Dairy science and technology

Dairy Scienceand TechnologySecond Edition

2006 by Taylor & Francis Group, LLC

FOOD SCIENCE AND TECHNOLOGY

A Series of Monographs, Textbooks, and Reference Books

Editorial Advisory Board

Gustavo V. Barbosa-Cnovas Washington State UniversityPullmanP. Michael Davidson University of TennesseeKnoxvilleMark Dreher McNeil Nutritionals, New Brunswick, NJRichard W. Hartel University of WisconsinMadison

Lekh R. Juneja Taiyo Kagaku Company, JapanMarcus Karel Massachusetts Institute of Technology

Ronald G. Labbe University of MassachusettsAmherstDaryl B. Lund University of WisconsinMadison

David B. Min The Ohio State UniversityLeo M. L. Nollet Hogeschool Gent, Belgium

Seppo Salminen University of Turku, FinlandJames L. Steele University of WisconsinMadison

John H. Thorngate III Allied Domecq Technical Services, Napa, CAPieter Walstra Wageningen University, The Netherlands

John R. Whitaker University of CaliforniaDavisRickey Y. Yada University of Guelph, Canada

76. Food Chemistry: Third Edition, edited by Owen R. Fennema77. Handbook of Food Analysis: Volumes 1 and 2, edited by

Leo M. L. Nollet78. Computerized Control Systems in the Food Industry, edited by

Gauri S. Mittal79. Techniques for Analyzing Food Aroma, edited by Ray Marsili80. Food Proteins and Their Applications, edited by

Srinivasan Damodaran and Alain Paraf81. Food Emulsions: Third Edition, Revised and Expanded, edited by

Stig E. Friberg and Kre Larsson82. Nonthermal Preservation of Foods, Gustavo V. Barbosa-Cnovas,

Usha R. Pothakamury, Enrique Palou, and Barry G. Swanson83. Milk and Dairy Product Technology, Edgar Spreer84. Applied Dairy Microbiology, edited by Elmer H. Marth

and James L. Steele85. 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. Salunkheand S. S. Kadam

87. Polysaccharide Association Structures in Food, edited byReginald 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 Ottersttter92. 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 Rahman95. 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: SecondEdition, Revised and Expanded, edited by Ching Kuang Chow

97. Seafood Enzymes: Utilization and Influence on PostharvestSeafood 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. Park102. Drug Residues in Foods: Pharmacology, Food Safety,

and Analysis, Nickos A. Botsoglou and Dimitrios J. Fletouris103. Seafood and Freshwater Toxins: Pharmacology, Physiology,

and Detection, edited by Luis M. Botana104. Handbook of Nutrition and Diet, Babasaheb B. Desai105. Nondestructive Food Evaluation: Techniques to Analyze

Properties and Quality, edited by Sundaram Gunasekaran106. Green Tea: Health Benefits and Applications, Yukihiko Hara107. Food Processing Operations Modeling: Design and Analysis,

edited by Joseph Irudayaraj108. Wine Microbiology: Science and Technology, Claudio Delfini

and Joseph V. Formica109. Handbook of Microwave Technology for Food Applications,

edited by Ashim K. Datta and Ramaswamy C. Anantheswaran


110. Applied Dairy Microbiology: Second Edition, Revised 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 OBrien 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 Marsili116. 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: SecondEdition, 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. Deshpande120. 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 Walstra122. Handbook of Food Enzymology, edited by John R. Whitaker,

Alphons G. J. Voragen, and Dominic W. S. Wong123. 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 Gnl Kaletun 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 byConstantina 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 Toldr

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. Marangoni141. Novel Food Processing Technologies, edited by

Gustavo V. Barbosa-Cnovas, 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-Cnovas

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 Onwulata147. Dairy Science and Technology: Second Edition, Pieter Walstra,

Jan T. M. Wouters and Tom J. Geurts


Pieter WalstraJan T. M. WoutersTom J. Geurts

Dairy Scienceand TechnologySecond Edition

A CRC title, part of the Taylor & Francis imprint, a member of theTaylor & Francis Group, the academic division of T&F Informa plc.

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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.)

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Preface

The primary theme of this book is the efficient transformation of milk into high-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 onthe 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 writtenas 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 knowledgeof 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 andtransformations (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 reorga-nized or rewritten. Factual information has been increased and partly moved toan Appendix.

Pieter WalstraJan WoutersTom Geurts

Wageningen, The Netherlands


Acknowledgments

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, Zrich, 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.


