Drying atlas drying kinetics and quality of agricultural products 9780128181621, 9780128181638, 2892892902, 3053053063, 3893893903, 0128181621, 012818163X

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DRYING ATLAS

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DRYING ATLAS Drying Kinetics and Quality of Agricultural Products Werner Mühlbauer Joachim Müller

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-818162-1 (print) ISBN: 978-0-12-818163-8 (online) For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

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Contents Preface xi Biographies xiii Acknowledgments xv

2.1.3 Thin-layer laboratory dryer 54 2.1.4 Thin-layer drying curves 56 2.1.5 Thin layer drying models 59 References 60

1

2.2 Quality kinetics

Production and processing

2.2.1 Impact of drying on quality 63 2.2.2 Optimization strategy 63 2.2.3 Standardized procedure 63 2.2.4 Reaction kinetics 65 References 66

1.1 Production 1.1.1 General aspects 3 1.1.2 Appropriate cultivars 3 1.1.3 Optimum stage of maturity 4 1.1.4 Production methods 4 1.1.5 Pre-treatments 6 References 7

3 Cereals

1.2 Drying

3.1 Barley (Hordeum vulgare L.)

1.2.1 General aspects 9 1.2.2 Drying parameters 9 1.2.3 Drying methods 14 References 34

3.1.1 Morphological characteristics 69 3.1.2 Production 69 3.1.3 Drying 70 3.1.4 Storage 70 3.1.5 Quality 70 3.1.6 Drying kinetics 71 3.1.7 Quality kinetics 73 3.1.8 Recommendations 73 References 73

1.3 Storage and packaging 1.3.1 General aspects 37 1.3.2 Storage conditions 37 1.3.3 Storage methods 40 1.3.4 Packaging 41 References 42

3.2 Maize (Zea mays L.)

1.4 Quality

3.2.1 Morphological characteristics 75 3.2.2 Production 75 3.2.3 Drying 76 3.2.4 Storage 76 3.2.5 Quality 77 3.2.6 Drying kinetics 78 3.2.7 Quality kinetics 80 3.2.8 Recommendations 83 References 84

1.4.1 General aspects 43 1.4.2 Utilization of dried products 44 1.4.3 Quality standards 45 1.4.4 Drying relevant parameters 46 1.4.5 Chemical composition 47 1.4.6 Important ingredients 49 References 50

2

3.3 Oat (Avena sativa L.)

Drying and quality kinetics

3.3.1 Morphological characteristics 3.3.2 Production 3.3.3 Drying 3.3.4 Storage 3.3.5 Quality

2.1 Drying kinetics 2.1.1 Optimization strategies 2.1.2 Standardized drying method

53 54

v

85 85 86 86 86

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Contents

3.3.6 Drying kinetics 88 3.3.7 Quality kinetics 89 3.3.8 Recommendations 89 References 89

3.4 Rice (Oryza sativa L.) 3.4.1 Morphological characteristics 91 3.4.2 Production 91 3.4.3 Drying 92 3.4.4 Storage 92 3.4.5 Quality 93 3.4.6 Drying kinetics 95 3.4.7 Quality kinetics 95 3.4.8 Recommendations 96 References 96

3.5 Rye (Secale cereale L.) 3.5.1 Morphological characteristics 99 3.5.2 Production 99 3.5.3 Drying 100 3.5.4 Storage 100 3.5.5 Quality 100 3.5.6 Drying kinetics 102 3.5.7 Quality kinetics 104 3.5.8 Recommendations 106 References 106

3.6 Wheat (Triticum L.)

4.2 Potato (Solanum tuberosum L.) 4.2.1 Morphological characteristics 131 4.2.2 Production 131 4.2.3 Drying 132 4.2.4 Storage 132 4.2.5 Quality 133 4.2.6 Drying kinetics 134 4.2.7 Quality kinetics 136 4.2.8 Recommendations 139 References 139

5 Oil crops 5.1 Coconut (Cocos nucifera L.) 5.1.1 Morphological characteristics 143 5.1.2 Production 144 5.1.3 Drying 144 5.1.4 Storage 144 5.1.5 Quality 145 5.1.6 Drying kinetics 146 5.1.7 Quality kinetics 148 5.1.8 Recommendations 149 References 150

