Dairy Science and Technology Second Edition doc

Food Emulsions: Third Edition, Revised and Expanded, edited by Stig E.. Listeria , Listeriosis, and Food Safety: Second Edition, Revised and Expanded, edited by Elliot T.. Food Analysis

Dairy Science

and Technology

Second Edition

A Series of Monographs, Textbooks, and Reference Books

Editorial Advisory Board

Gustavo V Barbosa-Cánovas Washington State University–Pullman

P Michael Davidson University of Tennessee–Knoxville Mark Dreher McNeil Nutritionals, New Brunswick, NJ

Richard W Hartel University of Wisconsin–Madison

Lekh R Juneja Taiyo Kagaku Company, Japan Marcus Karel Massachusetts Institute of Technology

Ronald G Labbe University of Massachusetts–Amherst Daryl B Lund University of Wisconsin–Madison

David B Min The Ohio State University Leo M L Nollet Hogeschool Gent, Belgium Seppo Salminen University of Turku, Finland James L Steele University of Wisconsin–Madison

John H Thorngate III Allied Domecq Technical Services, Napa, CA Pieter Walstra Wageningen University, The Netherlands John R Whitaker University of California–Davis

Rickey Y Yada University of Guelph, Canada

76 Food Chemistry: Third Edition, edited by Owen R Fennema

77 Handbook of Food Analysis: Volumes 1 and 2, edited by

Leo M L Nollet

78 Computerized Control Systems in the Food Industry, edited by Gauri S Mittal

79 Techniques for Analyzing Food Aroma, edited by Ray Marsili

80 Food Proteins and Their Applications, edited by

Srinivasan Damodaran and Alain Paraf

81 Food Emulsions: Third Edition, Revised and Expanded, edited by Stig E Friberg and Kåre Larsson

82 Nonthermal Preservation of Foods, Gustavo V Barbosa-Cánovas, Usha R Pothakamury, Enrique Palou, and Barry G Swanson

83 Milk and Dairy Product Technology, Edgar Spreer

84 Applied Dairy Microbiology, edited by Elmer H Marth

and James L Steele

85 Lactic Acid Bacteria: Microbiology and Functional Aspects, Second Edition, Revised and Expanded, edited by

Seppo Salminen and Atte von Wright

86 Handbook of Vegetable Science and Technology: Production, Composition, Storage, and Processing, edited by D K Salunkhe and S S Kadam

87 Polysaccharide Association Structures in Food, edited by

Reginald H Walter

88 Food Lipids: Chemistry, Nutrition, and Biotechnology, edited by Casimir C Akoh and David B Min

89 Spice Science and Technology, Kenji Hirasa

and Mitsuo Takemasa

90 Dairy Technology: Principles of Milk Properties and Processes,

P Walstra, T J Geurts, A Noomen, A Jellema,

and M A J S van Boekel

91 Coloring of Food, Drugs, and Cosmetics, Gisbert Otterstätter

92 Listeria , Listeriosis, and Food Safety: Second Edition,

Revised and Expanded, edited by Elliot T Ryser

and Elmer H Marth

93 Complex Carbohydrates in Foods, edited by Susan Sungsoo Cho, Leon Prosky, and Mark Dreher

94 Handbook of Food Preservation, edited by M Shafiur Rahman

95 International Food Safety Handbook: Science, International Regulation, and Control, edited by Kees van der Heijden, Maged Younes, Lawrence Fishbein, and Sanford Miller

96 Fatty Acids in Foods and Their Health Implications: Second Edition, Revised and Expanded , edited by Ching Kuang Chow

97 Seafood Enzymes: Utilization and Influence on Postharvest Seafood Quality, edited by Norman F Haard

and Benjamin K Simpson

98 Safe Handling of Foods, edited by Jeffrey M Farber

and Ewen C D Todd

99 Handbook of Cereal Science and Technology: Second Edition, Revised and Expanded, edited by Karel Kulp

and Joseph G Ponte, Jr.

100 Food Analysis by HPLC: Second Edition, Revised and Expanded,

edited by Leo M L Nollet

101 Surimi and Surimi Seafood, edited by Jae W Park

102 Drug Residues in Foods: Pharmacology, Food Safety,

and Analysis, Nickos A Botsoglou and Dimitrios J Fletouris

103 Seafood and Freshwater Toxins: Pharmacology, Physiology, and Detection, edited by Luis M Botana

104 Handbook of Nutrition and Diet, Babasaheb B Desai

105 Nondestructive Food Evaluation: Techniques to Analyze

Properties and Quality, edited by Sundaram Gunasekaran

106 Green Tea: Health Benefits and Applications, Yukihiko Hara

107 Food Processing Operations Modeling: Design and Analysis,

edited by Joseph Irudayaraj

108 Wine Microbiology: Science and Technology, Claudio Delfini and Joseph V Formica

109 Handbook of Microwave Technology for Food Applications,

edited by Ashim K Datta and Ramaswamy C Anantheswaran

and Expanded, edited by Elmer H Marth and James L Steele

111 Transport Properties of Foods, George D Saravacos

and Zacharias B Maroulis

112 Alternative Sweeteners: Third Edition, Revised and Expanded,

edited by Lyn O’Brien Nabors

113 Handbook of Dietary Fiber, edited by Susan Sungsoo Cho and Mark L Dreher

114 Control of Foodborne Microorganisms, edited by Vijay K Juneja and John N Sofos

115 Flavor, Fragrance, and Odor Analysis, edited by Ray Marsili

116 Food Additives: Second Edition, Revised and Expanded,

edited by A Larry Branen, P Michael Davidson, Seppo Salminen, and John H Thorngate, III

117 Food Lipids: Chemistry, Nutrition, and Biotechnology: Second Edition, Revised and Expanded, edited by Casimir C Akoh and David B Min

