Technology of Cheesemaking E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Society of Dairy Technology Series
• Sprache: Englisch

Seitenzahl: 1135

Veröffentlichungsjahr: 2011

Beschreibung

Now in a fully-revised new edition, this book covers the science and technology underlying cheesemaking, as practised today in the manufacture of hard, semi-soft and soft cheeses. Emphasis is placed on the technology, and the science and technology are integrated throughout. Authors also cover research developments likely to have a commercial impact on cheesemaking in the foreseeable future within the areas of molecular genetics, advanced sensor / measurement science, chemometrics, enzymology and flavour chemistry. In order to reflect new issues and challenges that have emerged since publication of the first book, the new chapters are included on milk handling prior to cheesemaking; packaging; and major advances in the control of the end user properties of cheese using key manufacturing parameters and variables. The volume has been structured to flow through the discrete stages of cheese manufacture in the order in which they are executed in cheese plants - from milk process science, through curd process science, to cheese ripening science and quality assessment. Overall, the volume provides process technologists, product development specialists, ingredients suppliers, research and development scientists and quality assurance personnel with a complete reference to cheese technology, set against the background of its physical, chemical and biological scientific base.

Leseprobe

Contents

Preface to the Technical Series

Preface to the Second Edition

Preface to the First Edition

Contributors

1 The Quality of Milk for Cheese ManufactureT.P. Guinee and B. O'Brien

1.1 Introduction

1.2 Overview of milk composition

1.3 Principles of cheese manufacture

1.4 Quality definition of milk

1.5 Factors affecting the quality of milk for cheese manufacture

1.6 Strategy for quality milk production

2 Conclusions ReferencesThe Origins, Development and Basic Operations of Cheesemaking TechnologyM. Johnson and B.A. Law

2.1 Introduction

2.2 The world market for cheese

2.3 The fundamentals of cheese technology

2.4 Basic cheese manufacture

2.5 The stages of cheesemaking

2.6 Cheese ripening/maturation

2.7 Reduced-fat versions of traditional cheeses

2.8 Whey technology for cheesemakers

2.9 The role of research and development in the future of cheese technology

2.10 Acknowledgements

3 The Production, Action and Application of Rennet and CoagulantsM. Harboe, M.L. Broe and K.B. Qvist

3.1 Historical background and nomenclature

3.2 Types of rennet and coagulants

3.3 Molecular aspects of the enzymes in rennet and coagulants

3.4 Technology of enzymes production

3.5 Analysis of coagulants

3.6 Legislation and approvals

3.7 Physical chemistry and kinetics of enzymatic coagulation of milk

3.8 Application of rennet and coagulants

3.9 Conclusions

4 The Formation of Cheese CurdT. Janhøj and K.B. Qvist

4.1 Introduction

4.2 Chemistry and physics of curd formation

4.3 Effect of milk composition on curd formation

4.4 Effects of milk pre-treatment on curd formation

4.5 Factors controlling curd formation in the vat

4.6 On-line measurement of curd firmness and syneresis

4.7 Cheese with reduced-fat content

5 The Production, Application and Action of Lactic Cheese Starter CulturesE. Høier, T. Janzen, F. Rattray, K. Sørensen, M.W. Børsting, E. Brockmann and E. Johansen

5.1 Introduction

5.2 Historical background

5.3 Production of starter cultures

5.4 Range of LAB used as starter cultures

5.5 Taxonomy of LAB

5.6 The types of lactic cultures

5.7 Modern approaches to the development of new starter cultures

5.8 Biochemistry of acidification by LAB

5.9 Proteolysis by LAB

5.10 Bacterionhaee of LAB

5.11 Development of phage-resistant starters

5.12 Future perspectives in starter culture development

6 Secondary Cheese Starter CulturesW. Bockelmann

6.1 Introduction

6.2 Surface-ripened cheeses

6.3 Classification of secondary starter cultures

6.4 Commercially available secondary cheese starter cultures

6.5 Surface ripening

6.6 Development of defined surface starter cultures

6.7 Proteolysis and lipolysis

6.8 Aroma

6.9 Conclusions

7 Cheese-Ripening and Cheese Flavour TechnologyB.A. Law

7.1 Introduction

7.2 The breakdown of milk proteins to flavour compounds in cheese

7.3 Breakdown of milk lipids in cheese

7.4 Lactose and citrate metabolism in cheese

7.5 The commercial drive for cheese-ripening and flavour technology

7.6 Commercial opportunities created by cheese-ripening and flavour technologies

7.7 Methods for the controlled and accelerated ripening of cheese

7.8 EMCs and cheese flavour products

7.9 Acknowledgements

8 Control and Prediction of Quality Characteristics in the Manufacture and Ripening of CheeseT.P. Guinee and D.J. O'Callaghan

8.1 Introduction

8.2 Principles of cheese manufacture

8.3 Cheese quality characteristics

8.4 Cheese quality: influence of chemical composition of milk

8.5 Cheese quality: effect of milk pre-treatments and manufacturing operations

8.6 Cheese quality: effect of cheese composition

8.7 Cheese quality: effect of ripening

8.8 Quality assurance in cheese manufacture

8.9 Conclusions

9 Technology, Biochemistry and Functionality of Pasta Filata/Pizza CheeseP.S. Kindstedt, A J. Hillier and J J. Mayes

9.1 Introduction

9.2 Measuring functional properties of pizza cheese

9.3 Manufacture of pizza cheese

9.4 Microbiological, proteolytic and physicochemical properties

9.5 Non-traditional methods of manufacture

10 Eye Formation and Swiss-Type CheesesA. Thierry, F. Berthier, V. Gagnaire, J.R. Kerjean, C. Lopez and Y. Noel

10.1 Introduction

10.2 Open texture and eye formation

10.3 Gas formation through propionic fermentation

10.4 Cheese structure and eye formation

10.5 Conclusions

11 Microbiological Surveillance and Control in Cheese ManufactureR Neaves and A.R Williams

11.1 Introduction

11.2 Milk for cheese manufacture

11.3 Heat treatment

11.4 Cheesemaking

11.5 Maturation of the curd

11.6 Specialist cheeses and cheese products

11.7 Cheese detects

11.8 Prevention and control

11.9 End-product testing and environmental monitoring

11.10 Microbiological techniques

11.11 Conclusions

12 Packaging Materials and EquipmentY. Schneider, C. Kluge, U. Wei β and H. Rohm

12.1 Introduction

12.2 Cutting of the cheese

12.3 Applications of cutting

12.4 Packaging of cheeses

12.5 Packaging machines

12.6 Conclusion

13 The Grading and Sensory Profiling of Cheese

13.1 Introduction to cheese-grading systems

13.2 Fundamentals of sensory processing

13.3 D.D. MuirGrading systems: defect versus attribute grading

13.4 The direct link: cheesemaking to consumer

13.5 Introduction to sensory profiling of cheese

13.6 Sensory vocabulary

13.7 Sample preparation and presentation

13.8 Assessor selection

13.9 Integrated design and analysis of data

13.10 Sensory character of commercial cheese

13.11 Development of flavour lexicons

13.12 Overview

13.13 Acknowledgements

Index

The Society of Dairy Technology (SDT) has joined with Wiley-Blackwell to produce a series of technical dairy-related handbooks providing an invaluable resource for all those involved in the dairy industry, from practitioners to technologists, working in both traditional and modern large-scale dairy operations. For information regarding the SDT, please contact Maurice Walton, Executive Director, Society of Dairy Technology, P. O. Box 12, Appleby in Westmorland, CA16 6YJ, UK.email: [e-mail]

