بسم اهللا الرحمن الرحيم

Effect of incubation and storage temperatures

on quality of set yoghurt

By

Abdallah Mohammed Ibrahim Mustafa

B.Sc. (Agric.) Honours

٢٠٠١

A thesis submitted to the University of Khartoum In partial

fulfillments of the requirement for the degree of M.Sc. (Agric.)

Thesis Supervisor:

Dr: Hassan Ali Mudawi

U of K

Faculty of agric

April ٢٠٠٤

List of contents

Contents Page Dedication. I Acknowledgment. II Abstract . III Abstract(Arabic). IIII Chapter one . ١ Introduction. ١ Chapter two. ٣ Literature review ٣ ٢٫١. Milk. ٣ ٢٫١٫١. The lipids of milk. ٤ ٢٫١٫٢. Carbohydrates. ٤ ٢٫١٫٣. Proteins. ٤ ٢٫١٫٤. Minerals. ٤ ٢٫٢. Milk products. ٤ ٢٫٢٫١. Fermentation. ٥ ٢٫٢٫٢. Fermentation reactions. ٦ ٢٫٣. Standardization of yoghurt. ٧ ٢٫٤. Yoghurt. ٧ ٢٫٤٫١. Composition of yoghurt. ٨ ٢٫٤٫٢. Flavouring of yoghurt. ٩ ٤٫٣. ٢. Types of yoghurt. ١٠ ٢٫٤٫٤. Starter culture. ١٠ ٢٫٤٫٥. Factors affecting quality of yoghurt. ١٣ ٢٫٤٫٥٫١. Type of milk. ١٤ ٢٫٤٫٥٫٢. Storage period. ١٤ ٢٫٤٫٥٫٣. Choice of starter culture. ١٦ ٢٫٤٫٥٫٣. Heat treatment. ١٦ ٢٫٤٫٥٫٥. Additives. ١٦ ٢٫٤٫٦. Manufacturing Method. ١٧ ٢٫٤٫٧. Manufacture of set yoghurt. ١٨

٢٫٤٫٧٫١. Clearing of milk. ١٨ ٢٫٤٫٧٫٢. Pasteurization.

٢٫٤٫٧٫٣. Homogenization.

١٨ ١٩

٢٫٤٫٧٫٤. Inoculation. ١٩ ٢٫٤٫٧٫٥. Incubation. ٢٠ ٢٫٤٫٧٫٦. Cooling. ٢٠ ٢٫٤٫٧٫٧. Storage. ٢١ ٢٫٤٫٨ .The HACCP-concept for dairy production. ٢١ ٢٫٤٫٨٫١ .Hazard. ٢٢ ٢٫٤٫٨٫٢. Critical Points. ٢٢ ٢٫٤٫٨٫٣. Critical Control Point. ٢٢ ٢٫٤٫٩. Planning HACCP. ٢٥ ٢٫٤٫١٠. Static hygiene conditions:Building. ٢٦ ٢٫٤٫١١. Building, hygienic handling of food. ٢٧ ٢٫٤٫١٢. Production, improve the technical

processing. ٢٨

Chapter Three. ٢٩ Materials and Methods. ٢٩ ٣٫١. Materials ٢٩ ٣٫٢. Methods. ٢٩ ٣٫٢٫١ Preparation of yoghurt mix. ٢٩ ٣٫٢٫٢. The manufacture of modified yoghurt sample. ٢٩ ٣٫٢٫٣. The Chemical analysis. ٣٠ ٣٫٢٫٣٫١. Density. ٣٠ ٣٫٢٫٣٫٢. Total solids. ٣١ ٣٫٢٫٣٫٣. Solid non fat (SNF). ٣١ ٣٫٢٫٣٫٤. Fat content.

٣٢

٣٫٢٫٣٫٥. Titratable acidity. ٣٢

٣٫٢٫٤. Chemical analysis of yoghurt. ٣٢

٣٫٢٫٤٫١. Fat content. ٣٢ ٣٫٢٫٤٫٢. Total solids.

٣٫٢٫٤٫٣. S.N.F (solid non fat). ٣٣

٣٫٢٫٤٫٤. Titratable acidity. ٣٣ ٤٫٢٫٤٫٥. Protein content. ٣٣ ٣٫٢٫٤٫٦. Lactose content. ٣٥ ٣٫٢٫٤٫٧. Ash content. ٣٥ ٣٫٢٫٤٫٨. PH. ٣٦ ٣٫٢٫٤٫٩. Viscosity . ٣٦ ٣٫٢٫٤٫١٠. Wheying-off. ٣٦ ٣٫٢٫٤٫١١. Sensory evaluation of yoghurt. ٣٦

٣٫٢٫٥. Experimental design and data analysis. ٣٧ Chapter four. ٣٨ Results and Discussion. ٣٨

٤٫١. mix formulations. ٣٨

٤٫٢. Incubation temperature. ٤٠

٤٫٣. Storage conditions. ٤١ ٤٫٤. Development of acidity for yoghurt mix.

Incubated at temperatures of ٤١ºC, ٤٣ºC and ٤٥ºC in

٤ hours stored at ٦c° for one day. ٤٢

٤٫٥. Effect of the wheying–off on quality of yoghurt. ٤٣

٤٫٦ Effect of storage temperatures on the Viscosity. ٤٤ ٤٫٧. Effect of storage temperature on the acidity. ٤٥ ٤٫٨. The interaction between the storage period temperature and incubation temperature on the yoghurt sensory quality .

٤٦

٤٫٩. Effect of the storage period on the quality attributes of yoghurt. ٤٧

٤٫١٠.Effect of the storage temperature on the quality attributes of yoghurt. ٤٩

٤٫١١. The interaction between the storage period and storage temperature on the whey separated. ٥٠

٤٫١٢. The interaction between the storage period and storage temperature on the yoghurt viscosity. ٥١

Chapter five. ٦٢ Conclusions and Recommendations. ٦٢ ٥٫١. Conclusions. ٦٢ ٥٫٢. Recommendations. ٦٣ References. ٦٤ Appendix. ٧١ Sensory evaluation of set yoghurt. ٧١

List of Tables

Table

Page

١. proximates of cow milk and yoghurt mix used for preparation of yoghurt samples

٣٨

٢.Effect of incubation temperatures on the quality attributes of yoghurt.

٤٠

٣. Effect of the storage period on the quality attributes of yoghurt.

٤١

٤. The interaction between the storage period temperature and Incubation temperature on the yoghurt level of acidity (% lactic).

٤٢

٥. The interaction between the storage period temperature and incubation temperature on the yoghurt whey separated.

٤٣

٦. The interaction between the storage period temperature and Incubation temperature on the yoghurt viscosity.

٤٤

٧. The interaction between the storage period and storage temperature on the yoghurt amount of acidity.

٤٥

٨. The interaction between the storage period temperature and incubation temperature on the yoghurt sensory quality .

٤٦

٩.Effect of the storage period on the quality attributes of yoghurt. ١٠. Effect of the storage temperature on the quality attributes of yoghurt.

٤٧ ٤٩

١١. The interaction between the storage period and storage temperature on the whey separated.

٥٠

١٢.The interaction between the storage period and storage temperature on the yoghurt viscosity.

٥١

List of Figures

Figure Page

١. Development of acidity for yoghurt mix, incubated at temperatures of ٤١°C, ٤٣ °C and ٤٥°C in ٤ hours for zero day.

٥٢

٢. Development of acidity for yoghurt mix, incubated at temperatures of ٤١°C, ٤٣°C and ٤٥°C in ٤ hours stored at temperature of ٦°C for one day.

٥٣

٣. Development of acidity for yoghurt mix, incubated at temperatures of ٤٣ ,٤١°C and ٤٥°C in ٤ hours stored a temperature of ١٠°C for one day.

٥٤

٤. Development of acidity for yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperature of ٦cº for days zero day,١٠, ٤،٦ ,١ days.

٥٥

٥.Development of acidity for yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperature of ١٠cº. days for zero day, ١٠, ٦ ,٤ ,١ days. ٦. Comparison between the developments of acidity for yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperatures of ٦cº, ١٠cº. days for zero day, ١٠, ٦ ,٤ ,١ days.

٥٦ ٥٧

٧. Development of viscosity for yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperature of ٦cº.days for zero day ,١٠, ٤،٦ ,١ days.

٥٨

٨. Development of viscosity for yoghurt mix, incubated at temperature ٤٥cºin ٤ hours in stored at temperature of ٦cº.days for zero day ,١٠, ٤،٦ ,١ days.

٥٩

٩.Comparison between The development of viscosity for yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperature of ٦cºand ١٠cº. days for zero day ,١٠ ,٦ ,٤ ,١ days.

٦٠

١٠.Comparison between the developments of acidity for yoghurt mix, incubated at temperature ٤٥C in ٤hours stored at temperature of ٦Cº, ١٠cº. days for zero day, ١٠ ,٦ ,٤ ,١ days.

٦١

.