118 Food Protein Analysis: Quantitative Effects on Processing,

R K Owusu- Apenten

119 Handbook of Food Toxicology, S S Deshpande

120 Food Plant Sanitation, edited by Y H Hui, Bernard L Bruinsma,

J Richard Gorham, Wai-Kit Nip, Phillip S Tong,

and Phil Ventresca

121 Physical Chemistry of Foods, Pieter Walstra

122 Handbook of Food Enzymology, edited by John R Whitaker, Alphons G J Voragen, and Dominic W S Wong

123 Postharvest Physiology and Pathology of Vegetables: Second Edition, Revised and Expanded, edited by Jerry A Bartz

and Jeffrey K Brecht

124 Characterization of Cereals and Flours: Properties, Analysis, and Applications, edited by Gönül Kaletunç

and Kenneth J Breslauer

125 International Handbook of Foodborne Pathogens, edited by Marianne D Miliotis and Jeffrey W Bier

126 Food Process Design, Zacharias B Maroulis

and George D Saravacos

127 Handbook of Dough Fermentations, edited by Karel Kulp

and Klaus Lorenz

128 Extraction Optimization in Food Engineering, edited by

Constantina Tzia and George Liadakis

129 Physical Properties of Food Preservation: Second Edition,

Revised and Expanded, Marcus Karel and Daryl B Lund

130 Handbook of Vegetable Preservation and Processing, edited by

Y H Hui, Sue Ghazala, Dee M Graham, K D Murrell,

and Wai-Kit Nip

131 Handbook of Flavor Characterization: Sensory Analysis,

Chemistry, and Physiology, edited by Kathryn Deibler

and Jeannine Delwiche

132 Food Emulsions: Fourth Edition, Revised and Expanded,

edited by Stig E Friberg, Kare Larsson, and Johan Sjoblom

133 Handbook of Frozen Foods, edited by Y H Hui, Paul Cornillon, Isabel Guerrero Legarret, Miang H Lim, K D Murrell,

and Wai-Kit Nip

134 Handbook of Food and Beverage Fermentation Technology,

edited by Y H Hui, Lisbeth Meunier-Goddik, Ase Solvejg Hansen, Jytte Josephsen,

Wai-Kit Nip, Peggy S Stanfield, and Fidel Toldrá

135 Genetic Variation in Taste Sensitivity, edited by John Prescott and Beverly J Tepper

136 Industrialization of Indigenous Fermented Foods: Second Edition, Revised and Expanded, edited by Keith H Steinkraus

137 Vitamin E: Food Chemistry, Composition, and Analysis,

Ronald Eitenmiller and Junsoo Lee

138 Handbook of Food Analysis: Second Edition, Revised

and Expanded, Volumes 1, 2, and 3, edited by Leo M L Nollet

139 Lactic Acid Bacteria: Microbiological and Functional Aspects: Third Edition, Revised and Expanded, edited by Seppo Salminen, Atte von Wright, and Arthur Ouwehand

140 Fat Crystal Networks, Alejandro G Marangoni

141 Novel Food Processing Technologies, edited by

Gustavo V Barbosa-Cánovas, M Soledad Tapia,

and M Pilar Cano

142 Surimi and Surimi Seafood: Second Edition, edited by

Jae W Park

143 Food Plant Design, edited by Antonio Lopez-Gomez;

Gustavo V Barbosa-Cánovas

144 Engineering Properties of Foods: Third Edition, edited by

M A Rao, Syed S.H Rizvi, and Ashim K Datta

145 Antimicrobials in Food: Third Edition, edited by

P Michael Davidson, John N Sofos, and A L Branen

146 Encapsulated and Powdered Foods, edited by Charles Onwulata

147 Dairy Science and Technology: Second Edition, Pieter Walstra, Jan T M Wouters and Tom J Geurts

CRC Press

Taylor & Francis Group

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© 2006 by Taylor & Francis Group, LLC

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Library of Congress Cataloging-in-Publication Data

Walstra, Pieter.

Dairy science and technology / Pieter Walstra, Jan T.M Wouters, T.J Geurts. 2nd ed

p cm (Food science and technology ; 146)

Rev ed of: Dairy technology / P Walstra … [et al.] c1999.

Includes bibliographical references.

ISBN 0-8247-2763-0 (alk paper)

1 Dairy processing 2 Milk 3 Dairy products I Wouters, Jan T M II Geurts, T J (Tom J.) III Dairy technology IV Title V Food science and technology (Taylor & Francis) ; 146.

SF250.5.D385 2005

Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Taylor & Francis Group

is the Academic Division of T&F Informa plc.

The primary theme of this book is the efficient transformation of milk into quality products This needs a thorough understanding of the composition andproperties of milk, and of the changes occurring in milk and its products duringprocessing and storage Moreover, knowledge of the factors that determine prod-uct quality, including health aspects and shelf life, is needed Our emphasis is on

high-the principles of physical, chemical, enzymatic, and microbial transformations.

Detailed manufacturing prescriptions and product specifications are not given, asthey are widely variable

Aimed at university food science and technology majors, the book is written

as a text, though it will also be useful as a work of reference It is assumed thatthe reader is familiar with the rudiments of food chemistry, microbiology, andengineering Nevertheless, several basic aspects are discussed for the benefit ofreaders who may be insufficiently acquainted with these aspects The book con-tains no references to the literature, but suggestions for further reading are given.The book is made up of four main parts Part I, “Milk,” discusses the chem-

istry, physics, and microbiology of milk Besides providing knowledge of theproperties of milk itself, it forms the basis for understanding what happens duringprocessing, handling and storage Part II, “Processes,” treats the main unit oper-

ations applied in the manufacture of milk products These are discussed in somedetail, especially the influence of product and process variables on the (interme-diate) product resulting A few highly specific processes, such as churning, arediscussed in product chapters In Part III, “Products,” integration of knowledge

of the raw material and of processing is covered for the manufacture of severalproducts The number of dairy products made is huge; hence, some product groupshave been selected because of their general importance or to illustrate relevantaspects Procedures needed to ensure consumer safety, product quality, and pro-cessing efficiency are also treated Part IV, “Cheese,” describes the processes and

transformations (physical, biochemical, and microbial) in the manufacture andripening of cheese Here, the processes are so specific and the interactions sointricate that a separate and integrated treatment is needed It starts with genericaspects and then discusses specific groups of cheeses

Several important changes have been introduced in this second edition Thereasons were, first, to improve the didactic quality of the book and, second, tomake it more useful as a reference source More basic and general aspects arenow treated, especially physicochemical and microbiological ones Part I hasbeen substantially enlarged, one reason why the title of the book has beenbroadened The nutritional aspects of milk components are now included, andthose of some products are enlarged A section on milk formation has been added

Naturally, the text has been updated Moreover, several parts have been nized or rewritten Factual information has been increased and partly moved to

reorga-an Appendix

Pieter Walstra Jan Wouters Tom Geurts

Wageningen, The Netherlands

First, we want to stress that much of the present book derives from the substantialcontributions that our then-coauthors, Ad Noomen, Arend Jellema, and Tiny vanBoekel, made to the first edition We are grateful that we could benefit from theirextensive expertise

Several people have provided information and advice Professors NormanOlson (University of Wisconsin, Madison), Marie Paulsson (Lund University, Swe-den), and Zdenko Puhan (Technical University, Zürich, Switzerland) scrutinized(parts of) the first edition and gave useful advice We consulted several colleaguesfrom our department, from NIZO Food Research (Ede, the Netherlands), andfrom the Milk Control Station (Zutphen, the Netherlands) We also receivedinformation from the following Dutch companies: Campina (Zaltbommel andWageningen), Carlisle Process Systems (formerly Stork, Gorredijk), FrieslandFoods (Deventer), and Numico (Wageningen) We thank all of the people involvedfor their cooperation and for the important information given

