

a new Hungarian cultivar, also had levels of anthocyanins that were six times higher than those of Montmorency (Wang et al., 1997). Morellos, red-fleshed cultivars, are preferred in Europe mainly for use in processed products such as juice and jam, though a small number are sold on the fresh market in Europe. Marasca cultivars are characterized by small, very dark-colored fruits that offer the best quality for making cherry wine and liqueurs (Lezzoni, 1996). In Europe, sour cherry cultivars commonly used for processing are Schattenmorelle, Cigany, Pandy, and Marasca (Kaack et al., 1996). In the U.S., Michigan is the largest producer of tart cherries, responsible for about 80% of the nation’s tart cherry supply. Current production records are given in Table 20.1. In 2001, the U.S. produced 370 million lb of tart cherries having a value of $50.7 million (Anon, 2002). However, due to unusual spring weather, U.S. tart cherry production in 2002 decreased by 84% from 2001, resulting in the lowest production since 1943 (Gunn, 2002). Most tart cherries are used exclusively in processing. About 60% of tart cherries are frozen, 30% are canned or otherwise processed, and 10% are used for juice, wine, and brine (Anon, 2002). For processed products, two cultivars predominate in the market — Bing for sweet cherries and Montmorency for sour cherries. Other varieties used for brining include Lambert, Royal Ann, Windsor, Schmidt, and Emperor Francis (Kaack et al., 1996). Increased interest in cherry products has been motivated by new studies that highlight the health benefits associated with consumption of cherries and other fruits. Cherries are good sources of antioxidants, anthocyanins, phenolic compounds, and melatonin, which may help to relieve the pain of arthritis, gout, and possibly fibromyalgia. Antioxidants in cherries can help fight cancer and heart disease (Cherry Marketing Institute, 2002).

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20.2 FRUIT QUALITY To obtain high-quality processed products, one must start with high-quality fruit. Although cherries are almost 90% water by weight, cherry trees require less water for good production than do apple or pear trees. Any substantial reduction in sunlight intensity may be accompanied by a lower level of photosynthesis that will adversely affect fruit set and growth. Cherry growth in an environment of high temperature and low humidity will lead to excess transpiration and subsequent reduction in fruit size. Cracking of fruit on the tree is due to osmotic absorption of water through the skin of the fruit. Cracking occurs when the fruit becomes wet with rain or fog, or upon immersion in water. This absorption of water may increase the cherry volume by as much as 10% before the skin fractures and cracks. Cherries that are likely to crack tend to have a higher proportion of pectic substances than those that do not crack. Cracking is more likely in fruit harvested late in the season. An increase in the temperature of the water on the fruit leads to increased incidence of cracking (Kertesz and Nebel, 1935). Although numerous types of cherry varieties are being bred for use in the fresh as well as processed market, the variety with the largest tonnage processed is Montmorency, a tart cherry. The typical Montmorency cherry reaches a level of approximately 24% total solids at full maturity. Soluble solid levels vary with harvest maturity and can be expected to peak at 15% in the Montmorency. Concurrently, total acidity, as percent malic acid, will reach 1.6 at harvest (Marshall, 1954). Although the main acid in cherries is malic acid, both citric and quinic acids have also been identified in the fruit (Romani and Jennings, 1971). The major difference between tart and sweet cherries is in their concentrations of acid. Sweet cherries have from 0.4 to 0.8% acid whereas tart cherries have from 1.5 to 1.8%. Nutrient analysis of red tart cherries has been done, and detailed results are available regarding vitamins, minerals, and basic chemistry (Leveille et al., 1974). Harvesting technologies have been studied for their impact on fruit quality (Cain, 1961; Woodroof and Luh, 1975; Arnold, 1969; Whittenberger et al., 1969; Arnold and Mitchell, 1970). Over 90 % of the mechanically harvested fruit attained a grade I or better. The slightly rougher handling in mechanical harvesting can induce a thickening and darkening of the epidermal cell walls, which is interpreted as a scald condition. Fresh cherries should be stored cold for maximum storage life. Optimum conditions are –1 to 0∞C at 90 to 95% relative humidity. Montmorency cherries have been shown to freeze at or below 28∞F (–2∞C). Under optimum refrigeration storage, Montmorency fruit can be expected to keep well for at least 2 weeks (Overhosler, 1926). The greatest weight loss during storage is due to transpiration. Storage conditions must include maintenance of high humidity in order to forestall weight loss. Extended storage of fresh sweet cherries under controlled atmosphere (CA) conditions has been shown to be possible (Wang and Vestrheim, 2002; Rumberger, 1971). Better flavor, reduced decay, and acid retention were listed as the main benefits when the fruit was stored at 0 to 2∞C for up to 3 weeks. Other work on CA storage of cherries showed mixed results (Porrit and Mason, 1965; Do et al., 1966). Relatively high levels of CO2 have proved to be effective. Singh (1970) showed that 10.5% CO2 and 2.5% O2 at 1∞C was an effective storage environment for cherries. Storage conditions and the related changes in fruit quality during CA storage of cherries have been reported (Stoll, 1977; Gudkovskii, 1978; Kalitka and Skripnik, 1983). Although hypobaric storage (storage under subatmospheric pressure) is not used commercially, it is an excellent method to store fresh cherries. Salunke and Wu (1973) found a 50% increase in typical storage life of cherries under hypobaric conditions (102 mm Hg). Significant studies on radiation of cherries in order to extend storage life have been concluded. (Eaton et al., 1970; Markakis and Nicholas, 1972; Panhwer, 1972). Overall assessments appear to indicate that excessive softening occurs even at levels of 50 krad, with an initial reduction of microorganisms, followed by increased microbial spoilage (Massey et al., 1965). In other work, gamma radiation was found to increase darkness, redness, and soluble solids content of the cherries.

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At the highest dosage, yellowness was increased, and in one cultivar, firmness was decreased. In some cultivars, certain dosages increased darkness and caused a shift in color from red towards green. Considerable fruit injury was caused by all dosages from 126 to 1000 krad (Eaton et al., 1970). Efforts to cool the fruit as rapidly as possible after harvest can have a significant impact on the storage life of cherries. Gerhardt et al. (1945) found that by rapidly hydrocooling cherries from 27∞C to 1∞C as soon as possible after harvest and by treating with an antifungal chemical, they were able to retain good quality for 30 d. Other workers have confirmed the importance of postharvest cooling and the potential of various antifungal treatments (Do et al., 1966; Post, 1968).

20.2.1 MODIFIED ATMOSPHERE PACKAGING (MAP)

OF

SWEET CHERRIES

Proper handling and cooling practices such as hydrocooling are essential to maintaining the quality of sweet cherries after harvest. Temperature and humidity management are still the most important factors that limit water loss and prolong the shelf life of cherries (Petracek et al., 2002). In recent years, sweet cherry quality has been maintained by the use of MAP, especially in the large production areas of the Western U.S. (Shelton, 1994). MAP bags have been shown to lower respiration rates of fruits and vegetables by altering the oxygen and carbon dioxide concentration in the bags (Rai et al., 2002). MAP bags can also prevent water loss and fruit shriveling by maintaining a high relative-humidity environment (Kappel et al., 2002). Other researchers have also shown that MAP maintained green stems and fruit firmness, both of which are critical for marketing cherries in retail stores (Kappel et al., 2002; Remón et al., 2000; Chen et al., 1981). The success of MAP depends on the physical properties of the film that determine permeability to oxygen and carbon dioxide, and on the respiration rate of the product, which is partially dependent on harvest date, maturity, variety, and other factors (Petracek et al., 2002). According to Kader et al. (1989) fruit ripening is significantly affected when O2 levels are below 8%, and a greater effect is seen as O2 levels decline. Increased CO2 levels (above 1%) inhibit fruit ripening and have a cumulative effect along with reduced oxygen levels. Incorrect use of MAP or not matching the product to the appropriate MAP film could result in anaerobic conditions, leading to product spoilage (Rai et al., 2002). Recent studies by Wargo et al. (2003) showed that Hedelfingen and Lapins sweet cherries maintained good quality at 3∞C for 4 weeks when stored in MAP bags with an internal gas composition of 4 to 10% oxygen and 7 to 8% carbon dioxide.

20.3 CANNED SWEET AND SOUR CHERRIES A traditional approach to preserving cherries is canning. Although the market for canned cherries has been flat or declining somewhat, it is still a significant market segment for the current cherry harvest. About 30% of the sour cherries and 12% of the sweet cherries in the U.S. are canned (see Table 20.2). New varieties of cherries continue to be developed to increase their quality and opportunities for use (Lane, 1990).

20.3.1 SOAKING Canning cherries requires that the fruit be prepared and washed prior to pitting. The cherries must be firmed by soaking them in chilled or iced water to prepare for the pitting process; room temperature water is not effective. When soaking cherries for pitting, drain weights generally decrease as soak time increases. Increasing soak time also causes a 5 to 10% decrease in total acidity and soluble solids. Lowering the temperature of the soak will produce fewer losses and a better, firmer cherry. Soaking, in general, will also increase the gross weight of the cherry prior to pitting. Soaking cools the fruit, firms it, removes residues, and provides an opportunity to separate trash and rotted fruit from the good fruit as well as facilitates the easy distribution of fruit into process. It also allows for temporary storage of the fruit. Once the fruit has been soaked and

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TABLE 20.2 Processed Utilization of Sweet and Tart Cherries in the U.S. (1999–2001) Processed Utilization (in Tons) 2000

2001

Canned Brined Othera

Sweet Cherries 10,900 10,000 63,785 56,600 15,165 16,660

9,000 48,180 16,660

Canned Frozen Otherb

Tart Cherries 42,450 47,700 68,950 72,150 14,750 19,950

45,950 88,500 18,650

Crop/Use

1999

Includes California canned utilization and other processed utilization (frozen, juice, etc.) from all states. b Juice, wine, brined and dried. a

Source: The Food Institute, 2002. The Almanac of the Canning, Freezing, Preserving Industries 2001–2002. 84th ed. Elmwood Park, NJ.

prepared for pitting, it is sized and color-graded, depending on the quality conditions of the pack desired. Some canneries prefer to refrigerate the cherries overnight or for over 3 h instead of soaking (Downing, 1996). Scald is a constant concern during this cooling operation. Studies have shown a direct relationship between fruit scald and oxygen content of the soaking tank water. Levels below 2 ppm of oxygen will allow the development of scald (Dekazos, 1966). Several factors affect the formation of scald. These include the initial handling of the fruit during harvest, postharvest bruising, and the overall heat transfer to the cherries since harvest. Fruit bruised during a normal harvest will develop serious scald within 4 h during summer heat (Woodroof and Luh, 1975).

20.3.2 PITTING Most sweet cherries are not pitted prior to canning, although tart cherries usually are. Pitting is accomplished using a mechanical pitter and is a unit operation that requires some care. The U.S. Food and Drug Administration (FDA) standard of quality for canned cherries specifies that no more than 1 pit be present in every 20 oz of canned cherries (The Food Institute, 2002). Incomplete pitting, resulting in pits that remain in the product, is a continuing problem. Whole pits and pit slivers have been found in products, thereby raising concerns about the efficacy of this operation. Although the industry is exploring alternatives in terms of technology for reducing stray pits in the product, adequate economical methods have not been developed. Weight loss during pitting operations typically is about 15%. The scattering of near-IR (infrared) radiation by intact cherries was investigated as a possible basis for pit detection (Law, 1973). The magnitudes of forward scatter and scatter at right angles were used to form a pit detection parameter. Surface color, degree of scalding, and size were found, in most cases, to have little effect on pit detection, although cherry orientation had a great effect. Some workers have suggested that a partial freezing of the fruit prior to pitting can significantly reduce fluid loss (Klein and Wittstock, 1988). A low-temperature blanch (at 60∞F for 5 to 20 min) has been reported to cause a significant increase in firmness of the pitted cherries prior to canning (LaBelle, 1971). A further increase in

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firmness was achieved by the addition of 0.04% calcium chloride. The pretreated hot-filled product was deemed firmer than the conventional cold-filled, exhausted, and retorted product. The effects of calcium brining at 500 to 1000 ppm and cold storage prior to processing have been reported by Carle et al. (2001) to improve the texture of canned cherries.

20.3.3 FILLING Once the cherries are pitted, they are ready to be filled into cans or jars. Some packs require slicing of the cherries, a process that has been accomplished mechanically and can be added to a process line (Ross, 1980). The filling operation is typically done either by hand or by using a shaker filler. After filling, the product is ready for syruping. Syruping levels used in the canning operation vary, depending on the quality and type of product, from simple water to fruit juice of up to 45∞ Brix solution. Different grades and market updates, including dietetic foods, require different syrup specifications. Once the syrup has been designed, it is preheated for use in the pack.

20.3.4 EXHAUSTING

AND

PROCESS

The syrup is applied at 65 to 77∞C and poured over the fruit in the can. The can is then routed to an exhausting tunnel where environmental temperatures are maintained at 88∞C either by water or steam. Duration in the tunnel is timed to deliver an internal fruit temperature of 77∞C at the time of exit. The purpose of exhausting is to remove air from the product, which enhances the stability and quality of the pack. Once exhausting is complete, the can is routed to a closing machine and loaded for processing. Actual process conditions of temperature and time depend on the size of the can, the pack weight, and the internal temperature upon process initiation. A can center temperature of 80 to 82∞C is usually recommended. Cooling the product after canning is very important in product quality preservation. Typical cooling level is a temperature no greater than 38∞C upon exiting the exhaust tunnel (Downing, 1996).

20.4 PRESERVATION BY FREEZING Frozen cherries have been used to provide the baking and other associated process industries with the opportunity to have high-quality cherries on a convenient, year-round basis. Frozen cherry packages offer flexibility to bakers and other manufacturers desiring to use fresh fruit-type products. Frozen cherries offer better color quality, improved flavor, and firmer texture than canned cherries (Cruess, 1958). The predominant cherry used in this operation is the Montmorency tart cherry although alternative varieties are being developed and marketed. The process for unit operations involved in freezing cherries begins with cooling and pitting the cherries, as detailed under the canning section. Before freezing, the cherries may be blanched at 100∞C for 40 to 60 sec in order to halt enzymatic oxidation, thereby offsetting browning. Other favorable treatments involve the immersion of fresh cherries in calcium chloride solutions prior to freezing to prevent loss of firmness (Alonso et al., 1995) via calcium bridging with pectic materials aided by the action of the enzyme pectin esterase. Van Buren (1973) suggested that blanching is beneficial in preserving the firmness of the tart cherry due to the alteration of pectic materials. The enzyme pectin methylesterase and calcium salts are again targeted as important for the maintenance of firmness (Van Buren, 1973). A typical blanch temperature for use in this phase is 71∞C, achieved by a steam tunnel process where the fruit passes individually on a belt through a steam environment for blanch operations (Voirol, 1972; Alonso et al., 1993). Sensory and objective tests have been used to evaluate blanching processes prior to IQF (individually quick frozen) freezing, as well as freezing products (Siegel et al., 1971; Morrison and Ellis, 1984). The use of microwave processing for rapid thawing and pitting of frozen unpitted red tart cherries has been studied, with mixed results (Weil et al., 1970).

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If blanching is not utilized, then chemical treatment must be used to neutralize the oxidizing enzymes of the cherry, offsetting browning. A possible chemical combination would be citric and ascorbic acid, with typical levels in the range of 350 mg/15 oz for citric acid and 200 mg/15 oz for ascorbic acid. The cherries are then ready for initial freezing. They are IQF for easy handling and movement into consumer retail packs of 16 oz, 32 oz, and so on, or into large-scale commercial tins for use by bakeries. The typical size of a bakery tin is 30 lb (14 kg). In the 30-lb tin or in other commercial loads, cherries are generally combined with sugar or syrup to give a 5 to 1 ratio of weight of fruit to sweetener. This traditional pack of 5 + 1 is typical of that provided to the bakery industry. The sugar or syrup coating adds a protective layer to the cherries, which reduces browning and prepares the product for bakery use. Weight addition is done based on the 5 to 1 ratio if dry sugar is used as the sweetener. Syruping is an alternative method. If syruping is used, the cherries are submerged in a 50 to 60∞ Brix solution and then chilled for freezing. Final storage of the frozen product is maintained at -18∞C or less. New equipment and methods are being developed to produce a better frozen product with more uses. An example is a semifrozen, flowable product for use with dairy items, including yogurts and ice creams (Anon., 1982). Direct immersion freezing of whole cherries has also been suggested as an alternative process with a claimed significant reduction in operating costs (Debon et al., 1974). Crivelli (1968) proposed that cherries that have been treated to prevent browning and flavor loss may be quick frozen without the addition of sugar or antioxidants. Bigareau Moreau cherries responded best to the process. The cherries (including stem and stone) were frozen in a tunnel at -35∞C with an air velocity of 4 m/sec and stored at -20∞C. Cherries were thawed by direct immersion into water and were comparable to fresh fruit. Dehydrofreezing is another alternative for the economical preservation of cherries (Monzini and Maltini, 1986; Pinnavaia et al., 1988). This process involves slowly drying the fruit at a low temperature and gradually replacing water with a fructose invert sugar syrup. The advantage of dehydrofreezing is related to the product’s lack of water — there is no seepage, and the product will not freeze solid.

20.5 BRINED CHERRIES Brining is primarily done with sweet cherries. The process (Figure 20.1) involves the use of sulfur dioxide, which causes bleaching of the cherry to a yellowish white color and a stabilization and inactivation of the enzyme systems. Further, it prevents microbial growth. Generally, fruit targeted for the brining operation is harvested slightly below full maturity. This delivers fruit that is firmer and less colored, which facilitates the bleaching process and also strengthens the fruit to withstand the brining process. Individual steps in the brining process begin with a sort of the fruit for bruised, scarred, or cracked tissue. The damaged fruits are removed, and the healthy cherries are then moved on to the brining solution, usually held in large tanks. Sometimes, in the Midwest, cellophane or polyethylene lined pits dug into the ground are used. Brining solutions are made with a target level of 1% sulfur dioxide solution. This solution may also be generated using a sulfur dioxide gas. An alternative to this is a sodium bisulfite solution. Both methods are acceptable, and the target is a final solution containing free sulfur dioxide at the level of 1%. A pH of 3.5 or higher in the brine solution should be maintained and monitored in order to prevent cracking. The addition of calcium chloride to the brine is typically initiated by using 5 lb of lime per hundred gal of water (0.6%). Storage of the cherries in these solutions can range from 6 weeks to 6 months or longer, in order to accomplish full brining. Reclamation of brine solutions is an important step to ensuring a minimum of pollution and waste disposal costs. Reuse of reclaimed brines has been shown to be effective many times over without adversely affecting the quality of the brined fruit (Panasiuk et al., 1977). Reclamation procedures generally include an adjustment of sulfur dioxide level, removal of dissolved natural pigments, and possibly pasteurization in order to inactivate pectinolytic enzymes.

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Harvested Cherries —ice (well) water cooling

De-Stemmer

Pitter

Fruit + 0.5% CaCl2 (100°C for 25 min.)

Waste

Pits & free run juice

Add Sodium Sulfite (0.5%) Add Potassium meta-bisulfite (0.3%)

— water wash

Fruit + 0.6% hot sodium bicarbonate (10 min.)

Target fruit pH —> 4.0 to 4.5 Add erythosine dye (0.05%) after 10 min heating

— 12 hour hold — repeat washings Add citric acid (to attain a pH of 3.5) Fruit & water Add storage brine (1% SO2 + 0.5% lime) — 4 to 6 week curing Fruit (in brine)

FIGURE 20.1 Cherry brining process.

Once the fruit is adequately brined, it can be moved on to various processes, including maraschino or glacé cherries. During the first several days of brining, the texture of the cherries deteriorates and then gradually becomes firm again. At the same time, pH changes can be significant; typically, a sharp increase is followed by a slow decrease until equilibrium is reached. A brining period of 4 to 6 weeks usually is adequate to ensure pH equilibrium and for fruit to reach a uniform straw color with modest levels of firmness. At that point, the fruit is ready for remanufacture in other products. Problems can arise during brining; a lack of firming in the calcium bisulfite solution, losses due to cracked skin, and solution pockets have been reported. Solution pockets are gaseous pockets formed in the brined cherries where the fruit appears to completely degrade. These solution pockets occur more frequently later in the season and as maturity increases. A sodium chloride and calcium chloride prebrine soak treatment may be effective in reducing the occurrence of solution pockets. It is further likely that softening of brined cherry fruit is due to some types of pectinolytic enzymes.

