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

YOGHURT
Science and Technology
YOGPR 6/1/99 4:43 PM Page i
© 2000 Woodhead Publishing Limited
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© 2000 Woodhead Publishing Limited
YOGHURT
Science and Technology
Second edition
A. Y. Tamime
Scottish Agricultural College Auchincruive,
Food Standards & Product Technology Department,
Ayr KA6 5HW,
Scotland
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
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England
Published in North and South America by CRC Press LLC
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First published 1985, Pergamon Press Ltd
Second edition 1999, Woodhead Publishing Ltd and CRC Press LLC
© 1999, Woodhead Publishing Ltd
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Cover design by The ColourStudio
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Printed by TJ International, Cornwall, England.
YOGPR 6/1/99 4:43 PM Page iv
© 2000 Woodhead Publishing Limited
Contents
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
1.4 Patterns of consumption
1.5 Methods of production and classification
1.6 References
2 Background to manufacturing practice
2.1 Introduction
2.2 Preliminary treatment of the milk base
2.2.1 Milk as a raw material
2.2.2 Separation of cellular matter and other contaminants
present in milk
2.2.3 Milk reception and storage
2.3 Standardisation of fat content in milk
2.4 Standardisation of the solids-not-fat content in milk

2.4.1 Traditional process
2.4.2 Addition of milk powder
2.4.3 Addition of buttermilk powder
2.4.4 Addition of whey powder and/or whey protein
concentrates
2.4.5 Addition of casein powder
2.4.6 Concentration by vacuum evaporation (VE)
2.4.7 Concentration by membrane filtration
2.4.8 Addition of non-milk proteins
2.5 Addition of stabilisers/emulsifiers
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2.5.1 General background
2.5.2 Miscellaneous properties and conditions
2.6 Addition of sweetening agents
2.6.1 General introduction
2.6.2 Types of carbohydrate sweetener
2.7 Addition of miscellaneous compounds
2.7.1 Penicillinase
2.7.2 Preservatives
2.7.3 Minerals, vitamins and/or fatty acids
2.8 Homogenisation
2.8.1 Effects on milk constituents
2.8.2 Aspects of processing
2.9 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 milk
2.9.4 Processing effects on the physical properties of
the gel

2.10 Fermentation process
2.10.1 Introduction
2.10.2 Starter organisms
2.10.3 Gel formation
2.11 Cooling
2.11.1 One-phase cooling
2.11.2 Two-phase cooling
2.12 Addition of fruit/flavouring/colouring ingredients
2.12.1 Fruits
2.12.2 Flavouring agents
2.12.3 Colouring matter
2.13 Packaging
2.13.1 Introduction
2.13.2 Functions of packages
2.13.3 Types of packaging materials
2.13.4 Comparative studies on permeability of different
yoghurt packages
2.13.5 Migration of monomers and other compounds
2.13.6 Tamper-evident packaging
2.13.7 Aluminium foil lids
2.13.8 Sterilisation of packaging materials
2.13.9 Outer or shipping container
2.14 Refrigerated cold storage, transport and distribution
2.14.1 The cold store
2.14.2 During transport
2.14.3 The retail shop and the consumer
2.15 Conclusion
2.16 References
3 Processing plants and equipment
3.1 Home or small-scale production

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3.1.1 Miscellaneous systems
3.1.2 Packaging system
3.2 Medium-scale production
3.2.1 Hand operated vat
3.2.2 Multipurpose vat
3.2.3 Mini dairy
3.2.4 Small-scale packaging machines
3.3 Large-scale production
3.3.1 Milk reception, handling and storage
3.3.2 Standardisation of fat content in milk
3.3.3 Fortification of milk solids
3.3.4 Homogenisation
3.3.5 Heat treatment
3.3.6 Fermentation/incubation of the milk
3.3.7 Cooling
3.3.8 Pumps
3.3.9 Miscellaneous fittings
3.3.10 Fruit handling and mixing units
3.3.11 Filling machines
3.3.12 Miscellaneous handling, chill cooling and refrigerated
cold storage
3.4 Mechanisation of yoghurt production and plant design
3.5 Continuous yoghurt production
3.5.1 Background
3.5.2 The NIZO process
3.5.3 Recent developments
3.6 Automation/process control
3.6.1 Levels of automation

3.6.2 Area/department 1
3.6.3 Area/department 2
3.6.4 Area/department 3
3.6.5 Area/department 4
3.6.6 Area/department 5
3.6.7 Area/department 6
3.6.8 Management information system
3.6.9 System architecture
3.6.10 System security
3.7 Building design, maintenance and services
3.7.1 General background and introduction
3.7.2 Location of a dairy plant
3.7.3 Layout of a dairy plant
3.7.4 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
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4.3 Factors involved in the selection and performance
of a detergent
4.3.1 Type/range of detergents used in the yoghurt
industry
4.3.2 Type of soiling matter
4.3.3 Water hardness and quality
4.3.4 Miscellaneous factors
4.4 Cleaning methods

4.4.1 Manual cleaning
4.4.2 Cleaning-in-place
4.4.3 Miscellaneous cleaning methods
4.5 Factors influencing the efficiency of cleaning
4.5.1 Type of soil
4.5.2 Method of cleaning adopted
4.5.3 Contact time
4.5.4 Concentration of detergent solution
4.5.5 Temperature
4.5.6 Flow rate or velocity
4.5.7 Acid wash
4.5.8 Plant design
4.5.9 Chemical composition of a detergent
4.6 Specific cleaning and sterilisation operations of yoghurt
processing equipment and utensils
Sterilisation aspects
4.7 Fundamentals of the sterilisation process
4.8 Methods of sterilisation and/or sanitation
4.8.1 Heat
4.8.2 Chemical agents
4.8.3 Filtration
4.8.4 Irradiation
4.8.5 Spraying, fogging or fumigation
4.9 Kinetics and mechanisms of microbial destruction
4.10 Means of assessing the sanitary condition of
a processing plant
4.10.1 Physical examination
4.10.2 Chemical examination
4.10.3 Bacteriological examination
Effluent treatment .

4.11 Background
4.12 Nature of pollution
4.13 Methods of effluent treatment
4.14 References
5 Traditional and recent developments in yoghurt production and
related products
5.1 Introduction
5.2 Standard commercial yoghurt
5.3 Yoghurt made from different mammalian milks
5.3.1 Goat’s milk yoghurt
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© 2000 Woodhead Publishing Limited
5.3.2 Sheep’s milk yoghurt
5.3.3 Buffalo’s milk yoghurt
5.3.4 Camel’s milk yoghurt
5.4 Pasteurised/UHT/long-life/heat shock yoghurt
5.4.1 Technology of manufacture
5.4.2 Processing effects on properties of product
5.5 Drinking yoghurt
5.5.1 Background
5.5.2 Processing aspects
5.5.3 Other beverage products
5.5.4 Carbonated products
5.6 Lactose hydrolysed yoghurt (LHY)
5.7 Concentrated/strained yoghurt
5.7.1 Introduction and nomenclature
5.7.2 Processing methods
5.7.3 Miscellaneous properties
5.7.4 Microstructure
5.7.5 Related products

5.8 Frozen yoghurt
5.8.1 Background, standards and marketing
5.8.2 Technology of manufacture
5.8.3 Related products
5.9 Dried yoghurt
5.9.1 Introduction
5.9.2 Processing methods
5.9.3 Kishk and related products
5.10 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
5.17 References
6 Microbiology of yoghurt and “bio” starter cultures
6.1 Introduction
6.1.1 Historical background and classification
6.1.2 Modification of starter cultures
6.1.3 Potential genetic modifications
6.2 Characteristics of growth
6.2.1 Milk as a medium for microbial growth
6.2.2 Associative growth
6.3 Factors affecting slow growth of starter cultures
6.3.1 Compounds that are naturally present in milk
6.3.2 Effect of incubation temperature and inoculation
rate
6.3.3 Mastitis milk and somatic cell count
6.3.4 Hydrogen peroxide (H

2
O
2
)
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6.3.5 Antibiotic residues
6.3.6 Detergent and disinfectant residues
6.3.7 Environmental pollution
6.3.8 Bacteriophages
6.3.9 Bacteriocins
6.3.10 Miscellaneous factors
6.4 Conclusion
6.5 References
7 Biochemistry of fermentation
7.1 Introduction
7.2 Carbohydrate metabolism
7.2.1 Homolactic fermentation
7.2.2 Heterolactic fermentation
7.2.3 Lactase activity
7.2.4 Production of lactic acid
7.2.5 Production of exopolysaccharide (EPS)
7.2.6 Production of flavour compounds
7.3 Protein metabolism
7.3.1 Constituent compounds of the milk protein molecule
7.3.2 Proteolytic enzymes
7.3.3 Proteolysis by the yoghurt and bio organisms
7.3.4 Products of proteolysis
7.4 Lipid/fat metabolism
7.4.1 Introduction

7.4.2 Changes in the level of free and esterified fatty acids
7.4.2 Changes in the level of volatile fatty acids
7.5 Vitamin metabolism
7.5.1 General background
7.5.2 Biosynthesis of folic acid
7.5.3 Biosynthesis of niacin
7.5.4 Biosynthesis of vitamin B
6
7.6 Miscellaneous changes
7.7 References
8 Preservation and production of starter cultures
8.1 Introduction
8.2 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 production
8.3.1 Growth characteristics
8.3.2 Concentration of cell biomass
8.4 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
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8.4.5 Bacteriophage resistant/inhibitory medium
(BRM/BIM)
8.5 Conclusion
8.6 References

9 Nutritional value of yoghurt
9.1 Introduction
9.2 Carbohydrates
9.2.1 Available carbohydrates
9.2.2 Unavailable carbohydrates
9.3 Protein
9.4 Lipids
9.5 Vitamins and minerals
9.6 Yoghurt and health
9.6.1 Therapeutic properties of yoghurt
9.6.2 Therapeutic properties of bio-yoghurt
9.7 Conclusion
9.8 References
10 Quality control in yoghurt manufacture
10.1 Introduction
10.2 Principles of HACCP
10.2.1 Brief introduction
10.2.2 Implementation of a HACCP system
10.3 Monitoring of process plant
10.4 Examination of raw materials
10.4.1 Liquid milk
10.4.2 Milk powder
10.4.3 Starter cultures for standard yoghurt
10.4.4 Starter cultures for bio-yoghurts
10.5 Quality appraisal of retail products
10.5.1 Analysis of chemical composition
10.5.2 Assessment of physical characteristics
10.5.3 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
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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
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© 2000 Woodhead Publishing Limited
This book is dedicated to our families
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© 2000 Woodhead Publishing Limited
Preface to second edition
When the first edition of this book was published in 1985, the retail markets in Aus-
tralasia, Europe and North America were dominated by just one product – stirred
fruit yoghurt, with natural set yoghurt occupying a well-defined niche. Some tradi-
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 the
basic products, but a more dramatic impact was achieved by the introduction of
mild-tasting bio-yoghurts. In these latter products, selected bacteria with prophy-
lactic/therapeutic properties are involved with the fermentation and, whilst many
aspects of the yoghurt-making process remain the same, the introduction of these

new cultures has led to some significant changes in both consumer attitudes and
manufacturing practices.
In light of these recent developments, it became apparent that a revision of this
book 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 this
technology has been covered by Mr J. Bird and Mr I. Chester who represent two of
the foremost equipment manufacturers in the world.We would like to acknowledge
their 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
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© 2000 Woodhead Publishing Limited
Preface to first edition
Although there are numerous fermented milks produced on a local basis around
the world, only yoghurt has achieved a truly international distribution. This popu-
larity stems from a number of sources: the pleasant, aromatic flavour of natural
yoghurt, its reputation as a foodstuff associated with good health, but perhaps above
all 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 the
retail markets, and since the introduction of fruit yoghurts during the 1950s sales
have climbed steadily upwards.
Today millions of gallons of yoghurt are produced each year, and yet because
manufacture is still, in essence, a natural biological process, success can never be
taken for granted. It is this capricious nature of the fermentation that makes it so
fascinating, and indeed if the system were not so prone to variation, then there

would 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 potential
difficulty remain that only a thorough appreciation of the nature of yoghurt can
provide those associated with its production and distribution with the confidence
that eliminates product failure.
It goes without saying that the best teacher is experience, but if this book can
offer some preliminary guidance on the intricacies of handling yoghurt, then its com-
pilation will have been worthwhile.
1983 A. Y. Tamime
R. K. Robinson
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© 2000 Woodhead Publishing Limited
1
Historical background
1.1 Introduction
Fermentation is one of the oldest methods practised by human beings for the trans-
formation of milk into products with an extended shelf life. The exact origin(s) of
the making of fermented milks is difficult to establish, but it could date from some
10–15000 years ago as the way of life of human beings changed from being food
gatherer to food producer (Pederson, 1979). This change also included the domes-
tication of animals (i.e. cow, sheep, goat, buffalo and camel), and it is most likely
that the transition occurred at different times in different parts of the world.Archae-
ological evidence shows that some civilisations (e.g. the Sumarians and Baby-
lonians in Mesopotania, the Pharoes in north-east Africa and the Indians in Asia)
were well advanced in agricultural and husbandry methods, and in the production
of fermented milks such as yoghurt.
Although there are no records available regarding the origin of yoghurt, the
belief in its beneficial influence on human health and nutrition has existed in many
civilisations over a long period of time. According to Persian tradition, Abraham
owed his fecundity and longevity to yoghurt and, in more recent times, Emperor

Francis 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 evo-
lution of this fermented product through the ages can be attributed to the culinary
skills of the nomadic people living in that part of the world. Today, fermented milk
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 restricted
usually to no more than a few months of the year. The main reason for this limited
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availability of milk is that intensive animal production has never really existed, so
that, as in early history, farming is in the hands of nomadic peoples who move from
one 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 vil-
lages where they could sell their animal produce. Another major factor is that the
Middle East has a subtropical climate and summer temperatures can reach as high
as 40°C. In such a climate, milk turns sour and coagulates within a short time of
milking, particularly as the milk is produced under primitive conditions. Thus, the
animals 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 the
hands of the milker is extremely high. Under these conditions the possibility of
transporting or even keeping milk for any length of time is non-existent. As a result
the bulk of the population consume milk only rarely, and even the nomadic people
have 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
Traditional name Country
Jugurt/eyran/ayran Turkey
Busa Turkestan
Kissel mleka/naja/yaourt Balkans
Urgotnic Balkan mountains
Leban/laban or laban rayeb Lebanon and some Arab
countries
Zabady/zabade Egypt and Sudan
Mast/dough/doogh Iran and Afghanistan
Roba/rob Iraq
Dahi/dadhi/dahee India
Mazun/matzoon, matsun, matsoni, madzoon Armenia
Katyk Transcaucasia
Yiaourti Greece
Cieddu Italy
Mezzoradu Sicily
Gioddu Sardinia
Tarho/taho Hungary
Viili Finland
Filmjolk/fillbunke/filbunk/surmelk/taettemjolk/tettemelk Scandinavia
Iogurte Brazil and Portugal
Skyr Iceland
Gruzovina Yugoslavia
Donskaya/varenetes/kurugna/ryzhenka/guslyanka Russia
Tarag Mongolia
Shosim/sho/thara Nepal
Yoghurt/yogurt/yaort/yourt/yaourti/ Rest of the world
yahourth/yogur/yaghourt (“Y” is replaced by
“J” in some cases)

After: Tamime and Deeth (1980), Accolas et al. (1978), Tokita et al. (1982) and Kosikowski and Mistry
(1997).
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furthermore, the coagulum is irregular, filled with gas holes and shows extreme whey
syneresis. Lactic acid bacteria, however, act on milk to produce a fermented product
which is pleasant to eat or drink; this latter product was usually referred to as sour
milk.
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 fermentation
process 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 fermenta-
tion, relying mainly on the indigenous microflora to sour the milk;
• heating the milk over an open fire to concentrate the milk slightly, so that the
final coagulum would acquire an attractive viscosity due to the modified prop-
erties of the casein, again a change which would have improved the quality of
the end product;
• seeding the heat-treated and cooled milk (blood or ambient temperature) with
sour milk from a previous batch, so enabling the thermophilic strains of lactic
acid bacteria to become predominant;
• gradual selection of lactic acid bacteria capable of tolerating high levels of lactic
acid 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 sour
milk 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 treat-
ment for milk and one such product became known as yoghurt from the Turkish
word “jugurt”; numerous variants of this word have appeared over the years and a

selection of alternatives is shown in Table 1.1.
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 the
type of milk used, the microbial species which dominate(s) the flora and their prin-
cipal metabolic products, Robinson and Tamime (1990) proposed a scheme of
classification 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 by
Tamime 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 keeping
quality.
The first step in this process turned out to be relatively simple because the con-
tainers traditionally used by the nomads for the production of yoghurt were made
from animal skins. In normal use the yoghurt would have been consumed fairly
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rapidly but, if left hanging in the skin for any length of time, the nature of the
product 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 end
result was a condensed or concentrated yoghurt with an acidity of >2.0% lactic acid
and a total solids content in the region of 25g100g
-1
; the original yoghurt might
have had a solids content of 12–13g100g
-1

and an acidity of around 1.5% lactic acid.
To the nomadic people, whose main sources of wealth and nourishment are the
animals that can be raised and the milk that they produce, the relative resistance of
the condensed yoghurt to spoilage must have appeared attractive.
Evidence of this trend can be found in Armenia where the mazun (Armenian
yoghurt) is usually pressed to yield a product called tan or than. Similarly, surplus
milk 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. Another
method 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 or
two, 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 salt
into 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 by
the addition of various quantities of salt. Another traditional way of prolonging the
keeping quality of concentrated yoghurt is employed in Lebanon, where the salted
product is made into small balls about 2cm in diameter and placed in the sun to
dry. Afterwards the yoghurt balls (which are partially dried) are placed in either
glazed earthenware pots or glass jars and covered with olive oil.The product is then
referred 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)
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© 2000 Woodhead Publishing Limited
season and it has a storage life of up to 18 months; the product is spread easily on
bread and consumed.
An alternative preservation process involves heating yoghurt for a few hours
over low fires of a special type of wood; the end product is referred to as smoked
yoghurt. 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 concentrated
yoghurt is processed even further to produce a totally different product of almost
indefinite keeping quality. This is a dried form of yoghurt; milk is processed into
yoghurt in the traditional manner and wheat flour, semolina or parboiled wheat,
known locally as burghol, is rubbed into it. The yoghurt–wheat mixture is shaped
into 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 pre-
pared by reconstituting the yoghurt–wheat mixture with water and then simmering
the mix gently over a fire. The consistency of this product, which is normally con-
sumed with bread, is similar to porridge.
The concentrated yoghurt can be also processed into a different product called
chanklich. Here again the product is partially dried, but is then mixed with spices
and herbs (presumably to assist in preservation). The mixture is then formed into
balls, placed into glass jars and finally covered with olive oil. It is evident that many
different products can be manufactured from yoghurt and Fig. 1.2 illustrates some
examples; 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 products
declined, except among certain communities in the Middle East. In their place, a
new generation of yoghurts emerged, with production typically centred on a large
modern 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

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© 2000 Woodhead Publishing Limited
Table 1.2 Per capita annual consumption (kg head
-1
) of fermented milks in some selected countries
Country
1975
a
1980 1985 1990 1993
B
b
Y
b
O
b
BYOB YO B Y OBY O
Australia – 1.0 – 1.8 – – 2.8 – – 3.5 – – 4.8 –
Austria 0.6 3.4 3.9 2.0 5.8 2.0 2.2 6.6 2.2 2.9 7.5 – 1.9 8.6 2.5
Belgium 6.7 5.1 – 2.8 4.9 – 2.3 6.0 0.06 1.9 6.5 1.2 2.3 5.7 3.9
Canada 4.4 0.7 – 0.6 1.7 – 0.6 2.5 – 0.5 3.2 – 0.4 3.1 –
Chile 1.4 2.5 3.9
Czechoslovakia 0.9 1.3 1.7 3.1 1.7 2.5 3.9 2.5 3.1
Denmark 23.1 5.9 7.1 9.8 9.1 7.8 8.4 8.0 7.5 6.9 7.8 6.9 5.6 8.3 6.8
Federal Germany 7.7 4.6 4.2 2.2 6.7 1.2 2.1 7.9 1.1 2.8
c
10.6
c
0.8
c
2.6

c
11.3
c
0.9
c
Finland 7.2 6.3 29.1 4.1 8.4 28.5 1.9 9.4 28.0 1.2 11.7 25.4 1.1 13.3 23.7
France 1.8 ¨ 7.8
d
ƨ 9.3
d
ƨ 12.7
d
ƨ16.4
d
ƨ 17.3
d
Æ
Iceland 1.7 5.7 6.2 14.1 9.9 14.7 9.8 16.1
India 3.7 – 18.8 4.0 – 23.1 4.7 –
Ireland 12.6 1.0 – 5.7 2.0 – ¨ 3.4
d
Æ 3.2 ¨ 3.1
d
Æ
Israel 3.4 10.7 4.7 9.6 6.8 9.1
Italy – 1.3 – 1.6 1.3 – 2.6 1.4 – ¨ 5.0
f
Æ
Japan – 0.8 1.7 – 1.0 1.4 – 2.9 5.0 – 3.9 3.9 – 4.8 3.7
Luxemburg 1.9 3.3 0.2 ¨ 5.1

d
ƨ6.8
d
Æ 1.8 6.1
Netherlands 10.5 14.2 – 9.5 17.8 – 8.5 18.1 – 10.7 21.8 – 9.0 20.7 –
Norway – 1.2 7.9 – 2.2 7.9 – 3.1 10.9 – 4.3 10.6 – 6.3
Poland ¨ 3.2
d
Æ 1.3 0.1 0.6 0.9 0.4 0.6
Spain 3.4 ¨ 6.0
d
Æ 5.5 8.0 ¨ 9.8
d
Æ
Sweden 4.2 2.3 17.6 0.1 4.2 19.7 0.03 5.4 21.9 0.002 7.4 21.7 7.5 21.1
Switzerland 5.5 10.9 – 1.0 13.8 – 1.2 16.2 – 1.7 17.3 – 1.7 17.0
UK – 1.7 – – 2.8 – – 3.1 – – 4.3 0.1 4.6 0.2
USA 9.0 0.9 – 1.9 1.2 – 2.0 1.8 – 1.4 2.1
Former USSR – – 7.2 – – 6.2 – – 7.5
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).