Part I

Milk

Chapter 1 Milk: Main Characteristics 3

1.1 Composition and Structure 3

1.1.1 Principal Components 3

1.1.2 Structural Elements 4

1.2 Milk Formation 7

1.3 Some Properties of Milk 11

1.4 Variability 12

1.5 Changes 13

Suggested Literature 16

Chapter 2 Milk Components 17

2.1 Lactose 17

2.1.1 Chemical Properties 17

2.1.2 Nutritional Aspects 19

2.1.3 Physicochemical Aspects 20

2.2 Salts 26

2.2.1 Composition and Distribution among the Phases 26

2.2.2 Properties of the Salt Solution 30

2.2.3 Colloidal Calcium Phosphate 32

2.2.4 Nutritional Aspects 33

2.2.5 Changes in Salts 33

2.3 Lipids 37

2.3.1 Constituent Fatty Acids 38

2.3.2 Lipid Classes 42

2.3.3 Nutritional Aspects 47

2.3.4 Autoxidation 48

2.3.5 Triglyceride Crystallization 52

2.4 Proteins 63

2.4.1 Chemistry of Proteins 63

2.4.2 Survey of Milk Proteins 72

2.4.3 Serum Proteins 76

2.4.4 Casein 79

2.4.5 Nutritional Aspects 83

2.5.1 Enzyme Activity 85

2.5.2 Some Milk Enzymes 88

2.5.3 Inactivation 92

2.6 Other Components 93

2.6.1 Natural Components 93

2.6.2 Contaminants 95

2.6.3 Radionuclides 97

2.7 Variability 98

2.7.1 Sources of Variability 98

2.7.2 Nature of the Variation 103

2.7.3 Some Important Variables 106

Suggested Literature 108

Chapter 3 Colloidal Particles of Milk 109

3.1 Basic Aspects 109

3.1.1 Surface Phenomena 111

3.1.2 Colloidal Interactions 118

3.1.3 Aggregation 122

3.1.4 Size Distributions 125

3.2 Fat Globules 127

3.2.1 Properties 127

3.2.2 Emulsion Stability 130

3.2.3 Interactions with Air Bubbles 134

3.2.4 Creaming 136

3.2.5 Lipolysis 139

3.3 Casein Micelles 140

3.3.1 Description 141

3.3.2 Changes 145

3.3.3 Colloidal Stability 150

3.3.4 Gel Formation and Properties 155

Suggested Literature 157

Chapter 4 Milk Properties 159

4.1 Solution Properties 159

4.2 Acidity 160

4.3 Redox Potential 162

4.4 Flavor 164

4.5 Density 166

4.6 Optical Properties 167

4.7 Viscosity 169

4.7.1 Some Fluid Rheology 169

4.7.2 Liquid Milk Products 173

Suggested Literature 174

Chapter 5 Microbiology of Milk 175

5.1 General Aspects 175

5.1.1 Microorganisms 175

5.1.2 Bacteria 176

5.1.3 Yeasts and Molds 179

5.1.4 Enumeration of Microorganisms 181

5.1.5 Growth 182

5.1.6 Milk as a Substrate for Microorganisms 187

5.2 Undesirable Microorganisms 190

5.2.1 Pathogenic Microorganisms 190

5.2.2 Spoilage Microorganisms 194

5.3 Sources of Contamination 197

5.3.1 Microbial Ecology 197

5.3.2 Microorganisms Present in the Udder 198

5.3.3 Contamination during and after Milking 199

5.4 Hygienic Measures 201

5.4.1 Protection of the Consumer against Pathogenic Microorganisms 202

5.4.2 Measures against Spoilage Organisms 202

Suggested Literature 203

Part II Processes Chapter 6 General Aspects of Processing 207

6.1 Introduction 207

6.2 Preservation Methods 209

6.3 Quality Assurance 212

6.3.1 Concepts 212

6.3.2 Hazard Analysis/Critical Control Points (HACCP) 214

6.3.3 Quality Assurance of Raw Milk 215

6.4 Milk Storage and Transport 217

6.4.1 Milk Collection and Reception 217

6.4.2 Milk Storage 218

6.4.3 Transport of Milk in the Dairy 221

6.5 Standardizing 222

Suggested Literature 223

Chapter 7 Heat Treatment 225

7.1 Objectives 225

7.2 Changes Caused by Heating 226

7.2.2 Reactions of Proteins 229

7.2.3 Reactions of Lactose 233

7.2.4 Heat Coagulation 236

7.3 Heating Intensity 242

7.3.1 Processes of Different Intensity 242

7.3.2 Kinetic Aspects 245

7.3.3 Inactivation of Enzymes 252

7.3.4 Thermobacteriology 255

7.4 Methods of Heating 263

7.4.1 Considerations 263

7.4.2 Equipment 265

7.4.3 Heat Regeneration 270

7.4.4 Control 271

Suggested Literature 272

Chapter 8 Centrifugation 273

8.1 Cream Separation 273

8.2 Removal of Particles 276

Suggested Literature 277

Chapter 9 Homogenization 279

9.1 Objectives 279

9.2 Operation of the Homogenizer 280

9.3 Effects of Turbulence 282

9.4 Factors Affecting Fat Globule Size 285

9.5 Surface Layers 287

9.6 Colloidal Stability 289

9.7 Homogenization Clusters 290

9.8 Creaming 292

9.9 Other Effects of Homogenization 293

9.10 Other Ways of Working 295

Suggested Literature 296

Chapter 10 Concentration Processes 297

10.1 General Aspects 297

10.1.1 Concentration of Solutes 297

10.1.2 Water Activity 300

10.1.3 Changes Caused by Concentrating 302

10.1.4 The Glassy State 303

10.1.5 Reaction Rates 304

10.2 Evaporating 307

10.3 Drying: General Aspects 314

10.3.1 Objectives 314

10.3.2 Drying Methods 316

10.4 Spray Drying 318

10.4.1 Drier Configuration 318

10.4.2 Atomization 319

10.4.3 Change of State of the Drying Air 322

10.4.4 Changes of State of the Drying Droplets 326

10.4.5 Two-Stage Drying 332

Suggested Literature 335

Chapter 11 Cooling and Freezing 337

11.1 Cooling 337

11.2 Freezing 338

Suggested Literature 340

Chapter 12 Membrane Processes 341

12.1 General Aspects 341

12.1.1 Types of Processes 341

12.1.2 Efficiency 343

12.1.3 Technical Operation 345

12.2 Ultrafiltration 346

12.2.1 Composition of the Retentate 346

12.2.2 Permeate Flux 349

12.3 Reverse Osmosis 351

12.4 Desalting 354

Suggested Literature 356

Chapter 13 Lactic Fermentations 357

13.1 Lactic Acid Bacteria 357

13.1.1 Taxonomy 357

13.1.2 Metabolism 360

13.1.3 Genetics 373

13.1.4 Bacteriocins 374

13.2 Acid Production 374

13.3 Bacteriophages 377

13.3.1 Phage Composition and Structure 377

13.3.2 Phage Multiplication 377

13.3.3 Phage Resistance Mechanisms 382

13.3.4 Inactivation 383

13.4 Ecological Aspects 384

13.5 Starters 385

13.5.2 Properties 388

13.5.3 Shifts in Flora 388

13.5.4 Traditional Starter Manufacture 390

13.5.5 Modern Starter Manufacture 394

Suggested Literature 396

Chapter 14 Fouling and Sanitizing 399

14.1 Deposit Formation 399

14.2 Cleaning 405

14.3 Disinfection 408

Suggested Literature 410

Chapter 15 Packaging 411

15.1 Distribution Systems 411

15.2 Packaging Materials 412

15.3 Filling Operation 415

Suggested Literature 417

Part III Products Chapter 16 Milk for Liquid Consumption 421

16.1 Pasteurized Milk 421

16.1.1 Manufacture 422

16.1.2 Shelf Life 427

16.1.3 Extended-Shelf-Life Milk 430

16.2 Sterilized Milk 431

16.2.1 Description 431

16.2.2 Methods of Manufacture 432

16.2.3 Shelf Life 436

16.3 Reconstituted Milks 437