Contents

Part IMilk

Chapter 1 Milk: Main Characteristics .....................................................3

1.1 Composition and Structure..........................................................................31.1.1 Principal Components .....................................................................31.1.2 Structural Elements .........................................................................4

1.2 Milk Formation............................................................................................71.3 Some Properties of Milk ...........................................................................111.4 Variability ..................................................................................................121.5 Changes......................................................................................................13Suggested Literature ...........................................................................................16

Chapter 2 Milk Components..................................................................17

2.1 Lactose.......................................................................................................172.1.1 Chemical Properties.......................................................................172.1.2 Nutritional Aspects ........................................................................192.1.3 Physicochemical Aspects...............................................................20

2.2 Salts............................................................................................................262.2.1 Composition and Distribution among the Phases.........................262.2.2 Properties of the Salt Solution ......................................................302.2.3 Colloidal Calcium Phosphate ........................................................322.2.4 Nutritional Aspects ........................................................................332.2.5 Changes in Salts ............................................................................33

2.3 Lipids .........................................................................................................372.3.1 Constituent Fatty Acids .................................................................382.3.2 Lipid Classes .................................................................................422.3.3 Nutritional Aspects ........................................................................472.3.4 Autoxidation ..................................................................................482.3.5 Triglyceride Crystallization...........................................................52

2.4 Proteins ......................................................................................................632.4.1 Chemistry of Proteins....................................................................632.4.2 Survey of Milk Proteins ................................................................722.4.3 Serum Proteins...............................................................................762.4.4 Casein ............................................................................................792.4.5 Nutritional Aspects ........................................................................83


2.5 Enzymes.....................................................................................................842.5.1 Enzyme Activity ............................................................................852.5.2 Some Milk Enzymes .....................................................................882.5.3 Inactivation ....................................................................................92

2.6 Other Components.....................................................................................932.6.1 Natural Components......................................................................932.6.2 Contaminants .................................................................................952.6.3 Radionuclides ................................................................................97

2.7 Variability ..................................................................................................982.7.1 Sources of Variability ....................................................................982.7.2 Nature of the Variation ................................................................1032.7.3 Some Important Variables ...........................................................106

Suggested Literature .........................................................................................108

Chapter 3 Colloidal Particles of Milk..................................................109

3.1 Basic Aspects...........................................................................................1093.1.1 Surface Phenomena .....................................................................1113.1.2 Colloidal Interactions ..................................................................1183.1.3 Aggregation .................................................................................1223.1.4 Size Distributions ........................................................................125

3.2 Fat Globules.............................................................................................1273.2.1 Properties .....................................................................................1273.2.2 Emulsion Stability .......................................................................1303.2.3 Interactions with Air Bubbles .....................................................1343.2.4 Creaming......................................................................................1363.2.5 Lipolysis ......................................................................................139

3.3 Casein Micelles .......................................................................................1403.3.1 Description...................................................................................1413.3.2 Changes........................................................................................1453.3.3 Colloidal Stability........................................................................1503.3.4 Gel Formation and Properties .....................................................155


Chapter 4 Milk Properties....................................................................159

4.1 Solution Properties ..................................................................................1594.2 Acidity .....................................................................................................1604.3 Redox Potential .......................................................................................1624.4 Flavor .......................................................................................................1644.5 Density.....................................................................................................1664.6 Optical Properties ....................................................................................1674.7 Viscosity ..................................................................................................169

4.7.1 Some Fluid Rheology..................................................................1694.7.2 Liquid Milk Products ..................................................................173



Chapter 5 Microbiology of Milk .........................................................175

5.1 General Aspects .......................................................................................1755.1.1 Microorganisms ...........................................................................1755.1.2 Bacteria ........................................................................................1765.1.3 Yeasts and Molds.........................................................................1795.1.4 Enumeration of Microorganisms.................................................1815.1.5 Growth .........................................................................................1825.1.6 Milk as a Substrate for Microorganisms ....................................187

5.2 Undesirable Microorganisms...................................................................1905.2.1 Pathogenic Microorganisms ........................................................1905.2.2 Spoilage Microorganisms............................................................194

5.3 Sources of Contamination .......................................................................1975.3.1 Microbial Ecology .......................................................................1975.3.2 Microorganisms Present in the Udder ........................................1985.3.3 Contamination during and after Milking ....................................199

5.4 Hygienic Measures ..................................................................................2015.4.1 Protection of the Consumer against

Pathogenic Microorganisms ........................................................2025.4.2 Measures against Spoilage Organisms........................................202


Part IIProcesses

Chapter 6 General Aspects of Processing............................................207

6.1 Introduction..............................................................................................2076.2 Preservation Methods ..............................................................................2096.3 Quality Assurance....................................................................................212

6.3.1 Concepts ......................................................................................2126.3.2 Hazard Analysis/Critical Control Points (HACCP)....................2146.3.3 Quality Assurance of Raw Milk .................................................215

6.4 Milk Storage and Transport ....................................................................2176.4.1 Milk Collection and Reception ...................................................2176.4.2 Milk Storage ................................................................................2186.4.3 Transport of Milk in the Dairy ...................................................221

