Foods: A Scientific Approach 0023219513

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FOODS A

Scientific

Approach

Third Edition HELEN CHARLEY

•

CONNIE WEAVER

CTCiT

-^^^-r^

O ocky Gc>/-/4«_4^

Foods A Scientific Approach

Digitized by the Internet Archive in

2009

http://www.archive.org/details/foodsscientificaOOchar

Food s A Scientific Approach HELEN CHARLEY Professor Emei-itus

ofFoods and Nutrition, Oregon

State University

CONNIE WEAVER Professor

and Head ofFoods and Nutrition, Purdue

THIRD

MERRILL, Upper Saddle

EDITION

AN IMPRINT OF PRENTICE HALL River,

New Jersey

•

Columbus, Ohio

University

Library of Congress Cataloging-in-Publication Data Charley, Helen.

Foods

a scientific

:

approach

Helen Charley, Connie Weaver.

/

cm.

p.

Includes bibliographical references

(p.

)

and

index.

ISBN 0-02-321951-3 Food.

1. .

II.

2.

Nutrition.

Cookery.

3.

I.

Weaver, Connie. 1950-

Title

TX354.C4723

1998

664—dc21

97-22861

CIP

AC Cover photo: Alfred Pasieka/Science Photo Library Editor: Kevin

M. Davis

Developmental Editor: Carol

S.

Sykes

Production Editor: Sheryl Clicker Langner Editorial/Production Supervision: Betsy Keefer

Design Coordinator: Karrie M. Converse Text Designer: Gary Gore

Cover Designer: Russ Maselli Production Manager: Pamela D. Bennett

Management: Karen

Electronic Text

L. Bretz

Director of Marketing: Kevin Flanagan

Marketing Manager: Suzanne Stanton Advertising/Marketing Coordinator: Julie Shough

This book was

set in

Adobe Garamond by

Carlisle

Communications, Ltd. and was printed and bound

by R.R. Donnelley and Sons Corp. The cover was printed by Phoenix Color Corp.

^=fe © 1998 by Prentice-Hall, Inc. ^S^S Simon & Schuster/A Viacom Company f^t^

Upper Saddle

All rights reserved.

No

River,

New Jersey 07458 book may be reproduced,

part of this

in

any form or by any means, without per-

mission in writing from the publisher. Earlier editions, entitled Press

Food Science,

©

1982 by John Wiley

Company.

Printed in the United States of America

1098765432 ISBN: 0-02-321951-3 Prentice-Hall International

(UK) Limited, London

Prentice- Hall of Australia Pty. Limited, Sydney

Prentice-Hall of Canada, Inc., Toronto Prentice-Hall Hispanoamericana,

S. A.,

Prentice-Hall of India Private Limited,

Mexico

New Delhi

Prentice-Hall of Japan, Inc., Tokyo

Simon

& Schuster Asia Pte.

Ltd., Singapore

Editora Prentice-Hall do Brasil, Ltda., Rio de Janeiro

&

Sons, Inc., and

©

1971 by The Ronald

Pref;ace A

Foods:

Scientific

Approach continues the emphasis of previous editions of Food Science

of the study of foods, drawing on the basic sciences of chemistry, and physiology. The objective in writing the text is to give the student an understanding of the complex nature ot and the changes that can occur in foods

on the

scientific aspects

physics, microbiolog)',

as

they are prepared, processed, and stored, whether

or in industry.

The

since the previous edition, facts Special coverage scientific activity.

carriers of

at

home,

in the industrial kitchen,

authors have attempted to glean from the literature, accumulated

and concepts that further

this objective.

given to topics that are of current interest or the focus of recent

is

Topics that are given special emphasis include the following: foods as

pathogens and elementary precautions

in

preventing foodborne

illnesses; the

neurophysiological basis for the ability to sense qualities in foods; sensory and instru-

mental methods of measuring food quality; carbohydrate- and protein-rich foods

as

glassy/rubbery polymers whose structure, rexture, and keeping qualities depend on temperature and the plasticizing effect of water; sugar and fat substitutes and adjustments

when

these are used in products; vegetable gums, their functions

biotechnology-derived products

The annotated

mosin).

(e.g.,

and

uses in foods;

and

FLAVR SAVR tomato and microbial-derived chy-

references for each chapter will aid the ambitious student

and the

time-pressed instructor in choosing supplemental reading selectively. Foods:

A

Scientific

Approach

is

written for a

have had general chemistry and are at

of elementary nutrition

is

theory.

The

The

course in foods for students

organic chemistry.

assumed. This book can serve

vanced students whose previous course of coverage"

first

least enrolled in

in foods

as a

who

A knowledge

supplementary

text for ad-

emphasized application and minimized

authors have attempted to maintain the "scientific depth and completeness as

observed by a reviewer in the previous edition.

authors appreciate the constructive suggestions from reviewers of portions of

the manuscript: Suzanne R. Curtis, University of Maryland; Ronald R. Eitenmiller,

K. Head, West Virginia University; ZoeAnn Holmes, Manfred Kroger, Pennsylvania State University; Jane Love, Iowa State University; Marilyn Mook, Michigan State University; Carole S. Setser, Kansas State University; and Martha B. Stone, Colorado State University. We are indebted to Pauline Douglas for transportation to and from libraries, who toted a ton of bound volumes, and who acted as arbiter for intractable sentences. Dr. Mark Hines' perspective on instruments currently used in food research and in food quality assessment in industry was helpful in preparing Chapter 1 University of Georgia;

Oregon

Mary

State University;

Helen Charley Connie Weaver

Brief Contents

PART

Chapter

I

FOOD QUALITY

PART

II

AND

LIQUIDS

Chapters

Water

Chapter

6 7

Chapter 8

PART

Chapter

III

AND

Evaluation ot Food

Sensory Perception of Foods

Chapter

CRYSTALS

1

2 Chapter 3 Chapter 4 Chapter

9

Measures and Weights Heating and Cooling Foods and Food

and Frozen Desserts Sugars, Alternative Sweeteners, and Confections

Starches and Vegetable

STARCHY FOODS

Chapter

Flour and

PART

51

Cocoa Beverage

Ice Crystals

Cereals

11

24 Safet)'

71

Coffee, Tea, and

Chapter 10

STARCHES

3 21

Gums

Dough Formation

1

90 07

118

130 162 174

Chapter 12

Leavening Agents

195

LEAVENING AGENTS

Chapter 13

Quick Breads

207

AND BREADS

Chapter 14

Yeast Breads

221

PARTV FATS AND

Chapter 15

Fats

IV

FOODS

FAT-RICH

and Oils

16 Emulsions Chapter 17 Pastry Chapter

243 268 280

VII

BRIEF

PART

Chapter 18

VI

PROTEINS

AND

PROTEIN-RICH

FOODS

Introduction to Proteins

Milk Chapter 20 Cheese Chapter 21 Eggs Chapter 19

22 Meat Chapter 23 Poultry Chapter 24 Seafood Chapter

PART

VII

Chapter 25

Shortened Cakes

CAKES

Chapter

PART

Chapter 27 Fruits

VIII

PLANT FOODS

26

Chapter 28 Chapter

Sponge, Angel Food, and ChifFon Cakes

Vegetables

29 Legumes

295 308 325 341 368 415 426

438 457

469 498 534

PART IX

Chapter 30

Geletin Gels

551

GELS

Chapter 31

Fruit Pectin Gels

556

CONTENTS

4

56

Contents

PART

I

Chapter 1

FOOD QUALITY

1

Evaluation of Food

3

SENSORY EVALUATION OF FOOD QUALITY Vocabulary of Sensory Analysis Sensory Testing of Foods

3

3

5

INSTRUMENTAL MEASUREMENT OF FOOD QUALITY Appearance Flavor

1

Texture

1

Sensory Perception of Foods

Chapter A,

APPEARANCE OF FOOD

21

23

Color of Foods

CHEMOSENSORY PROPERTIES Odor

23

24 29

Taste

Moutfifeel

38

TEXTURE OF FOODS Tactile Properties

Influence of Textures

O

39

39 on Flavor

40

44

Measures and Weights

TRADITIONAL AND METRIC MEASURING SYSTEMS

MEASURING UTENSILS Capacities

45

Tolerances

46

Accuracy

21

21

Visual Perception

Chapter

12

1

47

45

44

1

MEASURING TECHNIQUES CONTENTS

47

47

Liquids

48

Fats

48

Sugar

48

Flour

Heating and Cooling Foods and Food Safety

Chapter T:

HEATING FOODS Intensity of

51

Heat

51

52

Quantitative Aspects

53

Energy Transfer

54

Heating Foods with a Conventional Range

HEATING FOODS The Oven and Interaction of

5

How

A MICROWAVE OVEN

IN It

57

57

Functions

Food with Microwaves

58

59

Advantages and Disadvantages

DONENESS OF COOKED FOODS 60 COOLING AND REFRIGERATION OF FOODS 60 TEMPERATURE AND CONTROL OF PATHOGENS IN FOOD

TESTS FOR

Effects of

Heat

61

Effects of

Cold

62

PATHOGENS AND

PART

II

Chapter

y

THEIR RESPONSE TO HEAT

AND CRYSTALS

LIQUIDS

The Molecule

71 72

72

Hydrogen Bonding and

the States of

Water

FUNCTIONS OF WATER

IN

FOOD PREPARATION

A Medium for the Transfer of Heat A Dispersing Medium 79

WATER RELATIONS Water

Water

IN

— Bound or Free?

FOODS

82

82

84

Plasticizer

CHARACTERISTICS OF WATER THAT AFFECT Hardness of Water

pH

of

Water

79

79

83

Activity

Water as a

73

74

Water

64

69

Water

NATURE OF WATER

Boiling of

AND COLD

61

86

88

WATER AS A CLEANSING AGENT

88

ITS

USE

86

O

Chapter

and Chocolate Cocoa

Coffee, Tea,

90 CONTENTS

COFFEE

90 90

Characteristics of the Beverage

Constituents

Market Forms

for

94 95

Making Coffee 98

Staling of Coffee

TEA

93

of Coffee

Preparing the Brew

Methods

90

Bean (Grounds)

the Coffee

in

99 99

Kinds of Tea

99

Quality of the Beverage Constituents

in

99

Tea

Market Forms of Tea

1

02

Preparing Tea Beverage

Storage of Tea

1

02

1

03

CHOCOLATE AND COCOA BEVERAGE Conversion of Cocoa Bean Methylxonthines

in

Chocolate Products

104

Preparing Cocoa or Chocolate Beverage

/

Chapter

Ice Crystals

FACTORS AFFECTING Components

of the

ICE

Mix

1

The Freezing Process

1

1

1

04

and Frozen Desserts

CRYSTAL FORMATION

1

114

15

115

Freezing the Mix

1

1

5

QUALITY CHARACTERISTICS OF FROZEN DESSERTS

O

Chapter

Sugars, Alternative Sweeteners,

SUGARS AND OTHER SWEETENERS 118 CONSUMPTION OF SWEETENERS 119 ALTERNATIVES TO SUGAR 119 SUCROSE Source

121 121

Chemistry Solubility Effect of

107

1

STILL-FROZEN DESSERTS

Melting

107

07

HARDENING OF FROZEN DESSERTS The Mix

103

103

Chocolate

to

1

1

22

24

Sucrose on the Boiling Point of Water

and Coramelizotion

1

25

1

24

115

and Confections

118

XII

CANDIES

126

Determining Doneness of Candies

CONTENTS

Interfering

Agitation

26

1

Agents and Sucrose Crystal Formation

and Sucrose

Crystal Formation

Amorphous Candies

1

1

III

Chapter

J

43

Setting or Gelation of a

1

Cooked

48

Starch Paste

Factors Affecting Starch-Thickened Products

VEGETABLE GUMS Extracts

Exudates

1

1

52

152

155 56

1

57

from the Endosperm of Seed

Gums

1

U

57

158

Chemically Modified Carbohydrates

1

58

162

Cereals

UTILIZATION OF CEREAL GRAINS

STRUCTURE OF CEREAL GRAINS The

162 163

163

Cell

Parts of a Cereal

Grain

63

1

COMPOSITION AND NUTRITIVE VALUE OF CEREALS 164 ENRICHMENT AND FORTIFICATION OF CEREALS 167 MARKET FORMS OF CEREALS 167 CEREAL PRODUCTS THAT REQUIRE COOKING 167 READY-TO EAT CEREALS

COOKING CEREALS Purpose

1

1

69

Proportions of Liquid of Salt

1

Cooking Time

1

70

70

170

Effect of Alkaline

Cooking Water

Characteristics of

Cooked Cereal

POPCORN

168

169

69

Precautions

Amount

139

39

1

1

Starch as a Thickening Agent

Chapter 1

Gums

137

139

Starch Granules

Biosynthetic

34

Starches and Vegetable

Starch Chemistry

Gums

1

STARCHES AND STARCHY FOODS

STARCHES

Seaweed

28

34

Comparison of Candies and Frozen Desserts

PART

1

31

171

171 1

71

XIII

Dough Formation

Flour and

Chapter 1 1

174 CONTENTS

TYPES OF FLOUR Kinds of

174

Wheat

1

74 176

Effects of Milling

Classed by Use

1

78

FORMATION OF DOUGH Components

of Flour

1

Role of Water

183

Manipulation

184

Effects of

179

80

Sugar and Fat on Gluten

1

86

FLOUR IMPROVERS (DOUGH CONDITIONERS) 189 ENRICHMENT OF FLOUR PASTA 189

PART

LEAVENING AGENTS AND BREADS

IV

Leavening Agents

Chapter i 2^

STEAM AS A LEAVENING AGENT Proportions of

186

Water

to Flour

Gas-Holding Properties of

1

Batters

193 195

195

95

and Dougfis

1

95

196 LEAVENING AGENT CARBON DIOXIDE AS A LEAVENING AGENT AIR AS A

196

Release of Carbon Dioxide from Sodium Bicarbonate by Acid

BAKING POWDER Yield of

Carbon Dioxide

Acids Used

in

1

of Yeast

1

98

Sour Dough Bread

Chapter 1

D

INGREDIENTS Flour Liquid Salt

Sugar Eggs

Quick Breads IN

QUICK BREADS AND THEIR FUNCTIONS

207 207 208 208

208

204

205

207

Leavening Agent Fat

202

202

Production of Carbon Dioxide In

97

98

Baking Powders

BAKING SODA AND SOUR MILK 202 YEAST AS A SOURCE OF CARBON DIOXIDE Market Forms

1

198

208

207 207

3

xiv

CONTENTS

BALANCING INGREDIENTS IN QUICK BREADS 209 MANIPULATION OF INGREDIENTS IN QUICK BREADS 210

Utensils

212

Purposes

BAKING QUICK BREADS

212

Baking Time and Temperature

Changes

Effected

Batters

in

213

and Doughs

INDIVIDUAL QUICK BREADS Formulas

213

Popovers

2

Cream

21 3

213

1

214

Puffs

Muffins

216

Biscuits

219

Griddle Cakes and Waffles

Chapter 1

4

220

Yeast Breads

223

INGREDIENTS AND THEIR FUNCTIONS Yeast

223

Flour

223

224 225

Salt

Sugar

225

Yeast

225

Eggs

226

PROPORTIONS OF INGREDIENTS and Sugar

MANIPULATION OF YEAST DOUGH Basic

227 228

Scalding the Milk Dispersing the Yeast Flour

and

Liquid

Dough Development

228

228

FERMENTATION OF YEAST DOUGH Temperature Inflating the

231

Dough

231

232

Punching the Dough

PROOFING 232 BAKING 233 233

Temperature

Changes Doneness

227

227

Methods

Combining

226

226 226

Flour to Liquid Salt

223

224

Liquid

Fat

209

210

Techniques

Effected

233

234

The Baking Pan

235

231

QUALITY OF BREAD Aroma

235

XV

235 CONTENTS

235

Ortier Characteristics

STALING OF BREAD

235

PART V

AND

FATS

J

Chapter 1

Acids

244

Glycerides

246

FOODS

and Oils

Fats

CHEMISTRY OF FATS Fatly

FAT-RICH

241 243

243

CRYSTALS OF FAT

248

Polymorphism

248

Melting Points

249

CONSISTENCY OF FATS 252 COMPOSITION OF FOOD FATS 253 MODIFICATION OF NATURAL FATS 253 Hydrogenization

253

Interesterification

256

256

Acetylation

FUNCTIONS

IN

FOODS

FAT SUBSTITUTES

258

258

DETERIORATION OF FATS Absorption of Odors

260

Rancidity

262

Antioxidants

MEDIUM FOR THE TRANSFER OF HEAT

FATS AS A

265

Absorption of Fat

266

Using Fats

in

Chapter i

O

269 269

Functions of an Emulsion

EMULSIFYING AGENTS Manufactured

272

EMULSIONS

French Dressing

Mayonnaise

Cooked Dressing Other Emulsions

271

271

Naturally Occurring

COMMON

269

Emulsions

NATURE OF AN EMULSION Phases

264

264

The Frying Process

Hazards

260

260

275

275 277 277

275

269

xvi

OF EMULSIONS

STABILITY

VINEGAR

278

278

CONTENTS

1 /

Chapter

280

Pastry

INGREDIENTS AND THEIR FUNCTIONS 281

Salt

Fat

281 281

Liquid

PROPORTIONS OF INGREDIENTS

282

282

Salt

Fat

280

280

Flour

282 282

Liquid

MANIPULATION OF INGREDIENTS Dough

Stirring the

282

283

Cutting Fat Into the Flour

283

INTERRELATIONS BETWEEN INGREDIENTS Effects of

Kind of Fat and Type of Flour

Effects of Proportions of Fat

Fat

Is

Cut

283

284

and Water

Which

Effects of the Extent to

AND MANIPULATION

283 into Flour

285

ROLLING AND SHAPING PASTRY 285 BAKING PASTRY 286 CHARACTERISTICS OF PASTRY 287 Tenderness versus Toughness

287

Crispness

PUFF PASTRY

PART

289

PROTEINS AND PROTEIN-RICH

VI

Chapter i

FOODS

O

Structure

Groups

295

295

296

STRUCTURE OF PROTEINS 299

Primary Structure

300

Secondary Structure

300

Tertiary Structure

Quaternary Structure Conjugated Proteins

PROPERTIES

293

Introduction to Proteins

AMINO ACIDS R

287

287

Flakiness

302

302 302

299

295

7

FUNCTIONS

303

As Enzymes

xvii

303

304

Modifiers of Texture

Chapter

1

CONTENTS

304

Nonenzymafic Browning

9

308

Milk

COMPOSITION

308 DISPOSITION OF CONSTITUENTS

31

Ennulsion

310

1

313

Homogenization

PASTEURIZATION OF MILK TYPES OF DAIRY PRODUCTS

313 314

314

Fluid Milk

315

Cream

Evaporated Milk

315

Condensed Milk

31 5

315

Dried Milk Solids

316

Butter

316

Cultured Buttermilk

316

Yogurt

Sour Cream

317

Caseins, Coseinates,

MILK

MILK

310

Colloidal Dispersion

Filled

IN

310

Solution

and

and Whey

FOAMS

317

Proteins

Imitation Dairy Products

3

1

317

Foam Formation

317

Evaporated Milk Foams

31 8

318

Dried Milk Foams

Wfiipped Cream

318

MILK PROTEINS AS EMULSIFIERS 321 EFFECTS OF HEAT ON MILK Effects

on Casein Micelles

321

Effects

on Skin Formation

321

Effects

on Serum Proteins

321

320

HANDLING MILK AND FOODS MADE WITH MILK Holding Temperature and Safety

Exposure

322

to Light

Chapter /^\J

325

Cheese

TYPES OF NATURAL CHEESE Unripened Cheese Soft

322

322

325

326

Ripened Cheese

Semisoft Ripened Cheese

327 327

Firm and Hard Ripened Cheese

327

xviii

CONTENTS

FORMATION OF COTTAGE CHEESE PRODUCTION OF CHEDDAR CHEESE

327 334

334

Curd Formation

335

Ripening

PASTEURIZED PROCESS CHEESE

336

COMPOSITION OF CHEESES

337

USES OF CHEESE 337 CHEESE IN COOKING 337 337

Melting Cheese

339

Blending Cheese with Liquid

AL

Chapter

Eggs

STRUCTURE

341

341

Shell

341

Shell

Membranes

Albumen

341

341

342

Yolk

COMPOSITION

343

343

Egg White

343

Egg Yolk

QUALITY OF EGGS Deteriorative

Changes

345 345

Handling Eggs

to

Maintain Quality

Candling Eggs

to

Determine Quality

346 347

Uses of Eggs of Different Quality Grades Resistance of Eggs to Spoilage

Preserving Eggs by Freezing

347

347

and Drying

350

OF EGGS 350 EGGS AS EMULSIFIERS 351 EGGS AS BINDING, THICKENING, AND GELLING AGENTS

SIZE

Effects of

Heat on Egg Proteins

351

METHODS OF COOKING EGGS Cooked

in

353

Poached Egg Fried

353

Egg

354

Scrambled Egg

354

Custards

356

Soft Pie Filling

EGG FOAMS

356

Foam-Forming Properties of Egg White Stages to

Egg White Foams

Egg Yolk Foams Meringues

Omelet

Souffle

Proteins

357

Which Egg Whites are Beaten

Factors Affecting

Puffy

352

352

the Shell

361

361

362

and Fondue

EGG SUBSTITUTES

363

363

360

356

351

9

Chapters.