16.4 Flavor 437

16.5 Nutritive Value 439

16.5.1 Modification of Composition 439

16.5.2 Loss of Nutrients 440

16.6 Infant Formulas 441

16.6.1 Human Milk 441

16.6.2 Formula Composition and Manufacture 444

Suggested Literature 444

Chapter 17 Cream Products 447

17.1 Sterilized Cream 447

17.1.1 Manufacture 448

17.1.2 Heat Stability 448

17.1.3 Stability in Coffee 448

17.1.4 Clustering 450

17.2 Whipping Cream 452

17.2.1 Desirable Properties 452

17.2.2 Manufacture 453

17.2.3 The Whipping Process 454

17.3 Ice Cream 458

17.3.1 Manufacture 459

17.3.2 Physical Structure: Formation and Stability 462

17.3.3 Role of the Various Components 465

Suggested Literature 466

Chapter 18 Butter 467

18.1 Description 467

18.2 Manufacture 468

18.2.1 Processing Scheme 468

18.2.2 The Churning Process 471

18.2.3 Working 474

18.3 Properties 478

18.3.1 Microstructure 478

18.3.2 Consistency 480

18.3.3 Cold Storage Defects 485

18.4 Cultured Butter from Sweet Cream 486

18.5 High-Fat Products 489

18.5.1 Anhydrous Milk Fat 489

18.5.2 Modification of Milk Fat 490

18.5.3 Recombined Butter 492

18.5.4 Low-Fat Butter Products 494

Suggested Literature 495

Chapter 19 Concentrated Milks 497

19.1 Evaporated Milk 497

19.1.1 Manufacture 497

19.1.2 Product Properties 501

19.1.3 Heat Stability 502

19.1.4 Creaming 504

19.1.5 Age Thickening and Gelation 505

19.2 Sweetened Condensed Milk 507

19.2.2 Keeping Quality 509

Suggested Literature 512

Chapter 20 Milk Powder 513

20.1 Objectives 513

20.2 Manufacture 514

20.3 Hygienic Aspects 517

20.3.1 Bacteria in the Original Milk 517

20.3.2 Growth during Manufacture 519

20.3.3 Incidental Contamination 521

20.3.4 Sampling and Checking 521

20.4 Powder Characteristics 522

20.4.1 The Particle 522

20.4.2 Extractable Fat 522

20.4.3 Free-Flowingness 523

20.4.4 Specific Volume 525

20.4.5 Dissolution 526

20.4.6 WPN Index 529

20.4.7 Flavor 530

20.4.8 Conclusions 531

20.5 Deterioration 531

20.6 Other Types of Milk Powder 535

Suggested Literature 535

Chapter 21 Protein Preparations 537

21.1 Manufacture 538

21.1.1 Casein 539

21.1.2 Whey Protein 540

21.1.3 Other Products 542

21.2 Functional Properties 543

21.2.1 Solution Properties 544

21.2.2 Gels 546

21.2.3 Emulsions 548

21.2.4 Foams 549

Suggested Literature 550

Chapter 22 Fermented Milks 551

22.1 General Aspects 551

22.2 Types of Fermented Milks 552

22.2.1 Mesophilic Fermentation 552

22.2.2 Thermophilic Fermentation 553

22.2.3 Yeast–Lactic Fermentation 555

22.2.4 Molds in Lactic Fermentation 557

22.3 Cultured Buttermilk 557

22.4 Yogurt 558

22.4.1 The Yogurt Bacteria 559

22.4.2 Manufacture 562

22.4.3 Physical Properties 565

22.4.4 Flavor Defects and Shelf Life 568

22.5 Nutritional Aspects 569

22.5.1 Composition 569

22.5.2 Nutritional Value 570

22.5.3 Probiotics 571

22.5.4 Prebiotics 572

Suggested Literature 573

Part IV Cheese Chapter 23 Principles of Cheese Making 577

23.1 Introduction 577

23.2 Essential Process Steps 579

23.3 Changes Occurring 580

Suggested Literature 582

Chapter 24 Cheese Manufacture 583

24.1 Milk Properties and Pretreatment 583

24.1.1 The Raw Milk 583

24.1.2 Milk Treatment 584

24.2 Starters 586

24.3 Enzyme-Induced Clotting 588

24.3.1 Enzymes Used 588

24.3.2 The Enzyme-Catalyzed Reaction 590

24.3.3 Aggregation 591

24.3.4 Gel Formation 593

24.3.5 The Renneting Time 594

24.3.6 Clotting of Heat-Treated Milk 596

24.4 Curd Making 596

24.4.1 Clotting 597

24.4.2 Accumulation of Various Components 600

24.4.3 Concentrating before Clotting 601

24.4.4 Syneresis 603

24.4.5 Acid Production and Washing 608

24.4.6 Separation of Curd and Whey 610

24.6 Salting 615

24.6.1 Mass Transport during Salting 616

24.6.2 Important Variables 622

24.6.3 Distribution of Salt and Water after Salting 624

24.7 Curing, Storage, and Handling 625

24.7.1 Temperature 626

24.7.2 Air Conditions 627

24.7.3 Rind Treatment 627

24.7.4 Packaging 630

24.8 Cheese Composition and Yield 631

24.8.1 Variables Involved 632

24.8.2 Yield 636

24.8.3 Standardizing the Milk 638

Suggested Literature 638

Chapter 25 Cheese Ripening and Properties 641

25.1 Lactic Fermentation 641

25.2 Enzyme Sources 642

25.3 Proteolysis 644

25.3.1 Methods of Characterization 644

25.3.2 Milk Proteinases 645

25.3.3 Clotting Enzymes 646

25.3.4 Enzymes of Lactic Acid Bacteria 648

25.3.5 Enzymes of Nonstarter Organisms 650

25.3.6 Interaction between Enzyme Systems 650

25.3.7 Ultrafiltration of Cheese Milk 651

25.4 Lipolysis 653

25.5 Development of Flavor 654

25.5.1 Description 654

25.5.2 Formation of Flavor Compounds 655

25.6 Development of Texture 659

25.6.1 Structure 659

25.6.2 Consistency 661

25.7 Accelerated Ripening 669

25.8 Nutritive Value and Safety 672

Suggested Literature 675

Chapter 26 Microbial Defects 677

26.1 Coliform Bacteria 679

26.2 Butyric Acid Bacteria 680

26.3 Lactobacilli 683

26.4 Heat-Resistant Streptococci 684

26.5 Propionic Acid Bacteria 684

26.6 Organisms on the Rind 685

26.7 Other Aspects 686

Suggested Literature 686

Chapter 27 Cheese Varieties 687

27.1 Overview 687

27.1.1 Variations in Manufacture 688

27.1.2 Types of Cheese 694

27.2 Fresh Cheese 696

27.2.1 Quarg 697

27.2.2 Cottage Cheese 699

27.3 Gouda-Type Cheeses 702

27.3.1 Manufacture 702

27.3.2 Properties and Defects 709

27.4 Cheddar-Type Cheeses 712

27.4.1 Manufacture 712

27.4.2 Properties 716

27.5 Swiss and Pasta-Filata Types 718

27.5.1 Emmentaler 719

27.5.2 Mozzarella 722

27.6 Cheeses with a Specific Flora 724

27.6.1 Soft Cheese with Surface Flora 725

27.6.2 Blue-Veined Cheese 734

27.7 Processed Cheese 737

Suggested Literature 739

Part V Appendix Appendix A.1 Often-Used Symbols 743

A.2 Abbreviations 745

A.3 Conversion Factors 746

A.4 Physical Properties of Milk Fat 747

A.5 Amino Acid Composition of Milk Proteins 748

A.6 Amino Acid Sequences of Caseins 750

A.8 Trace Elements in Cows’ Milk 755

A.9 Physical Properties of Milk and Milk Products 756

A.10 Mass Density and Viscosity of Some Milk

Fractions 757

A.11 Heat Transfer 758

A.12 Data on Some Cheese Varieties 762

Part I

Milk

be used for the ‘normal’ milk of healthy cows, unless stated otherwise ally, a com-parison will be made with human milk.