I am grateful to my supervisor Dr: Hassan Ali Mudawi

for his comments ,advices and guiding without which his work

would not have been easily concluded.

full thanks to Engineer: Iffat Salah: Department of food

industries in Industrial Research & Consultancy Center (I

RCC).For his kind help and encouragement .

Thanks are also extended to the staff of Khartoum dairy

production co.ltd (K D P.C) for their assistance and support

throughout the practical phase, for them warm wishes

especially Fisal Abbas production manager.

Finally, thanks are due to everyone who made this work

possible and available.

Abstract

This study aimed to investigate the effect of incubation and

storage temperatures on quality of set yogurt made from cow milk

during storage life for ١٠ days. The yoghurt mix was prepared by

adding ١٠٠ Kg skim milk powder to ٣٤١ liters of distilled water and

then mixed for ٢٠ minutes and adding ٦ liters fresh cow milk. The

yoghurt mix was pasteurized at ٩٠Cº for ٢٠ minutes and then cooled

to ٤٥Cº. A ٢٪ of mixed starter culture type (CH-١, B-٣) was added.

Then was incubated at different incubation temperatures (٤١Cº, ٤٣Cº,

٤٥Cº) for ٤ hours. Physio-chemical charterctices such as acidity,

viscosity, wheying-off and sensory quality were carried out at zero

day and after one day in order to select optimum incubation

temperature Then optimum sample stored at two temperatures

(temperature of ٦c◦ and ١٠c◦) to study the effect on quality of yoghurt.

The results, showed that the incubation temperature of ٤٥°C gave

the best quality yoghurt compared with incubation temperatures

of ٤١Cºand ٤٥Cº as measured with the parameters of acidity,

wheying-off, viscosity and sensory quality.

Storage temperature of ٦°C gave the better quality yoghurt compared

with storage temperature of ١٠°Cº which resulted in large amount of

whey separated, slow viscosity and low acidity.

األطروحةملخص هدف البحث إلى دراسة تأثير درجات التحضين والتخزين المختلفة على

. أيام١٠جودة الزبادي العادي المصنع من الحليب البقرى في مدة تخزين

كجم من الحليب منزوع الدهن إلي ١٠٠تم تحضير الخلطة بإضافة

تمت . بقرى لتر من الحليب ال٦ دقيقة مع ٢٠ لتر ماء مقطر وخلطه لمدة ٣٤١

دقيقة و من ثم التبريد ٢٠ درجة مئوية لمدة ٩٠البسترة على درجة حرارة

( ٣-B ,١-CH) درجة مئوية إلضافة البادئ من نوع ٤٥لدرجة حرارة

٤٥ ، ٤٣ ، ٤١: ( تم التحضين على درجات تحضين مختلفة ، % ٢بنسبة

وم التالي أجريت في يوم التصنيع والي، ساعات٤درجة مئوية لمدة )

) الجودة الحسية،الشر ش، اللزوجة، الحموضة( اختبارات كيميائية وفيزيائية

الشر ش ، الحموضة: ( تحديد العينة األفضل حسب جودة الزبادي فيل

).ة الحسيالجودة، اللزوجة، المنفصل

( بعد اختيار العينة األفضل خزنت على درجات حرارة º ٦ ، º ١٠ (

على جودة الزبادي ومن نتائج البحث توصلنا إلى ها يرتأث لدراسةدرجة مئوية

:اآلتي

٤٣ ، ٤١( درجة مئوية مقارنة ب ٤٥أفضل درجة حرارة للتحضين هي -

.ة الحسيالجودةمن حيث الحموضة و الشر ش واللزوجة و )

١٠ مئوية مقارنة مع درجة الحرارة ة درج٦أفضل درجة تخزين هي

المنفصل الحموضة والشر شه كبيرة منالتي أعطت كمي مئوية ةدرج

. المتدنيةواللزوجة

CHAPTER ONE

INTRDUCTION

Yoghurt is a popular fermented milk product consumed world

wide. Modern yoghurt manufacture is mechanical, but it can also

be traditional as in many developing countries. Many people eat

yoghurt because they like it or due to its positive effect on

health. Yoghurt is produced in different forms such as whole milk

yoghurt, skim yoghurt, cream yoghurt, fruit yoghurt and liquid

yoghurt.

Milk for yoghurt production must have a low bacteria count,

should not contain enzymes and chemical substances, which may

slow down the development of the yoghurt culture and should not

contain penicillin and bacteria phage.

Yoghurt can be spoiled after any contamination and

growth of yeast and moulds used. Beside type of milk, heat

treatment, starter, incubation temperature and storage conditions can

affect the quality of set yoghurt. In the Sudan, the manufacturing of

yoghurt is inheritly dependent on traditional methods but later some

factories started to manufacte yoghurt using modern techniques that

lead to improvement in yoghurt production methodology and

consequently increased Production quantitatively and qualitatively. In

the Sudan, there are many different types of fermented dairy products

such as robe, mish and zabadi.

There are some defects occuring during the manufacture of set

yoghurt from cow milk such as high acidity associated with harsh

flavour and aroma, bitterness, bad taste, whey off and the defective

texture and bodyness (consistency). The aim of this study are to

try standardize optimum incubation time and temperature for set

yoghurt mix prepared as well as to investigate optimum storage

temperature of such product.

CHAPTAER TWO LITERATURE REVIEW

٢٫١. Milk:

Milk is important to human as it is needed by young and

old. Chemist define milk as an emulsion of fat in a watery solution of

sugar and mineral salt, with protein in colloidal suspension (Eckles, et

al., ١٩٥١).If asked to analyze an average sample of milk and

determine the chief constituents, the chemist would report the

following constituents:

Source: Eckles (١٩٥١)

Milk can be consumed fresh in the form of homogenized whole

milk, flavoured milk and fat –free milk (Webb et al., ١٩٨٠).

٢٫١٫١. The lipids of milk

MATERIAL PERCENTAGE

Water

Total solids

Fat

Protein

Lactose

Ash

٨٧٫٢٥

١٢٫٧٥

٣٫٨

٣٫٣

٤٫٨

٠٫٦٥

The lipids of milk are composed of about ٩٨٪ triglycerides, the

rest is composed of mono–and diglycerides, phospholipids, sterols,

hydrocarbons and free fatty acids.

٢٫١٫٢. Carbohydrates:

The carbohydrate present is lactose; anther carbohydrates

include glycerol groups to proteins and lipids.

٢٫١٫٣. Proteins:

The proteins of milk are of great importance in nutrition and

influence the behaviour and characteristics, two types of protein are

found casein and whey protein.

٢٫١٫٤. Minerals:

Mineral in milk are: Na, K, Ca, and Mg, and anions which

include phosphate, citrate, chloride, carbonate and sulphate.

٢٫٢. Milk products:

Milk products are manufactured by various methods. Fluid milk

include all of plain products ,with fat content varying between whole

and skim milk as flavoured and fermented milk (Bassette and

Acosta, ١٩٨٨).

٢٫٢٫١. Fermentation:

Fermentation was defined by Gale (١٩٤٨), as the process

leading to anaerobic breakdown of carbohydrates. Other major

compounds rather than carbohydrates such as organic acids, proteins

and fats are fermentable. In broader view, fermentation is an energy-

yielding process (Kosikiowski, ١٩٨٢). Milk fermentation generally

causes the break down of lactose to acid by Streptococci and

Lactobacilli bacteria so as to develop their characteristic flavor and

texture. Milk can be fermented by yeasts and moulds to produce

variety of products such as yoghurt, cheese, sour cream and butter

milk.

Microorganisms affect both the physico-chemical properties

and economic value of milk (Kilara and Shahandi, ١٩٨٧). The

physico-chemical changes influence milk properties such as flavour,

texture and nutritive value. The economic value of milk represented in

the increased shelf –life of product affect the feasibility of investment

in milk products (Kilara and Shahandi, ١٩٨٧).

Fermented milk products vary considerably in composition,

flavour and texture according to the nature of fermenting organisms,

the type of milk and the manufacturing process used

(Berlin,١٩٦٢; Chandan et al., ١٩٦٩).

Some factors must be carefully controlled during the

manufacturing process in order to produce a high quality yoghurt with

the required taste, aroma, viscosity and long life. These factors are:

choice of milk as type of milk is very important factor influencing the

quality of finished product, milk standardization, milk additives which

improve the viscosity and texture, homogenization, heat treatment and

culture preparation.

٢٫٢٫٢. Fermentation reactions:

Kosikowski (١٩٨٢) described the following major sugar and

citric acid reactions in milk:

١- Lactic acid fermentation: the lactic acid fermentation is the

most important one in milk.

٢- Propionic acid fermentation: the propionic acid fermentation

leads to the typical cheese flavour.

٣- Citric acid fermentation: it is responsible for the aromatic

flavour of butter milk, sour cream, cream cheese and some

cottage cheese.

٤- Alcoholic fermentation: as in kefir and koumiss which

contain ٣-١٪ ethyl alcohol.