XIX

A

Meat

368 CONTENTS

CONSUMPTION OF MEAT COMPOSITION 368 STRUCTURE OF MEAT

368

370

370

Muscle Fibers

374

Connective Tissue

376

Fat

IDENTIFICATION OF MEAT 377

Meat

Cuts of

376

376

Types of Meat

TENDERNESS OF MEAT

381

383

Basis of Tougliness

386

Conditioning (Aging) of Meat

386

Meat Tenderizers

INSPECTION

386

GRADES OF MEAT Yield

387

387

Quality Grades

387

Grades

COLOR OF MEAT Fresh

388

388

Meat

390

Cured Meat Pigments

STORING MEAT COOKING MEAT

391 391

Effects

on Meat Pigment and Color

Effects

on Meat Proteins and Tenderness

Effects

on the Fine Structure of Muscle

Effects

on Flavor

on

Methods

400

Content

Nutritive Value

401

401

Storage of Cooked Meat

Chapter

393

396

398

Effects of Initial Fat Effects

391

A Z)

406

415

Poultry

MARKET CLASSES OF POULTRY 415 PREPARING POULTRY FOR MARKET 41 INSPECTION AND GRADING 416 HANDLING RAW POULTRY 417 COMPOSITION AND STRUCTURE 417

COOKING POULTRY Methods

Doneness of Poultry Cooking Losses Frozen Poultry Flavor of

419

41

Cooked

421

422 422 Poultry

422

422 LEFTOVER COOKED POULTRY

YIELD

422

XX

J^^

Chapter

426

Seafood

CONTENTS

TYPES OF FISH

426

COMPOSITION AND NUTRITIVE VALUE 427 PURCHASING FISH 427 427

Market Forms Freshness

in Fish

SEAFOOD CONSUMPTION

ISSUES OF SAFETY IN and

Viral

Hazards

430

430

Fishborne Parasites

431 STRUCTURE OF FIN FISH MUSCLE INSTABILITY OF FISH MUSCLE TO FROZEN STORAGE 432 COOKING FISH

431

433

Methods

434

Assessing Doneness

437

PART

VII

CAKES

Chapter

A J)

Shortened Cakes

439

INGREDIENTS AND THEIR FUNCTIONS

439

439

Fat

442

Emulsifier

443

Sugar

445 445

Eggs Flour

446

Leavening Agent

447 447

Liquid Salt

PROPORTIONS OF INGREDIENTS Balancing Ingredients

in

Fat

and Low-Sugar Formulas

Adjustment for Altitude

447

a Conventional Coke

447

448

Single-Stage or Quick-Mix Formula

Low

429

429

Marine Toxins Bacterial

427

449

449

COMBINING INGREDIENTS FOR SHORTENED CAKE Objectives

Methods

450

BAKING

452

Changes

Effected

by Baking

Heat Penetration During Baking

Chapter

449

449

^O

452 453

Sponge, Angel Food, and Chiffon Cakes

DESIRABLE CHARACTERISTICS

457

457

INGREDIENTS AND THEIR FUNCTIONS

457

xxi

457 459 459 459

Eggs

Sugar Acid Flour

CONTENTS

459

Angel Food Coke Packaged Mix

MANIPULATION OF ANGEL FOOD CAKE BATTER Meringue

459

Incorporation of Flour

461

Making

the

MANIPULATION OF SPONGE CAKE BATTER MANIPULATION OF CHIFFON CAKE BATTER BAKING 463 463

PLANT FOODS

PART VIM

A/

Cell

467 469

Fruits

STRUCTURE OF PLANT TISSUE The

462 463

463

Baking Pan

Baking Time and Temperature

Chapter

459

469

472

Wall

475 475

Cytoplasm Vacuole Intercellular

476

Spaces

COMPOSITION OF FRUITS

476

476

Water

478

Carbohydrates Protein, Fat,

478

and Minerals

478

Vitamins

478

Organic Acids

RIPENING OF FRUIT

478

Slowing Ripening and Postponing Senescence

PIGMENTS

IN FRUITS

480

AND VEGETABLES

Chlorophyll and Carotenoids

481

481

481

Flavonoid Pigments

POST HARVEST FACTORS THAT INFLUENCE QUALITY Moisture Content and Texture Discoloration

and

Its

489

Prevention

490

PREPARATION OF FRUIT FOR SERVING Raw Cooked

CHANGES

492 IN FRUIT

493

Crispness

493

Tenderness

DRIED FRUIT Cooking Storing

492

492

494 494

494

CAUSED

BY

COOKING

493

489

XXM

ZO

Chapter

Vegetables

498

CONTENTS

COMPOSITION

498 498

Carbohydrates

500

Minerals and Vitamins

500

Organic Acids

TEXTURE OF VEGETABLES 501 QUALITY CHARACTERISTICS OF RAW VEGETABLES Storage

to

PREPARATION FOR COOKING 504

Preparation

in

504

504

Washing Waste

502

502

Maintain Quality

COOKING METHODS

505

505

Baking

506

Boiling in the Skin

506

Boiling Prepared Vegetables

506

Steaming

507

Panning or Stir-Frying

COOKING ON TEXTURE OF VEGETABLES

EFFECTS OF

507

Tenderness Texture of

507

507

Crispness

Cooked

Potatoes

511

FLAVOR OF VEGETABLES AND THE EFFECTS OF COOKING Sulfur

Compounds

PIGMENTS

in

515

Vegetables

AND

VEGETABLES

IN

THE EFFECTS OF

519

522

Corotenoids

524

Anthocyonins

525

Betalains

Anthoxanthins

525

Discoloration of Potatoes

NUTRITIVE VALUE

Chapter Z,

J

AND

526

PALATABILITY OF

COOKED VEGETABLES

Legumes

LEGUMES USED FOR FOOD 534 COMPOSITION AND NUTRITIVE VALUE 534 COOKING LEGUMES 540 540

Soaking

Cooking Time and Doneness Storage

Seasoning

542

543 543

Cooked Legumes

543

544

FLATUS AFTER INGESTING LEGUMES

527

534

VARIETIES OF

Canning

COOKING

519

Chlorophyll

Yields of

512

514

Mild Vegetables

544

PART

547

GELS

IX

CONTENTS

.

Chapter 5Q

551

Gelatin Gels

MANUFACTURE OF GELATIN GELATIN GEL FORMATION Dispersing Dried Gelatin

552

Gelation of a Gelatin Sol

552

551 551

USE OF GELATIN IN FOODS Fruit

and Vegetable

554

Jellies

Whips, Sponges, and Creams

554

554

Unmolding Gelatin Gels

Chapter 5\

553

553

Proportions of Gelatin

556

Fruit Pectin Gels

PECTIN CHEMISTRY

556

SOURCES OF PECTIN 557 COMPONENTS OF PECTIN GELS

558

558

Water

558

Acid

558

Pectin

Calcium Ions

558

559

Sucrose

CONTROLLING THE VARIABLES Hydrogen

Ion Concentration

IN

A PECTIN GEL

560

561

561

Sucrose Concentration

561

Pectin Concentration

AND TEMPERATURE 563 MADE WITH PECTIN CONCENTRATE OR POWDER MADE WITH FRUIT PECTIN EXTRACT 564

SETTING TIME JELLY JELLY

564

Testing for Pectin

Boiling the Jelly

564 564 565

Assessing Doneness

KEEPING QUALITY OF PECTIN JELLY Crystals

Index

in Jelly

563

564

Extraction of Pectin Clarification

xxiii

565

566

569

PART Food Quality

Evaluation of Food

Clualit)'

of food can be evaluated both with

human

senses

and with instruments.

senses are used to perceive the sensory properties of foods

and instruments

Human

are used to

quantity physical properties contributing to the sensory and nonsensory characteristics oi

food quality.

Every time food

is

eaten a judgement

decides that the food in question pass. Additionally, food

perception of foods

or

is

is

made. Consciously or otherwise the consumer

not oi acceptable

qualit)', that

it

shall or shall

not

tested for research purposes or for quality assurance.

is

Testing of food qualit)' sor)'

is

is

is

discussed in this chapter and the physiology of human sen-

discussed in Chapter 2.

SENSORY EVALUATION OF FOOD QUALITY Vocabulary of Sensory Analysis Before describing sensory testing of foods,

of our sensory responses.

Our

However, our vocabulary

to describe

Classification of

When

necessary to discuss classification systems

odor and texture

is

is

well developed.

woefully inadequate.

Odor

olfactory' receptors are isolated

may be

it is

vocabulary to describe colors and tastes

and characterized and scheme

possible to determine a natural classification

holds that there are no odor primaries

—

a limited set

their substrates specified,

for odors.

it

Current thinking

of elements that can be combined in

varying proportions to yield the spectrum of olfactory sensations. In this regard, olfaction is

more

like audition

than vision. Given the lack of understanding of basic olfictory mech-

anisms, classification of odors

Analogy odors

is

is

more

of an art than a science

and

little

agreement

exists.

often employed in an attempt to verbalize differences in odors. For example,

may be

characterized as nut-like, fruity,

oily,

or minty.

Classification of Texture

There

is

no widespread agreement on terminology

Consequently, analogy

is

for the textural attributes

often used to describe texture.

indicate the texture of fudge or the consistency of

monly made with cream. Velvety

is

Thus

the

word creamy

of foods. is

cream sauce, neither of which

used to is

com-

another such word used to characterize the mouthfeel

of

ice

creams and some cakes. Rubbery

is

used to describe some gels and the white of an

egg that has been boiled rather than hard cooked. PARTI

FOOD QUALITY

A 1963

and

texture classification system (Tables 1-1 for solid

and semisolid foods (16) and

later

1-2)

was developed by Szczesniak

expanded

new

the foundation of texture evaluation, but

This sensory texture profile

is still

foods have been suggested

(8). In this

in

to include liquid foods (14).

reference

scheme, textural characteristics are categorized

as

mechanical, geometric, or other. Primary mechanical characteristics include hardness, cohesiveness, viscosity,

and

elasticity, all of

which

are influenced

by the attraction between

constituents that

make up

a food. Adhesiveness, the fifth characteristic, applies to the at-

between

surfaces.

Secondary characteristics ol mechanical food texture include

traction

and gumminess. Geometrical characteristics relate to particle size and shape, and other characteristics include wetness and oiliness. Juiciness not only relates to water content, but the force with which water squirts out of cells upon chewing (15). A second approach termed the Spectrum Method (5) considers the natural time course of brittleness, chewiness,

sensations. Evaluations are

made

passes the lips to the afterfeel

Relatively

little

at

each stage of food ingestion from the time a sample

once the sample

research has been

is

done on

swallowed. texture perception, partly because of the

complexity ol sensations involved and partly because of the difficulty in preparing a range

of

substances that vary in only one

test

TABLE

component

of texture.

Some

attempts to develop

1-1

Relations between Textural Properties

and Popular Nomenclature

Mechanical Characteristics Secondary Parameters

Primary Parameters

Popular Terms Soft —> firm —>

Hardness Cohesiveness

hard

Brittleness

Crumbly -^ crunchy -^

Chewiness

Tender

Gumminess

— chewy^

brittle

tough

>

Short —> mealy -^ pasty

— gummy >

Thin -^ viscous Plastic

—

>

elastic

Sticky -^ tacky -^

gooey

Geometrical Characteristics Example

Class

and shape shape and orientation

Particle size

Gritty, grainy, coarse, etc.

Particle

Fibrous, cellular, crystalline, etc.

Other Characteristics Secondary Parameters

Primary Parameters

Dry —> moist -^ wet

Moisture content Fat content

From Szczesniak,

A

Popular Terms

S-,

M. A.

Bronst, H.

Oiliness

Oily

Greasiness

Greasy

H. Friedman.

1

963. "Development of standard rating scales for mecfianical

parameters of texture and correlation between the objective and

Food Science 28;397-403.

—> watery

tfie

sensory metftods of texture evaluation." Journal of

TABLE

-2

1

Sensory Mouthfeel Terms for

Classification of

Liquii

CHAPTER

Category

Typical

on

OF FOOD

Smooth, pulpy, creamy

surfaces

soft tissue

foamy

Carbonation-related terms

Bubbly, tingly,

Body-related terms

Heavy, watery,

Chemical

Astringent, burning, sharp

effect

Resistance to tongue Afterfeel Afterfeel

movement

Clean, drying, lingering, cleansing Refreshing, warming,

Temperature-related terms

Cold, hot

Wetness-related terms

Wet, dry

A

and Rheology. New

S.

1

979. "Clossificafion of mouthfeel

York:

fatty, oily

Slimy, syrupy, pasty, sticky

— mouth — physiological

From Szczesniak,

light

Mouthcoating, clinging,

Coating or oral cavity

Academia

Press,

cfiaracteristics of

beverages."

In P.

thirst

quenching,

Sfierman, ed

,

Food

filing

Texture

1-20. Classification system for beverages and semisolid foods.

physiochemical models for a particular component of texture have been successful. For example, the inverse of the frictional force of the food between the tongue and the roof of the

mouth

is

a

good predictor of smoothness

(3).

Sensory Testing of Foods Sensory evaluation has been defined

(6) as a "scientific discipline

analyze and interpret those responses to products (foods

by the senses of product d evelo p

sight, smell, taste, touch,

and

used to evoke, measure,

materials) that are perceived

and hearing. "^Sensoryjnethods

advertising claims. Objectives of sensory testing

anck&nalyticaK)lhe experimenter or totearnof is

may wish

know whether

there

consume rjesting. is

si

ipporr tpr

into

consuming

On

and

two general categori:^s)_affective^ whether the panelists prefer a product

fall

to learn

potential for acceptability by the

its

often called accep^nce o r

wish to

are useful for_

m ent or reformulation, qiialitv control, quality assurance, product sensory

specification, ravy materiaJs sensory specification, product optimization,

public. This affective testing

the other hand, the experimenter

a detectable difference

between or among samples

Caramelized sugar sugar frostings.

in-

continue to do so

the liquid that remains

Molten sugar, removed from the source of heat and then becomes a clear, glassy, noncrystalline, brittle solid. molten sugar

will

is

point.

If

and

I60°C (320°F). ^u^T^crystals^an be meltol^by heavy pan, placing the pan over low heat and shaking it so that sugar

point of sugar

putting dry sugar in a

on

The decompositionoFsiicrose by

of aldehydes and ketones

in

which

furfural

is

used to

which have been

make burnt-

heat gives rise to a complex mixture

and 5-hydroxymethyl

furfural are

nent constituents. The products of the pyrolysis of sucrose include, ture of cresols, eight of

identified (13).

promi-

in addition, a

When

soda

is

mix-

added

tol

caranieliaed-sugar, the heat plus_the3cidsj)resent releas e carbon dioxid e, bubbles of

which

inflate the

sucrose and water

makes when tint.

This

is

molten mass. is

a small

When

cooled

it is

porous and

brittle.

When

a syrup of

heated to the soft crack stage (named for the sound the hot syrup

amount of it

is

spooned into cold water),

not due to caramelization, but

is

it

takes

on

a pale

amber

attributed to liberation of furfural from

the sugar by the high temperature followed by the formation of polymers the syrup.

8

10

which

tint

CANDIES

126 PART LIQUIDS

Determining Doneness of Candies

II

AND

Boiling Point of the

Syrup

CRYSTALS

Once that

sucrose

is

in solution, the next step

away the

to boil

is

was added originally to ensure solution of the

right

amount of excess water

large sugar crystals.

One

ot the prob-

making any confection is to know how long to cook the syrup, that is, how much water to boil away to give a product of the desired consistency. The ghiet-ta ctor tha tdelems

in

^jer mines consistency in th efl nishe dprodu ct this the

temperature of the boiling syrup

is

is

the c oncentration of sugar in jhe syrup. For

an index. For either noncrystalline or crystalline

products, a thermometer can be used to determine

mometer mometer

is

when

not give a true reading unless the syrup

will

submerged

in the boiling syrup but not

the boiling syrup

For noncrystalline candies such will

as caramels, the object

be neither too thin nor too thick when

should be

just viscous

handled. Almost

all

enough

the water

it

that the pieces is

is

A ther-

is

is

level

read.

to concentrate the syrup so

room temperature. The syrup

has cooled to

when

done.

touching the pan, and the eye

with the top of the column of mercury when the temperature

it

is

boiling, the bulb of the ther-

is

cut will hold their shape and can be

evaporated horn syrup for bri tiks, glace, and

toffee.

The

high proportion of interfering substances permits these supersaturated syrups to supercool

and form candies that

are vitreous.

For crystalline candies the object

^ ^

is

to obtain in the finished

product the correct

ra-

and remaining saturated syrup. The following example will explain how this is accomplished. When sucrose syrup boils at 1 15°C (239°F), each 100 grams of water in the syrup has dissolved in it 669 grams of sucrose. When this syrup is cooled to 40°C (104°F), each 100 grams of water at this temperature can dissolve only 238 grams of sucrose. Thejig uid thus ho lds more 'fnliirc rhan it c an dissolvejithat temper ature and is said to be supersaturated. The difference between the amount of solute tKatthelOO grams of water holds and the amount it can dissolve at 40°C (669 minus 238, or 431 grams) is a measure of how 5U£ersaturatedt:he syrup is. This extra sucrose is held precariously in solution, and with a little encouragement, it will precipitate as tio

between sucrose

crystals until the

crystals

remaining syrup

just saturated.

is

syrup when

more supersaturated the The more sucrose that crystallizes,

it

The

higher the boiling point, the

more sucrose precipitated. amount of syrup that remains, and the

has cooled and the

the less the

firmer the candy.

The boiling_point

isjiotjij^rue-in dex to the c onc entration o f sucrose

must be cooked

ing point, toorwtien other sugars are present, the syrup

perature to concentrate the sucrose sufficiently. Ingredients other than sugar

syrup makes to give the

when

other sug-

w ith sucrose in a syrup, because they contribute to the elevation of the boil-

ats^are present

it

more

viscous.

may With

to a higher

not affect the boiling point, yet their presence a

more

tem-

A second point needs to be considered, too.

viscous syrup

less

in the

sucrose needs to precipitate

candy the desired consistency. This means that when

rri ilk

so lids, cocoa^^hocp-

latcj at, or d extrins a re_presenvthejucras£in the syru p neecLao.t be^so aancentrated- Thus syrup forTudge reaches the soft ball stage at a slightly lower boiling point than fondant (see Table 8-4).

Formulas for different candies and the temperature are given in Table 8-4.

to

which each should be brought

5

1

28

PART LIQUIDS

II

AND

CRYSTALS

Consistency of the Syrup (Cold Water

A second

index to the doneness of a candv-^yrup tfen~-takes into account these factors, as

well as the concentration of sucrose, tested near the

while the

Test)

test

is

is

tKesajnsistency

ofwe

cooled syrup. This should be

end of the cooking period. The~?yrup should be removed from the heat made. A small amount of the candy syrup is poured into fo^ water and its

behavior noted.

For example, syrup that

is

the container of cold water, but to the soft ball stage

is

at the soft ball stage it is

really soft.

so soft that

An end

it

can be collected from the bottom of

runs through the fingers. Syrup cooked

point that

is

so subjective

is

liable to errors in

judgment. Syrups for both fudge and fondant are sometimes overcooked when

method of assessing doneness

is

used.

Too

often the syrup for fudge

is

cooked beyond

this this

stage to the firm or even the hard ball stage. Table 8-5 gives the stages of doneness of a sugar

syrup as assessed by

its

Interfering Agents

consistency in cold water.

and Sucrose

Crystal Formation

Functions of Interfering Substances

The proportion of interfering of sugar,

if

substances present in a candy syrup influences the

any, that crystallizes

from the syrup

as well as the size

of the sucrose

amount

crystals.

Under the appropriate conditions, sucrose molecules in a syrup can align themselves manner unique to the sugar molecule to form crystals. Forces holding the molecules together in the crystals are hydrogen bonds between hydroxyl groups on contiguous molin a

ecules.

Axpo lecule of any sugar other than sucrose has a^ififerent shape,

interfere

TABLE 8-5 Consistency Tests for Doneness of Syrups

Boiling Point at Sea Level* Test

is

foreign. iind will

with sucrose molecules in solution joining or adding to sucrose

crystals already

srarted.

This

is

manv

in a syrup,

Thus

large.

and fructose even though they are part of the and fructose are present form instead of only a few that grow exceedingly

true for molecules of glucose

When

sucrosg molecule itselh

sucrose crystals tend to

foreign sugars favor the formation of

candies. Aside

more (and

from foreign sugars, other substances may

large sucrose crystals in crystalline candies. In fact, a

stances in a candy syrup

may

ALTERNATIVE

formation of

high proposition of interfering sub-

prevent crystallization altogether that makes possible non-

Proportions of Interfering Substances

Two methods are employed to ensure the optimum concentration of interfering substances in candies. The interfering substances may be added to the syrup or they may be formed

When care.

This

sugars

is

as the

candy cooks.

interfering substances as such are added, the quantity should be

is

measured with

A common way to provide interfering made from cornstarch. When molecules of starch

particularly true for crystalline candies.

to use corn syrup.

Corn syrup

enzymes catalyze the

is

and

are hydrolyzed, dextrins, maltose,

finally glucose are

formed. Either acid and heat or

reaction:

Enzymes

+

Starch

Water

—

—

>- Dextrins or Acid plus Heat

Enzyme-hydrolyzed corn syrup

is

+

Maltose

+

Glucose

marketed under the trade name of Sweetose.

If starch

has been converted to glucose completely, the syrup has a dextrose equivalent (DE) of 100 percent; syrups with lower

DE

have more dextrins and maltose and

less

glucose. For each

cup (250 milliliters) of sugar used to make fondant and fudge, one tablespoon (15 milliliters) of corn syrup will provide enough interfering substances to control effectively the size

of the

crystals. Hotiey,

which contains both glucose and

may

fructose,

be used to pro-

vide interfering sugars in candies. Interfering sugars, instead of being

some of

added

in a definite quantity,

the sucrose molecules as the candy syrup

with water molecules when the syrup of sucrose hydrolyzed,

is

is

may be formed from

boiled. Sucrose molecules will react

heated in the presence of acid. For each molecule

one molecule each of glucose and fructose

is

obtained:

Acid plus heat

H2O

C12H22O11

— or^QHiP^

+

QHijO^

Sucrose Sucrose

Water

Glucose

Fructose

'Invert sugar-'

This reaction (the reverse of that shown alyzed by the sults

is

enzyme

known

When

sucrase also.

earlier in the section

on chemistry) may be

The equimolar mixture of glucose and

is

invert sugar, the amount added is The amount of sucrose converted to in-

added to candy syrup to help form

even more

critical

vert sugar

depends upon the concentration of hydrogen ions (amount of

when

than in the case of corn syrup.

the candy syrup

must be taken

cat-

fructose that re-

as invert sugar.

acid

is

into consideration

acid) present

The alkalinity of the water and its neutralizing value when acid is used to hydrolyze the sucrose (4). One way

boiled (18).

CHAPTER 8 SUGARS,

smaller) crystals in crystalline

interfere with the

crv'stalline candies.

from sucrose

129

interfering sugars, such as glucose

SWEETENERS,

AND CONFECTONS

1

30

to control the

of ^^^^ LIQUIDS

"

AND

CRYSTALS

tartar.

some

amount of acid

This compound,

fondant

in

KHC4H4O6,

time that

and

acid

does acid present in the syrup around

as

make panocha, which

similar to fudge.

is

used in candy, the length of the cooking time

Some

invert sugar

is

is

critical (3).

and the candy

is

is

A cooking

likely to

be coarse

desired in candy to keep crystals small or the texture fine,

but prolonged cooking will yield too tallizes

measured amount of cream

too short gives insufficient invert sugar and a candy that

is

grainy.

is

to use a carefully

the acid salt of tartaric acid. Vinegar, used in

same purpose,

recipes for taffy, serves the

the crystals of brown sugar used to

When

is

is

much

too soft to handle.

invert sugar. In this case, too

One

reason for this

is

little

sucrose crys-

that invert sugar increases

With six percent invert sugar the solubility of sucrose at room 67 percent to 80 percent. At any given concentration of sucrose, fewer crystals would be expected to precipitate in the presence of invert sugar than in its absence. In addition, the two molecules that make up the invert sugar formed elevate the boiling point twice as much as does the sucrose from which it was derived. This means that the boiling point of the syrup in the presence of excess invert sugar is not a true index to the concentration of sucrose and hence to the doneness of the syrup. From 6 to 1 5 percent of invert sugar in fondant is sufficient to keep the crystals small (8); more than the solubility of sucrose.

temperature

this

is

(16-23%)

increased from

gives a

candy semifluid consistency.

A soft fondant has as a coating for

an advantage

bonbons.

A

if it is

to be

firmer fondant

is

melted and used to make mint wafers or

needed

molding cream centers

for

to be

melted fondant or melted chocolate. Chocolate-covered bonbons that are semifluid in consistency are made by incorporating the enzyme sucrase in the fondant centers before they are dipped in chocolate. This enzyme converts enough of the sucrose to invert

dipped

in

sugar during storage so that the sucrose crystals partially dissolve.

Effect of Interfering

Some

Substances on Consistency

interfering substances influence the consistency of a

candy

as well as the size

of the

sucrose crystals. Fat globules and the proteins from milk, as well as the solids from cocoa

and chocolate, influence the

viscosity of the syrup.

When the syrup is more viscous because

of the presence of these constituents, the crystal-to-syrup ratio need not be so high. It is for this reason that the boiling points for fondant and fudge differ. The final temperature for the syrup for the

fondant recipe given

in

Table 8-4

is

1

14°C, while that for fudge

is

and so have the same consistency or flow properties when tested in cold water, and the candies should have the same consistency. Fondant will have a higher ratio of sucrose crystals to saturated syrup; in fiidge the milk 1

12°C. Both syrups reach the soft

solids, the butter,

to

compensate

and the

solids

ball stage

from the cocoa contribute enough thickness to the syrup of crystals to syrup. Both fondant and fudge differ from

for the lower ratio

the original sugar in that the sucrose crystals are smaller and are

now suspended

in syrup.