6.5 Standardizing ...........................................................................................222Suggested Literature .........................................................................................223

Chapter 7 Heat Treatment ....................................................................225

7.1 Objectives ................................................................................................2257.2 Changes Caused by Heating ...................................................................226


7.2.1 Overview of Changes ..................................................................2267.2.2 Reactions of Proteins...................................................................2297.2.3 Reactions of Lactose ...................................................................2337.2.4 Heat Coagulation .........................................................................236

7.3 Heating Intensity .....................................................................................2427.3.1 Processes of Different Intensity ..................................................2427.3.2 Kinetic Aspects............................................................................2457.3.3 Inactivation of Enzymes..............................................................2527.3.4 Thermobacteriology.....................................................................255

7.4 Methods of Heating.................................................................................2637.4.1 Considerations .............................................................................2637.4.2 Equipment....................................................................................2657.4.3 Heat Regeneration .......................................................................2707.4.4 Control .........................................................................................271


Chapter 8 Centrifugation......................................................................273

8.1 Cream Separation ....................................................................................2738.2 Removal of Particles ...............................................................................276Suggested Literature .........................................................................................277

Chapter 9 Homogenization ..................................................................279

9.1 Objectives ................................................................................................2799.2 Operation of the Homogenizer................................................................2809.3 Effects of Turbulence ..............................................................................2829.4 Factors Affecting Fat Globule Size.........................................................2859.5 Surface Layers .........................................................................................2879.6 Colloidal Stability....................................................................................2899.7 Homogenization Clusters ........................................................................2909.8 Creaming..................................................................................................2929.9 Other Effects of Homogenization ...........................................................2939.10 Other Ways of Working...........................................................................295Suggested Literature .........................................................................................296

Chapter 10 Concentration Processes......................................................297

10.1 General Aspects .......................................................................................29710.1.1 Concentration of Solutes .............................................................29710.1.2 Water Activity..............................................................................30010.1.3 Changes Caused by Concentrating .............................................30210.1.4 The Glassy State..........................................................................30310.1.5 Reaction Rates .............................................................................304

10.2 Evaporating..............................................................................................307


10.3 Drying: General Aspects .........................................................................31410.3.1 Objectives ....................................................................................31410.3.2 Drying Methods...........................................................................316

10.4 Spray Drying ...........................................................................................31810.4.1 Drier Configuration .....................................................................31810.4.2 Atomization .................................................................................31910.4.3 Change of State of the Drying Air..............................................32210.4.4 Changes of State of the Drying Droplets ...................................32610.4.5 Two-Stage Drying........................................................................332


Chapter 11 Cooling and Freezing..........................................................337

11.1 Cooling ....................................................................................................33711.2 Freezing ...................................................................................................338Suggested Literature .........................................................................................340

Chapter 12 Membrane Processes ...........................................................341

12.1 General Aspects .......................................................................................34112.1.1 Types of Processes ......................................................................34112.1.2 Efficiency .....................................................................................34312.1.3 Technical Operation.....................................................................345

12.2 Ultrafiltration ...........................................................................................34612.2.1 Composition of the Retentate......................................................34612.2.2 Permeate Flux..............................................................................349

12.3 Reverse Osmosis......................................................................................35112.4 Desalting ..................................................................................................354Suggested Literature .........................................................................................356

Chapter 13 Lactic Fermentations ...........................................................357

13.1 Lactic Acid Bacteria ................................................................................35713.1.1 Taxonomy ....................................................................................35713.1.2 Metabolism ..................................................................................36013.1.3 Genetics .......................................................................................37313.1.4 Bacteriocins .................................................................................374

13.2 Acid Production.......................................................................................37413.3 Bacteriophages.........................................................................................377

13.3.1 Phage Composition and Structure...............................................37713.3.2 Phage Multiplication ...................................................................37713.3.3 Phage Resistance Mechanisms....................................................38213.3.4 Inactivation ..................................................................................383

13.4 Ecological Aspects...................................................................................38413.5 Starters .....................................................................................................385


13.5.1 Composition.................................................................................38513.5.2 Properties .....................................................................................38813.5.3 Shifts in Flora ..............................................................................38813.5.4 Traditional Starter Manufacture ..................................................39013.5.5 Modern Starter Manufacture .......................................................394


Chapter 14 Fouling and Sanitizing ........................................................399

14.1 Deposit Formation ...................................................................................39914.2 Cleaning...................................................................................................40514.3 Disinfection..............................................................................................408Suggested Literature .........................................................................................410

Chapter 15 Packaging ............................................................................411

15.1 Distribution Systems ...............................................................................41115.2 Packaging Materials ................................................................................41215.3 Filling Operation .....................................................................................415Suggested Literature .........................................................................................417