Occasion-This chapter is meant as a general introduction Nearly all that is mentioned —with the exception of parts of Section 1.2 — is discussed in greater detail in otherchapters However, for readers new to the field it is useful to have some idea of theformation, composition, structure, and properties of milk, as well as the variation —including natural variation and changes due to processing — that can occur inthese characteristics, before starting on the main text

1.1 COMPOSITION AND STRUCTURE

to the taste of milk More information on milk composition is given in Table 1.3

Lactose or milk sugar is the distinctive carbohydrate of milk It is a

disac-charide composed of glucose and galactose Lactose is a reducing sugar

The fat is largely made up of triglycerides, constituting a very complicated

mixture The component fatty acids vary widely in chain length (2 to 20 carbonatoms) and in saturation (0 to 4 double bonds) Other lipids that are present includephospholipids, cholesterol, free fatty acids, monoglycerides, and diglycerides

About four fifths of the protein consists of casein, actually a mixture of four

proteins: αS1-, αS2-, β-, and κ-casein The caseins are typical for milk and have somerather specific properties: They are to some extent phosphorylated and have little

or no secondary structure The remainder consists, for the most part, of the milkserum proteins, the main one being β-lactoglobulin Moreover, milk contains nume-

rous minor proteins, including a wide range of enzymes.

The mineral substances — primarily K, Na, Ca, Mg, Cl, and phosphate —

are not equi-valent to the salts Milk contains numerous other elements in trace

4 Milk: Main Characteristics

quantities The salts are only partly ionized The organic acids occur largely as ions or as salts; citrate is the principle one Furthermore, milk has many miscel-

laneous components, often in trace amounts.

The total content of all substances except water is called the content of dry

matter Furthermore, one distinguishes solids-not-fat and the content of fat in thedry matter

The chemical composition of milk largely determines its nutritional value; theextent to which microorganisms can grow in it; its flavor; and the chemicalreactions that can occur in milk The latter include reactions that cause off-flavours

1.1.2 S TRUCTURAL E LEMENTS

Structure can be defined as the geometrical distribution of the (chemical) nents in a system It may imply, as it does in milk, that the liquid contains particles.This can have important consequences for the properties of the system Forinstance, (1) chemical components are present in separate compartments, whichcan greatly affect their reactivity; (2) the presence of particles greatly affects somephysical properties, like viscosity and optical appearance; (3) interaction forcesbetween particles generally determine the physical stability of the system; and (4)the separation of some components (fat and casein) is relatively easy

is schematic and incomplete Some properties of the structural elements are given

Note: Typical for milks of lowland breeds.

a These values will rarely be exceeded, e.g., in 1 to 2% of samples of separate milkings of healthy individual cows, excluding colostrum and milk drawn shortly before parturition.

b Nonprotein nitrogen compounds not included.

in Table 1.2, again in a simplified form; the numerical data mentioned are meantonly to define orders of magnitude The table clearly shows that aspects of colloidchemistry are essential for understanding the properties of milk and the manychanges that can occur in it All particles exhibit Brownian motion; they have anelectrostatic charge, which is negative at the pH of milk Their total surface area

is large

Fat globules To a certain extent, milk is an oil-in-water emulsion But the

fat globules are more complicated than emulsion droplets In particular, the

surface layer or membrane of the fat globule is not an adsorption layer of one

FIGURE 1.1 Milk viewed at different magnifications, showing the relative size of

struc-tural elements (A) Uniform liquid However, the liquid is turbid and thus cannot be homogeneous (B) Spherical droplets, consisting of fat These globules float in a liquid (plasma), which is still turbid (C) The plasma contains proteinaceous particles, which are casein micelles The remaining liquid (serum) is still opalescent, so it must contain other particles The fat globules have a thin outer layer (membrane) of different constitution.

(From H Mulder and P Walstra, The Milk Fat Globule, Pudoc, Wageningen, 1974.)

Fat globule

Serum Membrane

Casein micelles

6 Milk: Main Characteristics

single substance but consists of many components; its structure is complicated.The dry mass of the membrane is about 2.5% of that of the fat A small part ofthe lipids of milk is found outside the fat globules At temperatures below 35°C,part of the fat in the globules can crystallize Milk minus fat globules is called

milk plasma, i.e., the liquid in which the fat globules float.

Casein micelles consist of water, protein, and salts The protein is casein.

Casein is present as a caseinate, which means that it binds cations, primarilycalcium and magnesium The other salts in the micelles occur as a calciumphosphate, varying somewhat in composition and also containing a small amount

of citrate This is often called colloidal phosphate The whole may be calledcalcium-caseinate/calcium-phosphate complex The casein micelles are notmicelles in the colloid-chemical sense but just ‘small particles.’ The micelles have

TABLE 1.2

Properties of the Main Structural Elements of Milk

Milk Plasma

Serum

Globular Proteins

Lipoprotein Particles

salts

Serum protein Lipids, proteins

To be considered as Emulsion Fine dispersion Colloidal

solution

Colloidal dispersion Content

centrifuge

Ultrafiltration Ultrafiltration Diffusion rate

(mm in 1h) a

Note: Numerical values are approximate averages.

a For comparison, most molecules in solution are 0.4 to 1 nm diameter, and diffuse, say, 5 mm

in 1 h 1 mm = 10 3 µ m = 10 6 nm = 10 7 Å.

an open structure and, accordingly, contain much water, a few grams per gram

of casein Milk serum, i.e., the liquid in which the micelles are dispersed, is milk

minus fat globules and casein micelles

Serum proteins are largely present in milk in molecular form or as very small

aggregates

Lipoprotein particles, sometimes called milk microsomes, vary in quantity

and shape Presumably, they consist of remnants of mammary secretory cellmembranes Few definitive data on lipoprotein particles have been published

Cells, i.e., leukocytes, are always present in milk They account for about

0.01% of the volume of milk of healthy cows Of course, the cells contain allcytoplasmic components such as enzymes They are rich in catalase

1.2 MILK FORMATION

Milk components are for the most part formed in the mammary gland (the udder)

of a cow, from precursors that are the results of digestion

Digestion Mammals digest their food by the use of enzymes to obtain simple,

soluble, low-molar-mass components, especially monosaccharides; small tides and amino acids; and fatty acids and monoglycerides These are taken up

pep-in the blood, together with other nutrients, such as various salts, glycerol, organicacids, etc The substances are transported to all the organs in the body, includingthe mammary gland, to provide energy and building blocks (precursors) formetabolism, including the synthesis of proteins, lipids, etc

In ruminants like the cow, considerable predigestion occurs by means ofmicrobial fermentation, which occurs for the most part in the first stomach or

rumen The latter may be considered as a large and very complex bio-fermenter.