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After brining, cherries destined for maraschino production may be placed in a sodium chloride solution to further remove skin discoloration (Wagenknecht and Van Buren, 1965; Anon, 1968). Fruit cocktail requires bleaching, firming, and dyeing of the fruit. Pitted cherries are firmed by soaking in hot 0.5% CaCl2. Sodium sulfite is added (0.5%) in order to bleach the original color, followed by the addition of 0.3% meta-bisulfite for a more complete bleaching. Once bleached, the cherries are neutralized by soaking in 0.6% sodium bicarbonate solution. When the fruit attains a pH of 4.5 to 5, erythrosine dye (U.S. Food Drugs and Cosmetics [FD&C] Red #3) at a level of 0.05% is added and the fruit is allowed to soak for 12 h. During this time, the dye penetrates the fruit tissue. Once colored, the fruit is rinsed with water and then acidified by the addition of citric acid to a water soak with a target pH of 3 to 3.7, which fixes the erythrosine dye. Proper handling during this process is essential to minimize bleeding of the dye when packed in a fruit cocktail mix. Bleeding of the dye is reported to occur if the fruit pH climbs above 3.5 (Chandler, 1965; Woodroof and Luh, 1975). Maraschino cherries have been leached, dyed, and syruped. Syruping of the fruit is done to achieve a typical sugar content of 48%. Once syruped, the fruit is drained and packed in a 45% sugar syrup with flavoring (benzaldehyde) and enough citric acid to produce a final pH of 3.6. Typically, the maraschino pack is vacuum sealed and pasteurized to a center temperature of 85∞C (Woodroof and Luh, 1975). Cherry juice of deep-colored, morello-type cherries has been extracted for use as a pigment source in the coloring of brined cherries. Successful application was reported, with a final color that was of slightly different hue and lightness than a standard artificially colored product (McLellan and Cash, 1979). Other work on maximizing color set has led to alternative processes involving skin laceration, alcohol-soluble dyes, and tea extracts (Wedral et al., 1986; Wissgott, 1986; Sapers, 1991). All coloring compounds used must be approved for food use. Some processors prefer to use carmine or natural colorants instead of FD&C Red #3.

20.6 CHERRY JUICE PROCESSING Cherry juice processing can lead to exceptional, high-quality, fully colored products without resorting to unusual varietal blends or specialized process techniques. As with any juice operation, quality of the product is highly dependent upon incoming raw material and quality of the process. Current trends in cherry juice processing favor the use of cold cherries that have been dumped into large bins, frozen bulk, defrosted at a later time, and then utilized for juice extraction. As one might expect, the quality of this product is quite poor. There are two general approaches to cherry juice production: One is a hot extraction process, and the other is a cold extraction process. Both yield reasonable quality products; however, the hot extraction process (heating to 70∞C) results in a somewhat higher color extraction and greater amounts of insoluble solids that must be removed in the final filtration operation. The cold extraction process involves taking fresh cherries and either pitting them or running them through a hammer mill that has been adjusted for minimum breakage of pits. Pit breakage leads to excessively strong maraschino-type or almond-type flavor, and pit fragments could tear filtration cloth segments in an automated or manual press system. Control of pit breakage also tends to leave larger chunks of fruit for juice extraction. Parteshko et al. (1971) suggested the use of glucose oxidase and sorbic acid for the stabilization of cherry juice. The juice retained its natural properties, and the addition of the substances stabilized the effect on the microflora of the juice. The addition of glucose oxidase further worked to eliminate the odor and flavor of the sorbic acid. Extraction of juice from cherries can be optimized for color with the addition of ascorbic acid and the use of the short-term mild blanch prior to pressing. Depending upon the variety used, the juice can vary significantly in chemical composition. The following ranges have been reported for total sugars (13 to 18%) and total acidity (0.5 to 1.9%) (Tressler et al., 1971). Cherry juice can

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be concentrated just as any other juice. No special conditions or problems are related to the concentration operation other than those associated with any fruit juice concentrate. A study by Bolin and Salunke (1971) evaluated various methods for concentration such as osmotic concentration, reverse osmosis, and foam mat dying. The use of cherries in beverages is an area of increased growth. Nani et al. (1990) conducted trials on apple and sour cherry juice blends. Blends contained varying amounts of Golden Delicious apple juice and Fanal sour cherry juice. Juices were assessed according to their pH, free acidity, composition, color, and sensory qualities after pasteurization and after storage for 1 to 4 months. Juices with 85 to 87.5% apple juice received the highest ratings. Cherry juice is generally clarified and filtered (Sahin and Bayindirli, 1993), but other related products may be produced starting with the raw juice. A high-solids nectar can be produced using fine mash for pressing. Use of homogenization could possibly assist in leading to a stable nectar. A full-solids cherry puree is sometimes produced for use in toppings and ice cream addition (Unilever, 1973). A partial concentration of cherry puree will produce a paste with a typical solids level of 30%. Past work on cherry juice processing has led to effective processes involving membrane filtration in order to achieve highly stable, deep-colored cherry juice and cherry-apple blends (Anon., 1986; McLellan and Brown, 1988).

20.7 DEHYDRATED CHERRIES Dehydration reduces the moisture content of cherries, making them ideal for shipping, baking, and other uses (Anon., 1983; Maltini et al., 1993). Dried cherries have a characteristic flavor, chewy texture and moisture content of 25%. Dehydrated fruit pieces are widely used in foods such as pastry, confectionery products, ice cream, frozen desserts, sweets, fruit salad, cheese, and yogurt. The compatibility of the fruit pieces with the food is dependent on equilibrium vapor pressures or the water activity of the components; it is necessary to avoid diffusion of moisture between the fruit and the food. Dehydration is beneficial in that it may conceal imperfections such as split, cracked, bird-pecked, or rain-damaged skin in otherwise useable cherries (Jacobs, 1951). Both sweet and sour cherries are used for dehydration. Cherry “raisins” are specialty products manufactured in Washington State utilizing Rainier and Bing sweet cherries (Kaack et al., 1996). The process involved in drying cherries is similar to that used in canning; however, fully ripened cherries are used in the dehydration process. Successful drying of cherries aims to accelerate the rate of drying, preserve color and flavor, inactivate enzymes, and improve the fruit’s ability to reconstitute. Several methods may be used to achieve drying. Cherries have been dipped in 0.5 to 2% solution of boiling sodium carbonate for 5 to 20 sec and then placed in a cool water rinse. Doing so resulted in tiny cracks on the skin’s surface that increased the drying rate and hindered case hardening (Mrak et al., 1946, Phaff et al., 1945). Effective dipping materials for increasing the drying rate have been reported to include the ethyl esters of fatty acids in the C10 to C18 range. Ethyl oleate was convenient to handle and was as effective as other compounds. Oleyl alcohol was also effective, as was oleic acid in neutral and acid solutions — but not in alkaline solutions. Commonly used food emulsifiers, wetting agents, and detergents were ineffective. Alkalies such as potassium carbonate were found to have no value in reducing drying time when added to the dip. Dipping waxy fruits for a few seconds in cold aqueous emulsion of ethyl oleate or another suitable compound reduced drying time in most cases to half or less that required for a waterdipped control, in the range of 38 to 66∞C (Ponting and McBean, 1970). Alderman and Newcombe (1945) found that holding pitted cherries under water at 57 to 63∞C for 3 to 4 min resulted in good color and texture preservation. When wetting agents of 0.2% concentration and 1% liquid pectin were added to the water, color and reconstitution were further improved. Weigand et al. (1945) found that a steam blanch combined with sulfuring was the best pretreatment.

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After the predehydration methods have been employed, the drying may begin through the use of a dehydrator. The cherries are normally placed on flats before being introduced into the dehydration chamber. Temperature (60 to 71∞C, dry bulb; 32 to 38∞C, wet bulb) and humidity (10 to 25%) are controlled in the chamber, and fans circulate the hot air to evenly dehydrate the fruit. Once the fruit has been dehydrated, the cherries may be prepared for packaging or storage. Cherries that have been placed in a vacuum for 3 to 5 min with 2000 to 3000 ppm of sulfur dioxide retain a brighter color in storage than those cherries that do not receive such treatment. Packaging should protect against insects and increased moisture absorption. In past work, cellophane and thermoplastic wax containers offered the best protection (Weigand et al., 1945); however, it is likely that many modern package films would offer good, if not better, protection. Cherries may be rehydrated by being placed in a 107∞C chamber for 5 min or by a slow simmer for 30 min in water (Weigand et al., 1945). However, Alderman and Newcombe (1945) found an improved method in which the cherries are simmered for 30 min in water at 79 to 82∞C. Three parts sugar is then added to four parts cherry and boiled for 3 min. Color is not diminished as readily, and the mixture is ready for the next steps in preparing pie filling.

20.8 DESSERT SPECIALTIES AND OTHER PRODUCTS Cherry pie filling is a typical extension of the canned pitted cherry pack. Receipts for this pack vary, depending upon starch manufacturer. Alternative and novel combinations have been developed. Wittstock et al. (1984) described a reduced-sugar cherry–apple filling that is prepared using apple pulp, cherry pulp, sugar, starches, colorants, and flavoring. The product has approximately 40% less sugar than standard fillings and is intended for use in various types of torte. Types of processed cherries have been investigated for use in cherry pie making. Evaluations were based on quality of the baked product and ease of use (Franks et al., 1969). There are several formulas that consider the compositional, functional, and economic aspects of the product, which may be used to produce a quality filling (Ulinski, 1989). Glacé cherries are crystallized fruit products and are popularly used with baking processes. A review of fruit pretreatment and the typical process of crystallizing (glacé style) has been published (Riedel, 1976). New cherry products are being introduced to increase sales of cherries. Cornell University is currently examining cherry varieties for their use in cherry wines and specialty products. Color and flavor are varied, depending upon variety, with some varieties producing exceptional, highquality products. MSU has developed two cherry sauces for commercial use by restaurants (Taylor, 1982). The sauces, tart cherry almond and dark cherry rum, contain modified starches, sweetener, flavorant, black cherry flavor, water, citric acid, and whole pitted sweet cherries. The sauces are hot-filled at 79 to 82∞C into 2 gal plastic pails before being shipped to commercial users. Cherry juice has also been incorporated into a meat curing process in order to enhance the taste and improve the effectiveness of seasonings or other additives (Stumpf and Stumpf, 1989). Some cherry juice powders have been produced using various unit operations. Foam mat drying, spray drying, and freeze drying are all possible methods. The extraction of anthocyanins for use as colorants has been explored with reasonable success (Chandra et al., 1993). Attempts have been made to use cherry in the production of flavored beers (Peill, 1976). Production of this specialty product necessitates neutralization of the inherent malt liquor flavor. The liquor is then sweetened, colored, flavored, and acidified with cherry juices as necessary. Cherries have also been used successfully in the production of spreads and cottage cheese sauce (Hendrick et al., 1969). Cherry candies at levels of 70 to 80% sugar have been made. A good review of these and other cherry products is presented by Pruthi et al. (1982). Cherry essence is a valuable product for use as an ingredient. Fresh cherry flavor is distinctly different from the popular maraschino cherry flavor. Recent work has found unique processes and applications for flavor material extracted from cherries (Karwowska, 1973; Anon., 1983; Labell, 1983; Andres, 1987; Smith, 1993).

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20.9 WASTE MANAGEMENT With ever-increasing pressure to minimize fluid and solid waste from food processing plants, every unit operation must be looked at for potential recycle opportunities, by-product potential, and waste stream pretreatment. A study of cherry processing plants concluded that, by eliminating the wastewater discharge from the depitting operation and by recycling the water, total wastewater treatment costs can be reduced by about 50% (Hassett and Klippel, 1973). Other studies confirm the need to address wastewater usage in soak tanks and pit flumes (Hang et al., 1971; Spatz and Trauberman, 1975); the impact on the cost of operations can be significant. The brining operation offers particular challenges to waste management. Alternative procedures for the disposal of waste-cherry-processing brine have been investigated. SO2 concentration in waste brine can be reduced to several hundred parts per million by neutralization with lime and filtration or sedimentation to separate precipitated CaSO3. Lower SO2 concentration can be achieved by oxidation with H2O2. Anaerobic storage of sludge and supernatant from neutralized brine may produce objectional odors. Brine to be disposed of in deep wells should be pretreated by neutralization and sedimentation (Sapers et al., 1977).

REFERENCES Alderman, D.C. and Newcombe, B. 1945. Dehydration of Montmorency cherries. Michigan Agricultural Experiment Station Quarterly Bulletin, 28: 97. Alderman, D.C. and Newcombe, B. 1945. Michigan Agricultural Experiment Station, unpublished. Alonso, J., Canet, W., and Rodriquez, M.T. 1993. Effect of various thermal pretreatments on the texture of frozen cherries (Prunus avium L.). Related enzyme activities. Zietschrift für Lebensmittel Untersuchung und Forschung, 196(3): 214–218. Alonso, J., Rodriguez, T., and Canet, W. 1995. Effect of calcium pretreatments on the texture of frozen cherries. Role of pectinesterase in the changes in the pectic materials. J. Agric. Food Chem., 43(4): 1011–1016. Andres, C. 1987. Fruit flavored concentrates provide food flavour systems. Food Process., USA, 48(3): 24–26. Anon. 1968. Oregon state develops cherry process. Pac. Fruit News, 146(4196): 6. Anon. 1982. Freeze flo leaves fruit cold — and smooth; Softer fruit for ice cream, yogurt. Dairy Rec., 83(9): 35. Anon. 1983. Dried cherries impart flavor, chewiness, aesthetic qualities to baked goods. Food Process., USA, 44(11): 42–43. Anon. 1986. RO membrane system maintains fruit juice taste and quality. Food Eng. Intl., 14(2): 54. Anon. 2002. USDA-NASS Agricultural Statistics 2002, Chapter V: Statistics of fruits, tree nuts, and horticultural specialties. National Agriculture Statistics Service, U.S. Department of Agriculture. Arnold, C.E. 1969. Physiological and histological changes in cherry fruit (Prunus cerasus L., cv. Montmorency) during mechanical harvesting, handling, and processing. Diss. Abstr. Int., Sect. B. The Sciences and Engineering, 30(4): 1442–1443. Arnold, C.E. and Mitchell, A.E. 1970. Histology of blemishes of cherry fruits (Prunus cerasus L., cv. Montmorency) resulting from mechanical harvesting. J. Am. Soc. Hortic. Sci., 95(6): 723–725. Bolin, H.R. and Salunke, D.K. 1971. Physicochemical and volatile flavour changes occurring in fruit juices during concentration and foam-mat drying. J. Food Sci., 36(4): 665–668. Cain, J.C. 1961. Mechanical harvesting of sour cherries: Effects of pruning, fertilizer, and maturity. Proc. N.Y. State Hortic. Soc., 198–230. Carle, R., Borzych, P., Dubb, P., Siliha, H., and Maier, O. 2001. A new process for firmer canned cherries and strawberries. Food Aust., 53(8): 343–348. Chandler, B.V. 1965. Fruit salad cherries. Food Presv. Q., 25(1): 16–18. Chandra, A., Nair, M.G., and Iezzoni, A.F. 1993. Isolation and stabilization of anthocyanins from tart cherries (Prunus cerasus L.). J. Agric. Food Chem., 41(7): 1062–1065. Chen, P.M., Mellenthin, W.M., Kelly, S.B., and Facteau, T.J. 1981. Effects of low oxygen and temperature on quality retention of “Bing” cherries during prolonged storage. J. Am. Soc. Hortic. Sci., 106(5): 533–535.

Sweet Cherry and Sour Cherry Processing

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Cherry Marketing Institute. 2002. Cherries: naturally good for you. Cherry Advantage Health News Update Issue 4. Crivelli, G. 1968. New research methods of freezing cherries. Industrie Agrarie, 6(12): 655–658. Cruess, W.V. 1958. Commercial Fruit and Vegetable Products. McGraw-Hill, New York. Debon, F., Hayward, G., Marcoe, R., and Robe, K. 1974. Reduces operating costs to 0.62¢/lb, saves workers per shift. Food Process., 35(12): 16. Dekazos, E.D. 1966. Relationship between scald Montmorency cherries and oxygen content in soak tanks. J. Food Sci., 31(6): 956–963. Do, J.Y., Salunkhe, D.K., Sisson, D.V., and Boe, A.A. 1966. Effects of hydrocooling, chemical and packaging treatments on refrigerated life and quality of sweet cherries. Food Technol., 20(6): 819. Downing, D.L. 1996. Canning of fruits. In A Complete Course in Canning and Related Processes 13th ed., Book 3. Processing Procedures for Canned Food Products. CTI Publications, Baltimore, MD. Eaton, G.W., Meehan, C., and Turner, N. 1970. Some physical effects of postharvest gamma radiation on the fruit of sweet cherry, blueberry, and cranberry. Can. Inst. Food Technol. J., 3(4): 152–156. Franks, O.J., Zabik, M.E., and Bedford, C.L. 1969. Sensory and objective comparison of frozen, IQF, dried, and canned Montmorency cherries in pies. Food Technol., 23(5): 675–677. Gerhardt, F., English, H., and Smith, E. 1945. Proc. Am. Soc. Hortic. Sci., 46: 191. Gudkovskii, V.A. 1978. Dlitel’noe khranenie plodov [Long-term Storage of Fruits]. Izdatel’stuo, Alma-Ata, Russia. Gunn, S. 2002. Cherry production. National Agriculture Statistics Service, U.S. Department of Agriculture. Hang, Y.D., Downing, D.L., and Splittstoesser, D.F. 1971. Sanitation and water usage in the processing of sour cherries. J. Milk and Food Technol., 34(9): 428–430. Hassett, A.F. and Klippel, R.W. 1973. Food Processing Wastewater Municipal Discharge or Separate Treatment? O’Brien & Gere Eng., Syracuse, NY, pp. 139–150. Hedrick, U.P. 1914. The Cherries of New York, New York Agricultural Experiment Station, Geneva, NY. Hendrick, T., Markakasis, P., and Wagnitz, S. 1969. Cherries in spreads and cottage cheese sauce. J. Dairy Sci., 52(12): 2057–2059. Iezzoni, A.F. 1996. Sour cherry cultivars: objectives and methods of fruit breeding and characteristics of principal commercial cultivars. In Cherries: Crop Physiology, Production and Uses. A.D. Webster and N.E. Looney (Eds.), CAB International: Cambridge, U.K., pp. 113–125. Jacobs, M.B. 1951. The Chemistry and Technology of Foods and Food Products, Vol. 2, 2nd ed., Interscience, New York. Kaack, K., Spayd, S.E., and Drake, S.R. 1996. Cherry processing. In Cherries: Crop Physiology, Production, and Uses. A.D. Webster and N.E. Looney (Eds.), CAB International: Cambridge, U.K., pp. 471–483. Kader, A.A., Zagory, D., and Kerbel, E.L. 1989. Modified atmosphere packaging of fruits and vegetables. Crit. Rev. Food Sci. Nutr. 28(1): 1–30. Kalitka, V.V. and Skripnik, V.V. 1983. Changes in content of anthocyanins during controlled atmosphere storage of cherries. Konservnaya i Ovoshchesushil’naya Promyshlennost’, 11: 46–47. Kappel, F., Toivonen, P., McKenzie, D.-L., and Stan, S. 2002. Storage characteristics of new sweet cherry cultivars. HortScience, 37(1): 139–143. Karwowska, K. 1973. Production and evaluation of natural fruit flavors. II. Production of natural fruit essences and study of their stability during storage. Prace Instytutow i Laboratoriow Badawczych Przemyslu Spozywczego, 23(3): 411–445. Kertesz, Z.I. and Nebel, B.R. 1935. Observations on the cracking of cherries. Plant Physiol., 10: 763. Klein, G. and Wittstock, E. 1988. Verfahren zum Entsteinen von Steinobst [Process for destoning of stone fruit]. German Democratic Republic Patent. Labell, F. 1983. Dry fruit flavors contribute to well-rounded fruit and chocolate products. Food Process., 44(11): 80–81. LaBelle, R.L. 1971. Heat and calcium treatments for firming red tart cherries in a hot-fill process. J. Food Sci., 36(2): 323–326. Lane, D. 1990. New cherry varieties. Proc. Wash. State Hortic. Assoc., 86: 234–238. Law, S.E. 1973. Scatter of near-infrared radiation by cherries as a means of pit detection. J. Food Sci., 38(1): 102–107. Leveille, G.A., Bedford, C.L., Kraut, C.W., and Lee, C.Y. 1974. Nutrient composition of carrots, tomatoes, and red tart cherries. Fed. Proc., 33(11): 2264–2266.

