yoghurt - science and technology 2ed 1999 - tamime & robinson

Preface to second edition Preface to first edition 1 Historical background 1.1 Introduction 1.2 Evolution of the process 1.3 Diversity of fermented milks 2.2 Preliminary treatment of t

Related titles on food science and technology from Woodhead Publishing:

General Feta and related cheeses

Eds R K Robinson and A Y Tamime (respectively Reading University and Scottish tural College, U.K.)

Agricul-Contents include: traditional methods for the manufacture of Feta cheese; industrial facture of Feta cheese; manufacture of Halloumi; manufacture of Egyptian, soft pickled cheeses; miscellaneous white-brined cheeses; cheeses made by direct acidification

manu-272pp 234 ¥ 173 mm hardback 1991

Chilled foods: a comprehensive guide

Eds C Dennis and M Stringer (respectively Director-General and Director of Food Science Division, Campden and Chorleywood Food Research Association)

“This book lives up to its title in reviewing a major section of the food industry.” tional Food Hygiene

Interna-Contents include: trends in consumer tastes and preferences; market place product edge; legislation; refrigeration of chilled foods; temperature monitoring and measurement; processing; chilled food packaging; chilled foods microbiology; conventional and rapid ana- lytical microbiology; microorganisms and safety in refrigerated foods; non-microbial factors affecting quality and safety; shelf-life determination and challenge testing; quality and con- sumer acceptability; cleaning and disinfection; hygienic design; total quality management 400pp 234 ¥ 173 mm hardback 1992 ISBN 1 85573 270 X

knowl-Food safety and quality Instrumentation and sensors for the food industry

Ed Erika Kress-Rogers (ALSTOM; formerly Leatherhead Food RA)

“In this book existing and forthcoming instrumentation systems are surveyed to provide a practical guide for those involved in designing, selecting and using such systems in the food industry International experts have presented their knowledge in an applied framework to

provide the most comprehensive workbook for practitioners ever written.” Food Science and Technology Abstracts

Contents include: colour measurement; compositional and texture analysis; rheological surement; analysis of water activity; ultrasound; infrared techniques; microwave measure- ment; laboratory instrumentation; chemical sensors, biosensors and immunosensors 740pp 234 ¥ 156 mm hardback 1993 ISBN 1 85573 363 3

mea-For more information contact Customer Services at Woodhead Publishing Ltd, Abington Hall, Abington, Cambridge CB1 6AH, England; tel: +44 (0)1223 891358 ext.30; fax: +44 (0)1223 893694; e-mail: wp@woodhead-publishing.com Please also visit our web site: www.woodhead-publishing.com

YOGHURT Science and Technology

R K Robinson

University of Reading,Department of Food Science & Technology,

Reading RG6 2AP,England

Cambridge England

YOGPR 6/1/99 4:43 PM Page iii

© 2000 Woodhead Publishing Limited

Published by Woodhead Publishing Limited Abington Hall, Abington

Cambridge CB1 6AH England

Published in North and South America by CRC Press LLC

2000 Corporate Blvd, NW Boca Raton FL 33431 USA

First published 1985, Pergamon Press Ltd Second edition 1999, Woodhead Publishing Ltd and CRC Press LLC

© 1999, Woodhead Publishing Ltd The authors have asserted their moral rights.

Conditions of sale This book contains information obtained from authentic and highly regarded sources Reprinted material is quoted with permission, and sources are indicated Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publishers.

The consent of Woodhead Publishing Ltd and CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale Specific permission must be obtained in writing from Woodhead Publishing Ltd or CRC Press LLC for such copying.

Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe British Library Cataloguing in Publication Data

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

Library of Congress Cataloging in Publication Data

A catalog record for this book is available from the Library of Congress.

Woodhead Publishing ISBN 1 85573 399 4 CRC Press ISBN 0-8493-1785-1

CRC Press order number: WP1785 Cover design by The ColourStudio Typeset by Best-set Typesetter Ltd., Hong Kong Printed by TJ International, Cornwall, England.

Preface to second edition

Preface to first edition

1 Historical background

1.1 Introduction

1.2 Evolution of the process

1.3 Diversity of fermented milks

2.2 Preliminary treatment of the milk base

2.2.1 Milk as a raw material2.2.2 Separation of cellular matter and other contaminants

present in milk 2.2.3 Milk reception and storage2.3 Standardisation of fat content in milk

2.4 Standardisation of the solids-not-fat content in milk

2.4.1 Traditional process2.4.2 Addition of milk powder2.4.3 Addition of buttermilk powder2.4.4 Addition of whey powder and/or whey protein

concentrates2.4.5 Addition of casein powder2.4.6 Concentration by vacuum evaporation (VE)2.4.7 Concentration by membrane filtration2.4.8 Addition of non-milk proteins

2.5 Addition of stabilisers/emulsifiers

YOGPR 6/1/99 4:43 PM Page v

© 2000 Woodhead Publishing Limited

2.5.1 General background2.5.2 Miscellaneous properties and conditions2.6 Addition of sweetening agents

2.6.1 General introduction2.6.2 Types of carbohydrate sweetener2.7 Addition of miscellaneous compounds2.7.1 Penicillinase

2.7.2 Preservatives2.7.3 Minerals, vitamins and/or fatty acids2.8 Homogenisation

2.8.1 Effects on milk constituents2.8.2 Aspects of processing2.9 Heat treatment

2.9.1 Destruction of micro-organisms/pathogens 2.9.2 Production of stimulatory/inhibitory factors 2.9.3 Changes in the physicochemical properties of milk2.9.4 Processing effects on the physical properties of

the gel2.10 Fermentation process2.10.1 Introduction 2.10.2 Starter organisms2.10.3 Gel formation2.11 Cooling

2.11.1 One-phase cooling2.11.2 Two-phase cooling2.12 Addition of fruit/flavouring/colouring ingredients 2.12.1 Fruits

2.12.2 Flavouring agents 2.12.3 Colouring matter2.13 Packaging

2.13.1 Introduction2.13.2 Functions of packages2.13.3 Types of packaging materials 2.13.4 Comparative studies on permeability of different

yoghurt packages 2.13.5 Migration of monomers and other compounds2.13.6 Tamper-evident packaging

2.13.7 Aluminium foil lids 2.13.8 Sterilisation of packaging materials2.13.9 Outer or shipping container2.14 Refrigerated cold storage, transport and distribution2.14.1 The cold store

2.14.2 During transport2.14.3 The retail shop and the consumer2.15 Conclusion

2.16 References

3 Processing plants and equipment

3.1 Home or small-scale production

3.1.1 Miscellaneous systems 3.1.2 Packaging system3.2 Medium-scale production

3.2.1 Hand operated vat3.2.2 Multipurpose vat3.2.3 Mini dairy3.2.4 Small-scale packaging machines3.3 Large-scale production

3.3.1 Milk reception, handling and storage3.3.2 Standardisation of fat content in milk3.3.3 Fortification of milk solids

3.3.4 Homogenisation3.3.5 Heat treatment3.3.6 Fermentation/incubation of the milk3.3.7 Cooling

3.3.8 Pumps3.3.9 Miscellaneous fittings3.3.10 Fruit handling and mixing units3.3.11 Filling machines

3.3.12 Miscellaneous handling, chill cooling and refrigerated

cold storage3.4 Mechanisation of yoghurt production and plant design3.5 Continuous yoghurt production

3.5.1 Background3.5.2 The NIZO process3.5.3 Recent developments3.6 Automation/process control

3.6.1 Levels of automation3.6.2 Area/department 13.6.3 Area/department 23.6.4 Area/department 33.6.5 Area/department 43.6.6 Area/department 53.6.7 Area/department 63.6.8 Management information system3.6.9 System architecture