5.2 Peanut (Arachis hypogaea L.)

3.6.1 Morphological characteristics 109 3.6.2 Production 109 3.6.3 Drying 110 3.6.4 Storage 110 3.6.5 Quality 110 3.6.6 Drying kinetics 112 3.6.7 Quality kinetics 113 3.6.8 Recommendations 115 References 115

4 Root crops 4.1 Cassava (Manihot esculenta Crantz) 4.1.1 Morphological characteristics 119 4.1.2 Production 119 4.1.3 Drying 120 4.1.4 Storage 120 4.1.5 Quality 121 4.1.6 Drying kinetics 123 4.1.7 Quality kinetics 126 4.1.8 Recommendations 128 References 128

5.2.1 Morphological characteristics 151 5.2.2 Production 152 5.2.3 Drying 152 5.2.4 Storage 152 5.2.5 Quality 153 5.2.6 Drying kinetics 154 5.2.7 Quality kinetics 155 5.2.8 Recommendations 156 References 156

5.3 Rapeseed (Brassica napus var. napus) 5.3.1 Morphological characteristics 157 5.3.2 Production 157 5.3.3 Drying 157 5.3.4 Storage 158 5.3.5 Quality 158 5.3.6 Drying kinetics 159 5.3.7 Quality kinetics 161 5.3.8 Recommendations 161 References 161

5.4 Soybean (Glycine max (L.) Merr.) 5.4.1 Morphological characteristics 5.4.2 Production

163 163



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5.4.3 Drying 164 5.4.4 Storage 164 5.4.5 Quality 164 5.4.6 Drying kinetics 165 5.4.7 Quality kinetics 166 5.4.8 Recommendations 167 References 167

5.5 Sunflower (Helianthus annuus L.)

7 Spices 7.1 Chili (Capsicum annuum L.)

5.5.1 Morphological characteristics 169 5.5.2 Production 169 5.5.3 Drying 170 5.5.4 Storage 170 5.5.5 Quality 171 5.5.6 Drying kinetics 172 5.5.7 Quality kinetics 173 5.5.8 Recommendations 173 References 174

7.1.1 Morphological characteristics 209 7.1.2 Production 209 7.1.3 Drying 210 7.1.4 Storage 210 7.1.5 Quality 211 7.1.6 Drying kinetics 212 7.1.7 Quality kinetics 214 7.1.8 Recommendations 216 References 216

7.2 Garlic (Allium sativum L.)

6 Vegetables 6.1 Carrot (Daucus carota) 6.1.1 Morphological characteristics 177 6.1.2 Production 177 6.1.3 Drying 178 6.1.4 Storage 178 6.1.5 Quality 179 6.1.6 Drying kinetics 180 6.1.7 Quality kinetics 181 6.1.8 Recommendations 182 References 183

6.2 Paprika (Capsicum annuum, C. frutescens) 6.2.1 Morphological characteristics 185 6.2.2 Production 185 6.2.3 Drying 186 6.2.4 Storage 186 6.2.5 Quality 187 6.2.6 Drying kinetics 189 6.2.7 Quality kinetics 190 6.2.8 Recommendations 192 References 192

6.3 Tomato (Solanum lycopersicum L.) 6.3.1 Morphological characteristics 6.3.2 Production 6.3.3 Drying 6.3.4 Storage 6.3.5 Quality 6.3.6 Drying kinetics

6.3.7 Quality kinetics 203 6.3.8 Recommendations 204 References 204

7.2.1 Morphological characteristics 219 7.2.2 Production 219 7.2.3 Drying 220 7.2.4 Storage 220 7.2.5 Quality 221 7.2.6 Drying kinetics 222 7.2.7 Quality kinetics 223 7.2.8 Recommendations 224 References 224

7.3 Onion (Allium cepa L.) 7.3.1 Morphological characteristics 227 7.3.2 Production 228 7.3.3 Drying 228 7.3.4 Storage 229 7.3.5 Quality 229 7.3.6 Drying kinetics 230 7.3.7 Quality kinetics 233 7.3.8 Recommendations 235 References 235

8 Stimulants 8.1 Cocoa (Theobroma cacao L.)

195 195 196 196 197 198

8.1.1 Morphological characteristics 8.1.2 Production 8.1.3 Drying 8.1.4 Storage 8.1.5 Quality 8.1.6 Drying kinetics

239 240 240 241 241 242

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8.1.7 Quality kinetics 244 8.1.8 Recommendations 244 References 244