٥- Butyric acid fermentation: it generates large amount of

carbon dioxide and hydrogen gas in the product.

Some authors have considered yoghurt to be a rich source

of vitamins while others showed that the content of many vitamins

decrease during yoghurt production (Deeth and Tamime,١٩٨١).

٢٫٣. Standardization of yoghurt:

Based on fat content there are two types of yoghurt, whole milk

yoghurt and skimmed milk yoghurt.

١.Whole milk yoghurt should contain ٣٪ fat.

٢.Partly skimmed milk yoghurt should contain less than ٣٪ and

more than ٠٫٥٪ fat.

٣.skimmed milk yoghurt should contain less than ٠٫٥٪fat

All types of yoghurt must posses at least ٨٫٥٪ solid non fat

(S.N.F) (Kosikowiski, ١٩٨٢). There are many about the importance

of fat in relation to both consistency and flavour of yoghurt. Galesloot

and Hassing (١٩٦٦), Ashton (١٩٦٣), Ritter and Siegenthaler (١٩٦٤),

Ashton (١٩٦٣), Ritter (١٩٦٤) and Davis (١٩٦٥) had remarked that

higher normal S.N.F in the mix necessitates increased acidity

development in order to initiate coagulation due to buffering action of

milk constituents.

٢٫٤. Yoghurt:

Yoghurt is popular fermented milk product consumed in many

parts of the world .It is produced in different forms such as whole

milk yoghurt, skimmed yoghurt, cream yoghurt, fruit yoghurt and

liquid yoghurt (Balasubramanyam and Kulkarn, ١٩٩١). Yoghurt has

become a highly popular food product over the last ١٥ to ٢٠ years .It

is known under many names in many countries e.g.

Mast : Iran

Laban: Lebanon, Saudi Arabia

Dahl : India

Zabadi: Sudan

Kosikowski (١٩٨٢).

Many people eat yoghurt because they like it or feel that it

will favorably affect their health. The digestive systems of some

people lack the enzyme lactase. As a result ,lactose will not be broken

down in the digestive system into simpler types of sugar ,and

therefore fresh milk will be harmful to these people. On the other

hand, they can eat cultured milk ,in which lactose is partly broken by

the bacterial enzymes. (Alpha Laval, ١٩٨٥). Yoghurt is highly

nutritious and easily digestible due to the predigested nutrients by

bacterial starters.

٢٫٤٫١. Composition of yoghurt:

In terms of overall composition ,yoghurt is similar to

milk. However there are many aspects in which the composition of

milk and yoghurt differs. These differences are seen in the increase of

soluble solids or changes happened by bacterial fermentation,

Vieseyre (١٩٦٤). Shalaby et al, (١٩٩١) evaluated the nutritive value of

yoghurt comparing it with the composition of milk and traditionally

prepared yoghurt. The following table shows comparison between

milk, modified yoghurt and traditional yoghurt.

Contents Milk Modified yoghurt Traditional yoghurt

Source: Shalaby (١٩٩١).

٢٫٤٫٢. Flavouring of yoghurt:

Yoghurt fortified with various flavouring and aroma additives

is very popular. Common additives are fruits and berries in syrup.

Fruits should be heat treated before use. The amount of fruits usually

about ١٥٪ .Yoghurt is sometimes flavoured by vanilla ,coffee, etc

sucrose or glucose are sometimes added together with flavouring

materials .Stabilizers and sweeteners may be used as additives in

yoghurt production. Vitamin C is sometimes also added.

Protein

Fat

Lactose

T.S

Ca++/m mol/l

Energy value

KJ/gm

٤٫٤٥

٥٫٥

٤٫٦٨

١٥٫٤٢

٢١٫٠٨

٣٠٫٦

٤٫٤٨

٥٫٦

٤٫٢٨

١٦٫١١

٤٩٫٨٧

٣٥٫٧

٤٫٦٤٪

٥٫٥٤

٤٫١

١٥٫٨٢

٥٣٫٥٢ m mol/l

٣٣٫٥ kJ/gm

٤٫٣. ٢. Types of yoghurt:

Deeth and Tamime (١٩٨١) found various types of yoghurt

according to their chemical composition, method of production,

flavour and nature of post-incubation processing. There are two main

types of yoghurt: set and stirred yoghurt (Deeth and Tamime, ١٩٨١).

In drink type yoghurt which is based on the stirred type, the

coagulum is broken down to liquid before filling .Frozen yoghurt

is inoculated and fermented in the same manner as a stirred yoghurt.

However cooling is achieved by pumping through a freezer in a

fashion similar to ice cream. In flavoured yoghurt usually fruits or

berries are added.

٢٫٤٫٤. Starter culture:

Dairy starter are cultures of harmless, active bacteria, grown

in milk or whey, which impart certain characteristics and qualities to

various milk products. The culture may compose one strain of

microorganism species (kosikoowski, ١٩٨٢). Yoghurt starter culture

is composed of Lactobacillus bulgaricus and Streptococcus

thermophillus it should have a proper balance between these

organisms, this is considered important, especially in the

manufacture of high quality yoghurt (Lee et al , ١٩٧٤).

Broudi and Collin (١٩٧٦) worked on Lebanese laban

(yoghurt) and found that five microorganisms are (or to be)

responsible for the fermentation: Str. Thermophillus, L. acidophilus,

Leuconostoc lactis, Kluyveromyces, fragiils and Saccharomyces

cerviseae.

They found that Str and lacto were responsible for the acid

production. Coccus grows faster than the rod type and is primarily

responsible for acid production, while the rod adds flavour and

aroma .As a result of combined growth, lactic acid grow faster than

that produced by either when growing alone and more

acetaldehyde also produced (Jay, ١٩٨٦). International standards are in

general agreement in that coli form count should not exceed ١٠ cfu/ml

in yoghurt (Salji et al; ١٩٨٧). The following table shows

characteristics of some of starter culture bacteria.

Table (ii)

Characteristics of some of starter culture bacteria.

Character Str. thermophillus Lact.bulgoricus

Gram stain + +

Shape Coccus Rod

Growth at١٠º

٤٠º

٤٥º

-

+

+

-

+

+

Fermentation of

Glucose

Galactose

Lactates

+

-

+

+

-

+

citrate metabolism - -

Ammonia from arginine - -

%G+Cª ٥١-٪٤٩ ٪٤٠٪

Source: Abdullah (١٩٩٢).

G+Cª = Guanine cytosine in DNA.

Symbols:

+ = ٩٠٪ or more strains are positive.

- = ٩٠٪ or more strains are negative.

٢٫٤٫٥. Factors affecting quality of yoghurt:

Many factors affect the quality of yoghurt such as milk

type ,heat treatment, starter culture and storage conditions.

٢٫٤٫٥٫١. Type of milk:

The total solid level in milk for yoghurt manufacture can vary

from as low as ٩٪ in skim milk yoghurt to over ٢٠٪ in other types of

yoghurt (Deeth and Tamime, ١٩٨١).

Kozhev, et al (١٩٧٢). Showed that the best yoghurt can be made from

milk containing ١٦-١٥٫٥٪ total solids. Total solids affect starter

activity in yoghurt manufacture especially in the titratable acidity and

the reduction in the coagulation time(Humphreys, and Maurean,

١٩٦٩).

The viscosity of yoghurt is almost dependent on the protein

content of milk, however, Deeth and Tamime (١٩٨١) concluded that

the addition of milk powder raised the protein level of milk.

Yoghurt made from high heat treatment powder has lower

level of the heat. labile vitamins than that made with low–heat

treated powder ( Deeth and Tamime, ١٩٨١).

Kosikowski (١٩٨٢) indicated that milk bases for yoghurt

display varying fat content but considerable amount is still made from

whole milk of about ٣٫٣٪ fat. Dairy starter are culture of harmless,

active bacteria, grown in milk or whey which impart certain

characteristics and qualities to various milk products. The culture may

be one strain of microorganism species called a single–strength

culture or a number of strains and /or species called a multi–strain or

mix–strain culture.

٢٫٤٫٥٫٢. Storage period:

Control of the storage temperature at ٤º-٥ºC is most

important since higher temperature can lead to defects such as

bitterness and low temperature can induce ice–crystal formation.

Reddy et al (١٩٧٦) found that storage at ٥ºC caused a decrease in the

level of vitamins. Spoilage will be dependent on the intrinsic and

extrinsic factors. The storage temperature will have a strong effect on

the selection temperatures for the dominant spoilage organism in a

particular food. Table (iii) showed different types of milk under

different storage temperatures. (Storgards, ١٩٦٤).

Raw milk stored at ٤°C

Pseudomonas sp. Especially Pseudomonas fluorescens. At low numbers: Off-odours and taste "metallic". At high numbers, off odours and taste "putrid" and sufficient quantity of heat stabile enzymes can be produced to interfere with cheese manufacture

Raw milk stored at room temperature

Many bacteria will grow using the lactose as an energy source. Streptococcus sp., Lactobacillus sp. and gut organisms. Souring is produced followed by coagulation of the milk protein (clotting)

Pasteurized milk

Many Gram negative (e.g. gut bacteria) are killed by pasteurization. Recontamination can occur in dairy or home. Spoilage often is due to gram positive Streptococcus sp. and Bacillus sp.