3.6.10 System security3.7 Building design, maintenance and services

3.7.1 General background and introduction3.7.2 Location of a dairy plant

3.7.3 Layout of a dairy plant3.7.4 Design and construction of dairy buildings 3.8 Conclusion

3.9 References

4 Plant cleaning, hygiene and effluent treatment

Cleaning aspects

4.1 Primary objectives

4.2 Principles of the cleaning process

YOGPR 6/1/99 4:43 PM Page vii

© 2000 Woodhead Publishing Limited

4.3 Factors involved in the selection and performance

of a detergent4.3.1 Type/range of detergents used in the yoghurt

industry4.3.2 Type of soiling matter4.3.3 Water hardness and quality4.3.4 Miscellaneous factors 4.4 Cleaning methods

4.4.1 Manual cleaning4.4.2 Cleaning-in-place4.4.3 Miscellaneous cleaning methods4.5 Factors influencing the efficiency of cleaning4.5.1 Type of soil

4.5.2 Method of cleaning adopted4.5.3 Contact time

4.5.4 Concentration of detergent solution4.5.5 Temperature

4.5.6 Flow rate or velocity4.5.7 Acid wash

4.5.8 Plant design4.5.9 Chemical composition of a detergent4.6 Specific cleaning and sterilisation operations of yoghurtprocessing equipment and utensils

Sterilisation aspects

4.7 Fundamentals of the sterilisation process4.8 Methods of sterilisation and/or sanitation4.8.1 Heat

4.8.2 Chemical agents4.8.3 Filtration4.8.4 Irradiation4.8.5 Spraying, fogging or fumigation4.9 Kinetics and mechanisms of microbial destruction4.10 Means of assessing the sanitary condition of

a processing plant4.10.1 Physical examination4.10.2 Chemical examination4.10.3 Bacteriological examination

Effluent treatment

4.11 Background4.12 Nature of pollution4.13 Methods of effluent treatment4.14 References

5 Traditional and recent developments in yoghurt production and related products

5.1 Introduction 5.2 Standard commercial yoghurt5.3 Yoghurt made from different mammalian milks5.3.1 Goat’s milk yoghurt

5.3.2 Sheep’s milk yoghurt 5.3.3 Buffalo’s milk yoghurt 5.3.4 Camel’s milk yoghurt5.4 Pasteurised/UHT/long-life/heat shock yoghurt

5.4.1 Technology of manufacture5.4.2 Processing effects on properties of product5.5 Drinking yoghurt

5.5.1 Background5.5.2 Processing aspects5.5.3 Other beverage products5.5.4 Carbonated products5.6 Lactose hydrolysed yoghurt (LHY)

5.7 Concentrated/strained yoghurt

5.7.1 Introduction and nomenclature5.7.2 Processing methods

5.7.3 Miscellaneous properties5.7.4 Microstructure

5.7.5 Related products5.8 Frozen yoghurt

5.8.1 Background, standards and marketing5.8.2 Technology of manufacture

5.8.3 Related products5.9 Dried yoghurt

5.9.1 Introduction5.9.2 Processing methods5.9.3 Kishk and related products5.10 Bio-yoghurt

5.11 Fat-substitutes yoghurt

5.12 Vegetable oil yoghurt

5.13 Chemically acidified yoghurt

5.14 Soy-milk yoghurt

5.15 Miscellaneous yoghurt products

5.16 Future developments and conclusion

6.3 Factors affecting slow growth of starter cultures

6.3.1 Compounds that are naturally present in milk6.3.2 Effect of incubation temperature and inoculation

rate6.3.3 Mastitis milk and somatic cell count 6.3.4 Hydrogen peroxide (H2O2)

YOGPR 6/1/99 4:43 PM Page ix

© 2000 Woodhead Publishing Limited

6.3.5 Antibiotic residues 6.3.6 Detergent and disinfectant residues6.3.7 Environmental pollution

6.3.8 Bacteriophages6.3.9 Bacteriocins6.3.10 Miscellaneous factors6.4 Conclusion

6.5 References

7 Biochemistry of fermentation

7.1 Introduction7.2 Carbohydrate metabolism7.2.1 Homolactic fermentation 7.2.2 Heterolactic fermentation7.2.3 Lactase activity

7.2.4 Production of lactic acid 7.2.5 Production of exopolysaccharide (EPS)7.2.6 Production of flavour compounds7.3 Protein metabolism

7.3.1 Constituent compounds of the milk protein molecule7.3.2 Proteolytic enzymes

7.3.3 Proteolysis by the yoghurt and bio organisms7.3.4 Products of proteolysis

7.4 Lipid/fat metabolism7.4.1 Introduction7.4.2 Changes in the level of free and esterified fatty acids7.4.2 Changes in the level of volatile fatty acids

7.5 Vitamin metabolism7.5.1 General background 7.5.2 Biosynthesis of folic acid 7.5.3 Biosynthesis of niacin 7.5.4 Biosynthesis of vitamin B67.6 Miscellaneous changes

7.7 References

8 Preservation and production of starter cultures

8.1 Introduction8.2 Methods of starter culture preservation 8.2.1 Liquid starters

8.2.2 Dried starters 8.2.3 Frozen starters 8.3 Technology of cell biomass production8.3.1 Growth characteristics 8.3.2 Concentration of cell biomass8.4 Production systems for starter cultures 8.4.1 Introductory remarks

8.4.2 Simple microbiological techniques 8.4.3 Mechanically protected systems 8.4.4 pH control systems

8.4.5 Bacteriophage resistant/inhibitory medium

(BRM/BIM) 8.5 Conclusion

9.4 Lipids

9.5 Vitamins and minerals

9.6 Yoghurt and health

9.6.1 Therapeutic properties of yoghurt9.6.2 Therapeutic properties of bio-yoghurt9.7 Conclusion

10.4 Examination of raw materials

10.4.1 Liquid milk 10.4.2 Milk powder 10.4.3 Starter cultures for standard yoghurt10.4.4 Starter cultures for bio-yoghurts 10.5 Quality appraisal of retail products

10.5.1 Analysis of chemical composition10.5.2 Assessment of physical characteristics10.5.3 Microbiological analysis

10.5.4 Assessment of organoleptic characteristics 10.6 Conclusion

10.7 References

Appendix I Different ways in which titratable acidity is expressed

and their relative values to % lactic acid

Appendix II Temperature conversion

Appendix III Volume units

Appendix IV Weight/mass units

Appendix V Miscellaneous units

YOGPR 6/1/99 4:43 PM Page xi

© 2000 Woodhead Publishing Limited

Appendix VI Work/energy and other related units Appendix VII Force and pressure units

Appendix VIII Length and area units Appendix IX Pearson square method and algebraic methods

This book is dedicated to our families

YOGPR 6/1/99 4:43 PM Page xiii

© 2000 Woodhead Publishing Limited

Preface to second edition

When the first edition of this book was published in 1985, the retail markets in tralasia, Europe and North America were dominated by just one product – stirredfruit yoghurt, with natural set yoghurt occupying a well-defined niche Some tradi-

Aus-tional products like labneh and drinking yoghurt were manufactured on a small scale

but, in general, the choice available to consumers was strictly limited

Over the last ten years, this scenario has changed Initially, competition for a share

of the lucrative market for fermented milks gave rise to numerous variants of thebasic products, but a more dramatic impact was achieved by the introduction ofmild-tasting bio-yoghurts In these latter products, selected bacteria with prophy-lactic/therapeutic properties are involved with the fermentation and, whilst manyaspects of the yoghurt-making process remain the same, the introduction of thesenew cultures has led to some significant changes in both consumer attitudes andmanufacturing practices