8.2 Coffee (Coffea L., Rubiaceae) 8.2.1 Morphological characteristics 247 8.2.2 Production 248 8.2.3 Drying 249 8.2.4 Storage 249 8.2.5 Quality 249 8.2.6 Drying kinetics 251 8.2.7 Quality kinetics 253 8.2.8 Recommendations 255 References 255

9 Fruits 9.1 Apple (Malus domestica Borkh.) 9.1.1 Morphological characteristics 259 9.1.2 Production 259 9.1.3 Drying 260 9.1.4 Storage 261 9.1.5 Quality 261 9.1.6 Drying kinetics 263 9.1.7 Quality kinetics 265 9.1.8 Recommendations 267 References 268

9.2 Apricot (Prunus armeniaca L.) 9.2.1 Morphological characteristics 269 9.2.2 Production 269 9.2.3 Drying 270 9.2.4 Storage 271 9.2.5 Quality 271 9.2.6 Drying kinetics 273 9.2.7 Quality kinetics 275 9.2.8 Recommendations 277 References 277

9.3 Banana (Musa × paradisiaca) 9.3.1 Morphological characteristics 279 9.3.2 Production 279 9.3.3 Drying 281 9.3.4 Storage 281 9.3.5 Quality 281 9.3.6 Drying kinetics 283 9.3.7 Quality kinetics 285 9.3.8 Recommendations 286 References 287

9.4 Fig (Ficus carica L.) 9.4.1 Morphological characteristics 289 9.4.2 Production 289 9.4.3 Drying 290 9.4.4 Storage 290 9.4.5 Quality 291 9.4.6 Drying kinetics 292 9.4.7 Quality kinetics 294 9.4.8 Recommendations 295 References 295

9.5 Grape (Vitis vinifera L.) 9.5.1 Morphological characteristics 297 9.5.2 Production 297 9.5.3 Drying 298 9.5.4 Storage 298 9.5.5 Quality 299 9.5.6 Drying kinetics 301 9.5.7 Quality kinetics 302 9.5.8 Recommendations 303 References 303

9.6 Litchi (Litchi chinensis Sonn.) 9.6.1 Morphological characteristics 305 9.6.2 Production 305 9.6.3 Drying 306 9.6.4 Storage 307 9.6.5 Quality 307 9.6.6 Drying kinetics 308 9.6.7 Quality kinetics 310 9.6.8 Recommendations 312 References 312

9.7 Longan (Dimocarpus longan Lour.) 9.7.1 Morphological characteristics 315 9.7.2 Production 315 9.7.3 Drying 316 9.7.4 Storage 317 9.7.5 Quality 318 9.7.6 Drying kinetics 319 9.7.7 Quality kinetics 321 9.7.8 Recommendations 322 References 323

9.8 Mango (Mangifera indica L.) 9.8.1 Morphological characteristics 9.8.2 Production 9.8.3 Drying 9.8.4 Storage 9.8.5 Quality 9.8.6 Drying kinetics

325 325 327 327 327 329



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Contents

9.8.7 Quality kinetics 332 9.8.8 Recommendations 334 References 334

9.9 Papaya (Carica papaya L.) 9.9.1 Morphological characteristics 337 9.9.2 Production 337 9.9.3 Drying 338 9.9.4 Storage 339 9.9.5 Quality 339 9.9.6 Drying kinetics 341 9.9.7 Quality kinetics 343 9.9.8 Recommendations 345 References 345

9.10 Pineapple (Ananas comosus (L.) Merr.) 9.10.1 Morphological characteristics 347 9.10.2 Production 347 9.10.3 Drying 348 9.10.4 Storage 348 9.10.5 Quality 349 9.10.6 Drying kinetics 350 9.10.7 Quality kinetics 351 9.10.8 Recommendations 353 References 353

9.11 Plum (Prunus domestica subsp. domestica) 9.11.1 Morphological characteristics 355 9.11.2 Production 356 9.11.3 Drying 356 9.11.4 Storage 357 9.11.5 Quality 357 9.11.6 Drying kinetics 358 9.11.7 Quality kinetics 360 9.11.8 Recommendations 362 References 362