٢٫٤٫٥٫٣. Choice of starter culture:

The starter culture is the term generally applied to the

organisms used to ferment cultured product, (chesse, yoghurt, kefir)

the organisms selected for this purpose need to produce the desired

effect in the product, (although you could use chesses starter in

yoghurt fermentation, the result would not be yoghurt). Starter of

yoghurt use only in manufacture of yoghurt. For normal commercial

yoghurt the starter must be capable of fermenting lactose and

producing lactic acid, little if any carbon dioxide is required and the

flavour and aroma must be fresh.

٢٫٤٫٥٫٤. Heat treatment:

Milk is heat treated in order to:

a). Improve the preparation of the milk as substrate for bacteria

b). Increase coagulum in finished product

c). Reduce whey separation

Alpha-Laval (١٩٩٥).

٢٫٤٫٥٫٥. Additives:

The addition of sugar often improves the “body” and “mouth

feel”. Stabilizers help to increase the viscosity of a yoghurt, they also

help to prevent the separation of whey from the yoghurt, a problem

known as synuresis. The most beneficial quantity as stabilizer to add

to a yoghurt mix has to be determined experimentally by each

manufacturer. Too much stabilizer in yoghurt can lead to the

formation of a rubbery texture and the yoghurt become a hard solid

mass. Common stabilizers are, gelatin, pectin, agar, starch used in

quantities in the order of ٠٫١٪ to ٠٫٥٪.

٢٫٤٫٦. Manufacturing Method:

The milk is clarified and separated into cream and skim

milk, then standardized to achieve the desired fat content. The various

ingredients are then blended together in a mix tank equipped with a

powder funnel and an agitation system.

The mixture is then pasteurized using a continuous plate heat

exchanger for ٣٠ min at ٨٥°C or for ١٠ min at ٩٥°C. These heat

treatments, which are much more severe than liquid milk

pasteurization, are necessary to achieve the following: produce a

relatively sterile and condusive environment for the starter culture,

denature and coagulate whey proteins to enhance the viscosity and

texture. The mix is then homogenized using high pressures of ٢٠٠٠-

٢٥٠٠ psi. Homogenization assists in mixing the stabilizers and other

ingredients and also prevents creaming and wheying off during

incubation and storage. Stability, consistency and body are enhanced

by homogenization. Once the homogenized mix has cooled to an

optimum growth temperature, the yoghurt starter culture is added.

A ratio of ١:١, Streptococcus to Lactobacillus, inoculation is

added to the jacketed fermentation tank. A temperature of ٤٣°C is

maintained for ٦-٤ hours with no agitation. This temperature is a

compromise between the optimums for the two microorganisms

(ST ٣٩°C; LB ٤٥°C). The titratable acidity is monitored until the it is

٠٫٨٥ to ٠٫٩٠٪. At this time the cooling and agitation begins, both of

which reduce the fermentation time. The coagulated product is cooled

to ٢٢-٥°C, depending on the product. Fruit and flavour may be

incorporated at this time, and then packaged. The product is now

cooled and stored at refrigeration temperatures (٥°C) to slow down the

physical, chemical and microbiological degradation.

٢٫٤٫٧. Manufacture of set yoghurt:

Cultured yoghurt manufacture is mechanical, but it can also

be traditional as in many developing countries. (Kosikowski, ١٩٨٢)

yoghurt is usually prepared as follows:

٢٫٤٫٧٫١. Clearing of milk:

When using milk powder heat treatment should be used

(Ashton, ١٩٦٣). Milk should be free from non toxic, bacteria phage

and of good chemical composition. (Peppler and Robert, ١٩٧٧).

٢٫٤٫٧٫٢. Pasteurization:

Foster (١٩٩٢) remarked that most lactic acid organisms

grew better in milk that had been autoclaved at ١١٥ºC for ١٥

minutes than that held at ٨٠ºC for ١٠ minutes. Heat treatment of

٨٠ºC for ٣٠ minutes denatured most whey protein in milk (Larson

and Rolleri, ١٩٥٥).

Green and Jezeski (١٩٥٦) noticed growth of flora in heat

treatment at ٩٠-٨٠ºC Acidity increased on heating for ٣٠ minutes

at ٧٠ºC. Davies and White (١٩٥٩) stated that pasteurization of

milk destroyed infective bacteria and produces an improved

consistency and firmness of yoghurt (Storgards, ١٩٦٤).

More rapid coagulation with an improvement in the

hydrophilic properties of protein occurred when milk was

homogenized and treated at ٨٥ for ٢٠ minutes (Grigorov, ١٩٦٦).

٢٫٤٫٧٫٣. Homogenization:

Homogenization improve consistency and stability of yoghurt

,Storgards and Aule (١٩٥٣). In ١٩٦٤, Pette and Smith noted that

homogenization is essentional for the production of a firm curd of

yoghurt. Mckanna (١٩٨٧), concluded that by homogenization before

pasteurization the pressure of ٢٠ mega Pascal (Mpa) was essential

for the production of long–life laban (yoghurt).

٢٫٤٫٧٫٤. Inoculation:

Most researchers agreed that Lactobacillus bulgaricus and

Streptococcus thermophillus are very important for yoghurt

manufacture (Ashton, ١٩٦٣). Some others used Lacto bacillus .Baker

and Hullet (١٩٨٩) used a culture consisting essentially of ١٥-

٢٥٪(w/w) L. acidophillus ٥٠-٣٠٪(w/w) L. bulgaricus and ٥٠-٣٠٪

(w/w) Strept thermophillus. Product become of good texture,

conventional taste and no wheying-off was observed on the surface

of the container. Jezeski (١٩٥٦) found that an inoculation rate of ٥-٢٪

reduced the time of coagulation from ١٠٠ to ٧٠ minutes. Kosikowski

(١٩٨٢), advised to use a١:١ L. Bulgaricus and Strep. thermophillus

and inoculation at the rate of ٥-٢٪ at ٤٥ºC (liquid culture).

Types of inoculums:

Streptococcus thermophillus was less viable than some lactic

organisms but withstood several months of storage in freeze–dried

form (Mattick and Hiscox, ١٩٣٩). A culture of L .bulgaricus and

str. Thermophillus was alive at ٥٠ºC to ٢٠ºc for ٣ minutes ,as

Kawashima, et al (١٩٦٣) was noted. Nikolov (١٩٦٤) presented freeze

– dried ampules of yoghurt culture for up to ٤ years when stored at ٣-

٦ºC. Humphrey and Maurean, (١٩٦٩) stated that successful growth of

cultures is largely dependant on the absence of both bacteria phage

and anti bacterial agents.

٢٫٤٫٧٫٥. Incubation:

Warm mixture transferred into containers, and then held at a

temperature of ٤٥ºC in the incubator for ٦-٣ hours until a titratable

acidity of ١٫٢-٠٫٩ (pH ٤٫٤) could be attained (kosikowiski, ١٩٨٢).

٢٫٤٫٧٫٦. Cooling:

Finally, yoghurt should be cooled in refrigerated rooms. The

rate of cooling should allow the desired acidity (٠٫٨٪) to be reached

but not surpassed when the temperature has been reduced to ٤ºc

.(Humphrey and Marurean, ١٩٦٩). Clibis and Schmall (١٩٦١)

suggested that cooling should start at PH ٤٫٧ -٤٫٥ in order to obtain a

final pH of (٣٫٨-٣٫٧).

٢٫٤٫٧٫٧. Storage:

Bertelsen (١٩٦٤) found that control of storage temperature at

٥-٤ºc is most important .Higher temperature can lead to defects such

as bitterness. Porter (١٩٧٥) suggested that when the yoghurt was set

firm it should transferred to a cold store and stored at (٨-٥ºc)

immediately. Gaafar (١٩٩٢) found that the decrease in acetaldehyde

and acetone (aroma) and the increase in acetic acid were closely

related to the rapid decrease in acceptability after ١٠-٨ days.

٢٫٤٫٨ The HACCP-concept for dairy production:

This HACCP concept has to be developed for all food products

of every factory. The five basic ideas of HACCP-concept

(Wilm,١٩٩٨) are:

١. Make a hazard analysis

٢. Determine the critical points (CPs) which might be of

hazard in the production of the food.

٣. Determine the CPs which may be critical control points.

being of high importance to the safety of the food and

which may be controlled safely using simple checks

named "Controlling".

٤. Define a control system of the critical points, using tests

which can be carried out during production in order to

interfere in cause of wrong production ."Monitoring".

٥. Define the way of verification to confirm that the

HACCP-system works."Verification"

٢٫٤٫٨٫١ Hazard:

With hazard every risk of the health of the consumer are meant.

Excluded are all events which are not related to health.

٢٫٤٫٨٫٢ Critical Points:

It is every point in the production of Food where risks of the

health of the consumer can be present.