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Maltini, E., Torreggiani, D., Brovetto, B.R., and Bertolo, G. 1993. Functional properties of reduced moisture fruits as ingredients in food systems. Food Res. Int., 26(6): 413–419. Markakis, P. and Nicholas, R.C. 1972. Irradiation of fruits and vegetables — 1966 to 1970. Isotopes Radiat. Technol., 9(4): 472–474. Marshall, R.E. 1954. Chap. 8: Chemical and physiological changes in cherry fruit. In Cherries and Cherry Products, John Wiley & Sons, New York. Massey, L.M. Jr., Robinson, W.B., Spaid, J.F., Splittstoesser, D.F., Van Buren, J.P., and Kertesz, Z. 1965. Effects of gamma radiation upon cherries. J. Food Sci., 30(5): 759–765. McLellan, M.R. and Cash, J.N. 1979. Application of anthocyanins as colorants for maraschino-type cherries. J. Food Sci., 44(2): 483–487. McLellan, M.R. and Brown, S. 1988. A study of morello cherry for use as a beverage base. Final Report to the NYS Agric. Res. & Dev. Grants Program, Food Science & Technology, Cornell University. Monzini, A. and Maltini, E. 1986. New trends in processing of horticultural products. Dehydro-freezing and osmotic treatment. Industria Conserve, 61(3): 265–272. Morrison, P.C. and Ellis, R.F. 1984. Rapid surface freezing of fruit preserves quality, prevents clumping. Food Process., 45(13): 60–62. Mrak, E.M., Perry, R.L., Phaff, H.F., Marsh, G.L., and Fisher, C.D. 1946. Dehydration of fruits. Univ. Calif. Agr. Expt. Sta. Bull., 698. Nani, R., Rizzolo, A., diCesare, L.F., and Picariello, M. 1990. New beverages based on clear juices. 1. Blends of apple juice and sour cherry juice. Industrie delle Bevande, 19(109): 402–405. Overholser, E.L. 1926. The cold storage behavior of cherries. Proc. Am. Soc. Hortic. Sci., 22: 54. Panasiuk, O., Sapers, G.M., and Ross, L.R. 1977. Recycling bisulfite brines used in sweet sherry processing. J. Food Sci., 42: 953–957. Panhwer, M.R. 1972. Preservation of food by radiation. Agric. Pak., 23(1): 5–12. Parteshko, V.G., Tomashevich, G.S., Evnitskaya, G.S., Sapronova, L.G., and Kravets, M.A. 1971. Stabilization of apple and morello cherry juice by glucose oxidase and sorbic acid. Tsudy Ukranskii Nauchno issledovatel’skii Institut Spirtovoi i Lilero vodochoi Promyshlennosti, 13: 136–141. Peill, A.J.C. 1976. Flavored beer: Will it succeed in the U.S.? Food Eng. Int., 1(5): 46–47. Petracek, P.D., Joles, D.W., Shirazi, A., and Cameron, A.C. 2002. Modified atmosphere packaging of sweet cherry (Prunus avium L., ‘Sams’) fruit: metabolic responses to oxygen, carbon dioxide, and temperature. Postharvest Biol. Technol., 24: 259–270. Phaff, H.J., Mrak, E.M., and Perry, R.L. 1945. New methods produce superior dried cut fruits. Food Ind., 17: 150–153, 234, 236, 238, 516–518, 600, 602, 604, 608, 634–637. Pinnavaia, G., Dalla-Rosa, M., and Lerici, C.R. 1988. Dehydrofreezing of fruit using direct osmosis as concentration process. Acta Alimertaria Polonica, 14(1): 51–57. Ponting, J.D. and McBean, D.M. 1970. Temperature and dipping treatment effects on drying rates and drying times of grapes, prunes, and other waxy fruits. Food Technol., 24(12): 1403–1406. Porritt, S.W. and Mason J.L. 1965. Controlled atmosphere storage of sweet cherries. Proc. Am. Soc. Hortic. Sci., 87: 128–130. Post, F.J. 1968. Influence of phosphate compounds and certain fungi and their preservative effects on fresh cherry fruit. Appl. Microbiol., 16(1): 138–142. Pruthi, J.S., Saxena, A.K., and Teotia, M.S. 1982. Cherries: II. Technological aspects. Indian Food Packer, 4: 36–67. Rai, D.R., Oberoi, H.S., and Baboo, B. 2002. Modified atmosphere packaging and its effect on quality and shelf life of fruits and vegetables — an overview. J. Food Sci. Technol., 39(3): 199–207. Remón, S., Ferrer, A., Marquina, P., Burgos, J., and Oria, R. 2000. Use of modified atmospheres to prolong the postharvest life of Burlat cherries at two different degrees of ripeness. J. Sci. Food Agric., 80: 1545–1552. Riedel, H.R. 1976. Production of crystallized or glaced fruits. Confectionery Prod., 42(9): 412, 414, 426. Romani, R.J. and Jennings, W.G. 1971. Stone fruits. In A.C. Hulme (Ed.), The Biochemistry of Fruits and Their Products, Vol. 2, Academic Press, London and New York. Ross, E.E. 1980. Fruit processing apparatus and method. U.S. Patent 4-221-104. Rumberger, G.G. (Brown Co.). 1971. Respiratory packaging. U.S. Patent 3,630,759. Sahin, S. and Bayindirli, L. 1993. The effect of depectinization and clarification on the filtration of sour cherry juice. J. Food Eng., 19(3): 237–245.

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Salunke, D.K. and Wu, M.T. 1973. Effects of subatmospheric pressure storage on ripening and associated chemical changes of certain deciduous fruits. J. Amer. Soc. Hortic. Sci., 98(1): 113–116. Sapers, G.M. 1991. Process for manufacture of non-bleeding maraschino cherries. U.S. Patent 5019-405. Sapers, G.M., Panasiuk, O., and Ross, L.R. 1977. Disposal of sweet cherry processing brines. J. Food Sci., 42(6): 1454–1456. Shelton, P. 1994. Cherry industry looks to modified atmosphere pack. Good Fruit Grower 5(5): 34. Siegel, A., Markakis, P., and Bedford, C.L. 1971. Stabilization of anthocyanins in frozen tart cherries by blanching. J. of Food Sci., 36(6): 962–963. Singh, B.J. 1970. Effects of controlled atmosphere (CA) storage on amino acids, organic acids, storage, and rate of respiration of Lambert sweet cherry fruit. Am. Soc. Hortic. Sci., 95(4): 458–461. Smith, M.A. 1993. Extraction of cherry flavoring from juice. U.S. Patent 5-234-708. Spatz, D.D. and Trauberman, L. 1975. How membranes separate profits from pollution. Food Eng., 47(10): 50–52. Stoll, K. 1977. Vorlagerung in kontrollierter Atmosphaere im Hinblick auf die industrielle Verarbeitung [Effects of controlled atmosphere storage on industrial processing]. Industrielle Obst und Gemueseverwertung, 62(5): 133–135. Stumpf, R.E. and Stumpf, R.W. 1989. Process for curing meat with fruit juice. U.S. Patent 4-806-373. Taylor, D.L. 1982. Cherries: They’re not just for dessert anymore. Food Eng., 54(5): 65. The Food Institute. 2002. The Almanac of the Canning, Freezing, Preserving Industries 2001-2002. 84th ed. Elmwood Park, NJ. Tressler, D.K., Charley, V.L.S., and Luh, B.S. 1971. Fruit and Vegetable Juice Processing Technology, 2nd ed., AVI Publishing, Westport, CT, pp. 282–301. Ulinski, T.J. 1989. Flavoring low water activity fruit fillings for bakery goods. Cereal Foods World, 34(4): 323–324. Unilever. N.V. 1973. Neth Appl., 7302-5282, Brit. Appl. 9528/72. Van Buren, J.P. 1973. Improves firmness without additives. Food Eng., 45(5): 127. Voirol, F. 1972. The Blanching of Vegetables and Fruits. Rache Products, London, U.K. Wagenknecht, A.G. and Van Buren J.P. 1965. Preliminary observations on secondary oxidative bleaching of sulfited cherries. Food Technol., 19(4): 658–661. Wang, H., Nair, M.G., Iezzoni, A.F., Strasburg, G.M., Booren, A.M., and Gray, J.I. 1997. Quantification and characterization of anthocyanins in Balaton tart cherries. J. Agric. Food Chem., 45: 2556–2560. Wang, L. and Vestrheim, S. 2002. Controlled atmosphere storage of sweet cherries (Prunus avium L.). Acta Agric. Scand., Soil Plant Sci., 52(4): 136–142. Wargo, J.M., Padilla-Zakour, O.I., and Tandon, K.S. 2003. Modified atmosphere packaging maintains sweet cherry quality after harvest. NY Fruit Q., 11(2): 5–8. Webster, A.D. 1996. The taxonomic classification of sweet and sour cherries and brief history of their cultivation. In Cherries: Crop Physiology, Production, and Uses; A.D. Webster and N.E. Looney (Eds.), CAB International: Cambridge, U.K., pp 3–24. Wedral, E.R., Hopefl, R.M., and Ivie, R.A. 1986. Colouring of fruit. European Patent 0-115-599-B1. Weigand, E.H., Litwiller, E.M., and Hatch, M.B. 1945. Dehydration of cherries and small fruits. Fruit Prod. J., 25: 9–14, 23. Weil, K.O., Moeller, T.W., Bedford, C.L., and Urbain, W.M. 1970. Microwave thawing of individually quick frozen red tart cherries prior to pitting. J. Microwave Power, 5(3): 188–191. Whittenberger, R.T., Levin, J.H., and Gaston, M.F. 1969. Cherries: A little bruising doesn’t have to hurt. Canner/Packer, 138(8): 26–27. Wissgott, U. 1986. Fruit colouring process. European Patent 0-115-600-B1. Wittstock, E., Neukirch, I., and Nobis, L. 1984. Fruit preparation with reduced sugar content — New filling for cakes and patisserie products. Baeker und Konditor, 32(3): 75–76. Woodroof, J.G. and Luh, B.S. 1975. Commercial Fruit Processing. The AVI Publishing, Westport, CT.

21 Plums and Prunes Laszlo P. Somogyi CONTENTS 21.1

Introduction ........................................................................................................................513 21.1.1 Nomenclature.......................................................................................................513 21.2 Plums..................................................................................................................................514 21.2.1 Canned Plums ......................................................................................................514 21.2.2 Frozen Plums .......................................................................................................515 21.3 Dried Plums (Prunes).........................................................................................................515 21.3.1 Harvesting ............................................................................................................516 21.3.2 Processing Fresh Plums.......................................................................................517 21.3.3 Packing and Storing.............................................................................................518 21.3.4 Dried Plums with Pits..........................................................................................518 21.3.5 Use of Preservatives and Other Food Additives .................................................518 21.3.6 Further Processing of Dried Plums .....................................................................518 21.4 Prune Juice .........................................................................................................................519 21.5 Prune Juice Concentrate.....................................................................................................520 21.6 Pitted Dried Plums .............................................................................................................522 21.7 Canned Dried Plums ..........................................................................................................524 21.8 Low-Moisture Plums..........................................................................................................524 21.9 Chemical Composition.......................................................................................................525 21.10 Nutrition, Health, and Food Ingredient Function..............................................................527 21.10.1 Nutrition and Health Benefits..............................................................................527 21.10.2 Potential of Plums as a Functional Food Ingredient ..........................................528 References ......................................................................................................................................528

21.1 INTRODUCTION The plum is a fleshy fruit (drupaceous) of the family Rosaceae and genus Prunus. In this family, the flesh of the fruit surrounds a hard pit or stone in which there is one seed. Prunus also includes several familiar stone fruits — apricot, cherry, and peach. There are more than 2000 varieties of plums, among which relatively few are of commercial importance.

21.1.1 NOMENCLATURE There is an old saying that “All dried plums are prunes but not all plums are prunes.” Botanically, all dried prunes are plums. In current usage, however, prune signifies a variety that can be and is normally dried without the removal of the pit. The word refers to both the fruit in its fresh state and to the dried product. Plum designates a variety primarily intended for uses other than drying — mainly for fresh consumption but also for use in canning, freezing, and crushing, and in jam and jelly products. Most plum varieties will ferment when dried with the pit.

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In recent years, however, as prunes and prune products are used increasingly as food ingredients by the food processors, the question of what defines a prune became an important marketing issue because marketers believe that the term dried plum has a more positive image to customers than dried prune. To improve the image of the fruit, the California Prune Board — representing the growers of California who produce 99% of the U.S. supply of prunes — petitioned the U.S. Food and Drug Administration (FDA) to approve the term dried plums as an alternative name for the prune. In 2000 the FDA approved the name change. (To mirror the name change, the association changed its name as well to California Dried Plum Board (Anon., 2000a).) According to the board, consumers reacted positively, a 5.5% increase in sales of dried plums occurring during the first year after the name change took effect, while during the previous year sales were down by 3.7%. To reflect the name change, in this chapter, the term dried plums will be mostly used (with the exceptions of juice and juice concentrate, because the leading products are still labeled prune juice). The two principal species of commercial plums are: (1) Prunus domestica, also known as the European plum, varieties of which tend to be purple to black (2) Prunus salicina, also known as the Japanese plum, varieties of which tend to be yellow to crimson

21.2 PLUMS Among the stone fruits, plums rank next to peaches in total production. The former Yugoslavia was the world’s leading producer of plums and dried plums, but the present distribution of plum production among the various newly independent republics is not available at this time. About 90% of the Yugoslav crops (primarily in Croatia) were processed into a brandy known as “Slivovitz.” Germany is second and the U.S. third in the volume of plum production. In the U.S., California is by far the leading producer of plums, followed by Washington, Oregon, Michigan, and Idaho. In the crop year 2000 to 2001, 188,000 tons were produced in 37,000 bearing acres, 90% of which was produced in California. In 2001 Idaho, Michigan, Oregon, and Washington produced a total of 21,000 tons of plums (Anon., 2002). During the same year, an additional 210,000 tons of dried plums (prunes) were produced in California. The drying ratio is approximately 3 lb of fresh plums to 1 lb of dried plums. In the years 2000 and 2001, the average price of plums was $239 and $274 per ton, respectively.

21.2.1 CANNED PLUMS The plum at one time was a favorite canned fruit, but it has been supplanted today by the peach, apricot, and pear. In the 1950s, over 2000 cases (24 No. 21/2 can case equivalent) were produced, primarily in the Northwestern U.S. (Anon., 2001a). Production of canned plums declined to 754 cases by 1989. (Data after 1989 are unavailable because government statistics for canned plums were discontinued after 1989.) More recently, sales of canned, bottled, and dried plums declined further. In 2001 only 3850 tons of plums were canned in the U.S. However, the plum is still an important canned fruit in the U.K. Several varieties are used for canning, the principal ones being the Green Gage plum and the Yellow Egg for the light-colored, and the Lombard for the dark-colored product (Luh et al., 1986). Color changes that plums undergo shortly before harvest are the most important indicator of maturity. Attempts have been made to develop objective measures of maturity such as soluble solids content and degree of firmness but further refinement in these techniques is required. The plums are washed, and the stems, decayed plums, and other foreign matter are removed. The plums are then passed over a sorting belt and moved through a size grader with vibrating screens having circular openings of 2.54, 3.17, 3.81, and 4.45 cm (1, 1.25, 1.5, and 1.75 in.) in

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diameter. A final inspection on the belt is made after size grading to remove imperfect fruits. The fruits are then filled directly into tin cans or glass jars. Boiling hot sugar syrup or hot water is added, depending on whether the plums are to be sweetened or remain unsweetened. The usual sweetener concentration of the syrup is 55∞ Brix for Fancy, 40∞ Brix for Choice, and 25∞ Brix for standard grades (Lopez, 1987). Cans of plums should be sealed at a temperature of 88 to 93∞C (190 to 198∞F), which is usually achieved by preheating or exhausting the cans before closing. The heat sterilization process will depend, naturally, on can size, fill temperature, and the solids content of the topping. The average process time for plums packed in water is 10 min at 100∞C (212∞F) for No. 2 cans and 25 min for No. 10 cans. In sugar syrup, the process time for No. 2 cans is 12 to 15 min, and for No. 10 cans between 28 and 35 min (Lopez, 1987). The cans are cooled in a rotary cooler with water spray to 38∞C (100∞F) after processing. Canned plums are covered by U.S. Department of Agriculture (USDA) Standards for Grades [7 Code of Federal Regulations (CFR) 52 §1781] and FDA Standards of Identity (21 CFR 145 §185).

21.2.2 FROZEN PLUMS In the 1940s, large quantities of plums and dried plums were frozen, usually in 13.5-kg (30-lb) tins, for remanufacture by fruit processors and bakers. The frozen pack has diminished since the 1950s to a small volume. In each of the years 2000 and 2001, approximately 650 tons of frozen plums were produced in the U.S. Recently, the development of individual quick frozen (IQF) highmoisture and pitted dried plums has led to some demand for frozen dried plums in transparent packaging material. Preparation for freezing of plums and dried plums is similar to that for peaches and other stone fruits, except that the fruit need not be peeled (Luh et al., 1986). Frozen plums are covered by USDA Standards for Grades (7 CFR 52 §2911).

21.3 DRIED PLUMS (PRUNES) As noted above, a large percentage of plums (prunes) are dried. By definition, a prune is a dried plum with moisture content reduced to between 19 and 35%. Dried plums have been considered a delicacy in their native France for centuries. The process of drying fresh fruits to produce dried plums on a large scale was practiced in Europe, primarily in France, Italy, and Austria, before the prune industry commenced in the U.S. In 1856, Frenchmen Louis Pellier introduced the La Petite d’Agen prune, a native of southwest France, to the Santa Clara Valley of California, where this major fruit processing industry started. Today, the D’Agen dried plum coming from California is known as the California French dried plum (formerly California French prune). The D’Agen plum is selected for drying because of its high sugar content and full-bodied flavor. Presently, California produces approximately 70% of the world’s supply of dried plums and 99% of the U.S. production (Anon., 2000a). High production is concentrated in the Sacramento, Santa Clara, Sonoma, Napa, and San Joaquin valleys of the state of California. Outside California, only insignificant quantities of plums are dried in the Northwestern U.S. Other major dried plum producing countries include Argentina, Chile, Croatia, and South Africa. Table 21.1 shows U.S. government statistics (Anon., 2002) for production figures and prices of California-grown dried plums in recent years. Among dried fruits, only raisins were produced in larger quantities than dried plums; 640,000 tons of raisins were produced in 1999. In the same year, the combined production of all other dried fruits in the U.S. (including apples, apricots, dates, figs, peaches, and pears) was approximately 40,000 tons (Anon., 2002). Table 21.2 shows domestic end product utilization of dried plums in the U.S. during the 2000 to 2001 crop year. The data indicate that pitted dried plums accounted for 53.2% of domestic prune

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TABLE 21.1 Production Volumes and Prices of California-Grown Dried Plums, 1999–2002 Year

Production Dry Basisa (1000 tons)

Grower Price ($/ton)

1999 2000 2001 2002

178 219 148 155 (168)

861 770 760 810

Note: N/A = not available. A total of 3 lb of fresh fruit will yield 1 lb of dried plums after drying. a

Source: Anon. USDA Nutrient Database for Standard Reference. Release 14. USDA National Agricultural Statistics Service, Washington. 2003. Website: www.nal.usda.gov.

TABLE 21.2 Domestic End-Product Utilization of Dried Plums in the U.S. during the 2000–2001 Crop Year Finished Product

Dried Plum Utilization (tons)

Dried plums, pitted Juice and concentrate Dried plums, unpitted Canned products Puree, butter, and diced dried plums Baby foods Government sales (whole dried plums) Domestic shipments total

48,805 29,463 7,937 1,076 442 778 3,177 91,678

Source: Plum Marketing Committee Report, 2002.

shipments. Like all convenience food products, the importance of pitted dried plums has been increasing steadily since the 1980s. Between August 1, 2000, and July 31, 2001, exports of dried plums from the U.S. were 81,300 tons, representing 54.9% of the 148,000 tons of total production. During this period, the leading export destinations were to Germany (19,294 tons), Japan (14,615 tons), and the U.K., Benelux and Italy (each between 6000 and 7000 tons). Other than California, Chile and France are the leading producers of dried plums.