Prevention of Crystal Formation

A

high proportion of interfering substances in a candy syrup

may

prevent crystallization

of sucrose altogether. This is desired in amorphous or noncrystalline candies such as caramels, taffy, toffee, and brittles. Compared with fondant and fudge, taffy has a high proportion of corn syrup, toffee a high proportion of fat, and caramels a high proportion of both. Table 8-3 gives the proportions of ingredients in different candies. Noncrystalline

candies are either very thick syrups or hard and glasslike. Their consistency depends chiefly

on how much the syrup was concentrated before

it

was removed from the

heat.

noncr}-stalline candies are sufficiently concentrated so they will not flow at ature.

Syrup

for toffee, a brittle candy,

to the soft crack or the hard ball stage, is

Syrup

desired.

The brown

Syrups for

heated to the hard crack stage, and that for taffy

is

depending on whether

a brittle or a chew)'

product

chewy consistency is heated to the firm ball stage. flavor of caramels and toffee are attributed to a reac-

for caramels that have a

color and characteristic

and sugar brought about by the high temperature.

tion betNveen protein

Aside from their use in candies, interfering sugars are useful in preventing tion of sucrose in syrups.

Syrup

for preserves

cooked

is

crystalliza-

until the sucrose concentration

is

near 65 percent. Slight overcooking gives a supersaturated syrup and the likelihood of large sucrose crystals forming eventually unless acid

the preserves cook. For this reason fruit pectin jellies the sucrose

lemon

juice

is

present to invert

is

added

some of the

watermelon

to

sucrose as

preserves. In

concentration must be at or close to 65 percent.

most

made

Jellies

with pectin concentrate have such a short boiling period (usually one minute) that very tle

invert sugar

is

produced and sucrose

and Sucrose

Agitation

may form

cr)'stals

in the jelly

lit-

during storage.

Crystal Formation

Fondant and Fudge

The

texture of a crv'stalline

candy or frosting

terfering agents present but also

is

influenced not only by the proportion of in-

by the number of

tion begins. If syrup for fondant or fudge

is

crystal nuclei that

beaten while

still

form once

when fondant syrup

to form, initiated

on the

is

do so on the

cr\'stals

that

first

The

form.

this large for the graininess of crystalline candies to

larger than

45 microns across

crystals

become

(1

detectably crystalline; those no larger

a large

number of crystal

urate d b efore an y crystals be gin to

pan

as the

which more

(122°F). Syrup for fudge

pan onto a the

flat

is

is

is

.

The syrup

sufficiently supersaturated.

Any As

larger as the syrup continues to cool.

is

drained is

sides

40°C (104°F) is is

of the

(2)

cooled to

and

50°C

poured from the

— not scraped—from

the pan, and

poured. Such syrups, as they cool, are

agitation will start crystal formation before the

This

is

still

is

form

illustrated in Figure 8-3a.

Fondant syrup allowed

supersaturated before beating

(Figure 8-3b).

cooled to

a result, the few crystals that

beating was initiated while the syrup was

ficiently

is

interfering substances are present,

in position before the syrup

slightly supersaturated.

of cr\'stals of only 6 microns!

form Crystals should be wiped from the

unstable and should be undisturbed.

syrup

size

nuclei, the syrup should be sufficiently supersat -

usually cooled in the pan; that for fondant

surface to cool.

thermometer

'*

centimeters) make fonmore than 25 microns across than 20 microns seem fine and creamy.

syrup for fondant cooks. Syrup for fondant

that for fudge, in

so large that

Crystals need

fact, crystals that are

This means that the tongue can detect a difference in the

To obtain

ruler.

be detected, however. Cr\'stals

micron equals 0.0001 or 10

dant seem grainy on the tongue (18). In

make fondant

the other

lint, or possibly seeded by sucrose crystals from syrup splashed These few grow exceedingly large because the other sucrose mole-

they can be seen by the unaided eye and their dimensions measured by a

no

On

allowed to stand undisturbed, eventually a few crystals begin

by dust or

sides of the pan.

cules that precipitate

not be

cr)'stalliza-

hot, too few crystals are initi-

ated and grow larger as the syrup cools, the result being coarse, grainy candy.

hand,

For

initially

this

grow

fondant,

hot (near I14°C or 237°F) and so only to cool to

initiated

131

room temper-

40°C (104°F) and become

suf-

forms numerous small crystals

CHAPTER 8 SUGARS, ALTERNATIVE

SWEETENERS,

AND CONFEaiONS

132 PART LIQUIDS

v^i!

II

AND

O-

CRYSTALS

*

-

Figure 8-3. Sucrose

•

tiJtcr- '>kJ^'

•

'




E

s

6 o D

U

:5 ro

-a

c o c

o o Q.

ca


o o — ^

o(N

u-i

the normal Rinctioning of the large intestine. Equally important in justifying the greater utilization

of cereals

that are polymers

the presence ot soluble

is

of the sugar xylose, and especially the P-glucans

mers of P-n-glucose 1

,3 linkage.

bilit\'

as in cellulose,

but the

1

,4

linkage

is

(4).

The

latter are poly-

interrupted occasionally with a

This accounts for the solubility of the (B-glucans (4) in contrast to the insolu-

of cellulose. These dietary

fibers, as

lowering the lipid and cholesterol

they are called, are considered to have a role in blood.

levels in the

ENRICHMENT AND FORTIFICATION The

nutritive value ol cereals can be

improved by enrichment or by

are enriched by replenishing certain nutrients

and

riboflavin, niacin, iron,

riched.

The

as

of 1998,

use of calcium and vitamin

removed

folic acid

D

is

is

fortification. Cereals

in processing. Inclusion ot thiamin,

mandatory

optional.

White

if a cereal

is

labeled en-

flour, grits, farina, pasta,

The objective of fortification is to use cereals as carriers of nutrients identified as likely to be deficient in the diet. The recommended level for fortification is approximately 25 percent of the Recommended

white

rice,

cornmeal, and white bread are routinely enriched.

Dietary Allowance (RDA), renamed Reference Daily Intake (RDI). Nutrients identified as appropriate to use in fortification include thiamin, niacin, riboflavin, vitamin (folacin),

B,,, folic

acid

vitamin A, vitamin D, calcium, iron, magnesium, and zinc. Ready-to-eat cereals

are considered appropriate vehicles for fortification. Ready-to-eat cereals are available fortified

with different combinations of the nutrients above.

MARKET FORMS OF CEREALS The

intact grain ol a cereal can be

advantage in

is

consumed, but

this

is

not the usual practice.

One

dis-

the long cooking time required. Equally important, cereals with the germ

may become

outer,

branny

rancid.

Many

individuals consider cereals

more

left:

palatable without the

For these reasons and for the sake of variety, grains are milled or oth-

layer.

erwise processed before they are marketed. There are two general approaches to milling.

The The

grain

may be

separated into

other approach

is

of increasingly reduced cereals.

some

its

structural

components, bran, germ, and endosperm.

to subdivide either the entire grain or size.

Products

Although some grains

are

made from

the

its

endosperm

endosperm

into particles

are referred to as refined

marketed raw and so require cooking, heat

stage in the processing of the array of ready-to-eat products

CEREAL PRODUCTS THAT REQUIRE COOKING

is

involved at

on the market.

(6)

Rice

Three forms of uncooked

White or polished

rice

rice are available. (5).

The

intact grain

is

known

as

brown

rice.

has had the bran removed. Converted or parboiled rice has been

soaked and then steamed to drive nutrients from the bran into the endosperm, after which dried and the bran removed. The conversion process gives the grains a golden makes them translucent. Rice is marketed according to the length of the grain. Long-grain rice accounts for almost % of the total, medium grain X or less, and short grain the grain

color and

is

1

67

Included are hemicelluloses, mainly xylans

fiber.

CHAPTER 10 CEREALS

1

68

no more than

rice constitutes

five percent.

are available, including glutinous rice, PART

A number ot specialty rices— some imported-

which

almost 100 percent amylopectin.

is

III

STARCHES AND STARCHY FOODS

Wheat Cracked wheat

endosperm 1

that

Bulgur

1-2).

The product

made by subdividing

is

is

is

made

from whole wheat.

not raw but time

is

the entire grain. Farina consists of middlings, the

when wheat is milled to produce flour (Fig. The grain is cooked, then dried and fractured.

most resistant to cracking is

required lor

it

to rehydrate in hot water.

Corn

The

removed by abrasion, as for polished rice, or it is loosened lye. The hull and residual lye are rinsed away from the soaked. Hominy consists of the whole endosperm; grits are bro-

hull of the corn kernel

is

by soaking in water that contains

endosperm

if

ken pieces of

the grain it.

is

Cornmeal

is

ground pieces of either white or yellow corn.

the finely

Oats kernels of oats, called groats, are steel-cut to form a granular cereal similar to cracked

The

wheat. Alternately, they are used to produce rolled oats.

The

groats are steamed to soften

pass between rollers that flatten them.

Two

types of rolled oats are avail-

them before they able

—

and quick cooking. The

regular

entire grain

divided to give quick cooking oats. Oat bran

is

is

used for the former; the groat

is

sub-

rich in P-glucans (4).

Barley

This grain with the bran removed

and

as

is

marketed

as pearl barley. It

an ingredient in fabricated cereal products. Barley

READY-TO-EAT CEREALS

is

a

is

used

as a thickener in

soups

good source of P-glucans.

(6,7)

Flaked For cornflakes, the endosperm of corn pressure with sugar, ter

is

salt,

removed before the

is

divided in half and these

grits are

cooked under

malt, and water until the particles are translucent. Part of the wagrits are

passed between rollers to flatten them. Toasting the flakes

at

high temperature produces blisters from the steam generated and reduces the moisture

to

below three percent so the flakes are Wheat flakes are made from intact

to facilitate uptake of water, after

flakes are

which the

They

are

steamed and then

grains are thoroughly

is

slightly cracked

cooked under

removed before the grains are passed between dried to a low moisture level as are cornflakes.

Part of the moisture

The

shelf-stable.

grains.

pressure.

rollers to flatten

them.

Granular

Grape Nuts® a stiff dough. loaves

are made from wheat and barley flours, salt, yeast, and enough water to form The dough is allowed to ferment for a few hours after which it is shaped into

and baked. This compact loaf is then broken

to yield the granules of

Grape Nuts

.

169

Shredded Shredded wheat

is

made from

intact grains.

They

posits parallel cuits.

rows of these shreds, layer on top of

Baking begins

at a

after

which the

chapter 10

bank of pairs of

rollers de-

CEREALS

are boiled for

cooked grains are fed to pairs of grooved shredding

rollers.

layer.

A

The

an hour,

layers are divided into bis-

high temperature to brown the surface and continues

temperature to reduce the moisture to the desired

at a

lower

level.

Puffed

Water vapor

is

used to puff cereals. In one procedure, the grains that are cooked until they

are translucent are heated to a high temperature within a few seconds. Generation of steam

enlarges the grains from

or grits

is

two

to five times. Alternately, a

cooked by steaming,

through the die of an extruder.

20 times. Milled

The

dough made from

which the dough

is

pieces are sealed in a

flour,

middlings,

shaped into pieces by forcing

popping

vessel or

rice

it

gun and heated

pressure. Release of the pressure inflates the pieces as much and pearled wheat may be cooked and pressure pufted, too.

and

to a high temperature to

after

as

15

Extruded For extruded cereal products, the dough

through the die at the

is

cooked under pressure and then forced to

end of the attached extruder

barrel.

The drop

exit

in pressure causes the

product to expand.

Instant

Instant cereals are precooked ing.

and dried before they

are marketed.

They

require

no cook-

Addition of hot water converts an instant product into a hot cereal ready tor serving.

Farina, rice,

When

and

rolled oats are available as instant cereals.

cereals are subjected to high temperature, as in pressure cooking, puffing,

may destroy some thiamin and lower starch may become indigestible, too (16).

toasting, the heat

Some of the

and

the nutritive value of the protein.

COOKING CEREALS Purposes Cereals are cooked to increase their digestibility and their cereal

is

creases the palatability of cereals

to six

palatabilit}'.

Chewing uncooked

wearing on the molars. Cooking softens the cellulose, but mainly cooking

volumes of water

by

its

are required to

in-

on the major component, starch. From two cook one volume of cereal because ot the uptake of effect

water by the gelatinizing and pasting starch.

Precautions

One of the goals in cooking cereals, as with

When

the dry cereal

is

added

using starch

to boiling water,

it

itself in

cooking,

is

to avoid lumps.

should be stirred only enough to prevent

the formation of lumps. Excessive agitation, either stirring or allowing the water to boil vigorously, results in an interior product.

1

When

70

ules are

PART

III

STARCHES AND STARCHY FOODS

grain

is

and some of the embedded starch gran-

milled, the cells are fractured

exposed on the surfaces of the individual

many of these

cooking,

particles. If the cereal

starch granules are dislodged.

is

agitated during

These thicken the liquid around the

individual particles of cereal. This gives a cooked cereal with individual pieces in a thick starch paste, a consistency that

consistency

ter

many find

the starch granules remain in place

if

Cooked

unpalatable.

on the

embedded

cereal has a bet-

surface of the pieces of cereal.

Flaked cereals are particularly susceptible to disintegration by agitation during cooking because flakes are inherently

more fragile than granules. The (B-glucans in rolled oats readily and form a viscous film around the cooked flakes. This is

dissolve in the cooking water

minimized when the cooking to

cook the

is

started in boiling water (23) or

when microwaves

are used

oats (22).

Proportions of Liquids Proportion of water to cereal depends in part upon the ity to

absorb water. Fine granular cereals require

size ol the particles

rice increases

an indication of the approximate amount

more than twice

When

of water.

is

this

in

volume, even when

low proportion of water

it is

steamed

it

will swell.

in only twice

used, the rice grains absorb

is

A pan with a tight-fitting lid and

their abil-

The amount of water

coarse cracked cereals four times, and flaked cereals two times.

needed to cook a cereal

and

times their volume of water,

five to six

all

However, its

volume

of it and the

If rice is cooked volume will be greater. The product will be more moist, also. Long-grain rice tends to swell more than short-grain rice; converted rice swells less than the same type polished. Brown rice swells somewhat less than polished. When cooked in milk, cereals swell more than when cooked in water. (Possibly phosphates

swell

is

limited.

with a higher proportion of water, the increase

from the milk

Amount The

are involved.)

of Salt

use ot

cup (250

more

low heat should be used.

in

salt lor flavor

salt,

is

Wz teaspoons

(7 milliliters) per

cup

all is

one teaspoon

(5 milliliters)

of salt per

except fine cereals, tor which

somewhat

optional. Approximately

of dry cereal will season

milliliters)

suggested.

Cooking Time

A number of factors

influence

(how much the grain water returns to a

is

boil,

how

long a cereal needs to be cooked. Size of the fragments

subdivided) and prior heat treatment are two such factors.

cooking time varies from

heat to 10 to 15 minutes over boiling water. ter,

a

When

Once

minimum

of 5 to 10 minutes on direct

cooking

completed over boiling wa-

is

additional cooking does no harm.

The cooking time of rice depends upon the variety. Some need to be cooked only 15 may require up to 30. Quick-cooking rolled oats cook in less time than

minutes, but others

do regular

rolled oats because the pieces are smaller. Directions

cooking time of

five

quick-cooking rolled rina, salt

and

pasta,

minutes over direct heat oats.

A number of quick-cooking cereal

have had disodium phosphate,

on the package specify a and one minute for

for regular rolled oats

products, including

Na2HP04, added

to them. This

rice, fa-

phosphate

speeds the cooking by enabling starch granules to reach gelatinization temperature

sooner.

Cooking Water

Effect of Alkaline

may

Polished rice or refined cereal

1

be cream colored or yellow tinted

lemon

(vinegar,

juice, or

cooking period,

not it

pours

Cooked

readily,

The

served.

cooking water, preferably

should be separate and distinct. They should be moist but

cereal

granular cereal should be free from lumps.

but

it

It

should not be so thin that

should flow enough to assume the shape of the dish in which

should not have a pasty consistency. The flavor of

cereal

late in the

Cooked Cereal

cooked flaked

sticky.

to the

when cooked in alamount of acid

small

keep the pigments in colorless form.

will

Characteristics of Pieces ot

cream of tartar) added

A

compounds.

kaline water because of the presence ot fiavonoid

all

cooked

it is

cereal

should be mild and nutlike.

On

the basis of cooked quality, rice

medium- or and tend

short-grain varieties are soft

to adhere.

falls

and translucent and

grain rice are slender

A number of factors

two

into

categories.

the

it

cooks, the amylose content of the starch, and

anatomy of the

varieties

of long-

remain separate. Most

and chalky and when cooked are moist and sticky that might account for differences in the cooking

amount

quality of rice have been investigated. These include the

sorbs as

Most

yield fluffy, dry grains that

its

amount and

kernel, particularly the

of moisture the rice ab-

gelatinization temperature,

and

components

that

distribution of

could limit the swelling of the starch. All of these factors point directly or indirectly to the starch

component. The quality of cooked

rice

inherent in the rice than from the cooking

appears to stem more from characteristics

method

Leftover cooked cereal need not be wasted. pie, as

topping for meat

creamed meat

—

all

ways

It

used.

can be used to make polenta or tamale

pies, or as a lining for a casserole

more

to include

made from

leftover stew or

cereal in the diet.

POPCORN The consumption

of popcorn,

one of the

first

snack foods, increased 72 percent over a 10

year period (10). Popularity of the microwaved product doubtless contributed to the increase (12). Successful

popping begins with the

vitreous (glassy) rather than a chalky

endosperm

packed, angular-shaped, starch granules

withstand the pressure that builds

The

kernels of

popcorn are

dosperm. Corn pops favors high

inflated

satisfactorily

(10—14%), depending on the

as the

only

variety.

is

grain.

essential.

Corn

kernels that can

The

is

a

tightly

must be able

pericarp (hull)

water inside the grain

to

vaporized.

by steam produced from moisture within the enthe moisture content is within a narrow range

when

A moisture content near

the upper

end of the range

volume; that near the lower end means fewer unpopped kernels

popcorn has had the moisture content adjusted before

Once

in a moisture-vapor-proof package.

pop have

Absence of voids around the

(8).

a

package

it is

is

(12).

put on the market, and

Quality it is

sold

opened, popcorn should be stored

in a sealed container.

The temperature yield

from popcorn

per, its ability to

to

(9).

which the popper

Optimum

is

preheated

is

another factor that influences the

temperature varies somewhat with the

hold heat, and the amount of corn popped

too hot, the corn scorches, and

if not

at

one time.

hot enough, the corn dries before

it

size

of the pop-

If the

popper

is

pops. Temperature

J]

CHAPTER 10 CEREALS

.

172

The

388°F). PART

STARCHES

III

AND

STARCHY FCXDDS

popper when the corn

popping should range from 173°

198°C (343° to 299°C or 470° to 570°F). A practical guide is to adjust the heat so that the corn begins to pop in one to three minutes after it goes into the popper. If once popping begins, all the grains pop within two minutes, the yield of popped corn will be maximum. Properly engineered electric corn poppers eliminate the guesswork from attempts to maintain optimum temperature. inside the

When

popper,

is

needs to be

itself,

at a

to

higher temperature (243° to

the contents of the kernel reach a sufficiently high temperature (near 177°C),

the accompanying high pressure ruptures the pericarp, exposing the interior of the grain to

atmospheric pressure. Apparently, the superheated water

izes in

The volume of the corn may

the starch granule and at the same time inflating the grain. increase tural

vapor-

in the starch granules

the hilum (8). This has the effect of pushing the cooked starch to the periphery of

20

30 times when

to

it is

popped. Films of gelatinized starch provide the struc-

framework of the porous popped kernels

compounds have been

Thirty-six

(15).

from popped corn

identified in the volatiles

(20).

Those considered important contributors to the aroma were pyrazines, fijrans, pyrroles, carbonyls, and substituted phenols. The odor of corn popped in a microwave oven does not diffrom that popped conventionally

fer materially

in oil,

but

does differ in

it

tactile character.

REFERENCES 1.

Akeson, W. R. and M. A. Stakmann. 1966. "Leaf protein concentrates:

2.

A

6.

Cereals

and How They Are Made. R.

with that from seed and animal crops."

and

F.

Economic Botany 20:244-50. Alternate uses

Association of Cereal Chemists. Pp. 15—^2.

of natural resources for food production.

Flaked, puffed, granular, shredded, and ex-

Caldwell, E.

and R.

P.,

B. Fast. 1990.

Breakfmt Cereab and

They Are Made. R. B. Fast and E. eds. St. Paul:

F.

"The 7.

Caldwell,

D.

Nielsen, and

J. S.

proteins

dosperm."

in

ties

J. -L.

U.

Klioo,

H. C.

8.

Physiology

53:851-55.

9.

Dziezak,

J.

salute to

1 1

W. P Bemis. 1954.

10.

Johnson,

1991. "Corn: Production, pro-

L.

and

cessing

utilization."

Cereal Science

A

and

Food Technology

45(6):74-75, 78, 80. Forms of able, including specialty rice.

rice avail-

in relation to

different temperatures.

1-55. Beta-

1991. "Romancing the kernel: rice varieties."

A. and

Technology 8:394-97. Yields of popcorn at

of cereal carbohydrates." In Dough

Pp.

A.

volumetric expansion of popcorn." Food

glucans in oats and barley. 5.

W.

Huelsen,

"Temperature of the popper

and Baked Products Texture, H. and J. M. Faubion, eds. New York:

and

Zeleznak

K.

popping." Journal of Cereal Science 1:43-52.

1990. "Rheological proper-

Van Nostrand Reinhold.

C,

R.

Character ot the kernel that favors popping.

of corn en-

particulates

Hoseney,

Abdelrahman. 1983. "Mechanism of popcorn

Wall. 1974. "Influence of

Rheology Faradi

cereals." In

of Cereal Science and Technology. Paul: American Association of Cereal

Chemists. Pp. 293-304. Details of their

High-lysine mutants. Doublier,

B. Fast

American

manufacture.

D.,

Plant

St. Paul:

Hoseney, R. C. 1986. "Breakfast

St.

rice, oats,

opaque-2 and floury-2 genes on formation ol

Caldwell, eds.

Principles

and barley included. Christiansen,

E.

truded products.

How

American Association of Cereal

Chemists. Pp. 1-14. Corn, wheat,

4.

In Breakfast

cereals."

of protein production per acre of forage

cereal grains." In

3.

1990. "Manufacturing technol-

Fast, R. B.

ogy of ready-to-eat

comparison

K.

Dekker. 1

1

Kulp, P.

Katz, S. H.,

1974.

and

Handbook of

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Technology. K.

eds.

New

J.

York:

Lorenz Marcel

65. Popcorn.

M.

L.

Hediger and

L. A. Valleroy.

"Traditional maize processing tech-

niques

in

new

tlic

world."

Scieiur

18.

184:765-73. Kftects ot liming on nutritive value. 12.

edition.

and R. C. Anantheswaran. "1988.

Lin, Y. E.

on popping of popcorn in a microwa\e oven. Joiiniu/ of Food Science '"

19.

grains.

Situation.

ferent countries.

Krunimel and

K.

J.

Dritschilo,

duction." Science 190:754-61

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Options

for

meeting the world's food needs. 1

5.

M. and H. G.

Reeve, R.

Walker. 1969. "The

microscopic structure of popped cereals." Cereal

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Chemistry

Structural

when popped. S. G., J. M. Gee, M. "Whittam,

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Ortord and

I.

Johnson. 1988. "Resistant

T.

starch: Its chemical effects

on

form

in foodstuffs

in

digestibility

vitro."

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Food

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Ruckman, Qualset.

J.

E.,

E R

Scheile

and C. O.

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as influenced

genotype and location.

"

Improved

of the hybrid.

by

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Journal

of

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and dry by mi-

21.

Woodbury, W. 1972. "Biochemical genetics and its potential for cereal improveBakers Digest 46(5):20-24,

27,

63. Protein quality; photosynthetic

effi-

ment.'"

ciency. J.

"Comparison

Weisz and P

J.

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oats."'

More

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Wood. 1991.

ot the effects of

microwave

starch

and 3-

Cereal Chemistry

(3-glucan

released

by

conventional method.