10 Medicinal plants 10.1 Basil (Ocimum basilicum L.) 10.1.1 Morphological characteristics 367 10.1.2 Production 367 10.1.3 Drying 367 10.1.4 Storage 368 10.1.5 Quality 368 10.1.6 Drying kinetics 369 10.1.7 Quality kinetics 371 10.1.8 Recommendations 372 References 373

10.2 Chamomile (Matricaria recutita L.) 10.2.1 Morphological characteristics 375 10.2.2 Production 375 10.2.3 Drying 376 10.2.4 Storage 376 10.2.5 Quality 376 10.2.6 Drying kinetics 377 10.2.7 Quality kinetics 378 10.2.8 Recommendations 379 References 380

10.3 Lemon Balm (Melissa officinalis L.) 10.3.1 Morphological characteristics 381 10.3.2 Production 381 10.3.3 Drying 381 10.3.4 Storage 382 10.3.5 Quality 382 10.3.6 Drying kinetics 383 10.3.7 Quality kinetics 384 10.3.8 Recommendations 386 References 387

10.4 Marjoram (Origanum majorana L.) 10.4.1 Morphological characteristics 389 10.4.2 Production 389 10.4.3 Drying 390 10.4.4 Storage 390 10.4.5 Quality 390 10.4.6 Drying kinetics 391 10.4.7 Quality kinetics 392 10.4.8 Recommendations 393 References 393

10.5 Peppermint (Mentha x piperita L.) 10.5.1 Morphological characteristics 395 10.5.2 Production 395 10.5.3 Drying 395 10.5.4 Storage 396 10.5.5 Quality 396 10.5.6 Drying kinetics 397 10.5.7 Quality kinetics 399 10.5.8 Recommendations 399 References 400

10.6 Sage (Salvia officinalis L.) 10.6.1 Morphological characteristics 10.6.2 Production 10.6.3 Drying 10.6.4 Storage 10.6.5 Quality 10.6.6 Drying kinetics

401 401 402 402 402 404

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10.6.7 Quality kinetics 405 10.6.8 Recommendations 406 References 406

10.7 St. John’s Wort (Hypericum perforatum L.) 10.7.1 Morphological characteristics 409 10.7.2 Production 409 10.7.3 Drying 410 10.7.4 Storage 410 10.7.5 Quality 410 10.7.6 Drying kinetics 411 10.7.7 Quality kinetics 412 10.7.8 Recommendations 412 References 412

10.8 Tarragon (Artemisia dracunculus L.) 10.8.1 Morphological characteristics 10.8.2 Production 10.8.3 Drying 10.8.4 Storage

415 415 415 416

10.8.5 Quality 416 10.8.6 Drying kinetics 417 10.8.7 Quality kinetics 418 10.8.8 Recommendations 419 References 420

10.9 Valerian (Valeriana officinalis L.) 10.9.1 Morphological characteristics 421 10.9.2 Production 421 10.9.3 Drying 422 10.9.4 Storage 422 10.9.5 Quality 422 10.9.6 Drying kinetics 423 10.9.7 Quality kinetics 424 10.9.8 Recommendations 424 References 425

Nomenclature 427 Index 429

Preface

Drying is the most common process for the preservation of all kinds of agricultural products. However, optimization of the drying process is rather complex, since heat and mass transfer phenomena occur simultaneously during the drying process. In addition, chemical and biochemical reactions occur during the drying process, which can significantly influence the quality of the dried product. In order to provide a unified procedure to determine the drying characteristics of agricultural commodities, highly accurate test benches and standardized procedures have been developed at the Institute of Agricultural Engineering of the University of Hohenheim, Stuttgart (Germany) and refined throughout more than 40  years of experimental work. The test benches allow the variation of temperature, humidity and air velocity in a range, which is relevant for all kinds of high-temperature dryers. Furthermore, drying processes can be investigated both in through-flow as well as in over-flow mode. Aside of the moisture content, the product temperature also is measured continuously providing valuable information about the impact of the drying process on the quality of the product. In addition, the influences of the physical properties of the product and the impact of the mechanical, thermal and chemical pre-treatments on the drying curves are determined. The thin layer drying curves of the various commodities can be used as a database for scientists to validate thin layer drying models. Special care was given, to investigate the impact of the drying parameters on the quality according to the official quality standards and the demands of food industry and consumers using standardized analytical methods. Quasi-continuous measurement of the quality parameters during the drying process allows to establish quality curves that can be used together with the drying curves to determine the reaction rate. The reaction rate describes the time gradient of biochemical processes and is the basis to develop reaction kinetic equations, allowing the mathematical description of the influence of the drying process on the different quality parameters. Thin layer drying models together with reaction kinetic equations are required to simulate and optimize the different types of high-temperature dryers in terms of capacity, energy consumption and quality as well as for process control.