In light of these recent developments, it became apparent that a revision of thisbook was long overdue, and it is to be hoped that readers will appreciate the intro-duction of bio-yoghurt and the additional information about this remarkable sector

of the dairy industry

Automation in yoghurt-making involves complex engineering and design and thistechnology has been covered by Mr J Bird and Mr I Chester who represent two ofthe foremost equipment manufacturers in the world We would like to acknowledgetheir assistance and that of all the companies who provided us with technical infor-mation and illustrations Last but not least, we are grateful to Mrs A Peacock (SAC)for her patience in typing the manuscript, and Mrs Y Gamble and E McCall (SAC)for their skills in taking the necessary photographs and drawing the illustrations

A Y Tamime

R K Robinson

Preface to first edition

Although there are numerous fermented milks produced on a local basis aroundthe world, only yoghurt has achieved a truly international distribution This popu-larity stems from a number of sources: the pleasant, aromatic flavour of naturalyoghurt, its reputation as a foodstuff associated with good health, but perhaps aboveall from the fact that the thick, creamy consistency makes it an ideal vehicle for fruit.Thus, it was the natural compatibility with fruit that really brought yoghurt into theretail markets, and since the introduction of fruit yoghurts during the 1950s saleshave climbed steadily upwards

Today millions of gallons of yoghurt are produced each year, and yet becausemanufacture is still, in essence, a natural biological process, success can never betaken for granted It is this capricious nature of the fermentation that makes it sofascinating, and indeed if the system were not so prone to variation, then therewould have been little motivation to produce this book at all Some aspects of pro-duction have, of course, become fairly standard, but so many areas of potentialdifficulty remain that only a thorough appreciation of the nature of yoghurt canprovide those associated with its production and distribution with the confidencethat eliminates product failure

It goes without saying that the best teacher is experience, but if this book canoffer some preliminary guidance on the intricacies of handling yoghurt, then its com-pilation will have been worthwhile

trans-of fermented milks such as yoghurt.

Although there are no records available regarding the origin of yoghurt, thebelief in its beneficial influence on human health and nutrition has existed in manycivilisations over a long period of time According to Persian tradition, Abrahamowed his fecundity and longevity to yoghurt and, in more recent times, EmperorFrancis I of France was said to have been cured of a debilitating illness by con-suming yoghurt made from goat’s milk (Rosell, 1932)

It is likely, however, that the origin of yoghurt was the Middle East, and the lution of this fermented product through the ages can be attributed to the culinaryskills of the nomadic people living in that part of the world Today, fermented milk

evo-products are manufactured in many countries (Campbell-Platt, 1987; Kurmann et

al., 1992), although few are of commercial significance.

1.2 Evolution of the process

The production of milk in the Middle East has always been seasonal, being restrictedusually to no more than a few months of the year The main reason for this limited

availability of milk is that intensive animal production has never really existed, sothat, as in early history, farming is in the hands of nomadic peoples who move fromone area to another following the pastures This type of existence forces nomads to

be in the wilderness for months at a time, far away from populated cities and lages where they could sell their animal produce Another major factor is that theMiddle East has a subtropical climate and summer temperatures can reach as high

vil-as 40°C In such a climate, milk turns sour and coagulates within a short time ofmilking, particularly as the milk is produced under primitive conditions Thus, theanimals are hand milked, no cooling of the milk is possible, and the risk of con-tamination by micro-organisms from the air, the animal, the feeding stuff or thehands of the milker is extremely high Under these conditions the possibility oftransporting or even keeping milk for any length of time is non-existent As a resultthe bulk of the population consume milk only rarely, and even the nomadic peoplehave to utilise the milk virtually as it is produced

However, it may well have been evident even at an early stage that the souring

of milk was by no means a uniform process Thus, the fermentation brought about

by non-lactic acid bacteria gives rise to a product which is insipid and stale and,

Table 1.1 Selection of yoghurt and yoghurt-like products that have been identified in the Middle East and elsewhere

countries

furthermore, the coagulum is irregular, filled with gas holes and shows extreme wheysyneresis Lactic acid bacteria, however, act on milk to produce a fermented productwhich is pleasant to eat or drink; this latter product was usually referred to as sourmilk.

The animals that are raised by the nomadic peoples of the Middle East are cows,goats, sheep and camels, and gradually the nomadic tribes evolved a fermentationprocess which brought under control the souring of these various milks In particu-lar, the process might have included:

• use of the same vessels, or the addition of fresh milk to an on-going tion, relying mainly on the indigenous microflora to sour the milk;

fermenta-• heating the milk over an open fire to concentrate the milk slightly, so that thefinal coagulum would acquire an attractive viscosity due to the modified prop-erties of the casein, again a change which would have improved the quality ofthe end product;

• seeding the heat-treated and cooled milk (blood or ambient temperature) withsour milk from a previous batch, so enabling the thermophilic strains of lacticacid bacteria to become predominant;

• gradual selection of lactic acid bacteria capable of tolerating high levels of lacticacid and of giving the product its distinctive flavour;

• eradication of any pathogenic micro-organisms present in the milk

Although the evolution of the process was strictly intuitive, the production of sourmilk soon became the established pattern of preservation, and since the early 1900s,defined micro-organisms have been used to prepare these products on a large scale

in factories Gradually other communities learnt of this simple preservative ment for milk and one such product became known as yoghurt from the Turkishword “jugurt”; numerous variants of this word have appeared over the years and aselection of alternatives is shown in Table 1.1

treat-1.3 Diversity of fermented milks

Around 400 generic names are applied to the traditional and industrialised

fer-mented milk products manufactured throughout the world (Kurmann et al., 1992).

Although these products may have different names, they are practically the same,and a more accurate list might include only a few varieties Taking into account thetype of milk used, the microbial species which dominate(s) the flora and their prin-cipal metabolic products, Robinson and Tamime (1990) proposed a scheme ofclassification for fermented milks which divided them into three broad categories:(a) lactic fermentations, (b) yeast–lactic fermentations and (c) mould–lactic fer-mentations (Fig 1.1) Recently, these products have been extensively reviewed byTamime and Marshall (1997)

Although yoghurt has many desirable properties, it is still prone to deterioration,especially at ambient temperature, within a matter of days, and one discernible trend

in the Middle East has been the search for simple techniques to extend the keepingquality

The first step in this process turned out to be relatively simple because the tainers traditionally used by the nomads for the production of yoghurt were madefrom animal skins In normal use the yoghurt would have been consumed fairly

con-rapidly but, if left hanging in the skin for any length of time, the nature of theproduct altered dramatically Thus, as the whey seeped through the skin and evap-orated, the total solids content of the yoghurt rose and with it the acidity The endresult was a condensed or concentrated yoghurt with an acidity of > 2.0% lactic acidand a total solids content in the region of 25 g 100 g-1; the original yoghurt mighthave had a solids content of 12–13 g 100 g-1 and an acidity of around 1.5% lactic acid.