1973. "Protein, lysine, and grain

yield of triticale

M.

crowaves; aroma and tactile quality.

22. Yiu, S. H., P.

L.

and Food Chemistry 18:926-28.

Popped conventionally

food protein pro-

in

compounds."'

volatile

Agricultural

J.

Kutzman. 1975. "Energ\'

J.

and land constraints

and

R. C. Landsay

P,

J.

in dif-

Libbey. 1970. "Popcorn flavor: Identification ol

W.,

food

1966. Per-

consumption of major foods

20. Wilradt,

in the nutritive

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D.

major

ot

NFS- 115, February

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Pimentel,

Statistical

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14.

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Mertz, E. T. and L. S. Bates. 1964. "Mutant

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the

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Studies

13.

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U.S.

Abstracts

25. Yiu, S. H.,

R

J.

Wood

"Effect of cooking rolled oats.""

on

and

starch

J.

Weisz. 1987.

and P-glucan of

Cereal Chemistry 64:373-79.

Cold-z'y. hot- water start.

173

Dough

Flour and

Formation

Wheat ally

make

the primary grain used to

is

made from

rye.

The

flour,

ahhough

proteins of wheat flour are superior for

they are incomplete.

The

limiting

amino

acid

is

lysine.

yields a grain with a lysine content higher than that

a

hmited amount of

making

flour

is

bread, but nutrition-

The wheat-rye

hybrid,

tritieale,

of the parent wheat. Information

is

on the performance of tritieale flour in bread making (24, 38) and also on the feaof using oil seed flours to supplement the proteins of wheat flour. Early work em-

available sibility

phasized the use ol soybean flour because ot

on other

Later, attention focused

safflower, sesame,

and sunflower

amount of wheat

flour replaced

the manipulation

is

limited,

altered (37). Bread

which breads made with nonwheat

Wheat

Most

(28, 32). is

high lysine as well as high protein content.

its

flours including cottonseed, cowpea, field pea, peanut,

if

farina in that for flour the grain

is

is

made with wheat

baked products

if

the

modified, and in some cases flour

if

the standard against

is

flours are measured.

from uncooked wheat

flours differ

will yield acceptable

the lormula

more

such

cereal products

as

cracked wheat and

extensively subdivided or milled.

of the grain are fractured with their contents exposed. Too, wheat flours properties due to the cultivar or blend of cultivars used

when

Many more

cells

differ in functional

they are milled. Success in

baking depends in part upon using the type ot flour best for the product, so some consideration of

how and why

flours differ

is

pertinent.

TYPES OF FLOUR Kind of

Wheat

Flours are classed according to the type of wheat (26) from which they are milled. There are three (

common

species of

Tritiaim aestivum) and club

third,

durum

wheat,

is

(

used to

classed according to color

wheat grown

in the

United

States.

Two, the

Triticum compactitm) wheats, are used to

make macaroni

products.

Wheats grown

of the surface of the kernel (white or

make

common flour.

The

lor flour can be

red), season

when

planted

Red wheat varieties, some soft and others hard, predominate. Soft red wheat is planted in the fall and so is referred to as winter wheat. Hard red wheat is planted in either spring or fall, depending on the growing conditions in the area. The endosperm of hard wheat shows greater resistance to cracking during the milling process. The difference between soft and hard wheat had been attrib(winter or spring), and whether they are hard or soft.

uted solely to the higher ratio of protein to starch in the

174

latter.

Recent evidence indicates,

1

however, that the hardness of hard wheat comes from greater continuity ot the protein matrix

within the

cells

and the

tighter

bonding of starch granules

These differences are shown in the scanning electron micrographs

of flour milled from the two types of wheat. tinuit}'

The protein

and the structure appears more open.

Many

(Fig. 11-1)

of particles

matrix in the soft wheat lacks con-

starch granules are exposed

dislodged. Starch granules in hard wheat flour appear firmly

embedded

in a

and some

continuous

endosperm cells is more likely to crack starch granules. and powdery; hard wheat flour feels gritty.

protein matrix so that fracture of Soft

wheat flour

teels soft

Figure 11-1. Scanning micrograph of

(o) soft

electron

winter

wheat flour and (b) hard red winter wheat flour. Magnified ' 800. (From

175

to this matrix (20, 35).

R.

C.

Hoseney and

P.

A.

Seib. Bakers Digest A7{6]:78.

Reprinted with permission from the

December 1973 Digest.

Chicago,

issue of Bakers IL.)

CHAPTER FLOUR AND 1

DOUGH FOR^AATION

1

76

Effects of Milling

way The chart in Figure 1 -2 shows in sequence the main steps in the conflour. Whole wheat flours are made from the en tire kernel. White flo urs

Flours differ not only in the kind of wheat from which they are made, but also in the PARTI

STARCHES AND STARCHY FOODS

they are milled (40). version of wheat to

1

the en dosperm. White flour accounts lor 97 percent ot the total flour consu"me3~WhCT! rlle^ndosperm of wheat is recAced by milling to pieces of a size to qualify as flour, usually a maximum of 72 percent of the grain is utilized. The other 28 percent constitutes shorts that includes bran and germ, much of which is used as food for animals.

come from

A 72

percent-extr action flour

is

known

as straight flour.

However, millers do not put the

and more crush-resistant pieces of endosperm mto high-quality flour. Those flours that are made of less than the entire endosperm are known as patent flours. The remainder of the endosperm yields lower-grade clear flours. The part of the endosperm from which clear last

come is also used to make breakfast cereals. Long patent flour s contain a high proportion of the endosperm.

flours

contain relatively

diagram

in Figure

fractions of flour.

less,

with a higher proportion of the endosperm

Short patent flours

left as clear flour.

The

11-3 shows the percentage of the wheat kernel found in the different

That portion of the endosperm

that

most

resists

cracking

is

higher in

Thus longer patent flours have a higher percentage of protein made from the same type of wheat.

protein and lower in starch.

than shorter patent flours

Air classificadon can be used to separate a flour into fractions with different ratios of protein to starch.

A controlled flow of air

and weight. Because heavier pieces the

same wheat

is

used to separate fce particles of flour according to

are higher in protein, this technique provides flours

differing widely in protein content (39). Before this innovation, the protein

content of the flour had to be controlled by the kind of wheat used and to a the proportion of endosperm crushed to

make

the flour.

By means of air

content averages 12 percent.

It

can vary with the

lesser extent

by

classification, flours

20 percent can be obtained from the same

that vary in protein content over a range of 5 to

wheat. White flour averages 65 to 70 percent starch and 8 to

is

size

from

relative

1

3 percent protein. Moisture

humidity of the

air to

which the

exposed. Flour contains approximately two percent pentosans and one to two percent

flour

lipids.

Particle Size

Pieces of endosperm in patent flour a sieve

must be small enough

with a mesh of 210 micrometers.

identity for white

wheat

flour as

the range in size of particles

is

(Fig.

1

1-4.)

This

for is

98 percent

to pass through

specified in the standard of

promulgated by the Food and Drug Administration. But

great.

The heterogeneous

character of conventional flour

is

shown in Figure 1 l-5a. Such flour packs and does not pour readily. In addition, its wettability is poor. Individual particles are too small to overcome the surface tension of water.

The mass

tends to float

between the

when combined with

finer particles.

Uneven

water,

buoyed up by

air

trapped in the spaces

access of individual particles of flour to water results

lumps. To eliminate these disadvantages of conventionally milled flour, instantized flour (also called instant-blending or quick-mixing flour) was developed (29). Instantized in

flour

is

made from conventional

all-purpose flour by bringing the particles into contact

with moisture so that they adhere to each other. Particles

of instantized flour must pass through

Only 20 percent of

The clumps of

a sieve

the flour particles can be small

flour are then dried.

with a mesh of 840 micrometers.

enough

to pass a

mesh of 74 micro-

meters. Particles of instantized flours are not only larger than ordinary flour, but they are

— .!2

iS

^

(1)

•i

g £ = o

3 o "

o 9 £ i CD

^

tfi

=

-o

178 PART

100

72%

III

of

100%

Wheat

STARCHES AND STARCHY FOODS

POUNDS OF WHEAT

28%

Straight, all steams

40o/,

j 14%

Wheat

of

Bran

Feed

14% Shorts

5

Fancy clear

a n Extra short or

§

fancy patent flour

°-

60%

D

Short or f.rst

patent flour

^^^^^

^so/^

Short patent flour

80% Medium patent

flour

90%

95%

Long patent flour

100%

Straight flour

Figure

1 1

-3. Milling of

wfieat. (From

Publishing

more uniform pack, pours

flour; yielcjs of milled fractions

C O. Swanson. Wheat Flour

Shorts

from 100 pounds of

Quality. Copyright

©

1

938, Burgess

Company, Minneapolis, MN.)

in size, as the illustrations in Figure

easily,

16% Bran 12%

and blends

1

l-5b show. Instantized flour does not

readily with cold Hquid. Flour in this

form

is

useful for thick-

ening gravies and other liquids. Instantized flour absorbs moisture more slowly than conventional flour and can tolerate

more mixing without making the product tough

(27).

Classed by Use

A

third classification of flours

is

according to use, as bread, all-purpose, pastry, cookie,

and cake flour. Milled in the conventional way, bread flour is a long extraction of hard wheats. Cake flour, at the other end of the scale, is a short patent of soft wheats. Although the particles for all conventionally milled flours must fall within a certain size range, some flours are finer than others. Bread flour is coarse and gritty compared with cake flour, which is fine and powdery, with a greater tendency to pack. To obtain particles as small as those of cake flour, extensive crushing of the endosperm is required so the pieces of flour pass a very fine sieve. All-purpose flour

an intermediate type, not

as coarse as

bread flour or

is,

as fine as

as the latter

cake

purpose flour there are an estimated one hundred billion (10 Pastry flour

than a

it

is

not quite as short a patent as cake flour, but

it

)

term implies,

flour. In a

cup of all-

pieces of endosperm.

resembles cake flour more

does all-purpose flour in composition and in baking properties. Cookie flour

is

long patent of a soft wheat.

Bread flour

is

eminently suited to making yeast breads. All-purpose flour makes and even yeast bread of high qual-

biscuits, muffins, waffles, gingerbread, coffee cake, ity.

Recipes for muffins, biscuits, and pastry especially adapted for soft wheat flour have

1

179 CHAPTER FLOUR AND 1

DOUGH FORMATION

Figure

1 1

-4. Photomicrograph of

1

mesh

6

160. (Photograph by

Original magnification

R.

silk I.

bolting cloth with particles of flour adhering.

Derby, General Mills,

Inc.

From Cereal Science Today.

October 1957. Reprinted by permission.)

been developed. Large quantities of

soft

wheat flour

are used

by the packaged mix

in-

dustry.

As

a consequence of differences in composition, flours vary in density.

flour weighs

more than

of cake flour. (See

a

cup of all purpose

flour,

which

in turn

A cup of bread

weighs more than a cup

Chapter 3 or the A.H.E.A. Handbook ofFood Preparation

for the weights

per cup of different flours.) Actually, a cupful of two brands of all-purpose flour

may

not

weigh exactly the same. The weight of a cup of all-purpose flour of the same brand may differ slightly

from year

to year

from variations

in

supply of wheat available to the

For substituting one flour for another, weight rather than measure

Durum wheat t)'pe

of wheat

is

is

harder than the hardest of hard wheats.

milled, the product

make all high-quality made from hard wheat bread used to

from other pasta

in that egg

is

is

called semolina.

When

A dough

is

miller.

a better basis.

the

endosperm of this and water is

of semolina

pasta products (macaroni, spaghetti, noodles). Pasta can be flour,

but the quality of the product

added

is

poor. Noodles differ

to the mix.

FORMATION OF DOUGH When particles of flour are wetted and then manipulated, a viscoelastic dough forms, for which the protein complex, gluten, is essential. Wheat flour is unique in its ability to form such a dough (19). Evidence has been presented that wheat gluten is an amorphous polymer that exists in the glassy state at room temperature when the level of moisture is as low as that which prevails in flour (21). The high proportion of liquid to flour used to make doug h is far in excess of that required to lower the glass transition temperature so that gluten becomes rubberlike. In the rubbery state, gluten can be manipulated to form dough. Much research effort has been expended to understand this dough-forming mechanism, as numerous papers on the subject attest (4-6, 10-12, 19, 22, 25, 33).

180 PART

STARCHES

III

AND

STARCHY FOODS

Figure

1

1-5. Photomicrographs

of (top) conventional flour,

unevenness of size of

showing

particles,

and

(bottom) instantized (agglomerated) flour, with particles of uniform size.

(Photographs by

General

R.

I.

Derby,

Mills, Inc.)

Components

of Flour

Proteins

The assortment of amino chemistry of proteins p-l

^^re f^han

40 percent

carboxyl group

free,

is

acids that

makes up the proteins of gkiten is unique (9, 23). (The 1 8.) A single amino acid^^lutami Tynak es up

discussed in Chapter

of"

the total.

Most of this amino

but as an amide, and

as

such

it is

acid

is

present, not with itssecoTR'

available for

hydrogen bonding with

the oxygens of hydroxyl, carboxyl, and carbonyl groups of proteins and other molecules.

Next

in quantity

the total

amino

a polypeptide

is

the

amino

acid mxilTne^which

acids in gluten.

The

makes up approximatel y 14 percent of

presence of proline puts constraints on the shape that

can assume. Other amino acids that

aflect the

dough-forming potential

of

which contributes flexibility, and leucine, which is important for hydrophobic interactions. Approximately two percent of the amino acids contain sulfur flour proteins are glycine,

1

group in cysteine. The low percentage of baand the even lower content of acidic ones result in a low net charge on most

either as a disulfide in cystine or a suiftiydry!

amino

sic

acids

CHAPTER

ot the molecules in the gluten complex.

Wheat

flour contains small

amounts of two

classes

soluble albumins, and dilute salt-soluble globulins (10). to

90%),

up

of metabolic proteins, water-

The main

portion of protein (85

the part essential for the formation of dough, consists of reserve protein stored

in protein bodies in the

endosperm of the wheat grain

the gluten complex, which can be

separa^

(33).

This reserve protein makes

into two fractions, rliadin .nnd gluten jji. j

Molecules of the gliadin fraction are smgle poK'peptides. Gliadins are soluble ethanol so they are classed as prolamins.

The

in

aqueous

alcohol-insoluble residue contains the high

molecular weight glutenins, which vary in tln^number of polypeptide subunits. Both the

and the glutenin

gliadin

fractions can be furtlVer subdivided (10), the gliadins into four

(LMW) component and a high odd number of sulfhydryl groups, so reactive sulfhydryl groups are available for both intra- and intermolecular disulfide formation and for sulfhydryl^ disulfide interchange believed essential for the doughforming mechanism (25). Differences in the dough-forming and bread-making potensubgroups and the glutenins into a low molecular weight

(HMW)

molecular weight

tial

one. Glutenins have an

of different flours resides in the

HMW glutenin.

A new system of nomenclature for the components of gluten has been introduced (34, 36),

though not without controversy

(11).

When

the complex molecules of glutenin are

depolymerized, they become soluble in aqueous ethanol,

partially

the proposal to call

sulfur gliadins are prolamins; the other three gliadins

containing prolamins; and the

The

as are the gliadins,

hence

the proteins of gluten prolamins. According to this system, low-

all

and the

LMW glutenins are sulfur-

HMW glutenins are high molecular weight prolamins.

gliadins appear to have a compact, globular shape, attributed to the presence of

numerous beta tutns in the polypeptide. How polypeptide subunits of glutenins, especially the component, are associated is still unsettled. Both linear and branched models

HMW

have been proposed, with either secondary or covalent (disulfide) linkages between subunits.

6

One model

(22).

Glutenin

that

II

shows how the subgroups might be arranged

of the scheme

(HMW glutenin)

is

made

is

shown

in Figure II-

of polypeptide subunits, each

held in compact shape by intramolecular disulfide bonds. These subunits are linked, in turn, in

more or

less linear

fashion by interpolypeptide disulfide bonds (13).

ulat weight glutenins (glutenin

to glutenin

II

I

Low

molec-

of the scheme) are believed to be united to each other

polypeptides by secondary bonds (hydrogen and

Van der Waals

and

linkages). In

this scheme, glutenin II would contribute elasticit}', and the mobile linkages of glutenin I would contribute the viscous element to gluten. A branched model has been proposed, based on fragments of polypeptides obtained when glutenin subunits are partially depolymerized (16). A linear model is shown in Figure 11-7 (33). The central portion of the polypeptide subunit is shown as a beta spiral (made of repeated hairpin or beta turns). Alpha helices are shown near either end of the polypeptide subunit and the polypeptides are linked head to tail by disulfide bonds between terminal cysteine residues. The fibrous character of glutenin is shown in the scanning electron micrograph of Figure 1 1-8.

Lipids

Lipids account for only a fraction of the weight of flour, but they are essential for the for-

mation of dough and breadmaking

181

(6). (Lipids are

discussed in Chapter 15.)

Most of the

1

AND DOUGH FLOUR

FORMATION

182 PART

III

STARCHES AND STARCHY FOODS

—-^-o-o-vders

Baking powders made with different acids are used by bakers and manufacturers of packaged mixes (3, 16), but only one type of baking powder is currently on the retail market.

However, cream of tartar, the potassium acid

salt

of tartaric acid:

COOH

H

—

C-

COOK

-OH

C — OH

-OH

OH COOH

COOH It is

available

and can be combined with sodium bicarbonate

A mix of X teaspoon of sodium bicarbonate and

powder.

be used in place of

The

reaction

1

/
(b)

(a) Figure 16-6. (a)

Ten

milliliters

Increase of

square centimeter micron

in

oil in

(b)

in interfacial

area when an emulsion

is

formed-

contact with water with an interfacial area of one

Ten

milliliters

diameter with an

of

interfacial

oil

emulsified in water as droplets 0.1

area 300 meters square. (Reproduced

from Paul Becker Principles of Emulsion Technology, by permission of Reinhold

Book Corporation, a subsidiary of Chapman-Reinhold,

Inc.,

New York,

1

955.)

fat

droplets

277 CHAPTER 16

EMULSIONS

Figure 16-7.

Scanning electron micrograph of

layers of electron-dense particles fat

around droplets of

from mayonnaise. (From C. M. Chang,

W.

D.

and O. Fennema, Canadian Inslitvte of Food Science and Technology Journal 5 A 26, 1972. Powrie,

Reprinted by permission.)

Electron micrographs of mayonnaise droplets of fat surrounded

by

layers

made with only egg yolk

of electron-dense particles

as

came from the low density lipoprotein of the yolk, but have contributed some of the electron-dense particles observed (1). gests that they

show

the emulsifier

(Fig. 16-7).

Their

size sug-

lipovitellins

may

Cooked Dressing Cooked

salad dressings are acidified liquids (water, milk, or fruit juice) thickened with

starch alone or with egg.

Such products involve basic principles of starch cookery

(Chapter 9) or of egg cookery (Chapter 21). The amount of fat included in cooked dressings is so small as to be readily emulsified by the proteins present. When cooked salad dressing

is

added

to

mayonnaise

percent, the product

in

amounts

must be labeled

to reduce the vegetable oil content

below 65

salad dressing.

Other Emulsions Mayonnaise-type salad dressings

in

which the concentrated emulsion of fat has been dion the market for a number of years. The

luted by cooked starch dispersion have been

health benefits of limiting the

ment of low

fat

amount of fat

in the diet spurred interest in the develop-

or fat-free salad dressings. Ingredients are selected for these modified dress-

ings for their ability to

mimic

the flow properties

and the mouthfeel of the innumerable gum and

droplets of fat found in conventional products. Hydrocolloids such as xanthan alginates

fill

this role, as

do maltodextrins and microparticulates.

Emulsions play an important of a good emulsion

in

sions are discussed in Chapter 25,

creams in

Chapter

than salad dressings.

role in foods other

cream puff batter

is

emphasized

cream and milk

in

as

Chapter

13.

The importance

Cake

batters as

emul-

emulsions in Chapter 19, and

ice

7.

Margarine and butter are examples of water-in-oil emulsions. Each contains a minimum of 80 percent fat with much of the remainder as water. When either is used for

minimal compared to that which occurs when other fats are heated with any water present. In butter and margarine, conversion of the emulsified droplets of sauteing, spattering

is

278

water to steam causes the bles releases the

foam. Rupture ot the film of emulsifier around the bub-

tats to

steam in small

jets.

PARTY FATS

AND

fat-r:ch

FOODS

STABILITY OF EMULSIONS Emulsions are unstable colloidal dispersions. rich layer

Instability

when

inversion, or coalescence. CteacQing occurs

above the more aqueous phase below.

less It is

is

dense

manifested as creaming, phase oil

droplets rise

to prevent

and form

creaming that milk

a fatis

ho-

mogenized (Chapter 19). Creaming accounts for the thicker upper layer in a carton of whipping cream that has been held in the refrigerator for a few days. The emulsion is not broken and the two layers can be redistributed by stirring the cream. The conversion of cream to butter is an example of phase inversion. When cream an oil-in-water emulsion is churned the emulsion breaks. At the same time, a water-in-oil emulsion is formed

—

—

with the freed butter

fat.

Coalescence involves rupture of the protective film around the

dispersed phase of an emulsion. Oil then unites with

A number of factors may cause an salt increases

ous jarring,

the surface tension of water, decreasing

as

when

a product

shipped,

is

oil,

water with water.

emulsion to break. Adding

may

its

may do

salt

so because

spreadability. Violent or continu-

break an emulsion. Allowing the surface to

dry removes one part of the protective layer around dispersed droplets. Fi££ziiigjnay cause

an emulsion to break, too. Conversion of water to of the emulsion and sharp edges of Freezing

may cause

the

ice crystals

crystals

removes the continuous phase

may puncture

fat to crystallize, too. Stability to

freezing

is

the layer of emulsifier.

an issue for salad dress-

ings used in prepared frozen foods. Salad dressings withstand freezing better if the oil used does not crystallize at

ther egg yolk or salt are used,

agent

and

waxy

if

low temperature,

and frozen storage

if relatively

rather than regular starch

is

high

used

as a

levels

of ei-

thickening

(9).

A broken

may be reformed by adding it slowly to liquid (one tablespoon of made with one cup of oil), or beating the mixture after each addiThe broken emulsion also may be gradually stirred into a stable emulsion. When a emulsion

water for mayonnaise tion.

good emulsion is emulsifying a broken one, the spoon or spatula used to combine the two. Destabilization of an emulsion destabilization of the emulsion

toppings, and ice cream. tants displace just

is

When

is

at

times desirable. In

necessary to stiffen the

foam

on the

and controlled whipped cream, whipped

fact, partial

in

these products are agitated, low molecular weight surfac-

enough of the protein

from adjacent droplets

force involved can be felt as a drag

that forms the emulsion originally to allow fat

to adhere.

VINEGAR Vinegar

is

a

common

ingredient in salad dressings. Cider, wine, malt, and distilled vine-

gar are four types.

The

wine)

which

is

acetic acid,

characteristic constituent of vinegar (literally, is

stages. First, a sugar solution is

produced by fermentation is

(12).

Vinegar

converted to alcohol by yeast and then

t'hi aigre,

is

or sour

fermented

two brew

in

this alcoholic

converted to an acetic acid solution. Microorganisms responsible for the second stage in

the conversion of sugars to acetic acid are those of the Acetobacter

made from

^mn^. Cider vinegar

is

apple juice, wine vinegar from grape juice, malt vinegar from malted grain, the

.

starch in the last converted to sugar by malt, or sprouted barley. Distilled vinegar

by fermentation of a dilute solution of alcohol. Other acids besides

is

made

form during

acetic

mentation, and these react with the alcohol to yield esters that contribute aroma to the

from each source has a

vinegar. Vinegar

on

characteristic flavor. Vinegars

the market are

standardized at five percent (50 grains) or four percent (40 grains) acetic acid. is

approximately

five

percent

and so

citric acid

is

comparable to vinegar

Lemon juice The pH

in acidity.

of distilled vinegar tends to be lower than that of other forms (10).

REFERENCES 1.

2.

3.

and

Science and Technology Joumal53:{3yA34-37

"Salad dressings stable to frozen storage."

Hansen, H. and

R.

L.

r)'pe.

1961.

Fletcher.

Food Technology 1 5:256—62. Factots

Corran,

W. 1934.

J.

oil.

"Emulsification by

10.

T

Kintner,

C, and M.

that in-

Mangel.

1952.

mustard." Spice Mi/I 57:175-77. Effective-

"Variation in hydrogen ion concentration

ness of mustard demonstrated.

and

Das, K. P, and

17:456-59. Cider, wine, malt, peach, pear,

E. Kinsella. 1990. "Stability

J.

total acidity in vinegar."

and

ot food emulsions: Physicochemical role ol

apricot,

protein and nonprotein emulsifiers." Advances

Krog, N.

Food

and

Science

Dickinson,

G.

Nutrition 34:81-201.

their

stability." In

and Foams.

Stainsby, eds.

Stainsby.

Dziezak,

12.

Pp. 1-44. Factors in-

York:

Mayer, Ernst. 1963. "Historic and modern

Food Technology

aspects of vinegar making."

J.

D. 1988. "Emulsifiers: The

inter-

Food

Tech-

stability."

174-80,

42(10): 172,

production of vinegar. 13.

182-83,

M.

Phillips,

C. 1981. "Protein conforma-

tion at liquid interfaces

and

role in stabi-

its

185—86. Surfactants, structure, and formulas:

lizing

applicauons.

Technology 3%\):'iQ-'y\, 54-57.

Fishback,

R.,

and

J.

L.

Kokini.

and mustard

flour

Food Mechan-

foams."

14.

Snell,

H. M., A. G. Olsen, and R.

Kremers. emulsifying Industrial

1935-

"Lecitho-protein.

ingredient

and

egg

in

Engineering

R.,

and N.

S.

on the

Parker.

1988.

E.

The yolk."

Chemistry

27:1222-23. Identification of the

a stabilizer. L.

and

emulsions

ism of stabilization.

1987.

on rheological properties of model O/W emulsions." Journal of Food Science 52: 1748—49. Effectiveness of mustard flour as "Effect of aging

Fisher,

E.

S.

Marcel Dekker.

17:582-84. Fermentation process in the

key to emulsion

nology

and

Larsson

K.

New

surfactants: Behavior at the interface.

volved.

facial

eds.

Pp. 127-80. Chemistry and structure of

Dickinson and Elsevier Science

compared.

1990. Food emulsifiers and

chemical and physical properties. In

Fribeg,

Advances in Food E.

New York:

Company.

1988.

Food Research

distilled vinegars

J.

Food Emubions.

and G.

E.,

Publishing

effective

emulsifier in egg yolk.

interaction be-

Ury, R. 1962. Unpublished data. Class in

tween emulsion droplets." In Advances in

Experimental Food Studies, Oregon State

Food EmuLions and Foams. E. Dickinson and G. Stainsby, eds. New York: Elsevier Science Publishing Company. Pp. 45-90.

Vincent, R.,

Types of surfactants; stabilizing action.

dispetsions of yolk kicimns." Journal ofFood

"Effect of surfactants

8.

York: Marcel

and emulsion

crease stability of emulsions to freezing.

Emulsions

7.

HLB

21.

rounding droplets of emulsified

"Emulsion

6.

P.

Electron micrographs of protective layer sur-

in

5.

E. Friberg, eds.

S.

Dekker.

Technical treatment. 4.

New

W. D. Powrie and C. M., O. Fennema. 1972. "Electron microscop\- of mayonnaise." Catiadiiin Institute of Food Chang,

Friberg, S. E., R. E.

Kayali. 1990. In

Coubran, and

I.

H.

Food Emulsions. K. Larsson

University.

W. D.

1966. "Surface

Powrie, and

activir)'

O. Fennema.

of yolk, plasma and

Science 3\:GA3-Ai. Surface activit)' sifying capability

279

fer-

and emul-

of various egg yolk fracuons.

CHAPTER 16 EMULSIONS

Pastry

less frequently made puff pastry. make such popular desserts as double-crust fruit pies, singleThe single crust pie may be baked, the cooked filling added,

Pastry includes conventional pie crust and the Conventional crust

is

used to

and fruit tarts. and then topped with meringue or whipped cream, as pumpkin pie the raw fdling is cooked in the crust as the crust soft pies,

is

used also in such entrees

shells, tarts,

and

as

as

for

cream

pie.

latter bakes.

meat pie and Quiche Lorraine. Puff pastry

top crust for meat

For custard and

Conventional crust is

used for patty

pies.

is one of the simplest of the batters and doughs in terms of the number of M«kmg4iighcquahtypastry should be a simple matter, because only four ingredients—'vlloui\fat, salt, aiidwateA-are used. That it is not simple is attested by the wide

Pastry crust

ingredients.

variation in the qualityliTpastry

An a fork

and the lack of confidence of many cooks.

important characteristic of good pastry

and should disintegrate

readily

due

when

is

tenderness. Pastry should cut easily with

bitten into, but

it

should not crumble. High-

been raised in blisters by teamlo rmecfa^thexrustbakes If the layers are thick and the blistersfew, the pastry will be tough. It the layers are thin and the blisters numerous, the pastry will be tender as well as flaky. Pastry should be crisp, not doughy or soggy. The edge of the pastry crust should be a golden brown and the center a paler brown (6). The degree to which each characteristic is actually obtained in a pastry depends upon the ingredients, their proportions, and the way they are manipulated. Crusts made from crumbs and softened table fat are simpler to make and are essentially fail-proof, which in qiulity pastry

is

flaky^ Flakes are

s

to layers of gluten that have

.

part accounts for their popularity.

INGREDIENTS AND THEIR FUNCTIONS Flour Flour

is

the

main ingredient

characteristics yields

more

sult

either

is

of the pastry

in pie crust. (4).

Two

kinds of flour

may

All-purpose flour, because of

be used: Each affects the

its

uted. In contrast to all-purpose flour, pastry flour does not yield as

to

make

280

higher protein content,

and the more gluten developed the more cohesive the dough. The rea tough or a flaky pastry, depending on how extensively the gluten is distrib-

gluten,

a tender

but crumbly pastry

(4).

much

gluten and tends

7

Salt Salt

281

is

used to season the Hour. Omitting the

salt

makes no difference

in pastry except in taste.

CHAPTER PASTRY

Fat Fat contributes tenderness (shortness) to pastry. Part of the toughness (lack ot shortness)

comes from the cooked starch paste. A cup of flour is about % starch. If /^ cup of starch were combined with two tablespoons of water as used in pastry and the mixture then rolled and baked, the product would be hard even without the protein of the flour. If a ball of dough made from flour and water were rolled and baked, it would be in pastry

—

—

tough and hard. The protein

in the flour yields gluten

with water and manipulated.

When

dough

the

wherever the flour

dampened

is

rolled and baked, the gluten

is

is

dena-

tured by heat and this contributes additional toughness. Fats tenderizepastry

by waterproofing the

particles of flour (15).

the starch in flour have an afFmit)' for water, that

the carbonyl

fat,

I

/^

*-^

groups are polar

I

These particular groups

fatty acids.

at strategic spots

in a

double bonds

as are the

molecule of fat make

it)

Both the protein and

contain polar groups. In a molecule of

it

in unsaturated

possible for the fat to unite

with polar groups on the surface of particles of

the molecule of fat (the major part of as a

is,

flour.

The remainder of

with no affinity for the flour (or the water) acts

mechanical barrier to prevent contact of the water molecules with the protein of the

flour. It is in this

Pure

fats

way

that fat waterproofs flour

and so

have more shortening power than do those

tain moisture.

Even with pure

fats

the characteristics of pastry depend

such as

upon

lard,

limits the

and margarine, con-

hydrogenated shortenings, and edible

the particular fat used

covering or spreading power than do plastic

development of gluten.

that, like butter

fats so a

(6, 8).

Liquid

fats

oils,

have more

high proportion of flour particles be-

come coated with fat. Softer fats spread more readily and contact more flour particles than do firmer ones. The higher the ratio of liquid to crystals, the greater is xhc covering power of fat (8).

This

is

influenced by the temperature of the

In addition to rating the

As

dough

a result,

making pastry

into layers. Oils,

Lard

is

on the other hand, tend

water contacts the flour with

but crumbly or even greasy pastry considered a superior

fat as well as

by

its

fatry acid

makeup.

tender, fats also contribute desirable flakiness

difficulty, little

results. Plastic fats

fat for

making pastry

by sepa-

to coat each particle of flour.

gluten

tend to

is

developed, and a tender

make

a

more

flaky product.

(12).

Liquid

The

liquid used in pastry

method Liquid

contains milk.

is

tributed throughout pastry (a)

or

(c) crisp, flaky,

and

to be too

tender.

To

brown,

(b)

by the to

stir-and-roll

form

a

dough.

compact, tough, and browning with

dough

is

to provide

dif-

produce the third rype, gluten should be distributed

throughout the mass of the dough in hundreds of small in pastry

oil

would not adhere

amount of cohesion to the dough. How extensively gluten is disand, of course, the amount of gluten, determine whether pastry

crumbly with a tendency

ficulty,

liquid, the flour panicles

needed to hydrate the flour so that during the mixing the gluten can be developed

sufficiently to give a certain

is

made with

usually water, but pastry

is

Without

steam to leaven

it

and so

areas.

A third purpose for the liquid

actually produce the flakes.

1

282 PARTV FATS

AND

PROPORTIONS OF INGREDIENTS Salt y^j fp^ biscuits,

FAT-RICH

FOODS

is

muffins, and most other quick breads,

Y.

teaspoon of salt per cup ot flour

J recommended.

11

1

usually

Fat

The proportion of fat customarily recommended varies from % to M cup for each cup ot flour. In one study (13) pastry made with plastic fat was nearest optimum in quality when the volume of fat was % the volume of flour. When liquid fat was used, the opti-

mum volume was less (3J^ tablespoons per cup of flour). A later study verified the greater effectiveness of oil versus shortening in

K cup of plastic

fat is

used,

it is

same time developing

at the

than % cup of fat

is

so

producing satisfactory pastry

difficult to stir in the

much

make

gluten as to

When

(2).

less

than

water and make a dough without

When more

the pastry tough.

used for each cup of flour, the pastry tends to be crumbly and greasy.

Within the recommended range, the smaller the proportion of fat, the greater is the likelihood of the development of too much gluten as the dough is worked after the addition of water. If cut into the flour adequately, % cup of fat per cup of flour will give tender, flaky pastry.

Liquid

Only

a

minimum

of water should be used. That small amount

to be flaky. If pastry has too

little

water,

it

is

necessary

the pastry

if

is

tends to be crumbly and browns very quickly,

optimum amount of water. Two tablespoons of water per cup of flour are sufficient to make the dough adhere and contribute flakiness with minimum risk of the pastry being tough. With the small amount of liquid used in pastry, errors in measuring in percentage can be large and so make a great difference in the as

does understirred pastry

quality of the pastry.

can

make

made with

A difference of

!^

the

teaspoon (three

of

a noticeable difference in the tenderness

milliliters)

the pastry.

of water per cup of flour

When more

blespoons of water per cup of flour are used, the chances of developing too in the fat to

than two

much

ta-

gluten

dough and so making the pastry tough are great (7). The smaller the proportion of flour, the more important it is to avoid excess water; otherwise, the pastry is likely to

be tough.

MANIPULATION OF INGREDIENTS One

cause for lack of confidence in making pastry

is

that considerable skill

the manipulation of ingredients. Even though ingredients are

method and flaky, or

on

required for

extent of manipulation determine whether pastry will be tender or tough,

mealy

skill in

is

in correct proportion, the

(7, 14, 17).

making

The

proportions suggested above put the

pastry. Until one's

technique

is

minimum demands

perfected, however,

it is

difficult to

achieve the particular combination of tenderness and flakiness desired. In a certain respect tenderness

which

is

probably the reason

it is

and

flakiness in pastry are

difficult to achieve

ens pastry, yet to obtain flaky pastry

some gluten

opposing

both in the same

is

necessary.

The

pastry. secret,

characteristics,

Gluten toughof course,

is

in

7

its

Even though two samples of pastry contain the same amount of gluten,

distribution.

thev

may differ

and

exist as a

in tenderness,

few thick

owing

283

to differences in the distribution, ff the flakes are large

layers, the pastry will

be tough;

if

the layers are

numerous and

CHAPTER PASTRY

tissue-paper thin, the pastry will be crisp yet tender.

Cutting Fat into the Flour

The

step in

first

A

evenly.

making pastry

plastic tat

may

is

to

sift

the salt with the flour to distribute the salt

be distributed throughout the flour by cutting

it

in

with a pas-

two knives, or two spatulas manipulated with a cutting motion like the blades of scissors. The technique is the same as that employed for biscuits (Chapter 13). The purpose of this cutting is to subdivide the fat and to increase its surtry blender, a pastry fork,

face area so that

coarse cornmeal are

embedded

touched by ter

more of the flour particles make contact with it. The mixture resembles when the fat is cut into the flour sufficiently. Some of the pieces of flour

in the fat,

fat.

Where

cannot reach the

proofed and the a

tough

show

its

pastry.

but between small chunks of

fat are layers

pieces of flour are in contact with fat or

flour.

less likely

The is

finer the fat

is

cut, the

more

of flour particles un-

embedded

in

it,

the wa-

flour particles are water-

an excessive development of gluten, which would result

Gluten in the pastry

in the

photomicrograph

in Figure 17-1

is

in

stained to

distribution.

Stirring the

Dough

once and in such a way as to distribute the The mixture should be stirred at once and with a wide circular motion. Otherwise, part of the dough will absorb more than its share of water. Some very wet, sticky spots result, whereas some flour gets no water. When this hap-

Water

is

added

to the flour-fat mixture

water over the mixture

pens, spots that are too

as

damp

dough adheres

overworked to get the spots that are too dry worked into makes tough pastry. Stirring should be discontinued when

are

the dough; the excessive stirring

the

all at

evenly as possible.

in large lumps.

INTERRELATIONS OF INGREDIENTS AND MANIPULATION To

achieve the desired characteristics in pastry, the kind of ingredients, their proportions,

and the extent sitate

to

varying the

which they

are

manipulated are interrelated. Different conditions neces-

amount of manipulation

EFFECTS OF KIND OF FAT

When

before and after the water

is

added.

AND TYPE OF FLOUR

oil is used instead of plastic fat, the flour is more readily waterproofed. In this case dough needs to be manipulated more after the liquid is added to develop enough gluten to make the pastry flaky and not too tender. The warmer a plastic fat, the more flour it can contact and the more tender is the pastry. Pastry flour, because it is low in gluten, tends to make a pastry that is so tender it is mealy (4). Dough made from pastry flour not only can tolerate more mixing but needs more after the water is added to develop some co-

the

hesiveness in the dough.

1

,

284

iLO'

^.^

PARTV FATS

AND

FAT-RICH

FOODS

Figure 1 7-1 . Photomicrographs of raw pastry dough stained to show gluten: (a)

standard dough, gluten

delicate

in

strands with indistinct edges;

dough

(fa)

with excess manipulation, gluten strands;

(c)

dough

amounts of gluten but strands 200. (From

Magnification

and

J.

in distinct

with excess water, large

Simpson, Journal of

indistinct.

Hirahara

S.

Home

Economics 53:684, Copyright © 961 American Home Economics Association, 1

Washington, D.C.)

(e)

Effects of Proportions of Fat

and Water

When a high

used, the fat should be cut into the flour

proportion of fat

is

should be manipulated more after the water too tender. flour

ring

When

a

is

more to limit the development of gluten. the dough should be kept to a minimum

and tough

crust.

added

low proportion of fat to flour

When

the water

but requires more to prevent

its

is

is

When

less

or the

so the pastry will be flaky

used, the fat should be cut into the a slight excess of water

to prevent excessive

is

added,

being too tender and crumbly and lacking

Limiting the amount of water and the manipulation of the dough flour, too.

stir-

development of gluten

skimped, the dough not only tolerates more

can compensate for a low ratio of fat to

dough

and not

after the

stirring,

flakiness.

water

is

added

Which Fat

Effect of the Extent to

The

extent to which the fat

mixing

water

after the

is

is

Is

Cut into the Flour

added.

much

developing so

tributed at this

dough subsequently. The

to distribute the water evenly

it is

gluten that the pastr\'

is

tough. If the

smaller.

It is

CHAPTER 17

in this flour that gluten

is

less

the

fat

is

cut

throughout the dough without

fat

more and

cut in

is

point, then the areas of flour not waterproofed by

ened by water will be

to

thicker the layers of flour between the pieces of tat, the

The

thicker are the layers of gluten developed in the into the flour, the trickier

285 dough

cut into the flour determines the tolerance of the

tat

and so

better dis-

free to

be damp-

developed. Obviously, the more

dough can be handled after the water is added without developing so much gluten that the dough is excessively cohesive and the pastry tough. In fact, the more such dough is stirred at this point, the more flaky and tender is the pastry. For example, pastry made by cutting in the fat with 100 strokes and incorporating the liqthe

cut into the flour, the

fat is

uid with 60 strokes

is

tensively, the

The making

fat

the

at least as flaky

the flour with 50 strokes

portions of both

more

and

dough has

and

as

tender as pastry

made by

and incorporating the water with 30 and if in addition, the

liquid are used,

fat

is

a high tolerance to handling after the water

discussion above applies to the standard

fat

into

minimum

pro-

cutting the

strokes. If

cut into the flour ex-

is

added.

method of combining

ingredients in

Other methods for making pastry have been developed (16). When ingrecombined by the water-paste method, for example, an attempt is made to con-

pastry.

dients are

amount and distribution of gluten in another way. In this method all of the liquid combined with enough of the fat-flour mixture to tie up most of the water. This lumpy paste in which gluten develops is combined with the remainder of the fat-flour mixture. Tender, flaky pastry can be made by this method, but no better than by the standard method (outlined above). For pastry made by the stir-and-roll method, melted fat or oil is used and milk is the liquid. The two are combined and then stirred into the flour to form trol the is

a dough. Ingredients

enough gluten

combined by

this

so the pastry will be flaky

method

require

more manipulation

to develop

and not excessively tender.

ROLLING AND SHAPING PASTRY After pastry

dough

is

stirred sufficiently in the bowl,

to a desirable thickness (slightly flour should be used (14).

on

less

it is

shaped into a

than % inch) on a clean,

the rolling surface, because

it

A pastry cloth under the dough facilitates rolling,

ball.

flat surface.

Then

it is

rolled

A minimum

of

increases the toughness of pastry

but the cloth

is

difficult to

launder

and keep sanitary. Rolling pastry between two sheets of waxed paper eliminates the need for flour. Lifting the rolling pin as it approaches the edge of the circle of dough prevents the formation of a beveled edge. In addition to flattening the ball of

dough

into a sheet

large enough to fit into a pie pan, rolling flattens out the small masses of gluten into very thin layers. The lumps of fat are also spread into layers. A cut down through a sheet of

raw dough properly stained would show thin layers of gluten with embedded starch granules alternating with and superimposed upon layers of fat. The thinner and more nurolled

and more tender is the pastry. dough is rolled into a sheet, it should be transferred, without stretching, to a pie pan, and the excess dough trimmed away. For a single-crust pie, the pastry may be baked on either the inside or the outside of the pie pan. Pricking the dough helps prevent merous the

layers, the flakier

After the

steam pockets forming between crust and pan.

PASTRY

— 286 PARTV FATS

AND

FAT-RICH

FOODS

BAKING PASTRY A hot oven oven too,

is

(218°C) [425°F]

is

also satisfactory if the

usually

recommended

baking time

for

baking

pastry.

A moderately hot

The baking time is influenced, made. A pan made of highly emissive dark or

is

lengthened.

by the material from which the pan is and uneven browning of the

dull metal or glass favors rapid

crust.

Lowering the tempera-

15°C (25°F) and increasing the baking time a few minutes will yield a more evenly browned crust. During baking, the fat melts and the gluten begins to set. Starch granules appear little altered in appearance due to the low level of moisture (9) (Fig 17-2). Part of the water in the layers of gluten in which the starch granules are embedded is converted to steam. Steam from layers of dough that are separated by layers of fat forms pockets around the fat and puffs up the layers of gluten. When these are coagulated by heat, the bubbles break, and layers of gluten with embedded starch granules, the whole coated with layers of fat, are left. Pockets of steam that form in dough as it bakes make the pastry porous and flaky. ture approximately

Some of the steam It is

pockets show as large blisters in the crust; others are quite small.

interesting to

watch pastry

able. After the pastry has

been

in the

as

it

bakes

if

an oven with a glass

oven for approximately

five

window

is

avail-

minutes, the dough be-

comes quite agitated. A pocket of steam forms in a particular area and lifts the dough until it no longer can stand the strain, at which point the inflated dough collapses and steam escapes. Thousands of miniature explosions, side by side, superimposed on each other, in unison, and in sequence, occur in the dough during the next five minutes in the oven. Gradually the number of explosions decreases and the agitation of the dough ceases as most of the water boils away. At this point the dough begins to set and the surface begins to

brown.

If all has

can be handled, cuts easily with

gone

well, the result

a fork,

is

a light, crisp, tender

and collapses with

product that

slight pressure in the

in short, a perfect pastry.

Figure 17-2. Incompletely pasted (gelatinized) starch granules isolated from pastry. Low moisture and high fat sharply restrict the pasting of starch in pie crust. (From R. C. Hoseney, W. A. Atwell, and D. R. Lineback, Cereal Foods World 22(2):57, 1977. Reprinted by permission.)

mouth

7

CHARACTERISTICS OF PASTRY

287

Tenderness versus Toughness

CHAPTER

determined by the amount and the distribution of gluten. The amount of gluten developed in the pastry is a composite of many factors: The kind of flour, the temperature of the ingredients, the kind of fat, the proportion of fat to flour and of liq-

PASTRY

Tenderness

in pastry

is

uid to flour, the extent to which the

dough

is

stirred after the

water

is

fat

is

cut into the flour, and the extent to which the

added. If pastry

tribute to mealiness include use of an oil or

the ter

using too

fat in excessively, is

added. Tough pastry

jective

on the board or

way

to assess

its

water,

is

mealy. Factors that con-

of pastry

flour,

and undermanipulating the dough

the result of too litde

fat,

too

much

cutting

after the

wa-

water, cutting fat into the

much manipulation

pastr)' cloth

used to break the pastry by an instrument pastry

it is

plastic fat, use

of the dough after the water is added. Use makes pastry rougher. Because pastry is crisp, an obtenderness (really the crispness or friability) is to measure the force

flour insufficiently, or too

of flour

is

little

too tender,

is

warm

known

as a

shortometer

(1) (Fig. 17-3).

Because

not homogeneous, the breaking strength of a number of samples must be mea-

sured to arrive at an average figure representative of the pastry.

Flakiness is due to layers of gluten with embedded starch granules separated by and puffed up by steam. Plastic fats favor the development of flakiness. The better the is distributed, the finer the flakes. Some manipulation of the dough after the water is

Flakiness in pastry fat fat

added

is

necessar)' to develop gluten. Tender, flaky pastry

so tender that

it

will

out the pastry make uted

less

is

desired, but

it

should not be

not hold together. Small, thin sheets of gluten distributed throughcrisp, flaky, yet

tender pastry (Fig. 17-4).

The same amount

distrib-

widely might give a tough crust. Puff pastry differs from conventional pastry in

the greater

numbers of layers of gluten separated by

layers

of fat (10).

Crispness Crispness in pastry results ing baking.

It is

when sufficient water is evaporated from the layers of dough durhow thick the dough is when rolled, how long the pastry is

influenced by

Figure

1

7-3.

A

shortometer used to measure the

breaking strength of crusts, cookies,

Wcxsdward.)

and

crisp,

baked products such as pie

crackers. (Photograph by John

1

288 PARTY FATS

AND

FAT- RICH

FOODS

(

.iiiui", ,^|i,..«ft.i

i.

I:

ti

jgiji^.r''

.,|||.i.Li^r7^-

,-

^;.(;,-l'-{ -J^~^\~^,,^„„.rv .-^;^-ii,7 it;.

,

(b)

(a)

(c)

Figure 1 7-5 Effects of folding on volume of puff pastry: (a) under-; (b) optimum; and (c] overfolding. (Adapted from J. T. Colburn and G. R. Ponkey, Boicers Digest 38(2):72, 964. Reprinted witfi permission from tfie April 964 issue of 1

Bakers Digest, Chicago,

1

IL.)

289

poured into the unbaked

made. Brushing the surface of the unbaked pastry crust with

beaten egg white and placing

be thickened before

it is

likely to give a

is

minimum

to bake the crust in

is

filling to the

when

(not hot!) so that

less likely to

ing lowers the setting temperature. Another

transfer the

fluid

Having the baking temperature too low

pie.

of water in the pastr)' crust to a

crust to be soggy. If the filling

is

CHAPTER PASTRY

1

290

board (approximately

five

minutes) until

it is

smooth and

PARTY FATS

AND

FAT-RICH

FOODS

elastic.

The

into a rectangle or square approximately /i-inch thick.

fat,

The dough

is

then rolled

cut into pieces,

spread

is

dough folded to cover the fat. The dough is sealed securely by pressing the edges. Then the dough, wrapped to prevent drying, is chilled in the refrigerator for at least half an hour. The chilled dough is rolled to a thickness of M inch, after which it is folded in thirds, one third above and the other below. The dough is given a quarter turn on the board and again rolled and folded. Chilling of the dough should follow to keep the fat from melting and so maintain the continuity of the layers. The rolling-folding, rolling-folding, plus chilling, routine repeated two more times forms enough thin layers to yield a flaky, tender pastry of optimum volume. After the last folding, the layered dough is rolled to a thickness to give the height desired in the baked product. An eight-fold increase in height is possible. Cream horns are over half the

dough and

made by winding

thin,

the other half of the

one-inch wide

strips ot

dough, edges overlapping

conical forms. For patty shells, a three-inch cookie cutter

which have the centers cut out by tops of the patty

shells.

The

a smaller cutter.

per and the edges moistened so the rings

set

patty shells and the horns are chilled one

last

Puff pastry

is

baked

The

plain rounds are placed

in a very

high

temperature

initial

around

used to

make

the rims and

a baking sheet lined with heavy pa-

on top of them

will adhere.

The assembled

time.

hot oven (232° to

imately five minutes, after which the temperature

The

latter are

on

slightly,

used to shape rounds, half of

is

results in the rapid

is

246°C

or 450° to 475°F) for approx-

lowered approximately 10°C (50°F). at the fat/ dough in-

production of steam

the layers of dough. After 10 minutes, the oven temper-

terface,

which separates and

ature

again lowered 10°C (50°F) for completion of baking. Total baking time varies with

is

lifts

the thickness of the pastry.

REFERENCES 1.

Bailey, ter."

2.

C. H. 1934. "An automatic shortome-

Cereal Chemistry 11:1 60-63. Instrument

devised to measure the breaking strength of

gealing points of

pastry.

HEIb! 1962.

Berglund,

T, and D. M. Hertsgaard.

P.

1986. "Use of vegetable els in cake, pie crust,

oils at

reduced

at

two

lev-

cookies and muffins."

Journal ofFood Science 5 1 :640^4.

3.

Two

oils

Colburn,

J.

T, and G.

"Margarines,

roll-ins,

characteristics

Denton, M.

1964.

Pankey.

R.

and puff pastry

short-

B.

Gordon, and R.

and

Fisher, plastic

J.

Sperry.

from medium-,

flour.

D. 1933. "Shortening value of

fats."

Industrial

third re-

and mix."

Economics 54:767-71.

their causes.

Simpson.

1961.

"Microscopic appearance of gluten

in pas-

try

S.,

dough and

and

its

I.

J.

relation to the tenderness

of baked pAsny.' Journal of Home Economics with excess manipulation, and dough with

Hornstein,

and from high-protein 5.

defects

Chemistry 10:156-60. Breaking strength of low-,

recipe

con-

value.

Desirable characteristics; factors for success;

excess water

made from

foods— a

—from

Home

Journal of

fats;

and shottening

"Finished

Pie crusts

1933. "Study of tendetness in pastries made from flour of varying strengths." Cereal

pastries

fat

53:681-86. Standard pastry dough, dough

of fats for puff pastry.

C

port.

Hirahara,

levels in pastry.

enings." Bakers Digest 58{2):66-68. Desirable

4.

Chemistry 25:1171-73. Breaking sttength

of pastries made with different

and Engineering

compared; photomicrographs

of raw dough.

1943.

L. R., F. B.

some commercial

An

King, and

F.

Benedict.

"Comparative shortening value of fats."

Food Research 8:\-l2.

attempt to account for differences in the

shortening value of fats.

"

9.

Hoseney, R. Seib. 1978.

C,

D. R. Lincback,

"Role of starch

in

R A.

,iiui

Bakers Digest 52(4):11-I4,

16.

10

Lagendijk,

and

J.,

J.

Cereal Chemistry

van Dolfsen.

dough on

1965.

and marfat

Piatt,

and

H.

and

Faridi

J.

M.

Faubion, eds.

New

the

S.

Fleming. 1923. "The ac-

phenomena."

newer

Industrial

in

relate the tenderizing effects

pastry

to

of

work by

fundamental

Harkins and coworkers on the orientation of molecules 16.

Rose,

T

S.,

at film interfaces.

M.

E.

Dressier,

and K. A.

Van Nostrand Reinhold. 309. Demands on flour used to make puff pas-

Johnston. 1952. "Effect of fat and water in-

one page of a long, involved discourse.

uniformity of tenderness of pistty." Journal

try;

I'.

corporation on the average shortness and

H. Buchanan.

of Home Economics 44:707—9. Comparison

1938. "The physical and chemical character-

of pastry made by the water-in-fat emulsion

Lowe,

istics

B.,

P.

M.

Nelson, and

of lard and other

J.

Station Research Bulletin 242:1-52.

value and refractive index of

fats

Iodine

and the

Matthews, R. H., and E. H. Dawson. 1963. "Performance of fats and

Three

Cereal

oils, lard,

etable oils

oils in

17.

Swanz, V. N. 1943. "Effects of cenain ables in techniques lard

pastry

on the baking

wafers."

Cereal

vari-

strength of

Chemistry

20:121-26. Effects of increasing the water, of

breaking strength of pastry.

biscuits."

and by the conventional method.

hits in relation to their

culinary value." Iowa Agricultural Experimetn

13

fat

Texture.

York:

12

W., and R.

attempt to

doughs and baked products." In

Dough Rheology and Baked Product

ma-

roll

and Engineering Chemistry 15:390-94. An

on the

states

thermal, mechanical and structural properof

Effects of

the breaking strength of short-

theories of surface

Levine, H., and L. Slade. 1990. "Influence

ties

:343^6.

1 1

tion of shortening in the light of the

characteristics of the pastry.

rubbery

and

McLaughlin,

bread.

on dough firmness." Cereal

of the glassy and

H.

nipulation and of flour used to

Chemistry A2:2'y^-b?>. N.iture of the

11

T,

I.

G. Halliday. 1936. "Factors influencing

the apparent shortening value of a fat."

in

pastry.

"Classification of puff-pastry fats

garines based

E.

40.

18,

Scanning electron micrographs of starch baked products, including

Noble,

baked foods.

pastry

and

Chemistry 40:291-302.

increasing the mixing time after the water was

added and of having ingredients

at

room

vs.

refrigerator temperature.

and two hydrogenated veg-

compared.

291

PARTY Proteins

and

Protein-Rich Foods

Introduction to Proteins

Proteins are nitrogen-containing organic compounds that constitute one group of essential nutrients.

Both plants and animals are sources oi protein

in the diet. Plants sup-

main sources. Animals obtain protein from plants or from other animals that have fed on plants from which they form new proteins ot a higher biological value. Proteins trom animal sources include milk, cheese, eggs, meat, poultry, and fish. Proteins are important not only for their nutritive ply proteins via seeds, with legumes

and

cereals as the

value but also because of the functions they perform in

many food

preparation

processes.

The amino

Proteins are polymers of amino acids.

acids in a protein molecule

and the

order in which they appear in the polymer influence the three-dimensional arrangement

polymer and the functional properties of the protein macromolecule.

of the

count of the amino acids found a discussion

and

in proteins

their structure

and

A

brief ac-

characteristics precedes

of the structure and properties of proteins. The functions of proteins

in the

preparation of foods follows.

AMINO ACIDS Structure

Amino

acids are organic acids with the following characteristic carboxyl group

O

— — OH //

C

Attached to this

this carboxyl

alpha carbon

is

carbon

the feature that distinguishes an ent groups of atoms bol

R

is

is

another carbon atom, designated the alpha carbon. To

joined, in addition to an

may

amino

acid

atom of hydrogen, an amino group ( — NHi), from a fatty acid. One of some twenty differ-

be attached at the fourth position of the alpha carbon.

The sym-

used to represent such groups. Thus the type formula

V I

R

C

O

— — OH //

C

NH2 295

296

can represent any amino acid. as a base, so

PART

PROTEINS

VI

The

AND

PROTEIN-RICH

The

carboxyl group ionizes as an acid and the amine group

an amino acid has both acidic and basic properties.

ionic

form that

H

prevails

depends on pH.

H

H

O

FOODS

O

//

//

//

OH NH, Lov/

When

NH,

NH-,

pH

Intermediate

pH

High pH

the positive and negative charges are in balance, the

point, in

which condition

it is

least soluble.

properties that affect the ionization of an

An R

amino

amino

acid

is

at its isoelectric

group may contribute acidic or basic

acid at a given

pH.

R Groups The chemical makeup of the R group distinguishes one amino acid from another (Table 18-1). An R group may be a single hydrogen at the fourth position of the alpha carbon, as — NHt) group, as in lysine, or a comin glycine, a chain of four — CH,) groups plus a (

(

R group is different, certain groups R groups consist of hydrocarbons, which are of the remaining R groups contain polar — OH)

plex ring structure, as in tryptophan. Although each

do have

characteristics in

common.

Six of the

nonpolar or hydrophobic. All but three or

(

— NHi)

(

groups that make them hydrophilic. The three

R groups that

are classed as

am-

phiphilic consist of a single hydrogen (glycine), a methyl group (alanine), or a heterocyclic ring (tryptophan).

The amino

acids that have hydrophilic

neutral.

Hydrophobic Valine

Amphiphilic

R

groups are acidic,

basic, or

FABLE 18-1 Structure of

Amino Acids

O

H I

H

C

H

\

//

OH

C

//

I

C

Glycine

CH,

C

CH,

OH

Glutamic Acid

OH

Methionine

OH

Cysteine

/ I

HO

NH,

O

H I

HqC

C

NH2

OH

C

CH3

Alanine

(CH2)

S

C

C

C

C

//

I

OH

C

O

H

O

H

//

H

-

NH2

NH, H

O

H

/

HS

Serine

CH2

C

C

I

OH

NH2

H

H

NH,

O

H

//

C

C

OH

NH,

CH-,

I

OH

C

Threonine*

S

CH,

C

—

/ OH

I

NH2 Cystine

H

H

O

H

I

//

C

C

CH3

NH2

CH,

OH

C

S

CHj-

C

—

/ OH

Valine' I

HO

H I

C

CH-,

NH2

I

CH,

C

HN II

//

OH

C

.

H2N

C

I

I

N

O

H

H (CH2)3

C

//

C

OH

Arginine

OH

Lysine'

Leucine I

NH2

NH,

CH,

H

//

C

CH,

CH,

C

O

H

H

I

OH

C

Isoleucine*

H2N

(CH2)4

C

//

C

I

CH3

NHj

NHj

O

H

>

^ C

I

CH2

C

//

C

OH

Aspartic Acid

/ HO 'Essential

NH, amino acids

297

TABLE 18-1 {continued) Structure of

Amino Acids

O

H I

HC^C I

C

CH2

NH

\

OH

C

Histidine'

I

I

N

//

NH2

/ CH H2

H2

C

C

\

\

C

C

H2

Q

Hydroxyproline has

^

an

OH

C

Proline

hydrogens

\ / N H 1

I

C

H

:^

H

C

\

/ C

C C

C

H

H

H

H

c

=

C

CH,

\

C c

—

H

C

H

\

^

C

O //

OH

Phenylalanine*

OH

Tyrosine

OH

Tryptophan

NH

C

/ HO

\

CH2

c

C

O //

C

NH

H

H H

^' C

c^ c

C

.c

c

'Essential

298

amino acids

CH2

C ^^^'

\ / \ N

H

O //

C

H

group

at the

carbon marked.

H H

OH

C

in

place of one of the

8

)

as in cysteine,

which can be con\erted

by oxidizing agents. In addition,

to a disulfide

299

sulfhydryl-disulfide interchange

(— SH)^=±(— S— S when both

CHAPTER

—

and disulfide groups are present. Proline and hydroxyproline are imino rather than amino acids. The nitrogen is linked to the end of the hydrocarbon side chain to form a cyclic structure. Presence of either imino acid in a is

possible

sulfhydryl

polypeptide puts restrictions on the

flexibility

of the polymer.

The R groups of constit-

uent amino acids are major determinants of both shape and properties of a protein molecule.

STRUCTURE OF PROTEINS Primary Structure

W hen

the carboxyl ot

one amino acid unites with the amino group of another, with the

elimination of one molecule oi water.

R

N H

c

C

--

Amino ac

OH

C

H

"

OH---

N

I

C

1

INTRODUaiON TO PROTEINS

300

The main chain of atoms, which

PART

PROTEINS

VI

AND

PROTEIN-RICH

FOODS

consists of repeating units

backbone of the protein molecule. Projecting

the

R groups. One

chain, like balls of fringe, are the the particular

R groups

in the polypeptide.

constitute

its

(amino

The

acids)

it

is

known

as

from either side of the main protein molecule differs from another in

contains and in the order in which they are united

and sequence of amino

kind, number,

primary structure

of— C — C — N—

alternately

acids in a protein molecule

(7).

Secondary Structure These high molecular weight polymers, unwieldy if fully extended, occupy space in a number of different shapes. Although rotation at the peptide bond is restricted because of its partial

double bond character, the bond that links the alpha carbon

molecule

Where and how a portion of a

polypeptide chain twists or bends

acid-side chains that are present. in

many

handed

The

proteins. coil,

The

alpha helix

is

is

^= O

is

possible.

the predominant secondary structure

portion of a molecule in this conformation exists as a tight, right-

with 3.7 amino acid residues per turn of the

at

backbone of the

structure

influenced by the amino

spiral (5,

10).

when

polypeptide in this arrangement are hydrogen bonds, which form

C

in the

More than one kind of secondary

free to rotate (12).

is

Holding the

a carbonyl

one point along the molecule approaches an imido group

group

.N

farther along the polypeptide backbone (Fig. 18-1). An imaginary line between the two atoms linked by a hydrogen bond would parallel the major axis of the helix, as shown in Figure 18-1. The beta-pleated sheet is the favored conformation where the molecule is

very hydrophobic like the

compact

(4).

coil

acid residues, effects a

Those segments assume an extended, helix. At intervals a beta

of the alpha 180° change

in direction

between carbonyl and imido groups of formation.

The random

pending on

their

amino

secondary structures

One

(7).

of the polypeptide (12).

parallel sections of the

molecule

another secondary structure. This term

stabilize the is

con-

used to charac-

acid

makeup, contain varying proportions of these three

(7).

secondary structure, the poly-L-proline

helix,

is

a loose,

left-handed coil charac-

of those segments of a molecule that contains a high proportion of proline or hy-

droxyproline. ring.

is

wavy shape, unamino Hydrogen bonds

turn, involving four

molecules that undergo rapid fluctuation in bond angles. Most proteins, de-

terize those

teristic

coil

rippled, or

This

The

alpha carbon and the nitrogen in these acids are part of the pyrolidine

restricts the flexibility

of the polypeptide and influences

its

secondary structure

A molecule of gelatin exists as a poly-L-proline helix. Another secondary structure, the

beta spiral (Fig. 11-7), has been proposed to account for the elasticity of gluten in bread

dough

(8).

Repeated beta turns account for the

residues per turn

is

a less

compact

coil

than

is

The

beta spiral with 13.5

tertiary,

protein molecules are

spiral shape.

the alpha helix.

Tertiary Structure

At

a

still

higher level of structural organization, designated

arranged to give compact, three-dimensional complexes that are globular shaped, such

ovalbumin of egg white and lactalbumin of milk. Proteins originate

in the

watery

as

medium

8

301 CHAPTER

1

INTRODUaiON TO PROTEINS

Hydrogen bond

!

A drawing

Figure 18-1. lix

of a portion of

of a protein molecule. (From

R. B.

ttie

Corey and

alpha heL.

Pauling, "The configuration of polypeptide chains teins," In

Proceedings of the International

Wool

in

pro-

Textile

B. Chemical Physics and Wool and Proteins. Copyright © Commonwealth Scientific and Industrial

Research Conference, Volume Physical Chemistry of 1

955, by the

Research Organization, Melbourne.)

of the

cell.

Folding or twisting of the molecule to form the tertiary structure tends to orient

the hydrophobic side chains to the interior of the structure to avoid contact with water. that are within a distance of three to five angstroms are subject to van

Hydrophobic groups der Waals attraction

bone

(7).

Bonding of polar amino acid

stabilize the tertiary structure

residues that project

of protein molecules,

also.

Such

from the back-

forces include hydro-

gen bonds between neighboring hydroxyl and carboxyl groups or between hydroxyl and

amino groups. Or the tween the sic

amino

—COO

attractive force

may be due to

salt

bridges (ionic bonds) that form be-

groups of an acidic amino acid radical and the

—NH:,

group of a ba-

acid radical, thus:

"OOC-

•NH-,

Covalent bonds

may

also serve to stabilize the tertiary structure

These include the disulfide bond

(

——— S

groups along the polypeptide backbone as a cross link.

S

may

)

that links

of protein molecules.

two cysteine

residues.

Or two R

be esterified to phosphoric acid, which serves

302 PART

PROTEINS

Quaternary Structure Proteins that consist of more than one polypeptide are characterized as having quaternary

VI

AND

PROTEIN-RICH

FOODS

structure.

Such

is

the molecule of collagen from the connective tissue of meat.

cule of collagen consists of three strands of gelatin

hand is

twist, to

form

a super helix (Fig. 18-2).

wound

like three-ply yarn,

Myosin found

another example of a protein with quaternary structure.

ferred to as the

tail,

consists of

two alpha

in the

One

muscle

The

mole-

fibers ol

meat

part of the molecule, re-

helical polypeptides twisted together as a helix.

Each polypeptide terminates at one end of the myosin molecule as head. Both myosin and collagen are asymmetric molecules, as is the collagen.

A

with a right

a globular-shaped

gelatin that forms

casein micelles of milk have quaternary structure (Chapter 19).

ferent casein molecules are assembled as submicelles

and

Three

these, in turn, are linked to

dif-

form

the micelles. Unlike the asymmetric myosin and collagen, casein micelles are roughly spherical in shape.

Conjugated Proteins

Some

proteins are

main one

in

combined with nonamino

lipids include lipovitellin

in

acid substances. Glycoproteins, such as the

egg white, contain small amounts of carbohydrates. Proteins complexed with

milk and phosvitin

and low-densiry lipoproteins ol egg yolk. The micelles of casein of egg yolk are examples of protein conjugated with

in the granules

phosphorus.

PROPERTIES The

highly polymerized protein molecules are within the size range of colloidal particles.

Molecules of such dimensions form colloidal dispersions in water. Such dispersions when they are in the liquid state are called sols. Repulsion due to like charges on such dispersed particles favors the stability

negative charges

of colloidal dispersions. Repulsion

on ionized carboxyl groups

ot acidic

amino

may be due

to an excess of

acid residues or to an excess

86 A

• Glycine

• Figure lices.

1

8-2.

Predominantly Imino Acids

Segment of a right-handed collagen

(Reprinted from M.

E.

triple helix

with three left-handed poly-L-proline he-

Nimni, 'The molecular organization of collagen and

the biophysical properties of the connective tissues," Biorheology

permission from Elsevier Sciences

Ltd,

1

its

7:52. Copyright

The Boulevard, Longford Lane, Kidlington

role in determining

©

OX5

1

990, with kind

1GB, UK.)

8

of positive

on nitrogen-containing groups of

charges

amino

basic

acid

residues.

303

Dispersions of polypeptides stabilized by electrical charge are sensitive to changes in the

pH

medium

of the

which they

in

point, the dispersion

is

arc dispersed. If the protein

Thus

destabilized.

thicken or curdle the cream served with

brought to

its

isoelectric

may

the acid from a tart fruit such as peaches

it.

Hydration of a protein macromolecuie VC'atcr

is

another source of stability of a protein

is

sol.

molecules hydrogen bond to polar groups of the backbone and to those of the amino

Other water molecules that are hydrogen bonded to those directly assoform a protective layer that prevents protein-protein contact jostled about by Brownian motion or by agitation. Both heat and salts may

acid side chains.

ciated with the polypeptide

when

they are

destabilize such dispersions.

When

the bonds that maintain the tertiary and secondary structures of a protein mol-

ecule are disrupted, the polypeptide loses

naturation, as

it is

its

characteristic spatial arrangement.

This de-

termed, exposes reactive groups that were concealed in the native

state.

Included are acidic, basic, hydrophobic, and sulfhydryl groups. Their exposure and their interactions subsequently fact,

many of the

may have unwanted

preliminary step. Heating

is

used frequently to denature proteins. Surface denaturation

and secondary structures that

effective, also. Tertiarj'

denatured by altering the pH.

poor

qualit)'

are stabilized

high concentration

Salts in

bonds by reducing agents may

disulfide

effects as well as others that are desired. In

functions that proteins perform in foods depend on denaturation as a

alter the

by

salt

may denature

bridges

may

is

be

protein. Breaking

conformation of a protein molecule. The

of yeast bread from dough that contains dead yeast

cells is

attributed to the

detrimental effects of the reducing agents from the yeast on the disulfide bonds in the gluten.

The compactness of

a loaf

of whole wheat bread

is

attributed to the action

on

gluten of glutathione, a reducing agent found in the germ.

FUNCTIONS amino acids are important constituents of the diet. Proteinfrom animal sources have a higher biological value than do those from plants

Proteins as sources of essential rich foods

such ties

as

and

legumes and

make important

cereals. Proteins

contributions to the sensory quali-

to the acceptability of foods, too.

As Enzymes Proteins as

foods

enzymes catalyze

An enzyme acts on

(9).

a variety

of reactions that affect color,

a specific

compound known

flavor,

as a substrate.

and texture of

To

the suffix

ase,

enzyme which denotes an enzyme, is catalyzes. Thus lior the type of reaction, such as hydrolytic or oxidative, that the enzyme and pase indicates an enzyme that acts on fat, amylase an enzyme that acts on starch, polyphenol oxidase an enzyme that catalyzes oxidation of polyphenolic compounds. Enzymes are responsible for numerous desirable changes in foods, but they produce some attached a prefix that indicates the substrate for the

unwanted

effects, too.

Enzyme

tributes to their senescence in yeast

and

activity that contributes to the ripening of fruits also

decay.

bread dough but undesirable

tenderize

meat but they

The

action of starch-hydrolyzing enzymes

in cold-stored potatoes

also lead to rancidity

is

con-

desirable

used for potato chips. Enzymes

of fats. Destabilization of the casein of milk in

CHAPTER

1

INTRODUaiON TO PROTEINS

304

the production of cheese

is

the result of enzyme action, as

is

the ripening of cheese during stor-

The enzyme that causes discoloration of fresh fruits and vegetables contributes to the color of black tea, cocoa, raisins, and prunes. Enzyme activity is temperature-dependent. An inage.

PART

PROTEINS

VI

AND

PROTEIN-RICH

FOODS

crease in temperature speeds the reaction until the temperature

the enzyme.

lemon

Enzyme

juice delays

activity

is

pH-dependent,

browning of such

too,

is

high enough to inactivate

which explains why a coating of

fresh fruit as peaches, pears,

tart

and bananas.

Nonenzymatic Brov^ning Proteins

may

participate in

nonenzymatic browning

with reducing sugars in a complex

which ments

result in the

series

of reactions,

called melanoidins.

The amino

acids that yield the

Maillard reaction,

and

Such reactions involving

most highly pigmented prodThe most reac-

tyrosine, in descending order.

tive sugars are lactose, ribose (a pentose), fructbse, (1).

as the

production of unsaturated, volatile products and brown colored pig-

ucts are lysine, glycine, tryptophan,

order

in certain foods. Proteins react

known

essential

amino

and glucose, again

in

descending

acids, especially lysine, lower the

and extent of browning, which may vary in color from light cream to dark brown, are temperature and pH. Browning increases with increase in pH above six and it increases with increase in temperature. Sugar-amine reactions contribute to the brown color of chocolate and cocoa and to browning of the crust of bread. The light tan color of heat-processed evaporated and of condensed milk is due to these reactions, as is browning of dried milk and dried egg white after extended storage at ambient temperature. Browning of the latter is prevented by treatment of egg white with an enzyme that eliminates glucose prior to denutritive value of a protein. Factors that influence the rate

hydration.

Modifiers of Texture Proteins

make

possible a variety of fabricated foods with unique textural properties.

They

contribute to the texture of foods through their action as gelling, foaming, and emulsifying agents

(6).

^^

Gels

The

conversion by heat of sols of globular proteins to gels that vary in consistency from

soft

and

foods.

jelly-like to rubbery, firm, or

hard

is

one way that proteins

affect the texture

of

A gel consists of two phases, a three-dimensional network of macro molecules and a

Thermal denaturation of a globular protein disand secondary structures of the molecule. This exposes reactive groups that can then form intermolecular disulfide and noncovalent bonds, including the beta sheet, that establish the network of the gel (17). Opposing forces that affect the integrity of a gel are protein-protein attraction as opposed to protein-water affinity and elasticity of the macromolecules of the network against the pressure of movement of ions released from the network (14). Shrinkage of the network or liquid phase immobilized by the network.

rupts the intramolecular bonds that stabilize the tertiary

mechanical disruption reduces sis.

its

ability to retain liquid, a

phenomenon known

Heat denatured gel-forming proteins include whey myosin of the muscle

lactoglobulin; egg white proteins, mainly ovalbumin; poultry,

as synere-

proteins from milk, chiefly beta-

and

fish;

and soy proteins

(11, 16, 17).

Some

denaturation

is

fibers

of meat,

necessary for gela-

8

many globular

tion, but

form

proteins

gels

with only limited unfolding. Gelation

com-

a

is

305

plex process only partly understood (3).

Proteins are

between theof)'

which

pairs

less

that this strengthening

is

a function of spacing

is

Two

when

susceptible to denaturation by heat

of hydrophobic groups

proteins that

is

is

due

sugar

is

present. Interaction

stronger in a solution of sucrose or other polyol. to the effect

The

of the polyol on the structure of water,

and orientation of the

—OH groups on

do not require heat denaturation

the polyol (2).

order to form gels are casein of

in

milk (Chapter 19) and gelatin (Chapter 30).

Foams

The foam-forming ability of proteins

influences the texture of such items as whipped topwhipped cream, meringues, ice cream, marshmaliows, souffles, bread dough, and cake batter. A foam is a rwo-phase system of bubbles of gas surrounded by a continuous film of liquid. In foods, usually the gas is air and the liquid is a protein sol. The first bubbles of air whipped or beaten into a protein sol are round and they remain so until the phase volume of the gas exceeds 0.74 (4). As more air is incorporated, pings,

the aqueous film that separates neighboring air cells thins.

naturing environment

(6).

foaming agent

tion as a

Exposed hydrophobic groups, (15), orient

A

thin interfacial film

essential if a protein

is

a de-

is

to func-

toward and are associated with the gas phase.

Hydrophilic groups are dissolved in the aqueous film. This surface activity lowers surface tension of water, facilitating

foam formation

Molecules of protein can make multiple

(4).

contacts with the interface, sections of the polypeptide between contacts forming loops

from the interface

that project face, too.

Pmtdns_are

and interaction of molecules

less

Even low

so,

air cells

foams

when

a

(13).

is

is

at the isoelectric point.

favored (llj.

foam

is

1

his

inter-

Repulsion

is

mteraction helps to

air cells (6).

are unstable dispersions, as evidenced

allowed to stand. Drainage

pressure of water in the spaces

lower than

Ends of the polypeptides may project from the

at the interface

aqueous film around

stabilize the

is

(1 1).

m ore effective foaming agents

is

by the accumulation of liquid be-

due

in part to gravity. In addition,

formed by the curved surfaces where three or more

that in the thin film, called lamella, in the interface

air cells

between adjacent

meet

air cells

The ensuing siphoning effect not only contributes to drainage but it exerts a pull on the may result in its rupture. Drainage is less when the interfacial film

denatured protein film that is

viscous. Sugar,

which

increases the viscosity of water,

Overheating makes some foams are

combined with other

stiff,

inelastic,

and

less

ingredients, air cells break

makes

more

for a

able to retain liquid.

and the foam

loses

stable

foam

(4).

When such foams

volume.

Emulsions In

addition

to

their

foam-forming

ability,

surface

active

proteins

with

their

hydrophilic/lipophilic character contribute to the texture of foods by their emulsifying action (7, 11, 13). Emulsification, including the role of proteins as emulsifiers, in

Chapter

16. Proteins are a part

is

contribute the smooth, thick creaminess to cream. As an emulsifier around the droplets of fat that are

formed when milk

cosity of milk. Emulsification as gravies, sauces,

and

discussed

of the layer of emulsifier around the droplets of fat that

is

homogenized, proteins contribute

of fat by the proteins of flour, milk, or eggs

in

many new to the vis-

such products

soft pie fillings contributes to the desirable tactile sensations these

CHAPTER

1

INTRODUaiON TO PROTEINS

306

items produce in the mouth.

PART

PROTEINS

VI

AND

higher proportion of fat and the increase in egg protein,

account for the

for emulsifying the increased fat,

compared

ference in texture of cream puffs

to

dif-

popovers and cake compared to muffins.

In themselves, proteins are essential dietary

PROTEIN-RICH

FOODS

The

low density lipoprotein,

especially the

components. Equally important, proteins

have major impacts on the sensory properties of foods. They help make nutritious food

more

palatable, contributing to the pleasure of eating.

REFERENCES 1.

Ashoor,

S.

"Maillard acids

and

H., and J. browning of

sugars." Journal of

49:1206-7. Effectiveness of eighteen 2.

Back,

J.

ondary

1984.

Zent.

B.

common amino Food Science

five sugars

amino acids. P., D. OakenfuU and M.

with

3.

of

effect.

Pp. 291-314. Action of Lee-Tuffneil. 1986.

globular

In

proteins."

R.

lases

10.

Mitchell and D. A. Ledward, eds.

New

A com11.

B., T.

E.

Kinsella. 1985. "Film

J.

12.

13.

between

and functional properties of food proteins." In Food Proteins. P. F. Fox and J. J. eds.

London and New

1981. "Protein conforma-

and

its

role in stabi-

,

54-57. Proteins

in the air

EM.

Richards,

1991. "The protein folding

Factors

Stainsby,

American

Scientific

that

264(1):

folding;

affect

sec-

tertiary structures.

G. 1986. "Foaming and emulsifiof Food

R. Mitchell and D. A.

Micromolecules.

J.

Ledward,

London and New

York:

Applied Science Publishing

Com-

eds.

cules at interfaces. 14.

T

Tanaka,

1981.

American 244:124-36,

work

sttuctures,

relationship;

"Gels." 138.

Scientific

Liquid/net-

expanding and contract-

ing forces.

stabilizing forces.

Miles, M.]., etal. 1991. "Scanning tunnel-

ing microscopy of a wheat seed storage protein reveals details

excel-

pany. Pp. 315-53. Action of macromole-

Science Publishers. Pp. 51-103. General

three-dimensional

many

emulsions and foams." Food Tech-

Elsevier

York:

Distributed by Elsevier Science, Applied

properties,

American 191(l):51-59.

cation." In Functional Properties

stability.

E. 1982. "Relationship

mole-

of protein

structure

M. C.

Phillips,

ondary and

structure

Condon,

Scientific

54-63.

forming behavior of

"

J.

hydro-

bubbles.

E.

food proteins. Journal of the American Oil Chemist Society 62:1358-66. Foam devel-

Kinsella,

cules."

problem." O'Neil, and

common

aqueous films (lamellae) that separate

molecules; illustrated.

opment and

"The

lizing

Scientific

American 197(3): 173-78, 180, 182, 184. Amino acids, polypeptides, and protein

J.

1954.

nology }5(\):50-5'\"Proteins."

W.

Corey and R. Hayward.

Pauling, L., R. B.

tion at liquid interfaces

film formation.

1957.

J.

oxidases.

structure of protein molecules;

Nutrition Research 34:1-79. Energetics of

German,

and

the Effects

and

lent illustrations.

Damodaran, S. 1990. "Interfaces, protein films and foams." Advances in Food and

P.

eds.

and

Development of the knowledge of the

York: Elsevier Applied

plex process only partially understood.

Doty,

Cook.

York: Marcel Dekker.

and C. D.

Science Publishing Co. Pp. 203-72.

8.

New

count for the

of

Y

and R.

Phillips

Clark, A. H.,

London and

7.

D.

R.

Processing.

Findley,

J.

6.

Flick, Jr.,

J.

in foods." In Protein Quality

polyols."

Functional Properties ofFood Macromolecules.

5.

G.

R. L.,

Biochemistry 18:5191-96. Attempt to ac-

"Gelation

4.

Ory

1989. "Enzymes that affect protein quality

B. Smith.

and

of the

mation. 9.

1979. "Increased thermal stability of proteins in the presence of sugars

Proceedings

structure."

National Academy of Science U.S.A. 88: 68-71. Evidence tor the beta spiral confor-

of an unusual supersec-

15.

Townsend,

A.,

and

S.

Nakai.

1983.

"Relationship between hydrophobiciry and

foaming

characteristics

of food proteins."

of Food Science 48:588-94. Foaming mechanism. Wang, C.-H., and S. Damodaran. 1991.

Journal

16.

"

Thermal gelation of globular proteins:

Influence ot protein contormation on gel

17.

Ziegler,

"The

G.

R.,

gelation

and

E. A. Foegeding. 1990.

of proteins." Advances

in

Food and Nutrition Research 34:203-98. Gelation mechanisms: gelatin, soy protein.s,

whey

proteins, myosin,

and egg

proteins,

ofAgricultural and Food Chemistry 39:433-38. Involvement of the

strength." Journal

beta-pleated sheet.

307

Milk

IVIilk

is

the raw material used in the manufacture of butter, cheese, yogurt,

dairy desserts. Fluid milk

with chocolate or cocoa. Milk

is

the

main ingredient

in

some

Items such as meat, poultry, eggs, vegetables, and cereals

Milk

is

and frozen

widely used as a beverage, either plain or flavored, especially

is

sauces, soups,

may

and puddings.

be cooked in or with milk.

valued for the functional properties of its components, especially the proteins that

foaming agents.

act as binding, emulsifying, or

Many

of the problems encountered

in the

making such products as cream of tomato or cream of asparagus soup, macaroni and cheese, or even yeast bread dough stem from the proteins in

use of milk for

cheese sauce, milk.

Milk

is

valued not

least for its nutrients.

COMPOSITION The composition of different forms of milk grams, or slightly

than 100

less

than

is

found

Table 19-1.

The

values are for 100

milliliters (X cup).

Because milk has a high percentage of water,

such

in

pound. This weight of fluid milk measures somewhat more

'A

as cakes

and bread. Milk

is

less

sweet than

its

it is

used

as the source

approximate

five

of water

in

foods

percent sugar content

might lead one to expect, because of the low sweetness of lactose (Chapter 2). Milk is a good source of high quality protein. The cow converts feed protein to food protein with an efficiency of 31 percent, the highest conversion content of milk

as

drawn

varies

for

any animal protein

with the breed of the cow.

It

(23).

The

fat

can be separated from the

aqueous phase of milk by centrifugation. Milk

is sold on the basis of fat content. Glycerides from others of animal origin in that they contain short-chain (C^-Cio) saturated fatty acids. These may give rise to desirable flavors in such products as cheese and to off flavors in rancid butter or dried whole milk. The color of the fat is influenced by the

of milk

fat differ

carotenoids in the feed. Milk

is

a

poor source of the mineral

iron, a

good source of phos-

phorus and magnesium, and an excellent source of calcium.

The

latter,

which

gives

A

by the fat) and some thiamin (derived from bacteria good source of niacin and an excellent source of riboflavin. the greenish yellow fluorescence to whey (the watery part of milk

Milk contains vitamin that thrive in the rumen).

It is

(carried a

from which much of the protein has been removed),

and by the flow of milk. The in the cow's pituitary.

308

latter

is

is

influenced by the feed of the

influenced by somatotropin, a

hormone

cow

synthesized

Cows given a supplement of microbe-produced bovine somatotropin

o

E

D D5

O

Q.

(A

E 1— o

O Q. E o

U

310

consume more

feed

and produce more milk

The

(10).

ascorbic acid content of milk varies

with the feed of the cow and the procedures used to prepare different forms of milk for the PART

PROTEINS

VI

market.

AND

PROTEIN-RICH

FOODS

DISPOSITION OF CONSTITUENTS IN MILK Solution

As

most foods, milk

true of

is

For one thing, milk

is

milk sugar,

The

lactose.

is

complex from the standpoint of

a solution. Dissolved in the

its

physical organization.

87 parts of water per 100 of milk

is

the

water-soluble vitamins thiamin, riboflavin, niacin, and ascorbic

acid are in solution in milk. Part of the minerals in milk are in solution. Included are chlorides, citrates,

potassium, magnesium, and sodium ions. Part of the calcium phosphate

is

in solution.

Colloidal Dispersion Dispersed in the aqueous phase, colloidally rather than in solution, are calcium and mag-

nesium phosphates and

citrates.

These consist of the caseins that

whey

or

serum proteins

The

proteins of milk (8) are colloidally dispersed-, too.

are precipitated

by acidifying milk

to a

pH

of 4.6 and the

which account

that remain dispersed (30). Caseins,

for approxi-

mately 80 percent of the proteins of milk, consist of alphaj, beta-, and kappa-casein. The ttj-casein consists

and K-casein remain

—

in the

of two fractions, a^,- and

cKji"

casein.

The

four caseins

are present in an average weight ratio of 3:0.8:3:1 (25).

whey

after the caseins are precipitated

—

otsj-, otj,-)

The

p-.

proteins that

with acid include P-lactoglobulin, a-

lactalbumin, serum albumin, and immunoglobulins. P-lactoglobulin

is

the

main whey

protein, present in a concentration similar to that of K-casein.

Casein Micelles

The

structures called micelles (25) (Fig. 19-1).

and with part of the salts of milk in These micelles are responsible for the opalescent

whiteness of milk. This micellar complex

is

caseins of milk are associated with each other

often referred to simply as casein. Exactly

the caseins are arranged in the micelle has been the subject of

much

research

how

and various

models have been proposed (19, 25, 28, 29). The model that most nearly accounts for the properties of the casein complex is shown schematically in Figure 19-2. Micelles are roughly spherical structures

made of submicelles. The

submicelles consist of aggregates of

molecules of a^i-, a^j'' P"i ^nd K-casein. The amino acid makeup of molecules of the caseins, with their high proline content (30), limits formation of alpha helix or beta sheet. Instead, the polypeptides, with their

numerous

(3

turns,

assume

a

compact, ellipsoidal

shape, with a length-to-diameter ratio of 4:1 for a^-casein, for example (28). Polar acid residues are clustered at one end of the molecule and hydrophobic ones

At the polar ends of the caseins

amino

at the other.

are serine residues esterified with phosphate. Ihe_£olai_£Jnd

_of K-casx '" rnn|;am^arnmplpY ttisaccharide Unit in addition.

assemble to form a submicelle, they are oriented

radially,

When

these casein molecules

hydrophobic ends toward the

center of the submicelle and polar and charged ends toward the periphery (Fig. 19-2a). In the formation of a casein micelle, colloidal calcium phosphate, chiefly, links adjacent submicelles by

way of phosphate groups on

the surface of the submicelles (Fig. 19-2b). Hence,

311 CHAPTER 19 MILK

Figure 19-1. Casein micelles from cow's milk. (Courtesy of R. S. Carroll, USDA, Regional Researcfi Center, Philadelphia. From Fundamentals of Dairy Chemistry, B. H. Webb, A. H. Johnson, and A. J. Alvord, ed, p. 443, 974. Avi Publishing Company. 250 Post Road East, Westport, CT, publisher.) 1

the casein micelles are sometimes referred to as calcium phosphocaseinate. Submicelles that contain K-casei n are oriented so that the calcium-insensiti ve

K -casein portion

is

on the

j urface of the micelle (Fig. 19-2c). TnFTesulting hydrophilic and charged surtace oTthe micelle keeps

it

colloidally dispersed.

Emulsion Milk

typifies

sion.

The

not only a solution and a colloidal dispersion

fat in

milk

is

(a sol),

but

it is

also

an emul-

present as small droplets or globules with an average diameter of

three to six microns (14). Fat globules

may

range in size from

less

than one to 10 microns,

the size influenced by the breed of cow. Jersey and Guernsey cows secrete larger fat globules than

do Holsteins.

Fat droplets in milk are prevented

from coalescing by a thin coat-

ing of emulsifier (a few millimicrons thick) around the fat globules at the liquid-fat interface.

Emulsions are discussed

The

layer

in

Chapter

of emulsifier around the

16.

fat

globules of milk

is

more complex

(14) than

is

and paprika around droplets of oil in French dressing or the orientation of lecithoprotein around fat droplets in mayonnaise. The structhe alignment of solid particles of mustard

membrane is pictured schematically in Figure 19-3. Lipids and proteins are main components of the membrane. Triacyl glycerols are the main lipids, but phospholipids and sterols are present, too. The proteins have a high content of hydrophobic leucine and of aspartic and glutamic acids. The enzymes alkaline phosphatase and lipase are associated with the membrane. The components ofthe membrane are deposited in an ture for this

the

312 PART PROTEINS

VI

AND

PROTEIN-RICH

FOODS

Non-linking

\

region

^^M^ (a)

Figure 1 9-2. Structure of a casein mishown schematically, [a) Cross-sec-

celle,

tion of

a submicelle with the three types of

casein molecules. Hydrophobic areas are

shaded,

Submicelles crosslinked by cal-

(b)

cium (Co), phosphate

(P),

and

citrate (Cit).

Nonlinking areas with K-casein are black, celle.

(c)

Partially

(From

T.

P.

in

completed casein mi-

Coultate. Food: The

Chemistry of Its Components. The Royal Society of Chemistry, London, 1989, p.

101

.

(c)

Reprinted by permission.)

orderly fashion, with high melting triacyl glycerols at the periphery of the

with carbohydrate side chains that are associated with lipids and proteins face of the

membrane

droplet

and

(13).

Fat globules are suspended throughout freshly

The

oil

at the outer sur-

globules associate in clusters that

become

drawn milk, but they do not remain so. them to the wa-

so large the forces binding

ter

phase are insufficient to counteract the effect of the difference in density between the

oil

phase and the water phase.

When

whole milk stands,

a process

known

as

A globulin,

clusters

creaming.

Cream

is

Fat droplets contribute viscosity to milk

cous the product. In goat's milk the to the top of the milk. fat

As a

one of the serum proteins, promotes

of fat droplets, being

fat

milk that

is

lighter, rise to the

clustering.

top of the milk,

extra rich in ernulsifiedfet_droglets.

and cream, the greater the number, the more

vis-

globules are so small that they are unable to float

result, goat's

milk does not cream. Carotene dissolved in the

globules gives the creamy tint to milk and cream.

313 CHAPTER 19 MILK

Fat

PI

h-O"-

asma

Protein

v^?;^-*— Bound water

Phospholipid High-melting triglyceride Cholesterol

Vitamin

Figure

1

9-3.

A

Diagram of

the structure of

tfie fat

globule

membrane

in milk.

(From Nicolai King. The Milk Fat Globule Membrane. 1955. Reprinted by permis-

Commonwealth

sion of the

Agricultural Bureau, Bucks, U.K.)

Homogenization To eliminate creaming, milk orifices that

reduce the

fat

is

homogenized. Milk

is

forced under pressure through fine

globules to an average diameter of

less

than two micrometers

The higher the pressure used to force the milk through the orifices, the smaller globules. With the formation of many smaller fat droplets, the surface of the fat in-

(Fig. 19-4).

the

fat

As the

fat droplets are subdivided by homogenization, the original supplemented by proteins from the aqueous phase of the milk. from the original protein and consist of casein subunits and serum proteins

creases enormously.

emulsifying material

These (11).

differ

The

is

small size ofthe

fat

drople tsjjid their grea ter density because of adsor bed casein

_£liQii nate visible _creaming>^

Homogenization

affects

milk in other ways. Homogenized mil k

opaque, and more viscous than unhomogenized milk thickened product less readily (32).

ules

and

to the

is

slightly thicker if made

These

witEme same

fat

with homogenizedrniUcand added

effects are due, at least in part, to greater surface area

composition of the new

fat

more

whiter,

is

content.

A starch

fat

of the

blends

fat

glob-

globule membranes.

PASTEURIZATION OF MILK Although market milk

is

produced under sanitary conditions,

(33) to eliminate pathogens that

might have contaminated

the raw milk to bottling the product. In fact, the as those that cause tuberculosis,

undulant

fever,

cow can be

and

it

it is

at

routinely pasteurized

any stage from drawing

the source of pathogens such

listeriosis.

Pasteurization

is

a

mild heat

treatment that eliminates pathogens, some spoilage microorganisms, and enzymes in milk.

314 PART

PROTEINS

VI

^^^^HKT

AND

PROTEIN-RICH

FOODS

(b)

(a)

Figure 19-4. milk. Smallest

dark

Fat globules

nonhomogenized

in (a)

gradation on the scale (Courtesy of

field illumination.

is

CP

milk

and

(t)

homogenized

two microns. Photomicrographs token with Division,

St.

Regis, Chicago,

IL.)

may be heated to 62°C (145°F) and held at that temperature 72°C (161°F) and held for 15 seconds. The latter, known as high-temperature-short-time pasteurization, does less damage to the flavor of milk. Ultrapasteurized milk has been heated to 138°C (280°F) for two seconds, which gives it a longer shelf life under refrigeration. The enzyme, phosphatase, serves as a built-in indicator by which the adequacy of pasteurization may be gauged. Pasteurized milk gives a neg-

To

effect pasteurization

for

30 minutes or heated

milk to

ative test for phosphatase.

So

sensitive

is

this test that the

presence of 0.1 percent raw milk

added to pasteurized milk can be detected, as can the fact that the pasteurization temperature was off by one degree Fahrenheit. The enzyme lipase is inactivated by pasteurization, which prevents homogenized milk from becoming rancid. Spoilage bacteria may survive pasteurization, so milk is cooled promptly and should be held under refrigeration. An out-

break of

listeriosis

traced to the

consumption of pasteurized milk

raises

questions about

the adequacy of pasteurization should milk be heavily contaminated with a pathogen

(9).

138°C (280°F) and then aseptically packaged may be held at room temperature for up to three months or until the seal is broken. The milk should then be refrigerated. Certified milk is produced under sanitary conditions that keep the bacterial Milk heated

to

count low. Unless the milk

is

pasteurized,

it

may

contain pathogens that cause foodborne

illness.

Milk is graded on the basis of bacterial count (number of bacteria per milliliter of Grade A milk must have a low bacterial count (a maximum of 20,000 per milliliter).

milk).

Most of the fluid milk on the retail market is Grade A. To keep the bacterial count low, milk and milk products must be stored at refrigerator temperature. Milk may be fortified with vitamins A and D. For the former, the level is 2000 International Units per quart (946 milliliters). Vitamin D milk must have 400 International Units added per quart (946 milliliters).

TYPES OF DAIRY PRODUCTS Fluid Milk Federal standards specify a

minimum

be raised at the option of a fat

state. Fat

content can be formulated. Milk

fat

content of 3.25 percent for whole milk. This

may

can be separated from milk, and milks that differ in is

marketed with

fat

contents of 2,

\'A,

1,

and

'A

per-

The

cent in addition to whole and skim milk.

enzyme

treated with the

latter

contains

less

than

)^

percent

hit.

hydrolyzes lactose to glucose and galactose,

lactase, that

is

Milk

315

avail-

CHAPTER 19

able for lactose-intolerant individuals.

MILK

Cream The

heavier the cream, the higher

the proportion of fat droplets to milk.

is

The

fat

con-

from half and half with approximately 10 percent fat, to coffee cream with approximately 18 percent tat, to light whipping cream with a minimum butterfat content tent increases

of 30 percent, and to heavy whipping cream with a

minimum

fat

content of 35 percent.

/^Evaporated Milk Evaporated milk

is

prepared from whole milk by preheating

it

to facilitate evaporation of

moisture and then removing 60 percent of the water under vacuum. centrate

added

then homogenized, sealed in a

is

to evaporated milk before

tin,

and

sterilized.

sterilized, stabilizes the a,-

it is

The

and 3-caseins against

by the high temperature even more

cipitation by either calcium ions or

does the K-casein present in the milk.

The

resultant con-

Carrageenan, a vegetable gum,

effectively

pre-

than

of the carrageenan (kappa form)

effectiveness

is

and the 3,6-anhydrogalactose units in the molecule and tan color of evaporated milk result from the reaction be-

attributed to the ester sulfate groups (17).

The

distinctive flavor

tween the proteins and the lactose of milk milk does not cream, and because can

is

unopened. Once the

seal

is

it

at the

high sterilization temperature. Evaporated

has been sterilized,

it

keeps indefinitely as long as the

broken, the contents become contaminated with mi-

croorganisms and the milk should be refrigerated and handled stitute

D

is

like fresh milk.

To recon-

evaporated milk, usually equal parts ol the concentrate and water are used. Vitamin

added

to evaporated

milk in an amount to give 400 International units per quart (946

milliliters).

Condensed Milk The market form water. Sugar

milk, which tion of sugar

is is it

of milk called condensed

added

is

in sufficient quantirv'

then canned.

The milk

is

not

made from whole milk by removing half the (approximately 44 percent) to preserve the sterilized,

but because of the high concentra-

keeps well.

Dried Milk Solids

Whole

milk, buttermilk, and skim milk are available in the form of dried powders.

last is called

for the

nonfat

dr>'

milk. Milk used to

make

dried milk

is

pasteurized.

The

That destined

manufacture of cottage cheese gets no additional heating. For the bread-making

in-

dustry, which uses the bulk of the dried milk produced, the milk is heated sufficiently (usually to 85°C or 185°F, for 20 minutes) to inactivate the loaf-depressant factor. Milk to be

dried for general purposes receives a

less

severe heat treatment before

it is

dried (5).

condensed under reduced pressure.

To evaporate the moisture from milk, it is first is then blown as a fine spray into a preheated vacuum chamber. The resulting powders have a moisture content of approximately two to three percent. To reconstitute these powders, 4'A ounces of dried whole milk or 3--: ounces of nonfat dry milk solids are made to a volume of one quart with water. Reconstituting milk powder is unnecessary

This concentrate

316 PART

PROTEINS

VI

AND

PROTEIN-RICH

FOODS

number of products. This is especially true of batters and doughs for which the milk powder may be sifted with the dry ingredients and water (equivalent to the amount of milk specified) added when the milk normally would be. Dried milk solids added to water as a fine powder tend to lump. To eliminate this problem some milk powder is exposed to water vapor, which causes the fine pieces to clump or agglomerate in much the same way that instantized flour is agglomerated. Water can then more easily find its way in the interstices that separate adjacent particles. Not only for a

are the particles in instant-dispersing

milk powder

has brought lactose to their surface. Lactose

is

but their exposure to moisture

larger,

more water-soluble constituent of the dried milk powder (3).

the

dried milk solids, which facilitates reconstituting

the

Butter Butter

is

obtained from cream by a process called churning.

whipped, which disrupts the membranes around the

The cream

fat droplets.

tinue to coalesce, the milk eventually separates into two phases

As the

—

is

fat

agitated or

droplets con-

the butterfat,

and the

aqueous phase with its dissolved and dispersed constituents. The membranes around some of the fat droplets remain. The clumps of fat are removed from the milk and the butterfat washed in several changes of cold water to remove the milk. Butter is usually salted and is

worked a

remove excess water. However, butter contains approximately 1 5 percent water, is emulsified. The minimum fat content of butter is 80 percent. The high

to

part of which

moisture content of butter makes place.

One

fatty acid so released

is

it

prone

to hydrolytic rancidity if

it is

stored in a

butyric, a molecule with a short chain.

It is

warm

volatile

and

has an unpleasant odor.

Cultured Buttermilk This form of soured milk

is

produced by treating pasteurized skim or part skim milk with

a bacterial culture that converts lactose to lactic acid.

thickens the product through

its

action

on

The

acid, a

minimum

of 0.5 percent,

the colloidally dispersed proteins. In addition,

organisms in the culture produce compounds that contribute aroma to the buttermilk.

One

such

compound

is

diacetyl, derived

from

citrate (24).

Yogurt product made by fermenting pasteurized milk with und Streptococcus themwphilus cultures (12, 31). The milk is usually fortified by adding powdered milk, which increases the viscosity of the mix and the nonfat milk solids above the minimum of 8.25 percent. The mix is heated for 30 minutes at 80° to 85°C (176° to 185°F) before it is incubated with the cultures. Heat denatures the

Yogurt

is

a custardlike or semifluid

Lactobacillus b ulgaric us

|3-lactoglobulin,

which

reacts

with the casein.

The

fine protein

network that forms when

coagulum that resists syneresis. Fermentation of the citrate in milk yields acetaldehyde, diacetyl, and acetic acid. Incubation time and temperature influence the contribution of the two organisms to the fermentation process and deacid accumulates constitutes a stable

termine whether the product will be predominantly sour or have an agreeable balance between sour taste and aroma (16). Federal standards specify milk fat contents of 3.25 percent

minimum

for yogurt,

cent for nonfat yogurt.

The

of 0.5 to 2 percent for low

fat

acidity calculated as lactic acid

is

yogurt and

less

0.9 percent

minimum.

than 0.5 perIn ad-

must be held under

dition to the product that

refrigeration, yogurt

is

available in frozen

317

form. Sweetener, flavoring, and fruits are optional ingredients. Federal standards for the CHAPTER 19

are yet to be established. frozen product ' *

MILK

Sour Cream Sour cream

is

same manner

cream cultured

light is

as

is

yogurt. Half-and-half sour cream produced in the

available.

Caseins, Caseinates,

and Whey

Proteins

These products manufactured from milk are available in dried form. They are used by the food industry as ingredients in a variety of foods, including baked products, cereals, cheese analogs,

comminuted meats, snack

bilized by either acid or ter

removal of the serum or whey,

washed curd

cial caseins. If the

before

it is

and whipped toppings. Casein micelles, destacoagulum also referred to as a curd. This curd, afwashed, dried, ground, and sieved to yield commer-

foods,

enzyme, form is

that

is

a

formed by

acidification of

dried, the sodium, potassium, or calcium

salts

milk

is

treated with alkali

of individual casein molecules

Whey, a byproduct of the manufacture of either whey protein concentrate (up to 50 percent 90 percent protein) (20).

(caseinates) are the result.

cheese,

is

available as

protein isolate (up to

Filled

and

either caseins or

protein) or

whey

Imitation Dairy Products

These range from

filled

milk in which part or

all

of the milk

of plant origin to nondairy creamers or coffee whiteners soy protein and vegetable

fat

has been replaced by fat

made with sodium

caseinate or

oil.

MILK FOAMS Foam Formation Foam

formation, including the role of proteins as foaming agents,

The

is

discussed in

nonuniform distribution of charged and hydrophobic groups (30). (5-casein is the most hydrophobic of the caseins, and it has the most nonuniform distribution of both acidic (hydrophilic) and hydrophobic groups. Such molecules collect at the air-water interface, lowering surface tension and facilitating foam Chapter

18.

caseins are proteins with a

When

milk is secreted, (3-casein molecules are present in the casein subby hydrophobic interaction. Chilling milk or cream weakens hydrophobic interaction, and some of the P-casein molecules may dissociate from the submicelles 1 ) and disperse in the aqueous phase together with the serum proteins. The serum formation (18).

micelles, held there

(

proteins are milk.

When

less

hydrophobic than

are the caseins, but they participate in the

foaming of

the ghosts of bubbles of milk foams were analyzed, P-lactoglobulin, a-lactal-

bumin, and P-casein were the chief components primary foaming agents.

(1),

suggesting that these proteins serve

as the

Fluid milk foams readily but the bubbles are large and

and soon disappear. ble.

It is

one thing

for a liquid to

rise to

foam and another

the surface of the milk for the

foam

A viscous liquid can more easily retain bubbles of air that are incorporated.

to be sta-

Increasing

the concentration of milk solids, as in evaporated milk or in only partially reconstituted

318

nonfat milk solids, provides the increased viscosity that enables the milk to retain gas bub-

whipped cream,

bles better. Fat globules in sufficient concentration, as in

PART

PROTEINS

VI

are

more

effec-

milk foam.

tive in stabilizing a

AND

PROTEIN-RICH

Evaporated Milk Foams

FOODS Evaporated milk whips best

if

it

has been chilled to the point where ice crystals have

formed from part of the water in the milk. This serves to further increase the concentration of solids and make the milk more viscous. Addition of acid in the form of lemon juice increases the viscosity of the milk through its effect on the dispersibility of the proteins. Whipped evaporated milk yields a glossy foam with fine cells and a large volume (three times the

unwhipped volume). The milk thickens

film of liquid

around the

air cells gets

as

more

whipped into it. The impede flow, much whipped evaporated milk

air cells are

many

thinner and the

air cells

mayonnaise thickens as more oil is beaten into it. Although a foam becomes very thick, it does not set. Such a foam must be served or used promptly. Upon standing.the milk drains from around the bubbles of air, which coalesce and then rise to the surface. In less than an hour the milk regains its fluid state, leaving little sign that it was once a foam. Chilling a whipped evaporated milk foam makes it last longer.

as

Dried Milk If dried

Foams

milk solids are combined with

much

water than would be needed to reconsti-

less

tute the milk, the viscosit)' of the milk concentrate

is

sufficient to retain bubbles

of

air

whipped into it. Nonfat dry milk yields a more stable foam than does dry whole milk. A whipped milk foam made with nonfat dry milk resembles beaten egg white more than it does either whipped evaporated milk or whipped cream. Some of the protein in the milk is denatured in the foam so that the foam sets. The structure is fragile, however. Upon standing, liquid drains from around the gas cells, leaving behind fragile films of denatured protein. Lemon juice beaten into a milk foam increases its stability. Increasing the viscosity

of a milk concentrate by dispersing in

gums lemon

(algin, karaya, locust

juice or

it

either gelatin or

bean, or tragacanth) gives a

one of a number of vegetable stable foam. Addition of

more

calcium sucrate also increases the viscosity of milk and improves the foam.

Whipped Cream In contrast to milks,

thickens

when

air

cream with

whips to

sufficient fat

bubbles are whipped into

it,

as

a fine, fairly stable

the fo am of whipped cr eam ^ufe ni^he rigid but fragile structure of the fat

globides^Iigned at the periphery of an

air

sion electron micrograph of one air bubble or at the air/serum interface

The

shown

basic concept of why

% of i century ago availability fat

is

(2, 6).

foam

bubble make physical contact.

foam

results

A

when

transmis-

(FC) with milkfat globules aligned

cell

in Figure 19-5.

cream can be whipped

dispersed in a sol of milk proteins.

Convertmg

X^T Air

bubbles,

foam was arrived at almost process became possible with the

to a stiff

Elaboration of details of the

of more sophisticated instruments. Cream

ffiaTuivolveraction at interfaces

foam. Cream

does evaporated milk, but, in addition,

a two-phase_systemjofe mulsified _ cream to a foam is a dynamic process is

many of which

are incorporated early in

the whipping process, are thought to be stabilized temporarily by a protective film of

(3-

lactoglobulin, a-lactalbumin, and (B-casein. Milkfat globules are in the aqueous layers that

define the air bubbles. As beating or whipping continues, subdivision of air bubbles increases their surface areas

and the aqueous film

that separates the bubbles

becomes

thin-

ner and thinner. This has the effect of orienting the fat globules around the air bubbles. In

319 CHAPTER 19 MILK

^^^mM^:^ ^y-^. Transmission electron micrograph of a foam

Figure 19-5. cream. Milk

fat

}^

globules

(MFG) are aligned

which they protrude. Coalesced milk

fat

cell

(FC)

in

wfiipped

at the air/ serum interface through

globules

(CMFG) are

at the lower

left.

Casein micelles fCM) are stained dark. (Micrograph courtesy of C. V. Moor From 993. Reproduced by S. Y. Lee and C. V, Moor, Journal of Food Science 58:1 25, 1

permission.)

chilled cream, sharp crystals of fat disrupt the milkfat globule

membrane and

the outflow

from adjacent globules unites the globules. Breaks in the milk fat globpossible a network of clumped fat globules at the air-serum interface membrane make ule cells. As a result, the foam stiffens. Controlled disruption of the milk fat globaround foam

of unsolidified

ule

membrane,

fat

essential for stiffening a

butter. For this reason, the

foam,

is

amount of whipping

the

first

step in churning

cream

to

make

or beating should be closely monitored.

Sheaxing-a^itiffibam-wkh^e blades of^a_bea ter dislodges clumped fat_globules, resulting in collapsed foam cells anclloss ojloam volume. Additional beating may cause complete

coUapseofthe toam "anci

A number of factors important one

is

its

separation into butterfat and buttermilk.

influence the ease with which cream

is

converted to a foam.

An

the concentration of fat globules. Figure 19-6 illustrates the effect of the

percentage of fat on the volume,

stiffness,

and

stability

of whipped cream

(6).

Heavy whip-

cream, but the latter whips to a larger

ping cream whips in less time than regular whipping volume (1). Cream with less than 30 percent fat requires excessive whipping and more serum drains from the foam. Addition of lemon juice thickens the cream and can help compensate for fewer fat globules. Temperature of the cream when it is whipped should be no higher than 7° to 10°C (45° to 50°F). Chilling makes the cream more viscous, but, more important, it causes part of the f^n the globules to crystallize. Only when the fat is at least pardy solidified are the globules able to stiffen the foam. Presence of the enzyme that promotes clustering of fat globules and creaming also favors foam formation. This enzyme is denatured by the heat of pasteurization so pasteurized cream does not whip as readily as raw cream. The more severe heat treatment used to prolong the shelf life of ultrapasteurized whipping cream increases

whipping time even more.

320 PART

PROTEINS

VI

AND

PROTEIN-RICH

FOODS

Figure

1

9-6.

centage of

and

Effects of per-

on the volume of whipped cream

fat

stability

(cream pasteurized and aged

24 hours

at

4.4°C (40°F). (From

A. C. Dohiberg and fHening.

New

J.

C.

York Agricultural

Bulletin No. 113.1 925. Reproduced by per-

Experiment Station

25%

20%

mission.)

30%

35%

been homogenized must be whipped longer to form a stifF foam. The new fat globule membranes differ from those of the original membranes. As a result, the new globules are not so readily adsorbed at the air-liquid interface that surrounds the air bubbles and globule-globule clumping is reduced (1). With the inclusion of

Cream

that has

proteins of the

a

low molecular weight

whips to a foam that

is

surfactant, a fabricated product

homogenized with milk protein unhomogenized dairy

indistinguishable in structure from that of

clear, low molecular weight surfactants such as acylated modify the interaction of homogenized milk fat globules. Sugar added to whipped cream before it reaches maximum stiffness delays the clumping of fat. There is somewhat less danger of overbeating cream once the sugar is added, however. The whipped cream must be beaten longer to make it equally stiff. Instant-foaming cream as an aerosol packed under pressure is available. As well as

cream

and

(1).

For reasons that are not

lactylated

monoacyl

glycerols

cream the mixture contains sweetener, vegetable gum (usually carrageenan), low molecular weight surfactant, and flavoring. Nitrous oxide is usually used as propellant. Instantfoaming, pressure-packed, nondairy toppings are also available.

MILK PROTEINS AS EMULSIFIERS The proteins of milk function more hydrophobic P-casein

as emulsifiers

of food emulsions. Both a^i- and especially the

are effective emulsifiers (7).

Their high surface

possible their adsorption at an oil-water interface of an emulsion as interface of a foam.

Of the

two main whey

it

proteins, (3-lactoglobulin

does

is

activity at

makes

an air-water

more hydrophobic

a-lactalbumin (15). In addition to their surface activity, proteins form flexible films that contribute to the stability of an emulsion. Emulsions are discussed in Chapter 16.

than

is

EFFECTS OF HEAT Effects

The

ON MILK

on Casein Micelles

CHAPTER 19

coll oidally dispersed casein

milk.

321

Moderate heat

as

mice lles are

used in cooking

fails

relatively insensitive to heat at the

to alter the stability

for the protein to precipitate. In fact, sweet fluid

ing point before the ca.sein complex

is

destabilized

perature u sed to sterilize evaporated milk

Decrease in the acidic food

pH by

may

may be

enough

tomatoes

may

tomato

of the casein micelles enough

held for four hours at the boil-

for clotting to occur.

heat.

neutralize charges

Thus, milk that

is

on

the casein micelles so that they are sensitive

not obviously sour

be as low as 4.0.

A number of techniques

is

may curdle when it is heated. made. The pH of proces.sed

have been recommended to pro-

from the acid and so prevent curdling of tomato soup. Included

to the

The higl^em-

destabilize the casein micelles, however.

Milk sometimes curdles when cream of tomato soup tect casein

of fresh

of milk by the action of lactic acid bacteria or by combination with an

may sufficiently

to denaturation

milk

pH

are

adding the

milk rather than the opposite, having both milk and tomato hot when com-

bined and serving the soup promptly, and thickening either the tomato juice or milk before they are If

combined.

cooked tomatoes are thickened with

a paste

made from

flour

and milk or cream, is com-

the acid from the tomatoes causes the milk to curdle as soon as the cold paste

bined with the hot tomatoes. As cooking continues, however, the curds disappear. great surplus of acid

from the high proportion of tomato to milk causes a

charge on the protein molecules of the milk, making

it

possible for

them

The

reversal

of

to be again col-

loidally dispersed.

Milk or cream used dehydrating ing syrup

all

When

effect

no doubt contribute

milk

is

nolic substances, the milk

is

is

Acid from brown sugar, the

to the instability of the casein.

may

curdle.

These polyphenolic compounds

mouth. The precipitation of the proteins

in

sometimes

re-

milk by polyphenolic compounds

The curdling of cream of asparagus soup and of scalloped

pota-

attributed to the presence of polyphenolic constituents in these foods. Salt in high con-

centration can cause milk proteins to precipitate, for example, cured

Effects

When

are

Foods that contain such compounds produce an astringent, puckery sen-

attributed to dehydration.

toes

curdles.

heated with foods that contain appreciable quantities of certain polyphe-

ferred to as tannins.

sation in the

make caramels sometimes

to

of the high concentration of sugar, and the high temperature in the boil-

ham baked

in milk.

on Skin Formation

milk

is

heated in an uncovered utensil, a skin forms on the surface. This

is

attrib-

uted to evaporation of water from the surface and concentration of casein that occludes

some milk fat and calcium salts. If milk is heated uncovered for some time and the skin removed as it forms, appreciable amounts of milk solids are removed from the milk. Another is that it tends to hold steam and thus makes the on the surface of hot milk minimizes the formation of a skin. It is for this reason that hot cocoa and hot chocolate are served topped with marshmallow or whipped cream or the beverage itself is whipped to induce a foam.

disadvantage of the formation of a skin

milk more

Effects

likely to boil over.

A foam

on Serum Proteins

Heat denatures the serum proteins of fresh milk, making them precipitate (20). Inactivation of phosphatase by the mild heat treatment used to pasteurize milk has been

MILK

322

mentioned. The enzyme

PART PROTEINS

VI

AND

PROTEIN-RICH

FOODS

lipase, a protein,

enzyme would

is

inactivated during pasteurization also.

Were

it

homogenized milk (14). The factor (or factors) in imheated milk that causes slackness and stickiness in yeast bread dough and low loaf volume (Chapter 14) requires more drastic treatment for denaturation than do most of the other serum proteins of milk. In the preparation of a number of food products milk is preheated before it is combined with other ingredients. When milk is heated, the denatured and coagulated serum not, this

proteins settle to the

phosphate that

is

rapidly bring about hydrolytic rancidity in

bottom of the container, carrying some of the

precipitated by heat too. SeLtling of thisp recipiratf

is

colloidal calcium

ori£otjhg-F€asans

milk s£orcbe^4p readily whenji-is^xpose cj ro high^hpar Milk should be heated

at a

mod-

pan with a thick bottom. more heat sensitive than is a-

erate or slightly lower setting in a steam-jacketed vessel or in a

Of the two main serum

proteins, P-lactoglobulin

is

lactalbumin. Both contain disulfide groups, but P-lactoglobulin also contains a reactive sulfhydryl group. Sulfur-containing

and methyl

sulfide,

serum proteins

of hydrogen sulfide

are the source

major components of the flavor of heated milk

(22). Reaction prod-

ucts of protein with the lactose of milk contribute to the flavor of heated milk, as well as its color.

Also contributing to the cooked

calactone, a

compound with

flavor,

but from the heated

fat, is

a buttery, coconutlike flavor characteristic of foods

8-de-

cooked

in butter (22).

I HANDLING MILK AND FOODS MADE WITH MILK Holding Temperature shelf life of fluid milk can be prolonged by keeping it at 4°C (40°F) or lower. Milk and foods made with high proportions of milk, such as whipped cream, sauces, puddings, and soft pie fillings, not only nourish humans but also provide good media tor the growth of microorganisms. Such foods have been implicated in a number of cases of illness (Chapter 4). One microorganism that is widely distributed is Staphylococcus aureus. Food

The

may become contaminated by contact with work surfaces, utensils, and hands. If such food is held in a warm place, these microorganisms grow and in the process develop a metabolic product that

toxic to

is

humans. Consumption of the food that contains the enterotoxin of variable severity and duration. Prompt cooling of such

results in gastrointestinal upsets

foods to at least

Even

isms.

so,

4°C (40°F)

will limit the

growth of these (and most other) microorgan-

such foods should be eaten soon after they are prepared for best quality.

Exposure to Light Light light

may have

a deleterious effect

on two vitamins

Milk exposed

in milk.

to fluorescent

of an intensity of 300 foot-ca ndles at a temperature of 4.4°C (40°F) for 48 hours loses

approximately

1 1

percent of it^