٢٫٤٫٨٫٣ Critical Control Point:

A Critical Control Point ("CCP") is a point in the production

line where a risk of hygiene may be put under control or eliminated.

With appropriate measures at that point the risk can be:

• Avoided.

• Eliminated.

• or reduced to an acceptable level.

Examples of critical control Points (CCPs) are:

• Income of raw materials.

• Storage and cooling of food.

• Recipes, handling and processing of food.

• Defrost, heating, warm hold phase and cooling.

• Distribution of food in restaurant, fast-food.

• pH of food.

• Correct separation between clean and unclean sectors.

• Cleaning and disinfection.

• Hygiene of the surroundings and hygiene of the stuff.

The main concern is to avoid contaminations originated from:

Biological factors:

• Bacteria, such as Salmonella, pathogen Escherichia coli,

Clostridium botulinum, The biological factors can be

avoided through cleaning and disinfection activities.

• They must be supervised by a controlling person making

periodic surface contact cultures and cultures from different

points of the production line.

Chemical factors:

• Residues originated from cleaning agents, insecticides and

other chemicals.It is very important to make sure that all

residues of cleaning and disinfection agents are eliminated

before product processing starts. This is done with rinsing

with clean drinking water. There should be a daily cleaning

and disinfection plan.

Physical factors:

• Splinters of metal, glass, bones, wood, stones and other

materials. As system to avoid the risk of physical factors

are filters, X-ray scanners and metal detectors are widely

used.

• All handling and controlling of the production of food

should be conform to the system of "Good Manufacturing

Practice (GMP).

Where do the microbes come from?

١. From the food materials themselves:

From the external surfaces of animals and plants during food

preparation.

During food preparation the microbes on the surfaces are easily

introduced to internal tissues.

٢. From the operators:

• We are continually shedding microbes via particles of skin and

hair and also our clothes. Hair nets, gowns and sometimes

gloves are all part of good hygiene practice to minimize

contamination of food with our normal flora.

٣. From the machinery:

Mixers, knives, chopping boards will become contaminated

with microbes during food preparation. Cleaning with appropriate

agents will reduce this contamination level. Sterilization removes it

completely.

٤. From the air:

The air contains ٨٠ microbes per cupic metre m٣ (depends a lot

on the activities in the room). Many microbes are specialized in

air-borne dissemination; e.g. mould spores. Thus that mouldy orange

or bread is likely to have acquired its spoilage organisms from the air.

٢٫٤٫٩. Planing HACCP:

To plan HACCP it should be proceeded as follows: the

following tables should be prepared :

• A detailed schematic description for every product should

contain :

• Definitions: Explain using terminologies and abbreviations.

• Cite the different foods which belong to the described

group.

• Characteristics: like flavor, smell, consistency, etc.

• Ingredients: All ingredients should be listed:

• Chemical and physical properties: Important analytical

values such as pH, acidity, viscosity should be established

as specification. Maximum and minimum of the deviations

should be included in this table.

• Purchasing department .

• Processing: such as pasteurization,sterilization etc should be

mentioned.

• Packing:glass ,tin can , etc

• Storage:Specification of the storage temperature,moisture

etc

• Shelf life.

• Indication for consumer for final preparation of the food

before eating .

• Special informations for specific consumer groups such as

diabetics .

• Description of processing .

• A table should be prepared with a complete information

about the production containing:

• Incoming of raw materials.

• Description of the processing.

• Temperatures,heating time, pressures.

• Temperature maximum time for storage.

The description of processing must be completed with a process

flow diagram of the whole proceeding with the marked CCPs, the

target values and tolerances and the corrective action should the

product fall out of specification.

٢٫٤٫١٠. Static hygiene conditions:Building:

The building where food is being handled according to hygiene

rules must be in a condition to avoid negative influence during

processing. Cleaning and disinfection of the rooms must be possible.

Necessary temperatures for handling,processing and storage must be

available. The building must be clean and the maintenance must have

been done. The floor must be easy to clean and to disinfect. It should

be watertight. The walls are to be covered with watertight material,

easy to clean and to disinfect. The ceiling should be covered wit

watertight material avoiding condensation water, dirt and molds.

There must be a sufficient number of hand washers with spender of

liquid soap and disinfection based on alcohol. There must be a

sufficient natural and artificial illumination of all rooms. There must

be a facility for the personal to change cloth. Windows must have fly

Screens. The rooms are not to be used for other activities. The surface

of tables and other surfaces which come in contact with food must be

easy to clean and to disinfect. To wipe there should be only one-way

towels ,not using it more than a day. Buckets and other cleaning

utensils should be of different color in order do avoid cross

contamination from one place to another. Machines and other utensils

for processing of food : All surfaces must be easy to clean and to

disinfect. Everything should be of stainless steel. Dynamic hygiene

conditions: Cleaning, disinfection and maintenance of the building ,

hygienic handling of food.

٤٫٢٫٤٫١١. Building, hygienic handling of food:

The building must be cleaned and disinfected properly.

Maintenance of the building includes painting of walls and ceilings

to avoid molds. Change broken coverings of walls and close all

unnecessary holes. Control the handling through all phases of the

production, from income control, from production to storage and

transportation.

٢٫٤٫١٢. Production, improve the technical processing:

Use good manufacturing practice principles for handling food

during processing.Make a separation of "dirty" and "clean" areas

making sure that the routes of transportation of "dirty" and "clean"

material does not cross each other (For example: keeping cooked meat

and uncooked meat together in one refrigerator or one cooling room.

this must be avoided)Avoid crossing unpacked food with already

packed food. Avoid people working in "dirty areas" to get to "clean

areas". This can be controlled by different hair coverings: Red for

dirty areas and green or white for clean areas. Wood utilities should be

avoided as they bear pathogen moulds, bacteria and virus.This

includes Tables, doors and einvironment: Health condition of the staff

which comes in contact with food . Medical inspection of all people

handling food is to be periodic repeated and documented. This

includes clinical inspection, bacteriological examination of faecis with

regard to Salmonella bacteria, daily control of the personal to avoid

purulent wounds, persons with diarrhea and other problems to come in

contact with food. These persons should be engaged in areas were

they cannot come in contact with open unpacked food. Diseases which

can spread by food include cholera, Enteritis infectiosa, paratyphoid,

dysentery, tuberculosis, dermatosis, salmonellosis and shigellosis.

(Wilm, ١٩٩٨).

CHAPTER THREE

MATERIALS &METHODS

٣٫١. Materials:

• All materials were obtained from Khartoum Dairy

Production Co. Ltd. ( K.D.P.C).

٢ .٣. Methods:

The chemical analysis was carried out using the facilities of

K D.P.C laboratory.

٣٫٢٫١. Preparation of yoghurt mix:

A١٠٠ kg of low heat skimmed milk powder (S.M.P)

reconstituted in ٣٤١ litres of distilled water blended for ٢٠ minuets,

pasteurized at ٩٠°C for ٢٠ minuets and cooled to ٤٥C°. The starter

culture was added and incubated at ٤٥C° for four hours then cooled

to less than ٢٠ºC.

٣٫٢٫٢. The manufacture of modified yoghurt sample:

All the modified yoghurt mix with total solid (T.S) of

١٤٫٣%, Fat of ٢٫٥% were heated to ٩٠ºC for ٢٠ minuets,

homogenized (٢٠٠ bar), cooled to ٤٥ºC, inoculated with ٢٪ mother

culture, (type CH١, B٣) and incubated at different

temperatures(٤١،٤٣،٤٥°C) for ٤ hours. After incubation the

modified yoghurt sample was transferred to refrigerator directly and

stored at two temperatures of ٦ and ١٠°C for ١٠ days. (Total acidity,

whey volume, sensory quality and viscosity) were carried out at zero

hour and after ٦ ,٤ ,١ and ١٠ day of storage.

٣٫٢٫٣. The Chemical analysis:

The Chemical analysis was carried out for raw cow milk,

toned milk and yoghurt mix .Tests carried out for raw milk include the

following:

٣٫٢٫٣٫١. Density:

* According to AOAC ١٩٩٠.

The density was determined by lactometer calibrarated,

over the range (١٫٠٣٥-١٫٠٢٥), (٣٥-٢٥ lactometer degree), lactometer

temperature was ٢٠ºC.

The density was determined as follows:-

• Determine temperature of milk =X.

• Determine temperature of lactometer =Y.

Then sample was poured in a graduated cylinder made up

the peak and let lactometer reading the degree = lactometer

reading.

Correction:

If temperature of milk more than ٢٠°C (temperature of

lactometer ) ٠٢ was added for each ١ºC to lactometer reading. If

temperature of milk less than ٢٠ºC (temperature of lactometer) ٠٢

were subtracted for each ١ºC to lactometer reading . The density was

calculated from the formula:

Density =١+ (C.L.R/١٠٠٠)

C.L.R: Correct Lactometer Reading

٣٫٢٫٣٫٢. Total solids:

*According to AOAC ١٩٩٠.