To the nomadic people, whose main sources of wealth and nourishment are theanimals that can be raised and the milk that they produce, the relative resistance ofthe condensed yoghurt to spoilage must have appeared attractive

Evidence of this trend can be found in Armenia where the mazun (Armenianyoghurt) is usually pressed to yield a product called tan or than Similarly, surplusmilk production in remote villages in Turkey is turned into concentrated yoghurt

by the daily addition of milk to yoghurt hanging in goat or sheep skins Anothermethod of concentration of yoghurt is where the product is placed in an earthen-ware vessel; the Egyptians call this product leben zeer

Nevertheless, even condensed yoghurt becomes unpalatable within a week ortwo, and it was for this reason that salted yoghurt rapidly became popular Salting

is an age-old method used by humans to preserve food, but the incorporation of saltinto concentrated yoghurt also acts as a neutralising agent to reduce the acid taste

of the product Thus, different types of concentrated yoghurt are made in Turkey bythe addition of various quantities of salt Another traditional way of prolonging thekeeping quality of concentrated yoghurt is employed in Lebanon, where the saltedproduct is made into small balls about 2 cm in diameter and placed in the sun todry Afterwards the yoghurt balls (which are partially dried) are placed in eitherglazed earthenware pots or glass jars and covered with olive oil The product is thenreferred to as winter yoghurt, that is, it is available when natural yoghurt is out of

YEAST-LACTIC FERMENTATIONS

LACTIC FERMENTATIONS

MOULD-LACTIC FERMENTATIONS

FERMENTED MILKS

Mesophilic

Thermophilic

Therapeutic

Kefir Koumiss Acidophilus - yeast milk Cultured buttermilk

Buttermilk Täfil Filmjölk Täetmjolk Långofil

Yoghurt Bulgarian buttermilk Zabadi Labneh Chakka

Bifighurt ® Acidophilus milk ABT Yakult BRA Biogarde ® Villi

Fig 1.1 Scheme of classification of fermented milk products (for details see Table 5.15)

ABT, Lactobacillus acidophilus, Bifidobacterium bifidum and Streptococcus thermophilus; BRA, Bifidobacterium infantis, Lactobacillus reuteri and L acidophilus Adapted from

Robinson and Tamime (1990) YOG1 6/1/99 4:41 PM Page 4

© 2000 Woodhead Publishing Limited

season and it has a storage life of up to 18 months; the product is spread easily onbread and consumed.

An alternative preservation process involves heating yoghurt for a few hoursover low fires of a special type of wood; the end product is referred to as smokedyoghurt This type of yoghurt is also preserved over the winter months by placing

it in jars and covering it with either olive oil or tallow

In some countries (Turkey, Lebanon, Syria, Iraq and Iran) the concentratedyoghurt is processed even further to produce a totally different product of almostindefinite keeping quality This is a dried form of yoghurt; milk is processed intoyoghurt in the traditional manner and wheat flour, semolina or parboiled wheat,known locally as burghol, is rubbed into it The yoghurt–wheat mixture is shapedinto small nuggets and placed in the sun to dry This product is called kishk and it

is sold either as nuggets or in a ground-up form as flour Kishk (as a dish) is pared by reconstituting the yoghurt–wheat mixture with water and then simmeringthe mix gently over a fire The consistency of this product, which is normally con-sumed with bread, is similar to porridge

pre-The concentrated yoghurt can be also processed into a different product calledchanklich Here again the product is partially dried, but is then mixed with spicesand herbs (presumably to assist in preservation) The mixture is then formed intoballs, placed into glass jars and finally covered with olive oil It is evident that manydifferent products can be manufactured from yoghurt and Fig 1.2 illustrates someexamples; the relationship between these various products is discussed further in

Chapter 5

1.4 Patterns of consumption

As refrigeration became widespread, so interest in these traditional productsdeclined, except among certain communities in the Middle East In their place, anew generation of yoghurts emerged, with production typically centred on a largemodern creamery, and success in the market place depending on the existence of a

Simmer over smoky fire Freeze

Dry

Churn

Mix with water

Separation

of whey

FROZEN YOGHURT

YOGHURT

SMOKED YOGHURT

DRINKING YOGHURT

YOGHURT CHEESE

STRAINED YOGHURT

YOGHURT BUTTER

or GHEE

DRIED YOGHURT

Fig 1.2 Schematic illustration showing the different processes for the manufacture of

yoghurt-related products

Table 1.2 Per capita annual consumption (kg head -1 ) of fermented milks in some selected countries

a Data for buttermilk also includes skimmed milk b B,Y,O: buttermilk, yoghurt and other fermented milks, respectively c Data includes German Democratic

Repub-lic d Data represent yoghurt and other fermented milk products.

Dash (–) indicates product is not manufactured; blank space indicates data are not available.

Data compiled from IDF (1977, 1982, 1987, 1992, 1995).

network of retail outlets with storage facilities at < 7°C Initially, production wasconfined to natural yoghurt and the market was limited, in large measure, to thosewho believed that yoghurt was beneficial to health Gradually, however, attitudestowards yoghurt changed, and the advent of fruit yoghurts during the 1950s gavethe product an entirely fresh image Instead of being a speciality item for the healthfood market, it became a popular and inexpensive snack food or dessert Pro-duction figures reflect the expanding market In the U.K., for example, the value

of yoghurt sold per annum in 1990 ran to around £400 million (sterling)

(Barrantes et al., 1994), and such figures are now commonplace around the world.

Indeed total production is still rising, a trend confirmed by the data shown in

Table 1.2

It is evident from Table 1.2 that fermented milks, and in particular yoghurt, arewidely consumed around the world and according to Kurmann (1984), the factorsthat can influence consumption are:

to be assessed in a cautious manner Nevertheless, fermented milk products madewith mesophilic lactic acid bacteria (see Fig 1.1) are widely consumed in the Scan-dinavian countries, while the yeast–lactic fermented milks are popular in the formerUSSR, eastern European countries and Mongolia

1.5 Methods of production and classification

The methods of production of yoghurt have, in essence, changed little over the yearsand although there have been some refinements, especially in relation to lactic acidbacteria, that bring about fermentation, the essential steps in the process are stillthe same, namely:

• Raising the level of total solids in the process milk to around 14 –16 g

100 g-1

• Heating the milk, ideally by some method that allows the milk to be held at hightemperature for a period of 5–30 min; the precise time will depend on the tem-perature selected

• Inoculating the milk with a bacterial culture in which Lactobacillus delbrueckii subsp bulgaricus and Streptococcus thermophilus are the dominant organisms.

• Incubating the inoculated milk, in bulk or retail units, under conditions thatpromote the formation of a smooth viscous coagulum and the desired aromaticflavour/aroma.

• Cooling and, if desired, further processing, e.g the admixture of fruit and otheringredients, pasteurisation or concentration (see Chapter 5)

• Packaging for distribution to the consumer under chilled conditions

At present there are many different types of yoghurt produced worldwide, andTamime and Deeth (1980) have proposed a scheme of classification that separatesall types of yoghurt into four categories based on the physical characteristic of theproduct This approach is illustrated in Table 1.3 However, these products and inparticular yoghurt are subdivided into different groupings based on the followingaspects:

• Legal standards (i.e existing or proposed) to classify the product on the basis

of chemical composition or fat content (full, semi-skimmed/medium orskimmed/low fat)

• Physical nature of the product, i.e set, stirred or fluid/drinking; the latter is sidered stirred yoghurt of low viscosity

con-• Flavours (plain/natural, fruit or flavoured; the latter two types are normallysweetened)

• Post-fermentation processing (vitamin addition or heat treatment)

Figure 1.3 illustrates a scheme for the classification of yoghurt based on the mentioned criteria

above-The fact that all commercial processes share this common “core” has led to theword yoghurt being applied to a whole range of products, for example, driedyoghurt, frozen yoghurt and even pasteurised yoghurt The inclusion of these

varieties under the banner of yoghurt offends some people, because yoghurt per se

must, by virtue of the process, contain an abundance of viable bacteria originatingfrom the starter culture However, popular usage appears to have determined that, as long as a carton is clearly labelled with information about the nature of the finishing process, for example, pasteurised yoghurt, the integrity of the basicproduct has not been compromised Common sense would suggest that this viewwill prevail