Other volumes in the Society of Dairy Technology book series:

Probiotic Dairy Products (ISBN 978 1 4051 2124 8)Fermented Milks (ISBN 978 0 6320 6458 8)Brined Cheeses (ISBN 978 1 4051 2460 7)Structure of Dairy Products (ISBN 978 1 4051 2975 6)Cleaning-in-Place (ISBN 978 1 4051 5503 8)Milk Processing and Quality Management (ISBN 978 1 4051 4530 5)Dairy Fats (ISBN 978 1 4051 5090 3)Dairy Powders and Concentrated Products (978 1 4051 5764 3)

This edition second published 2010First edition published 1999 Sheffield Academic Press© 2010 Blackwell Publishing Ltd

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Library of Congress Cataloging-in-Publication Data

Technology of cheesemaking / edited by Barry A. Law, A.Y. Tamime. – 2nd ed.p. cm.Includes bibliographical references and index.ISBN 978-1-4051-8298-0 (hardback : alk. paper)1. Cheesemaking. I. Law, Barry A. II. Tamime, A. Y.SF271.T36 2010637’.3-dc22

2009048133

A catalogue record for this book is available from the British Library.

Preface to the Technical Series

For more than 60 years, the Society of Dairy Technology (SDT) has sought to provide education and training in the dairy field, disseminating knowledge and fostering personal development through symposia, conferences, residential courses, publications, and its journal, the International Journal of Dairy Technology (previously published as the Journal of the Society of Dairy Technology).

In recent years, there have been significant advances in our understanding of milk systems, probably the most complex natural food available to man. At the same time, improvements in process technology have been accompanied by massive changes in the scale of many milk processing operations, and the manufacture a wide range of dairy and other related products.

The Society has embarked on a project with Wiley-Blackwell to produce a technical series of dairy-related books to provide an invaluable source of information for practicing dairy scientists and technologists, covering the range from small enterprises to modern large-scale operation. This ninth volume in the series, the second edition of Technology of Cheesemaking under the joint editorship of Barry Law and Adnan Tamime, provides a timely and comprehensive update on the principles and practices involved in cheese production. This new edition also introduces chapters on milk for cheesemaking, packaging technology for cheese and the prediction and control of the overall manufacturing process for hard cheeses.

This book provides a timely and valuable review of the progress being made in the greater understanding of the factors contributing to cheesemaking and how this knowledge may be applied to producing better and more consistent products.

Andrew WilbeyChairman of the Publications Committee, SDT

Preface to the Second Edition

The first edition of Technology of Cheesemaking aimed to assess critically the pool of scientific knowledge, which was then available to the cheesemaking industry as a tool for process and product innovation, quality improvement and safety. I had also hoped to provide an advanced text that would help those in higher education to understand the way that knowledge from strategic and applied research can be fed into commercial innovation in cheese manufacture and distribution. To this end, the second edition is intended to update that knowledge pool in the light of further demands for new data and technologies from an already mature industry seeking to further refine and expand its products and its production technologies. We have covered advances in the fields of coagulants, starter cultures and the manufacturing/maturation of a range of generic cheese varieties in order to update the original chapters, and also introduced new chapters in fields that have advanced dramatically over the ten years that have elapsed since the first edition.

New areas include specific coverage of milk pre-treatment science and technologies, emphasising the special needs of cheesemakers for a consistent and safe raw material. Emerging technologies that were confined to experimental studies ten years ago are now applied to improve the manufacturing properties of milk for cheesemaking.

We have also introduced an appraisal of the key stages of cheese manufacture which can be manipulated to control and maintain the consistency quality of cheese. Although this has produced some overlap with our coverage of milk quality for cheesemaking, we have felt justified in allowing this overlap because it emphasises the prime importance of milk preparation practice for the quality and consistency of cheese for the end user. We also acknowledge that many of these control techniques have been known and used by cheesemakers for many years, but it is only more recently that the science base has delivered a level of basic understanding of their underlying workings. This has led to refinements and new opportunities in cheese production recipes and line technologies, which we have included in this volume.

The first edition did not include packaging as a separate field for scientific and technological input into cheese manufacture. In recognition of the vital role of this area within the industry, and the knowledge-based advances in packaging methods specifically applicable to such a ‘live’ and varied product as cheese, the second edition includes a chapter dedicated to the knowledge and application of packaging materials and equipment. This includes general principles, through to specific challenges from cheese technology.

We would like to acknowledge the time and effort that the expert contributors have given to make this second edition possible. Many are the original authors who helped create the first edition, and our thanks go to them for adding to their already excellent work. We were also fortunate to have a number of new contributors, and we hope they will be as exciting by the results of their efforts as we are, in that this updated volume reflects another decade of progress in the industry and its base of science and technology.

Whilst reflecting on the satisfaction of delivering this volume, we must also pay special tribute to our colleague, Tony Williams, who passed away while the book was in final preparation. Tony, with his partner Paul Neaves, was an outstanding food microbiologist and a vital member of the team which delivered great practical benefits to food quality and safety through the astute gathering and application of basic knowledge to the sharp end of the food industry; the interface between the manufacturer and the retailer/consumer. Tony will be missed not only by those close to him, but also by professionals in food microbiology worldwide.

Barry Law and Adnan TamimeOctober 2009

Preface to the First Edition

Cheesemaking remains an art even today, when many of the once-variable stages of the process have been smoothed out by technology. The purpose of this book was to present the state of the art, to show where and how technology enhances the art, and to point the way towards further improvements in cheesemaking technology, which are achievable through exploitation of the basic science and technology. The book is about cheesemaking technology, and I hope that the reader will be able to feel the excitement of uncertainty band the satisfaction of success-through-understanding that cheesemakers experience when the product of their combined know-how and machinery emerges just as they say, the sellers of the cheeses and consumer would have liked.

It is not my intention to develop a complacent view of cheesemaking – it is hard work, and it sometimes goes wrong. However, I firmly believe that cheese technology supports one of the most advanced food manufacturing industries in the world, having overcome most of the problems of milk variability, microbiological control and culture failure that used to cause so much wastage and potential hazard for consumers. In this volume, we firstly describe and discuss cheesemaking technology from the point of view of cheesemakers. Authors explain the process step by step, showing how the universal elements of milk-conversion technology can be varied by process design and culture technology to yield so many wonderful and individual varieties of cheeses. The book is unique in this respect, and it will add to existing range of books and review articles by viewing cheese technology as the product of tradition, pragmatic development and the application of front-line science.