The Drying Atlas is a compilation of drying and quality curves gathered from our own research complemented by data from journal articles, books and reports. The Drying Atlas consists of two major parts: a general section and a specific section. The general section presents brief information on the morphology of the products. Physical and chemical properties of the products are listed in tables. Official quality standards for the different applications, most important ingredients and quality parameters mainly influenced by the drying process, are compiled. Appropriate cultivars for drying, optimum stage of maturity and impact of harvesting methods on product quality are described for the different products. Furthermore, the impact of mechanical, thermal and chemical pre-treatments for the different products, which can be used to accelerate the drying process, improve the quality and extend the shelf life of the product, are described. Detailed information is provided for the most common drying methods. The characteristic drying curves of the different drying methods are illustrated and the advantages and disadvantages of the drying methods are listed. Storage conditions, sorption isotherms and storage facilities give information about the optimal storage of the different products. The special section forms a database containing thin layer drying curves and related quality curves of 40 agricultural commodities compiled systematically and presented in a condensed way. Engineers and food scientists can use this information to develop and validate simulation models for heat- and mass transfer and biochemical processes, which are an important tool to optimize and control drying processes. Students and faculty members in agricultural engineering, food science and related subjects can use the Drying Atlas for teaching purposes as well as for research. Dryer manufacturers, and food scientists in the drying industry need to know the optimal temperatures, drying times, energy requirements and quality aspects concerning the drying systems and the product to be dried. The Drying Atlas provides a valuable source to answer those questions, which are decisive for economic and successful drying.

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Biographies

Dr.-Ing. Dr. h.c. Werner Mühlbauer Werner Mühlbauer received his master degree (Dipl.Ing.) in 1969 and his doctoral degree (Dr.-Ing.) in 1974 in mechanical engineering from the University of Stuttgart, Germany. His dissertation on grain drying was honored as the most outstanding dissertation in mechanical engineering in 1974. In 1986 he completed his habilitation at the Faculty of Agricultural Sciences of the University of Hohenheim, Stuttgart (Germany). From 1969 until 1989, he held the positions of senior researcher, lecturer and managing director at the Institute of Agricultural Engineering of the University of Hohenheim. In 1986, the Faculty of Agriculture at the University of Hohenheim awarded him the qualification of professor in agricultural engineering. In 1989, he was appointed as full professor for the newly founded department “Agricultural Engineering in the Tropics and Subtropics” at the University of Hohenheim. He established the new department and served as its director until his retirement in 2004. From 1996 to 2000, he was head of the Scientific Centre for Tropical Agriculture. In 1996, the Agricultural University of Bucharest (Romania) appointed him as doctor honoris causa for his “outstanding contribution to secure the food supply in developing countries”. Dr. Mühlbauer has been working in the field of drying agricultural commodities since 1970. His research covers almost all aspects of drying technologies (physical properties, drying theory, drying simulation, drying kinetics, impact of drying on quality, energy saving, development of drying methods, dryer testing and evaluation, etc.). Dr. Mühlbauer developed high accuracy test benches and standardized procedures to measure drying curves to predict drying behavior and impact on quality of most important drying products (cereals, root and oil crops, vegetables and spices, stimulants, fruits and medicinal plants). Based on his investigations, a low-temperature in-storage drying system for small grains was introduced in Germany. He also developed small scale low-temperature in-storage paddy dryers and initiated the dissemination of more than 100,000 units in South Korea between 1982 and 1985, which is considered as success story of the German development aid program. Since 1980, his research focuses on the development of solar dryers for various agricultural commodities.