This approach also implies that yoghurt manufacture must always include a mentation stage, that is a coagulum produced by the direct addition of lactic acidshould never be designated as a yoghurt or even yoghurt-like, yet it is this very stage

fer-Table 1.3 Proposed scheme for the classification of all yoghurt products

Category Physical state Yoghurt products

Fig 1.3 Generalised scheme for the classification of yoghurt

YOGHURT

Enzyme Hydrolysis Vitamin Fortification Vegetable Oils Heat Treatment

Plain/Natural Fruit Flavoured

Set Stirred Drinking

that can, in commercial practice, prove extremely temperamental Variations in milkcomposition, irregular behaviour of the starter organisms, faulty regulation of theincubation temperature, along with a number of other process variables, can all give rise to an end product that is deficient in respect of overall quality, and only athorough understanding of the fermentation can provide an operative with the foresight to reduce the risk of product failure It is with this background in mindthat the relevant issues have been isolated for discussion, for although the differentsteps in production are interrelated, it is convenient to discuss them within theconfines of an individual compartment The following chapters are a reflection ofthis view

1.6 References

accolas, j.p., deffontaines, j.p and aubin, f (1978) Le Lait, 58, 278.

barrantes, e., tamime, a.y., muir, d.d and sword, a.m (1994) Journal of the Society of Dairy Technology,

47, 61.

campbell-platt, g (1987) In Fermented Foods of the World, Butterworth, London.

idf (1977) In Consumption Statistics for Milk and Milk Products 1975, Doc No 93, International Dairy

Federation, Brussels, Belgium, pp 3– 4.

idf (1982) In Consumption Statistics for Milk and Milk Products 1966/80, Doc No 144, International

Dairy Federation, Brussels, Belgium, pp 8–10.

idf (1987) In Consumption Statistics for Milk and Milk Products 1985, Doc No 213, International Dairy

Federation, Brussels, Belgium, pp 4 – 6.

idf (1992) In Consumption Statistics for Milk and Milk Products 1990, Doc No 270, International Dairy

Federation, Brussels, Belgium, pp 4 –6.

idf (1995) In Consumption Statistics for Milk and Milk Products 1993, Doc No 301, International Dairy

Federation, Brussels, Belgium, pp 4 –6.

kosikowski, f.v and mistry, v.v (1997) In Cheese and Fermented Milk Foods – Origins and Principles,

Vol 1, Published by F.V Kosikowski – L.L.C., Westport, Connecticut, U.S.A., pp 87–108.

kurmann, j.a (1984) In Fermented Milk, Doc No 179, International Dairy Federation, Brussels, Belgium,

pp 8 –26.

kurmann, j.a., rasic, j.l and kroger, m (1992) In Encyclopedia of Fermented Fresh Milk Products, Van

Nostrand Reinhold, New York.

pederson, c.s (1979) In Microbiology of Food Fermentation, 2nd Edition, AVI, Connecticut, pp.

1–29.

robinson, r.k and tamime, a.y (1990) In Dairy Microbiology – The Microbiology of Milk Products,

Vol 2, 2nd Edition, Ed by Robinson R.K., Elsevier Applied Science Publishers, London, pp 291–343.

rosell, j.m (1932) Canadian Medical Association Journal, 26, 341.

tamime, a.y and deeth, h.c (1980) Journal of Food Protection, 43, 939.

tamime, a.y and marshall, v.m (1997) In Microbiology and Biochemistry of Cheese and Fermented Milk,

2nd Edition, Edited by Law, B Chapman & Hall, London, pp 57–152.

tokita, f., hosono, a., takahashi, f., ishida, t and otani, h (1982) Dairy Science Abstracts, 44, 728.

YOG1 6/1/99 4:41 PM Page 10

© 2000 Woodhead Publishing Limited

Background to manufacturing practice

2.1 Introduction

The process of yoghurt making is an ancient craft which dates back thousands

of years and possibly even to the domestication of the cow, sheep or goat, but it

is safe to assume that prior to the nineteenth century the various stages involved

in the production of yoghurt were little understood The survival of the processthrough the ages can be attributed, therefore, to the fact that the scale of manu-facture was relatively small, and hence the craft was handed down from parents

to children However, over the last few decades the process has become more rational, mainly due to various discoveries and/or improvements in such disciplinesas:

• microbiology and enzymology

• physics and engineering

• chemistry and biochemistry

Yet by today’s standards of industrial technology, the process of yoghurt making isstill a complex process which combines both art and science together

The micro-organisms of the yoghurt starter cultures play an important roleduring the production of yoghurt, for example, the development of acid and flavour.Their classification, behaviour and characteristics are discussed in detail in Chapter

7 However, in order to understand the principles of yoghurt making, it will be useful

to describe separately the various stages of manufacture and their consequenteffects on the quality of yoghurt The technology of the process, that is, the equip-ment required for small and large scale production, will be discussed in Chapter 3.The traditional and the improved methods for the manufacture of yoghurt areillustrated in Fig 2.1 It can be observed that the former process has several draw-backs, such as:

• Successive inoculations of the starter culture tend to upset the ratio between

Streptococcus thermophilus and Lactobacillus delbrueckii subsp bulgaricus, or

may lead to mutation beyond the 15–20th subculturing

• The low incubation temperature, for example, ambient, results in slowacidification of the milk (18 hours or more), compared with the optimum con-ditions of 40–45°C for 2–12–3 hours.

for example, whey syneresis, which can adversely affect the quality of yoghurt

• The traditional process provides no control over the level of lactic acid producedduring the fermentation stage

Nevertheless, despite these drawbacks it is obvious that the traditional process haslaid the basic foundation for the production of yoghurt as practised in the indus-try at the present time (see Fig 2.1) In reality, the basic changes depend on the following:

• the purity of the yoghurt starter cultures which can be obtained from cial starter manufacturers, starter banks or research establishments;

commer-• the ability of dairies to propagate these cultures in sterile milk under asepticconditions, so giving rise to active reliable starters; however, at present direct-to-vat inoculation (DVI) of the starter culture is widely used;

• the temperature of incubation can be accurately controlled, so that the rate ofacid development and the processing time is known in advance;

• the cooling of the yoghurt can be carried out quickly at the desired level ofacidity, and the quality of yoghurt is more uniform;

Boil milk to 2/3 of the original volume

to cause partial concentration

Cool to incubation temperature

Starter (previous day yoghurt)

Inoculate with starter culture

Incubate in bulk until coagulum is produced

(e.g overnight at room temperature)

Cool

Dispatch

Starter culture propagation

Produce set or stirred yoghurt (for detail refer to Figure 5.1) Inoculate with starter culture

Cool to incubation temperature

DVI

Heat Treatment Homogenisation

Preliminary treatment of milk (refer to text)

Fig 2.1 Generalised scheme illustrating the different methods for the production

of yoghurt YOG2 6/1/99 4:48 PM Page 12

© 2000 Woodhead Publishing Limited

• the development of easy methods for measuring the rate of acid development

in milk (using pH meters and/or acidimeters) enables even a semi-skilled ator to control the process adequately

oper-2.2 Preliminary treatment of the milk base

The bulk chemical composition of milk is mainly of water, but it also contains amixture of complex components such as proteins, carbohydrate, fats, minerals andvitamins which are the main source of food for the young mammal A detailedbreakdown of these components is shown in Fig 2.2 The characteristics of eachchemical component have been discussed elsewhere in detail and the reader is

Milk

Solids-

Vit C & B group Nitrogenous Substances Lactose Minerals

Proteins

Nonprotein Nitrogen

Heat Labile Heat Stable

Peptone

α-La, β-Lg &

Bovine Serum Albumin IgG1, IgG2,

IgM, IgA &

SIgA (FSC) a

Miscellaneous

β2 -Microglobulin Lactoferrin Glycoprotein

Fig 2.2 Typical example of the main chemical components of cow’s milk

a IgA could be also associated with another secretory component and the complex may occur in a

free state.