21.3.1 HARVESTING A diagram of the processing unit operations, from harvesting to drying plums, is depicted in Figure 21.1. In California, late August is the usual harvest time. Plums for drying are one of the few fruits

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Plums Checked for soluble solids, Pressure, and Fruit Drop in Orchard

Plums Harvested and Put into Harvest Bins

Bins Delivered to Dryer

Plums Dumped Out of Harvest Bins

Washed-Most Leaves and Twigs Removed

Plums Loaded onto Drying Trays

Full Trays Loaded onto Cars

Cars Placed into Drying Tunnels

Cars Removed After 18 to 20 Hours of Drying

(Plum Moisture Approx. 80%) (Plum Moisture Approx. 17–19%)

Trays Dumped and Prunes Loaded into Portable Bins

Dried Plums Stored 2 Weeks to Allow Equalization

Dried Plums Delivered to Packaging or Bulk Storage

FIGURE 21.1 Flowchart of unit operations for fresh plums — from harvesting to drying.

allowed to fully ripen before they are picked for processing. Fruit firmness, flesh, skin color, and sugar content of the extracted juice (determined by refractometer reading of the Brix value) are used as indices of harvest date. The soluble solids content of the juice must reach at least 22% prior to harvest for high-quality dried plums. Scott et al. (1993) found linear regressions between the eating quality of rehydrated dried plums (determined by sensory test) and the total soluble solids content and soluble solids to titrable acidity ratio of the fruit measured at harvest time. Currently, most of California prune production is harvested by machine. Dried plums are shaken from the tree by means of mechanical shakers and are caught on canvas frames or tarps and by self-propelled catching frames. Harvested fruit is generally put into large bins. These harvest bins are then hauled to dehydrator plants. In California, modern dehydrators have replaced the old method of sun drying that can take as long as 10 d.

21.3.2 PROCESSING FRESH PLUMS In the dehydration plant, the fruits are prepared for drying by cleaning with air blast and water sprays, followed by dipping in water. The fruits are then spread in a single layer on large wood

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trays and dehydrated — under carefully controlled conditions — to about 18% moisture. Mostly, forced-draft tunnel dehydrators are used, where the drying process usually requires from 24 to 36 h, depending on the size and solids content of the dried plums (Somogyi and Luh, 1986). The tunnel is operated under 74∞C (165∞F) dry bulb temperature, with wet bulb temperature 9∞C (15∞F) lower than the dry bulb at the cool end. In this process, 3 kg fresh fruit will yield 1 kg dried plums.

21.3.3 PACKING

AND

STORING

From the dehydrator, the dried plums are moved to the packaging unit, where they are first graded for size (measured as number of fruits per pound) and then inspected once more. The largest size grade has 20 to 30 fruits per lb, and the smallest grade has 90 to 100 fruits per lb. Dried plums store best when their moisture content is reduced to about 21%. This is generally referred to as a “natural condition” prune. Then the fruits remain in cool storage facilities until they are sold or utilized in further processing. Automated high-speed sorting systems to substitute for the traditional visual inspection are under investigation. Delwiche et al. (1993) tested a multicamera processor system capable of sorting dried plums for surface defects. Three line-scan cameras were used to view the dried plums. Each camera was connected to a subsystem computer to analyze and classify the images and remove defective dried plums by a pneumatic system.

21.3.4 DRIED PLUMS

WITH

PITS

If dried plums are marketed as “dried plums with pits,” then the fruits are taken from the warehouse and rehydrated to between 24 and 30% moisture, sterilized, inspected again, and packaged. The package sizes are usually 11.2 or 13.5 kg (25 or 30 lb) bulk cases for the institutional users, or 450-g or 900-g (1- or 2-lb) containers for the retail trade.

21.3.5 USE

OF

PRESERVATIVES

AND

OTHER FOOD ADDITIVES

The high sugar content of California French dried plums allows long shelf life for most products. Chemical preservative is required only for those fruits with moisture content higher than 25%. Bolin (1980) determined that dried plums should be pasteurized or treated with potassium sorbate if they are marketed at a water activity (aw) above 0.7. Almost exclusively, potassium sorbate applied by dip or spray is used as a preservative for dried plums. It is suggested that trials be done with solutions containing from 2 to 7% potassium sorbate, to achieve a deposit of the preservative on the fruit from 0.02 to 0.05% by weight (Chichester and Tanner, 1977). As for the use of other chemical additives, some industrial applications of dried plums require coatings to ensure easy handling of fruit pieces in high-speed processing operations (e.g., to improve free-flowing properties). Such coatings are applied as required by the customer’s specification and may include vegetable oils, monoglycerides and diglycerides, dextrose, cornstarch, rice powder, and so on. Calcium stearate is usually added, at concentrations between 0.1 and 0.05%, to lowmoisture prune products (< 4% moisture content) as a processing aid and free-flowing agent.

21.3.6 FURTHER PROCESSING

OF

DRIED PLUMS

From dried prunes, the industry processes several products including: 1. 2. 3. 4.

Prune juice from unpitted whole dried fruits Prune juice concentrates from unpitted whole dried fruits Fresh plum juice concentrate prepared from fresh plum juice USDA dried plum puree composed of prune juice concentrate, pitted and disintegrated dried plums, and water

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Alternatively, dried plums are pitted and marketed as: 1. Whole pitted dried plums 2. Pitted, canned dried plums Various forms of dry and low-moisture products are: 1. 2. 3. 4. 5. 6. 7.

Prune paste Prune fillings and toppings Low-moisture diced prune Low-moisture prune bits Low-moisture prune granules Low-moisture prune powder Prune fiber

Significant quantities of these prune products are sold to institutional users. Examples of the many food ingredient applications of prunes include (Anon., 1993a): 1. 2. 3. 4.

Diced prunes as a replacement for raisins in bakery products Prune paste as an ingredient for the manufacture of confectionery bars Prune puree serving as a fat replacement in baked foods Addition of prune juice to bread

Formulations for pastries, muffins, cakes, and so on, that utilize dried plums as fillings and as a “fat replacer” ingredient are described by Sanders (1993) and in bulletins of the California Dried Plum Board (Anon., 1993b).

21.4 PRUNE JUICE Prune juice prepared from California dried plums has been produced commercially since 1934 and consumed in substantial quantities in the U.S. (Woodroof, 1974). Currently, it is not a popular beverage outside the U.S. Only small quantities of single-strength prune juice are produced in Australia, and prune juice concentrate is manufactured in Australia, Chile, and Macedonia. Prune juice differs from other fruit beverages in that it is a water extract of dried fruit, rather than squeezed fresh produce (Luh, 1980). The concentration of single-strength commercial juice ranges from 18.5∞ to 21∞ Brix. The Federal Standards of Identity (21 CFR §146.187) require that prune juice contain not less than 18.5% (by weight) soluble solids extracted from dried plums. The FDA standards permit the following optional ingredients in prune juice: 1. One or any combination of lemon juice, lime juice, or citric acid as an acidulant 2. Honey, not less than 2% and not more than 3% by weight of the finished product 3. Vitamin C, from 30 to 50 mg per 177 ml (6 fl oz) serving of the finished product The Federal Regulation requires that the following label declaration appear on the container below the words “prune juice” or “dried plum juice”: “A water extract of dried plums.” Two methods are used commercially for making prune juice from dried plums. One of these, the diffusion method, is rarely used now in modern processing operations. Briefly, it involves extracting the soluble components from the dried fruit by three successive leachings with hot water. Each extraction step requires 2 to 4 h. The process begins with thoroughly washing the dried plums and dumping them into tanks. About 92 l (25 gal) of water is added to each 45 kg (100 lb) of prune, and the mixture is heated to 85∞C (185∞F) by steam coils. After the leachate is drained, the

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process is repeated twice with fresh water; less water is used in the second and third extraction steps. About 56 l (15 gal) water is added to each 45 kg (100 lb) of prune for the second extraction and about 37.5 l (10 gal) of water to 45 kg (100 lb) of fruit for the third extraction. At the completion of the third extraction, the three batches are combined and the liquid evaporated until its concentration reaches 19 to 21∞ Brix. The second prune juice processing technology, known as the disintegration method, is the current choice for processing because of the higher yields achieved and because the flavor of the juice is superior to that made by the diffusion process (Luh, 1980). The process consists of breaking up the washed dried plums by cooking in water for a minimum of 8 h with vigorous agitation. Pressure cookers may be used so that the cooking time can be reduced to as little as 10 min. Industry sources indicated that, by maintaining moderate temperature during cooking, following a short initial boiling (2 to 5 min), certain quality attributes of the finished product improve. Such milder heat treatment results in lighter color and greater viscosity of the finished juice. From the disintegrated fruit, the juice is separated, either by squeezing the pulp in a hydraulic press or by high-speed centrifugation (at approximately 4000 rpm). The extract is then clarified, by allowing the solids to settle and siphoning off the clear juice, or by filtering through a filter press using 1% infusorial earth, rice hulls, or other filter aid, or by a second centrifugation (at 5000 rpm). The resulting extract, a clear liquid, is collected in surge tanks and concentrated by heating in open vats or under vacuum to the desired 19 to 21∞ Brix concentration. When the extract reaches the desired concentration, it is heated in a heat exchanger to 88∞C (190∞F) prior to filling into the containers. Occasionally, citric acid is added to make the juice more tart. The addition of 900 g (2 lb) of citric acid per 378 l (100 gal) juice is recommended. Preheated juice is filled into cans or bottles, seamed or capped. Large cans (46 oz) and bottles (32 oz) are then conveyed through a cooling tunnel. Cooled containers of juice pass through an air curtain at the end of the cooling tunnel to remove pools of water and to dry the exterior of the container. Small cans (51/2 or 6 oz) are not cooled, but the seamed cans of juice pass through a washer to remove traces of juice on the can exterior. A diagram of a prune juice processing line using the disintegration method, combined with an advanced fruit solids recovery system, is depicted in Figure 21.2.

21.5 PRUNE JUICE CONCENTRATE To make a viscous form of prune juice concentrate, the regular juice must be depectinized first. It involves the addition of commercial pectic enzymes to the juice, with vigorous mixing to ensure good distribution of the low-volume liquid enzyme preparation. The juice and enzyme mixture is allowed to mix for 4 to 6 h. The juice is then filtered and concentrated under vacuum to approximately 60∞ Brix at a temperature not higher than 48∞C (120∞F). The product is preserved by freezing in 6 oz cans and marketed as a frozen prune juice concentrate (Luh et al., 1986). Prune juice concentrate is also made (for the industrial markets or for bulk shipments from California to the eastern states) for reconstitution into single-strength juice at 70∞ Brix (for domestic use) or 72∞ Brix (for export shipments) soluble solids concentrations. These products are shelf stable without requiring freezing or the addition of a chemical preservative. The bulk juice concentrate is packed in 19 l (5 gal) pails or 208 l (55 gal) drums, or transported in tank trucks to large-scale users. The bulk containers are made of enamel-coated steel or plastic. The weight of 70∞ Brix concentrate is 1.348 kg/l (11.25 lb/gal). Lang and Byer (1967) patented a technology for the recovery of the volatile constituents from prune juice and for fortifying the juice concentrates with flavor essences. The method is similar to that utilized for other fruit juice concentrates, but it has not been applied commercially to prunes. Because of the elaborate and costly equipment required to trap and recover volatile substances, flavor fortification cannot be justified for prune juice. Such a technique is suitable only for products of higher economic value such as berry concentrates and premium quality citrus concentrates.

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Fresh Water

French Prunes (in bins) Rinse Tank (−1,350 kg prunes/tank)

Water Drained Cook Tanks (with steam coil; cca. 9,500 liters each; 2–3° Brix Juice 1 hour boil, 10 hours simmering at 82°C) Pitter Pulp Surge Tanks

Pits

Centrifuge (4,000 rpm) Pulp

Water

Prune Pulp Solids Decanter Centrifuge Tanks with Mixer (4,000 rpm) Juice

Solid Wastes Empty Containers through Can Washer

Screen Juice

Filler

Surge Tanks Prune Juice Solids Centrifuge (5,000 rpm) Juice. 17–20° Brix Holding Tanks (5–6 hours at 70°C; Juice is blended within interconnected tanks, concentrated, if needed, to adjust solids content to > 18.5° Brix.) Juice Heat Exchanger ( >88°C)

Seamer Cans Converted Pasteurization (88°C) Cooling Tunnel (not used for 5-1/2 or 6 oz. cans) Case

Warehouse

FIGURE 21.2 Flowchart of an advanced prune juice process line.

Fresh Plum Juice Concentrate is manufactured with the same technology. The product is made of juice prepared from mature fresh plums. The 70∞ Brix concentrate is distributed frozen in 55gal drums or in 40-lb aseptic bags in box packages. The product is sold to food manufacturers to be used in baked goods, sauces and marinades, snack foods, and energy bars. Injected into meat or poultry, it is recommended as an inhibitor of food-borne pathogens and oxidation. Prune juice concentrate has been shown to be equal to or more effective than raisin juice concentrate as a natural preservative in whole grain bread (Sanders, 1993). In addition to mold inhibition, the prune juice concentrate has superior softness and moistness due to its high content of sorbitol and reducing sugars. USDA Dried Plum Puree, a smooth pliable fruit puree, is recommended and sold for reducing calories and fat in baked products. The product is composed of prune juice concentrate (52.7%), dried plums (17.2%), and water (30%). Potassium sorbate may be included as a preservative at a maximum concentration of 0.1%.

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Dried Plums Received Quality Control Testing

(Size,Offgrade, Foreign Material, Moisture)

Fumigation Size/Grading Sorted for Defects Quality Control Testing

(Size, Offgrade, Foreign Material, Moisture)

Dried Plumes Washed Dried Plums Processed

Steam Treatment

Sorting Belts Pits Removed Potassium Sorbate Application Packaging Quality Control Testing

(Inline Metal Detection and Checkweighing) (Moisture,Size,Off-grade, Sorbic acid,Foreign Material)

Pelletized Shipped

FIGURE 21.3 Flowchart for production of pitted dried plums.

21.6 PITTED DRIED PLUMS A flow diagram for the processing of pitted dried plums is shown in Figure 21.3. Dried plums are size-graded, sorted for defects, washed, steamed, and the pits removed by specially designed equipment. The same type of equipment is utilized in pitting prunes as for pitting cherries and olives. But unlike these fruits, dried plums are not freestone fruits, and therefore dried plums must be thoroughly preheated by steam treatment to ensure proper pit removal by the pitter equipment. The Ashlock pitter is the most frequently used pitting machine. Another suitable equipment is the Elliott pitter. Sunsweet Growers, Inc., the largest dried plum processor in California, has developed its own patented process improvement for prune pitting (U.S. Patent 3,260,291, 1966). The Dried Plum Administrative Committee has established rigid specifications as to the allowable percentage of whole pits and pit fragments in pitted dried plums. Only 2% of dried plums may contain pits or pit fragments larger than 2 mm. Actual practice indicates that the level of pit contamination is less than 0.002%. Processes for making diced dried plums, bits, paste, fiber, and fillings from pitted dried plums and the characteristics of the finished products are summarized in Table 21.3 (Somogyi, 1987).

Aesthetically pleasing, dark brown color; substantial elasticity; good “mouthfeel”; easy to coat, dip, and process

Plums are washed and dried in a series of controlled operations; then rehydrated, reinspected twice, graded for size, and sterilized before packaging Types I and II are pitted individually with little change in appearance Type III is pitted by a series of metal spikes 1/2 in. long and is characterized by a “flattened” appearance

Source: The California Dried Plum Board.

Benefits

Characteristics

Process

Whole Pitted Dried Plums

Homogeneous paste, including a special pliable and a lowmoisture stiff paste; dark amber color or natural red–golden color; also available in golden amber; total titratable acidity varies

Moisture ranges between 25 and 30%; interchangeable coatings made diced dried plums freeflowing; coatings include edible vegetable oils, diglycerides or monoglycerides, and dry coatings Substantial consistency, texture, and mouthfeel; free-flowing and easy to use; blends well into most products with little or no modification of formulas Guaranteed pit-free; easily adjustable consistencies and textures

Dried prune paste is forced through a finemesh sieve and formed into small cubes approximately 1/4 in. ¥ 1/4 in. random.

“Natural” dried plums are extruded through a fine-mesh screen to remove pits and to form paste

Pitted dried plums are cut into 1/4–1/2 in. pieces by dicing equipment

Guaranteed pit-free, no preservatives; freeflowing and easy to distribute

25% maximum moisture; pH value of 5.5; coated with light vegetable oil or alternative coatings; available in two colors: dark amber and golden amber

Dried Plum Bits

Dried Plum Paste

Diced Dried Plums

TABLE 21.3 Processing, Characteristics, and Benefits of California Dried Plum Products

Free-flowing; extremely long keeping properties; very high in fiber and nutrients; can be used “as is” or rehydrated; high water absorbency and moisture retention

Lekvar (prune paste with added sugar, pectins, and gums) can be exposed to high temperatures without running; synergistic with most standard bakery products

Various types are available with different percentages of solids, sugars, and starches

Pectins, gums, starch, and sugars are added to prune paste, enabling it to withstand oven temperatures

Pitted dried plums are pureed and drum-dried to 8% moisture. Product is screened to ensure uniformity of size

Three forms: flour, flakes, and granules; 45% fiber content; 8% maximum moisture, no chemicals used in processing

Dried Plum Fillings and Toppings

Dried Plum Fiber

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21.7 CANNED DRIED PLUMS Three types of canned, pitted dried plums are produced (Anon., 2000a): 1. Regular — packed in sugar syrup 2. Nectarized — water-packed with a smaller amount of topping liquid and about a third more dried plums per can than the regular canned prune pack 3. Moist-pack — regular pitted dried plums, at higher moisture content (35 to 42% moisture) than dried bulk plums — packed without addition of liquid Regular canned dried plums are usually processed from pitted California French dried plums of large size ranging between 110 and 132 counts/kg (50 and 60 dried plums to a pound). The dried plums contain about 18% moisture before the initiation of the canning process. Fruits are sorted on a belt to remove defective ones, washed, blanched in boiling water for 4 to 5 min, rinsed, and packed in cans. The input weights of the processed dried plums are 397 g (14 oz) per No. 21/2 can and 1.36 kg (48 oz) per No. 10 can. The syrup for canning is 24 to 30∞ Brix for heavy syrup and 30∞ Brix or higher for extra heavy syrup packs. After filling, the cans are exhausted at 93∞C (198∞F) for 12 to 15 min, then sealed at 88∞C (192∞F) or higher, and processed for 20 min at 100∞C (212∞F) for No. 21/2 cans and 35 min for No. 10 cans. The heat-processed products are cooled in a rotary water cooler to 38∞C (100∞F) (Lopez, 1987). The Federal Standards of Identity for canned dried plums (21 CFR §145.190) permit the addition of vinegar, lemon juice, or organic food acids. The shelf life of canned dried plums is greatly prolonged if the syrup used in canning is acidified. Citric acid at 0.4% concentration or an equivalent amount of other acidulant is recommended (Luh et al., 1986). Nectarized or “water-pack” dried plums are processed in the same way as the regular pack, except that cans of these plums contain one third more fruit, and the volume of the water topping is about half that of the syrup used in the regular pack. The resulting liquid contains about 32% soluble solids. All solids of the topping are leached out, and so dried plums in the finished product have a firmer texture than those in the regular product packed in sugar syrup. Moist-pack dried plums are also known as “dry-pack” dried plums (Luh et al., 1986). First, the pitted dried plums are heated in boiling water for 4 to 5 min, drained, and packed scalding hot in enameled cans. The lids are placed on the cans and given a first rolling operation. The cans are then exhausted for 20 min in live steam, sealed, and allowed to cool in the air. Besides the dried plums, no liquid topping is added to this product. The dried plums are usually cooked in water before serving.

21.8

LOW-MOISTURE PLUMS

Conventional dried plums may be dehydrated to low moisture in vacuum shelf dryers (Somogyi and Luh, 1986). The finished low-moisture dried plums contain less than 4% moisture. Because of the high sugar content of dried plums, dehydration to such a low moisture level can be achieved only under subatmospheric conditions. Moreover, drying under vacuum reduces oxidation during drying, and because the process is carried out at lower temperatures than with air drying, heat damage to the fruit (such as caramelization of sugars, destruction of carotene, etc.) is minimized. The process is initiated with perforation treatment of the whole, pitted and dried plums to achieve better dehydration and reconstitution character, followed by dehydration under vacuum. Low moisture diced plums, granules (8 mesh particle size), and dried plum powder (20 mesh) are also commercially available. One kilogram of low-moisture plums is equivalent to 1.5 kg of dried plums.