Dr. Mühlbauer initiated and coordinated bilateral research projects in 26 countries. Within his research activities, he developed several solar drying systems and supported his former students to establish their own companies. The multi-purpose solar tunnel dryer was commercialized and was distributed throughout more than 100 countries. The solar sewage sludge dryer is produced by a spin-out company of the University of Hohenheim. The World´s Number One in solar sewage sludge drying so far sold about 800,000 m2 of solar dryers in 28 countries all over the world. Dr. Mühlbauer has published 353 papers in national and international scientific journals; he holds six patents and gave more than 250 presentations at scientific conferences in 25 countries, mainly on drying of agricultural products. He supervised 31 doctoral theses, 168-MSc theses and 95-BSc theses. He is author of the Handbook on Grain Drying (in German), the only book on this topic worldwide containing all aspects of drying from drying theory to practical applications. Since his retirement in 2004, Dr. Mühlbauer is working as scientific adviser to a leading German drying company.

Dr. Joachim Müller Joachim Müller received his master degree (Dipl.-Ing. agr.) in 1985 and his doctoral degree (Dr. sc. agr.) in 1992 at the University of Hohenheim, Stuttgart (Germany). Subsequently he held the position as postdoctoral research fellow from 1992 to 1997 in the Department of Postharvest Technology and from 1997 to 2001 in the Department of Mechanization and Irrigation at the Institute for Agricultural Engineering in the Tropics and Subtropics at the University of Hohenheim. In 1999 he completed his habilitation at the Faculty of Agricultural Sciences of the University of Hohenheim. In 2001, Dr. Müller was appointed as full professor to the Department Agrotechnology and Food Sciences, Farm Technology at the Wageningen University (NL) and held this position until 2004. In 2004 he was appointed as full professor at the University of Hohenheim, Institute of Agricultural Engineering, where he has since been head of the Tropics and Subtropics Department. He has functioned as Director General of the Institute of Agricultural Engineering from 2012 to 2016. In 2018

xiii

xiv Biographies he has been additionally appointed Academic Director of the State Institute of Agricultural Engineering and Bioenergy at the University of Hohenheim. Dr. Müller is Editor-in-Chief for the Journal of Applied Research of Medicinal and Aromatic Plants and is a member of the editorial board for several other journals as well as a member of the scientific advisory council of the Fiat-Panis-Foundation, Ulm. He also acts as reviewer for the German Science Foundation (DFG), the Alexandervon-Humboldt Foundation and the German Ministry of Education and Research (BMBF). He is also a member of the Committee of Experts in Food Technology of the German Agricultural Society (DLG). Since his doctoral thesis on solar drying of medicinal plants, his research interests are focusing on drying of

agricultural commodities using various drying technologies such as convective-, osmotic-, microwave- and freeze drying. Applied research of Dr. Müller is always accompanied by fundamental research such as establishing sorption isotherms and drying curves on precision laboratory test benches. For process monitoring, he is developing non-invasive sensor systems for in situ measurements of product quality. Dr. Müller contributed chapters to three books in German and five books in English. He is author or co-author of 190 international, peer-reviewed publi­ cations. 20 doctoral theses, 149 MSc-theses and 67 BSctheses have been completed under his supervision. Under his guidance, five patents were issued between 2003 and 2015 and another four patent applications are pending.

Acknowledgments

The authors gratefully acknowledge the valuable contribution to the editorial assistance of Ingrid Amberg, Ann-Christine Schmalenberg, Dr. Parika Rungpichaya­ pichet, and Sabine Nugent; Dorothea Hirschbach-Müller for the excellent pictures of the products and for the

preparation of the diagrams. The authors appreciate the contribution of the graphic designers of unger + kreativ Strategen GmbH, Stuttgart (Germany), for producing the graphs of the structure of the products.

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P A R T   1

Production and processing

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C H A P T E R

1.1 Production 1.1.1  General aspects

Stone easy to separate from fruit flesh

Apricot, plum, litchi, longan, mango

The production methods of agricultural commodities have a significant impact on the quality of the dried product. The production chain starts with the selection of cultivars, which are appropriate for drying. The beginning of the harvest is determined by the producer when the optimum stage of maturity of the product is reached. After harvesting the product is transported to the farm or to the commercial drying enterprise where the product undergoes various pre-treatments depending on the species before the drying process can start.