Having established a detailed knowledge of cheesemaking per se, authors take the reader on to learn about rennets and coagulants – how they are made, standardised and used, and their concerted action (with lactic starter cultures) in forming the basis of all cheese, the curd. Following a logical progression, the book proceeds to consider how the lactic culture, the added moulds and non-lactics, and the eye-forming bacteria work in and on the ripening cheese in their different ways to convert the bland curd into the familiar cheddars, pizza cheeses, blue cheeses, camembert, Swiss-type cheeses and the aromatic smear-ripened varieties. Pressure on cheesemakers to produce both traditional and new varieties from increasingly uniform and controlled plant has pushed cheese technology to find new ways of accelerating and controlling the balance of cheese ripening – topics which this volume covers in depth.

Cheesemakers are ultimately answerable to, and dependent on, consumers for their livelihoods. This book includes chapters on food safety assurance as well on cheese grading and sensory assessment, showing how to ensure that the technology not only produces what the cheesemaker intends to do, but what the consumer expects and wants.

The book is for the enlightenment and support of a wide range of potential readers, ranging from the forever curious to cheesemakers who want to understand more clearly what they are making every day. It will be equally valuable to product development specialists seeking insights into the scope for innovation from the basic cheese technology, and to advanced students of food science and technology wishing to go beyond the standard cheese textbook. The experienced research scientist will find in these pages many examples of the working interface between research and applications, through which to establish communications with product development technologists. By including chapters by specialists in the technology of coagulants, cultures and ripening systems, we have widened the value of the book to include the interests of the dairy ingredients business.

I should like to acknowledge the contributors to this book. It is all very well to have an idea about a new approach to the integration of cheesemaking practice, technology and underlying science but, without the help of this group of expert and very busy people, the idea would be unattainable. Thank you all and please remember that any shortcomings in the quality of the book are the responsibility of me alone.

Barry A. Law

Contributors

Editor

Dr. A.Y. Tamime

24 Queens Terrace

Ayr KA7 1DX

UK

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Dr. F. Berthier

INRA

UR 342 URTAL, Technologie et Analyses

Laitières

39801 Poligny France

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Dr. W. Bockelmann

Federal Research Centre for Nutrition and

Food (BFEL) Location Kiel

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24121 Kiel

Germany

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Ms M.W. Børsting

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Dr. V. Gagnaire

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UMR 1253 Science et Technologie du Lait et de l’Oeuf

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35042 Rennes Cedex

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Dr. T.P. Guinee

Moorepark Food Research Centre

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Dr. M. Harboe

Chr. Hansen A/S10-12 Bøge AlléDK-2970 HørsholmDenmark

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Dr. A.J. Hillier

CSIRO Food and Nutritional Sciences

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Dr. T. Janhøj

Department of Food ScienceFaculty of Life Sciences

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Dr. T. Janzen

Chr. Hansen A/S10-12 Bøge AlléDK-2970 HørsholmDenmark

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Dr. E. Johansen

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Dr. M. Johnson

Wisconsin Centre for Dairy Research

Wisconsin University

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Dr. J.R. Kerjean

Actilait – Pôle OuestP.O. Box 5091535009 Rennes CedexFrance

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Dr. P.S. Kindstedt

Department of Nutrition and Food ScienceUniversity of Vermont253 Carrigan WingBurlightonVermont 05405-0086USA

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Dr. C. Kluge

Institute of Food Technology and Bioprocess EngineeringTechnische Universität DresdenBergstraße 120D-01069 DresdenGermany

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Professor B.A. Law

15 Dover PlaceParkdaleVictoria 3195Australia

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Dr. C. Lopez

INRA

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UMR 1253 Science et Technologie du Lait et de l'Oeuf65 Rue de Saint Brieuc35042 Rennes CedexFrance

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CSIRO Food and Nutritional Science

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Professor D.D. Muir

DD Muir Consultants26 Pennyvenie WayGirdle TollIrvine KA11 1QQUKTel. [phone]Fax (not available)E-mail: [e-mail]

Dr. P. Neaves

Williams & Neaves

The Food Microbiologists

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Délégation au Partenariat avec les EntreprisesP.O. Box 35327Domaine de laMotte 35653 Le Rheu France

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Dr. B. O’Brien

Dairy Production Research Centre

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Dr. D.J. O’Callaghan

Moorepark Food Research Centre

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Dr. K.B. Qvist

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Dr. F. Rattray

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Professor H. Rohm

Institute of Food Technology and Bioprocess Engineering

Technische Universität DresdenBergstraße 120D-01069 DresdenGermany

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Dr. Y. Schneider

Institute of Food Technology and Bioprocess EngineeringTechnische Universität DresdenBergstraße 120 D-01069 DresdenGermany

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Dr. K. Sørensen

Chr. Hansen A/S10-12 Bøge AlléDK-2970 HørsholmDenmark

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Dr. A. Thierry

INRA

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Dr. U. Weiß

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Mr. A.P. Williams

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1 The Quality of Milk for Cheese Manufacture

T.P. Guinee and B. O’Brien

1.1 Introduction

World production of milk in 2008 is estimated at ~576 × 106 tonnes (ZMP, 2008), with India/Pakistan, the Americas and Europe being the major producing regions. The proportions of total milk produced by cow, water buffalo, goat, ewe, camel and other are ~84.0, 12.1, 2.0, 1.3, 0.2 and 0.2, respectively (International Dairy Federation – IDF, 2008). Cows’ milk is the major milk used for cheese manufacture; however, significant quantities of cheese are also made from goat, sheep and water buffalo milks in some European Union (EU) countries, such as France, Italy and Spain.

Based on an estimated yield of 1 kg cheese 10 kg−1 milk, the percentage of total milk used for cheese is ~25%, but varies widely from ~70–90% in some European countries (Italy, France, Denmark and Germany) to ~0.5% in China. While cheese-like products are produced in most parts of the world, the principal cheese-producing regions are Europe, North America and Oceania. Cheese production has increased consistently over the last two decades at an annual average rate of ~1.5%. As discussed in Chapter 8, this may be attributed to a number of factors including increases in global population and per capita income, globalisation of eating trends/habits, changing lifestyles, growth in use of cheese as an ingredient in the food service (in pizza-type dishes, cheese burgers and salad dishes) and industrial sectors (cordon bleu entrees, co-extruded products with cheese and gratins).

The increase in consumption has been paralleled by a greater emphasis on improved quality and consistency with respect to the levels of particular nutrients (fat, protein, calcium -Ca2+ and sodium -Na+), physical properties (texture and cooking attributes), sensory characteristics and processability (size reduction attributes, such as shredability; ability to yield processed cheeses or other cheese products when subjected to secondary processing). Consequently, this has necessitated an increase in the quality and consistency of all inputs (milk composition/quality, enzyme activity/purity, starter cultures characteristics, for example, acid productivity, phage resistance, autolytic properties and flavour-imparting characteristics) and standardisation of the manufacturing process (cf. Chapter 8). In an overall context, milk quality for cheese manufacture may be defined as its suitability for conversion into cheese and deliver cheese of the desired quality and yield. The current chapter examines milk quality for cheese manufacture and the factors affecting it, together with broad-based strategies for improving quality and consistency.

1.2 Overview of milk composition

Milk consists of protein (caseins and whey proteins), lipid, lactose, minerals (soluble and insoluble), minor components (enzymes, free amino acids, peptides) and water (Table 1.1).