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Because they are free-flowing and highly hygroscopic, dehydrated plums require very careful handling and packaging that provides a good moisture barrier (Somogyi and Luh, 1986). To avoid severe caking of low-moisture prune products, final product inspection and filling are usually done in a dehumidified, air-conditioned room. Low-moisture diced plums and plum granules are recommended for bakery fillings and spreads. Low-moisture prune powder can be utilized as a sweetening and flavoring agent for whole wheat, rye, or pumpernickel breads, or the preparation of instant prune spread. Only one U.S. firm, the Vacu-Dry Co. (Sebastopal, CA), has ventured to develop low-moisture plum products. Its unique vacuum cylinder drying plant was able to produce fruit such as plums with high sugar content (Somogyi, 1999). In 2000 Vacu-Dry suspended operations, and at the present time we are not aware of any production of low-moisture plum products in the U.S. Low-moisture plum powder may be produced with the drum drying process. However, the highly sugary, sticky product would require sophisticated handling and the use of advanced anticaking food additives.

21.9 CHEMICAL COMPOSITION The composition of dried plum products are summarized in Table 21.4. As with other natural products, variations in composition may occur due to crop differences from year to year and differences in processing methods. Because dried plum juice contains over 17.5% sugars, it is high in calories. It provides 180 kcal per glass (240 ml, or 8 fl oz). By comparison, the same volume of other juices contain: orange, 110 kcal; pineapple, 130 kcal; and tomato, 41 kcal. Like most fruits, dried plums are very low in fat content. Carbohydrates are the primary source of calories provided by dried plums. Industry averages for total carbohydrates of dried plums are reported at 66 g/100 g fruit at 25% moisture. This figure includes the simple carbohydrates: fructose, 17%; glucose, 21%; and sorbitol, 15% (Sanders, 1993). GLC (Gas Liquid Chromatography) analysis showed that in 100 g dried plum, carbohydrates are composed of 21 g glucose, 16.1 g dietary fibers, 14 g fructose, 10.6 g sorbitol, 0.9 g sucrose, and 0.5 g starch (Kline et al., 1970). The low sucrose content is attributed to the presence of invertase, which has been implicated in the disappearance of sucrose during processing of dried plums (Wrolstad and Schallenberger, 1981). Dried plums are an outstanding source of dietary fiber (Somogyi, 1987) and offer all the multifunctional benefits of fiber. However, great variations exist among the dietary fiber data reported from different laboratories. Southgate, who is credited by others with the development of an “accurate” method for fiber determination, reported 16.1 g dietary fiber in 100 g dried plums containing 23.3% moisture (Paul and Southgate, 1978). The USDA Nutrient Database (Anon., 2002) lists 7.1 g “total dietary fiber” in 100 g uncooked 32.4% moisture prunes. Sanders (1993), referring to “industry averages,” listed 8.1 g total dietary fiber content in 100 g, 25% moisture dried plums. These variations of fiber contents reported by the various sources cannot be attributed solely to the natural variations in the composition of fruits. Rather, the lack of standardized tests, especially for the extraction procedures of soluble fibers, may be responsible for the discrepancies. The fiber content of dried plums consists of approximately 80% soluble material, mainly of pectin, hemicelluloses, and cellulose, along with some lignin and other components (Paul and Southgate, 1978). California dried plums contain between 1.2 and 2.0% organic acids, which are responsible for the low pH values of between 3.5 and 4.0. Malic acid is the primary acid present in dried plums and makes up approximately 97% of the total acid content, with traces of citric, benzoic, and chlorogenic acids also present (De Moura and Dostal, 1965). Malic acid is released more slowly than other organic acids, and thus has a greater carry-through during the chewing process (Sanders, 1993). Additionally, malic acid helps to inhibit microbial spoilage and can also serve as the acid component of chemical leavening systems.

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TABLE 21.4 Composition of Dried Plum (Prune) Products Value Per 100 g Edible Portion Dried Uncooked

Juice Canned

Canneda

Nutrient

Units

Puree

Low Moisture

Water Energy Total lipid (fat) Carbohydrate Fiber, total dietary Protein Ash

g kcal g g g g g

32.39 239.00 0.52 62.73 7.10 2.61 1.76

Proximate 81.24 71.00 0.03 17.45 1.00 0.61 0.68

70.67 105.00 0.20 27.80 3.80 0.87 0.46

30.00 257.00 0.20 39.00 3.30 2.10 2.60

4.00 339.00 0.73 89.07 8.00 3.70 2.50

Vitamin A Vitamin C Vitamin E Thiamin Riboflavin Niacin Pantothenic acid Vitamin B6 Folate total

IU mg mg mg mg mg mg mg mg

1,987.00 3.30 1,450.00 0.08 0.16 1.96 0.46 0.26 4.10

Vitamins 3.00 4.10 0.01 0.02 0.07 0.79 0.11 0.22 0.00

797.00 2.80 N/A 0.03 0.12 0.89 0.10 0.20 0.00

2,000.00 4.30 N/A 0.04 N/A 2.50 0.43 N/A N/A

1,762.00 0.00 N/A 0.12 0.17 3.00 0.42 0.75 2.00

Sodium, Na Calcium, Ca Iron, Fe Potassium, K Phosphorus, P Magnesium, Mg Copper, Cu Zinc, Zn Selenium, Se Manganese, Mn

mg mg mg mg mg mg mg mg mg mg

4.00 56.00 2.48 745.00 79.00 40.00 0.43 0.58 2.30 0.22

Minerals 4.00 12.00 1.18 276.00 25.00 14.00 0.07 0.21 0.60 0.15

3.00 17.00 0.41 226.00 26.00 15.00 0.12 0.19 N/A 0.10

23.00 31.00 2.80 852.00 72.00 N/A N/A N/A N/A N/A

5.00 72.00 3.52 1,058.00 112.00 64.00 0.61 0.75 N/A 0.31

Note: IU = international units; N/A = not available. a

Canned in heavy syrup pack, solids and liquid combined.

Source: USDA Nutrient Database for Standard Reference, Release 14, July 2002, to be found at the Website: http://www.nal.usda.gov.fni.

Because of the high acid content and low water activity, microbial spoilage does not occur in dried plums with less than 25% moisture. The high monosaccharide content of dried plums provides low water activity (aw), between 0.65 and 0.83 for a moisture range of 19 to 35% (Bolin, 1980). Dried plums provide vitamin A, iron, potassium, niacin, riboflavin, and vitamin B6. Dried plums provide 1987 international units (IU) of vitamin A per 100 g serving (199 retinol equivalent). As with other fruits (such as peaches, apricots, etc.), the vitamin A activity of prunes is due to the presence of the various pro-vitamin A carotenoid compounds. In dried plums, the primary provitamin A is beta-carotene, the yellow pigment of the flesh (Bolin, 1977). The beta-carotene content of the fruit is 1192 mg/100 g of fruit, which is equivalent to the above vitamin A activity (1 IU of vitamin A is equivalent to 6 mg of beta-carotene). Also, Bolin (1977) reported that the flesh portion

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of a prune contains 15% more beta-carotene than the skin. Sanders (1993) reported industry averages for beta-carotene content at 1.31 mg/100 g dried prunes. Besides the carotenoids and vitamin A, the content of other essential vitamins is insignificant in dried plums. Among mineral components of dried plums, potassium is present in a substantial amount (745 mg/100 g), while sodium content is very low (about 4 mg/100 g) (Anon., 2002). Also, prune juice is very high in iron content, with 3 mg per cup (240 ml or 8 fl oz) as compared to 0.5 mg per cup in most other fruit juices. Van Gorsel et al. (1992) investigated specific compounds in dried plum juice that may be useful to determine authenticity. They reported the predominance of certain free amino acids in processed prune juice, such as a-amino-n-butyric acid, citrulline, taurine, and O-phosphoethanolamine, and the presence of quinic acid. Other significant differences they found between prune juice and other common fruit juices are absence of citric and tartaric acids in the prune juice, the presence of only a low amount of malic acid, and the absence of anthocyanins in the other juices.

21.10 NUTRITION, HEALTH, AND FOOD INGREDIENT FUNCTION 21.10.1 NUTRITION

AND

HEALTH BENEFITS

Dried plums are a good source of dietary fiber, sorbitol, potassium, copper, boron and phenolic compounds, which are active in a web of interrelated physiological and health-promoting functions. Together, these constituents are involved in bone metabolism and may help regulate glucose metabolism, promote cardiovascular health, protect against cancer, and contribute to laxation. A characteristic of fruits such as dried plums is their mixture of different antioxidants. Different oxidative stressors require different antioxidants that work together for a greater total effect that rarely is achieved when individual antioxidants are isolated as supplements. Dried plums contain a number of phenolic antioxidant compounds — mainly neochlorogenic acid, a phenol in the hydroxycinnamate family. A study by Donovan et al. (1998) at the University of California at Davis determined the level of total phenolic compounds and the concentrations of specific phenolic compounds in California dried plums. The amount of total phenolics were 1840 mg/kg in pitted prunes and 441 mg/l in prune juice. These are equivalent to 106 mg phenolic per 240 ml serving of prune juice and 73 mg phenolic per 42 g servings of dried plums. These levels are higher than reported for many other fruits and commercial juices. Specific phenolic compounds identified in the prune include: • • • • •

Neochlorogenic, 1306 mg/kg in pitted prunes and 225mg/l in prune juice Chlorogenic acid, 436 mg/kg in pitted prunes and 193mg/l in prune juice; Rutin, 33 mg/kg in pitted prunes and 4 mg/l in prune juice. Coumaroylquinic acid, 15 mg/kg in pitted prunes and 4 mg/l in prune juice Coumaric acid, 10 mg/kg in pitted prunes and 4 mg/l in prune juice

Phenolic compounds, particularly hydroxicinnamates, have shown strong antioxidant properties and are believed to be very important for human health because some chronic diseases begin with oxidation damage of free radicals. Daily consumption of either dried plums or prune juice would increase daily intake of these phytochemicals. An antioxidant analysis called oxygen radical absorbance capacity (ORAC) measures the total antioxidant power of foods and other substances. A study conducted at the Human Nutrition Research Center at Tuft University using the ORAC method evaluated dried plums, a number of other fresh and dried fruits, and vegetables for total antioxidant capacity (McBride, 1999). The ORAC test scores for produce are most meaningful when the values represent the fruit as generally eaten. The ORAC test used 100 g (31/2 oz), equivalent to 10 to 12 dried plums. In this test dried plums ranked highest in antioxidant capacity among 22 different fruits and vegetables. In fact, the

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score of dried plums at 5770 was more than twice as high as the score for raisins, the secondranked fruit, which scored 2830 on the ORAC scale. The thesis that oxidative damage culminates in many of the maladies of aging is well accepted in the health community. Results of the Tuft University study suggest that young and middle-aged people may be able to reduce the risks of diseases of aging — including senility — simply by adding high-ORAC foods such as the prune to their diet. Dried plums are long known for their supportive role in normal digestive functioning. Currently, the precise substances in dried plums that help promote bowel movement remain unidentified. It is common to think of fiber as responsible for the laxative effect. Generally, it is believed that the laxative effect of dried plums is most likely due to a combined action of fiber, sorbitol, and phenolic compounds, which might affect electrolyte and water balance in the intestinal tract. A serving of 5 dried plums provides 3 g of dietary fiber, making dried plums a good fiber source. Dried plums contain both soluble and insoluble fibers, which together could act in a gentle way in the lower intestine to soften the stool, increase bulk, and promote bowel motility. Prune juice, which has less dietary fiber than dried plums, also promotes bowel function. Both dried plums and prune juice contain sorbitol, a sugar alcohol that can cause diarrhea in large doses — more than 50 g at a time. A serving of 5 dried plums would provide about 6 g of sorbitol. A 4 oz glass of prune juice would have about 8 g of sorbitol, well below the amount that could cause diarrhea. Further research is needed to clarify the role that dried plums play in laxation.

21.10.2 POTENTIAL

OF

PLUMS

AS A

FUNCTIONAL FOOD INGREDIENT

Dried plums might be utilized as a functional food ingredient. Recent academic studies suggested that they may function as a natural antioxidant, fight pathogens, and serve as an antimicrobial agent in processed food. A Texas A&M University study showed that dried plum ingredients were as effective in reducing oxidative rancidity in precooked sausage products as some traditional synthetic antioxidants compounds (Pszczola, 2001). A Kansas State University study showed that dried plum puree and fresh plum juice effectively inhibited or suppressed bacterial growth in ground beef (Anon., 2001b). Affected bacteria included Salmonella, E. coli, Listeria and Staphylococcus. The study revealed that ground beef containing 3% dried plum puree could kill more than 90% of any E. coli present within 3 d. Over 99% of E. coli were killed after 5 d. These findings could lead to inclusion of dried plums in the preparation of precooked meats.

REFERENCES Anon. Prune preparations as raw material for the food industry (in German). Gordian, 93(11): 169. 1993a. Anon. The do’s and don’ts for making reduced fat baked goods using dried plums. Technical Bulletin, California Prune Board, Pleasanton, CA, 1993b. Anon. Dried Plum from California. The California Dried Plum Board, Sacramento, CA, 2000a. Website: www.Californiadriedplum.org. Anon. Prune Board shows new wrinkle. Bus. J., December 15, 2000b. Anon. The Almanac. Edward E. Judge & Sons, Westminster, MD, 2001a. Anon. Dried plums reinvented, may be used to fight bacteria. TB & Outbreaks Week, October 2, 2001b. Anon. USDA Nutrient Database for Standard Reference. Release 14. USDA National Agricultural Statistics Service, Washington. 2002. Website: www.nal.usda.gov. Bolin, H. R. Effect of processing on nutrient composition and texture of dried plums. J. Food Qual., 1: 125–133, 1977. Bolin, H. R. Relation of moisture to water activity in dried plums and raisins. J. Food Sci., 45: 1190–1192, 1980.

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Chichester, D. F. and Tanner, F. W. Antimicrobial food additives, in Food Additives, 2nd ed., T. E. Furia, Ed., CRC Press, Boca Raton, 1977, chap. 3. Delwiche, M. J., Tang, S., and Thompson, J. F. A high-speed sorting system for dried plums. Trans. Am. Soc. Ag. Eng., 36(1): 195–200, 1993. De Moura, J. and Dostal, H. C. Nonvolatile acids of dried plums. J. Agric. Food Chem., 13(5): 433–435, 1965. Donovan, J. K., Meyer, A. S., and Waterhouse, A. L. Phenolic composition and antioxidant activity of prunes and prune juice (Prunus domestica). J. Agric. Food Chem., 46: 1247–52, 1998. Kline, D. A., Fernandes-Flores, E., and Johnson, A. R. Qualitative determination of sugars in fruits by GLC separations of TMS derivatives. J. Assoc. Off. Anal. Chem., 53(6): 1198–2001, 1970. Lang, A. A. and Byer, E. M. Production of flavor-enhanced apple and prune concentrates, U.S. Patent 3,310,409, 1967. Lopez, A. A complete course in canning, in Processing Procedures for Canned Food Products, 12th ed., The Canning Trade Baltimore, MD, 1987, Book 3. Luh, B. S. Nectars, pulpy juices, and fruit juice blends, in Fruit and Vegetable Juice Process Technology, 3rd ed., D. K. Tressler and M. A. Joslyn, Eds., AVI Publishing, Westport, CT, 1980. chap. 10. Luh, B. S., Feinberg, B., and Chung, J. J. Freezing fruits, in Commercial Fruit Processing, 2nd ed., J. G. Woodroof and B. S. Luh, Eds., AVI Publishing, Westport, CT, 1986. chap. 7. Luh, B. S., Kean, C. E., and Woodroof, J. G. Canning of fruits, in Commercial Fruit Processing, 2nd ed., J. G. Woodroof and B. S. Luh, Eds., AVI Publishing, Westport, CT, 1986. chap. 6. McBride, J. High-ORAC foods may slow aging. Agriculural Research Magazine Vol. 47 No. 2, 1999. Paul, A. A. and Southgate, D. A. T. The composition of foods, in MCE Special Report, No. 297, 4th ed., Elsevier-North Holland Biomedical Press, Oxford, U.K., 1978. Pszczola, D. Antioxidants: From preserving food quality to quality of life. Food Technol., 55(6): 51–58, 2001. Sanders, S. W. Dried plums: A multi-functional bakery ingredient. American Society of Bakery Engineers, Bulletin No. 228, pp. 973–979, 1993. Scott, K. J., Yuen, C. M. C., and Gun-Hee, K. Sensory quality of Australian d’Agen dried plums in relation to fruit maturity and chemical composition. J. Sci. Food Agric., 62(1): 95–97 1993. Somogyi, L. P. Dried plums, a fiber-rich ingredient. Cereal Foods World, 32(8): 541–544, 1987. Somogyi, L. P. Fruit dehydration, in Wiley Encyclopedia of Food Science and Technology, F. J. Francis Ed., John Wiley & Sons, New York, pp. 1142–1148, 1999. Somogyi, L. P. and Luh, B. S. Dehydration of fruits, in Commercial Fruit Processing, 2nd ed., J. G.Woodroof and B. S. Luh, Eds., AVI Publishing, Westport, CT, 1986, chap. 8. Van Gorsel, H., Li, C., Kerbel, E. L., Smits, M., and Kader, A. A. Compositional characterization of prune juice. J. Agric. Food Chem., 40(5): 784–789, 1992. Woodroof, J. G. Production and history of noncarbonated beverages. In Beverages: Carbonated and Noncarbonated, J. G. Woodroof and G. F. Phillips, Eds., AVI Publishing, Westport, CT, 1974, chap. 2. Wrolstad, R. E. and Schallenberger, R. S. Free sugar and sorbitol in fruits — A compilation from the literature. J. Assoc. Off. Anal. Chem., 64: 91–103, 1981.

22 Strawberries and Raspberries Charlotte L. Deuel and Anne Plotto CONTENTS 22.1 22.2

22.3

22.4

22.5

Introduction ........................................................................................................................532 Worldwide Production .......................................................................................................532 22.2.1 Growing Areas and Export Markets....................................................................532 22.2.1.1 Strawberries........................................................................................532 22.2.1.2 Raspberries.........................................................................................535 Botany and Horticulture.....................................................................................................536 22.3.1 Horticulture ..........................................................................................................536 22.3.1.1 Strawberries........................................................................................536 22.3.1.2 Red Raspberries .................................................................................536 22.3.1.3 Black Raspberries ..............................................................................536 22.3.2 Berry Varieties .....................................................................................................536 22.3.2.1 Strawberries........................................................................................536 22.3.2.2 Red Raspberries .................................................................................538 22.3.2.3 Black Raspberries ..............................................................................540 Harvesting ..........................................................................................................................540 22.4.1 Strawberries .........................................................................................................540 22.4.2 Red Raspberries ...................................................................................................540 22.4.3 Postharvest Handling ...........................................................................................541 Processed Strawberry and Raspberry Products .................................................................542 22.5.1 Frozen Purees and Puree Concentrates ...............................................................542 22.5.1.1 Single-Strength Purees.......................................................................542 22.5.1.2 Puree Concentrates.............................................................................542 22.5.2 Frozen Whole and Sliced Fruit ...........................................................................543 22.5.2.1 Individually Quick Frozen (IQF).......................................................543 22.5.2.2 Block Frozen Berries .........................................................................545 22.5.2.3 Thawing of Frozen Berries................................................................545 22.5.2.4 Canned and Aseptic Berry Products..................................................546 22.5.3 Juice Concentrate.................................................................................................548 22.5.3.1 Raw Material Receiving ....................................................................548 22.5.3.2 Berry Preparation ...............................................................................548 22.5.3.3 Heat Treatment...................................................................................548 22.5.3.4 Enzyme Treatment .............................................................................548 22.5.3.5 Pressing and Centrifugation...............................................................551 22.5.3.6 Filtration.............................................................................................551 22.5.3.7 Concentration and Essence Recovery ...............................................552 22.5.4 Dehydrated Berries ..............................................................................................552 22.5.5 Jams, Preserves, and Condiments .......................................................................553 22.5.6 Fruit Preparations, Fillings, Syrups, and Toppings.............................................554

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22.5.7 Beverages and Wines...........................................................................................554 22.5.8 Other Products .....................................................................................................555 22.6 Composition and Chemistry ..............................................................................................555 22.6.1 Color ....................................................................................................................555 22.6.2 Flavor ...................................................................................................................556 22.6.3 Microbiological....................................................................................................556 22.6.4 Nutrition...............................................................................................................556 22.6.5 Health Benefits ....................................................................................................556 22.7 Market ................................................................................................................................558 References ......................................................................................................................................559

22.1 INTRODUCTION Strawberries, red raspberries, and black raspberries are known and cultivated throughout the world and are prized for their fresh, fruity flavor and bright red color.