• Determine correct lactometer reading as described under

(٣٫٢٫٣٫١).

• Determine fat content as described under (٣٫٢٫٣٫٢).

The total solid was calculated from Richmond’s equation:

%T.S = (٠٫٢٥XC.L.R) + (١٫٢٢XF) + (٠٫٧٤)

Where:

C.L.R = Correct Lactometer Reading

F = fat content

٠٧٤ ,٠٫٠٢٥ and ١٫٢٢= constants

٣٫٢٫٣٫٣. Solid Non Fat (SNF):

*According to AOAC ١٩٩٠.

• Determine fat content as described under (٣٫٢٫٣٫٢).

• Determine total solids content as described under

(٣٫٢٫٤٫٢).

Solid non Fat was determined from the following equation:

% S.N.F= (%T.S_%F).

where:

S.N.F = Solid Non Fat

T.S =Total Solids

F = Fat.

٣٫٢٫٣٫٤. Fat content:

It was determined by Gerber method according to Bradly, et al

(١٩٩٢) as follows. In clean dry Gerber tube ١٠ml of sulphuric acid

(density ١٫١٨١٥gm\ml at ٢٠°C) was added, amyl alcohol (٢ml) was

poured, and then ١٠٫٩٤ ml of milk sample was added. Amyl alcohol

(٢ml ) was added to the tube ,followed by addition of distilled water,

the contents were thoroughly mixed till no white particles were seen

.The Gerber tube was centrifuged at ١١٠٠ revolutions per minuet

(rpm) for ٥ minutes, the fat content column was then read

immediately.

٣٫٢٫٣٫٥.Titratable acidity:

The acidity of milk was determined according to the AOAC

method (١٩٩٠). A ١٠ml of sample was placed in white porcelain

dish and ٥ drops of phenolphthalein indicator were added. Titration

was carried out using ٠٫١N NaOH until a faint pink colour appeared.

The titration figure was divided by ١٠ to get the percentage of

lactic acid.

٣٫٢٫٤. Chemical analysis of yoghurt:

٣٫٢٫٤٫١. Fat content:

The fat content was determined by Gerber method according to

Bradly, et al (١٩٩٢) as described earlier under( ٣٫٢٫٣٫٢).

٣٫٢٫٤٫٢. Total solids:

The total solids content was determined according to

modified method of AOAC (١٩٩٠). Three grams of the sample were

weighed into dry clean flat bottomed aluminium dish, and heated on

steam bath for ١٥-١٠ min. The dish was placed in an oven at ٧٠°C

over night, cooled in desiccators and weightings were repeated until

the difference between two readings was < ٠٫١mg. The total solids

content was calculated from the following equation:

T.S%= W١/ WX١٠٠

Where:

W١ = weight of sample a yoghurt after drying.

W = Original weight of yoghurt.

٣٫٢٫٤٫٣. S.N.F (solid non fat):

Was determined from the following equation

% S.N.F = T.S%_F% as described earlier under (٣٫٢٫٣٫٣).

٣٫٢٫٤٫٤. Titratable acidity:

The acidity of yoghurt was determined according to AOAC

(١٩٩٠) as described earlier under (٣٫٢٫٣٫٣).

٤٫٢٫٤٫٥. Protein content:

The protein content was determined by kjeldahl method

according to the AOAC (١٩٩٠) as follows:

*Digestion:

A ١٠ ml of yoghurt was weighted and poured in clean dry

kjeldahl flask. A ١ tablet of CUSO٤ was added, conc H٢SO٤ (٢٫٥ ml)

was also added to the flask. The flask was heated until a clear solution

was obtained after ٢ hours and left for another ٣٠ minutes. The flask

was removed and allowed to cool.

*Distillation:

The digested sample was poured in volumetric flask (١٠٠ml)

and diluted with distilled water .A ١٥ ml of ٤٠٪ NAOH was added,

the distillate was received in conical flask (١٠٠ ml) containing ١٠ml

of ٢٪ boric acid plus ٣ drops of indicator (bromocresol)

green + phenolphthalein red).

The distillation was continued until the volume in the flask was

٥٠ml then the flask was removed from the distillator.

*Titration:

The distillate was titrated with ٠٫١ N HCL until the end point

(red colour) was reached. The protein content was calculated from the

following equation:

N% = TX٠٫١ X ٠٫٠١٤X ١٠٠/W

Where:

T = Titration reading.

W = Weight of original sample.

% Protein content =N% X ٦٫٣٨.

٣٫٢٫٤٫٦. Lactose content:

It was determined by the Lane and Eynons method

according to AOAC (١٩٩٠) as follows:

Twenty five grams of sample was dissolved in distilled water,

clarified through filtering with acetate and k.oxlate, and then made up

to ٢٥٠ ml mark. A ٢٥ml of mixed fehling solution were pipetted

into ٣٠٠ ml conical flask.١٥ml of sugar solution was added and the

liquid was boiled on asbestos covered gauze.١ml of sugar solution

was added at ١٥-١٠ sec to the boiled liquid until the blue colour

was nearly discharged, ٥ drops of aqueous methylene blue solution

١٪ was added. The sugar solution was then added to boiled liquid

until the indicator was completely decolourized. The lactose content

was calculated from the sugar table.

٣٫٢٫٤٫٧. Ash content:

It was determined according to AOAC (١٩٩٠). Five grams

of sample were weighed into suitable crucible and evaporated to

dryness on steam bath ,then placed in multi furnace at ٦٠٠-٥٥٠C° for

٣ hours until ashes were carbon free. The sample was then cooled in

desiccators and weighed. The ash content was calculated by the

following equation:

Ash%= w١ / w X١٠٠

Where:

W١ = weight of Ash. W = Original weight of yoghurt.

٣٫٢٫٤٫٨. PH:

Was determined by digital PH Meter

٣٫٢٫٤٫٩. Viscosity:

Was determined by HAKK viscometer

Low speed : X sec

Fast speed :Y sec

Average =(X+Y) / ٢ =Z sec

Viscosity =Z sec x ٤٤٫٦٤٨

(٤٤٫٦٤٨ = viscosity factor)

٣٫٢٫٤٫١٠. Wheying-off:

Was determined by sucking the water on surface of the curd and

pouring in a graduated cylinder.

٣٫٢٫٤٫١١. Sensory evaluation of yoghurt:

The sensory evaluation was carried out by scoring method,

(panel test).according to Madanlal (١٩٧٧) as following:

Appearance and colour ٢

Consistency ٣

Flavour ٦

Taste ٤

Total score ١٥

A panel test was carried out by ٣ technician (trained) and two

technician (untrained).

Evaluation:

١٥-١٣ points = very good / excellent.

١٢-١٠ points =good.

٩-٧ points =fair.

less than ٧ points = bad.

٣٫٢٫٥. Experimental design and data analysis:

The design used in this study was the ٢ factor split-plot design and

samples have been taken in triplicate. Mean separation has been done

using least significant difference (L.S.D.) at ٥٫٠٠٪ probability.

(SAS.١٩٨٨).

CHAPTER FOUR

RESULTS AND DISCUSSION

Table ١

(proximates of cow milk and yoghurt mix used for preparation of

yoghurt samples).

T.S : Total solids.

S.N.F: Solid Non Fat.

T.A : Titratable acidity.

٤٫١. Mix formulation:

The results indicated that mix formulation with Total solid (T.S

) of ١٤٫٣١% and fat of ٢٫٥٪ gave the best quality (table.١). The

results disagreement with Kozhev, et al (١٩٧٢) who recommended

the best yoghurt to be made from milk containing ١٦-١٥٫٥٪ TS.

Density

gm/ml T.A (%lactic) S.N.F% Fat% T. S %

Sample

١٣٫٥ ٤٫٤ ٩٫١ ٠٫١٧ ١٫٠٢٩٠

Cow milk

١٤٫٣١ ٢٫٥ ١١٫٨١ ٠٫١٨ ١٫٠٣٤٤

Yoghurt mix

The result is in agreement with Deeth and Tamime, ١٩٨١) who

claimed that The total solid level in milk for yoghurt manufacture

can vary from as low as ٩٪ in skim milk yoghurt to over ٢٠٪ in

other types of yoghurt.

Table ٢ Effect of incubation temperatures on the quality attributes of yoghurt.

quality attributes of yoghurt.