Well sliceable

Apple, mango, papaya

Thin skin

Fig, grape, plum, tomato

Small size

Grape, banana

Large size

Apricot, fig, plum

Low fiber content

Mango, pineapple

Seedless

Grape

High dry matter content

Carrot

1.1.2  Appropriate cultivars

Well sliceable

Tomato

Low fiber content

Carrot

With the exception of rice and maize, the cultivar of cereals, roots and tubers, oil crops and stimulants has little influence on the drying behavior and the quality of the dried product. However, fruits, vegetables, spices and medicinal plants require specific properties to produce dried products with optimum quality. Therefore, appropriate cultivars with specific properties have to be selected, which can be dried easily and also guarantee the desired quality characteristics of the dried product (Table 1.1.1).

Uniform shape and size

Carrot, paprika

Intensive color

Carrot, tomato, paprika

High carotene content

Carrot

Low pungency content

Paprika

Low fruit juice and seed content

Tomato

Vegetables

Spices Light color

Onion, garlic

Papery skin easy to remove

Onion, garlic

TABLE 1.1.1  Required properties of products to achieve good drying quality.

Intensive pungency flavor

Onion

High capsaicin content

Chili

Required property

High coloring agents content

Chili

Product

Cereals

Stimulants

Uniform ripening

Rice

High caffeine and theobromine content Coffee, cocoa

Early maturing

Rice, maize

Low acid content

Cocoa

Pulp easy to remove

Coffee, cocoa

Fruits High sugar content

Fig, banana, apricot, grape, mango, pineapple, papaya

High carotene content

Mango

Drying Atlas. https://doi.org/10.1016/B978-0-12-818162-1.00001-8

Medicinal Plants and Herbs

3

High essential oil content

Medicinal plants, herbs

High content of active ingredients

Medicinal plants

© 2020 Elsevier Inc. All rights reserved.

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1.1.  Production

1.1.3  Optimum stage of maturity [1–3] The stage of maturity of the product at the beginning of the drying process is extremely important for the quality of the dried product.

1.1.3.1  Immature crops Premature harvest is causing problems during drying and also lowers the quality of the dried product such as: – Low germination rate (cereals, oil seeds) – Low milling yield (rice) – Low nutrient content (cereals, root crops, oil seeds, fruits) – Discolouration during drying (maize, rice, fruits) – Tough or rubbery texture (coconut, fruits) – Off-flavor (fruits, coffee, cocoa) – Low content of active ingredients (medicinal plants)

1.1.3.2  Overripe crops Delayed harvest leads to significant losses prior to harvest and also causes low quality of the dried product: – Infestation with microorganisms (cereals, oil seeds, medicinal plants) – Contamination with mycotoxins (cereals, figs) – High in-field losses caused by shattering, rodents and birds (cereals, oil seeds) – Off flavor (fruits, coffee, cocoa) – Discolouration (fruits) – Increased fiber content (cassava)

1.1.3.3  Fully mature crops The optimum stage of maturity is greatly influenced by the commodity. Depending on the crop, the stage of maturity can be described by the following essential requirements: Cereals – Accumulation of the nutrients is completed – Kernel transition from soft to hard dough stage is finalized – Color of the seed coat/husk changes from green to brown Root crops – Accumulation of the nutrients is completed – Low fiber content Oil seeds – Color of the pods changes from green to brown – Color of the seed changes from green to yellow, brown or black according to the cultivar

Vegetables – High dry matter content – High sugar content – High carotene content – Intensive color Spices – High content of coloring agents – Intensive pungency flavor – Skin can be easily removed Stimulants – Color of the skin/pod changes from green to yellow/ red – Consistency of the pulp – Easy separation of the pulp from the seeds Fruits Non-climacteric fruits (grape, longan, litchi, pineapple etc.) have to be harvested in the full mature stage; climacteric fruits (apple, apricot, banana, fig, mango, papaya, plum etc.) are harvested before full maturity and ripened naturally or artificially after harvesting until the fruit reaches the required properties: – Starch is completely converted into sugar – Low acid content – High sugar and acid ratio – Color of peel and flesh is fully developed – Flesh firmness is decreasing – Easy separation of the stone from the flesh Medicinal plants – Accumulation of the active ingredients is completed – Plant is in the beginning of the flowering stage

1.1.4  Production methods The production method also greatly affects losses and product quality. Especially the harvesting method greatly influences the drying process. Cereals and oil seeds [4–9] In industrialized countries and increasingly in developing countries, cereals and oil seeds are harvested at high capacity with fully automated combine harvesters, which have the following advantages: – – – –

Reduction of field losses Enable harvest at optimum maturity of the grains Harvest losses