Note: The milk also contains dissolved gases (O2, CO2 and N2), enzymes (lipases, reductases, proteases, phosphatases, lactoperoxidases, catalases, oxidases, etc.), cellular matter (epithelial cells, leucocytes), micro-organisms (bacteria, yeasts and moulds) and contaminants due to carelessness during milking

(straw, leaves, soil, disinfectant, etc.).

Adapted from Ling et al (1961), Larson and Smith (1974c), Walstra and Jenness (1984), and Scott

(1986).

referred to some reviews for a more complete discussion (Fox, 1992, 1994, 1997;Jakob, 1994; Pearce, 1995; Swaisgood, 1996).

2.2.1 Milk as a raw material

Milks of different species of mammals have been used for the production of yoghurt

Table 2.1 illustrates the major differences in the chemical composition of thesemilks As a result, variations in the quality of yoghurt do occur, depending on thetype of milk used For example, milk containing a high percentage of fat (sheep,buffalo and reindeer) produces a rich and creamy yoghurt with an excellent “mouth-feel” compared with yoghurt manufactured from milk containing a low level of fat,

or milk deprived of its fat content, for example skimmed milk The lactose in milkprovides the energy source for the yoghurt starter organisms, but the protein plays

an important role in the formation of the coagulum and hence the consistency/viscosity of the product is directly proportional to the level of protein present;yoghurt produced from unfortified mare’s and ass’s milk would be less viscous thanyoghurt made from sheep’s or reindeer’s milk

Although the flavour of yoghurt is mainly the result of complex biochemical tions initiated by microbial activity, the flavour of the milk base varies from species

reac-to species and this characteristic is reflected in the end product

Since cow’s milk is widely available in most countries around the world, theemphasis will be on the use of this type of milk for the manufacture of yoghurt,although even when considering cow’s milk, there are quite large differences in com-position (Table 2.2) The major constituents of milk are: water, fat, protein, lactoseand minerals (ash) A detailed breakdown of these components is shown in Fig 2.2.Inevitably, the chemical composition of fresh milk varies from day to day withinany particular breed depending on such factors as stage of lactation and age of thecow, milk intervals, season of the year and environmental temperature, breed ofcows and breeding policy, efficiency and intervals between milking, nutrition, hor-mones and/or disease of the udder The following are recommended for furtherreading regarding aspects of dairy cow husbandry (Larson and Smith, 1974a, b, c;Larson, 1978; Phillips, 1996) Figure 2.3 illustrates the monthly variations in the fat

Table 2.1 Chemical composition (g 100 g -1 ) of milk of ferent species of mammals

A M J J A S O N D J F M 3.6

Fig 2.3 Monthly variation (g 100 g -1 ) of the fat and protein contents of milk obtained

from the former Milk Marketing Boards in the U.K.

Since the ash and lactose contents in bulk milk vary little, figures of 0.75 and 4.5 g 100 g -1 , respectively, are taken as an annual averages The data were obtained between April 1993 and March 1994, before

these schemes were revoked on 31 October 1994.

England & Wales Milk Marketing Board (E&WMMB), Scottish Milk Marketing Board (SMMB), North of Scotland Milk Marketing Board (NSMMB), Aberdeen & District Milk Marketing Board

(A&DMMB), and Northern Ireland Milk Marketing Board (NIMMB).

Data compiled from Pickett (1996).

Table 2.2 Commercial (average expected) composition of cow’s milk (g 100 g –1 )

and protein contents of milk from the former Milk Marketing Board regions in1993–94 before these schemes were revoked on 31 October 1994 (Pickett, 1996) Inorder to overcome these inherent variations in composition, fresh liquid milk has

to be standardised and/or fortified:

• to comply with existing or proposed legal standards for yoghurt, that is, the centage of fat and/or solids-not-fat (see Chapter 10);

per-• to standardise the quality of yoghurt, that is, acidity, sweetness and tency/viscosity of the coagulum to meet the demands of the consumer; theformer two factors can be controlled during the production stages, but the con-sistency/viscosity of yoghurt is affected by the level of protein present in the milk and hence fortification of the milk solids-not-fat fraction is of primaryimportance

consis-2.2.2 Separation of cellular matter and other contaminants present in milk

Liquid milk may contain cellular material, for example, epithelial cells and cytes, which originates from the udder of the cow, and is, in some instances, due tocarelessness during milk production The milk is prone to further contaminationwith straw, leaves, hair, seeds, soil, etc The primary objective of a milk processor is

leuco-to remove such contaminants from the milk in order leuco-to ensure a better quality endproduct and while different methods are employed in dairies, the most universalsystem is the cloth filter However, this method of filtration does have its limitations,one of which is that it can only remove the large debris present in the milk

During the manufacture of some varieties of cheese, the presence of forming organisms and/or cellular matter can affect the quality of the product andsince the level of heat treatment of the cheese milk is limited to 72°C for 15 s,survival of the spores can lead to product loss

spore-Centrifugal clarification has been employed, with limited success, to removespores, but unfortunately the treatment tends to break the bacterial clumps and themilk sours more quickly However, the principle of centrifugation has been exploited

in a high speed separator known as a bactofuge and this type of separator canremove many undesirable micro-organisms from milk together with a very smallamount of the milk constituents In practice, the separated fraction (bactofugate)amounts to around 2–3% of the total throughput of milk The bactofuge is then subjected to a sterilisation treatment by live steam injection at 130–140°C for 3–4 sand, after cooling, is added back into the pasteurised cheese milk Another method of removing spore formers from cheese milk known as microfiltration hasbecome available Details and illustrations of this method of processing have beenreported by Tamime (1993) The application of this high heat treatment to only asmall portion of the milk overcomes the problems associated with the presence

of spore-forming organisms and at the same time does not affect the quality of thecheese

However, the use of bactofuge separators or microfiltration on a yoghurt cessing line is not really necessary since the heat treatment of the milk base (see

pro-Section 2.9.1) is high enough to eliminate, or at least reduce drastically, the desirable organisms in the yoghurt milk and, in any case, organisms of this type donot cause any major problems in the yoghurt industry Thus, the use of cloth filters

un-is more than adequate for raw milk In some instances, an in-line metal sieve has to

YOG2 6/1/99 4:48 PM Page 16

© 2000 Woodhead Publishing Limited

be installed when dried milk products are used to fortify the total solids in the milk; the metal sieves serve to separate any scorched or undissolved milk powderparticles.

2.2.3 Milk reception and storage

Milk collection from farms in developing and industrialised countries is carried out

in bulk, using a road tanker and, in some instances, rail tanker, or in churns; the ities available for milk reception at a typical dairy are discussed in Chapter 3.However, the current practice of milk handling in dairies involves (a) ensuring thatthe temperature is about 5°C, (b) perhaps subjecting the milk to various treatmentsbefore storage such as thermising at about 65–67°C and cooling to < 5°C, inoculat-ing the milk with lactic acid bacteria or other microfloras to control the growth of

facil-psychrotrophic bacteria (Fetlinski et al., 1982; Bianchi-Salvadori and Lavezzari,

1984), and/or (c) addition of formate or flushing with CO2 (Singh and Shankar, 1984;

Roberts and Torrey, 1988; Ruas-Madiedo et al., 1996; Espie and Madden, 1997) Muir

(1996) reviewed these methods of milk preservation and their effect on the quality

of fresh dairy products However, the use of CO2 can cause the deposition of milksolids in a plate heat exchanger and degassing is recommended before heat treat-ment (Calvo and de Rafael, 1995) Milk containing somatic cells > 250 000 ml-1 canaffect the organoleptic properties of yoghurt (Rogers and Mitchell, 1994), and whilstpreculturing the milk with proteolytic enzymes (from psychrotrophic bacteria orplasmin) or prolonged storage of milk for up to 6 days at about 7°C stimulates thegrowth of the starter culture, the yoghurt has substantially different physical prop-

erties (Reinheimer et al., 1990; Gassen and Frank, 1991; Srinivas et al., 1997; Prabba

and Shankar, 1997)

In warm countries milk tends to deteriorate faster due to methods of productionand handling A handbook has been published by the International Dairy Federa-tion (IDF, 1990) that addresses this topic in detail and the measures that are used

to minimise the bacterial spoilage of milk Furthermore, the lactoperoxidase (LP)system delays gel formation in cow’s milk by 1.5 hours and affects the flavour of theyoghurt; the body and texture characteristics are not affected (Mehanna and

Hefnawy, 1988; Kumar and Mathur, 1989; Abdou et al., 1994; Nichol et al., 1995; Nakada et al., 1996).

2.3 Standardisation of fat content in milk

The fat content (g 100 g-1) of yoghurt manufactured in different parts of the worldcan vary from as low as 0.1 to as high as 10 and in order to meet existing or pro-posed compositional standards for yoghurt, it is necessary to standardise the milk.For example, a typical average butterfat content in milk ranges from 3.7 to 4.2 g

100 g-1 (Fig 2.3), but the fat content of commercial yoghurt averages around 1.5 g 100 g-1 (medium fat yoghurt) or 0.5 g 100 g-1 (low fat yoghurt) The methodsemployed for standardisation are as follows:

• removal of part of the fat content from milk

• mixing full cream milk with skimmed milk

• addition of cream to full fat milk or skimmed milk

• a process which may combine some of the methods mentioned above, that is,the use of standardising centrifuges.

The components required to achieve a standard milk, using one of the abovemethods, can be easily calculated using the Pearsons Square method

How many litres of full cream milk (4 g fat 100 g-1) and skimmed milk (0.1 g fat

100 g-1) are required to produce 1000 l of yoghurt milk at 1.5 g fat 100 g-1?

0.1

3.9 +

The amount of skimmed milk required

The amount of cream required

1.5 50

0.1

49.9 +

The amount of cream required

Total 1000.0 l

2.4 Standardisation of the solids-not-fat content in milk

The percentage of solids-not-fat (SNF) (mainly the lactose, protein and mineralmatter) in milk for the manufacture of yoghurt is governed either directly by legalstandards of the country concerned, or indirectly by the manufacturer seeking toproduce an end product with certain physical properties and flavour In the case ofexisting legal standards, the required solids-not-fat content in yoghurt ranges from8.2 to 8.6 g 100 g-1 (see Chapter 10), and this minimum percentage seeks merely toprotect the consumer; that is, the SNF level is roughly comparable to the levelpresent in liquid milk From the manufacturer’s point of view, the physical

50

46 +

+

properties of yoghurt, for example, viscosity/consistency of the coagulum, are ofgreat importance and, in general, the higher the level of solids in the yoghurt mixthe greater the viscosity/consistency of the end product The relationship betweenthe level of solids in the milk and the consistency of yoghurt was studied by Tamime(1977), and he observed that this property was greatly improved as the milk solidsincreased from 12 to 20 g 100 g-1 Figure 2.4 shows this improvement in consistency

as measured by the penetrometer It must be emphasised that the greater the depth

of penetration, the softer the coagulum and vice versa However, the change in sistency between 16% and 20% tends to be less pronounced and hence there may

con-be little value, in terms of product quality, in using a solids level above 16 g 100 g-1.Since the 1970s, there have been many publications on the technology of yoghurtand other fermented milk products (Humphreys and Plunket, 1969; Robinson andTamime, 1975, 1986, 1990, 1993; Rasic and Kurmann, 1978; Tamime and Deeth, 1980;Olano and Ramos, 1982; Bottazzi, 1983; Kilara and Treki; 1984; Merilainen, 1987;

Shukla et al., 1987; Roginski, 1988; Tamime and Robinson, 1988; Morgensen, 1988; Chandan, 1989; Ferguson, 1989; Kroger et al., 1989, 1992; Schmidt, 1992;

Chandan and Shahani, 1993, 1995; Rossi, 1994; Varnam and Sutherland, 1994; Sarkar,1995; Tamime and Marshall, 1997; Oberman and Libudzisz, 1998) However, in aseries of articles, Vedamuthu (1991a–h, 1992a, b) has reviewed the topic extensively,while Mann (1984, 1985, 1987, 1990a, b, 1992a, b, 1994a, b) regularly publishes a

“Digest” of international dairy publications on yoghurt Furthermore, the tional Dairy Federation periodically publishes monographs updating the techno-logical and scientific aspects of fermented milks (IDF, 1984, 1988a, 1992a)

Interna-The level of solids in milk (including the fat content) for the manufacture

of yoghurt ranges from as low as 9 g 100 g-1 in low fat yoghurt to as high as

30 g 100 g-1in other types of yoghurt The best yoghurt is probably made from milkcontaining 15–16 g 100 g-1total solids (Tamime et al., 1987) and the composition of

50 100 150 200 250 300 350

After Tamime (1997).

YOG2 6/1/99 4:48 PM Page 20

© 2000 Woodhead Publishing Limited

most commercial yoghurts falls within the range of 14–15 g 100 g-1 Although

30 g 100 g-1 total solids has been suggested for the production of “super” yoghurt,the end product could well resemble “concentrated” yoghurt in its consistencyrather than normal yoghurt (see Chapter 5) Furthermore, if the total solids level inthe yoghurt mix is in excess of 25 g 100 g-1, it can adversely affect the availability

of moisture to certain strains of starter culture and this in turn can hinder their activity (Pulay and Krasz, 1974; Patel and Chakraborty, 1985)

As a result of increasing the level of SNF in the mix, the titratable acidity of themilk is raised due to the buffering action of the additional proteins, phosphates,citrates, lactates and other miscellaneous milk constituents (Walstra and Jenness,1984) and this function can lead to a reduced gel formation time (Table 2.3) Asimilar view is held by Davis (1973), who reported that doubling the SNF content

in milk resulted in a doubling of its titratable acidity However, different levels

of SNF in milk influenced the generation times and cell counts of the yoghurt starter culture; optimum conditions were 12 g and 14 g SNF 100 g-1 for L delbrueckii subsp bulgaricus and S thermophilus, respectively (Al-Dabbagh and Allan,

1989)

The fortification of the total solids in the yoghurt mix can be achieved by anumber of different methods, such as those described in the following sections

2.4.1 Traditional process

The application of heat to milk has long been practised traditionally, that is, boiling

to reduce the volume of the milk to two-thirds of its original value Although theobjective was to increase the concentration of total solids in the milk, the applica-tion of heat caused many physicochemical changes (refer to Section 2.9 on heattreatment) The degree of concentration achieved by the boiling process is rarelycalculated with any accuracy, but if, for example, the total solids level in the milk

is 13 g 100 g-1, the result of boiling the milk to reduce its volume to two-thirds will

be to raise the total solids content to around 19–20 g 100 g-1 This method offortification is still used in rural communities where the scale of yoghurt manufac-ture is very small

Table 2.3 Effect of total solids in the mix in relation to natural acidity (NA), titratable acidity (TA) and developed acidity (DA) after incubation at 42°C

Total solids (g 100 g -1 ) in Time of incubation % Lactic acid

2.4.2 Addition of milk powder

Milk powder (full cream or skimmed) is widely used in the industry to fortify liquidmilk for the manufacture of a thick smooth yoghurt (Bøjgaard, 1987) Since themajority of the commercial yoghurt produced in the United Kingdom is of the lowfat type, it is probable that skimmed milk powder (SMP) is the more popular ingre-dient The rate of addition to the yoghurt mix may range from as little as 1% to ashigh as 6%, but the recommended level is 3–4%, since the addition of higher levels

of milk powder may lead to a powdery taste in the yoghurt

Good quality yoghurt has been produced by fortification of the yoghurt mix with

(a) 2% SMP (Wolfschoon-Pomba et al., 1984; Resubal et al., 1987; Mehanna, 1988;

Mehanna and Hefnawy, 1990), (b) mixing raw milk with recombined milk at a ratio

of 1 : 1 (Kurwijila et al., 1983; Caric et al., 1986; Balasubramanyam et al., 1988), (c)

replacing half the water required for recombination of SMP with sweet whey

(El-Safty and El-Zayat, 1984) or using only Cheddar cheese whey (Krishna et al.,

1984), and (d) addition of high protein SMP to increase the level of protein to 5.2 g 100 g-1 (Mistry and Hassan, 1992)

In some developing countries, yoghurt is manufactured totally using SMP andanhydrous milk fat (AMF), about 99.9 g fat 100 g-1, and the normal practice is torehydrate the powder to about 12 g 100 g-1 SNF The use of SMP during the manu-facture of fermented milks is preferable to whole milk powder because of theproblem(s) associated with oxidised flavour in the latter product (Harper, 1985).The latest approach in SMP production is the use of protein adjustment in order toovercome the seasonal variation in the protein content in milk, and to improve func-tional characteristics and storage stability (Kieseker and Healey, 1996) However,

in some countries, for example Denmark and Italy, the fortification of the yoghurtmilk with powder(s) is not permitted, and hence other methods are employed toincrease the solids level

High protein milk powders (whole or skimmed) are available in some markets,and these are produced by ultrafiltration followed by diafiltration in order to reducethe lactose content before drying (see Table 2.4) (Bjerre, 1990; Mistry and Hassan,

(g 100 g -1 ) of different powders used for the manufacture of yoghurt

a Range of different powders.

Adapted from Tamime and Marshall (1997).

YOG2 6/1/99 4:48 PM Page 22

© 2000 Woodhead Publishing Limited

1991a, b; Mistry et al., 1992; Aguilar and Ziegler, 1994a, b) They have been used to produce firm yoghurts (El-Samragy et al., 1993a, b; Thomopoulos et al., 1993; Panfil- Kuncewicz et al., 1994; Getler et al., 1997), but are more expensive than SMP.

Since SMP is widely used for recombination during the manufacture of yoghurt,the specifications of the powder are important and can influence the quality of theproduct The current specifications for powders published by the American DairyProducts Institute (ADPI, 1990) are universally recognised; previously the organi-sation was known as the American Dry Milk Institute (ADMI) In general, powdersshould be free from any inhibitory agents and be of good microbiological qualityand physical standards Critical reviews of dairy powder specifications, including anupdate of standards, have been reported by Sjollema (1988) and Kjaergaard-Jensen(1990) Some specific requirements of SMP used for recombination have beenreported by Wilcek (1990) and include the following:

• whey protein nitrogen index, 4.5–5.9;

The quality of yoghurt was studied using different commerical types of SMP(Klupsch, 1987, 1989; Blondeau and Goursaud, 1992) and the characteristics of theproduct (i.e flavour, texture and acidity) differed considerably; some powders were

suitable for set rather than stirred-type yoghurts Recently, Chung et al (1997a, b)

reported that the use of old SMP affected the quality of yoghurt, so confirming thatpowder specifications can affect the quality of the manufactured yoghurt

2.4.3 Addition of buttermilk powder

Buttermilk powder (BMP) is a by-product of sweet cream butter manufacture, but

an acid type can also be obtained from the churning of cultured cream This low fatpowder is of value to the food and dairy industry because, due to the presence ofhigh levels of phospholipids, it has considerable emulsifying properties and its chem-ical composition is similar to SMP A method of manufacturing yoghurt from recom-bined dairy ingredients has been reported by Gilles and Lawrence (1979, 1982); thesuggested formula is: 25 kg AMF, 125 kg SMP, 10 kg buttermilk powder and 840 kgwater

Buttermilk powder, used up to 50% as a replacement for SMP in the facture of low fat yoghurt, was acceptable and similar to the control product

manu-(Vijayalakshmi et al., 1994) Fresh buttermilk fortified with SMP has been used

successfully to produce good quality yoghurt (El-Batawy et al., 1987; Vodickova

et al., 1987; Mansour et al., 1994/95), but the use of fresh buttermilk concentrated by

ultrafiltration (UF) or nanofiltration (NO) in low fat yoghurt production affectedthe consistency, flavour and aroma but not product stability (Reierstad, 1993)

2.4.4 Addition of whey powder and/or whey protein concentrates

This product originates in the cheese industry, and its utilisation in the food and thedairy industry was reviewed by Zadow (1983, 1994a, b), Alais and Blanc (1975),Smith (1976), Robinson and Tamime (1978), IDF (1988b) and Sienkiewicz andRiedel (1990) There are many different types of whey powder (WP) (e.g wheyprotein concentrates (WPC), isolate (WPI) or hydrolysate (WPH), denatured wheyprotein, whey protein fraction and non-protein nitrogen product) available on themarket and the characteristics of each are related to the processing techniqueapplied before the drying stages, for example, demineralisation, lactose removal,whey protein concentrate or straightforward drying The production and utilisation

of concentrated whey proteins have been reported by Howel et al (1990), Morr and

Foegeding (1990), Dybing and Smith (1991), Wilmsen (1991, 1992), Harper (1992),IDF (1992b), Caric (1994), Barbut (1995), Blenford (1996) and Urbiene andLeskauskaite (1996) According to Jelen and Horbal (1974), Hartman (1975),Nielsen (1976) and Spurgeon (1976), the recommended level of addition of wheypowder to the yoghurt mix is around 1–2%, since higher levels can impart an un-desirable whey flavour However, a process for the preparation of a yoghurt flavour

is based on fermenting cheese whey followed by drying (van der Schaft, 1991) andthe addition of such product to yoghurt improves its flavour and sweetens it.Since the 1970s, there have been great developments in whey technology toproduce various products of specific functional characteristics for yoghurt making.The heat stability of whey protein during the manufacture of yoghurt was reported

by Buchheim et al (1986), Jelen et al (1987), Patocka et al (1993) and Hollar et al.

(1995) However, whey protein powder was used to fortify the yoghurt mix at levels

ranging between 0.6% and 4% (Guirguis et al., 1984, 1987; Mehanna and Gonc, 1988; Rockell, 1989; Timmermans, 1993; Venkateshaiah and Jayaprakasha, 1995; Morris et

al., 1995; Venkateshaiah et al., 1996; Kailasapathy and Supriadi, 1996; Kailasapathy

et al., 1996a, b) and the results showed (a) that more acetaldehyde was produced,

(b) increased viscosity and reduced syneresis, (c) improved sensory attributes and(d) enhanced buffering capacity at low pH Good yoghurt could be produced from recombining SMP and sweet WP in a ratio of 75 : 25 (solids content about

12 g 100 g-1), but a higher ratio of 50 : 50 was recommended for yoghurt made with75% lactose hydrolysis; the latter product contained higher levels of soluble nitrogen due to:

• the addition of WP;

• the carry-over of yeast proteolytic activity in the b-d-galactosidase preparation,and

• the activity of the starter culture (Shah et al., 1993).

Replacement of SMP by whey–caseinate blends at 50% reduced the cost of facture and the yoghurt was acceptable, but the application of lactose hydrolysis

manu-during the manufacture of yoghurt has raised the cost slightly (Whalen et al.,

1988) Furthermore, different processes for the manufacture of yoghurt and relatedproducts using whey protein powder(s) in the mix have been patented by

YOG2 6/1/99 4:48 PM Page 24

© 2000 Woodhead Publishing Limited