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network of retail outlets with storage facilities at <7°C. Initially, production was
confined to natural yoghurt and the market was limited, in large measure, to those
who believed that yoghurt was beneficial to health. Gradually, however, attitudes
towards yoghurt changed, and the advent of fruit yoghurts during the 1950s gave
the product an entirely fresh image. Instead of being a speciality item for the health
food 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, are
widely consumed around the world and according to Kurmann (1984), the factors
that can influence consumption are:
• availability of milk
• food habits
• level of income
• advertising
• range of fermented milks available in the market
• distribution system
• relation to consumption of other dairy products
• religion.
However, the consumption of buttermilk is not properly classified in most countries
because: (a) traditional or natural buttermilk is the by-product of butter making
from ripened or cultured cream, (b) cultured buttermilk is produced by the fer-
mentation of skimmed milk with the addition of butter flakes, and (c) there is sweet
buttermilk which is not fermented; the data for buttermilk shown in Table 1.2 have
to be assessed in a cautious manner. Nevertheless, fermented milk products made

with 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 former
USSR, 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 years
and although there have been some refinements, especially in relation to lactic acid
bacteria, that bring about fermentation, the essential steps in the process are still
the same, namely:
• Raising the level of total solids in the process milk to around 14–16g
100g
-1
.
• Heating the milk, ideally by some method that allows the milk to be held at high
temperature for a period of 5–30min; 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.
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© 2000 Woodhead Publishing Limited
• Incubating the inoculated milk, in bulk or retail units, under conditions that
promote the formation of a smooth viscous coagulum and the desired aromatic
flavour/aroma.
• Cooling and, if desired, further processing, e.g. the admixture of fruit and other
ingredients, 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, and
Tamime and Deeth (1980) have proposed a scheme of classification that separates
all types of yoghurt into four categories based on the physical characteristic of the
product. This approach is illustrated in Table 1.3. However, these products and in
particular yoghurt are subdivided into different groupings based on the following

aspects:
• Legal standards (i.e. existing or proposed) to classify the product on the basis
of chemical composition or fat content (full, semi-skimmed/medium or
skimmed/low fat).
• Physical nature of the product, i.e. set, stirred or fluid/drinking; the latter is con-
sidered stirred yoghurt of low viscosity.
• Flavours (plain/natural, fruit or flavoured; the latter two types are normally
sweetened).
• Post-fermentation processing (vitamin addition or heat treatment).
Figure 1.3 illustrates a scheme for the classification of yoghurt based on the above-
mentioned criteria.
The fact that all commercial processes share this common “core” has led to the
word yoghurt being applied to a whole range of products, for example, dried
yoghurt, 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 originating
from 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 basic
product has not been compromised. Common sense would suggest that this view
will prevail.
This approach also implies that yoghurt manufacture must always include a fer-
mentation stage, that is a coagulum produced by the direct addition of lactic acid
should never be designated as a yoghurt or even yoghurt-like, yet it is this very stage
Table 1.3 Proposed scheme for the classification of all
yoghurt products
Category Physical state Yoghurt products
I Liquid/viscous Yoghurt
II Semi-solid Concentrated/strained
III Solid Frozen

IV Powder Dried
Adapted from Tamime and Deeth (1980), Robinson and Tamime
(1990) and Tamime and Marshall (1997).
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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
Full
Fat Medium
Low
Physical
Chemical
Flavour
Miscellaneous
that can, in commercial practice, prove extremely temperamental.Variations in milk
composition, irregular behaviour of the starter organisms, faulty regulation of the
incubation 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 a
thorough understanding of the fermentation can provide an operative with the
foresight to reduce the risk of product failure. It is with this background in mind

that the relevant issues have been isolated for discussion, for although the different
steps in production are interrelated, it is convenient to discuss them within the
confines of an individual compartment. The following chapters are a reflection of
this 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.
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