Part IIIProducts

Chapter 16 Milk for Liquid Consumption.............................................421

16.1 Pasteurized Milk......................................................................................42116.1.1 Manufacture .................................................................................42216.1.2 Shelf Life .....................................................................................42716.1.3 Extended-Shelf-Life Milk ...........................................................430

16.2 Sterilized Milk .........................................................................................43116.2.1 Description...................................................................................43116.2.2 Methods of Manufacture .............................................................43216.2.3 Shelf Life .....................................................................................436

16.3 Reconstituted Milks.................................................................................43716.4 Flavor .......................................................................................................43716.5 Nutritive Value.........................................................................................439

16.5.1 Modification of Composition ......................................................43916.5.2 Loss of Nutrients .........................................................................440

16.6 Infant Formulas .......................................................................................44116.6.1 Human Milk ................................................................................44116.6.2 Formula Composition and Manufacture .....................................444



Chapter 17 Cream Products ...................................................................447

17.1 Sterilized Cream ......................................................................................44717.1.1 Manufacture .................................................................................44817.1.2 Heat Stability ...............................................................................44817.1.3 Stability in Coffee .......................................................................44817.1.4 Clustering.....................................................................................450

17.2 Whipping Cream .....................................................................................45217.2.1 Desirable Properties.....................................................................45217.2.2 Manufacture .................................................................................45317.2.3 The Whipping Process ................................................................454

17.3 Ice Cream.................................................................................................45817.3.1 Manufacture .................................................................................45917.3.2 Physical Structure: Formation and Stability ...............................46217.3.3 Role of the Various Components ................................................465


Chapter 18 Butter ...................................................................................467

18.1 Description...............................................................................................46718.2 Manufacture .............................................................................................468

18.2.1 Processing Scheme ......................................................................46818.2.2 The Churning Process .................................................................47118.2.3 Working .......................................................................................474

18.3 Properties .................................................................................................47818.3.1 Microstructure..............................................................................47818.3.2 Consistency..................................................................................48018.3.3 Cold Storage Defects...................................................................485

18.4 Cultured Butter from Sweet Cream ........................................................48618.5 High-Fat Products....................................................................................489

18.5.1 Anhydrous Milk Fat ....................................................................48918.5.2 Modification of Milk Fat.............................................................49018.5.3 Recombined Butter......................................................................49218.5.4 Low-Fat Butter Products .............................................................494


Chapter 19 Concentrated Milks .............................................................497

19.1 Evaporated Milk ......................................................................................49719.1.1 Manufacture .................................................................................49719.1.2 Product Properties .......................................................................50119.1.3 Heat Stability ...............................................................................50219.1.4 Creaming......................................................................................50419.1.5 Age Thickening and Gelation .....................................................505

19.2 Sweetened Condensed Milk ....................................................................507


19.2.1 Manufacture .................................................................................50719.2.2 Keeping Quality...........................................................................509


Chapter 20 Milk Powder ........................................................................513

20.1 Objectives ................................................................................................51320.2 Manufacture .............................................................................................51420.3 Hygienic Aspects .....................................................................................517

20.3.1 Bacteria in the Original Milk......................................................51720.3.2 Growth during Manufacture........................................................51920.3.3 Incidental Contamination ............................................................52120.3.4 Sampling and Checking ..............................................................521

20.4 Powder Characteristics ............................................................................52220.4.1 The Particle..................................................................................52220.4.2 Extractable Fat.............................................................................52220.4.3 Free-Flowingness.........................................................................52320.4.4 Specific Volume ...........................................................................52520.4.5 Dissolution...................................................................................52620.4.6 WPN Index ..................................................................................52920.4.7 Flavor ...........................................................................................53020.4.8 Conclusions..................................................................................531

20.5 Deterioration ............................................................................................53120.6 Other Types of Milk Powder...................................................................535Suggested Literature .........................................................................................535

Chapter 21 Protein Preparations ............................................................537

21.1 Manufacture .............................................................................................53821.1.1 Casein ..........................................................................................53921.1.2 Whey Protein ...............................................................................54021.1.3 Other Products .............................................................................542

21.2 Functional Properties...............................................................................54321.2.1 Solution Properties ......................................................................54421.2.2 Gels ..............................................................................................54621.2.3 Emulsions ....................................................................................54821.2.4 Foams...........................................................................................549


Chapter 22 Fermented Milks .................................................................551

22.1 General Aspects .......................................................................................55122.2 Types of Fermented Milks ......................................................................552

22.2.1 Mesophilic Fermentation.............................................................55222.2.2 Thermophilic Fermentation .........................................................553


22.2.3 YeastLactic Fermentation ..........................................................55522.2.4 Molds in Lactic Fermentation.....................................................557

22.3 Cultured Buttermilk.................................................................................55722.4 Yogurt ......................................................................................................558

