EFFECT OF STORAGE ON THE MICROBIAL CHEMICAL · PDF fileTo my mother, who gave me ... Abdallah for his help in statistical analysis. ... milk dairy products. In this sence, goat's milk

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EFFECT OF STORAGE ON THE MICROBIAL CHEMICAL

AND SENSORY CHARACTERISTIS OF YOGHURT

MADE FROM COW'S MILK AND GOAT'S MILK

By

Mohammed Sid Ahmed Mahgoub Omer

B.Sc. (Honour) Animal Production

Faculty of Animal Production

University of Khartoum

A thesis submitted in partial fulfillment of the requirements for the degree

Of Master of Dairy Production and Technology

supervisor

Dr. Osman Ali Osman Alowni

Department of Dairy Production

Faculty of Animal Production

University of Khartoum

2010

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DEDICATION

To my father, who encouraged me to complete this work.

To my mother, who gave me continuous love, care and tender

during study.

To soul of my cousin Al –Tahir Osman Mahgoub.

To my brothers, sisters, friends and colleagues.

I dedicate this work.

Mohammed

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ACKNOWLEDGEMENT

All praise being to Allah, the almighty for his uncounted support.

Peace and blessing of Allah be on the soul of prophet and messenger,

Mohammed and his pious companions and followers.

I would like to thank and show my sincere gratitude to Dr. Osman

Ali Osman El owni for his helpful supervision, wise guidance and

continuous support.

My special thanks are due to Dr. Ibtisam Elyas M. El Zubeir, for

her help and kindness. I am greatly indebted to Prof. Mohammed Khair

Abdallah for his help in statistical analysis. I wish to thank the staff

members of the Department of Dairy Production. Also my thanks are

extended to the staff of Dairy Production labrotary, Faculty of Animal

Production.

I appreciate the kind help offered to me by Mr. Hashim El-Shikh

Mohammed, Mr. Hassan Mahgoub Nassr and Mrs. Elham Hassan

Alhussin from the Ministry of Agriculture, Animal Resources and

Irrigation. Northern State.

I am very grateful to my brother Rashid for financial support. And

my thanks extended to every person who contributed directly or indirectly

and whose is not mentioned here.

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LIST OF CONTENTS

Item Page

Dedication 2

Acknowledgment 3

List of content 4

List of tables 8

List figures 9

Abstract 10

Arabic Abstract 12

CHAPTER ONE 14

Introduction 14

CHAPTER TOW 17

Literature Review 17

2.1. Milk 17

2.2. Goat 17

2.3. Goat milk 19

2.3.1. Goat milk flavor 23

2.3.2. Goat milk Microbiology quality 24

2.3.3. Goat milk yoghurt 24

2.4. Fermentation 25

2.5. Yoghurt 26

2.5.1. Factor Affecting yoghurt quality 28

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LIST OF CONTENTS

Item Page

2.5.2. Manufacture of yoghurt 29

2.5.2.1. The basic requirements for making yoghurt 29

2.5.2.2. Standardization of fat content and fortification of solid-

non-fat content

30

2.5.2.3. Other ingredient 31

2.5.3. Processing of set yoghurt 31

2.5.3.1. Homogenization 31

2.5.3.2. Heat treatment 32

2.5.3.3. Inoculation and incubation of starter culture 33

2.5.3.4. Cooling 35

2.6. Starter culture 35

2.7. Nutritional and health of yoghurt 37

2.8. Sensory Evaluation 39

CHAPTER HTREE 40

3.1. Material and methods 40

3.1.1. Source of material 40

3.2. Manufacture of experimental yoghurt 40

3.3. Analysis of milk and yoghurt samples 41

3.4. Chemical analysis 41

3.4.1. Total solid content 41

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LIST OF CONTENTS

Item Page

3.4.2. Fat content 41

3.4.3. Ash content 42

3.4.4. Protein content 42

3.4.5. Titratable acidity 43

3.4.6. Lactose content 44

3.5. Microbiological examination 44

3.5.1. Sterilization 44

3.5.2. Type of culture media used for microbical examination 44

3.5.2.1. Plate count agar: (Scharlau 01-161) 44

3.5.2.2. M-17 medium: (Scharlau 01-247) 44

3.5.2.3. MRS broth: (Scharlau 01-135) 45

3.5.3. Enumeration of microorganism 45

3.5.3. 1. Total bacterial count 45

3.5.3.2. Sstreptococcus thermophilus 45

3.5.3.3. lactobacilus bulgaricus 45

3.6. Sensory evaluations 45

3.7. Statistical analysis 46

CHAPTER FOUR 47

Result 47

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LIST OF CONTENTS

Item Page

CHAPTER FIVE 63

Discussion 63

CHAPTER SIX 79

CONCLUSION AND RECOMMENDATION 79

Conclusion 79

Recommendation 79

Reference 80

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LIST OF TABLES

Table (1) Chemical composition and microbial content of cow’s milk

and goat’s milk before and after pasteurization.

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Table (2) Variation of some quality test of set yoghurt produced from

cow's milk during storage.

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Table (3) Variation of some quality test of set yoghurt produced from

goat’s milk during storage.

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Table (4) Variation of sensory evaluation test during storage of set

yoghurt produced from cow’s milk.

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Table (5) Variation of sensory evaluation test during storage of set

yoghurt produced from goat’s milk.

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Table (6) variation of some quality test between types of set yoghurt

produced from cow’s milk and goat’s milk.

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Table (7) Variation of sensory evaluation test between types of set

yoghurt produced from cow’s milk and goat’s milk.

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LIST OF FIGURES

Item Page

Figure (1) Variation of sensory evaluation test during storage

of set yoghurt produced from cow’s milk.

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Figure (2) Variation of sensory evaluation test during storage

of set yoghurt produced from goat’s milk.

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Figure (3) variation of chemical composition test between

types of set yoghurt produced from cow’s milk and

goat’s milk.

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Figure (4) variation of microbial test between types of set

yoghurt produced from cow’s milk and goat’s milk.

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Figure (5) Variation of sensory evaluation test between

types of set yoghurt produced from cow’s milk and

goat’s milk.

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Effect of storage on the Microbial, Chemical and Sensory

Characteristics of Yoghurt made from Cow's milk and Goat's milk

Mohammed Sid Ahmed Mahgoub Omer

M.Sc. Dairy production and Technology

Abstract: This study was carried out on cow's and goat's milk

set yoghurt at labrotary of Department of Dairy poduction, Faculty of

animal production, Univeasity of Kharoum. Chemical, microbiological

and sensory evaluations were carried out on the zero, 3rd, 6th, 9th and

12th days of storage.

Analysis of yoghurt samples made from cow's milk during storage

revealed significant (p< 0.05) variation in total solids, protein,

Streptococcus subsp count, Lactobacillus subsp count, color and texture.

High significant (p< 0.01) variation in fat, lactose, acidity, TBC and

flavor. Non significant variation was noticed in ash content.

Analysis of yoghurt samples made from goat's milk during storage

revealed significant (p< 0.05) variation in total solid, fat, titrable acidity,

TBC, Streptococcus subsp count, color, texture and flavor. Moreover

significant (p< 0.01) variation in protein, lactose and Lactobacillus subsp

count and non significant variation was noticed in ash content.

The comparison between two types of yoghurt showed significant

(p< 0.05) variation in protein and flavor and highly significant (p< 0.001)

variation in total solids, acidity, TBC, Streptococcus subsp count, color

and texture and non significant variation was noticed in fat, lactose, ash

content and Lactobacillus subsp count.

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This study concluded that quality of set yoghurt was affected

during storage and the cow's milk yoghurt sensory characteristics were

different from goat's milk yoghurt. For this reason this study

recommended that further studies and research are needed to improve the

quality and flavor of goat's milk yoghurt.

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الماعزمقارنة بين الزبادي المصنع من لبن األبقار ولبن

محمد سيد أحمد محجوب عمر

ماجستير إنتاج و تقانة األلبان

تخلص نيعها و : المس م تص ى ت رة الت ك الخث ادى متماس ى الزب ة عل ذه الدراس ت ه أجري

تج يم جودة المن ة الخرطوم لتق واني جامع اج الحي تخزينها و تحليلها فى معمل قسم ألبان آلية االنت

ة و ة و الميكروبيولجي يةالكيمائي ائى . الحس ل الكيم ى التحلي ة عل تملت الدراس د (أش بة الجوام نس

د نسبة الحموضة و ) الكلية و نسبة الدهن و نسبة البروتين و نسبة الالآتوز و نسبة الرماد و تحدي

ا العصوية (التحليل الميكروبيولجي ا السبحية و البكتري ا و البكتري ى للبكتري دد الكل و ) حساب الع

د ). ون و النكهة و القوامالل(التحليل الحسي أجرى التحليل الكيمائي و الميكروبيولجي و الحسي بع

.تسعة واثني عشرة يوما, ستة, التصنيع مباشرة وبعد ثالثة

على الزبادي المصنع من (p< 0.05)أظهرت الدراسة أن فترة التخزين تأثر معنويا

روتين ة و الب د الكلي ا لبن األبقار في محتواه من الجوام داد البكتري ا السبحية وأع داد البكتري و أع

ا أثر معنوي وام و ت ون و الق وية و الل وز و p< 0.01)(العص دهن و الالآت ن ال واه م ي محت ف

ي مستوي وي عل الحموضة و العدد الكلى للبكتريا والنكهة ولم تظهر الدراسة وجود اختالف معن

.الرماد

علي الزبادي المصنع من (p< 0.05)معنويا أظهرت الدراسة أن فترة التخزين تأثر

داد ا و أع ى للبكتري دد الكل دهن و الحموضة و الع ة و ال د الكلي واه من الجوام لبن الماعز في محت

ا أثر معنوي ة و ت وام والنكه ون و الق روتين p< 0.01)(البكتريا العصوية و الل واه من الب في محت

ي مستوي والالآتوز و أعداد البكتريا السسبحية ول وي عل م توضح الدراسة وجود اختالف معن

.الرماد

ة ات معنوي ك اختالف ة أن هنال وعين من (p < 0.05)أظهرت الدراسة المقارن ي الن عل

ة ات معنوي ا من p< 0.01)(الزبادي في محتواهما من البروتين والنكهة و اختالف في محتواهم

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م الجوامد الكلية و الحموضة و العدد الكلى للبكتري ا و أعداد البكتريا العصوية و اللون و القوام ول

داد اد و أع وز و الرم دهن و الالآت ا من ال ي محتواهم ة ف ات معنوي تظهر الدراسة وجود أختالف

. البكتريا السسبحية

ادي خلصت الدراسة إلي أن جودة الزبادي متماسك الخثرة تتأثر بسبب التخزين و أن زب

ار يختلف في الص اعز لبن االبق بن الم ادي ل ذه األسباب توصي الدراسة . فات الحسية عن زب له

.بمزيد من البحوث لتحسين جودة و نكهة زبادي لبن الماعز

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CHAPTER ONE

Introduction

The word yoghurt is derived from the Turkish word (Jugurt) and

it's a traditional food and beverage in the Bulkan and the Middle East

(Tamime and Deeth, 1980). Yoghurt is very popular fermented milk

product produced by lactic acid fermentation of milk by addition of

starter culture containing Streptococcus salivarius spp. thermophilus and

Lactobacillus delbruekii spp. bulgaricus. It's very versatile product that

suits all palates and meal occasions. Yoghurt has many forms including

drinkable (liquid) or solid, low fat or fat free, fruity or cereal flavored and

is a healthy and nutritious food (Tamime and Robinson, 2000 and

Mckinley, 2005).

Since the 1960s, the industrial production of fermented milks

(especially yoghurt) has increasingly developed world wide. Several

factors account for the success of yoghurt. It's natural image, it's

organoleptic characteristics (fresh and acidulated taste and characteristic

flavor) nutritional, prophylactic and therapeutic properties (Birollo et al.,

2000).

Milk from various mammals such as cow, buffalo, goat, sheep,

camel, etc. is used for different nutritional purposes, e.g., feeding to

young ones and preparation of some nutritional products such as milk

cream, butter, yogurt, ghee, sour milk, etc. (Webb et al., 1974; Hassan,

2005).

Today goat milk and its products play an important role in certain

parts of the world due to their beneficial health effects. Goat milk is

preferred more in the nutrition of babies, children and patients in many

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countries like Germany and France according to it's outstanding

physiological, microbiological and technological properties (Haenlein,

1993). The use of goat's milk becomes an opportunity to diversify the

dairy market since it allows us to develop added value fermented

products with particular characteristics, in comparison to cow milk

(Vargas et al., 2008).

The major differences between cow's milk and goat's milk are

related to the different properties of the different kinds of Casein (αS1-

casein, αS2-casein, κ- casein etc) and also the different structure and size

of fat globules and protein micelles (Tziboula – Clarke, 2003). All this

differences could lead to the milk behaving differently during the gelation

process and gel formation and thus, could affect the final quality of goat's

milk dairy products. In this sence, goat's milk yoghurt differs from cow's

milk yoghurt in some important properties like the firmness of the

coagulum, which tends to be soft and less viscous (Bozanic et al., 1998

and Karademir et al., 2002).

Although the basic composition of goat's milk is similar to the

composition of cow's milk, but physicochemical properties of both types

of milk differed significantly from each other. These differences come

from the distinctive structure, the composition and size of casein micelles,

proportions of individual protein fractions and higher quantity of mineral

salts and non-protein nitrogen compounds in goat's milk. It does not

remain without an influence on rheological properties of yoghurts from

goat's milk. Acid gel from goat's milk is more delicate in comparison to

gel from cow's milk (Zander, 1998). However, most of the research work

developed in this field deals with the manufacturing of special types of

cheese. Little information is available on the production of other products

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such as skimmed milk, flavored milk, yogurt, buttermilk, ice creams,

butter, condensed milk or powdered milk. The results obtained from

industrial use of cow's milk are not always suitable for the use of goat's

milk for the same purpose. Therefore, there is a need to develop specific

research for the use of goat's milk in the manufacture of the above-

mentioned products (Van Dender et al., 1990). Studies of changes in

quality characteristics during storage would enable producers to predict

the shelf life of the product more accurately (Salvador and Fiszman,

2004).

The aims of this study were to compare the chemical,

microbiological and sensory evaluations of set yoghurts from cow's and

goat's milk.

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CHAPTER TWO

Literature Review

2.1. Milk

The principal constituents of milk are water, fat, proteins, lactose,

and minerals. Milk also contains trace amounts of other substances such

as pigments, enzymes, vitamins, phospholipids, and gases (Michael,

2003). The quantitative composition of milk ranged as follow: water

85.5-89.5%, total solids 10.5-14.5%, fat 2.5-6.0%, proteins 2.9-5.0%,

lactose 3.6-5.5% and minerals 0.60-0.90% (Alfa-Laval, 1996).

Milk covers nutritional requirement for growing children,

convulsing adults, pregnant and lactating woman and for old people. Milk

can be used in many recipes and many milk products, for these reason the

value of milk as human food cannot be over emphasized (Matthewman,

1993). Milk is a precursor for many food products. It's value has been

enhanced by an enormous amount of research, especially over the past 50

years, to support the development and commercialization of dairy-based

products with an increasing variety of flavor, texture and shelf life

(Tamime, 2007). Milk quality and safety, extensions in shelf life, and

new product introductions have brought variety and convenience for the

consumer (Goff and Griffiths, 2006).

2.2. Goat

About 8000 B.C., goat was the first animal species to be

domesticated by the Sumerians in Mesopotamia. Goat had a strong

impact on all phases of the Sumerian's life. Goat was considered by

ancient people as a holy entity for worship at the side of gods. In modern

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times, goats play an important economic role in farming, providing food

for farmers in mountains, arid and semiarid areas (Hatziminaoglou and

Boyazoglu 2004). Goats rank third in terms of global milk production

from different animal species after cattle and buffaloes (Klinger and

Rosenthal, 1997). Although they rank second to cattle in number, goats

are more important to the subsistence needs and economic development

of peasant farmers because they provide a regular supply of meat, milk

and cash throughout the year (FAO, 1990). Milk and dairy products from

goats and sheep are very important for proper human nutrition, where

cow milk is not readily available or affordable. In some countries more

than one half or at least one third of all milk is supplied by goats and

sheep, which makes their contribution to sufficient protein and calcium

nutrition of people very significant (Haenlein, 2001).

Goat is one of milk sources that characterized by the economic

important, since goat can utilize feed roughages and crops residues by

products undesirable for human consumption convert into desirable food

(Devendra and Mcleroy,1982). Goats are reported to play special role in

the life of small holder farmers. Their small size makes it possible for

farmers to keep a large herd in small area (Boylan et al., 1996). Goat

plays an important role in income generation and nutrition provision

(Devendra, 1992).

Goats are very adaptable and are capable of utilizing wide range of

plants, which make them easy to keep (French, 1970). Goat is the most

versatile domestic animals in adaptation to arid and humid, tropical and

cold, and desert and mountain conditions (Gall, 1991; Quartermain, 1991

and Silanikove, 2000). Goats and sheep provide home supply and self-

sufficiency for families to avoid starving and malnutrition in protein,

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calcium, vitamins and energy. It has been noted that more people around

the world drink goat milk than cow milk (Campbell and Marshall, 1975

and Haenlein, 1981).

Between 1965 and 1994 the world goat population was estimated

to have increased from 373 million to 609 million head, the average

increase of eight million head per year (Nu Nu san and DeBoer, 1996).

FAO (2001) reported that the largest animal number increase for goats

during the last 20 years (1980-1999) from 458 million to 710 million

head, respectively. According to the latest estimate of livestock in Sudan

there are about 40.719 million heads of goat (Ministry of Animal

Resources, 2000). Four local breed types of goats are known in Sudan:

Nubian (the only specialized dairy goat), Desert, Nilotic dwarf and Tegri

(Hassan and Elderani, 1990).

2.3. Goat milk

Goat milk is a complex emulsion of fat in watery solution,

containing fat, proteins, lactose and minerals; being composed of 88.6%

water and 11.4% solids; containing 3.28% fat and 8.13% non fat. Non-

fat-solids are composed by 4.29% lactose, 3.20% proteins and 0.64% ash

(calcium, phosphorous, magnesium and potassium) (Martins et al., 2007).

Milk composition is affected by the goat’s breed, region and

sanitary conditions (free pasture or captivity), feeding characteristics,

health conditions and normal season lactation conditions (Jandal, 1996;

Wong, 1999; Gomes et al., 2004). In Poland, the mean content of basic

goat milk formed as follows: fat 2.25-5.52%, protein 2.58-4.15%, lactose

3.92-5.28%, ash 0.74-0.95%, dry matter 10.44-14.83% (Kudełka, 1996)

Goat milk sample was rated superior in terms of nutritional quality

with reference to calcium, magnesium, potassium, chloride and vitamins

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A, D, thiamine, riboflavin, choline, inossitol, nicotinic acid, B6, and B12.

It was also superior in some essential amino acids such as histidine,

methionine, phenylalanine and threonine. Total solids, protein, ash, short

and medium chain fatty acids, specific grafity and calorific value were

higher for goat milk, which was however lower in sodium, citrate and

vitamin C. Goat milk was lower in some essential amino acids namely

isoleucine, tryptophan and valine, including essential fatty acid a-linoleic

acid (Bille et al., 2000 and Haenlein, 2001). Minerals tended to be

absorbed better from goat’s milk than from cow’s milk ; goat's milk fatty

acids tended to be slightly better absorbed than the cow’s milk fatty acids,

especially C14:0 and C18:2 (Feverir et al.,1993).

Goat milk exceeds cow milk in monounsaturated fatty acids

(MUFA),polyunsaturated fatty acids (PUFA), and medium chain

triglycerides (MCT), which all are known to be beneficial for human

health, especially for cardiovascular conditions (Haenlein, 2004). The

nutritional advantage of goat milk fat compared with cow's milk has been

attributed to the high content of C6:0 to C10:0 fatty acids, lack of

agglutinin, a high percentage of the short- and medium-chain fatty acids

esterifies on the carbon 3 of the glycerol skeleton, and to a small size of

fat globules; hence making the dairy product easily digestible (Chilliard et

al., 2006). Average goat milk fat differs in content of its fatty acids

significantly from average cow milk fat (Jenness, 1980).

Goat milk fatty acids have become established medical treatments

for an array of clinical disorders (Haenlein, 2004). According to Alférez

et al. (2001) goat milk fats have a unique metabolic ability to limit

cholesterol deposits in arteries. Goat milk is more easily digested because

of the smaller size fat globules and different casein types, but there for

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often has a softer curd in cheese making and lower yield than does cow

milk (Haenlein, 2001). Le Jaouen, (1981) reported that the higher amount

of these small fat globules in the goat milk is responsible for the better

digestibility of goat milk.

There have been shown to be differences in the proportions of

alpha-S1, alpha-S2, beta and kappa casein between cow's milk and goat's

milk protein. Cow's milk protein is predominantly alpha-S1 casein, while

goat's milk protein is predominantly alpha-S2 casein. Both cow's and

goat's milk contain beta-lactoglobulin and alpha-lactalbumin. Beta-

lactoglobulin is mostly responsible for milk allergy (Tayllor, 1986;

Heyman and Desjux, 1992). Boulanger et al. (1984) demonstrated that in

casein of goat milk the same four proteins (α S1, α S2, β and κ-

casein) are present as in casein of cow milk, but individual differences

may occur in the content of α S1- casein, which seems to range from zero

in some samples, designated as “null type”, to very high levels in others

“high type”, with many intermediate classes. Subsequently, assay tests

indicated that αS1- casein can exist in null type milk in very low

concentration. Milk with low αS1- casein had a faster coagulation time,

whereas milk with high levels produced the firmer curd associated with a

better chemical composition (Ambrosoli et al., 1988).

Goat's milk has been said to be suitable alternative to cow's milk

for people with lactose intolerance and cow's milk protein intolerance, but

most of the evidence is an ecdotal, so there is some marginal differences

which distinguish goat's milk from cow's milk, leading to suggestions that

in certain cases goat's milk may be tolerated differently from cow's milk

(Frances, 2001). The lactose content of goat's milk appears to give slight

advantage over cow's milk for mildly lactose intolerant people, but there

22

is no clinical evidence to support this (Frances, 2001). Lactose is a sugar

found only in milk and milk products. Lactose must be broken down

(hydrolyzed) by the enzyme lactase, so that the two component sugars

may be absorbed into and used by the body. When the enzyme is partially

or totally deficient, lactose cannot be digested and absorbed. The lactose

is therefore unchanged when it reaches the large intestine. This can cause

symptoms of abdominal pain, cramps, gassy distension, flatulence and

diarrhoea. The diarrhoea is due to the ability of lactose to retain water in

the colon (Robinson, 2000). Lactose intolerance is usually inherited and

is racially distributed, being more common among people of Eastern

European, Asian and African origins. In these areas, milk drinking after

infancy is traditionally uncommon and levels of the lactase enzyme fall

during childhood (Rosado, 1997).

Goat milk and its products of yoghurt, cheese and powder have

three-fold significance in human nutrition: (1) feeding more starving and

malnourished people in the developing world than from cow milk; (2)

treating people afflicted with cow milk allergies and gastro-intestinal

disorders, which is a significant segment in many populations of

developed countries; and(3) filling the gastronomic needs of connoisseur

consumers, which is a growing market share in many developed countries

(Haenlein, 2004). The feeding of goat milk instead of cow milk as part of

the diet resulted in significantly higher digestibility and absorption of iron

and copper, thus preventing anemia (Barrionuevo et al., 2002).

Goat milk is known to have better qualities such as digestibility

and longer shelf life when processed than cow milk. Goat's milk can be

processed into different milk products. These are: yoghurt, fermented

23

milk (madila), cheese, butter (more difficult than that of the cow), and

cream (Ohiokpehai, 2003).

2.3.1. Goat milk flavor

Fat globules are smaller to much larger proportion, they cream up

only very slowly over several days, and their membrances are very

fragile, liberating easily lipase, then flavourful fatty acids, and causing

rancidity and off-flavor readily (Haenlein, 2001). Goat milk is

characterized with its offensive odor. This is especially from buck whose

odor floats strongly around the premises and can affect the flavor of the

milk. The unpleasant odor is obvious in milk if ventilation, milking

practices and cooling of milk are improper or insufficient (Eman et al.,

2009a).

Recently milked and cooled goat milk is odor free and hard to

distinguish from cow milk in odor and taste (Mowelm, 1988). According

to Namibian researchers, the main reason for not liking goat milk

products is the ‘goaty’ flavor/odour (Bille et al., 2000). The commercial

value of goat milk can be enhanced, especially for higher value milk

products, if its goaty flavor can be eliminated or reduced to an

unobjectionable level (Gupta, 2004).

The formation of the specific flavor of goat milk is closely linked

to the nature of the various constituents in the milk, and also to

biochemical and enzymatic factors. The latter depended on the

technological treatments applied to the milk and result in degradation of

its constituents. Lipase activity and spontaneous lipolysis play a major

role in the development of flavor in goat milk (Chilliard 1982a, 1982b).

Moreover the effect of the free fatty acids content has been established

(Skjevdal 1979 and Astrup et al., 1985).

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2.3.2. Goat milk Microbiological quality

The quality of goat milk may be considered as its potential to

undergo further processing and result in a product which lived up to the

consumers, expectations in terms of health (nutritional value), safety

(hygienic quality) and satisfaction (sensory attributes) (Jaubert and

Kalantzopoulos, 1996). Difficulties in managing the safety of milk derive

from the various sources of contamination. Undesirable organism may get

into milk either through the body (endogenously) or from some extended

source (exogenously) after milk has been drown (Lowenstein and Speck,

1983). It has become increasingly clear, internationally, that diseases in

dairy animals and the production and handling of milk under poor

hygienic conditions, can lead to wide spread outbreaks of human diseases

(Giesecke et al., 1994).

Some of the diseases that can be transmitted to humans from milk

include salmonellosis, tuberculosis, brucellosis, listeriosis, Qfever,

toxoplasmosis, streptococcus infections, staphylococcal infection and

campylobacter infection (Devendera and Burns, 1983 and Mowelm,

1988). Goat milk contains significantly lower bacterial counts than cow

or buffalo milk, and that variety of microbial organisms can be present in

goat milk without being pathogenic to humans (Haenlein, 1992).

2.3.3. Goat milk yoghurt

Nutritionally goat milk yoghurt had an advantage over cow milk

yoghurt due to it is higher nutrient density. Goat milk yoghurt was

preferred to cow milk yoghurt in appearance, texture and palatability,

while yoghurt from cow milk was preferred in aroma and flavor. The

preference for cow milk yoghurt was attributed to the higher content of

25

citrates in cow milk than goat milk; while the higher total solids goat milk

had favorable influence on yoghurt appearance, texture and palatability

(Bille et al., 2000). According to Stelio and Emmanuel, (2004) Caprine

yoghurt from milk from an Alpine breed, which had the lowest dry

matter, showed the lowest degree of firmness and total organoleptic

acceptance, showing this milk to be unsuitable for the production of

yoghurt.

The quality of yoghurts was markedly affected by the proportion of

goat’s milk in the mixture since the increase in the content of goat's milk

lead to important differences in terms of the physicochemical properties

of yoghurts, especially were regard to syneresis, flow properties, gel

firmness and whiteness (Vargas et al., 2008). Higher lactose content in

goat milk resulted in more acid goat milk yoghurt (Bille et al., 2000).

In comparison to cow and sheep milk yoghurts, goat milk yoghurt

had a looser consistency, higher acidity and was less acceptable

sensoricaly (Domagała, 2008). It is possible to process good quality

yoghurt from goat milk using low cost technology (Bille et al., 2000).

2.4. Fermentation

Communities in the Middle East and Asia are widely

acknowledged as having introduced fermented milks such as yoghurt into

their diet almost as soon as men began to domesticate animals. Some

fermented milks did, of course, become popular with local populations in

regions like Scandinavia and Russia (Koroleva, 1991). Originally

fermented milks developed as means of preserving nutrients (Beena,

2000). Fermented milks are manufactured throughout the world and

approximately 400 generic names are applied to traditional and

industrialized products (Kurmann et al., 1992).

26

The international dairy federation (IDF, 1992a- IDF, 1992b)

published general standards of identity for fermented milk that could be

briefly defined as follows : fermented milk as prepared from milk and/or

milk product (e.g. any one or combination of whole, partially or fully

skimmed, concentrated or powder whey, milk protein, cream and butter,

all of which have been at least pasteurized) by the action of specific

microorganisms, which results in reduction of the PH and coagulation.

These products include cultured batter milk, sour cream, yoghurt,

acidophilus milk, kefir and concentrated fermented milk products

(Hargrove and Maedonough, 1972).

The term ‘fermented milk’ or ‘cultured milk’ refer to products such

as yoghurt, sour milk, cultured butter milk and sour cream, which are

usually made from cow's milk by pure lactic acid fermentation.

Additionally, some products are made from milk from other species such

as ewes, goats or mares, and combined fermentation (by e.g. lactic acid

bacteria and yeast) results in products known as kefir or koumiss (Jaros

and Rohm, 2003). The considerable increase in demand for fermented

milks noted in recent years has resulted, to a great extent, from consumer

awareness of their beneficial effects. However, fermented milks are also

highly valued for their unique taste and aroma, which contributed to their

growing popularity as well (Saint-Eve et al., 2004).

Many parameters that critically affect the fermentation process and

product quality such as the activity of the lactic starter, the milk

contamination with lactic acid bacteria inhibitors, the adequacy of heat

treatment, and the effect of extrinsic factors such as incubation room

temperature (Soukoulis et al., 2007).

27

2.5. Yoghurt

The use of yogurt dates back many centuries, although there is no

accurate record of the date when it was first made. According to legend,

yoghurt was first made by the ancient Turkish people in Asia (Kurtz,

1981). Yoghurt is a fermented and coagulated milk product with a smooth

texture having mildly sour taste and pleasant flavor. It is obtained from

pasteurized or boiled milk by souring natural or otherwise using lactic

acid fermented bacteria (Soomro et al., 2003).

According to the code of federal Regulation of the FDA (FDA,

1996) yoghurt is defined as “food product by culturing one or more of the

optional dairy ingredients (cream, milk, partially skimmed milk and skim

milk), with characterizing bacteria culture that contains the lactic acid

bacteria, Lactobacillus delbureckii sub sp. bulgaricus and Streptococcus

thermophilus. The lactic acid lowers the PH, makes it tart, causing milk

protein to thicken and acts as a preservative since pathogenic bacteria

cannot grow in acid conditions (Eman et al., 2009b).

Yoghurts are prepared by the fermentation of milk by lactic acid

bacteria, which results in the pH of milk decreasing to pH < 4.6.

Industrially, yoghurts can be largely divided into 2 types Set-style

yoghurt is made in retail containers giving a continuous undisturbed gel

structure in the final product. In stirred yogurt manufacture, the gel is

disrupted by stirring (agitation) before mixing with fruit and then it is

packaged. Stirred yogurts should have a smooth and viscous texture

(Tamime and Robinson, 1999).

Yoghurt in different forms with diverse local names is made

throughout the world it's a fermented milk product, which has gained

great popularity throughout the world for its recognized sensorial,

28

nutritional, and health-promoting properties. A large variety of yoghurts,

resulting from technologically diversified approaches, as well as various

fruits and fruit flavours added, are available on the market today (Tamime

and Robinson, 1999 and Tarakci and Erdogan, 2003). Typical plain

yoghurt contained 3.5% fat, 12.06% total solids, 3.60% protein, 18.94%

moisture, 0.76% ash and 4.2% lactose (Athar, 1986). Tamime and Deeth,

(1980) reported that, the types differ according to their chemical

composition, method of production, flavor and texture of post-incubation

processing.

2.5.1. Factor affecting yoghurt quality

The composition of yoghurt is dependent on the type and source of

milk and a range of seasonal factors. For example: whole milk or

skimmed milk, season, lactation period and the feeding mode. It is also

significantly influenced by manufacturing conditions (such as

temperature and duration and equipment utilized) and on the presence of

other ingredients such as powdered milk or condensed milk (Blance,

1986). The successful production of yoghurt depends upon the processing

techniques i.e. correct selection of starter culture, heat treatment,

inoculation and incubation temperature, preservation, handling and

propagation of starter cultures that help to standardize and maintain

uniformity in the quality of end product (Anjum et al., 2007). One of the

most important parameter to determine the quality of the yoghurt is total

proteins (Kavas et al., 2003).

The most important factors that are influential in rheological

properties of yoghurt are: composition and quality of processing milk,

way and level of an enrichment of dry matter components, technological

parameters in production, the procedure with end-product during its

29

transport and storage. Very important is also selection of proper starter

culture responsible for acidification of milk and giving desirable sensory

properties of the product. The sources of flavor compounds in yogurt are

milk components (lactose, milk fat, proteins, citrates) and products of

their enzymatic degradation. However, it should be kept in mind that

other key factors are the quality and kind of milk, heat treatment

intensity, the content of fat, the method and parameters of incubation, as

well as the time and conditions of storage (Rasic and Kurmann, 1978;

Beshkova et al., 1998a; Tamime and Robinson, 1999; Bikowski, 1997).

2.5.2. Manufacture of yoghurt

The method of manufacture is still based on the system employed

by nomadic herdsmen many centuries ago. For example, the majority of

yoghurt consumed worldwide are manufactured with culture of bacteria

with growth optima of 37- 45 oC, and this characteristics derives from the

fact that the species in question, namely Lactobacillus delbureckii sub sp.

bulgaricus and Streptococcus thermophilus, evolved in the Middle East

where the ambient temperature in the summer months is often well in

excess of 35 oC, similarly, the universal methods of manufacturing

satisfactory yoghurt is based on the traditional process (Robinson et al.,

2006). Manufacturing methods vary considerably and for example,

depend on the country, the type of product manufactured, the raw

material used and the product formulation. However number of common

principles is general applied (Staff, 1998).

2.5.2.1. The basic requirements for making yoghurt

To ensure high quality end-product, the milk should have a low

bacterial count (i.e. maximum of 1.0×105 colony-forming-units (cfu) g1-).

30

Furthermore, the milk and other dairy ingredients should be free from

taints, antibiotic compounds, sanitizing agents and bacteriophages.

Somatic count should be < 4.0× 105 cells ml1- (Optimum < 2.5×105 cells

ml1-) (Tamime and Robinson, 1999; and Oliveria et al., 2002).

Fresh bovine milk is usually the base material for making yoghurt

in the western world, although ovine, caprine or buffalo milks can also be

employed. The fat content of most retail yoghurts lies in the range 1.0-4.5

g100ml 1-. The critical feature of the yoghurt is level of solids-non-fat

(SNF). The protein together with minerals, such as calcium and

phosphorus give rise to the basic gel structure of yoghurt (Tamime and

Robinson, 1999).

2.5.2.2. Standardization of fat content and fortification of solid-non-

fat content:

The fat content in yoghurt made in different parts of the world may

range from 0.1g to as high as 3.5-5.0g100ml 1- in order to meet existing

or proposed compositional standards. Therefore, it is necessary to

standardization as follow: (a) removal of all or part of the fat content (b)

mix all milk with skimmed milk. (c) Addition of cream to whole milk or

skimmed milk. (d) A process that may combine some of these methods

(Tamime and Robinson, 1999).

On an industrial scale, the elevation of the SNF can be achieved by

evaporation (EV) or ultra filtration (UF); reverse osmosis (RO) is an

optional process. The UF and EV process remove water and hence raise

the level of both fat and SNF in the yoghurt base, but UF does allow

some loss of lactose and minerals (Lankes et al., 1998 and Robinson; et

al., 2002). The alternative route is to add skimmed milk powder (SMP) to

31

the milk base, and system of hoppers, high-speed blenders and in-tank

mixing can be employed to ensure full and rapid incorporation of the milk

powder (Robinson and Tamime 1993; Fitzpatric et al., 2001; and

Fitzpatric and Cuthbert, 2004).

2.5.2.3. Other ingredient

It is general accepted that natural set yoghurt should comprise

nothing other than milk and the starter culture, but stirred fruit yoghurts

are permitted in some countries to contain stabilizers, fruit, flavors,

sweetening, agent, and preservatives ( Robinson et al., 2006).

2.5.3. Processing of set yoghurt

Once the desired composition of milk in terms of fat, SNF and, if

applicable, other ingredients has been achieved the milk will usually be

homogenized (Robinson et al., 2006).

2.5.3.1. Homogenization:

Whole milk is homogenized at pressure of 10-20 MPa in

temperature range of 55-65oC, usually prior to heat treatment, to prevent

creaming during fermentation. The process results in the disruption of the

milk fat globules, which are stabilized by specific fat globules membrane

consisting mainly of proteins, phospholipids and neutral glycerides into

much smaller one (Jaros and Rohm, 2003).

Homogenization breaks down fat into smaller globules which

prevents the formation of cream line . This improves the consistency and

viscosity of yoghurt, thus a greater stability to synersis can be obtained

(Rasic and Kurman, 1978; Tamime and Deeth, 1980; Tamime and

Robinson, 1985). Furthermore, homogenization of yoghurt mix breaks up

32

powdered ingredients resulting in uniform distribution of the ingredients

(Vedamuthu, 1991). The covering of the homogenization-induced,

enlarged fat globule surface area with fragments of milk proteins leads to

the development of the secondary fat globule membrance, which is of

great importance for the characteristics of fermented dairy products

(Schkoda, 1999).

2.5.3.2. Heat treatment

Yoghurt mix is normally heated at higher temperature and longer

time than normal pasteurization, ranging from 90 to 95o C for 5 to 10

min, to help improve product consistency through whey protein

denaturation (Mottar et al., 1989; Rasic and Kurman, 1978; Tamime and

Deeth, 1980 and Tamime and Robinson, 1985). Heating of the base milk

is essential in yoghurt manufacture, and temperature-time condition may

be varied to adjust physical properties of yoghurt products (Joras and

Rohm, 2003). Heat treatment significantly affected viscosity and

acetaldehyde development without influencing incubation time and

acidity (Soukoulis et al., 2007).

The objectives of heat treatment of yoghurt mix are to kill

pathogenic microorganism and to in activate lipase and hence to prevent

lipolysis (Rasic and kurman, 1978). Milk heat treatment considered to be

critical factor for texture formation. Heating induces whey protein

denaturation so that whey proteins can associate casein micelles. Whey

proteins are bound to caseins through disulfide linkages and hydrophobic

interactions (Law, 1996).

Other essential actions of the heating stage are:

33

(a) Partial breakdown of the whey proteins to amino acid that stimulate

the activity of starter culture.

(b) An expulsion of oxygen from the milk that is beneficial for the

growth for the microaerophilic starter bacteria.

(c) A reduction in the indigenous microflora in the milk that might

otherwise compete against the added bacteria (Robinson et al., 2006).

High heat treatment of the milk base leads to faster gelatin and

firmer gels (Lee and Lucey, 2003). Yoghurt prepared with unheated or

inadequately heat-treated milk, is characterized by poor texture, weak gel

and firmness, and increased susceptibility against wheying off (Tamime

and Robinson, 1999).

2.5.3.3. Inoculation and incubation of starter culture

After heat treatment stage, the milk will be cooled to 42-43 oC

ready for the addition of the starter culture consisting of a 50:50 mixture

of Lactobacillus delbureckii sub sp. bulgaricus and Streptococcus

thermophilus (Robinson et al., 2006). These organisms grow in a

protocooperative relationship, resulting in rapid acidification by

stimulating each other (Joras and Rohm, 2003). Depending on type and

activity of the starter cultures, other metabolites such as carbon dioxide,

acetic acid, diacetyle, acetaldehyde, large molecular weight

exopolysaccharides or several other compounds are produced besides

lactic acid, resulting in the characteristic properties of the products

regarding flavor, texture and aroma. Since Streptococcus thermophilus is

weakly proteolytic its growth is stimulated by the rods, which liberate

free amino acids and small peptides from casein. The cocci in turn

encourage the growth of Lactobacillus delbureckii sub sp. bulgaricus by

34

producing formic acid and carbon dioxide (Matalon and Sandine, 1986

and Rajagopal and Sandine, 1990).

The result of this microbial activity is that the acidity of the milk

will have risen to around 1.0-1.2 g 100ml1- lactic acid (around PH 4.2-

4.3) after 3-4 hours. At this acidity the milk proteins will have coagulate

to form afirm gel (Lucey and Singh, 2003 and Lucey and Singh,1997).

Lactobacillus. delbureckii sub sp. bulgaricus is more capable in booth

acid and acetaldehyde production compared to Streptococcus

thermophilus (Singh and Sharma, 1982).

The essential features are temperature control during incubation

and means of cooling the product on a preset PH has been reached

(Robinson et al., 2006). The determination of incubation time is an

essential technical parameter in industrial yoghurt production. Due to the

complexity of the fermentation process and the great number of factors

entangled in yoghurt coagulation, prediction of the incubation step is

difficult, so it is a common practice to control it empirically. In addition,

definition of the optimal incubation time is significant not only in

reducing the manufacturing cost but also in avoiding deterioration of the

quality characteristics of the final product. The end point of the

fermentation process is usually defined by the PH value (Soukoulis et al.,

2007). However, the need to avoid contamination of the milk with

undesirable bacteria, yeasts and moulds during inoculation is universal,

and number of systems has been developed to achieve this aim (Tamime,

2002). Once the milk has been inoculated, it will follow, one of two

routes: it will be filled into cartons for incubation as set yoghurt or it will

be fermented in bulk tank stirred yoghurt (Robinson et al., 2006).

35

2.5.3.4. Cooling

When the yoghurt reaches the required acidity i.e. around 0.8-1.0

percent lactic acid, cooling of the coagulum commences and the intention

is to reduce the temperature of the coagulum to below 20oC within an

acceptable time span. Thus below 20oC the metabolic activity of the

starter organisms is sufficiently reduced to prevent the yoghurt for

becoming unpalatable due to excessive acidity. Hence initiation of

cooling depends on the level of lactic acid required in the end product

(usually between 1.2 and 1.4 percent lactic acid) and the rate of cooling

that can be achieved with the available equipment and in manner that

does not damage the texture of the yoghurt (Robinson,1981).

Knowledge of the behavior of yoghurt during long storage is

important, because its shelf life is based on whether the products display

any of the physical, chemical, or sensory characteristics that are un

acceptable for consumption (Salvados and Fiszman, 2004).

2.6. Starter culture

The classical yoghurt starter culture is a mixture of Streptococcus

thermophilus and Lactobacillus delbureckii sub sp. bulgaricus, with

acocci-rods ratio of usually 1:1 (Hassan and Frank, 2001; Hutkins, 2001).

The two organisms interact synergistically. This interaction depended on

the fact that Streptococcus thermophilus grows more rapidly than

Lactobacillus delbureckii sub sp. bulgaricus in milk, and ferment lactose

homofermentatively to give L (+) lactic acid as principle product. In

addition, carbon dioxide is liberated by the breakdown of urea in the milk

by urease, and usually, formic acid (up to 40µg mL1-); all three

metabolites stimulate the growth of Lactobacillus delbureckii sub sp.

36

bulgaricus (Robinson, 2000). Lactobacillus delbureckii sub sp.

bulgaricus can hydrolyse casein- especially β-casein- by means of a cell-

wall-bound protinase to release polypeptides and, by further enzymatic

activity, free amino acids as well (Beshkova et al., 1998b).

The practical result of the synergy is that both species grow rapidly

and actively metabolise sufficient lactose to lactic acid to complete the

fermentation of milk to yoghurt within 3-4 hours. One species alone

might take 12-16 hours to produce the same level of acidity (Tamime et

al., 1984). Metabolites liberated by two species give yoghurt a flavor that

is distinctly different from any other fermented milk. An acetaldehyde at

level up to 40ml L1- is major component of the flavour profile, and the

major pathway for its production by Lactobacillus delbureckii sub sp.

bulgaricus and to lesser extent, Streptococcus thermophilus, is conversion

of theronine to glycine by threonine aldolase (Zourari et al., 1992; and

Marshall and Tamime, 1997).

Some strains of the two species can also produce appreciable levels

of extracellular polysaccharide materials, such as the glucans, or

polymers involving glucose, galactose and rhamnose as the constituent

sugars (Robinson, 1999; Devusty et al., 2003). The presence of these

metabolites enhances considerably the viscosity and hence consumer

appeal of the retail yoghurt, but a number of factors, such as composition

and structure of polysaccharide, the amount produced and the acidity of

the milk, all influence the properties of the final product (Laws and

Marshall, 2001; and Zoon, 2003).

The most common inoculating material used by the modern dairy

plants is the culture comprising Streptococcus thermophilus and

Lactobacillus bulgaricus. These microorganisms grow together

37

symbiotically and are responsible for the production of good taste and

aroma in yoghurt. An incubation temperature lies somewhere between 39

ºC and 45 ºC for the optimum acid production by the two species (Anjum

et al., 2007). Lactic acid bacteria are fastidious microorganisms and their

growth is often restricted in milk because of its paucity in essential

nutrients, thus the success of milk fermentation relies most often upon the

synergy between Streptococcus thermophilus and Lactobacillus

bulgaricus. Because both bacteria are able to grow alone in milk, this

indirect positive interaction is called proto-cooperation (Courtin and Rull,

2004).

Traditionally, yoghurt is manufactured using Streptococcus

thermophilus and Lactobacillus delbrueckii spp. bulgaricus as starter

cultures. These organisms are claimed to offer some health benefits

however, they are not natural inhabitants of the intestine. Therefore, for

yoghurt to be considered as a probiotic product. Lactobacillus delbrueckii

spp. bulgaricus and Streptococcus thermophilus are at a daily dose of 109

cfu and several authors have indicated that a minimal concentration of

106 cfu/g of a product is required for a probiotic effect (Kumar and Singh,

2007; and Birollo et al., 2000). France and Spain established the

requirement of a minimum viable lactic acid bacteria number during

yoghurt’s shelf-life of 5x108 cfu ml1-. Other countries have established

values of 106 cfu ml1- (Switzerland and Italy), 107cfu ml1- (Japan), 108 cfu

ml1- (Portugal) and 107cfu ml1- (Turkey) (Birollo et al., 2000 and

Anonym, 2001).

2.7. Nutritional and health yoghurt

The nutritional and therapeutic effects of yoghurt are well known

and mainly attributed to fermentative change in the milk and/or the

38

metabolic effects of the yoghurt microflora (Irkin and Eren, 2008).

Yoghurt has richer composition than milk due to its production conditions

and more different substances exist in its combination compared to milk

because of fermentation. Thus its nutritional property increases and

digestion gets easy as it contains particularly viable yoghurt bacteria and

their metabolites, many undesired microorganisms couldn’t grow up in

yoghurt and existence of these bacteria has been correlated with several

benefits for consumer. Hence, yoghurt is accepted to be safety product

(Rasic and Kurman, 1978).

Fermentation improved food safety, nutritional quality through the

biosynthesis of vitamins, essential amino acids and proteins. Also through

fermentation the digestibility of proteins and carbohydrate is improved.

Furthermore, harmful toxic substances are broken down and the

bioavailability of minerals is improved (Baltcock and Azam-Ali, 1998).

The nutritional and health benefits of yoghurt are numerous. It is a good

source of proteins, energy (calories), vitamins and minerals. As a

fermented product, it may also have therapeutic value and may also result

in reduced incidences of lactose intolerance (Fernandez-Garcia. et al.,

1994 and Robinson and Dombrowski, 1983). Certain therapeutic

properties associated with yoghurt have increased both its production and

consumption all over the world. Many health benefits like protection

against gastrointestinal upsets, lowering cholesterol, improved lactose

digestion, enhanced immune response, better protein, iron and calcium

assimilation are due to live bacteria present in yoghurt (Marona and

Pedrigon, 2004)

39

2.8. Sensory Evaluation

The importance of milk grading lies in the fact that dairy products

are only as good as the raw materials from which they were made. It is

important that dairy personnel have knowledge of sensory perception and

evaluation techniques. The identification of off-flavors and desirable

flavors, as well as knowledge of their likely cause, should enable the

production of high quality milk, and subsequently, high quality dairy

products (Goff, 2008). According to Delahunty (2002) the sensory

properties of dairy products, categorized as flavor, texture and appearance

attributes, determine consumer acceptability and willingness to repeat

purchase of a product, with some additional contribution from their

nutritional value and wholesomeness. A majority of sensory properties

are complex by definition as they are stimulated by the integrated

involvement of many different compositional and structural properties of

the product which means that they cannot be adequately detected or

represented by instrumental or chemical techniques.

It is important to define flavor, taste and odor. From a sensory

perspective, flavor is the total non-textural, non-visual and non-aural

perception of food as it is eaten. It comprises taste, which is governed by

sensors on the tongue, and odor, which is governed by sensors in the

olfactory system on the upper surface of the nasal cavity (Meilgaard et

al., 1999). However, due to the sophisticated functioning of the human

sensory systems, even a slight change in composition can be detected as a

change in sensory character and, therefore, sensory evaluations, in one

form or another, has become routinely applied in the dairy industry, in

particular for quality control (Delahunty, 2002).

40

CHAPTER HTREE

Material and methods

3.1.1. Source of material:

Fresh cow’s milk was obtained from the University of Khartoum

dairy farm, and goat’s milk was obtained from local farm at Shambat.

The milk samples were collected, cooled and transported to the Dairy

laboratory, Faculty of Animal Production, Khartoum University for

analysis and processing.

The yoghuty (Streptococcus thermophilus and Lactobacillus

bulgaricus) were kindly supplied by Khartoum Dairy Products Company,

Khartoum North. Plastic cups were purchased from the local market.

Yoghurt was manufactured from cow’s and goat’s milk, which subjected

to the same procedures.

3.2. Manufacture of experimental yoghurt:

The milk was heated to 95°C for 10 minutes. Heated milk was

allowed to cool with constant gentle agitation to an incubation

temperature (42 – 43) °C. A starter culture taken from previously

manufactured yoghurt was added at rate of 3% (w/v). The inoculated milk

was distribution into plastic cups were inculated at 45°C for four hours.

Then the set yoghurt cups were removed from the incubator and cooled to

10°C and kept for 12 days. The samples were analyzed for chemical,

microbiological, and sensory evaluation during storage at interval of zero,

3rd, 6th, 9th, and 12th days.

41

3.3. Analysis of milk and yoghurt samples:-

3.4. Chemical analysis:-

3.4.1. Total solid content (T.S %):

The total solid content was determined according to the method of

AOAC (1990). Three grams of the milk samples were weighted in dry

clean flat bottomed aluminum dish and heated in steam bath until there is

little or no free liquid movement in dish (< 25 minutes) the dishes were

placed in an oven at 100°C for three hours. Then cooled in a desicator

and weighed quickly. Weighing was repeated until the difference between

the two readings was less than 0.1mg.

The total solid (T.S) content was calculated as follow:

T.S% = W2 –W/W1-Wx100

W = weight of dish

W1 = weight of dish + milk test portion

W2 = weight of dish + dry milk

3.4.2 Fat content:

Fat content was determined using Gerber methods (Bradley et al.,

1992). 10 ml of sulphuric acid (specific gravity 1.820-1.825 at 15.5°C)

were measured into Gerber butyrometer. From a well mixed sample, 11

ml of milk or yoghurt was gently added into the butyrometer tube. One

ml of amyl alcohol was added and lock stopper was inserted securely

42

with the stopper end up. The Gerber tube was grasped and shacked with

precaution until the curd was completely digested. The Gerber tubes were

centrifuged at 1100 revaluation per minute (rpm) for 4 minutes. The

butyrometers were placed in a water bath (60-63°C) for 5 minutes. The

fat percent was then read out directly from the fat column.

3.4.3. Ash content:

The Ash content was determined according to the method

described in the AOAC (1990). Five grams of the samples were weighed

in crucible and evaporated to dryness on steam bath. The crucibles were

then placed in muffle furnaces at 550°C until ash were carbon free (2-3

hours), then crucibles were cooled in a desicator and weighed. The ash

content was calculated using the following equation.

Ash% = W2 –W/W1-Wx100

Where:

W = weight of dish

W1 = weight of dish + milk test portion

W2 = weight of dish + ash

3.4.4. Protein content:

Protein content of milk samples was determined according to

Kjeldahl method as described by AOAC (1990). Five ml of each milk

samples were weighed in dry Kjeldahl flasks. Kieldahl tablets of CuSO4

and concentrated H2SO4 (25ml) were added to the flasks. The flasks were

heated until clean solutions were obtained (1.8-2.25 hours). The flasks

were then removed and allowed to cool.

43

The digested milk samples were diluted by added 300 ml distilled

water to flask. Then 50 ml H3BO3 4% solution with indicator

(bromocresol green / Methyl red) were added to graduated 500ml

titiration flask and placed flask under condenser tip so that tip is well

below H3BO3 solution surface. Seventy five mlNaOH50% were drown

side wall of Kjeldahl flask with no agitation. The flask was immediately

connected to distillation bulb on condenser. The flask was vigorously

swirled to mix content thoroughly; until all NH3 has been distilled.

Titratationat H3BO3 was done by receiving solution with standard 0.1 HCl

solutions to first trace of pink.

The protein content was calculated as follows:

N%= T x 0.1 x 0.014 x 100/W

P%= N% x 6.38

Where:

T= Reading of titration.

W= Weight of original sample.

P= Total protein.

3.4.5. Titratable acidity:

The acidity of milk samples was determined according to Foley et

al. (1974). By measuring 10 ml of milk into a white porcelain dish. Then

0.5 ml of a 1.6% solution of phenolphthalein were added and Titrated

with N/9 caustic soda until a faint pink color which lasts for not less than

30 secs was obtained. The titration figure was divided by 10 to give the

acidity of the sample expressed as percentage lactic acid (W/V).

44

1 ml of N/9 NaoH = 1 ml of N/9 lactic acid= 0.01g lactic acid

y ml of N/9 NaoH = 0.01g x y lactic acid

0.01y in 10ml i.e % = o.1y

Where y is the titration figure.

3.4.6. Lactose content:

The lactose content was determined by subtracting the sum of

protein%, fat%, and ash% from total solid T.S%.

Lactose% = T.S% - (Protein% + Fat% + Ash %)

3.5. Microbiological examination:-

3.5.1. Sterilization:-

Glassware such as Petri-dishes, test tubes, pipettes and flasks were

sterilized in hot oven at 160°C for one hour (Harrigan and Mc Cance,

1976). Ringer solution used in the preparation of serial dilution was

sterilized by autoclaving at 121°C for 15 minutes (Harrigan and Mc

Cance, 1976).

3.5.2. Type of culture media used for microbical examination:

All media were prepared according to manufacturer’s instruction:

3.5.2.1. Plate count agar (Scharlau 01-161):

This media was prepared according to Harrigan and Mc Cance

(1976), 23.5 grams of the medium were dissolved in 1000 ml distilled

water, and then it was sterilized by autoclaving at 121°C for 15 minutes.

The medium was then distributed using the pour plate technique.

3.5.2.2. M17 medium (Scharlau 01-247):

The medium was prepared by suspending 66 gms in 1000 ml

distilled water. It was bring to boiling with constant stirring until

complete dissolution and sterilized by autoclaving at 121°C for 15

minutes.

45

3.5.2.3. MRS broth (Scharlau 01-135):

The medium was prepared by suspending 57 gms of powder in

1000 ml of distilled water and it was soaked. It was heated to boiling and

sterilized by autoclaving at 121°C for 15 minutes.

3.5.3. Enumeration of microorganism:

3.5.3.1. Total bacterial count:

The pour plate technique using plate count agar medium, was used

for total bacterial count. The plates were incubated at 32±1°C for 48±3

hours and colonies were counted according to Houghtby et al. (1992).

3.5.3.2. Streptococcus thermophilus:

This was carried out using modified M17 medium, 0.1ml from suitable

dilutions were spread on the surface of sterile modified M17 medium. The

plates were incubated at 37°C for 48±3 hours according to Frank et al.

(1992).

3.5.3.3. Lactobacilus bulgaricus:

This was carried out using modified MRS medium, 0.1ml from suitable

dilutions were spread on the surface of sterile modified MRS medium. The

plates were incubated at 37 °C for 48 ± 3 hours according to Frank et al.

(1992).

3.6. Sensory evaluations:

The sensory evaluation of yoghurt was done by participant uses a

nine-point scale (9 for ‘like extremely’ down to 1 for ‘dislike extremely’)

to score each attribute (Lawless and Heyman, 1999). The following

quality properties were evaluated: color, flavor and texture, using

untrained panelists. Samples of yoghurt for sensory evaluation were

presented in plastic cups of a volume of 40 gms.

46

3.7. Statistical analysis:

All the data of this experiment were analyzed statistically by using

complete randomized design (CRD). The analysis of variance and the

significant differences between means were determined using Duncan

Multiple Range Test using SPSS (Statistical Package for Social Science)

version 10. Figures were done using Microsoft excel.

47

CHAPTER FOUR

Result

Table (1) shows the comparison of chemical composition and

log10 count of microbial content of cow’s and goat’s milk before and

after pasteurization.

The mean total solids values of cows, milk were 13.87±0.12% and

the minimum was 13.83% and the maximum was 13.91% for

unpasteurized milk and for pasteurized milk were 13.84±0.19%, 13.67%

and 14.01% for mean, minimum and maximum values, respectively.

The mean total solids values of goat’s milk were 13.52±0.06% and

the minimum was 13.34% and the maximum was 13.69% for

unpasteurized milk and for pasteurized milk were 13.68±0.09%, 13.50%


The fat content of cow’s unpasteurized milk has mean of

4.60±0.08% and the minimum was 4.48% and the maximum was 4.72%,

while fat content in pasteurized milk were 4.58±0.05%, 4.45% and 4.70%

for mean, minimum and maximum values, respectively.

The fat content of goat's unpasteurized milk has mean of


while fat content in pasteurized milk were 4.55±0.13%, 4.43% and 4.67%

for mean, minimum and maximum values, respectively.

The protein content of cow's unpasteurized milk has mean of


while protein content in pasteurized milk were 3.76±0.18%, 3.67% and

3.85% for mean, minimum and maximum values, respectively.

48

The protein content of goat’s unpasteurized milk has mean of


while protein content in pasteurized milk were 3.50±0.09%, 3.41% and


The lactose content of cow’s unpasteurized milk has mean of


while lactose content in pasteurized milk were 4.87±0.05%, 4.80% and


The lactose content of goat's unpasteurized milk has mean of

4.81±0.06% and minimum was 4.74% and the maximum was 4.88%,

while lactose content in pasteurized milk were 4.88±0.06%, 4.81% and

4.95% for mean, minimum and maximum values, respectively .

The ash content of cow’s unpasteurized milk has mean of 0.75±

0.03% and the minimum was 0.72% and the maximum was 0.79%, while

ash content in pasteurized milk were 0.73±0.03%, 0.70% and 0.77% for

mean, minimum and maximum values, respectively.

The ash content of goat’s unpasteurized milk has mean of


while ash content in pasteurized milk were 0.75±0.03%, 0.71% and


The acidity percent of cow’s unpasteurized milk has mean of


while the acidity percent in pasteurized milk were 0.19±0.01%, 0.18%


49

The acidity percent of goat’s unpasteurized milk has mean of


while the acidity percent in pasteurized milk were 0.185±0.01%, 0.18%


The mean for log total bacterial count (T.B.C) in unpasteurized

cow’s milk was 7.33±0.02 and minimum was 7.30 and maximum was

7.35, while in pasteurized milk were 6.02±0.06, 5.99 and 6.04 for mean,

minimum and maximum values, respectively.

The mean for log total bacterial count in unpasteurized goat’s milk

was 7.31±0.01 and the minimum was 7.28 and the maximum was 7.33,

while in pasteurized milk were 5.99±0.06, 5.96 and 6.01 for mean,

minimum and maximum values, respectively.

50

Table (1) : Chemical composition and microbial content of cow’s milk and goat’s milk before

and after pasteurization :

Items types Cow milk Goat milk treatment Means±sd min max Means±sd min max

Total solids (%)

before 13.87±0.12 13.83 13.91 13.52±0.06 13.34 13.69 after 13.84±0.19 13.67 14.01 13.68±0.09 13.50 13.85

Fat (%) before 4.60±0.08 4.48 4.72 4.58±0.01 4.45 4.70 after 4.58±0.05 4.45 4.70 4.55±0.03 4.43 4.67

Protein (%) before 3.58±0.09 3.49 3.67 3.47±0.07 3.38 3.56 after 3.76±0.18 3.67 3.85 3.50±0.09 3.41 3.59

Lactose (%) before 4.82±0.07 4.76 4.88 4.81±0.06 4.74 4.88 after 4.87±0.05 4.80 4.94 4.88±0.06 4.81 4.95

Ash (%) before 0.75±0.03 0.72 0.79 0.74±0.04 0.71 0.77 after 0.73±0.03 0.70 0.77 0.75±0.03 0.71 0.78

Acidity (%) before 0.17±0.01 0.16 0.17 0.18±0.01 0.17 0.19 after 0.19±0.01 0.18 0.19 0.185±0.01 0.18 0.19

logTBC before 7.33±0.02 7.30 7.35 7.31±0.01 7.28 7.33 after 6.02±0.06 5.99 6.04 5.99±0.06 5.96 6.01

51

Table (2) shows variations of some quality tests of cow’s milk set

yoghurt during storage. The minimum of total solids was 13.55% and the

maximum was 13.86%. The total solids of set yoghurt revealed

significant (p< 0.05) variations during storage.

The fat content of set yoghurt showed values of 4.48% and 4.63%

for minimum and maximum values, respectively. The fat content of set

yoghurt revealed significant (p< 0.01) variations during storage.

The protein content of set yoghurt showed values of 3.39% and

3.80%, for minimum and maximum values, respectively. The protein

content of set yoghurt revealed significant (p< 0.05) variations during

storage.

The lactose content of set yoghurt showed values of 4.52% and

4.85, for minimum and maximum values, respectively. The lactose


storage.

The ash content of set yoghurt showed values of 0.71% and 0.74%,

for minimum and maximum values, respectively. The ash content of set

yoghurt revealed not significant variations during storage.

The minimum of acidity percent of set yoghurt was 0.91% and the

maximum was 1.35%. The acidity of set yoghurt revealed significant (p<

0.01) variations during storage.

The minimum count of log total bacterial count (TBC) was 10.55

and the maximum was 12.59. The (TBC) of set yoghurt showed


52

The minimum count of log Streptococcus subsp. was 8.94 and the

maximum was 11.71. The Streptococcus subsp. of set yoghurt showed


The minimum count of log Lactobacillus subsp. was 9.39 and the

maximum was11.58. The Lactobacillus subsp. of set yoghurt showed


Table (3) show variations of some quality tests of goat’s milk set

yoghurt during storage. The minimum of total solids was 13.10% and the

maximum was 13.73%. The total solids of set yoghurt revealed


The fat content of set yoghurt showed values of 4.40% and 4.60%,

for minimum and maximum values, respectively. The fat content of set

yoghurt revealed significant (p< 0.05) variations during storage.

The protein content of set yoghurt showed values of 3.31% and

3.66%, for minimum and maximum values, respectively. The protein


storage.

The lactose content of set yoghurt showed values of 4.31% and

4.81, for minimum and maximum values, respectively. The lactose


storage.

The ash content of set yoghurt showed values of 0.71% and 0.74%,

for minimum and maximum values, respectively. The ash content of set

yoghurt revealed not significant variations during storage.

53

The minimum of acidity percent of set yoghurt was 1.18% and the

maximum was 1.51%. The acidity of set yoghurt revealed significant (p<

0.05) variations during storage.

The minimum count of log total bacterial count (TBC) was 11.18

and the maximum was 12.20. The (TBC) of set yoghurt showed


The minimum count of log Streptococcus subsp. was 9.41 and the

maximum was 11.59. The Streptococcus spp. of set yoghurt showed


The minimum count of log Lactobacillus subsp. was 8.85 and the

maximum was 11.68. The Lactobacillus subsp. of set yoghurt showed


54

Table (2): Variations of some quality tests of set yoghurt produced from cow's milk during

storage:

Items (0) (3) (6) (9) (12) L.S

mean±sd mean±sd mean±sd mean±sd mean±sd

Total Solids

(%)

13.84±0.11 c 13.86±0.08 c 13.77±0.15 b 13.59±0.16 b 13.55±0.10 a *

Fat (%) 4.60±0.08 b 4.58±0.13 b 4.63±0.10 c 4.48±0.10 a 4.50±0.08 a **

Protein (%) 3.71±0.17 c 3.66±0.10 c 3.80±0.09 c 3.53±0.17 b 3.39±0.14 a *

Lactose (%) 4.79±0.15 b 4.83±0.07 b 4.52±0.22 a 4.85±0.09 c 4.83±0.21 b **

Ash% 0.74±0.03 a 0.74±0.04 a 0.73±0.02 a 0.73±0.03 a 0.71±0.02 a N.S

Acidity (%) 0.91±0.12 a 1.21±0.06 b 1.31±0.19 b 1.32±0.07 b 1.35±0.07 b **

Log

Streptococcus

10.10±0.34 c 11.71±0.26 c 10.29±0.78 b 10.53±0.34 b 8.94±.053 a *

Log

Lactobacillus

9.57±.022 a 11.58±.036 c 10.29±0.40 b 10.63±0.19 b 9.39±0.09 a *

Log TBC 10.55±0.60 a 12.59±0.55 d 11.63±0.52 c 11.77±0.43 c 11.12±0.07 b **

*=p≤0.05

**=p≤0.001

NS=P>0.05

Means with the same raw being similar subscript letter are not significantly (P> 0.05) affected.

55

Table (3): Variations of some quality tests of set yoghurt produced from goat’s milk during

storage:

Items (0) (3) (6) (9) (12) L.S


Total Solid (%) 13.73±0.10 a 13.60±0.09 a 13.55±0.10 b 13.27±0.29 c 13.10±0.32 c *

Fat (%) 4.58±0.10 b 4.60±0.14 b 4.58±0.10 b 4.55±0.20 b 4.40±0.08 a *

Protein (%) 3.48±0.18 b 3.53±0.17 b 3.44±0.22 b 3.66±0.31 c 3.31±0.10 a **

Lactose (%) 4.81±0.14 c 4.73±0.20 b 4.69±0.30 b 4.31±0.38 a 4.68±0.17 b **

Ash (%) 0.74±0.03 a 0.74±0.04 a 0.73±0.02 a 0.73±0.03 a 0.71±0.02 a N.S

Acidity (%) 1.18±0.07 a 1.31±0.10 b 1.43±0.19 b 1.51±0.13 b 1.51±0.15 b *

Log

Streptococcus

9.62±0.21 a 11.59±0.30 b 9.92±0.40 a 10.61±0.24 a 9.41±0.10 a *

Log

Lactobacillus

10.11±0.36 b 11.68±0.31 c 10.32±0.43 b 10.59±0.29 b 8.85±0.59 a **

Log TBC 11.30±1.00 a 12.16±0.03 b 12.20±0.11 b 11.80±0.43 b 11.18±0.12 a *

*=p≤0.05

**=p≤0.001

NS=P>0.05


56

Table (4) and figure (1) shows variations of sensory evaluation

tests of cow’s milk yoghurt during storage. The minimum of color was

6.47% and the maximum was 7.27%. The color of set yoghurt showed

significant (p< 0.05) variations during storage. The minimum of flavor

was 6.60% and the maximum was 7.57%. The flavor of set yoghurt

showed significant (p< 0.01) variations during storage. The minimum of

texture was 6.77% and the maximum was 7.37%. The flavor of set

yoghurt showed significant (p< 0.05) variations during storage.

Table (5) and figure (2) shows variations of sensory evaluation

tests of goat’s milk yoghurt during storage. The minimum of color was

7.27% and the maximum was 7.73%. The color of set yoghurt showed

significant (p< 0.05) variations during storage. The minimum of flavor

was 6.03% and the maximum was 6.53%. The flavor of set yoghurt

showed significant (p< 0.05) variations during storage. The minimum of

texture was 6.60% and the maximum was 7.07%. The flavor of set

yoghurt showed significant (p< 0.05) variations during storage.

57

Table (4): Variations of sensory evaluation tests during storage of set yoghurt produced

from cow’s milk:

Items (0) (3) (6) (9) (12) L.S


Color 7.10±1.19 c 7.27±0.87 c 7.13±1.04 c 6.80±1.19 b 6.47±0.90 a *

Flavor 7.57±0.86 c 7.30±1.15 c 6.97±0.93 b 6.93±1.29 b 6.60±1.28 a **

Texture 7.37±0.77 c 7.07±1.29 b 7.07±0.94 b 7.03±1.16 b 6.77±0.77 a *

*=p≤0.05

**=p≤0.001

NS=P>0.05

Nine-point scale (9 for " like extremely" down to 1 for " dislike extremely" )


58

Table (5): Variations of sensory evaluation tests during storage of set yoghurt produced

from goat’s milk:

Items (0) (3) (6) (9) (12) L.S


Color 7.73±0.83 c 7.50±1.10 b 7.53±1.40b 7.33±0.99 a 7.27±0.98 a *

Flavor 6.43±1.25 d 6.53±1.14 d 6.03±1.19 a 6.40±1.16 c 6.20±1.40 b *

Texture 7.07±0.91 b 6.83±0.83 a 6.63±1.16 a 6.60±1.04 a 6.70±1.15 a *

*=p≤0.05

Nine-point scale (9 for "like extremely" down to 1 for" dislike extremely" )


59

Table (6) and figure (3-4) shows variations of some quality tests

between two types of yoghurt. The mean of total solids for cow’s milk

yoghurt was 13.72±0.14%, while the mean total solids for goat’s milk

yoghurt was 13.49±0.33%. The total solids of set yoghurt revealed highly

significant (P< 0.001) variations between the two types.

The mean of fat content for cow’s milk yoghurt was 4.56± 0.11%

while the mean fat content for goat’s milk yoghurt was 4.54±0.14%. The

fat content of set yoghurt showed not significant variations between the

two types.

The mean of protein content for cow’s milk yoghurt was

3.62±0.19%, while the mean protein content for goat’s milk yoghurt was

3.48±0.22%. The protein content of set yoghurt showed significant (P< 0.05) variations between the two types.

The mean of lactose content for cow’s milk yoghurt was

4.79±0.21%, while the mean of lactose content for goat’s milk yoghurt

was 4.68±0.31%. The lactose content of set yoghurt showed not

significant variations between the two types.

The mean of ash content for cow’s milk yoghurt was 0.73±0.03;

similarly the mean of ash for goat’s milk yoghurt was 0.73±0.03. The ash

content of set yoghurt revealed no significant variations between the two

types.

The mean of acidity percent for cow’s milk yoghurt was

1.22±0.19%, while the mean of acidity percent for goat’s milk yoghurt

was 1.39±0.18%. The acidity percent of set yoghurt revealed highly

significant (P< 0.001) variation between the two types.

60

The mean of los TBC for cow’s milk yoghurt was 11.53±0.82,

while the mean of log TBC for goat’s milk yoghurt was 11.73±0.60. The

TBC of set yoghurt revealed highly significant (P< 0.001) variations

between the two types.

The mean count of log Streptococcus subsp. for cow’s milk

yoghurt was 10.31±1.01, while the mean of log Streptococcus subsp. for

goat’s milk yoghurt was 10.23±0.85, the count of Streptococcus subsp. of

set yoghurt showed highly significant (P< 0.001) variations between the

two types.

The mean count of log Lactobacillus subsp. for cow’s milk yoghurt

was 10.29±0.85, while the mean of log Lactobacillus subsp. for goat’s

milk yoghurt was 10.31±1.00 the count of Lactobacillus subsp. of set

yoghurt revealed not significant variations between the two types of

yoghurt.

Table (7) and figure (5) showed variations of sensory evaluation

tests between the two types of set yoghurt. The mean of color for cow’s

milk yoghurt was 6.94±0.97, which the mean of color for goat’s milk

yoghurt was 7.47±1.02. The color of set yoghurt showed highly

significant (P< 0.001) variations between the two types. The mean of

flavor for cow’s milk yoghurt was 7.07±1.15 while the mean of flavor for

goat’s milk yoghurt was 6.32±1.23. The flavor of set yoghurt revealed

significant (P< 0.05) variation between the two types of yoghurt. The

mean of texture for cow’s milk yoghurt was 7.06±1.01, while the mean of

texture for goat’s milk yoghurt was 6.77±1.03. The texture of set yoghurt

showed highly significant (P< 0.001) variation between the two types of

yoghurt.

61

Table (6) : variation of some quality test between types of set yoghurt produced

from cow’ smilk and goat’s milk .

Items Cow Goat L.S

mean±sd mean±sd

Total solid % 13.72±0.14 b 13.49±0.33 a 0.000 ***

Fat % 4.56±0.11a 4.54±0.14 a 0.709 NS

Protein % 3.62±0.19 b 3.48±0.22 a 0.030 *

Lactose % 4.79±0.21 a 4.68±0.31 a 0.116 NS

Ash % 0.73±0.03a 0.73±0.03a 0.912 NS

Acidity % 1.22±0.19a 1.39±0.18 b 0.000 ***

Log Streptococcus 10.31±1.01 b 10.23±0.85 a 0.000 ***

Log Lactobacillus 10.29±0.85 a 10.31±1.00 a 0.085 NS

Log TBC 11.53±0.82 a 11.73±0.60 b 0.000 ***

*=p≤0.05

***=p≤0.0001

NS=P>0.05

Means with the same raw being similar subscript letter are not significantly (P> 0.05)

affected.

62

Table (7): Variations of sensory evaluation tests between types of set yoghurt produced

from cow’s milk and goat’s milk:

Items Cow Goat L.S

mean±sd mean±sd

Color 6.94±0.97 a 7.47±1.02 b 0.000 ***

Flavor 7.07±1.15b 6.32±1.23 a 0.013 *

Texture 7.06±1.01 b 6.77±1.03 a 0.000 ***

*=p≤0.05

***=p≤0.0001


Score = nine-point scale (9 for "like extremely" down to 1 for" dislike extremely").

63

CHAPTER FIVE

Discussion

Table (1) shows the composition of goat’s milk. These results are

in range with that of Kudełka (1996) who reported that in Poland, the

mean content of basic goat milk formed as follows: fat 2.25-5.52%,

protein 2.58-4.15%, lactose 3.92-5.28%, ash 0.74-0.95%, dry matter

10.49-14.83%, but higher than the findings of Stelios and Emmanuel

(2004) they reported that caprine milk from an alpine breed had the

lowest value. It’s also supported Domagała and Juszczak (2004) expet the

ash content was higher. These results are lower than that of Bille et al.

(2000). The result was confirmed Jandal (1996); Wong (1999) and

Gomes et al. ( 2004) who reported that milk composition is affected by

the goat’s breed, region and sanitary conditions (free pasture or captivity),

feeding characteristics, health conditions and normal season lactation

conditions.

Table (1) shows the log count of TBC of goat’s milk. The result

obtained was higher, that because large numbers are often related to poor

farming practices and poor cleaning system (IDF 1994). The result was

higher value than that of Muehlherr et al. (2001) who reported that the

log count of TBC for bulk milk tank samples from dairy goat farms was

4.69 cfu/ml. These results were in agreement with Bille et al. (2000) who

reported that goat milk was also higher in microbial load than cow milk,

which was attributed to difference in milking technique and the goats

were milked by hand and the cows were milked more hygienically by

machine.

64

Table (1) shows total solids and fat content of the cow’s milk. The

results obtained were in agreement with total solids reported by Webb et

al. (1974) and Hassan (2005) and disagreement for fat reported by Webb

et al. (1974) and Hassan (2005) who reported that the total solids in the

milk ranged from 10% to 17%, which include fat and non-fat materials.

The amount of fat materials is 3% to 4% and the amount of non-fat

material is in the range of 7% to 10%.

Table (1) shows the protein content of cow’s milk. The result was

in agreement with that of Webb et al. (1974) and Hassan (2005) who

found that the total protein content of milk range from 2% to 4%, and the

milk protein have the high nutritional value and the principal component

of the milk proteins is casein, which constituents about 75% of all milk

proteins.

Table (1) shows the lactose and ash content of the cow’s milk.

These results are in range for lactose and high for ash with that of Webb

et al. (1974) and Hassan (2005) who found that the lactose in the milk

was from 2% to 5%, while ash content was about o.65%.

Table (1) shows the acidity percent of the cow’s milk. The

obtained result was in agreement with that of Rehman and Salaria (2005)

they reported that the lactic acid ranged from 0.15±0. 03% to 0.26±0.03%

Table (1) shows the log count of TBC of the cow’s milk. The result

was higher value than that of FDA (2008) reported that the standard limit

for standard plate count as 20,000 CFU/ml

The results of cow’s milk composition were in range reported by

Alfa-Laval (1996) who reported that the quantitative composition of milk

65

ranged as follow: water 85.5-89.5%, total solids 10.5-14.5%, fat 2.5-

6.0%, proteins 2.9-5.0%, lactose 3.6-5.5% and minerals 0.60-0.90%.

Table (2) shows significant variations of total solids during storage.

The result was in agreement with Anjum et al. (2007) who reported that

treatment and storage period had significant effect on the total solids of

yoghurt samples prepared by locally isolated starter culture and

commercial starter culture. It is evident from the result that reduction in

total solids throughout storage period might be due to change of lactose

into lactic acid by lactose fermenting bacteria in yoghurt. These results

were confirmed Tamime and Robinson, (1985). In general the present

result was lower than that of Hussain et al. (2009) who reported that the

average total solids content of natural yogurt was of 19.2 with a standard

deviation of 0.035.

Table (2) shows significant variations of fat content during storage.

The result was in agreement with that of El-abbassy and Sitohy (1993)

they reported that during storage the fat/Dm% decreased as aresult of

probable fat hydrolysis. The result was disagreement with Anjum et al.

(2007) who reported that the fat content of yoghurt, displayed statistically

not significant difference for reduction in fat content at the end of storage

period that might be due to production of volatile fatty acids by yoghurt

organisms.

Table (2) shows significant variations of protein content during

storage. The result was in agreement with that of Tamime and Robinson

(1985) they reported that the protinase activity of Lactobacillus

bulgaricus hydrolyses the casein to yield polypeptides and broken down

by the peptidases of Streptococcus thermophilus with liberation of amino

acids. This supported El-abbassy and Sitohy (1993) they reported that

66

during storage, there was slight decrease in TN of yoghurt samples. El-

abbassy and Sitohy (1993) and El-Shibiny et al. (1979) reported that non-

protein-nitrogen (NPN) content of yoghurt samples gradually increased

during storage period, this could be attributed to limited hydrolysis of

milk protein. In general the protein content of milk cow yoghurt was

lower than that stated by Turkish Food Codex Fermented Milk Bulletin

(Anonym, 2001) that the minimum protein content requirement was 4%.

Table (2) shows significant variations of lactose content during

storage. The result was in accordance with Anjum et al. (2007) who

reported that lactose content of the sample manifested a decreasing trend.

It’s also supported Toba et al. (1983) who reported that lactose content of

yoghurt mix progressively decreased during storage period and glactose

content of mixed progressively increased during storage.

Table (2) shows not significant variations of ash content during

storage. The result was discordance with that of Shanley (1973) who

found that the protein and ash contents of yoghurt decreased with

progress of storage period. The result was lower than that of Nahar et al.

(2007) who reported that the ash percent of cow’s milk dahi was

0.809±0.04. It also supported Lingathurai et al. (2009) who reported that

the average level of ash was 0.80%. The result was higher value than that

of El-Bakari and El-Zubeir (2009) they reported that the mean of ash

content of plain yoghurt was 0.678±0.146.

Table (2) shows the acidity percent of yoghurt during storage. The

present study revealed increased acidity during storage; increase in

acidity was due to the formation of lactic acid by bacteria present in

yoghurt during storage period Anjum et al. (2007). It also supported

Hussain et al. (2009) and Guller and Mutlu, (2005) who observed an

67

increase in total titrable acidity during storage period. The result was

disagreement with Salvador and Fiszman (2004) they found that the PH

value barely changed over storage time indicating that the yoghurt

samples did not developed much acidity under any of storage condition

studied.

Table (2) shows significant variations of number of Streptococcus

subsp. and Lactobacillus subsp. during storage. These results were

revealed decreased in number during storage, a number of factors that

affect the loss of viability of probiotic organisms in yoghurt, including

acidity of products, acid produced during refrigerated storage (also

known as post acidification), level of oxygen in products and oxygen

permeation through the package, sensitivity to antimicrobial substances

produced by starter bacteria, lysogenic charcter of bacteria and lack of

nutrients in the milk Dave and Shah (1997) and Tamime et al. (2005). It

also supported Salvador and Fiszman (2004) they reported that at 10co a

considerable reduction in both cultures was observed at the end of

storage. The results were disagreement with Vargan et al. (1989) who

stated that there were no reports on the changes in yoghurt flora during

storage of fermented soy milk. In general these results are higher than

that of Irkin and Eren (2008) they reported that viable Streptococcus

thermophilus and Lactobacillus bulgaricus number were between 107-108

cfug1- for yoghurt producing with starter culture.

Table (2) shows significant variations of number of TBC during

storage. The result was in agreement with that of Reps et al. (2008) who

found that, the decrease of number of living bacteria was observed in

yoghurt during storage. The result was higher value than finding of El-

Bakari and El Zubeir (2009) they reported that the means of

68

microbiological measurement for the plain yoghurt samples were log

9.10±9.86.

Table (3) shows significant variations of total solids during storage.

The result was agreement with that of Anjum et al. (2007) who reported

that total solids decreased gradually during storage period in both type of

samples. The result was disagreement with that of Kavas et al. (2003)

who reported that it is accepted that the increase during 14 days on total

solids content were not significant and attributed to the evaporation, it’s

supported Akalin (1993) who reported that the increase determined

during the storage period is normal.

Table (3) shows significant variations of fat content during storage.

The result was in agreement with that of Koestanti1and Romziah, (2008)

they reported that the decreasing of fat in yoghurt happened due to the

breakage of lipid during fermentation process splitting-up fresh milk

becomes deep yoghurt, so that fat content decrease. The result was

disagreement with Anjum et al. (2007) who reported that fat% did not

change during storage. The result was higher value than that of Guler and

Mutlu (2005) they reported that the fat content of bio-yoghurt made from

goat’s milk was 3.1%.

Table (3) shows significant variations of protein content during

storage. The result was in agreement with that of Serra et al. (2009) who

reported that in all treatments studied, caseins were hydrolyzed and

hydrophobic peptides were increased during storage, as reflected by the

increase in soluble nitrogen at the end of the storage. The result

disagreement with Koestanti and Romziah (2008) they reported that at the

fermentation process, biomass microbe Lactobacillus bulgaricus and

Streptococcus thermophilus increase, thus the sum of microbe protein

69

increase, that automatically increasing protein inside the yoghurt. El-

shibiny et al. (1979) reported that the increase in free amino acids of

yoghurt during storage suggests limited proteolysis of milk protein.

Table (3) shows significant variations of lactose content during

storage. The result was in accordance with Anjum et al. (2007) who

reported that lactose content decreased as dose of starter culture and

storage period increased, it’s supported Goodenought and Kleyn (1976)

they reported that the decrease in lactose content during storage is due to

production of lactic acid.

Table (3) shows not significant variations of ash content during

storage. The result was disagreement with that of Hidiroglou and Proulx

(1982) they reported that milk Ca, P and Mg contents were all highest

during the first day of storage then decrease sharply at 2nd day. The result

was lower than that of Nahar et al. (2007) who reported that the ash

percent of goat’s milk dahi was 0.784±0.06; it’s supported Bille et al.

(2000) who reported that ash content of goat milk was 0.83%.

Table (3) shows the acidity percent of yoghurt during storage. The

present study was revealed increased acidity during storage; the result

was in accordance with that of Kavas et al. (2003) who reported that the

acidity increase in yoghurt during the storage was also to be significant.

It’s supported El-abbassy and Sitohy (1993) they found that the acidity

increased and PH decreased gradually in yoghurt samples until the end of

storage period. Fernandez-Garcia et al. (1994) found that the content of

organic acids in yoghurt during fermentation and cooled storage of

yoghurt continuously changed, and this affect PH of yoghurt during

storage.

70

Table (3) shows significant variations of number of Streptococcus

subsp. and Lactobacillus subsp. during storage. The results were revealed

decreased in number during storage; that could be due to the lower

storage temperature and over acidification have been reported to limit the

growth of Lactobacillus delbureckii spp.blugaricus Kenifel et al. (1992).

The results were in accordance with that of Ekinic and Gurel (2008) they

reported that the viable counts of Streptococcus thermophilus in control

during storage changed from 8.33 log (cfu g1-) on day 1to 6.33 log (cfu

g1-) on day 15. The results were in agreement with that of Kumar and

Singh (2007) they reported that yoghurt to be considered as a probiotic

product. Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus

thermophilus are at a daily dose of 109 cfu. It’s also supported Irkin and

Eren (2008) they reported that yoghurt which are produced by starter

culture have high numbers of yoghurt bacteria means that yoghurt

produced by using starter culture have higher therapeutic and/or

antimicrobial properties beside their organoleptic characteristics.

Table (3) shows significant variations of number of TBC during

storage. The result agreed with that of Sun and Griffiths (2000) they

reported that during storage and distribution, the cell number significantly

decreases due to the over produced lactic acid. It’s also supported Sofu

and Ekinci (2007) they reported that total plate counts were lower for

whole-fat yoghurt samples, possibly due to the decrease in PH during

storage. The result was higher value than that of Nahar et al. (2007) who

showed that the total viable bacteria count per ml of dahi prepared from

goat milk was 5.859±0.05 (log value).

Table (4) and figure (1) shows significant variations of colors

during storage. The result agreed with Vargas et al. (2008) who reported

71

that the compaction of the solid matrix and the increase in the syneresis

index during storage explain these color change, it’s also supported

Hutchings (1999) who reported that the changes in color coordinates can

be attributed to the different level of opacity.

Table (4) and figure (1) shows significant variations of flavor

during storage. The result was observed confirmed the finding of

Oberman (1985) who found that diacetyle reductase enzyme becomes

responsible for loss of the flavor after long storage. Foda et al. (2007)

reported that prolonging cold storage period affect the flavor significantly

could be due to the strong taste. The results were disagreement with that

of Salvador and Fiszman (2004) they reported that no significant changes

in relation to the storage time were found in color and flavor intensity for

either type of yoghurt.

Table (4) and figure (1) shows significant variations of texture

during storage. The result disagreement with that of Becker and Puhan

(1989) they found that high protein content gives higher firmness values,

indicating that this characteristic was not greatly affected by the different

storage conditions. It’s also supported Herrero and Requena (2006) they

found that the textural properties of yoghurt showed no significant

differences throughout the shelf period of 28 days. Abu-Jdayil and

Mohemed (2002) reported that throughout storage, protein rearrangement

was continuing, and more protein-protein contacts were being

established, leading to increasing viscosity during storage.

Table (5) and figure (2) shows significant variations of colors

during storage. The result agreed with Sofu and Ekinci (2007) they

reported that the color block with grayish-greenish yellow color was

dominant in both whole- and low-fat yogurts at the end of storage. The

72

presence of these colors is associated with microbial spoilage of the food

product. The color analysis data were parallel to PH and microbial count

data. The result obtained was disagreement with the finding of Salvador

and Fiszman (2004) they reported that no significant changes in relation

to time were found in color, flavor intensity and sweetness for either type

of yoghurt. The score of all sensory parameters significantly decreased

after the addition of goat milk, except whiteness and creaminess which

increased significantly when more goat’s milk was added.

Table (5) and figure (2) shows significant variation of flavor during

storage. The result observed confirmed the finding of Ekinci and Gurel

(2008) they reported that, in general, the level of carbonyl compounds

decreased during cold storage, this could be associated further with

metabolic activity of the starter cultures during the storage period. It’s

also supported Ozer (2006) and Radi et al. (2009) who reported that

acetaldehyde, which is mainflavor substance in yoghurt, metabolized to

ethanol via alcohol dehydrogenase of Streptococcus thermophilus. Flavor

scores at zero time were significantly higher than of two weeks.


during storage. The result was in accordance with Mumtaz et al. (2008)

who reported that texture was affected significantly during storage in all

experimental yoghurts. The result was disagreement with Radi et al.

(2009) who reported that the different yoghurt samples showed similar

texture after two weeks of storage as that of zero time. This supported

Herrero and Requena (2006) they found that the texture properties were

maintained constant throughout the shelf- life of the product.

Table (6) and figure (3) shows significant variations of total solids

between goat’s and cow’s milk yoghurt. This might be due to the

73

increased total solids in cow’s milk compared to goat’s milk. The result

was in agreement with Soukoulis et al. (2007) who reported that yoghurt

quality may be improved by properly selecting the parameters associated

with fermentation process and compositional properties of the milk base.

Similarly it supported Brendehaug and Abrahamsen (1986) they reported

that the goat milk is characterized by very large inconstancy in it is

chemical composition during lactation period which results in differences

in dry matter (DM) content of that raw material. The results were

disagreement with that of Kavas et al. (2003) who reported that milk kind

and concentration method are not effective on total solids content of the

yoghurt samples.

Table (6) and figure (3) shows not significant variations of fat

content between goat’s and cow’s milk yoghurt. The results were in

agreement with that of Kavas et al. (2003) who reported that milk kind

and storage affects were found not to be significant on the fat contents of

the yoghurt samples. The results were disagreement with the that of

Salvador and Fiszman (2004) they reported that the major difference

between cow’s milk and goat’s milk were those concerning lipid and

casein content, which was significantly higher for goat’s milk, similarly it

supported Jenness (1980) who reported that average goat milk fat differ

in content of its fatty acids significantly from average cow milk fat.

Table (6) and figure (3) shows significant variations of protein

content between goat’s and cow’s milk yoghurt. This might be due to the

differences in the protein content of the cow’s milk and goat’s milk. The

results were in agreement with that of Domagała (2008) who reported

that an important role is also played by the composition and

physicochemical properties of milk which yoghurt prepared from. It’s

74

also confirmed Morón et al. (2000) and Chandan et al. (1992) who

reported a comparative study on the composition of proteins in goat and

cow milk and, specifically, of their amino acid profile. The latter

parameter was similar in both types of milk, except with respect to

sulphur amino acids, which were present in a higher concentration in the

goat milk protein in comparison with cows. Other differences were also

found, but these concerned more the physico-chemical nature of the

proteins, and in particular, the different concentrations of the casein

fraction in the two types of milk. Such differences could produce a

different degree of utilization at the digestive level. The results were

disagreement with finding of Kavas et al. (2003) who reported that the

concentration method and the milk kind were found not to be effective on

the protein contents of the yoghurt samples.

Table (6) and figure (3) shows not significant variations of lactose

content between goat’s and cow’s milk yoghurt. The results were

disagreement with that of Bille et al. (2000) who reported that higher

lactose content in goat milk resulted in more acid goat milk yoghurt than

cow milk yoghurt under the same treatment. It’s also supported Salvador

and Fiszman (2004) they reported that goat’s milk has higher total

protein, lactose, ash, and total solids content than cow milk. On the other

hand Fox and Mc Sweeney (1998) and Antunac et al. (2001) reported that

goat milk has lower concentration of lactose than cow milk.

Table (6) and figure (3) shows not significant variations of ash

content between goat’s and cow’s milk yoghurt. The results were in

agreement with that of Gupta (2004) who reported that the mineral

content of goat milk and cow milk is generally similar. The results were

disagreement with that of Nahar et al. (2007) who reported that the

75

maximum ash percent was seen in buffalo milk dahi followed by cow

milk dahi, and lowest in goat milk dahi. It’s also supported Rutherfurd et

al. (2006) who reported that the mineral composition of the prepared goat

milk formula was higher than that of the prepared cow milk formula for

most minerals.

Table (6) and figure (3) shows significant variations of acidity

percent between goat’s and cow’s milk yoghurt. The acidity of goat’s

milk yoghurt was higher than cow’s milk yoghurt. The faster acidification

and lower PH value in goat milk yoghurt could be explained by the

enhancement of growth, acidity progress and peptidase activity of

Lactobacillus delbureckii ssp. blugaricus in goat’s milk Rysstad and

Abramamsen (1987); Bozanic et al. (1998) and Tamime and Robinson

(1999). It’s also supported Domagał (2008) and Abramamsen and

Rysstad (1991) who reported that yoghurt from goat milk had a higher

acidity on comparison to the acidity of cow milk yoghurt, this can be

explained by a faster increase of acidity in goat milk due to its lower

buffering capacity and higher content of non protein nitrogen (NPN) and

vitamins, which are needed for fast microorganism development.

Table (6) and figure (4) shows the number of Streptococcus subsp.

and Lactobacillus subsp. between goat’s and cow’s milk yoghurt. The

results were higher than that obtained by Noni et al. (2004) who reported

that Lactobacillus bulgaricus and Streptococcus thermophilus bacteria

count were between 107-108 cfu g1- for 10 days yoghurt samples. These

were also confirmed of Irkin and Eren (2008) they reported that viable

Streptococcus thermophilus and Lactobacillus bulgaricus number were

determined between 107-108 cfu g1- for yoghurt produced with starter

culture. It’s also IDF (1988) reported that the beneficial effects of the

76

regular ingestion of yoghurt on the consumer’s health have always been

related to the presence of high concentration of viable lactic acid bacteria

in the product. Several countries have established minimum values of

lactic acid bacteria for yoghurts and/or fermented milks during shelf-life.

The value ranged between 1x106 to 5x108 cfu g1-. The present study

showed that not significant variations of Lactobacillus bulgaricus

between the two types of yoghurt. On the other hand Streptococcus

thermophilus cow’s milk yoghurt was higher than that of goat’smilk

yoghurt that could be due to the lysogenic character of Streptococcu

thermophilus is important on bacteria viability during storage period of

yoghurts and high fat content of yoghurts are more inhibitory of probiotic

bacteria Husson et al. (2000) and Vinderola et al. (2000)

Table (6) and figure (4) shows significant variations of number of

TBC between goat’s and cow’s milk yoghurt. The results were higher

than that observed by Hussain et al. (2009) who found that the average

total viable count of natural yoghurt was log 4.6x108 cfu g1-. The

resultswere revealed that the TBC of goat’s milk yoghurt were higher

than that of cow’s milk yoghurt. the results were in agreement with that

of Bille et al. (2000) who reported that goat milk was also higher in

microbial count than cow’s milk, this was attributed to the difference in

milking technique whereby the goat’s were milked by hand and the cow’s

milk were milked hygienically by machine. It’s also confirmed Muehlherr

et al. (2001) who reported that the microbiological counts for bulk tank

milk from goat’s milk was high. The results were agreement with that of

Nahar et al. (2007) who reported that highest average total viable count

was recorded for dahi sample of buffalo milk, cow milk and goat’s milk,

respectively.

77

Table (7) and figure (5) shows significant variations of color

between goat’s and cow’s milk yoghurt. The color of goat’s milk yoghurt

score was high than cow’s milk yoghurt, that might be due to the white

color of goat’s milk. Similary it’s supported Vargas et al. (2008) and

Alichandidis and Polychroniadous (1996) they reported that the absence

of β-carotene in goat milk together with its elevated proportion of small

fat globules as compared to cow’s milk. The results were in accordance

with that of Salvador and Fiszman (2004) they reported that the score of

all sensory parameters significantly decreased after the addition of goat

milk, except whiteness and creaminess which increased significantly

when more goat’s milk was added.

Table (7) and figure (5) shows significant variations of flavor

between goat’s and cow’s milk yoghurt. The flavor of goat’s milk

yoghurt was lower than that of cow’s milk yoghurt, and this might be due

to the “goaty” flavor or high acidity. The results were in agreement with

that of Vargas et al. (2008) who reported that goat’s milk yoghurt was as

less consistent and more acid, with non-typical taste and flavor. It’s

supported Karademir et al. (2002) who found that goat’s milk contains a

higher level of short chain fatty acids than cow’s milk, which explain the

characteristics flavors of caprine dairy products. It’s also supported Bille

et al. (2000) who reported that citrate plays role in flavor of milk and

milk products, in which goat milk was poor in sodium and citrate.

Similary it’s confirmed Pazáková et al. (1999) who reported that goat

milk yoghurt had a markedly (a goat) flavor which negatively influenced

the sensory quality. The results werevdisagreement with that of Mowelm

(1988) who reported that recently milked and cooled goat milk is odor

free and hard to distinguish from cow milk in odor and taste.

78


between goat’s and cow’s milk yoghurt. The texture of goat’s milk

yoghurt was lower than that of cow’s milk yoghurt, and this might be due

to the different types of milk. The results were in agreement with that of

Stelio and Emmanuel (2004) they reported that the texture differences

between goat’s and cow’s milk yoghurt are attributed to the kind of milk

used and their compositional differences. It’s also confirmed Domagała

(2008) who found that yoghurts from goat milk have a loose and weak

consistency, high syneresis and higher acidity than yoghurts from cow

and sheep milk. The results were disagreement with that of Bille et al.

(2000) who reported that goat milk yoghurt texture was much superior to

cow milk yoghurt that because dry matter content of goat milk was

reflected in higher viscosity and superior texture of its yoghurt samples.

79

CHAPTER SIX

CONCLUSION AND RECOMMENDATION

Conclusion:

The result of this study concluded that quality of set yoghurt was

affected during storage. The cow's milk yoghurt sensory characteristics

were different from goat's milk yoghurt. This study suggested utilization

of goat's milk yoghurt acceptance by consumers.

Recommendation:

. More attention should be given to production, collection, transport and

cooling of goat's milk.

. Standardization of goat's milk for yoghurt manufacture should be

observed to meet specification.

. Yoghurt must be kept under cold condition.

. Further studies and research are needed to improve the quality and

flavor of goat's milk yoghurt.

80

Reference

Abrahamsen, R. K. and Rysstad, G. (1991). Fermentation of goat’s milk with

yoghurt starter bacteria: A review, Cultured Dairy Products Journal., 26:

20-26.

Abu-Jdayil, B. and Mohameed, H. (2002). Experimental and modeling studies of

the flow properties of concentrated yogurt as affected by the storage time.

J. Food Eng., 52 (4): 359-365.

Akalin, S. (1993). Investigations on the production of fermented milk products

like yoghurt and determination of some properties. Ph.D. thesis. Faculty

of Agriculture, Ege University, Izmir, Turkey.Cited by Kavas, G; Uysal,

H; Kilic, S; Akbulut, N. and Kesenkas, H. (2003). Some properties of

yoghurt produced from goat milk and cow-goat milk mixtures by

different fortification methods. Pakistan Journal of Biological Sciences.,

6 (23): 1936-1939.

Alfa-Laval, (1996). Dairy Handbook. Food Engineering AB, P.O. Box 65, S-221

00 Lund, Sweden. Cited by Michael, L. (2003). The effects of

composition and processing of milk on foam characteristics as measured

by steam frothing. (M Sc.) Faculty of the Louisiana State University and

Agricultural and Mechanical College. USA.

Alférez, M. J. M; Barrionuevo, M; López Aliaga, I; Sanz Sampelayo, M. R;

Lisbona, F; Robles, J. C. and Campos, M. S. (2001). Digestive utilization

of goat and cow milk fat in malabsorption syndrome. J. Dairy Res., 68

(3): 451-461.

Alichandidis, E. and Polychroniadous, A. (1996). Special feature of dairy

products from ewe and goat milk from the physicochemical and

organoleptic point of view, production and utilization of ewe and goat

81

milk. Proceedings on the IDF/ Greek National Committee of

IDF/CIRVAL Seminar, International Dairy Federation, Brussels,

Belgium. pp 21-43.

Ambrosoli, R; Stasio, L. and Mazzocco, P. (1988). Content of α S1- casein and

coagulation properties in goat milk. J. Dairy Sci., 71(1): 24-28.

Anjum, R. R; Zahoor, T. and Akhtar, S. (2007). Comparative study of yoghurt

prepared by using local isolated and commercial imported starter culture.

J. Res Sci. B. Z. Univ, 18 (1): 35-41.

Anonym, (2001). Turkish Food Codex Fermented Milk Bulletin. 24512, Ankara,

T. C. Official Gazette.

Antunac, N; Havranek, J. L. and Samarzija, D. (2001). Effect of breed on

chemical composition of goat milk. Czech Journal of Animal Science., 46

(6): 268-274.

Association of Official Analytical Chemists (AOAC), (1990). Official Methods

of Analysis, 15th ed. Association of Official Analytical Chemists.

Arlington, Virginia. USA.

Astrup, H. N; Steine, T. A. and Robstad, A. M. (1985). Taste, free fatty acids and

fatty acid content in goat milk. Acta Agric. Scand., 35: 315-320

Athar, I. H (1986). Preparation of cheese and yoghurt (Dahi) at household level. Brochure. Pak. Agri. Res. Council, Islamabad, Pakistan, pp: 24-35.

Baltcock, M and Azam-Ali, S. (1998). Opportunities for fermented food

products in developing countries. Intermediate Technology Food Chain.,

23: 3-4.

Barrionuevo, M; Alférez, M. J. M; López Aliaga, I; Sanz Sampelayo, M. R. and

Campos, M. S. (2002). Beneficial effect of goat milk on nutritive

82

utilization of iron and copper in malabsorption syndrome. J. Dairy Sci.,

85: 657-664.

Becker, T. and Puhan, Z. (1989). Effect of different process to increase the milk

solids non-fat content on the rheological properties of yoghurt.

Milchwissenchaft., 44: 626-629.

Beena, A. K. (2000). Health benefits of fermented milks in: ICAR. Winter school

on recent developments in fermentation of milk; Nov 6-29; Kerela, India.

Thrissur: Kerela Agriculture University. pp. 88-95.

Beshkova, D. M; Simova, E. D; Frengova, G. I; Simov, Z. L. (1998a). Production

of flavour compounds by yoghurt starter cultures. J. Industrial

Microbiology. Biotechnology., 20: 180–186.

Beshkova, D. M; Simova, E. D; Frengova, G. I; Simov, Z. L. and Adilov, E. F.

(1998b). Production of amino acids by yoghurt bacteria. Biotechnology

Progress., 14: 963-965.

Bikowski, Z. (1997). Modern methods in the production technology of yoghurts.

In: Materials of VI Scientific Session: Advance in Technology,

Engineering and Management of Dairy Industry, Olsztyn, Poland. pp. 29–

51.

Bille, P. G; Vovor, M. N; Goreseb, J. and Keya, E. L. (2000). Evaluating the

feasibility of adding value to goat milk by producing yoghurt using low

cost technology method for rural Namibia. J. Food Technol Afr., 5 (4):

139-144.

Birollo, G. A; Reinheimer, J. A. and Vinderola, C. G. ( 2000). Viability of lactic

acid microflora in different types of yoghurt. Food Research

International., 33: 799-805.

Blance, B. (1986). The nutritional value of yogurt. Int. J. Immunotherapy., (1):

25-47.

83

Boulanger, A; Grosclaude, F. and Mahe, M. F. (1984), "Polymorphism of caprine

alpha-S1 casein and alpha-S2 casein", Genetique Selection Evolution.,

(16):157-175.

Boylan, W. J; Makhambera, T. B. E; Kamwanja, L. A; Swartz, H. A. and Patten,

S. E. (1996). Breedings goats in the tropics to enhance child nutrition and

health. Proceeding of the VI International Conference on Goats, Bijing,

China, (1): 51-53.

Bozanic, R; Tratnik, L. and Maric, O. (1998). The influence of goat milk on the viscosity and microbiological quality of yoghurt during storage, Mljekarstvo., (48): 63-74.

Bradley, R. L; Arnold, Jr. E; Barbano, Jr. D. M; Semerad, R. G; Smith, D. E. and

Vines, B. K. (1992). Chemical and physical methods. pp. 433–531 In:

Standard Methods for the Examination of Dairy Products. 16th ed. R. T.

Marshall, ed. Am. Public Health Assoc.,Washington, DC.

Bramely, A. J. and McKinnon, C.H. (1990). The microbiology of raw milk. In:

Dairy Microbiology. Applied Scince. Publishers limited, England. (1):

163-208.

Brendehaug, J. and Abrahamsen. R. K. (1986). Chemical composition of milk

from herd of Norwegian goats. J. Dairy Res., 53: 211-221.

Campbell, J. R. and Marshall. R. T. (1975).The science of providing milk for

man. Mc Graw – Hill Book Co. Publ; New York, NY .

Chandan, R; Attaie, R. and Sahani, K. M. (1992). Nutritional aspects of goat

milk and its products. In: Proceeding of the Fifth International

Conference on Goats, Vol. II. Indian Council of Agricultural Research

Publishers, New Delhi, India. pp. 399- 420.

84

Chilliard, Y. (1982a): Physiological variations in lipase activities and

spontaneous lypolysis in bovine, caprine and human milk: A literature

review. Lait, 62: 126-154. Cited by Jaubert. J. Biochemical characteristics

and quality of goat milk. CIHEAM-Options Mediterraneennes., 71-74.

Chilliard, Y. (1982b): Bibliographic review of the physiological variations of

lipase activities and spontaneous lipolysis in cows' milk, goats' milk and

human milk. Lait, 62: 1-31. Cited by Jaubert. J. Biochemical

characteristics and quality of goat milk. CIHEAM-Options

Mediterraneennes., 71-74.

Chilliard, Y; Rouel, J; Ferlay, A; Bernard, L; Gaborit, P; Raynal-Ljutovac, K;

Lauret, A. and Leroux, C. ( 2006). Optimizing goat's milk and cheese

fatty acid composition. pp. 123–145 In: Improving the Fat Content of

Foods. Williams, C. and Buttriss, J. Wood-head Publishing Ltd.,

Cambridge, UK.

Courtin, P. and Rul, F. (2004). Interaction between microorganisms in simple

ecosystem: Yoghurt bacteria as study model. Lait., 84: 125-134.

Dave, R. I. and Shah, N. P. (1997). Viability of yogurt and probiotic bacteria in

yogurt made from commercial starter cultures. Int. Dairy. J., 17: 31-41.

Delahunty, C. M. (2002). Analysis: Sensory Evaluation. Elsevier Science Ltd.

Cork, Ireland.

Devendra, C. and Mcleroy, G. B. (1982). Goat and sheep production in tropics.

Longman Scientific and Technical. Longman Group, U.K.

Devendera, C. and Burns, M. (1983). Feeding and nutrition. In: Goat production

in the tropics. Common wealth Agriculture. Bureaux: 90-115.

85

Devendra, C. (1992). Goat and rural prosperity. Proceedings of the Fifth

International Conference on Goats. Indian Council of Agricultural

Research Publishers, New Delhi, India. pp 6-25.

Devuyst, L; Zamfir, M. F; Adriany,T; Marshall, V; Degeest, B and Vaningelgem,

F. (2003). Exopolysaccharide-producing Streptococcus thermophilus

strains as functional starter cultures in the production of fermented milks.

Int. Dairy J., 13: 707-717.

Domagała, J. (2008). Sensory evaluation and rheological properties of yoghurts

prepared from goat, cow and sheep milk. Electronic Journal of Polish

Agricultural Universities., 11(3).

Domagała, J. and Juszczak, L. (2004). Flow behaviour of goat’s milk yoghurts

and bioyoghurts. Electronic Journal of Polish Agricultural Universities.,

7 (2).

Ekinci, F. Y. and Gurel, M. (2008). Effect of using propionic acid bacteria as an

adjunct culture in yoghurt production. J, Dairy Sci., 91(3): 892-899.

El-abbassy, M. Z. and Sitohy, M. (1993). Metabolic interaction between

Streptococcus thermophilus and Lactobacillus bulagricus in single and

mixed starter yogurt. Nahrung., 37 (1): 53-58.

El-Bakari, J. M. and El Zubeir, I. E. M. (2009). Chemical and microbiological

evaluation of plain and fruit yoghurt in Khartoum State. Int. J. Dairy Sci.,

4: 1-7.

El-Shibiny, S; El-Dien, H. F. and Hofi, A. A. (1979). Effect of storage on the

proteins of zabadi. Egyptian. J. Dairy Sci., 7(1):1-7

86

Eman, L; Moustafa, M; El-Demerdash, E. and Hashem, M. E. (2009a). Survey

study on goat milk from different breed rose under Egyptian conditions.

Egyptian Journal of Sheep and Goat Sciences., 4 (1): 89 – 98

Eman, M; El-Diasty, R; El-Kaseh, M. and Salem, R. M. (2009b). The effect of

natamycin on keeping quality and organoleptic characters of yoghurt.

Arab J. Biotech., 12 (1): 41-48.

FAO (1990). Production yearbook, Vol. 44. FAO Statistics Series, No. 99. Food

and Agriculture Organization of the United Nations, Rome, Italy.

FAO (2001). Production Yearbook 1999. Food and Agriculture Organization of

the United Nations, Vol. 53. Statistical Series No. 156, Rome, Italy, pp.

251.

FDA (1996). Yoghurt. 21CFR 131-200, Code of Federal Regulations. US. Dept.

of Health and Human Services, Washington, DC.

FDA (2008). Milk and Cream. 21 CFR 131.110, Code of Federal Regulations.

US. Dept. of Health and Human Services, Washington, DC. Cited by

Vargas. J. L. (2009). Determination of fluid milk quality. (M Sc.). Faculty

of the Louisiana State University.

Fernandez-Garcia, E and Mc Gregor, J. U. (1994). Determination of organic

acids during the fermentation and cold storage of yogurt. J. Dairy Sci.,

77: 2934-2939.

Feverir, C; Mourot, J; Jaguelin, Y; Mounier, A. and Lebreton, Y. (1993).

Utilisations digestives compares des laits UHT de chevre et de vache,

Effets nutritionnels de la gelification. Utilisation du modele porcin. Lait,

73: 581-592. Cited by Haenlein, G. F.W. (2001). Past, present, and future

perspectives of small ruminant research. J. Dairy Sci., 84: 2097–2115.

87

Fitzpatric, J. J; Weidendorfer, K; Weber, M. and Teunou, E. (2001). Practical

considerations for reconstituting dairy powder to high solids content in

stirred-tank. Milchwissenschaft., 56: 512-516.

Fitzpatric, J. J. and Cuthbert, R. (2004). Effect of temperature on reconstitution

of milk powders to high solids content in a stirred-tank.

Milchwissenschaft., 59: 55-58.

Foda, M. I; A bd El-A ziz, M. and Awad, A. A. (2007). Chemical, rheological

and sensory evaluation of yoghurt supplemented with turmeric. Int. Dairy

Sci., 2 (3): 252-259.

Foley, J; Buckley, J. and Marphy, M. F. (1974). Commercial testing and product

control in the dairy industry. University College Cork, Cork, Ireland.

Fox, P. F. and Mc Sweeney, P. L. H. (1998). Dairy Chemistry and Biochemistry.

1st ed. Lactose, Table 2.1. pp. 21.

Frances, R. (2001). Goat's milk a suitable hypoallergenic alternative. British

Food Journal., 103 (3): 198-208.

Frank, J. F; Christen, G. L. and Bullerman, L. B. (1992). Tests for groups of

microorganisms. In Standard Methods for the examination of Dairy

Products. Marshal, R. T. (1992). 16th ed. American Public Health

Association. Washington. DC. pp. 271-283.

French, M. H. (1970). Observations on goat. Agricultural Studies N0. 80.

Chapter 6 pp. 93-129. FAO Rome, Italy.

Gall, C. F. (1991). Breed differences In: Adaptation of Goats. Elsevier Science

Publ. Amsterdam. The Netherlands, World Animal Science Series. pp.

413-429.

88

Giesecke, W. H; Dupreez, J. H. and Petzer, I. M. (1994). Practical mastitis

control in dairy herds. Butterworths Durban. 12(22): 157-217

Goff, H. D (2008). Dairy Science and Technology: Education series. Cited By

Vargas, J. L. (2009). Determination of fluid milk quality. (MS). Louisiana

State University and Agricultural and Mechanical College.

Goff, H. D. and Griffiths, M. W. (2006). Major advances in fresh milk and milk

products: Fluid milk products and frozen desserts. J. Dairy Sci., 89:1163-

1173.

Gomes, V; Libera, A. M; Madureira, K. M. and Araujo, W. P. (2004). Influence

of lactation stage on goat (Capra hircus) milk composition. Brazilian

Journal of Veterinary Research and Animal Science., 4: 339-342.

Goodenought, E. R. and Kleyn, D. H. (1976). Qualitative and quantative changes

in carbohydrates during the manufacture of yoghurt. J. Dairy Sci., 59 (1):

45-47.

Guler, A and Mutlu, B. (2005). The effects of different incubation temperatures

on the acetaldehyde content and viable bacteria counts of bio-yogurt

made from ewe’s milk. Int. J. Dairy Tech., 58: 174- 179.

Gupta, R. B. (2004). Effect of cyclodextrins on the flavour of goat milk and its

yoghurt. Thesis. Auckland University of Technology. Auckland USA.

Haenlein, G. F. W. (1981). Dairy goat industry of the United State. J. Dairy Sci.,

64: 1288-1304.

Haenlein, G. F.W. (1992). Role of goat meat and milk in human nutrition. In:

Proceedings of the Fifth International Conference on Goats, Vol. II, part

II. Indian Council of Agricultural Research Publishers, New Delhi, India.

pp. 575–580.

89

Haenlein, G. F. W. (1993). Nutritional value of dairy products of ewe's and goat's

milk, production and utilization of ewe and goat milk, IDF proceeding.

pp: 159-178.

Haenlein, G. F.W. (2001). Past, present, and future perspectives of small

ruminant research. J. Dairy Sci., 84: 2097–2115.

Haenlein, G. F.W. (2004). Goat milk in human nutrition. Small Ruminant

Research., 51:155–163.

Hargrove, R. E. and Alford, F. A. (1972). Composition of milk products. In:

Fundamentals of dairy chemistry, 2nd ed. Edited by Webb, B. H., Jonson,

A. H. and Alford, J. A. The AVI Publishing Company INC. West Port

Connecticut, USA. pp. 58-86.

Harrigan, W. F. and Mc Cance, M. E. (1976). Labrobary methods. In: Food and

dairy microbiology. Academic Press Inc. London.

Hassan, N. I. and ElDerani, O. H. (1990). Goat resource in Arab World, 2.

Republic of Sudan. ACSAC. Health and Prod., 4 (2):126.

Hassan, S. S. (2005). Quality assurance of various dairy products. Thesis

Department of Chemistry, University of Peshawar, Peshawar, Pakistan.

Cited by Imran, M; Khan, H; Hassan, S. S. and Khan, R. (2008).

Physicochemical characteristics of various milk samples available in

Pakistan. J Zhejiang Univ Sci B., 9 (7): 546–551.

Hassan, A. A. and Frank, J. F. (2001). Starter culture and their use. In: Marth .E.

H and Steel. J. L. Applied Dairy Microbiology, Marcel Dekker, New

York, 151-206.

Hatziminaoglou, Y. and Boyazoglu, J. (2004). The goat in ancient civilizations:

from the fertile crescent to the Aegean Sea. Small Ruminant Research.,

45: 163-178.

90

Herrero, A. M. and Requena, T. (2006). The effect of supplementing goat’s milk

with whey protein concentrate on textural properties of set-type yoghurt.

J. Food Sci. Technol., 41: 87–92.

Heyman, M. and Desjux, J. F. (1992). Significance of intestinal food protein

transport. J. Pediatric Gastroenterology and Nutrition., 15: 48-57.

Hidiroglou, M and Proulx, J. C. (1982). Factors affecting the calcium, Mg and

P contents of cow milk. Canadian. J. Comparative Med., 46 (2): 212-214.

Houghtby, G. A; Maturin, L. J. and Koenig, E. K. (1992). Microbiological count

methods. In: Standard Methods for the examination of Dairy Products.

Marshal, R. T. (1992). 16th ed. Of the AmericanPublic Health Association

C; Inc; Washington. DC. pp. 213-244.

Hussain, I; Rahman, A. and Atkinson, N. (2009). Quality comparison of

probiotic and natural yogurt. Pakistan Journal of Nutrition., 8 (1): 9-12.

Husson, C. H; Mengaurd, J; Gripon, J. C; Benbadi, L. and Chartier, P. C. (2000).

Characterization of Streptococcus thermophilus strains that undergo lysis

under unfavourable environmental conditions. J. of Food Microbiology.,

55: 209-213.

Hutchings, J. B. (1991). Food colour and appearance. 2nd ed. Aspen Publishers,

Inc; Gaithersturg, MD, USA.

Hutkins. R. W. (2001). Metabolism of starter culture. In Marth. E. H and Steel. J.

L. Applied Dairy Microbiology, Marcel Dekker, New York, 207-241.

IDF (International Dairy Federation) (1988). Fermented Milks. Science and

Technology. Bulletin of the IDF No. 227, Brussels.

IDF (1992a). General standards of identity for fermented milks. Brussels.

91

International Dairy Federation, standard No. 163

IDF (1992b). General standards of identity for milk products obtained from

fermented milks heat treated after fermentation. Brussels. International

Dairy Federation, Standard No.164.

IDF (1994). Recommendation for the hygienic manufacture of the milk and milk

based product. Bulletin of International Dairy Federation. No 292,

Brussels, Belgium.

Irkin, R. and Eren, U.V. (2008). A research about viable Lactobacillus

bulgaricus and Streptococcus thermophilus numbers in the market

yoghurts. World Journal of Dairy and Food Sciences., 3 (1): 25-28.

Jandal, J. M. (1996). Comparative aspects of goat and sheep milk. Small

Ruminant Research., 22:177-185.

Jaros, D and Rohm, H. (2003). Controlling the texture of fermented milk dairy

products: The case of yoghurt. Dairy processing improving quality.

Edited by Gerrit Smit. Woodhead publishing limited. Cambridge England

pp. 155-184.

Jaubert, G. and Kalantzopoulos, G. (1996). Quality of goat milk for cheese and

other products. Proceedings of the VI International Conference on Goats,

Beijing, China, (1): 274-284.

Jenness, R. (1980). Composition and characteristics of goat milk: Review 1968–

1979. J. Dairy Sci., 63: 1605–1630.

Karademir, E; Atamer, M; Tamucay, B. and Yaman, S. (2002). Some properties

of goat milk yoghurts produced by different fortification methods.

Milchwissenschaft., 57 (1): 261-263.

92

Kavas, G; Uysal, H; Kilic, S; Akbulut, N. and Kesenkas, H. (2003). Some

properties of yoghurt produced from goat milk and cow-goat milk

mixtures by different fortification methods. Pakistan Journal of

Biological Sciences., 6 (23): 1936-1939.

Klinger, I. and Rosenthal, I. (1997). Public health and safety of milk and milk

products from sheep and goats. Revised Scientific Technology., 16 (2):

482-488.

Kneifel, W; Ulberth, F; Erhard, F. and Jaros, D. (1992). Aroma profiles and

sensory properties of yoghurt and yoghurt-related products. Screeing of

commercially available starter culture. Milchwissenschaft., 47: 362-365.

Koestanti, S. E. and Romziah, S. (2008). Improving yoghurt quality of goat milk

by implemented complete feed omega 6 on goat feeding program. The

15th Congress of FAVA, 27-30 October. Bangkok, Thailand.

Koroleva, N. S. (1991). Products prepared with lactic acid bacteria and yeasts.

Therapeutic properties of fermented milks (ed. R. K. Robinson) .pp. 159-

180. Applied Science Publishers, London.

Kudełka, W. (1996). Wpływ wybranych czynników na podstawowe wyróżniki

jakosci mleka koziego [The influence of selected factors on basic quality

indicators of goat’s milk]. PhD. Diss., AE, Cracow [in Polish]. Cited by

Domagała, J. (2008). Sensory evaluation and rheological properties of

yoghurts prepared from goat, cow and sheep milk. Electronic Journal of

Polish Agricultural Universities., 11(3).

Kumar, A. and Singh, H. (2007). Recent advances in microencapsulation of

probiotics for industrial applications and targeted delivery. Trends in

Food Science and Technology., 18: 240-251.

93

Kurmann, J. A; Rasic, J. L. and Kroger, M. (1992). Encyclopedia of fermented

fresh milk products. New York: Van Nostrand Reinhold.

Kurtz, A. (1981). Sut. Teknolojisi. Ataturk Universitesi Yaymlari, No: 573.

Ataturk Universitesi Basim Evi. Cited by Hussain, I; Rahman, A. and

Atkinson, N. (2009). Quality comparison of probiotic and natural yogurt.

Pakistan Journal of Nutrition., 8 (1): 9-12.

Lankes, H; Ozer, B. and Robinson, R. K. (1998). Effect of method of

fortification on the texture of natural yoghurt. Milchwissenschaft., 53:

510-513.

Law, A. J. R. (1996). Effect of heat treatment and acidification on the

dissociation of bovine casein micelles. J. Dairy Res., 63: 35-48.

Lawless, H. T, and Heyman, H. (1999). Sensory evaluation of food: Principles

and practices, Aspen Publishers Inc; Gaither Sburg, Maryland.

Laws, A. P. and Marshall, V. M. (2001). The relevance of exopolysaccharides to

the rheological properties of milk fermented with roby strains of lactic

acid bacteria. Int. Dairy J., 9: 709-721.

Lee, W. and Lucey, J. A. (2003). Rheological properties, whey separation and

microstructure in set-style yoghurt: Effects of heating temperature and

gelation temperature. Journal of Texture Studies., 34: 515-536.

Le Jaouen, J. C. (1981). Milking and the technology of the milk products. In:

goat production. pp: 345-376. ed. Gall C. Acamdemic Press London.

Lingathurai, S. P; Vellathurai, S; Vendan, E. and Anand, A. A. P. (2009).

A comparative study on the microbiological and chemical composition of

cow milk from different locations in Madurai, Tamil Nadu. Indian

Journal of Science and Technology., 2: 0974- 6846.

94

Lowenstein, M. and Speck, S. J. (1983). External service. In: Extention Goat

Hand Book 4. USDA, Washington D.C, pp 1-14.

Lucey, J. A. and Singh, H. (1997). Formation and physical properties of acid

milk gels: A review. Food Research International., 30: 529-542.

Lucey, J. A. and Singh, H. (2003). Acid coagulation of milk. Advanced Dairy

Chemistry – Proteins (ed P. F. Fox and P. H. L. MC Sweeney), 2: 1001-

1026. Aspen Publishers, Gaithersburg.

Marona, D. and Pedrigon, G. (2004). Yoghurt feeding inhibits promotion and

progression of cancer. Med. Sci. Monit., 10: 96-104.

Marshall, V. M. E. and Tamime, A. Y. (1997). Physiology and biochemistry of

fermented milks. Microbiology and biochemistry of cheese and

fermented milk (ed. B. A; Law), 2nd ed, pp. 153-192, Blackie Academic

and Professional, London.

Martins, R. C; Borges, S.C; Lopes, V. V; Pereira, R. N. and Vicente, A. A.

(2007). Monitoring goat milk quality during pasteurisation and ohmic

treatment using UV-VIS-SWNIR spectroscopy. Written for presentation

at the 2007 CIGR Section VI International Symposium on Food and

Agricultural Products: Processing and Innovations. Naples, Italy. 24-26.

Matalon, M. E. and Sandine, W. E. (1986). Lactobacillus bulgaricus,

Streptococcus thermophilus and yoghurt: A review. Cultured Dairy

Products Journal., 21: 6-12.

Matthewman, R. W. (1993). Dairying. The tropical agriculturist, published by

the Macmillan Press Ltd.1st ed. London, UK.

McKinley, M. C. (2005). The nutrition and health benefits of yoghurt. Int.

J. Dairy Technol., 58 (1): 1-12.

95

Meilgaard, M; Civille, G. V. and Carr. B. T. (1999). Sensory evaluation

techniques. 3rd ed. CRC. Press LLC, Boca Raton.

Michael, L. (2003). The effects of composition and processing of milk on foam

characteristics as measured by steam frothing. (M. Sc.) Faculty of the

Louisiana State University and Agricultural and Mechanical College.

USA.

Ministry of Animal Resources and Fisheries, (2002). Republic of Sudan. Statistic

and Information Department.

Morón, D; Martín Alonso, J. J; Gil Extremera, F; Sanz Sampelayo, M. R. and

Boza, J. (2000). Composition of goat milk and cow milk produced in the

Iberian southeast. Comparative study. In: Proceeding. 7th International

Conference on Goats, Vol. II, Tours (France), 15-21 May 2000, and p.

617.

Mottar, J; Bassier, A; Joniau, M. and Baert, J. (1989). Effect of heat induced

association of whey proteins and casein micelles on yoghurt texture. J.

Dairy Sci., 72: 2247-2256.

Mowelm, A. (1988). Goat milking. In: Goat Farming. Farming Press Book.

United Kingdom: 109-127.

Muehlherr, J; Zweife, C; Corti, S; Blanco, J. E. and Stephan, R. (2003).

Microbiological quality of raw goat’s and ewe’s bulk-tank milk in

Switzerland. J. Dairy Sci., 86 (12): 3849-3856.

Mumtaz, S. S; Rehman, U; Huma, N; Jamil, A. and Nawaz, H. (2008).

Xylooligosaccharide enriched yoghurt: Physicochemical and sensory

evaluation. Pakistan Journal of Nutrition., 7 (4): 655-659.

96

Nahar, A; Al-Amin, M; Alam, S. M. K; Wadud, A. and Islam, M. N. (2007).

Comparative study on the dahi (yoghurt) prepared from cow, goat, and

buffalo milk. Int. J. Dairy Sci., 2 (3): 260-267.

Noni, I. D; Pellegrino, L. and Masotti, F. (2004). Survey of selected chemical

and microbiological characteristics of (plain or sweetened) natural

yoghurts from the Italian Market. Lait., 84: 421-433.

Nu Nu san, and De Boer, A. J. (1996). Changing role of goats in a dynamic

world of agriculture. Proceedings of the VI international conference on

goats, Beijing, China, 1: 229-239.

Oberman, H. (1985). Fermented milks. in: Microbiology of Fermented Foods.

(editor B. J. B.Woods). Elsevier Applied Science Publisher. London U K,

1: 167-195.

Oliveira, C. A. F; Fernandes, A. M; Neto, O. C. C; Fonesca, L. F. L; Silva, E. O.

T. and Balina, S. C. (2002). Composition and sensory evaluation of whole

yoghurt produced from milk with different somatic cell counts. Australian

Journal of Dairy Technol., 57: 192-196.

Ohiokpehai, O. (2003). Processed food products and nutrient composition of goat

milk. Pakistan Journal of Nutrition., 2 (2): 68-71.

Ozer, B. A. (2006). Yoghurt Bilimive Teknoljisi Sidas Medya, Sanal, Urfa,

Turkey. Cited by Ekinci, F. Y. and Gurel, M. (2008). Effect of using

propionic acid bacteria as an adjunct culture in yoghurt production. J,

Dairy Sci., 91(3): 892-899.

Pazáková, J; Burdová, O; Turek, P. and Laciaková, A. (1999). Sensorial

evaluation of yoghurt produced from cow, sheep and goat milk. Czech J.

Food Sci., 17 (1): 31-34.

97

Quartermain, A. R. (1991). Evaluation and utilization of goat breeds. In: K.

Maijala, (ed.), Genetic Resources of pig, sheep and goat. Elsevier Science

Publ., Amsterdam. The Netherlands, World Animal Science, B8, 451-

470.

Radi, M; Niakusari, M. and Amiri, S. (2009). Physiological, texture and sensory

properties of low-fat yoghurt produced by using modified wheat starch as

a fat replacer. Journal of Applied Science., 9: 2194-2197.

Rajagopal, S. N. and Sandine, W. E. (1990). Associative growth and proteolysis

of Streptococcus thermophilus and Lactobacillus bulgaricus in skim

milk. J. Dairy Sci., 73 (1): 894-899.

Rasic, J. L. and Kurman, J. A. (1978). Yoghurt: scientific Grounds, Technology,

manufacture and preparation. Technical Dairy Publishing House.

Copenhagen-Denmark.

Rehman, Z. U. and Salaria, A. M. (2005). Effect of storage conditions on the

nutritional quality of UHT processed buffalo milk. J. Chem. Soc. Pak., 27

(1): 73–76.

Reps, A; Jankowska, A. and Wiśniewska, K. (2008). The effect of high

pressures on the yoghurt from milk with the stabilizer. Journal of Physics:

Conference Series., (121): Part, 14

Robinson, J. L. and Dombrowski, D. B. (1983). Effect of orotic acid ingestion on

urinary and blood parameters in humans. Nutr. Res., 3: 407-415.

Robinson, R. K. (1981). Cooling of the yoghurt. In: Modern Dairy Technology

Vol. (2). Elsevier Applied Science Publishers. London and New Jersey

98

Robinson, R. K and Tamime, A. Y. (1993). Manufacture of yoghurt and other

fermented milks. Modern dairy Technology (ed. R. K. Robinson), 2nd ed.

2: 1-48 Elsevier Applied Science. London.

Robinson, R. K. (1999). Factor affecting the consistency of stirred, fruit

yoghurts. Dairy Industries International., 64 (7): 23-25.

Robinson, R. K. (2000). Yoghurt. Encyclopyaedia of Food Microbiology (ed R.

K. Robinson, C. A. Batt and P.D.Patel). Vol. 3. pp. 784-791, Academic

Press, London.

Robinson, R. K; Tamime, A. Y. and Wszolek, M. (2002). Microbiology of

fermented milk. Handbook of Dairy microbiology (ed. R. K. Robinson)

3rd ed. pp. 367-430.

Robinson, R. K; Lucey, J. A. and Tamime A. Y. (2006). Manufacture of yoghurt.

Fermented milks. Edited by A.Y.Tamime. 1st Published 2006. Blackwell

Publishing. pp. 53-75.

Robinson, F. (2001). Goats milk – a suitable hypoallergenic alternative. British

Food Journal., 103 (3): 198-208.

Rosado, J. L. (1997). Lactose digestion and maldigestion: implications for

dietary habits in developing countries. Nutrition Research Reviews., 10:

137-149.

Rutherfurd, S. M; Darragh, A. J; Hendriks, W. H; Prosser, C. G. and Lowry, D.

(2006). Mineral retention in three-week-old piglets fed goat and cow milk

infant formulas. J. Dairy Sci., 89: 4520-4526.

99

Rysstad, G. and Abrahamsen, R. K. (1987). Formation of volatile aroma

compounds carbon dioxide in yoghurt starter grown in cow’s milk and

goat’s milk. J. Dairy Res., 54: 257-266.

Saint-Eve, A; Koran, E. P. and Martin, N. (2004). Impact of the olfactory quality

and chemical complexity of flavouring agent on the texture of low fat

stirred yogurts assessed by three different sensory methodologies. Food

Qual. Pref., 15: 655–668.

Salvador, A. and Fiszman, S. M. (2004). Textural and sensory characteristics of

whole and skimmed flavored set-type yoghurt during long storage. J.

Dairy Sci., 87: 4033-4041.

Schkoda, P. (1999). Serambindung und Rheologie fermentierter Milchprodukte –

Modellierung von struktur paramterern. VDI-Verlag, Dusseldorf.

Serra, M; Trujillo, A. J; Guamis, B. and Ferragut, V. (2009). Proteolysis of

yogurts made from ultra-high-pressure homogenized milk during cold

storage. J. Dairy Sci., 92: 71-78.

Shanley, M. K. (1973). Analysis of free sugars in yoghurt. Aust. J. Dairy

Technol., 28 (2): 58-60.

Silanikove, N. (2000). The physiological basis of adaptation in goats to harsh

environments. Small Ruminant Research., 35: 181-193.

Singh, J. and Sharma, D. K. (1982). Yoghurt starters in skim milks. 1. acid and

flavour production and protelytic activity by yoghurt starters. Cultured

Dairy Products Journal., 17: 22 – 25.

Skjevdal, T. (1979). Flavour of goat's milk: A review of studies on the sources

of its variations. Livestock Production Science., 6 (4): 397-405.

100

Sofu, A. and Ekinci, F. Y. (2007). Estimation of storage time of yogurt with

artificial neural network modeling. J. Dairy Sci., 90: 3118-3125.

Soomro, A. H; Rain, A. A. and Kashkeli, M. (2003). Industrial yoghurt and

indigenous dahi. Online J. Biol. Sci., 3(1): 86-90.

Soukoulis, C; Panagiotidis, P; Koureli, R. and Tzia, C. (2007). Industrial yogurt

manufacture: monitoring of fermentation process and improvement of

final product quality. J. Dairy Sci., 90: 2641-2654.

Staff, M. C. (1998). Cultured milk and fresh cheese. In: The technology of dairy

product.2nded. Early, R. Blackie Academic and Professional. London.

123-157

Stelio, K. and Emmanuel, A. (2004). Characteristics of set type yoghurt made

from caprine or ovine milk and mixtures of the two. International Journal

of Food Science and Technology., 39: 319–324.

Sun, W and Griffiths, M. W. (2000). Survival of bifidobacteria in yogurt and

simulated gastric juice following immobilization in gellan-xanthan beads.

Int. J. Food Microbiol., 61: 17-25.

Tamime, A.Y. and Deeth, H. C. (1980). Yoghurt: Technology and biochemistry.

J. Food. Protect., 43 (12): 939-977.

Tamime, A.Y; Kalab, M. and Davies, G. (1984). Microstructure set-style yoghurt

manufactured from cow’s milk fortified by various methods. Food

Microstructure., 3: 83-92.

Tamime, A.Y and Robinson, R. K. (1985). Yoghurt: Science and Technology.

Pergamon Press Ltd. Oxford, UK.

Tamime, A. Y and Robinson, R. K. (1999). Yoghurt: Science and Technology.2nd

ed. Woodhead, Cambridge, UK.

101

Tamime, A. Y and Robinson, R. K. (2000). Yoghurt science and Technology.

CRC Press, Washington, DC. pp. 1-128

Tamime, A.Y. (2002). Microbiology of starter cultures. Dairy microbiology

Handbook (ed. R. K. Robinson) 3rd ed. pp. 261-266. Wiley science Inc.

New York.

Tamime, A. Y; Saarela, M; Sondergaard, K; Mistry V. V. and Shah, N. P.

(2005). Production and mainteinance of viability of probiotic

microorganisms . In: A. Y. Tamime (Ed) Probiotic Dairy Products.

Blackwell publishers. Oxford. UK pp: 39-97.

Tamime, A.Y. (2007). Structure of dairy products. Blackwell Publishing Ltd.

State Avenue, Ames, USA. PP. 134-169.

Tarakci, Z. and Erdogan, K. (2003). Physical, chemical, microbiological and

sensory characteristics of some fruit-flavored yogurt. Y. X. U. Vet. Derg.,

14: 10-14.

Taylor, S. L. (1986). Immunologic and allergic properties of cow's milk proteins

in humans. J. Food Protection., 49: 239-250.

Toba, T; Watanabe, A . and Adachi, S. (1983). Quantitative changes in sugars,

especially oligosaccharides, during fermentation and storage of yogurt . J.

Dairy Sci., 66: 17-20.

Tziboula-Clarke, A. (2003). Goat milk. In: Roginnski, H; Fuquay, J. W. and

Fox, P. F. Editors, [Encyclopedia of dairy sciences Vol. 2, Academic

Press, London, UK (2003). pp. 1270-1279].

Van Dender, A. C. F; Moreno, I. and Garcia, S. (1990). Avaliação do uso de

culturas filantes e/ou diluição para fabricar iogurte de leite de cabra.

Colet. ITAL, Campinas, 20(1): 83.

102

Vargan, L. H. M; Reddy, K. V and DA Silva, R. S. F. (1989). Shelf-life studies

on soy-whey yogurt- a combined sensory, chemical and microbiological

approach. Lebensm. Wiss.Technol., 22: 133-137.

Vargas, M; Chafer, M; Albors, A; Chiralt, A. and Gonzalez-Martinez, C.

(2008). Physicochemical and sensory characteristics of yoghurt produced

from mixtures of cow's and goat's milk. Int. Dairy. J., 18 (12): 1146-1152.

Vedamuthu, E. R. (1991). The yoghurt story – past, present and future.1. Dairy

Food Environ. San., 11: 202-203.

Vinderola, C. G; Bilo, J. A. and Reinhemir, J. A. (2000). Survival of probiotic

microflora in Argentinian yoghurts during refrigerated storage. Food

Resarch International., 33: 97-102.

Webb, B. H; Johnson, A. H. and Alford, J. A. (1974). Fundamental of dairy

chemistry. 2nd ed. Westport, C T: AVI Publishing Co.

Wong, N. P. (1999). Fundamentals of dairy chemistry (3rd ed), Aspen

Publishers, Gaithersburg, MD. pp. 511–582.

Zander, L. (1998). Reologia w nauce o żywności. [Rheology in food science].

Materiały XXIX Sesji Naukowej KTiChŻ PAN, Olsztyn, 25-27 [in

Polish]. Cited by Domagała, J. and Juszczak, L. (2004). Flow behaviour

of goat’s milk yoghurts and bioyoghurts. Electronic Journal of Polish

Agricultural Universities., 7 (2).

Zoon, P. (2003). Viscosity smoothness and stability of yogurt as affected by

structure and EPS functionality, Fermented milk. IDF special issue 0301.

pp. 280-289. International Dairy Federation. Brussels.

103

Zourari, A; Accolas, J. P. and Desmazeaud, M. J. (1992). Metabolism and

biochemical characteristics of yogurt bacteria: A review. Le Lait., 72:

134.

104

Figure (1): Variation of sensory evaluation test during storage of set

yoghurt produced from cow’s milk.

5.8

6

6.2

6.4

6.6

6.8

7

7.2

7.4

7.6

7.8

0 3 6 9 12

Type

Days

Color

Flavor

Texture

105

Figure (2): Variation of sensory evaluation test during storage of set

yoghurt produced from goat’s milk.

0

1

2

3

4

5

6

7

8

9

0 3 6 9 12

Type

Days

Color

Flavor

Texture

106

Figure (3): variation of chemical composition test between types of

set yoghurt produced from cow’s milk and goat’s milk.

0

2

4

6

8

10

12

14

16

Total solid %

Fat % Protein %

Lactose %

Ash % Acidity %

Types

Parameter

Cow

Goat

107

Figure (4): variation of microbial test between types of set yoghurt

produced from cow’s milk and goat’s milk.

9

9.5

10

10.5

11

11.5

12

Log Streptococcus Log Lactobacillus Log TBC

Types

Parameter

Cow

Goat

108

Figure (5): Variation of sensory evaluation test between types of

set yoghurt produced from cow’s milk and goat’s

milk.

5.6

5.8

6

6.2

6.4

6.6

6.8

7

7.2

7.4

7.6

Colour Flavour Texture

Types

Parameter

Cow

Goat

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EFFECT OF STORAGE ON THE MICROBIAL CHEMICAL · PDF fileTo my mother, who gave me ... Abdallah for his help in statistical analysis. ... milk dairy products. In this sence, goat's milk