The casein fraction coexists with the insoluble minerals as a calcium phosphate–casein complex. The water and its soluble constituents (lactose, native whey proteins, some minerals, citric acid and minor components) are referred to as serum. During cheese manufacture, the milk is subjected to a partial dehydration, involving controlled expulsion of serum and concentration of fat, caseins (and in some cases denatured, aggregated whey proteins) and some of the minerals. The methods engaged to affect the dehydration include limited destabilisation and aggregation of the calcium phosphate casein in the form of a gel network which

Table 1.1 Compositional and gelation characteristics of cows’ milks.

Source: Compiled from O’Brien et al. (1999b–d), Mehra et al. (1999) and Hickey et al. (2006b) for manufacturing milks.

CharacteristicMeanRangeGross composition  Dry matter (g 100 g−1)12.0411.52–12.44Fat (g 100 g−1)3.553.24–3.90Lactose (g 100 g−1)4.424.21–4.56Total protein (g 100 g−1 )3.252.99–3.71True protein (g 100 g−1 )3.062.77–3.47Casein (g 100 g−1)2.512.29–2.93Whey protein (g 100 g−1)0.540.48–0.64Non-protein nitrogen (N) (g 100 g−1 N)5.334.79–6.16Urea (mg 100 g−1)27.6022.00–37.50Ash (g 100 g−1)0.740.71–0.77Calcium (mg 100 mL−1)118108–137Iron (mg 100 mL−1)976460–1490Magnesium (mg 100 mL−1)10796–117Chloride (mg 100 mL−1)10095–116Vitamins/vitamin components  β-Carotene (μg g−1 fat)3.180.48–8.37Thiamine (μg mL−1)0.180.09–0.35Riboflavin (μg mL−1)0.880.19–1.85Vitamin A (μg g−1 fat)9.412.18–27.85Vitamin E (μg g−1 fat)25.566.84–42.15Iodine (I) (μg mL−1)0.280.20–0.51Cobalt (Co) (μg mL−1)0.960.44–1.70Gelation propertiesa  RCT (min)6.154.50–7.44A30 (mm)46.8043.00–51.381/k20 (mm−1)0.230.3–0.19Other components  Total free fatty acids (mg kg−1 fat)37692629–5108

aBased on the analysis using the Formagraph (Type 1170, Foss Electric, Denmark) on milks at pH 6.55 and rennet-treated at a level corresponding to ~0.18 mL L−1 (Chymax Plus, Pfizer Inc., Milwaukee, WI); RCT is an index of rennet coagulation (gelation) time, A30 of the curd firmness after 30 min, and 1/k20 of gel firming rate.

encloses the fat and serum via specific enzymatic hydrolysis of the casein, acidification (by fermentation of milk lactose to lactic acid by added bacterial cultures), elevated temperature and various mechanical operations as discussed in Chapter 8. Amongst others, the degrees of casein aggregation and dehydration are critical parameters controlling the properties and quality of the final cheese.

Although manufacturing procedures for most cheese types are very defined (at least in large modern cheesemaking facilities) in terms of technology applied and the type and levels of operations imposed on the milk (cf. Chapter 8), variations in cheese quality do occur. Seasonal variation in the composition and quality of milk are considered to be crucial factors contributing to the inconsistency in quality. Consequently, an overview of milk composition in terms of its relevance to cheese manufacture is presented below. The main focus of this chapter is on cows’ milk, which accounts for an estimated 95% of total milk used in cheese manufacture; the characteristics of other milks are discussed elsewhere (Anifantakis, 1986; Juárez, 1986; Remeuf & Lenoir, 1986; Muir et al., 1993a,b; Garcia-Ruiz et al., 2000; Bramanti et al., 2003; Huppertz et al., 2006; Kuchtik et al., 2008; Caravaca et al., 2009).

1.2.1Casein

The nitrogenous fraction of cows’ milk typically consists of casein, whey protein and non-protein nitrogen (urea, proteose-peptones, peptides) at levels of ~78, 18 and 4 g 100 g−1, respectively, of total nitrogen (Table 1.1).

Casein, which is typically present at a level of 2.5 g 100 g−1 in cows’ milk, is the main structural protein of both rennet- and acid-induced milk gels (Table 1.1). The casein is heterogeneous, comprising four main types: αs1, αs2, β and κ, which represent ~38, 10, 35 and 15 g 100 g−1 of the total casein, respectively (Fox & McSweeney, 1998; Fox, 2003; Swaisgood, 2003). Model studies in dilute dispersions indicate that the individual caseins vary in the content and distribution of phosphate (Table 1.2); the respective number of (serine) phosphate residues per mole of casein are ~8, 10–13, 5 and 1 for αs1-, αs2- β- and κ-caseins, respectively. The serine phosphates bind calcium and calcium phosphate, and consequently, different caseins have different calcium-binding properties. Generally, αs1-, αs2- and β-caseins bind calcium strongly and precipitate at relatively low calcium concentrations (~0.005–0.1 M CaCl2 solutions), inclusive of the calcium level in milk (30 mM); in contrast κ-casein is not sensitive to these calcium concentrations and can, in fact, stabilise up to 10 times its mass of the calcium-sensitive caseins.

Casein in milk exists in the form of spherical-shaped colloid particles (~40–300 nm diameter), known as casein micelles (Fox & Brodkorb, 2008; McMahon & Oommen, 2008). Different models have been proposed for the structure of the casein micelle on the basis of the location of individual caseins (in response to their calcium sensitivity) and the calcium phosphate. These include:

sub-micelle model (Schmidt, 1982), in which sub-micelles are ‘cemented’ together by colloidal calcium phosphate (CCP) and κ-casein-rich sub-micelles are mainly concentrated at the surface of the micelle; the hydrophilic C-terminal region of the κ-casein orient into the serum as a highly hydrated ‘hairy layer’ that is in a state of constant flux and confers stability to the micelle by steric repulsion;

Table 1.2 Characteristics of cows’ milk proteins of relevance to cheese manufacture.

Source: Compiled from Mulvihill & Donovan (1987), Fox & McSweeney (1998), Fox (2003) and Swaisgood (2003).

dual bonding model (Horne, 1998), in which the interior of the micelle is composed of αs- and β-caseins which form a lattice through interactions between hydrophobic regions (hydrophobic-induced) and between hydrophilic regions containing phosphoserine clusters (that attach to CCP clusters), while κ-casein molecules located at the surface interact hydrophobically with the other caseins (αs- or β-) and orient their highly hydrophilic regions (hairs) into the serum;tangled, cross-linked web model (Holt & Horne, 1996), comprising a ‘tangled’ mass of rheomorphic casein chains cross-linked by calcium phosphate nanoclusters, similar in casein composition throughout but with the chains becoming more diffuse at the micelle periphery (on moving outwards from the dense centre); andinterlocked lattice model (McMahon & Oomen, 2008), featuring a system of interlocking sites composed of anchoring calcium phosphate nanoclusters (several hundred per micelle), which bind the phosphoserine domains of αs- and β-caseins; the hydrophobic ends of these caseins orientate away from the calcium phosphate nanocluster and interact hydrophobically with other α- and β-caseins, while κ-casein is predominantly surface located because of its lack of phosphoserine domains (to bind to the calcium phosphate nanoclusters) and its highly charged C-terminal regions (which prevents strong electrostatic interactions).

In all of the above models, the arrangement of casein within the micelle is such that the interior is mainly occupied by the calcium-sensitive caseins (αs- and β- and κ-casein is principally located at the surface, with its hydrophilic C-terminal region (caseinomacropeptide) oriented outwards toward the serum phase in the form of protruding negatively charged hairs, which create an electrokinetic potential of ~−20 mV and confer stability to the micelle by electrostatic repulsion, Brownian movement and a consequent steric repulsion (de Kruif & Holt, 2003; Horne & Banks, 2004). The κ-casein C-terminal projecting from the micelle surface has been considered as an extended polyelectrolyte brush (de Kruif, 1999), a region containing 14 carboxylic acid groups and immersed in a milk serum with a high ionic strength (~0.08 M) due to the presence of various ions (e.g. potassium, sodium, chloride, phosphate, citrate). Consequently, electrostatic interactions (between the C-terminal regions) at physiological conditions are very short and highly screened (by the high ionic strength). This is conducive to a high degree of ‘solvency’ and extension of the κ-casein C-terminal hairs and to the stability of the micelle as a whole. Moreover, the C-terminal region of the κ-casein is glycoslyated to varying degrees (Table 1.2; Saito & Itoh, 1992; Mollé & Leonil, 1995; Fox & McSweeney, 1998; Mollé et al, 2006), containing galactose, N-acetylgalactosamine (GalNAc) and/or N-actetylneuraminic (sialic) acid (NANA) (Dziuba & Minkiewicz, 1996). These may further enhance the ability of κ-casein to increase micelle stability by steric impedance and electrostatic repulsion via their contribution to increase in water binding (to carbohydrate moieties) and to negatively charged carboxylic groups (on the NANA molecule). O’Connell & Fox (2000) found that the level of glycosylation of κ-casein and protein surface hydrophobicity increased as a function of micelle size.

While a predominant surface location of κ-casein confers stability to the casein micelle in native milk, it renders it susceptible to aggregation/flocculation by processes which reduce the solvency of (and collapse/flatten) the κ-casein hairs or remove them, and thereby enable contact between the more hydrophobic micelle cores, for example cleavage of the κ-casein by acid proteinases, reducing the negative charge by acidification, reducing ionic strength by microfiltration/diafiltration at native pH. However, the interactions between the micelle cores are modified by many factors, including pH, composition of the serum phase, ionic strength, protein concentration and conditions to which milk is subjected (heat, acidification, ultrafiltration/diafiltration homogenisation, shearing).

The casein micelles on a dry weight basis consist of ~7 g 100 g−1 ash (mainly calcium and phosphorous), 92 g 100 g−1 casein and 1 g 100 g−1 minor compounds including magnesium and other salts. They are present in milk at 1014−1016 mL−1, are highly hydrated (~3.7 g H2O g−1 protein), are spherical and have a diameter of ~80 nm (100–500 nm), a surface area of ~8 × 10−10 cm2 and a density of ~1.063 g cm−3 (Fox & McSweeney, 1998).

1.2.2Whey protein

Whey protein in cows’ milk is typically ~0.6–0.7 g 100 g−1 and consists of four main types – β-lactoglobulin (β-Lg), α-lactalbumin (α-La), immunoglobulin(s) (Ig) and bovine serum albumin (BSA) at levels of ~54, 21, 14 and 6 g 100 g−1 of total (Table 1.2). The properties of the individual whey proteins have been extensively reviewed (Table 1.2; Mulvihill & Donovan, 1987; Brew, 2003; Fox, 2003; Hurley, 2003; Sawyer, 2003). In milk, they exist as soluble globular proteins and are characterised by a relatively high level of intramolecular disulphide bonding, and β-Lg and BSA each contain one cysteine residue per mole. On heat-induced denaturation, the whey proteins can interact via thiol-disulphide bonds with other whey proteins and with κ-casein. The latter results in the formation of κ-casein/β-Lg aggregates either at the surface of the casein micelle or in the serum phase or both (cf. Chapter 8). The size and location (serum/micelle surface) of these aggregates are affected by severity of heat treatment of milk, pH at heating, ionic strength, calcium level and casein-to-whey protein ratio. The degree of interaction and size/location of aggregates have a profound effect on the structure and physical properties of rennet- and acid-induced milk gels, and hence on cheeses (see Chapter 8). For example, a high level of casein-whey protein interaction, induced by high heat treatment of the milk (e.g. 95°C for ≥ 1−2 min, ~≥40% denaturation of total whey protein; Guinee et al., 1995), is highly favoured in the manufacture of yoghurt and smooth-textured cheeses with a high moisture-to-protein ratio, such as cream cheese and ultrafiltration-produced Quark. In these products it increases protein recovery and moisture binding (reduce syneresis), contributes smoothness and enhances yield (Guinee et al., 1993). In contrast, high heat treatment of milk is unsuitable for acid-curd cheeses with a granular structure (Cottage cheese) or for Quark manufactured using a mechanical separator, as it impedes whey expulsion during separation and makes it difficult to achieve the desired dry matter and texture characteristics. High heat treatment of milk is generally undesirable for rennet-curd cheeses as denatured protein at levels of ≥25% of total (at heat treatments of 82°C for 26 s, or greater) impedes the ability of the milk to gel on rennet addition, causes marked deterioration in melt properties of the cheese (Rynne et al., 2004) and reduces the recovery of fat from milk to cheese (see Chapter 8). However, a higher-than-normal heat treatment that gives a moderate degree of whey protein denaturation may be desirable as a means of modulating the texture of reduced fat cheese, e.g. reduce firmness (Guinee, 2003; Rynne et al., 2004).

1.2.3Minerals

Cows’ milk contains ~0.75 g 100 g−1 ash, which comprises K+, Ca2+, Cl−, P5+, Na+ and Mg2+ at concentrations (mg 100 g−1) of ~140, 120, 105, 95, 58 and 12, respectively (Table 1.2; White&Davies, 1958a; Chapman & Burnett, 1972;Keogh et al., 1982; Grandison et al., 1984; O’Brien et al., 1999c). These minerals are partitioned to varying degrees between the serum (soluble) and the casein (colloidal or insoluble) in native milk (pH ~6.6−6.7) at room temperature. Serum concentrations as a percentage of the total concentration for each of the minerals are ~100, 100, 100, 66, 43 and 34 for Na+, K+, Cl−, Mg2+, P2+ and Ca2+, respectively. The partition concentrations of Ca2+ and P2+ between the colloidal and soluble states in native milk is controlled mainly by the degree of ionisation of the casein (micelle), which in milk may be considered as a very large dominant anion that regulates the degree of binding of the counterion calcium, to an extent affected by the concentration of calcium per se and those of citric acid and phosphate. A major difference between the calcium salts of citrate (tricalcium citrate − Ca3(C6H5O7)2) and phosphate (tricalcium phosphate − Ca3(PO4)2) is their solubility, with the solubility product of the latter being very low (2.07 × 10−33 mol L−1 at 25°C) compared to the former (3.23 × 10−3 mol L−1 at 25°C).

Cows’ milk typically contains ~120 mg 100 mL−1 calcium (~30 mM), which exists as colloidal inorganic calcium (~12.5 mM), caseinate calcium (8.5 mM), soluble unionised calcium (6.5 mM) and serum ionic calcium (2.5 mM). Calcium attached to the casein micelle, referred to as micellar calcium phosphate, is composed of the colloidal inorganic Ca2+ (more frequently denoted CCP) and caseinate Ca2+. The former occurs as a calcium phosphate complex attached indirectly to the organic serine phosphate groups, while the latter is attached directly to casein via the dissociated ε-carboxyl groups of acidic amino acids including aspartic (pKa ~3.9) and glutamic (pKa ~4.1) acids. Owing to the high molarity of glutamic and aspartic acids (~25 and 7 mM) in milk (with a casein content of 2.5 g 100 g−1), it can be inferred that only ~26 g 100 g−1 of the available ε-carboxyl groups are titrated with calcium and that these groups could potentially bind with added calcium to increase the susceptibility of the casein to aggregation, especially on rennet treatment. The sensitivity of the individual caseins to calcium precipitation as found from model studies in dilute solutions varies and tends to increase with the number of moles of both phosphate and glutamic acid per mole of casein. Hence, the concentration of Ca2+ at which the individual caseins precipitate is lowest for αs2-casein (<2 mM), intermediate for αs1-casein (3–8 mM) and β-casein (8–15 mM), and highest for κ-casein, which remains soluble at all of these concentrations and can prevent the precipitation of the other caseins (Aoki et al., 1985).

In the context of the milk salt system, the milk may be viewed as a ‘soup’ consisting of a large colloidal anion (calcium phosphate casein) dispersed in a serum containing various soluble salt and ionic species (calcium citrate, sodium phosphate, potassium and ionic calcium). The insoluble (colloidal salts associated with the casein) and soluble (serum) salts exist in equilibrium. While the soluble citrate and phosphate compete with the casein for calcium ions (resulting in the formation of calcium citrate and insoluble calcium phosphate), the polyvalent casein is the main player controlling the equilibrium concentrations of salts. However, slight changes in pH and concentrations of serum salts (e.g. as a consequence of natural variation or fortification) can affect the equilibrium balance, and consequently the charge and reactivity of the casein.

1.2.4Milk lipids

Cows’ milk typically contains ~3.7 g 100 g−1 lipid, but the level varies significantly (from ~3.0 to 5.0 g 100 g−1) with breed, diet, health, stage of lactation and animal husbandry. Triacylglycerols, denoted as milk fat, represent ~96–99 g 100 g−1 lipid. The remaining (1–2 g 100 g−1) consists of phospholipids (0.8 g 100 g−1), diacylglycerols, sterols (0.3 g 100 g−1) and trace quantities of carotenoids, fat-soluble vitamins and traces of free fatty acids (FFA) (Jensen, 2002; Huppertz et al., 2009). The fat in milk exists in the form of dispersed globules (~2–6 μm average volume weighted diameter) (Wiking et al., 2004), surrounded by a lipoprotein membrane (milk fat globule membrane, MFGM) (Keenan & Maher, 2006). The MFGM stabilises the enclosed fat against coalescence and fusion (and hence, phase separation) and access from lipases, such as the lipoprotein lipase (LPL) naturally present in native milk, or from lipases of contaminating microorganisms, such as Pseudomonas spp. (Ward et al., 2006). Inadvertent damage of the membrane, as, for example, by manhandling of the milk (e.g. excessive shearing, turbulence, cavitation; see Section 1.5.4), is highly undesirable in cheese manufacture. It leads to free fat in the cheese milk, lower recovery of milk fat to cheese, lipolysis of the fat by lipases that survive pasteurisation treatment, high levels of FFA and undesirable flavours (e.g. bitter, soapiness, metallic), especially in some cheese types (e.g. Emmental, Gouda, Cheddar). In the latter cheeses, only low to moderate levels of FFA are required for satisfactory flavour (Cousin & Marth, 1977; Woo, 1983;Gripon, 1993;Brand et al., 2000; Collins et al., 2004; Ouattara et al., 2004; Deeth & FitzGerald, 2006; see also Chapter 8). Nevertheless, there are a number of applications in cheese manufacture where the cheese milk is homogenised, resulting in physical breakage of the MFGM and its replacement by a newly formed membrane composed of casein and whey proteins, and smaller fat globules (Huppertz & Kelly, 2006). The reformed fat globule, owing to its smaller size (~1.0 μm), is stable to flocculation and creaming, but does not isolate the enclosed fat from lipolytic enzymes. These properties are exploited in the manufacture of cheeses (see Chapter 8):

high-fat acid-curd cheeses, such as Cream cheese, where the smaller fat globules prevent flocculation and creaming during the relatively long incubation/gelation period and where the reformed fat globule membrane enables the fat globule to behave as a fat-filled protein particle, become an integral part of the gel network during acid gelation and contribute to the desired texture characteristics (Guinee & Hickey, 2009; cf. Chapter 8); and

Table 1.3 Free fatty acid profile of milk fat triacylglycerols.

Source: Compiled from Jensen (2002) and MacGibbon & Taylor (2006).

aValues in parentheses indicate the range of values reported in the literature.

rennet-curd cheeses where a high level of lipolysis is desirable (e.g. blue-type cheeses), where added lipases or lipases from secondary starter cultures can access the fat more easily, bring about selective hydrolysis of the triacylglycerols and release the FFA that lead to the desired flavour.

The principal fatty acids in milk fat on a total weight basis are C16:0 (palmitic), C18:1(oleic) and C14:0 (myristic) in decreasing order (Table 1.3). While the shorter chain fatty acids (C4:0 to C12:0) are present in lower quantities on a weight basis, they are primarily responsible for the piquant flavour of hard Italian cheeses, such as Parmesan and Romano, or the sharp goaty/sheep-like flavours of soft goat milk cheeses. These fatty acids are hydrolysed from the milk fat triacylglycerols by lipase enzymes, which gain access owing to damage of the MFGM during cheese manufacture and maturation. The principal sources of these lipases are added exogenous enzymes (added rennet paste, pregastric esterase), secondary flora (Brevibacterium linens, Penicillium roqueforti, Geotrichiun candidum; see also Chapter 6), starter culture lactic acid bacteria and culture adjuncts (Lactococcus spp., Lactobacillus helveticus) (Collins et al., 2004; Hickey et al., 2006b; Santillo et al., 2007; Hashemi et al., 2009; Jooyandeh et al, 2009).

1.3 Principles of cheese manufacture

Cheese is a concentrated protein gel, which occludes fat and moisture. Its manufacture essentially involves gelation of cheese milk, dehydration of the gel to form a curd and treatment of the curd (e.g. dry stirring, cheddaring, texturisation, salting, moulding, pressing). The moulded curd may be consumed fresh (shortly after manufacture, for example within 1 week) or matured for periods of ~2 weeks to 2 years to form a ripened cheese. The gelation of milk may be induced by:

selective hydrolysis of the κ-casein at the phenyalanine105–methionine106 peptide bond by the addition of acid proteinases, referred to generically as rennets (chymosin, pepsin);acidification (using starter cultures or food-grade acids and/or acidogens), at a temperature of 20–40°C, to a pH value close to the isoelectric pH of casein, i.e. ~4.6; and/ora combination of acid and heat, for example heating milk at pH ~5.6 to ~90°C.

1.3.1Rennet-induced gelation

On treatment of milk with chymosin (rennet), the κ-casein is hydrolysed, with the primary cleavage point being the peptide bond phenylalanine105–methionine106, and the liberation of the highly charged, hydrophilic methionine106–valine169 caseinomacropeptide into the milk serum (whey). This results in an effective ‘shaving’ of the hairy layer from the micelle surface, a marked reduction in the negative surface charge to ~−10 mV, and an increase in the attractive forces between, or ‘stickiness’ of, the para-casein micelle surfaces. Consequently, the latter begin to aggregate when sufficient κ-casein is hydrolysed (~80–90 g 100 g−1 of total; Green et al., 1978; Dalgleish, 1979), resulting in the formation of clusters/aggregates of para-casein micelles that fuse gradually and eventually ‘knit’ into a restricted, periodic repeating, solid-like viscoelastic gel network (Fig. 1.1). The enzymatic stage of rennet coagulation and the aggregation of enzymatically altered sensitised para-casein micelles overlap. While the exact contribution of calcium to rennet coagulation is unclear, it is likely that the casein calcium (which in effect may be considered as pre-bound ionic calcium) is the principal agent inducing cross-linking and aggregation of the para-casein micelles into a gel. The serum ionic calcium in milk is in equilibrium with the casein calcium. Hence, apart from reflecting the level of casein-bound calcium, serum ionic calcium probably plays little, or no, direct role in rennet-induced casein aggregation and gelation of milk. Similarly, the progressive increase in gel firmness of rennet-treated milks on the addition of calcium chloride (ionic calcium) while retaining a constant pH (Fig. 1.2) probably reflects the consequent increases in the levels of casein calcium and CCP rather than an increase in the serum ionic ion calcium per se. Hence, it is noteworthy that on concentration of milk by evaporation, the calcium ion activity slightly decreases from ~ 1.0 to 0.75 mM L−1 while the levels of micellar calcium increase (Nieuwenhuijse et al., 1988). Rennet-induced gelation of milk is hindered by a variety of factors, which either:

restrict access of the rennet to its substrate (κ-casein), for example complexation of denatured whey protein with κ-casein at the micelle surface, as a result of high heat treatment of the cheese milk (Fig. 1.1; Guinee, 2003);act as obstacles to the aggregation and fusion of rennet-treated casein micelles, for example κ-casein/β-Lg appendages at micelle surface, or serum κcasein/β-Lg particles (Guyomarc’h, 2006);

Fig. 1.1 Effect of pasteurisation temperature on changes in storage modulus G’ during the rennet gelation of milk. Note: Milks were heated to various temperatures (in °C) for 26 s prior to rennet addition: 72 (•), 74.6 (), 75.9 (°) or 78.5 (Δ); the milks were cooled to 31°C, adjusted to pH 6.55 if necessary with lactic acid solution (5 g 100 g−1), treated with chymosin (Chymax Plus, Pfizer Inc., Milwaukee, WI) at a rate of 0.18 mL of undiluted rennet per litre of milk; all milks had similar contents of protein (3.3 g 100 g−1) and fat (3.4 g 100 g−1); G’ was measured dynamically using low-amplitude strain oscillation rheometry (controlled stress rheometer).

Fig. 1.2 Changes in curd firmness at 60 min (A60; •) and curd firming rate (1/k20; ) of skimmed milk as a function of the level of added calcium chloride. Note: All milk samples (~3.45 g protein 100 g−1 of milk) were adjusted to pH 6.55 prior to measuring the rennet gelation properties at 31°C on the Formagraph (Type 1170, Foss Electric, Denmark); the following parameters were measured k20, a measure of time from the onset of gelation to a output signal width of 20 mm, and A60, the width of the output signal at 60 after rennet addition.

reduce the ‘stickiness’ of rennet-altered casein micelles, for example increased ionic strength (e.g. by the addition of NaCl to the cheese milk as in Domiati cheese) (Awad, 2007; Huppertz, 2007), negative charge (high pH); and/orreduce the degree of bonding between touching micelles, for example reducing the level of calcium by the addition of ethylenediaminetetraacetic acid (EDTA) or other chelants (Shalabi & Fox, 1982; Mohammad & Fox, 1983; Choi et al., 2007), ion exchange (Mei-Jen-Lin et al., 2006) and/or dialysis (Wahba et al., 1975), or by a naturally low level of Ca2+ as in late lactation milks or milks from cows with subclinical mastitis (White & Davies, 1958a).

Following gel formation, the resultant milk gel is subjected to a number of operations that promote the release of whey, an approximate tenfold concentration of the casein, fat and micellar calcium phosphate components, and a transformation to a curd with much higher dry matter content than the original milk gel (45 g 100 g−1 for Cheddar curd at whey drainage). These operations include cutting the gel into pieces (referred to as curd particles, ~0.5–1.5-cm cubes), stirring and heating the particles in expressed whey, reducing the pH of the aqueous phase inside the curd particle by fermentation of lactose to lactic acid (by the lactic bacteria in the starter culture added to the milk prior to rennet addition), and physical draining of the whey from the curd particles by pumping the curd particle–whey mixture onto perforated screens (cf. Chapter 8). Following whey drainage, the curd particles knit together into a cohesive mass of curd, which is treated to enhance further whey expulsion and concentration to the desired dry matter content of the cheese variety being manufactured; these treatments differ according to variety but typically include further lactose fermentation and pH reduction, cutting the curd mass into pieces (slabs), moulding the pieces to the desired shape and weight of finished cheese, salt addition and pressing. During the dehydration process of the gel, protein concentration and aggregation continues via various types of intra- and intermolecular interactions (Lucey et al., 2003), including calcium bridging (between glutamate/aspartate residues, calcium–CCP bridges between phosphoserine residues), hydrophobic interactions between lipophilic domains and electrostatic interactions (other than calcium bridging). The strength of these interactions is modulated by ionic strength, pH, calcium and temperature, and hydrolysis of proteins to peptides, which alters the hydrophile/lipophile balance of the proteinaceous fraction.

Following manufacture, rennet-curd cheeses are usually matured or ripened by holding under specific conditions of temperature and humidity for periods which range from ~2 to 4 weeks for soft cheeses (for Camembert-type cheeses) to ~2 years for some hard cheeses (for Parmesan-style cheeses). During this period, a host of physico-chemical changes take place which transform the ‘rubbery/chewy’-textured fresh cheese curd to the finished cheese with the desired variety quality characteristics, for example a soft, smooth, short and adhesive texture with a mushroom-like flavour and creamy mouth-feel for Camembert, or a long, elastic sliceable texture and mild, sweet flavour for Leerdammer cheese. These physico-chemical changes include:

glycolysis, conversion of residual lactose to lactic acid by the starter culture and of lactic acid to other compounds, such as acetic acid and propionic acid by secondary starter cultures such as Propionobacteria freudenreichii subsp. shermanii in Emmental-style cheese;proteolysis, hydrolysis of caseins to peptides and free amino acids by proteinases and peptidases present in the cheese (residual rennet; plasmin, and proteinases and peptidases from the cells of starter culture and non-starter lactic acid bacteria); andlipolysis, involving the hydrolysis of triacylglycerols to FFA, di- and monoacylglycerols by lipases and esterases from various sources, including native milk LPL, added pregastric esterases and/or secondary cultures.

The physico-chemical and biochemical changes that occur during ripening are discussed in Chapter 8 and several comprehensive reviews are available (Collins et al., 2004; McSweeney & Fox, 2004; Upadhyay et al., 2004; Kilcawley, 2009).

Of particular interest in relation to milk composition and cheese quality is the impact of the proportion of intact αs1-casein content in milk on casein aggregation, strength of the rennet- induced milk gel and texture of the final cheese. The sequence of residues 14–24 is a strongly hydrophobic domain and confers intact αs1-casein with strong self-association and aggregation tendencies in the cheese environment (Creamer et al., 1982); interestingly, this domain also has 3 mol of glutamate, which are expected to contribute to intra- and intermolecular calcium bridges. It has been suggested that self-association of αs1-casein in cheese, via these hydrophobic ‘patches’, leads to extensive cross-linking of para-casein molecules and thus contributes to the overall continuity and integrity of the casein matrix in the cheese curd (de Jong, 1976,1978; Creamer et al., 1982;Lawrence et al., 1987). The early hydrolysis of αs1-casein at the phenylalanine23-phenylalanine24 peptide bond, by residual rennet retained in the cheese curd following manufacture (~10% of that added), results in a marked weakening of the para-casein matrix and reductions in fracture stress and firmness of the cheese during maturation (deJong, 1976,1977; Creamer & Olson, 1982; Malin et al., 1993;Tunick et al., 1996; Fenelon & Guinee, 2000). This hydrolysis is a key step in mediating the conversion from a fresh rubbery curd to a mature cheese with the desired textural and cooking properties (meltability) (cf. Chapters 7–10).

1.3.2Acid-induced gelation

The caseins in milk are insoluble at their isoelectric points (pH ~4.6) at temperatures >~8°C (Mulvihill, 1992). This property is exploited in the formation of acid-curd cheeses, such as Cottage cheese, Quark and Cream cheese, the manufacture of which involves slow quiescent acidification of the cheese milk to pH ~4.6–4.8 by starter culture, acidogens (e.g. glucono-δ-lactone) at temperatures of 20–30°C (Guinee et al., 1993; Lucey & Singh, 1997; Fox et al., 2000; Farkye, 2004; Lucey, 2004; Schulz-Collins & Zenge, 2004). Acidification results in a number of physico-chemical changes promoting hydration/dispersion or dehydration/aggregation effects on the casein micelle, with the ratio of these effects changing as the pH declines during the acidification (fermentation) process. Reducing the pH from 6.6 to ~5.2–5.4 results in a decrease in the negative charge of the micelles due to titration of negative charges with H+ ions. Nevertheless, this is generally not accompanied by the onset of gelation because of:

solubilisation of micelle ‘cementing’ agent CCP (fully soluble at pH ~5.2 at 20°C);diffusion of all caseins from the micelle to the serum (owing to a decrease in the degree of electrostatic interaction between phosphoserine residues of αs- and β-caseins and the CCP nanoclusters);increases in ionic strength of the serum phase; andhydration of the casein micelles.

However, further reduction in pH in the range ~5.2–4.6 results in aggregation of casein and gel formation, as forces promoting dispersion of casein micelles are overtaken by the sharp reductions in the negative charge and hydration of the casein, the collapse in steric effect associated with the κ-casein C-terminal ‘hairs’ and the increase in hydrophobic interactions. The onset of gelation typically occurs at pH ~5.1 and further reduction in pH toward 4.6 coincides with the eventual formation of a continuous gel structure with sufficient rigidity to enable separation of whey from the curd by physical means (e.g. breakage, stirring, and whey drainage, or centrifugation). The increase in gel rigidity coincides with a sigmoidal increase in the elastic shear modulus of the gelling cheese milk, as the pH continues to decrease towards 4.6 during incubation and the fermentation of lactose to lactic acid (Fig. 1.3).

High heat treatment of the cheese milk (e.g. 95°C for ≤1 min) leads to an increase in the pH at the onset of gelation (from ~4.7 to 5.3) and the rigidity of the resultant gel (cf. Chapter 8; Vasbinder et al., 2003; Anema et al., 2004). These changes coincide with increases in the level of whey protein denaturation and its covalent interaction with κ-casein,

Fig. 1.3 Effect of milk treatment (heated to 90°C for 30 min − •, or unheated − °) on changes in modulus G’ during acid gelation of milk. Note: The milks (1.5 g fat 100 g−1, 3.2 g protein 100 g−1)were cooled to 40°C and a thermophilic starter culture (Streptococcus thermophilus) was added at a rate of 2.5 g 100 g−1; G’ was measured dynamically using low-amplitude strain oscillation rheometry (controlled stress rheometer).

via thiol-disulphide interchange. This interaction occurs both at the surface of the micelle, resulting in the formation of filamentous appendages projecting from the micelle surface, as well as in the serum phase when κ-casein dissociates from the micelle into the serum and interacts with β-Lg to form soluble complexes that sediment as the pH is reduced during fermentation and/or rennet treatment. The different types of interactions are influenced by pH and the level of whey proteins (Donato & Guyomarc’h, 2009; cf. Chapter 8). In situ denatured whey protein increases the concentration of gel-forming protein, the spatial uniformity of the gel matrix and the number of stress-bearing strands in the matrix. Denatured whey proteins, whether in the form of filamentous appendages (κ-casein/β-Lg) that occur at the surface of micelle and ‘flatten’ on pH reduction or that occur as serum-soluble κ-casein/β-Lg particles that sediment on pH reduction (and/or rennet treatment), act as obstructions that physically obstruct/prevent a high level of interaction of the native casein micelles and, thereby, lead a more continuous gel structure with higher rigidity. The increase in micelle size resulting from complexation with denatured whey protein (Anema & Li, 2003) is conducive to an earlier touching of the casein micelles during the acidification/gelation process and the onset of gelation at a higher pH. The changes in gel structure associated with high heat treatment of milk lead to significant increases in the stiffness (G’) and visual smoothness of the resultant acid gel (Fig. 1.3), a principle which has long been exploited in the manufacture of yoghurt.

In the manufacture of acid-curd cheeses, the milk gel is cut or broken, and whey removal is achieved by various means including centrifugation, ultrafiltration and/or straining the broken gel in muslin cheese bags. In some varieties (e.g. cream cheese), whey separation is further enhanced by heating the broken gel to temperatures of ~80°C prior to centrifugation or to ~50°C prior to ultrafiltration or straining. Treatments of the curd differ with cheese variety. In the manufacture of Quark, the temperature of the concentrated curd (~18 g 100 g−1