22.4.1 The Yogurt Bacteria.....................................................................55922.4.2 Manufacture .................................................................................56222.4.3 Physical Properties ......................................................................56522.4.4 Flavor Defects and Shelf Life .....................................................568

22.5 Nutritional Aspects ..................................................................................56922.5.1 Composition.................................................................................56922.5.2 Nutritional Value..........................................................................57022.5.3 Probiotics .....................................................................................57122.5.4 Prebiotics .....................................................................................572


Part IVCheese

Chapter 23 Principles of Cheese Making ..............................................577

23.1 Introduction..............................................................................................57723.2 Essential Process Steps ...........................................................................57923.3 Changes Occurring ..................................................................................580Suggested Literature .........................................................................................582

Chapter 24 Cheese Manufacture............................................................583

24.1 Milk Properties and Pretreatment ...........................................................58324.1.1 The Raw Milk..............................................................................58324.1.2 Milk Treatment ............................................................................584

24.2 Starters .....................................................................................................58624.3 Enzyme-Induced Clotting........................................................................588

24.3.1 Enzymes Used .............................................................................58824.3.2 The Enzyme-Catalyzed Reaction ................................................59024.3.3 Aggregation .................................................................................59124.3.4 Gel Formation..............................................................................59324.3.5 The Renneting Time....................................................................59424.3.6 Clotting of Heat-Treated Milk ....................................................596

24.4 Curd Making............................................................................................59624.4.1 Clotting ........................................................................................59724.4.2 Accumulation of Various Components .......................................60024.4.3 Concentrating before Clotting.....................................................60124.4.4 Syneresis ......................................................................................60324.4.5 Acid Production and Washing.....................................................60824.4.6 Separation of Curd and Whey.....................................................610


24.5 Shaping and Pressing ..............................................................................61324.6 Salting ......................................................................................................615

24.6.1 Mass Transport during Salting ....................................................61624.6.2 Important Variables......................................................................62224.6.3 Distribution of Salt and Water after Salting ...............................624

24.7 Curing, Storage, and Handling................................................................62524.7.1 Temperature .................................................................................62624.7.2 Air Conditions .............................................................................62724.7.3 Rind Treatment ............................................................................62724.7.4 Packaging.....................................................................................630

24.8 Cheese Composition and Yield ...............................................................63124.8.1 Variables Involved .......................................................................63224.8.2 Yield.............................................................................................63624.8.3 Standardizing the Milk ................................................................638


Chapter 25 Cheese Ripening and Properties .........................................641

25.1 Lactic Fermentation.................................................................................64125.2 Enzyme Sources ......................................................................................64225.3 Proteolysis................................................................................................644

25.3.1 Methods of Characterization .......................................................64425.3.2 Milk Proteinases ..........................................................................64525.3.3 Clotting Enzymes ........................................................................64625.3.4 Enzymes of Lactic Acid Bacteria ...............................................64825.3.5 Enzymes of Nonstarter Organisms .............................................65025.3.6 Interaction between Enzyme Systems ........................................65025.3.7 Ultrafiltration of Cheese Milk.....................................................651

25.4 Lipolysis ..................................................................................................65325.5 Development of Flavor............................................................................654

25.5.1 Description...................................................................................65425.5.2 Formation of Flavor Compounds ................................................655

25.6 Development of Texture ..........................................................................65925.6.1 Structure.......................................................................................65925.6.2 Consistency..................................................................................661

25.7 Accelerated Ripening ..............................................................................66925.8 Nutritive Value and Safety ......................................................................672Suggested Literature .........................................................................................675

Chapter 26 Microbial Defects................................................................677

26.1 Coliform Bacteria ....................................................................................67926.2 Butyric Acid Bacteria ..............................................................................68026.3 Lactobacilli ..............................................................................................68326.4 Heat-Resistant Streptococci.....................................................................684


26.5 Propionic Acid Bacteria ..........................................................................68426.6 Organisms on the Rind............................................................................68526.7 Other Aspects...........................................................................................686Suggested Literature .........................................................................................686

Chapter 27 Cheese Varieties ..................................................................687

27.1 Overview..................................................................................................68727.1.1 Variations in Manufacture ...........................................................68827.1.2 Types of Cheese ..........................................................................694

27.2 Fresh Cheese............................................................................................69627.2.1 Quarg ...........................................................................................69727.2.2 Cottage Cheese ............................................................................699

27.3 Gouda-Type Cheeses ...............................................................................70227.3.1 Manufacture .................................................................................70227.3.2 Properties and Defects.................................................................709

27.4 Cheddar-Type Cheeses ............................................................................71227.4.1 Manufacture .................................................................................71227.4.2 Properties .....................................................................................716

27.5 Swiss and Pasta-Filata Types ..................................................................71827.5.1 Emmentaler..................................................................................71927.5.2 Mozzarella ...................................................................................722

27.6 Cheeses with a Specific Flora .................................................................72427.6.1 Soft Cheese with Surface Flora ..................................................72527.6.2 Blue-Veined Cheese.....................................................................734

27.7 Processed Cheese.....................................................................................737Suggested Literature .........................................................................................739

Part VAppendix

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.7 Some Properties of Lactose ................................................754

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 MilkFractions ......................................................................................757

A.11 Heat Transfer .......................................................................758

A.12 Data on Some Cheese Varieties ..........................................762


Part I

Milk


31 Milk: Main Characteristics

Milk is dened as the secretion of the mammary glands of mammals, its primarynatural function being nutrition of the young. Milk of some animals, especiallycows, buffaloes, goats and sheep, is also used for human consumption, either assuch or in the form of a range of dairy products. In this book, the word milk willbe used for the normal milk of healthy cows, unless stated otherwise. Occasion-ally, a com-parison will be made with human milk.

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 eld 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

1.1.1 PRINCIPAL COMPONENTS

A classication of the principal constituents of milk is given in Table 1.1. Theprincipal chemical components or groups of chemical components are those presentin the largest quantities. Of course, the quantity (in grams) is not paramount in allrespects. For example, vitamins are important with respect to nutritive value; en-zymes are catalysts of reactions; and some minor components contribute markedlyto 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 complicatedmixture. 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 fths of the protein consists of casein, actually a mixture of fourproteins: S1-, S2-, -, and -casein. The caseins are typical for milk and have somerather specic properties: They are to some extent phosphorylated and have littleor no secondary structure. The remainder consists, for the most part, of the milkserum proteins, the main one being -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



quantities. The salts are only partly ionized. The organic acids occur largely asions 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 drymatter. 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 avor; and the chemicalreactions that can occur in milk. The latter include reactions that cause off-avours.

1.1.2 STRUCTURAL ELEMENTS

Structure can be dened as the geometrical distribution of the (chemical) compo-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.

Figure 1.1 shows the main structural elements of milk. Of course, the gureis schematic and incomplete. Some properties of the structural elements are given

TABLE 1.1Approximate Composition of Milk

ComponentAverage Contentin Milk (% w/w)

Rangea

(% w/w)Average Content in

Dry Matter (% w/w)

Water 87.1 85.388.7 Solids-not-fat 8.9 7.910.0 Fat in dry matter 31 2238 Lactose 4.6 3.85.3 36Fat 4.0 2.55.5 31Proteinb 3.3 2.34.4 25

casein 2.6 1.73.5 20Mineral substances 0.7 0.570.83 5.4Organic acids 0.17 0.120.21 1.3Miscellaneous 0.15 1.2

Note: Typical for milks of lowland breeds.

a These values will rarely be exceeded, e.g., in 1 to 2% of samples of separatemilkings of healthy individual cows, excluding colostrum and milk drawn shortlybefore parturition.b Nonprotein nitrogen compounds not included.


1.1 Composition and Structure 5

in Table 1.2, again in a simplied form; the numerical data mentioned are meantonly to dene 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 areais large.

Fat globules. To a certain extent, milk is an oil-in-water emulsion. But thefat globules are more complicated than emulsion droplets. In particular, thesurface layer or membrane of the fat globule is not an adsorption layer of one

FIGURE 1.1 Milk viewed at different magnications, showing the relative size of struc-tural elements (A) Uniform liquid. However, the liquid is turbid and thus cannot behomogeneous. (B) Spherical droplets, consisting of fat. These globules oat in a liquid(plasma), which is still turbid. (C) The plasma contains proteinaceous particles, which arecasein micelles. The remaining liquid (serum) is still opalescent, so it must contain otherparticles. 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.)

C 50000

B 500

A 5

Milk

Plasma

Fatglobules

Fatglobule

SerumMembrane

Caseinmicelles



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 35C,part of the fat in the globules can crystallize. Milk minus fat globules is calledmilk plasma, i.e., the liquid in which the fat globules oat.

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 amountof 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 1.2Properties of the Main Structural Elements of Milk

Milk

Plasma

Serum

Fat Globules Casein MicellesGlobular Proteins

LipoproteinParticles

Main components Fat Casein, water,salts

Serum protein Lipids, proteins

To be considered as Emulsion Fine dispersion Colloidal solution

Colloidal dispersion

Content(% dry matter)

4 2.8 0.6 0.01

Volume fraction 0.05 0.1 0.006 104Particle diametera 0.110 m 20400 nm 36 nm 10 nmNumber per ml 1010 1014 1017 1014Surface area (cm2/ml milk)

700 40000 50000 100

Density(20C; kg m3)

920 1100 1300 1100

Visible with Microscope Ultramicroscope Electron microscope

Separable with Milk separator High-speedcentrifuge

Ultraltration Ultraltration

Diffusion rate(mm in 1h)a

0.0 0.10.3 0.6 0.4

Isoelectric pH 3.8 4.6 45 4

Note: Numerical values are approximate averages.

a For comparison, most molecules in solution are 0.4 to 1 nm diameter, and diffuse, say, 5 mmin 1 h. 1 mm = 103 m = 106 nm = 107 .


1.2 Milk Formation 7

an open structure and, accordingly, contain much water, a few grams per gramof casein. Milk serum, i.e., the liquid in which the micelles are dispersed, is milkminus fat globules and casein micelles.

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

Lipoprotein particles, sometimes called milk microsomes, vary in quantityand shape. Presumably, they consist of remnants of mammary secretory cellmembranes. Few denitive data on lipoprotein particles have been published.

Cells, i.e., leukocytes, are always present in milk. They account for about0.01% of the volume of milk of healthy cows. Of course, the cells contain allcytoplasmic components such as enzymes. They are rich in catalase.

Table 1.3 gives a survey of the average composition and structure of milk.

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 pep-tides and amino acids; and fatty acids and monoglycerides. These are taken upin 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 rst stomach orrumen. 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 dependson the composition of the feed. Proteins are broken down into amino acids. Therumen ora 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 greaterpart 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


8M

ilk: Main C

haracteristicsTABLE 1.3Composition and Structure of Milka

SERUM

Water 790 g Organic acids Proteins

citrate 1600 mg casein +

Carbohydrates formate 40 mg -lactoglobulin 3.2 glactose 46 g acetate 30 mg -lactalbumin 1.2 gglucose 70 mg lactate 20 mg serum albumin 0.4 gothers oxalate 20 mg immunoglobulins 0.8 g

others 10 mg proteose peptone +Minerals others

Ca, bound 300 mg Gases Nonprotein nitrogenousCa, ions 90 mg oxygen 6 mg compoundsMg 70 mg nitrogen 16 mg peptides +K 1500 mg Lipids amino acids 50 mgNa 450 mg glycerides + urea 250 mgCl 1100 mg fatty acids 20 mg ammonia 10 mgphosphate 1100 mg phospholipids 100 mg others 300 mgsulfate 100 mg cerebrosides 10 mg Enzymesbicarbonate 100 mg sterols 15 mg acid phosphatase

others peroxidaseTrace elements many others

Zn 3 mg Vitamins, e.g. Phosphoric esters ~300 mgFe 120 g riboflavin 2 mg OthersCu 20 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.

FAT GLOBULE

triglycerides diglycerides monoglycerides

Glycerides40 g0.1 g10 mg

60 mg

60 mg100 mg0.3 mg

Fatty acidsSterolsCarotenoidsVitamins A, D, E, KWaterOthers

CASEIN MICELLE

caseinproteose peptone

lipaseplasmin

CaphosphatecitrateK, Mg, Na

Protein

Salts

WaterEnzymes

850 mg1000 mg150 mg

26 g+

~80 g

MEMBRANE

LIPOPROTEINPARTICLE

lipidsprotein

enzymeswater

LEUKOCYTE

Many enzymese.g., catalase

Nucleic acidsWater

water +protein 700 mgphospholipids 250 mgcerebrosides 30 mgglycerides +fatty acids 15 mgstreols 15 mgother lipidsenzymes alkaline phosphatase xanthine oxidase many othersCu 4 gFe 100 g

2 g



small globules, which grow while they are transported to the apical end of thecell. They become enrobed by the outer cell membrane (or plasmalemma) whilebeing pinched off into the lumen. This type of secretion is called merocrine,which means that the cell remains intact.

Table 1.4 gives some information about the synthesis of specic components.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, speciccomponents. Some substances, notably water and small lipophilic molecules, can

FIGURE 1.2 Stylized diagram of a mammary secretory cell. Below is the basal part, ontop the apical part of the cell. The cell is bounded by other secretory cells to form theglandular epithelium. See text for further details. (From P. Walstra and R. Jenness, DairyChemistry and Physics, Wiley, New York, 1984. With permission.)

Ribosomes

Basementmembrane

Golgi apparatus

Cytosol

Lysosome

Outer cellmembrane(plasmalemma)

Golgi vesiclewith caseinmicelles

Endoplasmicreticulum

Mitochondrion

Nascentfat globule

Junctionalcomplex

Microvillus

5 m

Nucleus

LUMEN


10 M

ilk: Main C

haracteristicsTABLE 1.4Synthesis of Important Milk Components

Milk Component Precursor in Blood Plasma Synthesis of Component

NameConcentration

(% w/w) NameConcentration

(% w/w)In the

Secretory Cell?Specificfor Milk?

Specific for the Species?

Water 86 Identical 91 No No No Lactose 4.7 Glucosea 0.05 Yes Yes NoProtein

Caseins 2.60.320.120.01

Yes Yes Yesb-lactoglobulin Amino acids 0.04 Yes Yes Yes-lactalbumin Yes Yes YesLactoferrin Yes No YesSerum albumin 0.04 Identical 3.2 No No YesImmunoglobulins 0.07 Most are identical 1.5 No No Yes

Enzymes Trace Various Yesc Noc YesLipids

Triglycerides 4 Partly Partly

Phospholipids 0.03 Some lipids 0.3Citric Acid 0.17 Glucosea 0.05 Yes No NoMinerals Identical No No No

Ca 0.13 0.01Pd 0.09 0.01Na 0.04 0.34K 0.15 0.03Cl 0.11 0.35

a Glucose can also be formed in the secretory cells from some amino acids.b All proteins are species specic, but comparable proteins occur in the milk of all ruminants.c Is not true for all enzymes.d In various phosphates.

Acetic acid-Hydroxy butyric acid

A

0 010 006

.

.ccylglycerols



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 1.2and 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 whichthe freshly formed milk is secreted. From there, the milk can flow through smallducts 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 youngor by the milker. The udder is not fully emptied.

Lactation. When a calf is born, lactation i.e., the formation and secretionof 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. ThepH of milk is about 6.7 at room temperature. The viscosity is low, about twice thatof water, which means that milk can readily be mixed, even by convection currentsresulting from small temperature fluctuations. The dissolved substances give milkan 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 (20) equalsabout 1029 kgm3 at 20C; it varies especially with fat content.

Milk as a Dispersion. Milk is also a dispersion; the particles involved aresummarized 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, whicheither occurs due to gravity or more efficiently is induced by centrifugation.



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 similarin composition to skim milk.

Likewise, casein micelles can be concentrated and separated from milk, forinstance, by membrane ltration. 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 donein cheese making, the casein micelles start to aggregate, forming a gel; whencutting the gel into pieces, these contract, expelling whey. Whey is also similarto milk serum but not quite, because it contains some of the fat globules and partof the -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, arange of liquid milk products of various compositions can be made. Some exam-ples are given in Table 1.5.

Flavor. The avor of fresh milk is fairly bland. The lactose produces somesweetness and the salts some saltiness. Several small molecules present in verysmall quantities also contribute to avor. The fat globules are responsible for thecreaminess of whole milk.

Nutritional value. Milk is a complete food for the young calf, and it canalso provide good nutrition to humans. It contains virtually all nutrients, most ofthese in signicant quantities. However, it is poor in iron and the vitamin Ccontent is not high. It contains no antinutritional factors, but it lacks dietary bre.

Milk as a Substrate for Bacteria. Because it is rich in nutrients, manymicroorganisms, especially bacteria, can grow in milk. Not all bacteria that needsugar can grow in milk, some being unable to metabolize lactose. Milk is poorin 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 varyin size and, to some extent, in composition, and the same applies to casein micelles.

Natural Variation. The main factors responsible for natural variation in milkare the following:

Genetic factors: Breed and individual. The stage of lactation: This can have a signicant effect. Especially

the milk obtained within 2 or 3 d after parturition tends to have a verydifferent composition; it is called colostrum or beestings.


1.4 Variability 13

Illness of the cow: Especially severe mastitis (inammation of theudder) 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 affectmilk yield. However, the effect of the cows diet on milk compositionis fairly small, except for milk fat content and composition.

In a qualitative sense, cows milk is remarkably constant in composition.Nevertheless, individual milkings show signicant 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 classied as follows:

1. Physical changes occurring, for instance, when air is incorporatedduring 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 airwater 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 ocs 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 0C part of the fat remainsliquid. The presence of fat crystals can strongly diminish the stabilityof fat globules against clumping.

2. Chemical changes may be caused by the presence of oxygen: Severalsubstances may be oxidized. In particular, light may induce reactions,often leading to off-avors. 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



proteolysis; and phosphatases, which cause hydrolysis of phosphoricacid esters.

4. Microbial changes are often the most conspicuous: The best-knowneffect 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 4C 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 changesin composition and properties of the milk, as it is intended to do. These changes canbe 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 briey 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 dependson temperature and duration of heating. Low pasteurization (e.g., 15 s at 74C)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 90C, 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 118C) 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 145C 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 ow-through centrifuge called a cream sep-arator, yields skim milk and cream. The skim milk has a very low fat content, 0.05to 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 leadsto a considerable reduction in fat globule size. Such milk creams very slowly butis 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. Manyproperties are altered; the pH decreases, for example.

Membrane processes may be applied to remove water; this is called reverseosmosis. Ultraltration 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, causesconsiderable alteration. Part of the lactose is converted to lactic acid, causing a


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