Incubation temperature (c°)

Amount of acidity (%)

Whey separated (ml)

Viscosity (c p)

Sensory quality(total scores)

٤١ ºc ٠٫٧٢ c ٠٫٥٠ a ٣٦٤٫٦٠ c ١٤٫٣٧ c ٤٣ ºc ٠٫٧١ b ٠٫٤٠ b ٤٥٠٫٢٠a ١٤٫٥٠ b ٤٥ ºc ٠٫٧٥ a ٠٫٣١ c ٤٧٦٫٢٠ b ١٤٫٦٧ a

L.S.D (P≤ ٠٫٠٥) ٠٫٠٠١٩٢٤ ٠٫٠٠١٩٢٤ ٠٫٠٠١٩٢٤ ٠٫٠٠١٩٢٤

Mean values having different superscript letters within each column differ significantly (P≤ ٠٫٠٥) ٤٫٢. Incubation temperature:

Table (٢) shows that the results indicated that the best

incubation temperature is ٤٥ºC which gave the best quality with

regard to the (total acidity (T.A) whey volume, viscosity and the

average values of sensory quality) in comparison with temperatures of

٤١ºC and ٤٣ºC (large amount of whey separated and decrease in

consistency). The results are in agreement with kosikowiski, (١٩٨٢)

who reported that warm mixture should be transferred into containers

and then held at a temperature of ٤٥ºC in the incubator for ٦-٣ hours

until a titratable acidity of ١٫٢-٠٫٩% could be attained.

Table ٣ Effect of the storage period on the quality attributes of yoghurt

Storage

quality attributes of yoghurt

period (days)

Amount of acidity (% lactic)

Whey separated(ml)

Viscosity (cp)

Sensory quality(total scores)

Zero day ٠٫٦٤c ٠٫٠٠c ١٤٥٫١٠c ١٤٫٢٣c One day at ٦ ºc ٠٫٧٣b ٠٫٠٦b ٦٢٥٫٩٠a ١٤٫٨٠a One day at ١٠ ºc ٠٫٨١a ١٫١٤a ٥٢٠٫٩٠b ١٤٫٥٠b

L.S.D (P≤ ٠٫٠٥)

٠٫٠٠١٩٢٤ ٠٫٠٠١٩٢٤ ٠٫٠٠١٩٢٤ ٠٫٠٠١٩٢٤

Mean values having different superscript letters within column differ significantly (P≤ ٠٫٠٥). ٤٫٣. Storage conditions:

Table (٣) shows that the results indicated that the storage

temperature of ٦ºC gave the best quality with regard to (T.A, whey

volume, viscosity and total score for sensory quality) compared to

temperature of ١٠ºC which gave large amount of whey separated and

bitterness ). The results are nearly in agreement with Berteloson

(١٩٦٤) who showed that control of storage temperature at ٥-٤ºC is

most important and higher temperatures can lead to defects such as

bitterness.

Table ٤ The interaction between the storage period temperature and Incubation temperature on the yoghurt level of acidity (% lactic).

Incubation temperature Storage period (days) ٤١ ºc ٤٣ ºc ٤٥ ºc Zero day ٠٫٦٥ g ٠٫٦٠ h ٠٫٦٨ f One day at ٦ ºc ٠٫٧٢ e ٠٫٧٢ e ٠٫٧٥ d One day at ١٠ºc ٠٫٨٠ c ٠٫٨١ b ٠٫٨٤ a L.S.D (P≤ ٠٫٠٥)

٠٫٠٠١٠٣١

Mean values having different superscript letters within columns and rows differ significantly (P≤ ٠٫٠٥). ٤٫٤.Development of acidity in yoghurt mix. incubated at

temperatures of ٤١ºC, ٤٣ºCand ٤٥ºC in ٤ hours stored at ٦°c for

one day.

Table ( ٤ ) shows that the development of acidity in yoghurt

mix, incubated at temperature of ٤٥ºC gave a better results

(optimum acidity without whey separated) compared with

temperatures of ٤٣c˚and ١٠c˚.

Table ٥ The interaction between the storage period temperature and

incubation temperature on the yoghurt whey separated.

Incubation temperature Storage period (days) ٤١ ºc ٤٣ ºc ٤٥ ºc Zero day ٠٫٠٠e ٠٫٠٠e ٠٫٠٠e One day at ٦ ºc ٠٫٢٠d ٠٫٠٠e ٠٫٠٠e One day at ١٠ ºc ١٫٣٠a ١٫٢٠b ٠٫٩٤c L.S.D (P≤ ٠٫٠٥) ٠٫٠٠١٠٣١

Mean values having different superscript letters within each column and row differ significantly (P≤ ٠٫٠٥). ٤٫٥. Effect of the wheying–off on quality of yoghurt:

Table (٥) shows that the whey separated increased at storage

temperature of ١٠ºC more than at temperature of ٦ºC .That

means the temperature of ٦ºC gave better quality compared to

temperature of ١٠ºC (good coagulum. Texture). The results are in

agreement with Shukla et al., ١٩٨٨) who found that the wheying–off

is a major defect in yoghurt therefore stabilizers and additives of milk

powder usually (٢٫٥-٠٫٥٪) are used to check wheying–off in

yoghurt.

Table ٦ The interaction between the storage period temperature and

Incubation temperature on the yoghurt viscosity.

Incubation temperature Storage period (days) ٤١ ºc ٤٣ ºc ٤٥ ºc Zero day ١٣٣٫٩٠ h ١٤٥٫١٠ g ١٥٦٫٣٠ f One day at ٦ ºc ٥٨٨٫١٠ c ٦٢٥٫١٠ b ٦٩٢٫٠٠ a One day at ١٠ ºc

٤٠١٫٨٠ e ٥٨٠٫٤٠ d ٥٨٠٫٤٠ d L.S.D (P≤ ٠٫٠٥) ٠٫٠٠١٠٣١

Mean values having different superscript letters within columns and rows differ significantly (P≤ ٠٫٠٥). ٤٫٦. Effect of storage temperatures on the Viscosity:

Table (٦) shows a comparison between development of

viscosity in yoghurt mix, incubated at temperature of ٤٥ºC and stored

at temperatures of ٦˚C and ١٠ºC. The viscosity of yoghurt held at

temperature of ٦ºC gave better than that held at temperature of ١٠ºC

however, low temperature increased viscosity and improved product

firmness and stability.

Table ٧ The interaction between the storage period and storage temperature

on the yoghurt amount of acidity.

Storage temperature Storage period ٦ ºc ١٠ ºc Zero day ٠٫٦٨i ٠٫٦٨i One day ٠٫٧٥h ٠٫٨٤f Four days ٠٫٧٨g ١٫١٢c Six days ٠٫٨٥e ١٫٤٥b Ten days ١٫١٠d ١٫٥٠a

L.S.D (P≤ ٠٫٠٥)

٠٫٠٠١٠١٢

Mean values having different superscript letters within columns and rows differ significantly (P≤ ٠٫٠٥). ٤٫٧. Effect of storage temperature on the acidity:

Table (٧) shows a comparison between development of acidity

in yoghurt mix, incubated at temperature of ٤٥ C◦ and stored at

temperature of ٦ºC and ١٠ºC. The acidity of yoghurt held at

temperature of ٦ºC was better than that held at temperature of ١٠ºC.

Higher temperature (temperature of ١٠◦C) can lead to defects such as

bitterness and low temperature (temperature of ٦◦C), can reduce the

fermentation and allow the desired acidity to be reached.

Table ٨ ٤٫٨. The interaction between the storage period temperature and

incubation temperature on the yoghurt sensory quality .

Incubation temperature

Storage period(days) ٤١ ºc ٤٣ ºc ٤٥ ºc Zero day ١٤٫٠٠h ١٤٫٢٠g ١٤٫٥٠e One day at ٦ ºc ١٤٫٧٠c ١٤٫٨٠b ١٤٫٩٠a One day at ١٠ ºc ١٤٫٤٠f ١٤٫٥٠e ١٤٫٦٠d L.S.D (P≤ ٠٫٠٥)

٠٫٠٠١٠٣١

Mean values having different superscript letters within columns and rows differ significantly (P≤ ٠٫٠٥). Table ٨ shows that the results indicated that the best incubation temperature of ٤٥cº and storage temperature of ٦cºwhich gave the best quality with regard to sensory evaluation in comparison with temperatures of ٤١cº and ٤٣cº and storage temperature of ١٠cº.The results are in agreement with porter (١٩٧٥) who suggested that when the yoghurt was set firm it should be transferred to a cold store and stored at (٨-٥cº) immediately.

Table ٩ ٤٫٩. Effect of the storage period on the quality attributes of yoghurt.

quality attributes of yoghurt Storage period

Amount of acidity (%lactic)

Whey separator (ml)

Viscosity (c p)

Zero day ٠٫٦٨e ٠٫٠٠e ١٥٦٫٣٠e One day ٠٫٧٩d ٠٫٤٧d ٦٣٠٫٤ d Four days ٠٫٩٥c ١٫٢٠c ٦٩٢٫٠٠c Six days ١٫١٥b ٢٫٢٥b ٧٣١٫١٠b Ten days ١٫٣٠a ٣٫١٥a ٩٠٤٫١٠a L.S.D (P≤ ٠٫٠٥)

٠٫٠٠١٢٤٢ ٠٫٠٠١٢٤٢ ٠٫٠٠١٢٤٢

Mean values having different superscript letters within each column differ significantly (P≤ ٠٫٠٥). ٤٫٩٫١. Table ٩ shows that the amount of acidity increased at ten days more than at zero ,one, four and six days. The results are in agreement with Gaafer (١٩٩٢) who found that the decrease in acetaldehyde and acetone (aroma) and the increase in acetic acid were closely related to the rapid decrease in acceptability after ١٠-٨ days. ٤٫٩٫٢. Table ٩ shows that whey separated increased at ten days more than at zero,one, four and six days. The results are disagreement with Pette and Smith (١٩٦٤) who noted that homogenization is essentional for the production of affirm curd of yoghurt.