It contains numerous bacteria that can digest cellulose, thereby breaking downplant cell walls, providing energy and liberating the cell contents From celluloseand other carbohydrates, acetic, propionic, butyric and lactic acid are formed, whichare taken up in the blood The composition of the organic acid mixture depends

on the composition of the feed Proteins are broken down into amino acids Therumen flora uses these to make proteins but can also synthesize amino acids fromlow-molar-mass nitrogenous components Further on in the digestive tract themicrobes are digested, liberating amino acids Also, food lipids are hydrolyzed inthe rumen and partly metabolized by the microorganisms All these precursors canreach the mammary gland

Milk Synthesis The synthesis of milk components occurs for the greater

part in the secretory cells of the mammary gland Figure 1.2 illustrates such acell At the basal end precursors of milk components are taken up from the blood,and at the apical end milk components are secreted into the lumen Proteins areformed in the endoplasmic reticulum and transported to the Golgi vesicles, inwhich most of the soluble milk components are collected The vesicles grow insize while being transported through the cell and then open up to release theircontents in the lumen Triglycerides are synthesized in the cytoplasm, forming

others 10 mg proteose peptone +

Ca, bound 300 mg Gases Nonprotein nitrogenous

Ca, ions 90 mg oxygen 6 mg compounds

bicarbonate 100 mg sterols 15 mg acid phosphatase

Zn 3 mg Vitamins, e.g Phosphoric esters ~300 mg

Fe 120 µ g riboflavin 2 mg Others

Cu 20 µ g ascorbic acid 20 mg many others

a Approximate average quantities in 1 kg milk Note: The water in the casein micelles contains some small-molecule solutes.

10 mg

60 mg

60 mg

100 mg 0.3 mg

lipase plasmin

Ca phosphate citrate

K, Mg, Na

Protein

Salts

Water Enzymes

850 mg

1000 mg

150 mg

26 g +

~80 g

MEMBRANE

LIPOPROTEIN PARTICLE

lipids protein enzymes water

enzymes alkaline phosphatase xanthine oxidase many others

small globules, which grow while they are transported to the apical end of thecell They become enrobed by the outer cell membrane (or plasmalemma) while

being pinched off into the lumen This type of secretion is called merocrine,

which means that the cell remains intact

Most are synthesized in the cell Others are taken up from the blood but, generally,not in the same proportion as in the blood; see, especially, the salts This meansthat the cell membranes have mechanisms to reject, or allow passage of, specificcomponents Some substances, notably water and small lipophilic molecules, can

FIGURE 1.2 Stylized diagram of a mammary secretory cell Below is the basal part, on

top the apical part of the cell The cell is bounded by other secretory cells to form the

glandular epithelium See text for further details (From P Walstra and R Jenness, Dairy Chemistry and Physics, Wiley, New York, 1984 With permission.)

Ribosomes

Basement membrane

Golgi apparatus Cytosol

Lysosome

Outer cell membrane (plasmalemma)

Golgi vesicle with casein micelles

TABLE 1.4

Synthesis of Important Milk Components

Name

Concentration

Concentration (% w/w)

In the Secretory Cell?

Specific for Milk?

Specific for the Species?

Lipids

0 01

0 006

pass the cell more or less unhindered Some other components, such as serumalbumin and chlorides, can ‘leak’ from the blood into the milk by passing throughthe spaces between secretory cells Also, some leukocytes somehow reach thelumen Finally, cell remnants, such as part of the microvilli depicted in Figure 1.2

and tiny fragments of cytoplasm that occasionally adhere to a fat globule, aresecreted and form the lipoprotein particles of Table 1.2

Excretion The glandular epithelium, consisting of layers of secretory cells,

form spherical bodies called alveoli Each of these has a central lumen into which

the freshly formed milk is secreted From there, the milk can flow through small

ducts into larger and still larger ones until it reaches a cavity called the cistern.

From the cistern, the milk can be released via the teat A cow has four teats andhence four separate mammary glands, commonly called (udder) quarters.Excretion of the milk does not happen spontaneously The alveoli have tocontract, which can be achieved by the contraction of muscle tissue around thealveoli Contraction is induced by the hormone oxytocin This is released intothe blood by stimulation of the teats of the animal, be it by the suckling young

or by the milker The udder is not fully emptied

Lactation When a calf is born, lactation — i.e., the formation and secretion

of milk — starts The first secretion greatly differs in composition from milk (seeSubsection 2.7.1.5) Within a few days the milk has become normal and milk yieldincreases for some months, after which it declines The yield greatly varies amongcows and with the amount and the quality of the feed taken by the cow For milchcows, milking is generally stopped after about 10 months, when yield has becomequite low The duration from parturition to leaving the cow dry is called thelactation period, and the time elapsed after parturition is the stage of lactation

1.3 SOME PROPERTIES OF MILK

Milk as a Solution Milk is a dilute aqueous solution and behaves

accord-ingly Because the dielectric constant is almost as high as that of pure water, polarsubstances dissolve well in milk and salts tend to dissociate (although this dis-sociation is not complete) The ionic strength of the solution is about 0.073 M The

pH of milk is about 6.7 at room temperature The viscosity is low, about twice that

of water, which means that milk can readily be mixed, even by convection currentsresulting from small temperature fluctuations The dissolved substances give milk

an osmotic pressure of about 700 kPa (7 bar) and a freezing-point depressionclose to 0.53 K The water activity is high, about 0.995 Milk density (ρ20) equalsabout 1029 kg⋅m−3 at 20°C; it varies especially with fat content

Milk as a Dispersion Milk is also a dispersion; the particles involved are

summarized in Table 1.2 This has several consequences, such as milk beingwhite The fat globules have a membrane, which acts as a kind of barrier betweenthe plasma and the core lipids The membrane also protects the globules againstcoalescence The various particles can be separated from the rest

The fat globules can be concentrated in a simple way by creaming, which

either occurs due to gravity or — more efficiently — is induced by centrifugation

12 Milk: Main Characteristics

In this way cream and skim milk are obtained Skim milk is not identical to milkplasma, though quite similar, because it still contains some small fat globules.Cream can be churned, leading to butter and buttermilk; the latter is rather similar

in composition to skim milk

Likewise, casein micelles can be concentrated and separated from milk, for

instance, by membrane filtration The solution passing through the membrane isthen quite similar to milk serum If the pores in the membrane are very small, alsothe serum proteins are retained When adding rennet enzyme to milk, as is done

in cheese making, the casein micelles start to aggregate, forming a gel; whencutting the gel into pieces, these contract, expelling whey Whey is also similar

to milk serum but not quite, because it contains some of the fat globules and part

of the κ-casein split off by the enzyme Casein also aggregates and forms a gelwhen the pH of the milk is lowered to about 4.6

Moreover, water can be removed from milk by evaporation Altogether, a

range of liquid milk products of various compositions can be made Some ples are given in Table 1.5

exam-Flavor The flavor of fresh milk is fairly bland The lactose produces some

sweetness and the salts some saltiness Several small molecules present in verysmall quantities also contribute to flavor The fat globules are responsible for thecreaminess of whole milk

Nutritional value Milk is a complete food for the young calf, and it can

also provide good nutrition to humans It contains virtually all nutrients, most ofthese in significant quantities However, it is poor in iron and the vitamin Ccontent is not high It contains no antinutritional factors, but it lacks dietary fibre

Milk as a Substrate for Bacteria Because it is rich in nutrients, many

microorganisms, especially bacteria, can grow in milk Not all bacteria that needsugar can grow in milk, some being unable to metabolize lactose Milk is poor

in iron, which is an essential nutrient for several bacteria, and contains someantibacterial factors, such as immunoglobulins and some enzyme systems More-over, milk contains too much oxygen for strictly anaerobic bacteria Altogether,the growth of several bacteria is more or less restricted in raw milk, but severalothers can proliferate, especially at high ambient temperatures

1.4 VARIABILITY

Freshly drawn milk varies in composition, structure, and properties Even within themilk from a single milking of one cow, variation can occur The fat globules vary

in size and, to some extent, in composition, and the same applies to casein micelles

Natural Variation The main factors responsible for natural variation in milk

are the following:

• Genetic factors: Breed and individual.

• The stage of lactation: This can have a significant effect Especially

the milk obtained within 2 or 3 d after parturition tends to have a very

different composition; it is called colostrum or beestings.

• Illness of the cow: Especially severe mastitis (inflammation of the

udder) can have a relatively large effect The milk tends to have anincreased content of somatic cells

• Feed: The amount and the quality of the feed given strongly affect

milk yield However, the effect of the cow’s diet on milk composition

is fairly small, except for milk fat content and composition

In a qualitative sense, cows’ milk is remarkably constant in composition.Nevertheless, individual milkings show significant differences in composition.The variation is small in milk processed at the dairy, because this consists ofmixtures of the milk of a large number of cows from many farms

Other Causes As soon as the milk leaves the udder, it becomes contaminated,

for instance, with oxygen and bacteria (milk within the udder of a healthy cowtends to be sterile) Contamination with other substances can occur The temper-ature of the milk generally decreases These factors can lead to changes in milkproperties Far greater changes occur during long storage and in milk processing(see the next section)

1.5 CHANGES

Milk is not a system in equilibrium It changes even while in the udder This ispartly because different components are formed at various sites in the mammarysecretory cell and come into contact with one another after their formation.Furthermore, several changes can occur due to the milking, the subsequent low-ering of the temperature, and so on Changes may be classified as follows:

1 Physical changes occurring, for instance, when air is incorporated

during milking: Because of this, additional dissolution of oxygen andnitrogen occurs in milk Moreover, a new structural element is formed:air bubbles Milk contains many surface-active substances, predomi-nantly proteins, which can become attached to the air–water interfaceformed Furthermore, by contact with the air bubbles, fat globules maybecome damaged, i.e., lose part of their membrane Fat globules maycream Creaming is most rapid at low temperature because the globulesaggregate to large flocs during the so-called cold agglutination (Sub-section 3.2.4) On cooling, part of the milk fat starts to crystallize, themore so at a lower temperature But even at 0°C part of the fat remainsliquid The presence of fat crystals can strongly diminish the stability

of fat globules against clumping

2 Chemical changes may be caused by the presence of oxygen: Several

substances may be oxidized In particular, light may induce reactions,often leading to off-flavors Composition of salts can vary, for example,with temperature

3 Biochemical changes can occur because milk contains active enzymes:

Examples are lipase, which causes lipolysis; proteinases, which cause

14 Milk: Main Characteristics

proteolysis; and phosphatases, which cause hydrolysis of phosphoricacid esters

4 Microbial changes are often the most conspicuous: The best-known

effect is production of lactic acid from lactose, causing an obviousdecrease in pH Numerous other changes, such as lipolysis and pro-teolysis, may result from microbial growth

Cooling of the milk to about 4°C is generally applied to inhibit many of thechanges mentioned, especially growth of microorganisms and enzyme action Inmany regions, the milk is already cooled at the farm, directly after milking, in aso-called bulk tank The milk should be kept cold during transport to the dairyand subsequent storage

Processing At the dairy milk is always processed Of course, this causes changes

in composition and properties of the milk, as it is intended to do These changes can

be drastic, as the following examples will show, and it can be questioned whether theresulting product can still be called milk; however, it is standard practice to do so.The most common processes applied are briefly described in the following paragraphs

Heat treatment is virtually always applied, primarily to kill harmful bacteria.

It also causes numerous chemical and other changes, the extent of which depends

on temperature and duration of heating Low pasteurization (e.g., 15 s at 74°C)

is a fairly mild treatment that kills most microorganisms and inactivates someenzymes but does not cause too many other changes High pasteurization (e.g.,

15 s at 90°C, but varying widely) is more intense; all vegetative microorganismsare killed, most enzymes are inactivated, and part of the serum proteins becomeinsoluble Sterilization (e.g., 20 min at 118°C) is meant to kill all microorganisms,including spores; all enzymes are inactivated; numerous chemical changes, such asbrowning reactions, occur; and formic acid is formed UHT (ultrahigh-temperature)heating (e.g., at 145°C for a few seconds) is meant to sterilize milk while mini-mizing chemical changes; even some enzymes are not inactivated fully

Separation, usually by means of a flow-through centrifuge called a cream

sep-arator, yields skim milk and cream The skim milk has a very low fat content, 0.05

to 0.08% Milk skimmed after gravity creaming has a much higher fat content

Unless stated otherwise, the term skim milk will refer to centrifugally separated milk.

By mixing skim milk and cream, milk may be standardized to a desired fat content

Homogenization (i.e., treatment in a high-pressure homogenizer) of milk leads

to a considerable reduction in fat globule size Such milk creams very slowly but

is also altered in other respects All types of sterilized milk or, more generally,all long-life liquid milk products are homogenized in practice

Evaporation removes water, producing milk that is more concentrated Many

properties are altered; the pH decreases, for example

Membrane processes may be applied to remove water; this is called reverse

osmosis Ultrafiltration separates milk into a concentrate and a permeate that israther similar to milk serum Electrodialysis removes some inorganic salts

Fermentation or culturing of milk, usually by lactic acid bacteria, causes

considerable alteration Part of the lactose is converted to lactic acid, causing a

Beverage Milk

Beverage Milk

Skim Milk b

Whipping Cream

Evaporated Milk

Whey (Sweet) b

Buttermilk (Sour) c

Yogurt (Plain) d

Note: Approximate examples; values at room temperature.

16 Milk: Main Characteristics

decrease in pH to such an extent that the casein becomes insoluble This makesthe milk much more viscous The bacteria also produce other metabolites, thekind and concentrations of which depend on the bacterial species

Cheese making As mentioned, milk can be clotted by adding rennet, which

contains a specific proteolytic enzyme The enzyme transforms the casein micelles

in such a way that they start to coagulate The resulting gel can be broken intopieces; stirring the material then results in the formation of curd particles andwhey The curd contains the micellar casein and most of the fat, the liquid wheycontains most of the water-soluble components of the milk and some protein splitoff casein by the rennet The curd is further processed to form cheese

various ways and of some liquid milk products, including whey More extensiveinformation is given in the Appendix

Suggested Literature

Most aspects are further discussed in later chapters, and literature will be mentioned there.

A general reference for many aspects treated throughout the book: H Roginski,

J.W Fuquay, and P.F Fox, Eds., Encyclopedia of Dairy Sciences, Vols 1–4,

Academic Press, London, 2003.

A good monograph on milk synthesis, secretion, and collection, although slightly outdated

in a few aspects: B.L Larson, Ed., Lactation, Iowa State University Press, Ames,

Iowa, 1985.

In this chapter, the properties of the various (classes of) components of milk arediscussed The emphasis is on chemical properties and reactivity, although someother aspects are included such as crystallization Synthesis of the componentsand nutritional aspects are briefly mentioned The chapter ends with a section onthe natural variation in milk composition Chapter 3 emphasizes the physicalaspects of milk’s structural elements

The reader should be cautioned that people often speak of a componentwhereas what is being referred to is actually part of a larger molecule; for instance,

‘the linoleic acid content of milk’ generally refers to linoleic acid esterified inthe triglycerides, rather than to the ‘free’ fatty acid implied

or in cerebrosides (see Table 2.8)

Lactose can be separated from milk or, in industrial practice, from whey, byletting it crystallize Crystalline lactose is produced in large amounts, and it ismainly used in foods and in pharmaceuticals; nearly all pills contain lactose as

a filling material Lactose is also used as raw material for a range of chemical orenzymatic derivatives, such as lactitol, lactulose, and oligosaccharides

2.1.1 C HEMICAL P ROPERTIES

Lactose is a disaccharide composed of D-glucose and D-galactose The aldehydegroup of galactose is linked to the C-4 group of glucose through a β-1, 4-glycosidiclinkage (Figure 2.1) Both sugar moieties occur predominantly in the pyranosering form Chemical reactions of lactose involve the hemiacetal linkage between

18 Milk Components

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C1 and C5 of the glucose moiety, the glycosidic linkage, the hydroxyl groups,and the –C–C– bonds

Furthermore, lactose is a reducing sugar As shown in Figure 2.1, the O–C1

bond in the glucose moiety can break, leading to an open-chain form that has analdehyde group It is also shown that conversion of the α-anomer into the β-anomer,and vice versa, does occur via the open-chain form This phenomenon is called

mutarotation Presumably, less than 0.1% of the lactose in fresh milk is in the

open-chain form At high temperatures, and also at high pH values, this is a muchhigher proportion, say, between 1 and 10% Because the aldehyde group is by farthe most reactive one of lactose, this means that the reactivity of the sugar then isgreatly enhanced

Suitable reagents or enzymes can cause mild oxidation of lactose, whereby

the aldehyde group is converted to a carboxyl group Somewhat more vigorousoxidation ruptures the glycosidic linkage and produces carboxyl groups in the

remaining sugars Gentle reduction of lactose converts the aldehyde group to an

alcohol group More intense reduction cleaves the glycosidic linkage and results

in the formation of alcohol groups in the remaining sugars Hydrolysis of lactose

by acid does not occur easily If it occurs (high temperature and low pH), manyother reactions take place as well

Several reactions of lactose occur when milk is heated Lactose may isomerize

into lactulose That means that the glucose moiety converts to a fructose moiety

(Figure 2.1) Isomerization of the glucose moiety into mannose may occur aswell, yielding epilactose, but the latter compound is formed in only trace amounts

In these isomerization reactions milk components are active as catalysts (It is

FIGURE 2.1 Chemical structure of β-lactose and of lactulose, and the mutarotation of the glucose moiety of lactose.

4 3

1 O OH

OH HO

6 CH2OH

2 3 4 4

2

3

6 O O OH

OH OH

OH

OH

OH OH

(Galactose) MUTAROTATION

β

4 3

O O

OH OH HO

6 CH2OH

(Glucose)

Pyranose form Furanose form

O O

CH2OH

OH OH HO

α

O O

CH2OH

HO OH HO

Open form

(Galactose) (Fructose)

LACTULOSE

OH O

CH2OH

OH O H

O β-LACTOSE

O

not fully clear as to which milk components are involved.) The quantity oflactulose in heated milk products can be used as an indicator of the intensity ofthe heat treatment.

Other reactions occurring during heat treatment are caramelization and

Maillard reactions, which are to some extent related The latter occur in the presence

of amino groups, especially the ε-amino group of lysine residues in proteins Thesereactions can lead to formation of flavor compounds and brown pigments, and to

a decrease in the nutritionally available lysine They are discussed in some detail

in Subsection 7.2.3

Sweetness: a lactose solution is approximately 0.3 times as sweet as a sucrose

solution of the same concentration In milk, the sweetness is, moreover, to someextent masked by the protein, primarily casein Consequently, unsoured whey has

a sweeter taste than milk If the lactose in milk is hydrolyzed into glucose andgalactose, the sweetness is considerably enhanced

2.1.2 N UTRITIONAL A SPECTS

Lactose primarily provides the young animal with energy (about 17 kJ per gram

of lactose), but it has other functions, such as giving a sweetish flavor Lactosecannot be taken up into the blood; it must first be hydrolyzed into glucose andgalactose This occurs slowly, which prevents a sudden large increase of the glucoselevel of the blood after ingesting a substantial amount of milk High blood glucoselevels are considered to be detrimental Moreover, some sugar (galactose and lac-tose) can reach the large intestine (colon), where it serves as a carbon source forseveral benign colon bacteria

Lactose is hydrolyzed by the enzyme lactase, more precisely, β-galactosidase(EC 3.2.1.23), which is secreted in the small intestine Naturally, the sucklingyoung needs this enzyme, but after weaning, the amount of enzyme produceddecreases to an insignificant level This is not so for all humans The estimatesvary, but in at least 60% of people over 4 years old the enzyme activity is greatlyreduced (to 5 to 10%), and they thus poorly metabolize lactose; these people are

called lactose mal-absorbers Drinking milk considerably enhances the activity

of their colon flora Roughly half of the mal-absorbers then develop significant

problems, ranging from flatulence to severe diarrhea; these are called lactose

intolerant They cannot drink milk in significant quantities (say, 100 ml per day).

The proportion of lactose mal-absorbers varies widely among regions, from, say,

10 to 90% In populations in which milk has been part of the diet for numerousgenerations (people living in, or originating from, most of Europe and parts ofCentral and Western Asia, India, and Eastern Africa), mal-absorbers are scarce;

in other populations they are very common It has often been observed thatfermented (sour) milk products hardly produce lactose intolerance, although theystill contain about two thirds of the original lactose; the explanation is not fullyclear Another possibility is to treat milk with lactase: the lactose then is almostfully hydrolyzed into glucose and fructose (moreover, some oligosaccharides areformed) As mentioned, it markedly increases the sweet taste of the milk