22.2 WORLDWIDE PRODUCTION 22.2.1 GROWING AREAS

AND

EXPORT MARKETS

22.2.1.1 Strawberries The U.S. is the largest producer of strawberries in the world, followed by Spain, Poland, and Japan (Table 22.1). Total U.S. production decreased from 2000 to 2002 but still accounts for about 800,000 t. The U.S. has a large fresh strawberry market (86% of crop). Spain grows primarily for the fresh market in Europe. China’s production has grown significantly over the last decade to 120,000 t and it is likely to become a larger player in the world processed berry market in the future. California grows about 85% of the total strawberry production in the U.S. The overall production in California decreased from 2000 to 2002 but still accounts for 700,000 t. Oregon produces 15,000 to 18,000 t and North Carolina grows 9,000 to 10,000 t (Table 22.2).

TABLE 22.1 Worldwide Strawberry Crop Production — 1998–2003 Metric Tonnes Country

1998

1999

2000

2001

2002

2003

U.S. Spain Poland Japan Italy Korea Republic of Russian Federation Mexico China Turkey Germany

743,750 308,300 149,858 181,100 178,000 155,521 128,000

831,258 377,527 178,211 203,100 185,852 152,481 115,000

862,828 343,105 171,314 205,300 195,661 180,501 128,000

749,520 326,000 242,118 208,600 184,314 202,966 125,000

893,670 328,700 154,830 210,500 150,890 209,938 130,000

835,300 262,500 160,000 208,000 158,774 209,938 145,000

118,805 5,436 120,000 81,545

137,736 8,003 129,000 109,194

141,130 9,108 130,000 104,276

130,688 9,637 117,000 110,130

142,245 9,949 120,000 110,000

150,261 9,949 120,000 110,000

Sources: FAOSTAS data, 2004. http://faostat.fao.org/faostat/servlet/

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TABLE 22.2 U.S. Strawberry Production by State — 2000–2002 Metric Tonnes State California Oregon North Carolina Washington Pennsylvania New York Michigan Total

2000

2001

2002

740,182 16,845 10,500 5,864 2,954 2,955 3,772 866,818

630,000 18,273 8,909 7,273 3,909 2,727 2,636 735,818

686,591 15,000 N/A N/A N/A N/A N/A 782,773

Note: N/A = Data not available. Source: From Oregon Agricultural Statistics Service; Agricultural Statistics Board, NASS, USDA. Vegetables, July 2002.

TABLE 22.3 Processed Berry Production — Pacific Northwest, U.S. — 1999–2001 Metric Tonnes State

1999

2000

2001

Oregon Washington

Strawberries 18,136 15,227 4,363 4,955

17,273 6,182

Oregon Washington

Red Raspberries 5,886 6,000 29,704 30,568

6,636 32,500

Black Raspberries 1,314 1,727

1,727

Oregon

Source: From Oregon Agricultural Statistics Service; Economy Research Service, USDA — Fruit and Tree Nuts Situation and Outlook Yearbook — 2002. www.nass.usda.gov/or/berry02.pdf.

Processed strawberries account for about 150,000 t in California. Strawberry production begins in California in February and peaks between May and June; harvest continues until October. The strawberry season is several months longer than in the Northwest, where it starts in June. In 2001, Oregon strawberry production totaled 18,000 t with about 17,000 t going to processed products. Washington produced 7,300 t and processed 6,200 t (Table 22.2 and Table 22.3). Other significant producers of strawberries are North Carolina, Pennsylvania, New York, and Michigan. Processed strawberry production in the U.S. decreased from 230,000 t in 1998 to 173,000 t in 2001, about 25% of the total crop (Table 22.4). The total strawberry production was maintained due to a steady demand for fresh strawberries.

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TABLE 22.4 U.S. Strawberry Utilization — 1998–2001 Metric Tonnes Utilization

1998

1999

2000

2001

Total Processed % Processed

745,318 229,954 31

822,909 239,182 29

839,818 205,909 23

757,409 173,591 23

Source: From National Agricultural Statistics Service, USDA. www.usda.gov/nass/pubs/agr02/02_ch5.pdf.

TABLE 22.5 Domestic Consumption of Processed Strawberry Products — 2000

Country

Total Production (t)

Processed (t)

U.S. Poland Mexico Japan

839,818 238,500 121,254 205,430

211,503 110,231 43,090 521

Processed % of Total

Imports Processed (t)

Exports Processed (t)

Exports % of Total

Processed Consumed Domestically (t)

25 46 36 0.2

35,448 50 180 29,662

19,449 96,201 32,869 —

2 40 27 —

227,502 19,591 10,402 30,183

Source: From Foreign Agricultural Service, USDA. Data is not available for other countries.

Poland and Mexico process 35 to 45% of their crop and export 80 to 90% (Table 22.5). Poland follows the U.S. in frozen strawberry production at 100,000 t (Table 22.6). Spain, Mexico, and China each produce 35,000 to 50,000 t. Japan processes very little of its domestic crop, and depends on imports from the U.S., Mexico, and China for its processed strawberry product needs (Table 22.6). Poland and Spain’s primary markets are in Europe, while Mexico and the U.S. focus on

TABLE 22.6 Frozen Strawberries — Production and Exports — 2001 Total Crop (t)

Frozen Products (t)

Frozen% of Total Crop

Frozen Exports (t)

Exports – % of Total Crop

U.S.

760,000

174,000

23

33,000

4

Spain

325,500

52,000

16

45,000

14

Poland

238,174

103,000

43

90,000

38

Mexico

126,279

49,000

39

39,000

31

China

120,000

36,000 (estimate)

30

21,000

18

Country

Source: From Foreign Agricultural Service, USDA, Gain Reports, 2002.

Export Markets Japan, Canada, Australia, Korea, France Netherlands, Germany, France, Italy, Belgium Germany, Netherlands, France, Denmark, U.K. U.S., Japan, Canada, Australia, Austria Japan, Netherlands, Australia, Germany, U.K.

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TABLE 22.7 U.S. Exports — Frozen Strawberries — 1998–2001 Metric Tonnes Country

1998

1999

2000

2001

Japan Canada Other Total

13,830 8,514 4,723 27,070

14,660 8,464 2,204 25,335

9,850 7,794 1,787 19,431

9,095 7,782 16,179 33,056

Source: From U.S. Department of Commerce, U.S. Bureau of the Census, December 2001.

North American and Pacific markets. Japan and Canada have been primary export markets (Table 22.7) for the U.S. 22.2.1.2 Raspberries North American red raspberries are predominantly grown in the Pacific Northwest (Oregon and Washington) and British Columbia. Oregon processes 6,000 t and Washington processes 30,000 t (Table 22.3). Black raspberries account for 1,700 frozen t in Oregon. Overall, 40,000 t harvested in Oregon and Washington in 2001 were destined for both the fresh and processed markets. This represents 10% of the world production (Table 22.8). The seven largest producers of red raspberries in the world are the Russian Federation, the former Yugoslav Republics, U.S., Poland, Germany, Hungary, and Chile (Table 22.8). Much of the Yugoslav, Polish, and Hungarian stock is exported to other European countries such as Germany and Austria for concentration of juices and preservation as jams, preserves, and sauces.

TABLE 22.8 Worldwide Red Raspberry Production — 1999–2003 Metric Tonnes

Russian Federation Serbia and Montenegro U.S. Poland Germany Chilea Ukraine Hungary Canada United Kingdom France a

1999

2000

2001

2002

2003

100,000 60,000 49,351 43,195 35,500 28,700 (estimate) 13,786 22,277 15,650 11,000 7,020

102,000 56,059 51,256 39,727 33,700 29,000 (estimate) 19,723 19,804 16,247 9,500 8,743

90,000 77,781 54,885 44,818 29,200 29,000 (estimate) 19,137 13,306 11,658 9,800 8,549

100,000 94,366 52,889 45,026 29,000 —

108,000 94,400 53,000 45,000 29,000 —

16,400 10,000 14,291 8,400 7,999

16,000 10,000 13,900 9,000 8,000

Oregon Raspberry and Blackberry Commission (estimates).

Source: From FAO Statistics, 2004. http://faostat.fao.org/faostat/servlet/.

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22.3 BOTANY AND HORTICULTURE 22.3.1 HORTICULTURE To a botanist, strawberries and raspberries are not true berries. True berries, such as blueberries, cranberries, grapes, and citrus are simple fruits that develop from a single, entire ovary. An ovary is the part of the flower that contains or will become the seed or seeds of the plant. Strawberries and raspberries are really aggregate fruits or multiple fruits. 22.3.1.1 Strawberries Strawberries belong to the family Rosaceae and the genus Fragaria. Most domesticated cultivars are crosses between Fragaria chiloensis and Fragaria virginiana and also typed as Fragaria ananassa. Plantings of strawberries are made in the fall or spring, depending on the location. In California and warmer climates, fall plantings are preferred. Varieties may be of the spring-bearing type or the ever-bearing type. In colder regions, plants are usually put into the ground in the spring unless the roots in a fall planting can be established prior to a cold winter. The plants are available bare roots and are carefully planted in bed rows. Good soil drainage is important to prevent root rot in the plants. Plastic mulch helps to conserve moisture, provide needed warmth to the soil for maximum growth, and protect the berries from soil-borne insects. Strawberries are aggregate fruits of a perennial plant that begins bearing fruit in its second year after planting. Cultivars vary in fruit-bearing characteristics; some bear all the year round, some once a year. 22.3.1.2 Red Raspberries Red raspberries are called “cane berries” because they grow on 3- to 6-ft erect stalks with many short thorns. Known as bramble fruits, they are members of the Rosaceae family, genus Rubus. Most cultivars originated from Rubus idaeus. A raspberry is an aggregate fruit composed of bright red drupelets that are fleshy and contain seeds. Raspberries grow best in climates with cool summers and moderate winters. The plant yields fruit in its second year after planting. The canes are biennial, producing for 2 years. Pruning and training are important to the continuous production of a raspberry plant (Nagy, 1993). 22.3.1.3 Black Raspberries Black raspberries are domesticated forms of Rubus occidentalis, which is indigenous only to North America. Production of black raspberries is decreasing due to less cold hardiness than red raspberries, plant diseases, and variability in crop (Nagy, 1993).

22.3.2 BERRY VARIETIES 22.3.2.1 Strawberries In California, the predominant varieties used in processing are Chandler and Selva. Other varieties are Camarosa, Diamante, Seascape, Oso Grande, Driscoll, Tioga, and Douglas. In 1983, Douglas was the predominant variety followed by Aiko, but both varieties have almost disappeared as has the processing of strawberries in California. Strawberries grown in the Pacific Northwest were specifically bred for processing. Common varieties for processing are Totem, Hood, Redcrest, and Puget Reliance. Other varieties are Benton, Shuskan, Redgem, and Bountiful (Table 22.9 and Table 22.10). The Totem variety, developed in British Columbia, is favored for juice production because it contributes significantly to the overall color. The Hood variety was very popular for many years as a fruit used for preserves. It has

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TABLE 22.9 Characteristics of Northwest U.S. Strawberry Cultivars Strengths

Predominant cultivar in the Pacific Northwest. Rich color is desired for juice concentrate and purees.

Light-colored for processing

Highly colored, attractive, and flavorful for fresh market or processing

High-quality, acid fruit for processing

Fresh for processing

Totem High yield Large fruit Firm Caps easily Good internal color Good frozen quality Benton Fruit does not darken Fruit “holds well” in field

Shuksan Large attractive fruit Tough skin Excellent internal color

Weaknesses

Darkness Acid fresh flavor

Low yield Poor internal color Poor texture Too soft Occasional malformed fruit

Moderately productive Difficult to cap

Redcrest High yield Large fruit Excellent internal color Extremely firm Hood Early season Excellent internal color Excellent flavor Caps easily Redgem

Suited to IQF processing Bountiful Late season processing berry with concentrated ripening Source: From Moore, P. 1993. Small fruit cultivar characteristics. Proceedings of the Western Washington Horticultural Association. 83: 106–109; Stahler, M. 1992. USDA small fruit breeding. Proceedings of the Oregon Horticultural Society. 83: 189–193.

excellent color and flavor. It is not as suitable for individually quick frozen (IQF) fruit products. The Benton variety is not as rich in color. A new variety, Redcrest, is finding acceptance as an upcoming commercial variety. The following varieties are grown in Europe: •

Senga Sengan — Poland, the largest producer of strawberries in Europe and the world’s biggest exporter of frozen strawberries, grows the Senga Sengan variety that is preferred

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TABLE 22.10 Compositional Analyses of Benton and Totem Strawberries Benton Composition Total anthocyanin (mg/100 g) Flavanols (mg catechin/100 g) Leucoanthocyanins (abs units/g) Total phenolics (mg gallic acid/g fruit) (abs 280 nm) Nonflavanoid phenolics (abs unit) Ascorbic acid (mg/100 g) Soluble solids (∞Brix) Titratable acidity (meq/100 g) pH PPO (nmol/min/g) b-Glucosidase

Fully Ripe 32.7 46.2 3.45 1.5 0.83 7.8 40.2 7.8 127 3.33 465 3

Totem

Overripe 58.8 58.2 3.98 2.27 1.12 10.1 39 10.4 115 3.48 400 2

Fully Ripe 50.3 55.2 3.93 1.31 0.81 7.2 43.8 7.6 119 3.44 302 3

Overripe 80.6 68.2 4.83 2.06 1.11 9.2 42 8.7 111 3.57 201 3

Note: abs = absorbance. Source: From Pilando, L.S., Wrolstad, R.E., and Heatherbell, D.A. 1985. Influence of fruit composition, maturity, and mold contamination on the color and appearance of strawberry wine. J. Food Sci. 50: 1121–1125.

• • • • •

by European preserve manufacturers (Strick, personal communication). This round berry is dark red-brown, juicy, sweet-sour, full of aroma, and firm Fratina — heart-shaped fruit with dark red color, moderately firm texture, and pleasant sweet-sour flavor and aroma Fracunda — slightly sour berry of medium red color Gorella — relatively firm berry with sweet sour flavor and little aroma Havelland — red fruit that is juicy and slightly sour, possessing characteristic strawberry aroma Framura — medium-dark red fruit with fairly firm texture, juiciness, somewhat sour flavor, and strong strawberry aroma

22.3.2.2 Red Raspberries The raspberry has been traced to the city of Troy that existed over 2000 years ago. Pliny the Elder wrote about the raspberry in 45 A.D., describing it by the Greek name Ida after Mount Ida where the berry was gathered at the time. The blossom was used for medicinal purposes as an eye ointment or to soothe stomach ailments. By the fourth century A.D., raspberries were being cultivated. Raspberry seeds have even shown up in Roman fort site excavations. John Parkinson describes the berries as “raspis-berries” in his Paradisi in Sole. The name is likely to have been derived from the Anglo–Saxon Resp, which means shoot or sucker. Today, the name Ida, probably derived from the Ide mountains in Turkey, continues in the genus and species name for raspberry, Rubus idaeus (Jennings, 1988). Raspberries are known to be juicy and full of sweet, tart, characteristic raspberry flavor and aroma. The main varieties in the northwest U.S. and in the British Columbia area in Canada are Willamette and Meeker (Table 22.11). Other varieties are Chilcotin, Comox, and Chilliwack (Table 22.11 and Table 22.12), which are noted for their excellent bright color. In Chile, raspberry production is increasing, and Willamette and Meeker varieties are also dominant. The Willamette is a darker red berry than the Meeker. It has been used for some time but now is decreasing in favor of the Meeker. The Willamette is a large berry, with a long shape and firm

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TABLE 22.11 Characteristics of Northwest U.S./Canadian Raspberry Cultivars Variety

Leading variety, suitable for processing and fresh market; midseason

Dark fruit preferred for processing but darkens too rapidly for fresh market; early season

Midseason

New variety — very large attractive fruit with excellent flavor

Midseason

Fresh market cultivar; midseason

Strengths Meeker Medium yield Medium size Medium solids Consistent productivity Machine harvestable Good flavor Willamette High fruit color Fruit flavor Machine harvestable Early

Chilliwack Firm attractive fruit Machine harvestable High solids Tulameen Large fruit Excellent flavor Bright color Machine harvestable Comox High productivity Firm, attractive fruit Intermediate size Chilcotin Medium yield Does not darken rapidly Long harvest season

Weaknesses

Soft

Fruit color Moderate productivity Soft Small Low yield Low solids

Low yield

Moderate fruit firmness

Difficult to machine harvest Low to medium solids

Primarily fresh market cultivar Small size Soft Acid flavor Low-soluble solids

Source: From Moore, P. 1993. Small fruit cultivar characteristics. Proceedings of the Western Washington Horticultural Association. 83: 106–109; Daubeny, H. 1992. Prospects for new raspberry and strawberry varieties. Proceedings of the Western Washington Horticultural Association. 82: 110–111.

texture. The rich red color was favored by processors and consumers of raspberry juice concentrate. The Meeker variety is a thimble-shaped, bright red fruit with an excellent raspberry flavor. It is preferred as a freezing berry. In the northeast U.S., northern Europe, and Russia, cold hardiness is a key requirement for raspberry varieties. In Canada, varieties such as Ottawa, Viking, Chief, Latham, and Honeyking are selected for their hardiness. The Boyne variety has been used for processing in the northeast

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TABLE 22.12 Analysis of Red Raspberry Cultivars Variety

pH

Soluble Solids

Titratable Acidity

Meeker Willamette Chilliwack Comox

3.09 2.94 3.01 2.86

11.8 8.7 12.3 9.6

0.94 1.27 1.22 1.48

Source: From Moore, P. 1994. Development of new raspberry cultivars for the Pacific Northwest. Progress Report to the Oregon Raspberry and Blackberry Commission. pp. 54–59.

U.S. In Russia, cultivars include Novost Kuzmina, Pavlovskaya, Yunost, and Rakyeta. In Eastern Europe (Poland, Hungary, former Yugoslavia, and Bulgaria), growers have utilized many varieties that have been bred for the climate in the region. Malling Exploit and Krupna Dvoroda are grown in former Yugoslavian countries. Rubin Bulgarski, Newburgh, and Shopska Alena are produced in Bulgaria. In Hungary, varieties such as Malling Exploit, Nagymarosi, and Fertöd 4 are known for their excellent flavor (Jennings, 1988). Raspberry cultivars grown for processing in Europe (Table 22.13) are Malling Jewel, Malling Admiral, Norfolk Giant, Zeva, Glen Clova, Bulgarian Ruby, and Schoenemann. Many of the European varieties originated in Britain. 22.3.2.3 Black Raspberries A North American indigenous species, Rubus occidentalis, has been domesticated over the years to produce today’s black raspberry. Black raspberries were developed and found greater propagation and introduction in the 1800s. Typically, the Munger variety is grown for processing. It consists of large, plump, firm fruit. Little crumbliness or seediness is seen in this variety. In addition, it is resistant to diseases.

22.4 HARVESTING 22.4.1 STRAWBERRIES Strawberries are picked by hand and transported in flats to the processing plant. For purees and juices, the berry cap is often attached to the berry because it is later removed in the pulping and finishing processes. If the berries are destined for the freezer, they need to be of a higher grade and free of extraneous material and defects, with good firmness and color.

22.4.2 RED RASPBERRIES When picked, the core of the raspberry remains on the branch. Raspberries are hand-picked for the fresh market. Because they are so fragile, they do not transport well, though more transportresistant varieties are in development. For the industrial processing market, raspberries are machineharvested. Over 75% of the red raspberries produced in the Pacific Northwest are machineharvested. The advantages of machine harvesting are cost savings for the grower and a better product for processing, including better color and higher soluble solids than in handpicked berries. During harvesting, about 20% of the crop is lost on the ground. Current yields are approximately 2.5 t/acre. The large harvesting machines are typically based on a U-shaped frame that straddles the canes in each row. Finger-like rods or rotating heads gently loosen the berries from the canes. The tension

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TABLE 22.13 Characteristics of European Raspberry Cultivars Variety Glen Clova

Malling Jewel

Zeva

Schoenemann

Characteristics Early season Good for processing Small–medium berry size Firm in texture Medium to pale red Midseason Medium–large fruit Medium red color Seed flavor Firm fruit Early midseason Medium–large fruit Dark red Firm Pleasant sweet–sour flavor Strong raspberry aroma Late season Large fruit Dark red Firm Slightly sour Little aroma Good for juice

Source: From Nagy, S. (Ed.). 1993. Fruit Juice Processing Technology. AgScience, Auburndale, FL. pp. 26, 436–514.

of the rods is adjusted so that only ripe berries are removed. Several passes may be made through the field before most berries are removed. Research is currently focused on improved cultural techniques to increase the machine harvesting efficiency. In the Pacific Northwest, raspberries are traditionally grown on hills, with plants set 2 ft apart in rows spaced 10 ft apart to allow passage of harvesters. A hedgerow system consisting of a solid row of canes is currently under investigation as a method to increase harvester efficiency and yield per acre.

22.4.3 POSTHARVEST HANDLING The shelf life of fresh strawberries is dependent upon time and temperature relationships and the degree of infection with fungi such as Botrytis cinerea and Rhizopus. Ambient postharvest storage conditions will reduce usable fruit, increase Howard mold count, darken the berry color, and increase the soluble solids due to desiccation. Processors transport the berries from the field to the processing plant as quickly as possible and process the berries within a few hours. Other facilities control decay by cooling quickly after harvest and storing the strawberries at cool storage temperatures of 35∞F (2∞C) and at high humidity. Factors such as weather conditions, field condition, humidity, and packing container can affect shelf life. Typically, strawberries are hand-harvested. Mechanical harvesting is also used in some regions. Bulk-harvested berries have a shelf life of 6 to 8 d if they are forced-air cooled and stored at 35∞F (2∞C). Treatments with sulfur dioxide have been shown to minimize mold growth (Smith, 1986).

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Raspberries are also susceptible to fungi infections in the field and during postharvest storage. As noted with strawberries, raspberries should be stored at temperatures just above freezing to maximize shelf life. A study using sulfur dioxide generator pads showed a reduction in visible mold growth but little reduction in Howard mold count. At low temperatures (35∞F; 2∞C), color was maintained, but at higher temperatures (70∞F; 21∞C), there was some bleaching of the anthocyanins with the sulfur dioxide (Spayd et al., 1984).

22.5 PROCESSED STRAWBERRY AND RASPBERRY PRODUCTS 22.5.1 FROZEN PUREES

AND

PUREE CONCENTRATES

22.5.1.1 Single-Strength Purees Frozen berry purees are used in jams and preserves, and also in beverages such as nectars. Many industrial ingredients such as bakery fillings or formulated fruit pieces contain puree as the source of fruit solids. Puree is processed from fresh berries that are transported directly from the grower or packer to the processor or from frozen sortouts or sieved berry pieces that do not meet standards for IQF, straight pack, or fresh fruit. The berries should be clean and free of rot or significant mold and possess typical color and flavor. Strawberries may still have berry caps attached because the berry will be screened prior to packing as a puree. Some small levels of extraneous material such as leaves may also be present in the stock material to be used for strawberry, red, or black raspberry puree. The berries are pregraded for Howard mold count, Brix, pH, titratable acidity, and visual color and appearance. Based on industry standards, the berries are graded as “puree stock.” The berries are dumped from the plastic crates or flats into a cleaning chamber. Using high air volume exhaust, a McLaughlan or similar air blower pulls extraneous materials from the berries as they are conveyed through the chamber. After washing with chlorinated water, the berries are sorted and inspected. The inspectors remove rot and major defects. The empty crates are washed thoroughly and returned to the field for the next picking cycle. After inspection, the berries are either frozen in drums or totes for further processing or conveyed to a chopper or disintegrator. After chopping, the berry pulp is sieved or screened in a pulper or finisher to remove extraneous material (leaves, caps), and a homogeneous puree is produced. For a seedless strawberry puree, a screen with openings of 0.027 in. or 0.033 in. is used. Sieves of 0.045 in. or 0.60 in. are used to produce strawberry puree with seeds. Seedless raspberry puree is produced using a screen of 0.045 in.; screens of 0.060 in. or 0.125 in. are used for a raspberry puree with seeds. The berry purees may be pasteurized at 190∞F (88∞C) for 11/2 to 2 min prior to cooling to 60 to 70∞F (15 to 21∞C). Some purees may be packed without pasteurization to maintain fresh strawberry flavor. Enzymatic browning may occur with some varieties during thawing of nonpasteurized purees. Packing types are high-density polyethylene pails, polyethylene-lined kraft cardboard cases, or food grade steel drums lined with low-density polyethylene liners. The containers are placed immediately into a blast freezer at –5 to –10∞F (–20 to –23∞C) and then transferred into cold storage rooms at 0∞F (–18∞C). Typical pack sizes in the U.S. are 6.5-lb (2.93-kg) tubs, 28-lb (12.6-kg) pails, or 400-lb (180-kg) drums. 22.5.1.2 Puree Concentrates The puree concentrate process is outlined in Figure 22.1. Puree concentrate has the advantage of increased soluble solids content. Often, the concentration is referred to as -fold (¥). A twofold (2¥) raspberry puree concentrate would be approximately 20∞ Brix, or twice the soluble solids of singlestrength puree. A typical strawberry puree concentrate is 4¥ or 28∞ Brix. Sometimes, sucrose may be added to the puree prior to concentration to achieve higher soluble solids in the finished product and a sweeter puree. The sugar also helps to stabilize the anthocyanin pigments. The advantage of puree concentrates is a reduction in total weight and, thus, decreasing storage and shipping costs.

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Receive raw fruit

Rinse fruit

Inspect fruit

Chop fruit

Heat pulp

Digest pulp with enzymes

Remove seeds

Pasteurize

Concentrate

Cool

Fill pails or drums

FIGURE 22.1 Puree concentrate process.

The jam, preserve, filling, and confection industry requires higher fruit solids to reduce process time, maintain fruit flavor, and achieve a low water activity level (0.5 to 0.7 aw) to minimize microbiological growth and migration of moisture in a finished bakery product. Typically, the viscosity of puree concentrate is higher due to pectin levels. Some manufacturers add pectinase enzymes to reduce pectin levels in purees prior to concentration. The depectinized purees are pasteurized and then concentrated in multieffect vacuum evaporators designed to handle pulpy products. Purees are thawed for 40 to 48 h in refrigerated storage prior to use. Care must be taken to use the berry purees while they are still in the semithawed state to avoid microbiological growth.

22.5.2 FROZEN WHOLE

AND

SLICED FRUIT

22.5.2.1 Individually Quick Frozen (IQF) Strawberries must be firm, ripe, fully colored, and flavorful to be selected for the IQF process. The berries are picked at optimum ripeness and transported to the processing plant. If mechanical freezing is used, berries are forced-air cooled at 35∞F (2∞C) to remove field heat. It takes 15 to 30 h to lower the product temperature to 35∞F (2∞C), resulting in 4 to 5% loss in product weight due to dehydration (Acharya et al., 1989). After being transferred from flats or crates, the berries are gently washed and cleaned prior to inspection. Unripe, soft, or unacceptable berries are sorted on the inspection belt. Inspection lines and equipment are rinsed clean and sanitized on a regular basis

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(every 2 to 4 h). Individual fruits are quick frozen in a blast air freeze tunnel (–40∞F; –40∞C), liquid carbon dioxide or nitrogen freezer, or on trays in a blast freezer (–5 to –10∞F; –20 to –23∞C). Quick freezing reduces ice crystal size and thus minimizes drip loss upon thawing. The original shape of each berry is maintained, and the berries are free-flowing, allowing ease of use. Glassy state is a term often used to describe a product at a temperature where molecules are slowed to the point where they are no longer reacting with each other (Berne, 1994). Some processors are implementing this technology into their freezing operations and storage facilities. Each fruit has a different minimal critical temperature that results in longer shelf life, better color and flavor, and reduced drip loss in the final product. Different methods of freezing are chosen, depending on availability of the refrigerant or cryogenic gas and also the developing regulations governing the environmental policies. Mechanical systems depend on refrigerants such as chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFCs), ammonia, or liquid carbon dioxide in a closed loop. The Environmental Protection Agency (EPA) in the U.S. and other government agencies worldwide agreed in 1992 to phase-out CFCs by 1996 and HCFC by 2030 because of their connection with depletion of the ozone layer and the trapping of heat in the atmosphere, resulting in global warming (Rowlands, 1993). The Montreal Protocol of 1987 and the Copenhagen Agreement of 1992 are international agreements that have determined schedules for phaseout of these refrigerants. Ammonia is being chosen as the replacement for HCFC and CFC in spite of safety hazards in the event of leakages. The capital costs are high for mechanical systems, but operational costs are less than those of cryogenic freezing. Mechanical freezers operate at –40∞F (–40∞C) and freeze the berries to 10∞F (–13∞C) in 10 to 15 min. A dehydration loss of 1 to 2% occurs in this process. Some of the disadvantages of mechanical freezing are product dehydration, product breakage, evaporator coil frosting in mechanical chillers reducing performance, and lower quality than a cryogenically frozen product (Acharya et al., 1989). Liquid nitrogen (LN2) or carbon dioxide (CO2) is used in cryogenic freezing systems. In Figure 22.2, a cryogenic immersion freezer is attached to a cryogenic tunnel freezer. Liquid nitrogen is often used for freezing berries. Instantaneous freezing occurs as the gas state changes from liquid to gas. The boiling point is –320∞F (–196∞C), resulting in faster freezing than carbon dioxide freezing. Some raspberry processors combine a liquid nitrogen immersion unit with a nitrogen postcool tunnel freezer. A combination of cryogenic and mechanical freezing called cryomechanical is also utilized. Freezing costs are reduced by utilizing the cryogenic unit for crust freezing the berries, following up with passage through a mechanical tunnel that completes the freezing of the berry. The quantity of cryogenic gas per unit of berries is lessened, therefore reducing the cost. Precooling the berries is not required using this process. The benefits of quick freezing, such as moisture retention and minimization of ice crystal damage, are realized using this process. Processors who currently have a mechanical tunnel are finding that a cryogenic prestep enables them to operate more efficiently and produce a better product. For strawberries and raspberries, a fluid bed mechanical freezer is most appropriate. Berries are also frozen individually by spreading them in thin layers on trays in a blast freezer and then packing them in cartons once they are frozen. The freezing time is longer than the above processes, resulting in increased ice crystal damage. The capital costs are lower with this method, but the labor inputs are higher. Because drip loss is a concern for many applications such as yogurt

FIGURE 22.2 Diagram of a liquid nitrogen freezer (courtesy of Air Products and Chemicals, Inc.).

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TABLE 22.14 Effect of Air Velocity on Freezing Time Air Velocity (ms-1)

Freezing Time (min)

1.7 2.3 4.2 5.4 6.1

43.1 31.8 25.4 23.2 19.3

Source: From Muftugil, N. 1986. Theoretical and experimental freezing times of strawberries. Int. J. Refrig. 9: 29–30. International Institute of Refrigeration, 177 boulevard Malesherbes, 75017 Paris, France. E-mail: [email protected]. Web site: www.iifiir.org.

fruit preparations, the processors will need to consider their customer’s needs as they choose freezing systems. In one study, strawberries were frozen in a blast freezer at –30∞F (–34∞C) at different air velocities. The freezing time to –10∞F (–23∞C) was measured. Freezing times decreased with higher air velocities (Muftugil, 1986a). The results are presented in Table 22.14. 22.5.2.2 Block Frozen Berries 22.5.2.2.1 Straight Pack Strawberries are picked by hand without berry caps and are transported in flats to the processor. The berries are graded for Howard mold count, color, flavor, texture, Brix, and other quality components. After cleaning and sorting, the strawberries are left whole or sliced, diced, or crushed, according to the industrial customer’s specification. The prepared berries are packed into plastic tubs, pails, or 55-gal drums. These finished products are then blast frozen at –40∞F (–40∞C) prior to storage in cold storage rooms at less than 0∞F (–18∞C). Overripe fruit is soaked in calcium lactate prior to packing. Using objective texture measurement and sensory testing, an increase in firmness can be observed. During the freeze–thaw cycle or in heating the berries, pectin methyl esterase is activated, and the pectic substances are deesterified. The calcium ions then bind themselves between pectin polymers in the middle lamella region of the cells. The resulting effect is a perceived firmer texture (Main et al., 1986). 22.5.2.2.2 Sweetened These products are typically used in ice cream, yogurt, or bakery preparations or fillings. Whole, sliced, or crushed berries are blended with sugar for ratios such as 4 + 1 (berries + sugar), 3 + 1, or 7 + 1. A usual procedure is to “cap” or sprinkle sugar on the surface of the berries after they have been filled into pails or drums. In the U.S., the quality of the berries is USDA Grade A or B. Wrolstad et al. (1990) found that sucrose has a protective effect on the anthocyanin pigments. Polymeric color and browning color development decreased with increased sucrose content. This effect may be due to inhibition of polyphenoloxidase. The protective effect is likely to occur during the freezing and thawing cycles, as well as storage. Adding corn syrups to berries has also been shown to have a stabilizing influence on the anthocyanin pigments (Beery, 1978). 22.5.2.3 Thawing of Frozen Berries During freezing, a change of state occurs as water changes to ice. Depending on the freezing method, the drip loss and damage to the fruit can vary significantly. The cells lose their liquid

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TABLE 22.15 Effect of Thawing Method on Drip Loss Thawing Method Variable 1.5 2.1 2.5 3.1 3.5 Variable 20∞C 30∞C 40∞C 50∞C 60∞C Variable 30∞C 40∞C Variable 22∞C 6.5∞C

Drip Loss (%)

air velocities (22∞C) (ms–1) 6.0 6.5 5.8 5.4 4.3 temperatures — water bath 5.3 5.1 8.4 9.8 12.5 temperatures — circulating air 6.8 9.0 temperatures — no air circulation 7.1 4.2

Source: From Muftugil, N. 1986. Thawing of frozen strawberries. Int. J. Refrig. 9: 31–33. International Institute of Refrigeration, 177 boulevard Malesherbes, 75017 Paris, France. E-mail: [email protected]. Web site: www.iifiir.org.

contents, and the cell components no longer have the water-holding capacity of fresh fruit. With structural damage, there is a loss of intercellular gases and cell juices and a perceived loss of freshness. The thawing cycle has been shown to affect drip loss or leakage. The thawing cycle is longer than the freezing cycle. Different thawing methods (room temperature, circulating air, cool storage, water bath, and convection oven) were evaluated in a study. The results showed that the temperature of the frozen berries rises rapidly to the melting point and remains at this temperature until all the ice crystals are melted. The temperature then rises rapidly after complete thawing. Temperature has a significant effect on the thawing time. Water is a faster thawing medium than air. Increasing air velocity decreases the thaw cycle at a given temperature. Drip loss increases with higher thaw temperatures. Air velocity was not found to affect drip loss. The results are seen in Table 22.15. For best results, frozen berries should be thawed slowly in cool storage or a walk-in refrigerator. After thawing for the time periods noted in Table 22.16, chilled fruit should be used within 2 d, and room temperature fruit within 2 h. Changes in sucrose, fructose, and glucose contents that are naturally present in frozen strawberries were found to occur during thawing. Sucrose content decreased by 70% due to invertase activity. Sucrose is inverted to fructose and glucose. Other mechanisms of sugar hydrolysis such as sucrose synthase activity or acid hydrolysis were not observed. The total free sugar content remained constant (Skrede, 1983). 22.5.2.4 Canned and Aseptic Berry Products 22.5.2.4.1 Canned Products Raspberries and strawberries for canning are picked as firm as possible and transported to the plant where they are graded. After cooling for several h at 35∞F (2∞C) to firm the berries, they are washed

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TABLE 22.16 Thawing Times for Frozen Berries Form

Pack Size

Time

Sliced sweetened berries Sliced sweetened berries Whole IQF berries Sweetened puree Whole/sliced puree

6.5 lb 30-lb pail 30-lb carton 30-lb pail 55-gal drum

10–15 h 36–48 h 30–40 h 40–48 h 168–216 h (7–9 d)

Source: From California Strawberry Commission.

and sorted. Inspectors remove mushy, moldy, green, and damaged berries from the inspection belt. Raspberries are handled as little as possible, so they are not screened prior to packing. Strawberries are graded over screens to remove oversized or undersized berries. The canning berries are filled into cans, and the cans are weighed. Weights are adjusted for each can to assure the proper level of berries in each can. Hot corn syrup or sucrose syrup is then added to each can. A steam jet exhausts each can as it is conveyed to the seamer. The lids are then seamed on the cans. Production codes are affixed to each can. The cans are transported to a cooker that oscillates the can to bring the internal temperature to at least 190∞F (88∞C). An oscillating cooler is then used to cool the cans to approximately 95∞F (35∞C). The cans are dried and palleted until labeling. The labels are glued onto each can. The cans are placed in trays of eight cans and then wrapped with plastic shrink-wrap film. The trays are stacked onto pallets and stored in a cool, dry warehouse until shipping. 22.5.2.4.2 Aseptic Products Juice is prepared by extracting fresh fruit or reconstituting juice concentrates to single-strength juice. Juice is introduced into the deaerator, which removes entrained oxygen. Loss of color and vitamin C results in the heating and subsequent storage if oxygen is not removed. Deaeration should not remove fruit volatiles. The juice is typically heated using a plate heat exchanger system to 200∞F (93∞C) and held at that temperature for several minutes to destroy spoilage organisms. The juice is then cooled to 75∞F (24∞C) and filled into aseptic packages using aseptic fillers. The process is controlled to avoid product recirculation due to insufficient heating or filler stoppages. Indirect tubular heat exchangers are used for higher viscosity liquids and purees that would cause fouling of plates in a plate heat exchanger. Scraped-surface heat exchangers consist of scraper blades on a dasher, which is rotated within a cylinder. This indirect heat process is used for heat-sensitive products such as particulates and purees. A heat exchange medium such as steam is applied to a jacket on the outside of the cylinder. A rotary pump transfers the puree or chunky product into the heating cylinder. The temperature is brought to 200∞F (93∞C), and then the product is held in a holding tube for 30 sec to several minutes, depending on piece size. The product is then cooled to 75∞F (24∞C) in water or glycolcooled heat exchange cylinders (APV, 1992). A new process called ohmic heating was found to significantly improve the quality of chunky fruit in aseptic processes such as fruit for yogurt. The product is heated using direct electrical resistance heating. The particulates and liquid phase are heated simultaneously by the passage of electric current through the product. Due to even heating, the overall process time is reduced, and the product quality is improved. Commercial systems are found in the U.K., Japan, U.S., and France (Parrott, 1992). Commercially sterile containers are used for packaging aseptic products. It is important that all seals are sterile and potential for contamination is eliminated. Types of aseptic packages are

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aseptic cartons, form-fill-seal tubs, bottle-shaped containers, and bag-in-box systems. Retail and industrial size packages are possible with aseptic processing.

22.5.3 JUICE CONCENTRATE 22.5.3.1 Raw Material Receiving Diagrams showing unit operations for berry juice and juice concentrate processing are shown in Figure 22.3 and Figure 22.4. Berries are delivered to the plant as fresh berries and are graded prior to processing. Precooling and cool storage may be necessary if plant capacity does not allow immediate processing. The berries may also be inspected and frozen in bulk drums for later processing. Frozen fruit is then thawed prior to processing in a tempering room or for several days at room temperature. Proper thawing times are important to avoid fermentation of the frozen berries. The berries are mechanically cleaned in a machine with air blowers to remove leaves or other debris. It is important to minimize the growth of mold in both raspberries and strawberries before processing into juice. A study of strawberry juice concentrate made from Benton variety strawberries showed that mold content had a significant effect on the finished quality. Fungal enzymes, such as polyphenoloxidase and glycosidase, are likely catalysts in the destruction of anthocyanin pigments. Also, with increased browning due to polyphenoloxidase activity, the ratio of anthocyanin color intensity to brown color (browning ratio) decreases. Howard mold counts are used by processors to assess the positive fields containing mold fragments under microscopic analysis. Typical species of mold found in strawberries are Rhizopus and Botrytis cinerea. Greater mold contamination decreased color intensity, increased browning, and produced hazes that were more difficult to remove in processing (Rwabahizi and Wrolstad, 1988). 22.5.3.2 Berry Preparation The berries are washed minimally with low-pressure water sprays to remove dirt and are then inspected for excessive mold, rot, green, and other defective berries. The berries are chopped to a mash or pulp in a distintegrator or roller-type mill (Figure 22.5). 22.5.3.3 Heat Treatment Blanching has been indicated as a method to improve the color in strawberry juice concentrate due to deactivation of polyphenoloxidase and glycosidase. The process must be controlled to minimize degradative processes that are accelerated by heat (Rwabahizi and Wrolstad, 1988). Heat treatment also acts to deactivate anthocyanases that will break down the anthocyanin pigments present in strawberries and raspberries and also polyphenoloxidases that can contribute to browning of macerated pulps. The cell wall membranes are denatured, allowing the cell contents to pass through the membranes. The pulp is heated in a continuous heat exchanger, typically of the tubular, spiral, or augur type. A preheating treatment is necessary for the pectolytic enzymes to optimally break down the cell structure and allow the juice to be extracted from the fruit. The optimal temperature for enzyme treatment is around 120∞F (50∞C). 22.5.3.4 Enzyme Treatment The pulp is treated with enzymes prior to pressing to allow maximum juice yield. The enzyme is metered into the digestion tanks or added just after the heat treatment. Traditional methods consist of treatment with pectolytic enzymes such as pectin methyl esterase, polygalacturonase, and pectinylase. Maceration enzyme preparations also contain hemicellulase, cellulase, and amylase. Recently, it has been noted that side chain activity of these enzyme preparations may adversely

12 TRESTER CONTAINER Pomace container 13 ROHSAFT TANK Raw juice tank

5 MAISCHEERHITZER Mash heater 6 ENZYMDOSIERUNG Enzyme dosing

FIGURE 22.3 Processing line for berry juice production (courtesy of Bucher Guyer Ltd.).

7 MAISCHE TANK Mash tank

11 TRESTER-AUSTRAGSCHNECKE Pomace conveying screw

4 BEERENMUHLE KVS 2–80 Berry mill

11

9

CH-8166 NIEDERUENINGEN

BUCHER-GUYER AC

BUCHER

DATUM NAME GEZ. 11.02.03 M. Hauser . GEPR . FREIG

10 TRESTER BUNKERSCHNECKE Pomace collecting screw

10

3 SCHRAGSCHNECKE Inclined srew

7

9 BUCHER HPX 5005i UNIVERSAL PRESSE BUCHER HPX 5005i UNIVERSAL PRESS

5

2 DAMPFZUFUHR Steam injection

4

8

8 MAISCHERUEHRWERK Agitator Mash tank

2

3

6

1 BEERENSILO Silo for berries

1

BLATT 1 VON 1

2.K-93-075-00-A3 A3

FLIESSSCHEMA/Flow Sheet Beerenlinie/Berry processing line

12

13

Strawberries and Raspberries 549

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Processing Fruits: Science and Technology, Second Edition

Receive raw fruit Rinse fruit Inspect fruit Chop fruit Heat pulp Add Enzymes Digest pulp with enzymes Press pulp Centrifuge pulp Clarify/Filter Juice

Solids to waste or byproduct line

Pasteurize juice Concentrate

Cool concentrate

Recover essence

Blend in essence Store in tanks Fill pails or drums

FIGURE 22.4 Juice concentrate process.

FIGURE 22.5 Pulper-finisher mill for preparation of berry pulp (courtesy of Brown International Corporation).

Strawberries and Raspberries

551

FIGURE 22.6 Hydraulic press (courtesy of Bucher Guyer Ltd.).

affect the anthocyanin contents in the finished juice. The enzyme manufacturers each have a blend of enzymes that are suited to berry juice manufacture. The pH and temperature of the pulp is critical to its efficiency. The pH of raspberry and strawberry pulp will not typically adversely affect the enzyme activity. However, temperature is critical for maximizing the capacity of the process. The fruit pulp is treated with the enzymes until a negative pectin result occurs. Generally, treatment time in stainless steel tanks at 120 to 125∞F (49 to 52∞C) consists of 1 to 2 h. 22.5.3.5 Pressing and Centrifugation Selection of the proper press for a juice operation is dependent on capital funds available, desired product yields, and availability of press aids. The types of presses that have been used to extract berry juice are horizontal hydraulic, mechanical horizontal basket, horizontal pneumatic, horizontal and vertical screw type, and belt. Horizontal hydraulic presses manufactured by Bucher–Guyer are found worldwide. The advantages are high production capacity, the absence of a need for press aids, and minimum suspended solids content. The Bucher Multipress model is finding application in smaller plants (Figure 22.6). The press is filled with a central mash feed. As the press is filled, some free run juice may be extracted. The pressing cycle consists of an initial pressing phase where low pressures extract free run juice, a pressure rise phase of up to 2 bars, and a final pressing phase. The drum is rotated to loosen the cake. After the pressing cycle, the pomace is discharged. Leaching may be used to increase yield of sugar solids and to extract further color. Screw presses such as the Jones or Reitz have been used for berries but require the introduction of press aids such as rice hulls, wood fiber, or paper for maximum effectiveness. The horizontal pneumatic press known as Wilmes Tankpress uses a pressure of 2 to 6 bar to extract juice. The pressure is uniformly distributed on the mash as it is being pressed. Belt presses (Belmer, Klein, Ensink, and Hamako) have been used to press mashes by pressing between fine mesh belts, which are compressed to thinner and thinner belt gaps. Centrifugation is finding several applications in juice processing. Some plants have converted from press systems to self-cleaning centrifuges. Press aids have been eliminated. Yields are higher, and waste is decreased. One plant reports a processing rate using four centrifuges of 1000 tons/d. Decanter centrifuges separate fiber solids and clear juice (Nagy, 1993). 22.5.3.6 Filtration Traditionally, plate filters operated under pressure have been used to clarify juices. The plates are coated with a layer of diatomaceous earth, which filters colloids, particles, high-molecular-weight carbohydrates, and protein complexes from the juice. The clarity of a single-strength juice is

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measured in nephelometric turbidity units (NTU) in a Hach Turbidimeter. A visual examination under strong light will also reveal particles that were not completely filtered from the juice or were reintroduced into the juice in the latter stages of the process. Another process that has been tested and used commercially on strawberry and raspberry juice is ultrafiltration. One study used hollow fiber membranes with a cut-off of 10,000 molecular weight. The result is a greater loss of anthocyanin pigments using the ultrafiltration process. Ultrafiltration also removes a significant level of polymerized pigments. Melanoidin pigments, however, smaller than the molecular-weight cut-off, are not removed to a significant degree. Browning also increases in storage due to nonenzymatic changes rather than enzymatic. Earlier studies with molecularweight cut-offs of 10,000 to 50,000 found that polyphenoloxidase was removed; therefore, ultrafiltered juice would not be susceptible to postfiltration or concentration enzymatic browning (Rwabahizi and Wrolstad, 1988). Heating of the enzyme in the concentration process would also deactivate it. 22.5.3.7 Concentration and Essence Recovery The following types of evaporators are available for juice or puree concentration processes: batch pan and plate-type evaporators such as rising–falling film, falling film, Paravap, and Paraflash systems. Batch pan systems are used in some jam processes. Soluble solids are increased to 65% or above in a jacketed vacuum kettle. The rising–falling film plate package consists of a single pass rising and falling film principle. The liquid is vaporized as it contacts steam-heated plates and is discharged to the vapor–liquid separator. This separator condenses flavor volatiles from the vapor using differential temperatures of vapor condensation. Several effects are typically used in tandem to concentrate to high-soluble solids of 65∞ Brix. The falling film plate evaporator’s advantages over the rising–falling film evaporator are decreased residence times and higher evaporation capacity. A double pass is possible with this system. The Paravap system is used to concentrate both juices and purees. A corrugated plate pattern in the plate heat exchanger allows fluids to vaporize at high velocities, resulting in greater liquid surface area for mass transfer. Products that are highly viscous can be evaporated because only moisture is transported in the vapor. Two or three effects are typically used (APV, 1989). A new process, Osmotek, is a cold, direct osmotic concentration process utilizing a thin membrane (25 to 100 mm) with a molecular-weight cut-off of about 100 Da. High-fructose corn syrup is used as the agent to facilitate osmotic concentration as it flows countercurrent to the juice. The quality of raspberry juice concentrate is very close to the single-strength quality. Heat labile juices such as strawberry would benefit from this cold process because flavor and color degrades with exposure to heat (Wrolstad et al., 1993). Strawberry juice concentrate is concentrated to soluble solids to meet customer specifications, which may be 36, 51, or 65∞ Brix. Raspberry juice concentrate is offered at 45, 65, and 68∞ Brix. Puree concentrates vary from 20 to 40∞ Brix. Essence is condensed from the evaporated vapors by controlling condenser liquid temperatures. Essence recovery is important to the quality of the juice and puree concentrate. It may be added back as essence returned or sold separately as essence separate. Storage of the essence should be in refrigerated conditions (35 to 40∞F) (2 to 4∞C). Essence should not be frozen because the volatiles will separate from the water phase, resulting in loss in the freezer or upon thawing.

22.5.4 DEHYDRATED BERRIES Berry flavor and color degrades with exposure to prolonged elevated temperatures. Also, the juices of both strawberries and raspberries tend to drip with heating and progression of ripening. Therefore, conventional air-drying techniques, such as continuous belt dryer with counterflow air movement,

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are not used in dry berries. Tray dryers are used to produce batches of whole dried strawberries. The flavor tends to have cooked notes, and the color is typically browner than fresh due to degradation of the anthocyanins. Freeze-drying has been the typical method for drying strawberry and raspberry pieces. The berries are individually quick frozen to minimize cell damage and then spread onto drying trays. The freeze-dry chamber is filled with the trays, and the shelf temperatures during a cycle are varied to maintain an optimal product temperature. Studies have shown that the most important processing variable is temperature. The packaging selected must be a high-moisture barrier film such as foilpaper laminate or foil polyethylene laminate. With exposure to air, freeze-dried berries absorb moisture very quickly, and the physical structure changes (Pääkkönen and Mattila, 1991); therefore, the water activity of the final product is critical to maintenance of crisp texture in the freeze-dried berries. Cereal and confection products often contain freeze-dried berry products. Another patented technology, refractance window drying, has recently been introduced by the MCD Company to dry purees and juices in a thin layer on a belt resting on hot water (205∞F)(127∞C). The water acts as the heat transfer medium, and the fruit dries quickly in 3 to 10 min. This continuous system is less labor-intensive and requires less energy than freeze-drying, but the throughputs are not as high for the equipment size and plant space that are required. The flavor and color quality, because of quick drying, often exceeds that of the freeze-dried product. Drum dryers are also used to dry raspberry and strawberry purees. Because of high temperatures (> 220∞F) (> 105∞C) for a timer period of 1 to 5 min, the color and flavor is often browner and more caramelized than in freeze-dried or refractance window dried products. Spray-drying of strawberry and raspberry juices is done at high temperatures for a short time. Typically, a carrier must be added such as maltodextrin, starch, or corn syrup solids to prevent clumping during drying and in storage. A fine powder is produced by this process, which is suitable for instant drinks.

22.5.5 JAMS, PRESERVES,

AND

CONDIMENTS

The effect of immature strawberries on the shelf life of strawberry jam was studied at the University of Arkansas. Color and flavor of various combinations was found to be initially acceptable but deteriorated over 6 and 12 months’ storage at 25 and 35∞C (77 to 95∞F). Color acceptability was related more to total anthocyanin content rather than the level of ripe puree in the jam. The storage temperature variable had a greater influence on the changes in total anthocyanin content than the maturity of the initial fruit. The method of production of jam consisted of boiling 2 kg of strawberry puree for 2 min and then adding a sugar mixture that consisted of 8 g of a slow set pectin and 2 kg of sugar. The puree and sugar were concentrated to 68% soluble solids, and then 15 or 30 ml of a 25% malic acid solution was added to achieve 0.75 and 1.5% acidity (Spayd and Morris, 1981). Some strawberry varieties will produce preserves with better color than other varieties. Abers (1979) found that phenolics such as leucoanthocyanins and flavanols contribute to greater color deterioration. A variety such as Hood with high anthocyanin pigment content and lower ascorbic acid content is expected to produce less Maillard browning in storage. Various methods of cooking are used to produce jam products. Open air systems cook the pulp and sugar mixture until the syrup is clear and the Brix is at least 65∞. Another process using a rotary evaporator operated under low pressure is also used. Flavor comparisons of these two methods showed that a number of flavor volatiles are lost in an open air system; therefore, a vacuum kettle approach where volatiles are condensed and added back is likely to produce a jam with a more acceptable flavor quality (Lesschaeve et al., 1991). Pectin is traditionally used to produce the gel that stabilizes the fruit pieces in solution. Jams are used as spreads on breads and are expected to exhibit a viscous nature. High methoxylated (HM) pectins in combination with high-sugar solids and low pH are used for most jam products. Low-methoxyl pectins are used in low-sugar products and require calcium ions for gelation. A

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study with varying levels of pectin (0, 0.05, 0.1, 0.2, and 0.4%) shows that increasing amounts of pectin increase the viscosity of a strawberry jam. Other factors such as strawberry flavor, sweetness, acidity, and sharpness decrease with increasing levels of pectins. Fresh, overripe, cooked, and caramel notes decreased, and candied fruit notes increased with increased levels of HM pectin. The degree of esterification of the pectin significantly affects the viscosity. When the degree of esterification (DE) was decreased from 83 to 66, the viscosity of the jam decreased. A recommended level of HM pectin in this study was 0.2%. Low-methoxyl pectin must be used at 0.4 to 0.6% to achieve the same results in a 60∞ Brix jam (Guichard et al., 1991).

22.5.6 FRUIT PREPARATIONS, FILLINGS, SYRUPS,

AND

TOPPINGS

Fruit preparations are used in dairy products such as yogurt and ice cream. The solids content is lower than for bakery fillings. Usually, fruit preparations are either hot-filled or aseptically packed and consist of fruit, sweeteners, starch, or other stabilizers and flavorings. Fruit toppings are also produced from whole fruit, purees, and juice concentrates with added sweeteners and starches. Usually, a light viscosity is desired to flow over ice cream or pancakes. Fruit fillings are formulated with raspberry or strawberry purees and juice concentrates and other sweeteners, thickeners, and flavors to meet a certain specification. Applications are cookies, breakfast bars, donuts, toaster pastries, and so on. Often, the target is a stable shelf life in nonrefrigerated conditions. Stability can be achieved through addition of preservatives such as sodium benzoate, calcium propionate, or potassium sorbate but also through reduction of water activity to less than 0.72. Solids content is critical to reduction of water activity, as well as the addition of water-binding ingredients such as glycerin, sorbitol, or fructose. Strawberry syrup was produced in one study by adding sucrose to single-strength strawberry juice until the Brix was 51∞. This syrup was then compared with syrups fortified with anthocyanins extracted from strawberries and/or ascorbic acid. Even though the fresh strawberry juice contained 11 mg ascorbic acid/100 g juice, the unfortified strawberry syrup had no detectable levels of ascorbic acid. The pH of the syrups was approximately 3.3. The titratable acidities were approximately 0.17 to 0.19. The fortified syrup contained 80 mg ascorbic acid. Addition of ascorbic acid accelerated the destruction of anthocyanins in storage. Anthocyanin fortification reduced the rate of pigment breakdown. Syrups with added ascorbic acid showed increased browning and reduction of the browning ratio. A condensation reaction between ascorbic acid and anthocyanins in strawberry juice is suggested as the initial stage in this degradation. High anthocyanin to ascorbic acid ratio in a strawberry juice or syrup favors greater stability of the anthocyanin. Ascorbic acid is not stable in processed strawberry products. However, if the anthocyanin contents are elevated through fortification or higher initial levels, there may be a slightly protective effect on ascorbic acid levels (Skrede et al., 1992).

22.5.7 BEVERAGES

AND

WINES

Flavored yogurt drinks utilize strawberry and raspberry juice concentrates and pectin to stabilize the solution. Carrageenans have also been shown to stabilize the proteins in a fruit juice–milk drink. One study evaluated pectins at six levels (0, 0.1, 0.2, 0.3, 0.4, and 0.5%) and three levels of raspberry juice concentrate (1, 5, and 10%) on rheology of yogurts. Increased pectin levels had a linear response in increased viscosity in the yogurt. The yogurt was more shear stable. Viscosity also increased with increasing levels of concentrate, but the yogurt was less shear stable (Ramaswamy and Basak, 1992). Yogurt preparations, therefore, are formulated to minimize syneresis and loss of viscosity. Strawberry wines will show varying degrees of degradation in color from the appealing rose color to the undesirable brown. Benton and Totem varieties were compared in the production of a strawberry wine. Totem strawberries tend to be richer in anthocyanins than Bentons. Wine was produced from clarified strawberry juice by first adding 25 ppm SO2 and allowing the juice to stand

Strawberries and Raspberries

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for 2 h. The soluble solids were then adjusted to 22∞ Brix with sucrose. After the addition of 1% ammonium phosphate and 1% champagne strain yeast culture, the juice was allowed to ferment at 60∞F, until the wine reached 0.1 to 0.2% reducing sugars. Following racking and the addition of 10 ppm SO2, the wines were bottled and stored (Pilando et al., 1985). After fermentation, the anthocyanin content decreased to 3 to 9% of the original pigment content. Coprecipitation with the yeast probably contributed to the loss of anthocyanins. Browning gradually increases in wines, with greater browning using overripe berries and berries exhibiting mold contamination (Pilando et al., 1985).

22.5.8 OTHER PRODUCTS Tissue culture has been evaluated as a method to produce intense strawberry flavors. Natural strawberry flavor is used extensively in the food industry to flavor beverages, confections, bakery fillings, yogurts, ice creams, cake mixes, and so on. The production of a liquid suspension that could generate elevated levels of flavor is currently being researched (Hong et al., 1989). Raspberry flavor may be produced by yeast fermentation (Boeker et al., 2001). The oil of berry seeds is being analyzed for its contents and was found to be outstandingly rich in polyunsaturated fatty acids; berry oil could therefore have an interest for the natural foods market (Knapp, 2002).

22.6 COMPOSITION AND CHEMISTRY 22.6.1 COLOR Strawberry color is based on the presence of two major anthocyanin pigments: pelargonidin-3glucoside and cyanidin-3-glucoside. A total of eight pelargonidin- and three cyanidin-based anthocyanins were found in 39 strawberry cultivars (Baker et al., 1994). Pelargonidin-3-glucoside was the most prevalent pigment, ranging from 100% to 82%. Cyanidin-3-glucoside was present in all but three of the 39 cultivars analyzed (Baker et al., 1994). Strawberry pigments are very unstable due to the following chemical changes: hydrolysis of unstable aglycones, degradation of intermediaries, formation of copigment complexes with flavonoids, and degradation due to polyphenoloxidase. High temperatures, ascorbic acid, pH, lack of sucrose, heavy metals, oxygen, light, and nonenzymatic and enzymatic browning have all been shown to degrade or cause bathochromic shifts in anthocyanin color (Wesche-Ebeling and Montgomery, 1990; Markakis, 1982; Gross, 1987; Perera and Baldwin, 2001). Even with blanching and the inactivation of polyphenoloxidase, intermediate oxidation polymers or quinones that were formed in earlier enzymatic browning reactions will initiate reactions that lead to loss of anthocyanins (Wesche-Ebeling and Montgomery, 1990). However, in juices, strawberry color can be stabilized through addition of sucrose or other sugars. Cyanidin-3-sophoroside is the major pigment in most red raspberries (Boyles and Wrolstad, 1993; de Ancos et al., 2000a; Torre and Barritt, 1977). Other significant pigments are cyanidin-3glucosylrutinoside, cyanidin-3-rutinoside, and cyanidin-3-glucoside, as well as pelargonidin-3sophoroside (Boyles and Wrolstad, 1993). A content of higher cyanidin-3-glucoside, low monomeric anthocyanin content and high polymeric color in raspberry juice is an indication of pigment degradation due to poor processing techniques and storage (Boyles and Wrolstad, 1993). In frozen raspberries, the pigment degradation varied with the cultivars, each having a different anthocyanin profile (de Ancos et al., 2000a). Juice pH, organic acid concentration, sugars, and initial anthocyanins are all factors that affect anthocyanin stability. Additionally, cyanidin-3-glucoside was reported to be the most unstable anthocyanin during processing (Boyles and Wrolstad, 1993; de Ancos et al., 2000a). Black raspberries can be distinguished from red raspberries by the presence of cyanidin-3sambubioside and cyanidin-3-xylosylrutinoside (Torre and Barrit, 1977).

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TABLE 22.17 Typical Microbiological Standards

Frozen whole and sliced berries Frozen berry puree Juice concentrate

SPC

Yeast

Mold

Coliform

10,000–20,000/g 2,000–20,000/g 1,000–10,000/g

500–5,000/g 200–5,000/g 100–1,000/g

500–5,000/g 50–5,000/g 50–1,000/g