٤٫٩٫٣. Table ٩ shows that the viscosity increased at ten days more

than at zero,one, four and six days. The results are in agreement with

Grigorov (١٩٦٦) who found that more rapid coagulation and

improvement in the hydrophilic properties of protein occurred when

milk was homogenized and treated at ٨٥cº for minutes.

Table ١٠ ٤٫١٠. Effect of the storage temperature on the quality attributes of yoghurt.

quality attributes of yoghurt

Storage temperature(c°)

Amount of acidity (%lactic)

Whey separated (ml) Viscosity (c p)**

٦ ºc ٦٦٧٫٤٩ ٠٫٣٤ ٠٫٨٣ ١٠ ºc ٥٨٠٫٤٢ ٢٫٤٩ ١٫١٢ Mean ٦٢٣٫٩٥٥ ١٫٤١ ٠٫٩٧ .S.D ٤٧٫٩٧٣ ٠٫٣٢٨ ٠٫٢٩٤

Table ١٠ shows that Standard Deviation (S.D.) = ٠٫٠٢٩٤ and all different values appear after comparing with the mean. ** C P = Centi poise Table ١١ ٤٫١١. The interaction between the storage period and storage

temperature on the whey separated.

Storage temperature Storage period(days) ٦ ºc ١٠ ºc Zero day ٠٫٠٠ h ٠٫٠٠ h One day ٠٫٠٠ h ٠٫٩٤ e Four days ٠٫٢٠ g ٢٫٢٠ c Six days ٠٫٥٠ f ٤٫٠٠ b Ten days ١٫٠٠ d ٥٫٣٠ a

L.S.D (P≤ ٠٫٠٥)

٠٫٠٠١٠٣١

Mean values having different superscript letters within columns and rows differ significantly (P≤ ٠٫٠٥).

Table ١١ shows least significant difference (L.S.D.) =

٠٫٠٠١٠٣١and mean values for the interaction between storage Period and storage temperature have significant difference on whey separated.

Table ١٢ ٤٫١٢. The interaction between the storage period and storage

temperature on the yoghurt viscosity.

Storage temperature

Storage period (days) ٦ ºc ١٠ ºc Zero day ١٥٦٫٣٠ i ١٥٦٫٣٠ i One day ٦٩٠٢٫٠٠ e ٥٨٠٫٤٠ h Four days ٧١٤٫٤٠ d ٦٦٩٫٧٠ f Six days ٨٥٩٫٥٠ c ٦٠٢٫٨ g Ten days ٩١٥٫٣٠ a ٨٩٣٫٠٠ b

L.S.D (P≤ ٠٫٠٥)

٠٫٠٠١٠٣١

Mean values having different superscript letters within columns and

rows differ significantly (P≤ ٠٫٠٥).

Table ١٢ shows that least significant difference (L.S.D.) = ٠٫٠٠١٠٣١and mean values for the interaction between storage period

and storage temperature have significant difference on the viscosity.

Amount of acidity for Zero day

0.56

0.58

0.6

0.62

0.64

0.66

0.68

0.7

41cº 43cº 45cº

storage temperatures

aci

dity

(%la

ctic

)

Amount of acidityfor Zero day

Fig ١. Development of acidity in yoghurt mix, incubated at different temperatures** in ٤ hours for zero day. **Different temperatures = (٤١ºc,٤٣ºc,٤٥ºc)

0.71%0.71%0.72%0.72%0.73%0.73%0.74%0.74%0.75%0.75%0.76%

41cº43cº45cº

incubation temperatures

acid

ity(%

lact

ic)

Fig ٢. Development of acidity in yoghurt mix, incubated at different temperatures** in ٤ hours stored at temperature of ٦°C for one day. **Different temperatures = (٤١ºc,٤٣ºc,٤٥ºc)

Fig ٣. Development of acidity in yoghurt mix, incubated at different temperatures** in ٤ hours stored a temperature of ١٠°C for one day. **Different temperatures = (٤١ºc,٤٣ºc,٤٥ºc)

0

0.002

0.004

0.006

0.008

0.01

41c, 43c, 45c incubation temperature

aci

dity

(% la

ctic

)

IncubationtemperatureAmount of acidityfor one day

Fig ٤. Development of acidity in yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperature of ٦cº for days zero day,٤،٦ ,١ and ١٠ days.

Amount of acidity during storage at temperature of 6cº

0.00%

0.20%

0.40%

0.60%

0.80%

1.00%

1.20%

Zero 1, 4, 6, 10,

Storage days

acid

ity(%

lact

ic)

Amount of acidity during storage at temperature of 10cº

0.00%

0.20%

0.40%

0.60%

0.80%

1.00%

1.20%

1.40%

1.60%

Zero 1, 4, 6,10

,

Storage days

aci

dity

(%la

ctic

)

Fig ٥. Development of acidity in yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperature of ١٠cº. Days for zero day, ٦ ,٤ ,١ and ١٠ days.

0.00%

0.20%

0.40%

0.60%

0.80%

1.00%

1.20%

1.40%

1.60%

Zero1,4,6,10

,

storage days

acid

ity(%

lact

ic)

Fig ٦. Comparison between the developments of acidity in yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperatures of ٦cº, ١٠cº. Days for zero day, ٦ ,٤ ,١ and ١٠ days.

01002003004005006007008009001000

Zero1,4,6,10,

storage days

visc

osity

(c p

)

Fig ٧. Development of viscosity in yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperature of ٦cº.Days for zero day ,٤،٦ ,١ and ١٠ days.

Fig ٨. Development of viscosity in yoghurt mix, incubated at temperature ٤٥cºin ٤ hours in stored at temperature of ٦cº.Days for zero day ,٤،٦ ,١ and ١٠ days.

Viscosity during storage at temperature of 10ºc

0

200

400

600

800

1000

storage days

visc

osity

(c p

)

Fig ٩. Comparison between The development of viscosity in yoghurt mix, incubated at temperature ٤٥cºin ٤ hours stored at temperature of ٦cºand ١٠cº. Days for zero day ,٦ ,٤ ,١ and ١٠ days.

0

100

200

300

400

500

600

700

800

900

1000

1 2 3 4 5 6 7 8 9

storage days

visc

osity

(c p

)

0.00%0.20%0.40%0.60%0.80%1.00%1.20%1.40%1.60%

Zero1,4,6,10,

storage days

acid

ity(%

lact

ic)

Fig ١٠. Comparison between the developments of acidity in yoghurt mix, incubated at temperature ٤٥C in ٤hours stored at temperature of ٦Cº, ١٠cº. Days for zero day, ٦ ,٤ ,١ and ١٠ days.

CHAPTER FIVE

CONCLUSIONS AND RECOMMENDATIONs

٦٫١. Conclusions:

The study of the effect of different incubation and storage

temperatures on quality of set yoghurt during storage life of ١٠ days

reached the following results:

١. The incubation temperature of ٤٥ºC gave generally better

quality compared to incubation temperatures of ٤١ºC and

٤٣ºC.

٢. The storage temperature of ٦ºC gave generally better quality

compared to storage temperature of ١٠ºC.

These results indicated in details that:

١. Incubation temperature of ٤٥ºC gave best product in terms

of consistency (texture and body) and viscosity and level of

whey separated.

٢. Incubation temperatures of ٤١ºC and ٤٣ºC gave large

amount of whey separated, low viscosity and body less.

٣. Storage temperature of ٦ºC gave better product in terms of

consistency (texture and body) and viscosity and level of

whey separated.

٤. Storage temperature of ١٠ºC gave large amount of whey

separated, low viscosity and body less.

٦٫٢. Recommendations:

The recommendations of this study are as follows:

١. The incubation temperature and time were recommended to

be at temperature of ٤٥ºC and for ٤ hours, respectively.

٢. The storage temperature was recommended to be at

temperature of ٦ºC during storage life for ١٠ day at factory

conditions.

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APPEX (١)

Sensory evaluation of set yoghurt

History:

Type of milk:

Number of sample:

Put mark in ( )

١- appearance and colour

٥ ( ) -٤ ( )-٣ ( ) -٢ ( )-١- ( )

Defects

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_

_

٢. texture

٤ ( ) -٣ ( )-٢ ( ) -١-(

)

٥- ( )

٣. flavour

٥ ( )-٤ ( ) -٣ ( ) -٢ ( ) -١-( )

Defects

_

_

٤- Taste

٥ ( )-٤ ( ) -٣ ( ) -٢ ( ) -١-( )

Defects

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_

General evaluation

١- bad

٢- fair

٣- good

٤- very good

٥- excellent

Remarks: