Fluid Milk Processing

(Dairy Technology)
Student Handbook
for
CLASS
XI

CENTRAL BOARD OF SECONDARY EDUCATION, DELHI

Shiksha Kendra, 2 Community Centre, Preet Vihar, Delhi-110092, India
 Fluid Milk
Processing
(Dairy Technology)

Student Handbook for

CLASS
XI

Central Board of Secondary Education, Delhi

Shiksha Kendra, 2 Community Centre, Preet Vihar, Delhi-110092 India
Fluid Milk Processing Student’s Handbook for Class XI

Price : Rs.

First Edition 2013 CBSE, India

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Fluid Milk Processing (Student Handbook for Class XI)

 Preface
I ndia has emerged as the largest milk producing country in the world with annual milk
production estimated as 127 million tonnes. Dairy industry is of crucial importance to
India due to a number of reasons. Firstly, the dairy industry is a source of livelihood
and provides sustainable income to farmers. Secondly, milk is closely associated with
food security and is the first food for millions of infant. Thirdly, dairy products are
a major source of cheap and nutritious food to millions of people in India. Fourtly,
the dairy processing sector ranks first in terms of processed foods with 37 per cent
of the total milk production is being processed. India is not only one of the largest
producers of dairy products, but also has the largest consumer base. The activities
of dairy processing involve milk collection and chilling, processing, production of
milk products, packaging, quality assurance, storage, distribution and marketing.
These activities require skilled persons, professionals and trained man power. The
course on “Fluid Milk Processing” for class XI students has been introduced with
the following objectives:

zz This is a basic course to equip students with knowledge and skills in the area
of milk processing. Major topics covered in this book are milk, quality of milk,
reception of milk, market milk and various related processes like clarification,
cream separation, homogenization, pasteurization, sterilization, packaging, plant
utilities, effluent treatment etc.

zz Students will get an exposure to vocational/professional course on dairy

processing and technology. Course will motivate students to come up as
entrepreneurs in the area of dairy processing.

zz This course is an effort to sensitize student to the field of milk processing.

Students will get an opportunity to choose career in the area of dairy technology
and they may further pursue diploma (Dairy Technology)/B. Tech. (Dairy
Technology)/B. Tech. (Food Technology).

zz This course has been designed to provide entry level job skills to the students
and will help to meet the human resource requirement for dairy processing
sector.

Vineet Joshi, IAS

Chairman
CBSE

Fluid Milk Processing (Student Handbook for Class XI)

 Acknowledgements
Advisors

l Dr. S. Ayyappan, Secretary, DARE & Director General, ICAR, Krishi Bhavan, New Delhi-110001
l Sh. Vineet Joshi, IAS, Chairman, CBSE, Delhi

Special Acknowledgement
l Dr. Rameshwar Singh, Project Director (DKMA), Directorate of Knowledge, Management in Agriculture,
5th Floor, Krishi Anusandhan Bhawan - I, Pusa, New Delhi-110002
l Dr. A.K. Srivastava, Director, National Dairy Research Institute (NDRI), Karnal- 132001, Haryana
l Mr. Gian Muterja, General manager, Model Dairy Plant, NDRI, Karnal
l Mr. A.K. Sharma, Dairy Superintendent, Experimental Dairy, NDRI, Karnal

TECHNICAL Group
l Dr. Pradip Behare, Scientist, Technical Co-editor, NDRI, Karnal-132001
l Er. P.S. Minz, Scientist, Technical Editor, NDRI, Karnal-132001

Author Group
l Dr. Pradip Behare, Scientist, NDRI, Karnal-132001
l Er. P.S. Minz, Scientist, NDRI, Karnal-132001
l Dr. Surajit Mandal, Scientist, NDRI, Karnal-132001
l Mr. M.H. Sathish Kumar, Scientist, NDRI, Karnal-132001
l Mr. Dikshit A. Chichudde, Sr. Officer, Mother Dairy, Delhi

Editing and Coordination

l Dr. Biswajit Saha, Programme Officer, Vocational Education, CBSE, Delhi

l Shri Dharampal Singh, Former Director (EDUSAT & Vocational Education), and Consultant
(Agriculture), CBSE, Delhi
l Mrs Pragya Gaur, Consultant (Science), CBSE, Delhi

iv Fluid Milk Processing (Student Handbook for Class XI)

Fluid Milk Processing (Student Handbook for Class XI) v
vi Fluid Milk Processing (Student Handbook for Class XI)
 CONTENTS
Chapter 1
C O N T E N T S
Milk: Composition, Properties and Nutritional Role 1

Chapter 2
Quality of Milk: Grading of Milk-Dye Detection Test, Platform
Test, Sensory Evaluation, Milk and Public Health, Common Milk
Borne Disease, Spoilage-Causes and Prevention, Adulterants
and Their Detection 11
Chapter 3
Milk Reception Operations at Dairy Plant 31
Chapter 4
Fluid Milk Varieties and Special Milks 38
Chapter 5
Cream Separation 48
Chapter 6
Clarification
55
Chapter 7
Milk Pasteurization 59
Fluid Milk Processing (Student Handbook for Class XI) vii
 C O N T E N T S
Chapter 8
Milk Homogenization 72

Chapter 9
Sterilization 78
Chapter 10
Cleaning and Sanitation of Dairy Equipments,
Clean-in-Place (Cip) 82
Chapter 11
Effluent Treatment and Dairy Plant Waste Disposal 90
Chapter 12
Dairy Utilities-Steam, Water, Refrigeration, Electricity 96
Chapter 13

102
Packaging – Packaging Materials and Specifications, Machines,
Systems and Operational Aspects

Chapter 14
Fluid Milk Processing Dispensing of Fluid Milk Through Bulk
Vending, Bottles, Cartons and Pouches 116

viii Fluid Milk Processing (Student Handbook for Class XI)

Chapter I

Milk: Composition, Properties and

Nutritional Role

Objective
The purpose of this chapter is to understand the milk, its constituents,
physic-chemical properties, and nutritional aspects of milk.

Introduction
Milk is an important part of daily diet of vast population on the earth, due to its high
nutritional value. It is secreted by the mammalians for the nourishment of their new born.
According to Food Safety and Standards Authority of India (FSSAI), milk is defined as
“a secretion derived from complete milking of healthy milch animals.” It shall be free
from colostrums. Milk of different classes and of different species/designations shall
conform to the standards prescribed in the act (Table 1.1). Chemically speaking “milk is a
substance in which fat is present in emulsion, casein (major milk protein) together with
some minerals in colloidal suspension and lactose, mineral salts and whey proteins in
watery solution”.

Dairy animals are capable of producing milk for

nourishment of their off-spring due to
presence of mammary glands

Fluid Milk Processing (Student Handbook for Class XI) 1

 Table 1.1. Standards for different types of milk as per FSSAI

Class of milk ** Minimum (%)

Milk fat Milk Solid-Not-Fat (SNF)

Buffalo milk 5.0 to 6.0* 9.0

Cow milk 3.0 to 4.0* 8.5

Goat or sheep milk 3.0 to 3.5* 9.0

Mixed milk 4.5 8.5

Standardized milk 4.5 8.5

Recombined milk 3.0 8.5

Toned milk 3.0 8.5

Double toned milk 1.5 9.0

Skim milk Not more than 0.5 8.7

* Varies in different states

** When milk is offered for sale without indication of the class, the standards prescribed for buffalo milk
shall apply

Composition of Milk
Milk is nearly complete natural food on the earth. It contains water, fat, protein, lactose
and minerals as major constituents. The proportions of these constituents vary with species
of the animal. For example, Mare’s milk contains only about 1.6% fat, while Reindeer’s
milk contain 22.5% fat. Apart from this, the composition also varies with several other
factors, viz., the breed of animal, the feeding pattern, season, lactation period, age of
animal, environment, etc. The average compositions of milk from different species are
given in the table 1.2.

Total solids in a milk is the Sum of milk fat and

solid-not-fat (TS= fat + SNF)

Milk also contains several minor constituents like salts (Ca, PO4, Cl, Na Mg, K, S, and
citrate) and trace elements that are very important from physiological and nutritional point

2 Fluid Milk Processing (Student Handbook for Class XI)

of view. Several enzymes, non-protein nitrogenous substances, vitamins and pigment are
also found in milk.

Table 1.2. Average compositions (per cent) of milk from different species

Constituents Cow Buffalo Human Goat Mare Sheep

Water 87.20 82.76 87.43 86.5 89.1 79.4

Fat 3.70 7.38 3.75 4.5 1.6 8.6

Protein 3.50 3.60 1.63 3.5 2.7 6.7

Lactose 4.90 5.48 6.98 4.7 6.1 4.3

Ash 0.70 0.78 0.21 0.8 0.5 1.0

Total Solids 12.8 17.24 12.57 13.5 10.90 20.6

Constituents of Milk
The major component of milk is water while remainder consists of fat, protein and lactose.
Milk also contains minerals, vitamins, specific blood proteins and enzymes in smaller
quantities. The major and minor constituents of milk are briefly explained below;

Fat
Milk fat is composed of various lipid, that comprises 98% triglycerides as a major lipid
and 2% other minor lipids like diglycerides, monoglycerides, cholesterol, phospholipids,
free fatty acids, cerebrosis and gangliosides. Particularly the fatty acids of milk fat vary
widely in their length (4 to 20 carbon atoms) and unsaturation (0 to 4 double bonds). Milk
fat also has a relatively high content of short-chain saturated fatty acids such as butyric
(C4) and capric (C10) acids. These fatty acids are important to the flavour of milk products
and off-flavours that may develop in the milk.

Protein
There are two groups of milk proteins casein and whey or serum proteins. The former
constitutes about 80% while latter 15 % of the total milk protein. Non-protein nitrogen
constitutes about 5% of the total nitrogen of milk. Caseins can be fractionated in to four
distinct proteins such as as1, as2, b, and k- caseins. These caseins play important role in

Fluid Milk Processing (Student Handbook for Class XI) 3

cheese making. The principal whey protein fractions are b-lactoglobulin, bovine serum
albumin (BSA), a-lactalbumin and immunoglobulins.

Lactose
Lactose is a disaccharides present in milk as major milk sugar. It is composed of glucose
and galactose. It imparts sweet taste to the milk. Lactose also acts as carbon source for
majority of the spoilage causing organisms growing in milk. In products like dahi, yoghurt,
lassi, shrikhand etc., lactose is the major components get converted in to lactic acid by
the action of lactic acid bacteria.

Salts/Minerals
Milk salts consists mainly of chlorides, phosphates, citrates, sulfates and biocarbonates
of sodium, potassium, calcium, and magnesium. Some of the milk salts (chlorides,
sulfates, and compounds of sodium and potassium) are soluble and are present in milk
as ions dissolved in milk whey. Others salts such as calcium and phosphate are much
less soluble and at the normal pH (6.6-6.7) of milk exist partly in dissolved and partly in
insoluble (i.e. colloidal) form in close association with the casein micelles. A large number
of mineral elements such as, zinc, iron, and manganese are present in normal milk in
trace amounts.

Vitamins and Enzymes

Milk also contains many vitamins that are fat soluble (e.g. A, D, E and K), water soluble,
vitamin B complex (e.g. e.g. thiamine or B1, riboflavin or B2, niacin, pathothenic acid,
pyridoxine or B6, biotin, B12, folic acid etc.) and vitamin C. Absence of vitamins in the diet
over prolonged period causes deficiency diseases. The enzymes which are found in milk
include lipase, proteases, peroxidase, catalase, lactoperoxidase and acid phosphatase.
Some of the minor constituents play important function.

Water
It constitutes the medium in which the other milk constituents are either dissolved or
suspended. Most of it is free and only a very small portion is in the bound form, being
firmly bound by milk proteins, phospholipids etc.

4 Fluid Milk Processing (Student Handbook for Class XI)

Natural Inhibitory Substances in Milk
Several types of inhibitory substances are secreted in milk primarily for preventing infection
in mammary gland or conferring resistance to new born. However, the same substances
can also help in preservation of milk for some time, in natural form. One of the natural
antimicrobial systems LP-system has been investigated thoroughly for extending the shelf-
life of raw milk. Both specific and non-specific types of substances are secreted in milk
and are shown in table1.3.

Table 1.3. Natural inhibitory substances present in milk

Inhibitory Description Role

Substances

Immunoglobulins Types of antibodies and component of immune Neutralize toxins, suppress bacterial growth
(Ig) system produced locally in the udder (IgA) or and assist in the process of phagocytosis
transferred to milk from circulation (IgG)

Leucocytes One of the types of cell present in milk. Total cell Indicates normal or abnormal milk. These cells
count of milk from uninfected udder ranges from 1 inhibit invading bacteria by phagocytosis
to 5 lakhs/ml, of which 10% are polymorphonulcear
leucocytes (PMN)

Complement Component of immune system. About nine Associated with bactericidal (killing effect on
components of complements are found to be organisms) activity
present in human milk

Bifidus factor Bifidus factor is a nitrogen containing oligosaccharide Supports the growth of Bifidobacteria in infants,
present in human milk which helps in maintenance of intestinal
health

Lactoferrins Lactoferrins are iron binding proteins, The Inhibit the growth of bacteria by depriving
concentrations of lactoferrin in human and bovine them of iron
milk are 2-4 and 0.02-0.35 mg/ml, respectively

Lysozyme Enzyme present in much higher concentration Exhibit lytic action on cell wall component of
(30 mg/100 ml) in human milk than bovine milk gram positive bacteria
(0.01 mg/100 ml)

Lactoperoxidase/ Lactoperoxidase is an enzyme naturally secreted When all the three components are available,
thiocyanate / in milk in concentration of about 30µg /ml. the system gets activated and produces
hydrogen peroxide Thiocyanate is the substrate for this enzyme to act unstable substances which are bacteriostatic
system and is present in varying concentrations of 1 to 10 (Inhibit organisms growth).
(LP-System) ppm. Hydrogen peroxide acts as a catalyst.

Miscellaneous The other inhibitory substances in milk include Vitamin binding proteins inhibit the growth of
substances vitamin binding proteins (vitamin B12 and folate) bacteria by depriving them of vitamin.
and fatty acids

Fluid Milk Processing (Student Handbook for Class XI) 5

Colostrums
The first secretion collected from the udder at the beginning of lactation known as colostrums.
Colostrum or ‘fore milk’ differs from that of normal milk with respect to its composition and
physic-chemical properties. Unlike milk upon heating colostrum get coagulated and can
be observed like rubbery curd. Almost all milk constituents except lactose are present in
higher quantities in colostrums. Colostrum has about 17.5 % protein (5.08 % casein and
11.34 % albumin), 5.10 % fat, 2.19 % lactose, 1.1% ash and 26.20 % total solids. It has
a strong odour, bitter taste, slight reddish yellow colour, higher concentration of fat and
protein, particularly immunoglobulins and a low content of lactose. The successive milking
leads to the composition rapidly approaches to that of normal milk, however, it does not
become fully normal until about 15 days after calving.

Physico-Chemical Properties of Milk

Milk can be described as a delicately balanced biochemical fluid. It contains more water
than any other constituents and therefore, the properties of milk are primarily those
of an aqueous system. Water is the continuous phase in which other constituents are
either dissolved or suspended. Lactose and a portion of the mineral salts are found in
solution, proteins and the remainder of the mineral is colloidal suspension and fat as an
emulsion.

Flavour
This is composed of smell (odour) and taste. The flavor of milk is blend of the sweet
taste of lactose and salty taste of minerals. The phospholipids, fatty acids and fat of milk
also contribute to the flavor.

Colour
The colour of milk ranges from yellowish creamy white (cow milk) to creamy white (buffalo
milk). It is a blend of the individual effects produced by i) the colloidal casein particles
and dispersed fat globules, both of which scatter light and ii) the carotene which imparts
a yellowish tint. The skim milk has a bluish and whey has a greenish yellow colour.

Acidity and pH of Milk

Freshly drawn milk is amphoteric in nature. The acidity of milk is determined by titration
method using alkali and phenolphthalein indicator (Fig.1.1). It is also called natural acidity and

6 Fluid Milk Processing (Student Handbook for Class XI)

is caused by the presence of casein, citrate,
acid phosphate etc., in milk. Developed or
Burette
real acidity of milk is due to the production
of lactic acid as a result of bacterial action
on lactose. The titratable acidity is usually Lab. stand

expresses as a percent of lactic acid. The Boss & Clamp

acidity of cow milk varies from 0.13 to 0.14%
Pipette
lactic acid and buffalo milk from 0.15 to 0.17
% lactic acid.

The pH of normal milk usually varies from

6.4 to 6.6 for cow milk and 6.7 to 6.8 for N/9 Sodium
Hydroxide Beaker
buffalo milk. Higher pH values for fresh milk Conical Indicator
indicate that the animal suffered from udder Flask

infection (mastitis) and lower values indicate Fig.1.1. Titratable acidity apparatus
bacterial action.

Viscosity
The viscosity of milk is mainly due to the milk proteins
and fat present as colloidal system and to a minor
extent due to lactose and salts in solution with water.
It is measured by viscometer (Fig.1.2). Viscosity of milk
is expressed in centipoises (cp). The relative viscosity
of cow milk, buffalo milk, skim milk and whey is 2.0 cp,
1.80 cp, 1.50 cp and 1.20 cp respectively at 20°C and
it decreases with an increase in temperature.

The absolute viscosity of water at 20°C is equal to 1.005

cp (centipoises). The viscosity of milk is influenced by
state and concentration of protein, fat, temperature and
age of the milk.
Fig.1.2. Viscometer for determination
of viscosity of milk
Boiling Point
Any liquid boils at the temperature at which its vapour pressure is equal to that of the
atmosphere. Since the vapour pressure of a solution is always less than the vapour pressure
of the pure solvent, the boiling point of a solution will always be higher than that of the

Fluid Milk Processing (Student Handbook for Class XI) 7

pure solvent. Pure water boils at 100°C. Milk constituents are therefore, responsible for
elevation of the boiling point of milk. Milk generally boils at 100.15°C.

Freezing Point
Milk freezes at temperature slightly lower than water due to the presence of soluble
constituents such as lactose, soluble salts etc., which lower or depress the freezing point.
The average freezing point depression of cow milk may be taken as 0.547°C and buffalo
milk 0.549°C.

Density and Specific Gravity

Density of substance is its mass (weight) per unit volume, whereas specific gravity is the
ratio of density of the substance to density of a standard substance (water). Since the
density of a substance varies with temperature, it is necessary to specify the temperature
when reporting densities or specific gravities. The specific gravity of milk is usually expressed
at 15.6°C with help of lactometers (Fig.1.3). Three types of lactometers are generally used
i.e.. Zeal, Quevenne and ISI lactometer. The average specific gravity ranges from 1.028
to 1.030 for cow milk, 1.030 to 1.032 for buffalo milk and 1.035 to 1.037 for skim milk.
The specific gravity of milk is affected by its constituents, each of which has a different
specific gravity (water-1.00, fat-0.93, protein-1.346, lactose-1.666, salts -4.12, Solid-not-fat
1.616). As milk fat is the lightest constituents, the more there is of it the lower the specific
gravity will be, and vice versa.

Fig.1.3. Determination of specific gravity of milk by ISI lactometer

8 Fluid Milk Processing (Student Handbook for Class XI)

Factors Affecting Composition and Physico-Chemical Properties
of Milk
Milk differs widely in composition. All milks contain the same kind of constituents but
in varying amounts. Milk from individual cows shows greater variation than mixed herd
milk. The variation is always greater in small herds than in large ones. In general milk
fat shows the greatest daily variation, then comes protein followed by ash and lactose.
Factors affecting milk composition are summarized in a table 1.4.

Table 1.4 Factors affecting composition and physico-chemical properties of milk

Factors Description

Species Each species yields milk of a characteristic composition

Breed Breeds producing largest quantity of milk yields lower percentage of fat

Individuality Each animal tends to yield milk of a composition that is characteristics of

the individual

Interval of milking Longer milking interval is associated with more milk with a lower fat

Completeness of milking Complete milking gives normal composition while incomplete gives lower

Frequency of milking Frequency of milking whether two, three of four times a day does not affect
composition in greater extent

Day-to-day milking It may show variations for the individual animal

Disease and abnormal Milk yield decreases and alters the milk composition
condition

Stage of lactation The first secretion after calving (Colostrum) is very different from milk in its
composition and general properties

Feeding It has temporary effect on milk composition

Season The percentages of fat and SNF (solid-not-fat) show some variations during
the course of the year

Age The fat percentage in milk declines slightly as the cow grows older

Excitement Yield and composition of milk are liable for transient fluctuation during periods
of excitement

Administration of drugs and Certain drugs may bring temporary change in the fat percentage. Injection
hormones or feeding of hormones results in increase of both milk yield and fat
percentage

Fluid Milk Processing (Student Handbook for Class XI) 9

 Nutritional Value of Milk
Milk is nearly a complete food except that it is deficient in iron, iodine, copper and vitamin
C. It has very high nutritive value and gives about 75 to 100 kilocalories per 100 g. Milk
has sufficient quantity of major proteins like casein, ± a-lactalbumin, b-lactoglobulin and
bovine serum albumin. They contain all essential amino acids and are useful in body building
and other normal cell functions. The major milk sugar is lactose, whose primary function
is supply energy, but it also helps to establish milk acidic reaction in the intestine which
check growth of unwanted bacteria in the intestine and facilities assimilation of minerals

Milk fat has high energy value of 9.3 kilo calories/g. Besides, it contains significant amounts
of essential fatty acids. Apart from this, fat contributes to the flavour and physical properties
of milk which increase consumer preference for the milk.

Milk is an excellent source of essential minerals especially Ca and P which is very useful
in bone formation. On an average milk contains, Ca-123, Mg-12, PO4-95, Na-58, K-141,
CI-119, S-30, and citric acid-160 mg/100 ml. These minerals also play very important role
in other physiological functions.

Vitamins are essential for normal healthy growth and reproduction of living organisms.
Milk is a good source of Vitamin A, D, thiamine and riboflavin. However, it is deficient
in Vitamin, C. Milk also contains a number of trace elements (those which are present
in very small quantity, measured as few µg per litre), which may have important role in
supporting nutritionally fastidious organisms.

REVIEW QUESTIONS

1. What is milk? What factors affect its composition?

2. Which naturally occurring inhibitory substances are present in milk?

3. How colostrum is different from normal milk?

4. What is the role of milk constituents?

5. Which minor components are deficient in milk?

6. Which apparatus is used to measure specific gravity of milk?

10 Fluid Milk Processing (Student Handbook for Class XI)

Chapter 2

Quality of Milk: Grading of Milk-

Dye Detection Test, Platform Test, Sensory
Evaluation, Milk and Public Health, Common
Milk Borne Disease, Spoilage-Causes and
Prevention, Adulterants and Their Detection

Objective
In this chapter, we will learn
l Grading of milk based on certain rapid test
l Public health significance of milk
l Detection of adulterants in milk

Introduction
Milk testing and quality control is an essential component of any milk processing industries.
Milk being made up of 87% water is prone to adulteration by middleman suppliers and
farm levels. Moreover, its high nutritive value makes it an ideal medium for the rapid
multiplication of bacteria, particularly under unhygienic production and storage at ambient
temperatures. For making good quality dairy products, good quality raw milk and other
required materials are indispensable. Therefore, for assuring the quality of raw milk, certain
basic quality parameter tests are to be carried out at various stages of transportation of
milk from the producer to the processor and finally to consumer. Milk quality control is

Fluid Milk Processing (Student Handbook for Class XI) 11

 the use of approved tests to ensure the application of approved practices, standards and
regulations concerning the milk and milk products. The tests are designed to ensure that
milk products meet accepted standards for chemical composition and purity as well as
levels of different micro-organisms.

Testing milk and milk products for quality and monitoring those milk products, processors
and marketing agencies adhere to accepted codes of practices costs money. The rationales
are:

a) Milk producer: The milk producer expects a fair price in accordance with the quality
of milk she/he produces.

b) Milk processor: The milk processor who pays the producer must assure himself/
herself that the milk received for processing is of normal composition and is suitable
for processing into various dairy products.

c) Consumer: The consumer expects to pay a fair price for milk and milk products of
acceptable to excellent quality.

d) Public and Government agencies: These have to ensure that the health and
nutritional status of the people is protected from consumption of contaminated and
sub-standard foodstuffs and that prices paid are fair to the milk producers, the milk
processor and the final consumer.

All the above-is only possible through institution of a workable quality testing and assurance
system conforms to national or internationally acceptable standards.

1) Grading of Milk-dye Detection Test

a) Methylene Blue Reduction Test
The dye reduction tests are based on the bacterial dehydrogenase activity. The
dehydrogenases are a group of enzymes capable of transferring of hydrogen atoms from a
substrate to a biological acceptors or redox sensitive dyes. Aerobic group of organisms use
oxygen as electron acceptor. Thus during their growth and metabolism consume dissolve
oxygen from the media and decrease the medium redox potential. The larger the initial
number of microorganisms and higher the growth and metabolic rate, there will be faster
the reduction of redox potential in the medium. Rate of oxygen consumption and duration
of exhaustion are varying proportionately to the initial number of microorganisms in the
system. A number of dyes can change their colour/ appearance in response to the change
in redox potential of the medium and are called redox sensitive dyes, e.g. methylene blue,

12 Fluid Milk Processing (Student Handbook for Class XI)

resazurin etc. In general, the redox dyes are in coloured state at higher redox potential
(e.g. at oxidized redox potential, methylene blue remains as blue coloured form) and at
reduced redox potential they becomes colourless or leucoform (e.g. at reduced redox
potential methylene turns into colourless or leucoform).

Milk inside the udder is devoid of oxygen and remains at reduced redox potential. During
milking, dumping, cooling, etc. a lot of oxygen is incorporated in milk and remains as
dissolved state and thus, the redox potential of milk is increased as high as 0.3 V. At this
redox potential, methylene blue exists as blue coloured (oxidized form). Bacterial growth
in milk reduces the redox potential to a level of 0.06-0.10 V and at this redox potential,
methylene blue is reduced and changes in leucoform or colourless state.

The methylene blue reduction test is based on the fact that the color imparted to milk by
the addition methylene blue will be disappeared. The removal of the oxygen from milk
and the formation of reducing substances during bacterial metabolism cause the color to
disappear. A certain quantity of milk and methylene blue dye is taken and mixed in a air
tight container and incubate the system, the microorganisms in milk will grow and consume
oxygen and thus, the milk oxidation and reduction potential will goes down to reduce the
methylene blue dye to leuco-form or colourless state. The total time taken to reduce the
methylene blue dye is called methylene blue reduction time (MBRT), which is inversely
related to the initial microbial load in the milk sample. Higher MBRT indicates the lower
initial microbial load in milk and vice-versa. The reasons for the oxygen consumption
are the bacteria. Though certain species of bacteria have considerably more influence
than others, it is generally assumed that the greater the number of bacteria in milk, the
quicker will the oxygen be consumed, and in turn the sooner will the color disappear.
Thus, the time of reduction is taken as a measure of the number of organisms in milk
although actually it is likely that it is more truly a measure of the total metabolic reactions
proceeding at the cell surface of the bacteria. However, the methylene blue reduction
test has lost much of its popularity because of its low correlation with other bacterial
procedures. This is true particularly in those samples which show extensive multiplication
of the psychrotropic species.

Table 2.1. Grading of milk: Standard MBRT and quality chart for milk (BIS standard)

Grade of milk MBRT (h)

Very good 5 and above
Good 3 and 4
Fair 1 and 2
Poor ½ or less

Fluid Milk Processing (Student Handbook for Class XI) 13

 b) Resazurin Test
Resazurin test is the most widely used test for hygiene and the potential keeping quality
of raw milk. Resazurin is a dye indicator. Under specified conditions Resazurin is dissolved
in distilled boiled water. The Resazurin solution can later be used to test the microbial
activity in given milk sample.

Resazurin can be carried out as:

i. 10 min test.

ii. 1 hr test.

iii. 3 hr test.

The 10 min Resazurin test is useful and rapid, screening test used at the milk platform.
The 1 hr test and 3 hr tests provide more accurate information about the milk quality, but
after a fairy long time. They are usually carried out in the laboratory.

Table 2.2. Readings and results (10 minute resazurin test)

Resazurin disc No. Colour Grade of milk Action

6 Blue Excellent Accept

5 Light blue Very good Accept

4 Purple Good Accept

3 Purple pink Fair Separate

2 Light pink Poor Separate

1 Pink Bad Reject

0 white Very bad Reject

2) Platform Tests

a) Organoleptic Tests
The organoleptic test permits rapid segregation of poor quality milk at the milk receiving
platform. No equipment is required, but the milk grader must have good sense of sight,
smell and taste. The result of the test is obtained instantly, and the cost of the test are

14 Fluid Milk Processing (Student Handbook for Class XI)

low. Milk which cannot be adequately judged organoleptically must be subjected to other
more sensitive and objective tests.

Protocol

1. Open the can/ container of milk.

2. Immediately smell the milk.

3. Observe the appearance of the milk.

4. If still unable to make a clear judgement, taste the milk, but do not swallow it. Spit
the milk sample into a bucket provided for that purpose or into a drain basin, flush
with water.

5. Look at the can lid and the milk can to check cleanliness.

Obervations/Judgement

Abnormal smell and taste may be caused by:

zz Atmospheric taint (e.g. barny/ cowy odour).

zz Physiological taints (hormonal imbalance, cows in late lactation- spontaneous

rancidity).

zz Bacterial taints.

zz Chemical taints or discolouring.

zz Advanced acidification (pH < 6.4).

b) Indicator Test
zz The acidity developed in milk due to bacterial activity is measured in terms of pH
value as indicated by special indicator dyes, e.g. brom thymol blue and brom-cresol
purple.

c) Sediment Test
zz Milk is passed through a funnel containing a filter disc and the amount of dirt and dust
collected is compared visually or by weight. The test indicates the gross impurities
and dirt in milk as a result of unhygienic conditions of production.

Fluid Milk Processing (Student Handbook for Class XI) 15

 d) Clot on Boiling (C.O.B) Test
The test is quick and simple. It is one of the old tests for too developed acid in milk (pH
< 5.8) or abnormal milk (e.g. colostral or mastitis milk). If a milk forms clot on raising the
milk temperature to boiling indicate the increasing the acidity of milk or abnormal milk
composition (colostral milk). Such milk cannot be heat processed and must therefore be
rejected.

e) Alcohol Test
The test is quick and simple. It is based on instability of the milk proteins when the levels
of acid and/or rennet are increased and acted upon by the alcohol. Also increased levels
of albumen (colostrum milk) and salt concentrates (mastitis) results in a positive test i.e.
coagulation of milk upon mixing with alcohol.

f) Alcohol-Alizarin Test
The procedure for carrying out the test is the same as for alcohol test but this test is
more helpful. Alizarin is a colour indicator changing colour according to the acidity of milk
(Table 2.3). The Alcohol-Alizarin solution can be bought readymade or be prepared (0.4
g alizarin powder in 1 lit of alcohol (61%)).

Table 2.3 Colour Indicator for Alcohol-Alizarin test

Parameter Normal milk Slightly acid Milk Acid milk Alkaline Milk

pH 6.6 – 6.7 6.4 – 6.6 6.3 or lower 6.8 or higher

Colour Red brown Yellowish-brown Yellowish Lilac

Appearance of milk No coagulation no lumps No coagulation Coagulation No coagulation

f) Measurement of Titratable Acidity (% Lactic Acid)

Bacteria that normally develop in raw milk produce more or less of lactic acid. In the acidity
test the acid is neutralised with 1/9 N NaOH. The amount (ml) of alkali is measured and
titratable acidity is calculated (% lactic acid). Fresh milk contains in this test also “natural
acidity” which is due to the natural ability to resist pH changes .The natural acidity of milk
is 0.16 - 0.18%. Higher acidity signifies developed acidity due to the action of bacteria.

16 Fluid Milk Processing (Student Handbook for Class XI)

g) Gerber Fat Test
The fat content of milk and cream is the most important single factor in determining the
price to be paid for milk supplied by farmers in many countries.

Also, in order to calculate the correct amount of feed ration for high yielding dairy cows,
it is important to know the butterfat percentage as well as well as the yield of the milk
produced. Further more the butterfat percentage in the milk of individual animals must be
known in many breeding programmes.

Butterfat tests are also done on milk and milk products in order to make accurate adjustments
of the butterfat percentage in standardised milk and milk products.

h) Inhibitor Test
Milk collected from producers may contain drugs and/or pesticides residues. The significant
amounts in milk may inhibit the growth of lactic acid bacteria used in the manufacture of
fermented milk, besides being a health hazard. The milk sample is evaluated for starter
culture activity test, where after heat treatment, milk is fermented with selected lactic starter
culture and the developed acidity is measured. The control milk sample (free from inhibitory
substances) is run as control and from the acidity quality of milk is judged accordingly.

3) Sensory evaluation
Understanding of the principles of sensory evaluation is necessary for grading milk. Five
primary senses are used in the sensory evaluation of dairy products: sight, taste, smell,
touch and sound. The greatest emphasis, however, is placed on appearance (colour e.tc.),
taste and flavour.

Soon after milk is received on the platform of dairy the lid of can is opened and milk is
stirred up with a plunger to test for smell whether pleasant or unpleasant. Then it is tested
for appearance (colour e.tc.), taste and flavour.

a) Colour

Observe the colour of the milk. If abnormal in colour, it should be held over for subjection
to confirmatory tests.

Fluid Milk Processing (Student Handbook for Class XI) 17

 b) Odour and Taints

Smell the milk in the container immediately after removing the lid. In case of foul or
abnormal smell, hold over the milk for subjection to confirmatory tests.

The sense of taste

Taste buds, or receptors, are chiefly on the upper surface of the tongue, but may also
be present in the cheek and soft palates of young people. These buds, about 900 in
number, must make contact with the flavouring agent before a taste sensation occurs.
Saliva, of course, is essential in aiding this contact. There are four different types of nerve
endings on the tongue which detect the four basic “mouth” flavours -sweet, salt, sour,
and bitter. Samples must, therefore, be spread around in the mouth in order to make
positive flavour identification. In addition to these basic tastes, the mouth also allows us
to get such reactions as coolness, warmth, sweetness, astringency, etc.

The sense of smell

We are much more perceptive to the sense of smell than we are to taste. For instance,
it is possible for an odouriferous material such as mercaptain to be detected in 20 billion
parts of air. The centres of olfaction are located chiefly in the uppermost part of the nasal
cavity. To be detectable by smell, a substance must dissolve at body temperature and be
soluble in fat solvents.

Note: The sense of both taste and smell may become fatigued during steady use. A good
judge does not try to examine more than one sample per minute. Rinsing the mouth with
water between samples may help to restore sensitivity.

Milk grading techniques

Temperature should be between 60-70° F (15.5-21° C) so that any odour present may be
detected readily by sniffing the container. Also, we want a temperature rise when taking
the sample into the mouth; this serves to volatize any notable constituents.

Noting the odour by placing the nose directly over the container immediately after shaking
and taking a full “whiff” of air. Any off odour present may be noted.

Need to make sure we have a representative sample; mixing and agitation are
important.

18 Fluid Milk Processing (Student Handbook for Class XI)

Agitation leaves a thin film of milk on the inner surface which tends to evaporate giving
off odour if present.

During sampling, take a generous sip, roll about the mouth, note flavour sensation, and
expectorate. Swallowing milk is a poor practice.

Can enhance the after-taste by drawing a breath of fresh air slowly through the mouth
and then exhale slowly through the nose. With this practice, even faint odours can be
noted.

Milk has a flavour defect if it has an odour, a foretaste or an aftertaste, or does not leave
the mouth in a clean, sweet, pleasant condition after tasting.

Characterization of Flavour Defects

Lipolytic or hydrolytic rancidity

Rancidity arises from the hydrolysis of milkfat by an enzyme called the lipoprotein lipase
(LPL). The flavour is due to the short chain fatty acids produced, particularly butyric acid.
LPL can be indigenous or bacterial. It is active at the fat/water interface but is ineffective
unless the fat globule membrane is damaged or weakened. This may occur through
agitation, and/or foaming, and pumping. For this reason, homogenized milk is subject to
rapid lipolysis unless lipase is destroyed by heating first; the enzyme (protein) is denatured
at 55-60°C. Therefore, always homogenize milk immediately before or after pasteurization
and avoid mixing new and homogenized milk because it leads to rapid rancidity.

Some cows can produce spontaneous lipolysis from reacting to something indigenous to
the milk. Late lactation, mastitis, hay and grain ratio diets (more so than fresh forage or
silage), and low yielding cows are more suseptible.

Lipolysis can be detected by measuring the acid degree value which determines the
presence of free fatty acids. Lipolytic or hydrolytic rancidity is distinct from oxidative
rancidity, but frequently in other fat industries, rancid is used to mean oxidative rancidity;
in dairy, rancidity means lipolysis.

Characterized: soapy, blue-cheese like aroma, slightly bitter, foul, pronounced aftertaste,
does not clear up readily

Fluid Milk Processing (Student Handbook for Class XI) 19

 Oxidation

Milk fat oxidation is catalysed by copper and certain other metals with oxygen and air.
This leads to an autooxidation reaction consisting of initiation, propagation, termination.
RH — R + H initiation - free radical
R + O2 —— RO2 propagation
RO2 + RH — ROOH + R
R + R — R2 termination
R + RO2 — RO2R

It is usually initiated in the phospholipid of the fat globule membrane. Propagation then
occurs in triglycerides, primarily double bonds of unsaturated fatty acids. During propagation,
peroxide derivatives of fatty acids accumulate. These undergo further reactions to form
carbonyls, of which some, like aldehydes and ketones, have strong flavours. Dry feed,
late lactation, added copper or other metals, lack of vit E (tocopherol) or selenium (natural
antioxidates) in the diet all lead to spontaneous oxidation. It can be a real problem especially
in winter. Exposure to metals during processing can also contribute.

Characterized: metallic, wet cardboard, oily, tallowy, chalky; mouth usually perceives a
puckery or astringent feel

Sunlight

Often confused with oxidized, this defect is caused by UV-rays from sunlight or flourescent
lighting catalyzing oxidation in unprotected milk. Photo-oxidation activates riboflavin which
is responsible for catalyzing the conversion of methionine to methanal. It is, therefore, a
protein reaction rather than a lipid reaction. However, the end product flavour notes are
similar but tends to diminish after storage of several days.

Characterized: burnt-protein or burnt-feathers-like, “medicinal”-like flavour

Cooked

This defect is a function of the time-temperature of heating and especially the presence of
any “burn-on” action of heat on certain proteins, particulary whey proteins. Whey proteins
are a source of sulfide bonds which form sulfhydryl groups that contribute to the flavour.
The defect is most obvious immediately after heating but dissipates within 1 or 2 days.

Characterized: slightly cooked or nutty-like to scorched or caramelized

20 Fluid Milk Processing (Student Handbook for Class XI)

Transmitted flavours

Cows are particulary bad for transmitting flavours through milk and milk is equally as
susceptible to pick-up of off flavours in storage. Feed flavours and green grass can be
problems so it is necessary to remove cows from feed 2-4 hrs before milking. Weeds,
garlic/onion, and dandelions can tranfer flavours to the milk and even subsequent products
such as butter. Barny flavours can be picked up in the milk if there is poor ventilation and
the barn is not properly cleared and cows breathe the air. These flavours are volatile so
can be driven off through vacuum de-aeration.

Characterization: hay/silage, cowy/barny

Microbial

There are many flavour defects of dairy products that may be caused by bacteria, yeasts,
or moulds. In raw milk the high acid/sour flavour is caused by the growth of lactic acid
bacteria which ferment lactose. It is less common today due to change in raw milk microflora.
In both raw or processed milk, fruity flavours may arise due to psychrotrophs such as
Pseudomonas fragi. Bitter or putrid flavours are caused by psychrotrophic bacteria which
produce protease. It is the proteolytic action of protease that usually causes spoilage in
milk. Malty flavour is caused by S. lactis var. maltigenes and is characterized by a corn
flakes type flavour. Although more of a tactile defect, ropy milk is also caused by bacteria,
specifically those which produce exopolysaccharides.

Miscellaneous defects
zz astringent

zz chalky

zz chemical/medicinal - disease - associated or adulteration

zz flat - adulteration (water)

zz foreign

zz salty - disease associated

zz bitter - adulteration

Fluid Milk Processing (Student Handbook for Class XI) 21

 4) Milk and Public Health
Milk is an excellent medium for the growth of a large variety of bacteria. Bacteria need
considerable amounts of nutrient such as water, carbohydrate, fat and other substances for
their growth. Milk contains all of these nutrients. Microorganisms are capable of causing
deterioration in flavour, physical appearance of milk and transmission of infectious diseases
to the consumers. The various organisms get into milk through unhygienic, carelessness
and unsanitary practices of the farmers, processors and distributors. Discoloration,
sliminess, ropiness, putrefaction, rancidity and many other defects are caused by various
microorganisms growing in the milk and milk products. Bacterial contamination of raw
and pasteurized milk is considered to be a great problem for dairy milk. The important
genera of bacteria normally found in milk are, Microbacterium, Micrococcus, Streptococcus,
Staphylococcus, Lactobacillus, Bacillus, Clostridium, Arthrobacter, Actinomyces, Coxiella,
Pseudomonus etc. Most of these organisms are free living, widely distributed in soil,
feeds, cows, buffaloes, goats, dairy utensils etc. Contamination usually occurs at the
farm where milk is produced. Contamination of raw milk can arise from several sources
including, soiled udders, inadequate cleaned milking equipment, and poor handling and
processing of samples. Prolonged or improper holding of dairy products may permit microbial
contamination to increase. Bovine mastitis may cause contamination with Staphylococcus
aureus, Streptococcus agalactiae, E. coli and other microorganisms. Poor cleaning of the
milking equipment may cause contamination with streptococci, coliforms, or heat resistant
Bacillus spp. Spoilage of pasteurized or raw milk by proteolytic psychrotrophic bacteria
can occur on prolonged storage below 7°C.

5) Common Milk Borne Disease

Pathogenic bacteria in milk and significance

Some micro-organisms may cause food poisoning (pathogenic microorganisms), either by
intoxication and/or infection. Intoxication implies the production of poisons in the food prior
to its consumption. Infection means the establishment, active growth, and multiplication
of such microorganisms in the human body. Often rather large numbers are needed to
cause an infection, but sometimes, as in the case of Salmonella typhimurium, the MID
(minimum infection dose) may be as small as one bacterium.

The human pathogens transmitted through milk are classified into food infection and
food poisoning groups. In food infection milk act as a carrier of the microorganisms, this

22 Fluid Milk Processing (Student Handbook for Class XI)

enters in human body through milk. It takes time to a person to become ill and fairly small
numbers of microorganisms may suffice to cause illness. In food poisoning, preformed
toxins in milk are responsible. Consumers rapidly fall ill. Large numbers of the pathogenic
microorganisms are usually needed to cause food poisoning. Hygienic milk production
practices, proper handling and storage of milk and mandatory pasteurization has decreased
the threat of milk borne diseases such as tuberculosis, brucellosis, and typhoid fever. There
has been a number of food borne illnesses resulting from the ingestion of raw milk, or
dairy products made with milk that was not properly pasteurized or was poorly handled
causing post-processing contamination.

It should also be noted that moulds, mainly of species of Aspergillus, Fusarium , and
Penicillium can grow in milk and dairy products. If the conditions permit, these moulds
may produce mycotoxins which can be a health hazard.

The following bacterial pathogens are still of concern today in raw milk and other dairy
products:

Bacillus cereus
Listeria monocytogenes
Yersinia enterocolitica
Salmonella spp.
Escherichia coli O157:H7
Campylobacter jejuni

Table 2.4. Pathogenic bacteria in raw milk – significances and control

Pathogens Disease Control Measures

Mycobacterium bovis/ Tuberculosis Clean milk production

M. tuberculosis Low temperature storage
Pasteurization
Hygienic processing

Brucella abortus/ Brucellosis Clean milk production

B. melitensis Thermization before low temperature storage for
processing
Pasteurization
Hygienic processing

Fluid Milk Processing (Student Handbook for Class XI) 23

 Pathogens Disease Control Measures

Coxiella burnetii Q fever Clean milk production

Low temperature storage
Pasteurization
Hygienic processing

Staphylococcus aureus Enterotoxin Clean milk production

Thermization before low temperature storage for
processing
Pasteurization
Hygienic processing

Escherichia coli Some serotypes Clean milk production

pathogenic for men, Thermization before low temperature storage for
faecal contamination processing
Pasteurization
Hygienic processing

Listeria monocytogenes Listeriosis Clean milk production protocol reduces the initial number
in raw milk
Pasteurization

Bacillus cereus Enterotoxin Clean milk production

UHT sterilization of milk
Use of bactofugation and microfiltration in processing
operation

Clostridium perfringens Gas gangrene/ survive Clean milk production

pasteurization UHT sterilization of milk
Use of bactofugation and microfiltration in processing
operation

6) Spoilage-Causes and Prevention

Spoilage Organisms
The microbial quality of raw milk is crucial for the production of quality dairy foods.
Spoilage is a term used to describe the deterioration of a foods’ texture, colour, odour
or flavour to the point where it is unappetizing or unsuitable for human consumption.
Microbial spoilage of food often involves the degradation of protein, carbohydrates,

24 Fluid Milk Processing (Student Handbook for Class XI)

and fats by the microorganisms or their enzymes. In milk, the microorganisms that are
principally involved in spoilage are psychrotrophic organisms. Most psychrotrophs are
destroyed by pasteurization temperatures, however, some like Pseudomonas fluorescens,
Pseudomonas fragi can produce proteolytic and lipolytic extracellular enzymes which
are heat stable and capable of causing spoilage. Some species and strains of Bacillus,
Clostridium, Cornebacterium, Arthrobacter, Lactobacillus, Microbacterium, Micrococcus ,
and Streptococcus can survive pasteurization and grow at refrigeration temperatures which
can cause spoilage problems.

Saprophytic Bacteria in Milk and Significances

According to the main points of attack on the major milk constituents, the saprophytic
bacteria are subdivided as follows:

a) Microorganisms degrading milk carbohydrate (lactose) are classified as glycolates,

e.g. Streptococci, Lactobacilli, Coliforms

b) Microorganisms degrading proteins are classified as proteolytes, e.g. pseudomonas,

enterobacteriaceae, aerobic spore-formers.

c) Microorganisms degrading lipids are classified as lipolytes, e.g. pseudomonas,

micrococci, aeromonas, corynebacteria.

The effect of growth of saprophytic bacteria in milk may be important in three ways as
follows:

a) The change in milk composition may interfere with manufacture, if a fermentation is

involved in the manufacture process, and this may affect the yield and quality of the
product, e.g. cheese.

b) The flavour of the raw milk may be adversely influenced (e.g. rancidity) and this may
directly affect the flavour of the product e.g. pasteurized milk or cream.

c) Heat-stable bacterial enzymes may continue to act in the product, particularly during
long storage, and adversely affect the stability and/ or flavour of cream and UHT
milk.

Fluid Milk Processing (Student Handbook for Class XI) 25

 Table 2.5. Saprophytic bacteria in milk - significance and control measures

Group of Representative Significances Control Measures

microorganisms organisms

Lactic acid bacteria Lactococcus spp. At room temperature ferment Pasteurization of milk to
Lactobacillus spp. lactose to lactic acid and kill mesophilic lactic acid
Strpotococcus milk gets sour and become bacteria
thermophilus unfit for processing due to Storage of milk at low
the loss of heat stability temperature (4°C)

Coliforms Escherichia coli Ferment lactose using Pasteurization kill all the
Enterobacter hetero-lactic pathway coliforms
aerogenes Degraded proteins Hygienic processing of raw
Klebsiella spp. Produce gas and cause milk
Citrobacter spp. “unclean flavour” in milk Clean milk production
Indicator of post processing protocol reduces the initial
contamination and hygienic number in raw milk
operations

Psychrotrophs Pseudomonas, Grow at refrigerated Thermization of raw milk

Achromobacter, temperature before low temperature
Flavobacterium, Produce heat stable storage for processing
Alcaligenes proteases and lipases Pasteurization of raw milk
which break down the Hygienic processing of raw
milk proteins and fats milk
Cause off flavour and taste, Clean milk production
off colour protocol reduces the initial
number in raw milk

Heat resistant Microbacterium Spoil heat treated milk Clean milk production
bacteria lacticum, by sweet curdling, off protocol reduces the
Micrococcus spp. flavour, clumping of fat initial number in raw
Thermophilic globules milk
streptococci, sproe- UHT sterilization of
froming bacillus: milk
Bacillus spp. (B. Use of bactofugation
cereus, B. subtilis), and microfiltration in
Clostridium spp. processing operation
(Cl. tyrobutyricum)

26 Fluid Milk Processing (Student Handbook for Class XI)

7) Adulterants and Their Detection

Modes of Adulteration
zz Removal of fat by skimming

zz Addition of skim milk

zz Addition of water

zz Addition of starch and cane sugar for raising density

zz Addition of neutralizers and other preservatives to increase keeping quality

a) Detection of skim milk

zz Lower fat percentage

zz Higher density of milk

zz Higher ratio of SNF: fat

b) Detection of starch

zz Take 3 ml milk in a test tube.

zz Boil the milk sample.

zz Cool it and add a few drop of iodine solution (1%).

zz Appearance of blue colour indicates the presence of starch.

zz Blue colour disappears when the sample is boiled.

c) Detection of cane sugar

zz Take 1 ml milk in test tube.

zz Add 1ml HCl.

zz Add 0.1g resorcinol powder and mix well.

zz Place the test tube in boiling water for 5-10 min.

zz Appearance of red colour indicates the presence of cane sugar in milk sample.

Fluid Milk Processing (Student Handbook for Class XI) 27

 e) Detection of neutralizers

zz Take 1 ml milk in test tube.

zz Add 5 ml alcohol.

zz Add a few drops of rosalic acid solution (1%) and mix well.

zz Appearance of rose red colour indicates the presence of carbonate.

f) Detection of skim milk powder

zz If colour of milk by addition of nitric acid turns light violet it indicates mixing of skim
milk powder otherwise normal milk shows yellow colour.

g) Detection of gelatin

zz Add picric acid and shake well and observe the precipitation which in case is of
yellow colour, indicates mixing good amount of gelatin and if is shows light brown
colour it indicates mixing in very little quantity.

h) Detection of total solids

This adulteration can be determined by following Richmond’s formula:

T.S. =CLR/4 + 1.2F + 0.14

Where,
T.S. = total solids in milk (%)
CLR = correct lactometer reading of milk (20°C)
F = fat in milk (%)

i) Detection of water in milk

The water adulteration in milk can be detected by following tests:

A) Nitrate test

zz Water from ponds, river, wells and hand pump contains nitrate, therefore, if milk is
adulterated with such water this test is positive.

28 Fluid Milk Processing (Student Handbook for Class XI)

zz Treated tap water normally does not contain nitrate.

zz This test is not useful in case milk is adulterated with tap water.

B) Lactometer test

Milk has a specific gravity (1.028 -1.032). When it is adulterated with water, the specific
gravity of milk is shifted from its normal value. The lactometer test is designed to detect
the change in specific gravity of such adulterated milk.

zz Mix the milk sample gently and pour it gently into a measuring cylinder (300-500).

zz Let the Lactometer sink slowly into the milk. Read and record the last Lactometer
degree (ºL) just above the surface of the milk.

zz If the temperature of the milk is different from the calibration temperature (Calibration
temperature may be 20°C) of the lactometer, calculate the temperature correction.

zz For each ºC above the calibration temperature add 0.2ºL; for each ºC below calibration
temperature subtract 0.2 ºL from the recorded lactometer reading.

zz Milk has specific gravity of 1.026 -1.032 and lactometer reading range of 26.0
-32.0ºL.

zz Reading consistently lower than expected indicates the adulterated milk.

C) Freezing Point Determination

Freezing point is the most constant of all measurable properties of milk. Small change in
milk composition (adulteration of milk with water) will shift the freezing point of milk from
its normal values (-0.54ºC). In case milk is adulterated with water the fat and total solids
in milk are reduced causing a rise in freezing point. This can be determined by use of
standardized apparatus Hortvet cryoscope.

l00 (T -T 1)
Water added (%) in milk = ——————
T

T and T1 is freezing point of pure and adulterated milk.

Fluid Milk Processing (Student Handbook for Class XI) 29

 REVIEW QUESTIONS

1. What is the need of testing milk and milk products for quality?

2. How is the adulteration of milk with pond water determined?

3. Define spoilage of milk and milk products. How do saprophytic organisms cause
spoilage of milk and milk products?

4. Enlist the various methods of controlling spoilage causing and pathogenic organisms
in milk and milk products.

5. Enlist the pathogens which are transmitted through milk and milk products.

6. Differentiate between alcohol test and alcohol-allizarine test.

7. Dye reduction test is a good indicator of quality of raw milk – why?

8. Importance of determination of specific gravity of milk.

9. Enlist the various flavour defects observed in milk and milk products.

10. How is the addition of neutralizers in milk detected?

30 Fluid Milk Processing (Student Handbook for Class XI)

Chapter 3

Milk Reception Operations at Dairy

Plant

Objectives
To know the procedure of sampling and testing of milk at dairy reception
dock

Introduction
The quality of incoming milk greatly influences the quality of the processed milk. It is well
known that the initial quality of milk on the receiving dock depends on the production and
handling of milk at the farm. When milk is received at dock it should be clean, sweet,
pleasant flavor, obtained from healthy animal, produced under hygienic conditions, free
from colostrums and other contaminants, promptly cooled (5°C or below) and transported
in refrigerated conditions. Abnormal milks (obtained from diseased animal or altered
composition) and milk exhibiting high developed acidity should be rejected at the dock as
it reduces the heat stability of milk and creates problem during subsequent processing.

Milk may be delivered to the milk plant/dairy in cans or tankers via road or rail (Fig.3.1
to Fig.3.3). The milk in these containers has to be graded, emptied, measured by
weight or volume, sampled and bulked to provide continuity of supply to the subsequent
processes.

Milk Reception
At the dock the milk coming from villages or chilling centers which has been stored in
bulk coolers (Fig.3.4), comes either in cans or insulated tankers are unloaded by manual

Fluid Milk Processing (Student Handbook for Class XI) 31

 Fig.3.1. Milk reception dock Fig.3.2. Stainless steel milk tanker

Fig.3.3. Milk received in cans Fig.3.4. Milk stored in bulk coolers at chilling centers

or mechanical process. In manual process the cans are unloaded to the tipping point,
where the leads/covers are removed and the milk is inspected. They are then tipped
manually and both cans and lids passed on to a can-washer via a drip saver or drain
rack. In case of high throughput required the procedure is mechanized and the cans are
unloaded directly from the truck on to the conveyor (power-driven or by gravity-roller) and
the tipping, sampling and weight-recording may be completely automatic. The milk received
is then weighed and stored in large capacity storage tanks or silos. If the milk is received
from milk chilling centers, it has already been graded, weighed, sampled and cooled. It
may be weighed and sampled again or the Centers report may be used.

Milk reception should be planned and the equipment so chosen that intake operations
are properly carried out. This is especially very important where large volumes of milk are
received. Delays in process cause deterioration of milk, which may increase labour cost and

32 Fluid Milk Processing (Student Handbook for Class XI)

operating cost of the can-washer. The deliveries of milk should follow a schedule. The aim
should be to complete milk reception within 3-4 hours especially in Indian conditions.

Operations
The milk reception operation includes unloading, grading, sampling, weighing and testing
of milk.

Unloading
Once the milk filled cans and tanker enters in to the dairy premises the primary objective
is to unload the cans and tankers to avoid any delay in receiving operation. The milk cans
are assembled in definite order according to each supplier. If a milk tanker is being used,
it is first properly positioned so that connections can be made conveniently.

Grading
This refers to the classification of milk on the basis of quality. Milk of poor grade has to
be rejected. Grading also helps in price fixing of milk. The grading is based on organoleptic
or sensory tests such as smell, taste, colour and appearance, acidity and sediment test.
These are included under platform tests. Platform tests are those tests which are performed
to check the quality of the incoming milk on the receiving platform so as to make a quick
decision regarding acceptance or rejection of milk at the dock.

Sampling
Drawing of accurate and representative sample of milk for subsequent chemical and
bacteriological is very important. While strict precautions regarding sterility of stirrer,
sampler, container etc., are required for obtaining a bacteriological sample, dryness and
cleanliness of the above equipment should suffice for a chemical sample.

The prerequisite of sampling is thorough mixing of the milk with plunger or stirrer, which
can be operated manually or mechanically in the milk in cans or tankers (Fig.3.5). in case
of milk in can a representative sample may also be drawn after quick dumping of the milk
in to the weigh tank, whereby it gets mixed so thoroughly that a representative sample
may be taken without further mixing in labeled sample bottle (Fig.3.6). The sample may be
individual, composite (mixture of two or more individual lots of milk) and drip (representing
the entire days supply). The samplers may be dipper, tube, automatic vacuum or drip. For
chemical analysis many a times samples are preserved using suitable preservatives and

Fluid Milk Processing (Student Handbook for Class XI) 33

 Fig.3.5. Plunger for Fig.3.6. Milk in sample bottle for Fig.3.7. Milk samples for
mixing of milk analysis analysis stored in refrigerator

stored in a refrigerator before the analysis (Fig.3.7). The common preservatives used are
mercuric chloride, formalin and potassium dichromate.

Weighing
This is an essential step in accounting for milk receipts and disposal, making payments for
milk. The milk in-cans are dumped in to the weigh tank either manually or mechanically,
where it get weighed and dumped in to dump tank (Fig.3.8 & Fig.3.9). The milk in tankers
may be measured by volume by passing it through a flowmeter and its measurements are
converted in to weight by multiplying the volume with an agreed density. Another common
alternative to road tankers is to use of weigh-bridge, the tanker being weighed once when
it is full, and again after it has been emptied.

Fig.3.8. Milk weighting tank Fig.3.7. Milk dump tank

34 Fluid Milk Processing (Student Handbook for Class XI)

Testing
The quality of raw milk is tested in a quality control laboratory (Fig.3.10) or dairy dock at
the dairy plant before its acceptance. These tests check the suitability of milk for further
processing and human consumption. The milk is also graded on the basis of certain tests.
The various routine tests employed for raw milk testing are summarized in table 3.1.

Fig.3.10 Quality control laboratory for milk testing

Among all the above tests which are summarized in table 3.1, smelling the milk in can at
the receiving dock and COB, if needed, are routinely used to decide acceptability of the
milk by the dairies in our country.

Clot-on-Boiling test is done by taking 2 ml milk in a test tube and boiling it over a burner
flame. If precipitates or clotted particles appear, the milk is not accepted as it indicates
developed acidity and such milk may clog pasteurizer plates. Certain dairies also fix the
standards for level of acidity, above which the milk may not be acceptable.

The microbiological quality is also routinely tested by MBRT or RRT, standard plate count
and coliform count. However, these tests are not used still for pricing the milk in India.
The MBRT is very popular test in our country, as it has good correlation with the rate of
souring under atmospheric handling that reflects the level of its keeping quality. The test
has also reasonable correlation with the standard plate count at 370C. The test is based
on the principle that when methylene blue in its oxidized from (blue) is added to milk,
gets reduced (colourless) due to metabolic activity of micro-organisms. The time required
for such reduction is related with the number and type of micro-organisms in milk.

Fluid Milk Processing (Student Handbook for Class XI) 35

 Table 3.1 Quality control tests performed for milk at
reception point and dairy plant

Name of test Purpose Significance

Platform Test

Organoleptic test To check taste, smell and physical Indicate acceptability

appearance of the milk. Also or rejection of milk.
Acidity
determine heat stability and pH of Milk which do not
Sediment test
milk to know suitability of milk for comply with the test or
Alizarin alcohol test processing (making market milk and unable to maintain the

10 min. resazurin test dairy products). To observe types of pre-determined level/

organisms present in milk standards have to be
Direct microscopic count (DMC)
rejected

Microbiological quality tests

Methylene blue reduction test (MBRT) To determine extent of microbial Milk having higher
load present in milk indirectly by dye bacterial load or higher
Resazurin reduction test (RRT)
reduction tests or enumerate number in particular types
Total plate count (standard plat count)
of organisms in milk by viable plate of group are either
Coliform count count methods. rejected or processed

Thermoduric count immediately

Also check specific group of

Thermophilic count organisms like coliforms, thermoduric

& thermophilic (heat resistant
Psychrotrophic count
bacteria), psychrotropic (cold loving
Proteolytic count bacteria), protein degrading and fat
Lipolytic count degrading bacteria present in milk

Others

Test for mastitis To know whether the milk is obtained Mastitis milk or milk
from animal suffering from mastitis. containing antibiotics
Test for antibiotic residues
Also to check antibiotics administered are not to be used
to animals are entered in to milk or mixed with normal
or not milk

36 Fluid Milk Processing (Student Handbook for Class XI)

 REVIEW QUESTIONS

1. Which operations are carried out at milk reception dock?

2. What are platform tests? How COB is performed?

3. How milk samples are preserved before their chemical analysis?

4. How will you check microbial load in milk samples?

5. How will you check heat stability of milk?

Fluid Milk Processing (Student Handbook for Class XI) 37

 Chapter 4

Fluid Milk Varieties and Special

Milks

Learning objectives
- To know about various types of fluid milk and their preparation
- To know about special milks and their preparation

Introduction
The term market milk refers to processed milk which is being sold in the market. Market
milk should satisfy the mandatory legal requirements of the nation. Although, India stands
first in milk production, only 16-17% of the milk produced is processed by organized sector.
India is the world’s most significant consumer of fluid milk with annual consumption of 47.1
million tonnes in 2010. The majority of India’s milk production is consumed as fluid milk
rather than processed in other products as is the case in other regions. In India, Food
Safety and Standards Authority of India (FSAAI) prescribe various standards to processed
milk which is intended to sell in India. These standards are compulsory standards and
all manufacturers in India must follow this. The FSSAI has been established under Food
Safety and Standards Act, 2006. It is the manufacturers’ responsibility to declare the type of
milk, nutritional value, manufacturing date, ‘use before’ date, storage prescription, maximum
retail price (MRP) and customer care number on the package of all food commodities.
According to FSSAI, the following types of fluid milk can be packed and marketed in India,
with their minimum fat and solids-not-fat (SNF) level as mentioned against each type.

Apart from the above mentioned types (Table 4.1), there are three more categories such

38 Fluid Milk Processing (Student Handbook for Class XI)

as cow milk (3.5% fat and 8.5% SNF) buffalo milk (6.0% fat and 9.0%SNF) and mixed
milk (4.5% fat and 8.5% SNF), however some relaxation in fat level is permitted in some
states (minimum 3.0% fat in case of cow milk in Mizoram and Orissa and to minimum
5.0 % fat in case of buffalo milk in several states).
Table 4.1. Different grades of milk

Sl. No. Type Fat (%) SNF (%)

1 Toned Milk (TM) Minimum 3.0 Minimum 8.5

2 Double-toned milk (DTM) Minimum 1.5 Minimum 9.0

3 Standardized milk (STDM) Minimum 4.5 Minimum 8.5

4 Skimmed milk (SM) Not more than 0.5 Minimum 8.7

5 Full cream milk (FCM) Minimum 6.0 Minimum 9.0

Processing of Raw Milk to Prepare TM/DTM/STDM/SM/FCM

Processing of raw milk includes several operations to be given to it, in order to make it
safe for human consumption and to meet the legal requirements. Processing also improves
the physico-chemical and microbiological quality of the product. These operations include
filtration, separation, standardization, homogenization, pasteurization and sterilization.
Most of these operations are synchronized with the HTST pasteurizer, thereby facilitating
a continuous process operation.

In India milk production is scattered and mostly restricted to small and medium scale
farmers who are holding one or few animals. Raw milk (unprocessed milk) is collected
from these farmers (also referred as milk producers) by village cooperative society
or private entrepreneurs. Milk is then pooled and chilled at chilling centre or at main
processing plant, whichever is closer. Chilled milk is then stored in the raw milk silo for
further processing.

In the processing plant, raw milk is pumped from raw milk silo to float controlled balance
tank (FCBT) of pasteurizer unit using stainless steel pump of suitable capacity. Milk then
passes to regenerative heating section-I of pasteurizer, wherein milk is heated from 5°C
to 40°C. At this temperature, milk is routed to the filter assembly for filtration. Filtration
is carried out using duplex filter unit, in which one filter is usually kept as standby to
facilitate continuous processing during cleaning operations. After filtration, milk passes into

Fluid Milk Processing (Student Handbook for Class XI) 39

 the cream separator. Several types of separators are available, among which, those with
online standardization units help to standardize the milk to desired fat level. Otherwise, milk
needs to be standardized to desired fat and SNF level before taken into HTST pasteurizer
for production of toned, double toned milk etc. After separation and/or standardization, milk
moves back to pasteurizer for further heating. Milk is then heated from 40°C to 65°C in
regenerative heating section-II and then taken out for homogenization. Homogenization is
suitable for high fat milk, wherein milk fat size reduces to less than two micron size and
thus reduces the cream plug formation during the storage. Cream plug formation occurs
due to the difference in density of fat and SNF. Homogenization also brightens the colour
of milk due to more scattering of light and reduces the curd tension, thereby improving
the digestibility of milk. After homogenization, milk is again moved back to pasteurizer
for further heating. Milk is heated to final pasteurization temperature of 72°C or higher in
heating section using hot water as heating medium. After heating section, milk moves to
the holding section, wherein it is held at temperature not less than 72°C for 15 seconds.
At the end of the holding section, flow diversion valve (FDV) is fixed to verify whether
each and every particle has reached the pasteurization temperature or not. If any part
of milk has not attained the pasteurization
temperature, FDV diverts that part of milk to
FCBT for reprocessing, thus ensuring the safety
of pasteurized milk. Properly heat treated milk
moves back to regenerative cooling section-II
and then regenerative cooling section-I and
finally chilled to less than 5°C in chilling section.
Chilled water is used as cooling medium
in chilling section. Minimum back pressure
is necessary in heating section to prevent
boiling of milk. It should be higher than the
pressure of heating medium or chilling medium
in pleat heat exchanger to avoid any risk of
cross contamination. Chilled milk is stored
in pasteurized milk silos, from where it is
transferred to packaging section after ensuring
its safety and quality parameters.

The production flow chart that may be used NOTE: * Homogenization is optional for low fat
products like DTM/TM and applicable in high fat
for pasteurized toned, double toned and milk like STDM/FCM

standardized milk is given in Fig.4.1. Fig.4.1. Flow chart for the production of toned,
double toned and standardized milk

40 Fluid Milk Processing (Student Handbook for Class XI)

Standardization of fat and SNF is the only difference among the different types of market
milk. During standardization, the milk fat is removed if it is excess than desired and added
in the form of cream to milk, if it is less. Similarly SNF is also standardized to desired
level by adding skim milk or skim milk powder (SMP) in case of shortage. Excess SNF
can be adjusted by adding milk of low SNF than is desired.

Production of Reconstituted Milk

The term reconstituted milk is used when milk is prepared from the dry powder or milk
powder. It may be prepared from either SMP and/or whole milk powder (WMP).

There are no preset standards for reconstituted milk. However, it should meet the other
appropriate general standards of SM/TM/DTM/STDM after reconstitution. The schematic
diagram of reconstitution process is shown in Fig.4.2.

Fig.4.2. Schematic diagram of reconstitution process

The milk powder is dissolved in lukewarm water at 40°C in a double jacketed vat equipped
with an agitator for uniform mixing of the contents. The heating medium circulated in the
jacket is usually hot water. Dry powder is added through venturi system. Water being a
major raw material for reconstituted milk, the water quality which is used for dispersion
of milk powder is important and should be checked routinely for its chemical and physical

Fluid Milk Processing (Student Handbook for Class XI) 41

 quality parameters. It must also be free from harmful microorganisms. Soft water is
preferred because high mineral content in water disturbs the colloidal stability of milk at high
temperature processing during later stages. After addition of powder into water, contents
are recirculated and agitated till complete dissolution of powder. The complete hydration
of milk powder is very much essential in order to prevent chalkiness (a defect) in the
finished product. The hydration time varies with the powder quality. Good quality powder
must hydrate completely within 30 minutes. After complete hydration, reconstituted milk
is transferred to float controlled balance tank of the pasteurizer for heat processing. The
pasteurized reconstituted milk is stored in pasteurized milk storage tank till its packaging.
At commercial level, reconstituted milk is used only for standardization purpose.

Production of Recombined Milk

According to FSSR-2011, recombined milk is defined as the homogenized product prepared
from milk fat, non-fat-milk solids and water. The recombined milk shall be pasteurised
and shall show a negative phosphatase test. It should have minimum 3.0% fat and 8.5%
SNF.

The scarcity of fresh milk in lean season is the reason for production of recombined and
reconstituted milk so as to meet the market demand. This also helps to utilize the milk
solids that get converted and stored in the form of SMP, WMP, butter and butteroil due
to excess collection of milk during the flush season. Recombined milk can be produced
in hilly areas and other places where there is limited availability of raw milk.

The milk fat sources for recombined milk are generally butter oil or anhydrous fat and/or
unsalted butter. Therefore, homogenization is an essential step in preparing recombined
milk since fat source is added into the water. The solids-not-fat source dissolves easily in
water, but the fat source needs homogenization to disperse in the water. Homogenization
helps in creating an oil-in-water emulsion that is native to milk. The schematic diagram
of reconstitution process is shown in Fig.4.3. Homogenization is done at 65°C in a two
stage homogenizer at a pressure of 150 kg/cm2 and 50 kg/cm2 in first and second stage
respectively. Therefore milk is heated to 65°C in the HTST pasteurizer and routed from the
regeneration section out into the homogenizer at this temperature. After homogenization,
milk is again goes back to HTST pasteurizer for completing the pasteurization.

42 Fluid Milk Processing (Student Handbook for Class XI)

 Fig.4.3. Schematic line diagram of recombined milk processing

Activities Suggested
1. Visit your local market and perform the followings.

a. collect the various types of fluid milks available

b. Note down the colour of the pouch used for different fluid milk varieties

c. Note down the prices per 500ml of each fluid milk varients

d. Label declarations on each pouch including fat and SNF level

2. Visit local dairy plant and see the process lines and manufacturing process

REVIEW QUESTIONS

1. What are the FSSR-2011 standards suggested for different fluid milks?

2. Which milk contains highest total solids?

3. Write briefly on the process of preparation of recombined milk

Fluid Milk Processing (Student Handbook for Class XI) 43

 4. Write the differences between recombined and reconstituted milk

5. Write the schematic diagram of reconstitution process and explain

Special Milks
Learning Objectives
- To know about the manufacturing process of various special milks
- To know about the various ingredients used various special milks

a. Flavoured Milk
It is a special type of milk produced to attract the consumers, particularly children.
Flavoured milk generally contains sugar, permitted food grade flavors and colours. The
product is heat treated in order to ensure its safety to consumers and also to extend the
keeping quality. Flavoured milk is marketed as pasteurized flavoured milk packaged in low
density polyethylene (LDPE) pouches, ultra-high temperature (UHT) treated flavoured milk
packaged aseptically in a multi-layer laminates and sterilized flavoured milk packaged in
glass or retortable plastic bottles. Pasteurized flavoured milk has a limited shelf-life of two
days at refrigerated temperature, whereas UHT treated and sterilized flavoured milk will
stay safe for 4-6 months at ambient temperature. Sugar, suitable colour and flavours are
added to increase the flavor and appearance of the product. The commercial products
are available in a variety of flavours such as rose, elaichi (cardamom), badam (almond),
strawberry or butterscotch.

FSSR-2011, prescribes the standards to flavoured milk in terms of fat and SNF. Flavoured
milk shall meet the minimum fat and SNF level of double toned milk/ toned milk/
standardized milk. Therefore, manufacturer has to clearly declare the type of milk on its
label. Flavoured milk may also contain nuts, coffee, chocolate in addition to cane sugar.
Sugar is added at the rate of 7-8% based on the flavor. Chocolate and coffee flavoured
milk requires slightly higher amount of sugar for better palatability. Only permitted and
food grade colour and flavours should be used, the level varying with the type of flavor.
Flavour is usually dosed at the rate of 0.1 to 0.2% level and maximum colour addition is
100 parts per million (ppm) for artificial colours. Natural colours like â-carotene is added
as per requirement. Following are the various artificial colour compounds used to obtain
different colours in flavoured milk (Table 4.2).

44 Fluid Milk Processing (Student Handbook for Class XI)

 Table 4.2. Artificial colours used in flavoured miles

S. No. Colour desired Colour compound

1 Yellow Tartarzine

2 Red Ponceau 4 R

3 Orange Sunset Yellow

4 Blue Brilliant Blue

5 Green Tartarzine + Brilliant Blue

6 Pink Carmosine

Pasteurized flavoured milk involves treatment like filteration, homogenization, standardization,

pasteurization and packaging similar to general market milk. Milk is standardized to higher
fat and SNF levels so that when sugar and other ingredients are added later fat and
SNF level meets the desired level. Standardized milk is then homogenized in two stage
homogenizer to avoid cream plug or cream layer formation during the storage of the
product. After, mixing with other ingredients like sugar, flavor and colour, the milk is filtered
and pasteurized at slightly higher temperature than the market milk. The pasteurization
treatment is usually at 76-78°C/ 15 seconds, with immediate cooling thereafter to 4°C.
The pasteurized flavoured milk is then packaged in the form-fill-seal (FFS) machine in the
form of 200 ml or 500 ml pouches and stored at refrigerated temperature. The detailed
process flow diagram is given in Fig. 4.4.

Sterilized flavoured milk is prepared to extend the shelf life of the flavoured milk, steps
are similar to pasteurized flavoured milk till filtration. Thereafter, flavoured milk is filled into
the bottles of 200 ml capacity and capped. These bottles are placed on the trolley of the
retort sterilizer and sterilized at temperature of 117-118°C for 20 minutes. Steam is used
as the heating medium for the retort sterilizer. After sterilization, bottles are allowed to
cool to room temperature, labeled and packed in secondary package (cardboard trays) for
distribution and transportation. UHT treated flavoured milk is also prepared in the same
manner, except here the product is expose to high temperature like 135 to 140°C for few
seconds and packaged aseptically in a multi-layered cartons.

Fluid Milk Processing (Student Handbook for Class XI) 45

 Fig.4.4. Process flow diagram of flavoured milk

b. Fortified Milk
The term ‘fortification’ means the addition of nutrients at levels higher than those found
in the original food. Prevalence of vitamin A deficiency in India is above than the cut off
level suggested by Food and Agricultural organization (FAO). Similarly, population who are
suffering from other micronutrient deficiency (Iron, Calcium and Vitamin D) are also at higher
percentage in India. Therefore, fortification is adopted as part of public health measures
targeted at addressing specific micronutrient inadequacies. Milk can be considered as
good vehicle for fortification because of its availability and ability to reach large population

46 Fluid Milk Processing (Student Handbook for Class XI)

at affordable price. Generally, milk is fortified with vitamin A, vitamin D, calcium and iron.
Vitamin A is essential for better vision and vitamin D is necessary for calcium absorption
in our body. Calcium supports the bone health while iron fortification helps in reducing
the risk of anaemia. Generally, vitamin A is fortified at the rate of 2000IU and vitamin D
is fortified at the rate of 400IU per liter of milk respectively. Many dairy co-operatives and
private manufacturer do have the practice of fortification of milk with Vitamin A. Calcium
fortified milk is also available in market with additional calcium. Iron fortification to milk is
a challenging area because it affects sensorial parameters of milk and research work is
under progress at National Dairy Research Institute, Karnal to explore the possibility of
iron fortified milk.

Activities Suggested
1. Visit local market and collect the following information
a. Various flavoured milks available with their brand name
b. Colour used for each flavor
c. Nutritional information and ingredients listed in their label

2. Visit dairy plant near to you and see the production lines of flavoured milk

REVIEW QUESTIONS

1. Explain with the flow diagram the process of sterilized flavoured milk
manufacturing

2. What are the different flavours that are commonly used in flavoured mik
preparation

3. List various food grade colour chemicals and their respective colours

4. What do you understand by the term fortified milk and explain briefly the uses of
this?

Fluid Milk Processing (Student Handbook for Class XI) 47

 Chapter 5

Cream Separation

Introduction
To learn about cream separation
and centrifugal separators

Principle
Before the invention
of separators, cream was
separated by keeping milk in a
vessel until the cream floated to
the top and could be skimmed
off by hand. This method was
time consuming, caused souring
of milk, allowed partial removal
of cream and thus was not
suitable for industrial application.
Therefore centrifugal separators
are used for cream separation
(Fig.5.1).

Centrifugation separation is
based on Stoke’s law which
relates various factors for
determining particle terminal
Fig.5.1. Centrifugal cream separator

48 Fluid Milk Processing (Student Handbook for Class XI)

velocity. In case of sedimentation, terminal velocity is also known as settling or sedimentation
velocity. The Stoke’s law is defined by the following equation:

———— equ (i)

Where,
= Particle terminal velocity
[Particle will move vertically downwards if upwards if ]
= Density of particle (kg/m3)
= Density of fluid (kg/m3)
= Fluid viscosity (Ns/m2)
R = Radius of the particle (m)
g = Acceleration due to gravity (m/s2)

In case of fat separation, Stoke’s equation is given as:

———— equ (ii)

= Fat separation velocity

= Density of milk plasma (kg/m3)
= Density of fat globule (kg/m3)
= Milk plasma viscosity (Ns/m2)
R = Radius of the fat globule (m)
g = Acceleration due to gravity (m/s2)

if d = fat globule diameter then equ (ii) can be rewritten as:

———— equ (iii)

Or,

———— equ (iv)

Fluid Milk Processing (Student Handbook for Class XI) 49

 It can be inferred that velocity of fat separation will increases with:

zz Increasing fat globule diameter

zz Increasing difference between milk plasma and fat density

zz Decreasing milk plasma viscosity

For centrifugal separation of fat globules, the separation velocity is given by the
equation:

———— equ (v)

Where,
R = Distance between fat globule and axis of rotation of centrifuge
= Angular velocity

Centrifugal separators are widely used in the dairy industry. Cream separators work on
the principle of centrifugal separation. Some of the applications of centrifugal separation
in dairy processing units are as follows:
zz Clarification: Removal of impurities and foreign objects like straw, curd particles etc
from raw milk
zz Skimming: Removal of cream from milk
zz Standardization: Adjusting the fat content of milk and milk products to desired level
zz Bactofugation: Removal of bacteria from milk
zz To recover fat from buttermilk or whey
zz Whey clarification
zz Quarg separation

Equipment
The core component of centrifugal separator is conical discs which are stacked one over
the other. There are about 120 or more conical discs having an angle of 45 to 60 degrees.
The discs are separated by separation channels. The gap between two consecutive disc
is in the rage of 0.5 to 2.0 mm. The milk enters at the base of disc stack inside the
rapidly revolving bowl of the separator. The speed of rotation of separator bowl can vary
from 2000 rpm to 20,000 rpm depending on the capacity. The discs are provided with

50 Fluid Milk Processing (Student Handbook for Class XI)

distribution holes (Fig.5.2). The distribution holes are positioned one above the other
making channel for the ascending liquid (Fig.5.3). Due to the centrifugal force the light
phase moves towards the axis of rotation and the heavy phase towards the bowl wall
(Fig.5.4). The milk plasma has higher specific gravity (1.036) than that of fat (0.9). During
centrifugation, plasma is thrown towards the outer periphery and channelized into skim
milk out let. The lighter phase i.e. cream is channelized towards the axis and is pumped
out through cream outlet by pairing disc (Fig.5.5). Paring disc is used to pump cream or
skim milk towards the outlet.

Fig.5.2. Disc of cream separator

Fig. 5.3. Stacking of discs

Fluid Milk Processing (Student Handbook for Class XI) 51

 Fig.5.4. Separation of milk plasma and fat in a disc bowl centrifuge

Fig.5.5. Centrifugal cream separator

The capacity of the cream separator is in terms of skimming and standardization. During
skimming entire cream from the whole milk is separated by the cream separation. The fat
content in skimmed milk may be as low as 0.04-0.05%. In standardization the operating
parameters of cream separator is set to obtain milk of a desired fat content. Since more
energy is required for skimming, therefore skimming capacity is always less than the
standardization capacity. Cream separators can be operated at a lower capacity than rated
capacity by lowering the rpm.

52 Fluid Milk Processing (Student Handbook for Class XI)

Classification of Cream Separator

According to Temperature
zz Warm separator: The optimum milk temperature for fat separation is 52 to 55oC, as
the dynamic viscosity of the milk is virtually stable in this temperature range. Higher
temperature above this range will cause protein precipitation, which is undesirable.

zz Cold separator: For cold separation operating temperature is between 4 and 20 °C.
Cold separation method has lower energy consumption and prevents growth of
microorganism.

According to Milk Feed

zz Top feed: Provision of whole milk inlet is from the top of the centrifuge bowl
(Fig.5.6).

zz Bottom feed: Milk is fed at the bottom of bowl through the hollow spindle
(Fig.5.7).

Fig.5.6. Top feed cream separator Fig.5.7. Bottom feed cream separator

Fluid Milk Processing (Student Handbook for Class XI) 53

 Efficiency
The performance of a cream separator is evaluated by the degree of cream separation
from the whole milk. Degree of cream separation (E) is given as:

Suggested Reading
http://creamseparatorgallery.webs.com/articles.htm

REVIEW QUESTIONS

1. What is the primary purpose of cream separation?

2. Briefly describe principle of cream separation?

3. With a neat diagram explain working of cream separator.

4. Give classification of cream separator

54 Fluid Milk Processing (Student Handbook for Class XI)

Chapter 6

Clarification

Objective
To improve physical quality of milk by clarification. Removal of extraneous
matters presenting milk improves overall acceptability.

Introduction
Filtration is for removal of material lighter than milk such as wood, cellulose, packaging
material residue etc., whereas clarification is done to remove components heavier than
milk. Milk clarification is the process of removing undesirable foreign matter such as dirt,
curd particles, blood corpuscles, epithelial cells, bacteria sediment, sludge etc from the
milk. To some extent bacteria also get removed as slime during the clarification process.
Clarification cannot be considered an effective means of bacteria removal.

Principle and Methods

Clarification of milk is done by the principle of centrifugal separation. The principle can
be understood by a simple experiment. Take water in cylindrical container (Fig.6.1a). Add
sand to water until the water becomes turbid (Fig.6.1b). The container is then rotated
about its axis at a very high rpm (Fig.6.1c). Due to centrifugal force, heavier sand particles
separate out towards the periphery of container (Fig.6.1d). Same principle is applied for
clarification of milk. For clarification milk is passed through a centrifugal clarifier.

Fluid Milk Processing (Student Handbook for Class XI) 55

 Fig.6.1. Experiment to demonstrate centrifugal separation

Equipment
The clarifier consists of conical discs stacked over each other which rotate inside the
clarifier bowl. Milk is introduced into the separation channels at the outer edge of the
disc stack, flows radially inwards through the channels towards the axis of rotation and
leaves through the outlet at the top (Fig.6.2). Particles, which are more dense than the
continuous milk phase, are thrown back to the perimeter. The sludge gets collected in the
space around the disc and milk being lighter moves up towards the outlet. The amount of
solids that collect will vary, however, it must be manually removed from the centrifuge at

56 Fluid Milk Processing (Student Handbook for Class XI)

regular intervals. From the studies it has been established that warm clarification of milk,
e. g. at 50 to 55°C is preferred to cold clarification.

Modern clarifiers are of self-cleaning type which allows for continuous operation (Fig.
6.3a). The clarifier bowl has discharge ports which open up periodically. These discharge
ports remain closed under pressure. Release of pressure opens the port and sludge is
evacuated from the space (Fig.6.3b). Such desludging results in about 0.05-0.10% of milk
being lost.

Fig.6.2. Clarifier

Fig. 6.3 Self desludging clarifier

Fluid Milk Processing (Student Handbook for Class XI) 57

 REVIEW QUESTIONS

1. What is the primary purpose of clarification?

2. Briefly describe principle of clarification?

3. With a neat diagram explain working of clarifier.

4. With a neat diagram explain working of self clearing clarifier

58 Fluid Milk Processing (Student Handbook for Class XI)

Chapter 7

Milk Pasteurization

Objective
Pasteurization is a heat treatment to destroy all pathogenic microorganisms
present in the milk. This process makes milk safe to consume. This chapter
deals with pasteurization process and equipment for pasteurization.

Introduction
The process of pasteurization was developed by French microbiologist Louis Pasteur in
1862. This process was used by Louis to prevent souring in the fermented beverages.
This results in better shelf-life of the beverages.

Pasteurization is a thermal process of heating milk to a certain temperature, holding at

that temperature for a specified period of time followed by rapid cooling. The purpose of
pasteurization is to destroy pathogenic or disease causing microorganism. According to
WHO definition “Pasteurization is a heat treatment aimed to reduce the number of harmful
microorganisms in milk and cream to a level at which they do not constitute a significant
health hazard”. It is intended to result in an extended shelf-life of milk and in only minimal
chemical, physical and organoleptic changes. Pasteurization conditions are designed to
effectively destroy the organism Mycobacterium tuberculosis. Pasteurization of milk and
cream results in a negative phosphatase reaction.

Difference Between Pasteurization and Sterilization

Pasteurization: Thermal process for destruction of pathogenic microorganisms

Fluid Milk Processing (Student Handbook for Class XI) 59

 Sterilization: Sterilization is a more severe heat treatment designed to destroy all
contaminating bacteria. It is generally done at high temperature than pasteurization.

Milk produced from healthy milch animals has low microbial count. During handling and
transportation, milk may be contaminated by various microorganisms. Milk is good medium
for growth of bacteria and other pathogenic microorganism because of its excellent nutrient
profile (Table 7.1). Therefore milk must be thermally treated to make it safe for human
consumption. Perishable nature of milk is due to the following factors:

a. Nutritional profile of milk

b. Warm temperatures due to tropical climate in India

c. Substantial contamination with microorganisms during milking, handling and

transportation. Microorganism which can affect the storing quality of milk are:

zz Bacteria

zz Molds

zz Yeasts

zz Viruses

d. Prolonged time before cooling or processing. Therefore milk should be cooled to 4o

within 2 hours after milking.

Pasteurization of milk is required before packaging, distribution and marketing due to

following reasons:

a. Health issues: Pasteurization as a thermal treatment makes milk safe for human
consumption by destroying of pathogenic microorganism.

b. Food safety regulations: Food safety agency of many countries have made
pasteurization compulsory for packaged milk (either in pouches or bottles).

c. To increase the keeping quality of milk: Shelf life of milk can be improved by
destroying spoilage microorganism. Thermal treatment also deactivates enzymes
like lipase present in milk, which causes fat breakdown in milk. Pasteurized milk is
safe to drink for up to 5-20 days depending on the storage temperature. Shelf life of
around 20 days can be achieved if pasteurized milk is continually refrigerated below
5oC. Pasteurization improves the shelf life of milk and allow processed milk to be
distributed and marketed to far off distances.

60 Fluid Milk Processing (Student Handbook for Class XI)

 Table 7.1. Average composition of milk

Constituents per 100 g

Cow Milk Buffalo Milk

Water 86.6 84.2

Fat 4.6 6.5

Protein 3.4 3.9

Lactose (carbohydrate) 4.9 5.2

Ash 0.6 0.8

Pasteurization process may vary from country to country depending upon the regulations.
Effective pasteurization of milk depends on the reliability of the time-temperature-pressure
relationships. Microorganisms exhibit a range of temperature over which it can grow.
Microorganisms can sustain certain maximum temperature but further rise in temperature
will cause gradual destruction of microorganisms. The microbial destruction kinetics depends
on the time-temperature combination employed for pasteurization.

Time-Temperature Combination for Pasteurization

Time temperature combination is very critical for the pasteurization process. The effect of time
and temperature combination is different against the bacteria (Table 7.2). High temperature
and prolonged heating is not recommended because of the following reasons:

zz High temperature and prolonged heating causes cooked flavor which is undesirable
to the consumers.

zz Formation of creamline is undesirable.

zz Physical and chemical changes should be minimum.

Milk pasteurization process was designed to provide a minimum temperature and time
combination needed to inactivate the most heat-resistant, non-spore-forming, pathogenic
microorganism. Initially the target organism was Mycobacterium bovis or M. tuberculosis
responsible for tuberculosis. In early 1950s, the minimum pasteurization temperature was
increased to destroy a slightly more heat-resistant organism Coxiella burnetti, responsible
for Q-fever. Few regulatory agencies are planning to raise the required temperature of

Fluid Milk Processing (Student Handbook for Class XI) 61

 pasteurization to that now used in ultra-pasteurization for safeguard from more heat
resistant bacteria. In case it happens in future, ultra pasteurization will be redefined as
pasteurization.

To assess the adequacy of heat treatment during pasteurization, inactivation of Alkaline

Phosphatase (ALP) enzyme is used as an indicator. The heat stability of ALP is greater
than that of pathogens, the enzyme serves as an indicator of product safety.

Table 7.2. Time temperature combination for pasteurization and sterilization

Temperature Time Pasteurization Type

63°C 30 minutes Batch Pasteurization

72°C 15 seconds High Temperature Short Time Pasteurization (HTST)

89°C 1.0 second Ultra Pasteurization (UP)

90°C 0.5 seconds Ultra Pasteurization (UP)

94°C 0.1 seconds Ultra Pasteurization (UP)

96°C 0.05 seconds Ultra Pasteurization (UP)

100°C 0.01 seconds Ultra Pasteurization (UP)

138°C 2.0 seconds Ultra-high temperature (UHT) Sterilization

Methods

a. Batch Method (LTLT)

In Low Temperature Long Time (LTLT) method, milk is heated to 63°C in a jacketed tank or
vat with agitator. Milk in the vat is heated using hot water or steam. At 63°C temperature,
milk is held for 30 minutes and is allowed to cool. This method is used for processing
milk upto 5000 liters.

b. Continuous (HTST)
High Temperature-Short Time (HTST) pasteurization is the most widely used method for
commercial processing of milk. Milk is heated to a temperature of at least 72°C and is
kept at that temperature for not less than 15 seconds. Milk is then immediately cooled to
a temperature of less than 4°C.

62 Fluid Milk Processing (Student Handbook for Class XI)

Equipment
HTST pasteurization involves heating of milk at least 72oC and holding it for 15 sec. Plate
heat exchanger (PHE) are used for milk pasteurization. PHE uses stainless steel plates
(SS-316) to transfer heat between milk and heating/cooling medium.

In PHE, fluids are exposed to a much larger surface area over the plates resulting in
higher heat transfer compared to conventional heat exchangers. It has a very high heat
transfer coefficient on both sides of plates and are cost effective, easy to maintain and
compact in design.

Plate Heat Exchangers

Stainless steel plates (Fig.7.2) are used in a plate heat exchanger to transfer heat between
two fluids. Gap between the plates are 3-6 mm wide through which hot and cold fluids are

Fig.7.1. Plate heat exchanger

Fluid Milk Processing (Student Handbook for Class XI) 63

 flown alternately (Fig.7.3). Plates of the heat exchanger provide a large surface area for
heat transfer and are corrugated to cause turbulence in the flow. The fluids spread over
the plates in the form of thin film. Due to large surface area and turbulence in flow, the
rate of heat transfer is very high compared to other heat exchangers. Heat transfer from
the hot to cold fluid is rapid and there is a quick rise in temperature of cold fluid.

Fig.7.2. Stainless steel plate used in plate heat exchanger (Source: Wikipedia)

Fig.7.3. Flow arrangement of hot and cold fluid in a plate heat exchanger (Source: Wikipedia)

64 Fluid Milk Processing (Student Handbook for Class XI)

HTST Pasteurizer
Pasteurizer is the most important equipment for fluid milk processing (Fig.7.4). The different
sections and components in HTST pasteurizer are shown in Fig.7.5. There are four main
sections in a HTST pasteurizer: a. Regeneration, b. heating, c. holding d. chilling.

Fig.7.4. HTST pasteurizer

Fig.7.5. Components of a HTST pasteurizer

Fluid Milk Processing (Student Handbook for Class XI) 65

 Construction of HTST Pasteurizer

i. Regenerative Heating
Raw milk at 4°C in the constant level balance tank (Fig.7.6) is pumped into the regeneration
section of pasteurizer. Incoming milk is pre-heated to approximately 55 to 68°C by heat
given up by hot pasteurized milk flowing in a counter current direction on the opposite
sides of the plates. A positive displacement timing pump delivers milk to the rest of the
HTST system under positive pressure.

Fig.7.6. Milk balance tank and feed pump

66 Fluid Milk Processing (Student Handbook for Class XI)

ii. Heating
In heating section raw milk is heated upto 72 °C by hot water (flow rate : 1:1.2 times of
milk) on opposite sides of the plates.

iii. Holding
Milk is kept at 72°C for 15 sec, this is known as holding. Holding of milk can be in a
chamber in the heat exchanger or in an external tube. External tube holding is generally
preferred for milk pasteurization. Milk is flown through this holding tube (Fig.7.7) and is
kept for at least 15 sec. The maximum velocity of milk in the holding tube depends on:

a. The speed/flowrate of the timing pump

b. Length and diameter of the holding tube

c. Friction factor of the internal surface of tube

Fig.7.7. Holding tubes

Fluid Milk Processing (Student Handbook for Class XI) 67

 iv. Flow Diversion Valve (FDV)
A temperature sensor is located at the end of the holding tube and before the flow diversion
valve (FDV). A FDV is shown in Fig.7.8. The temperature sensor sends the signal to
the recorder-controller. The temperature of the milk is continuously recorded on a chart
recorder by a pen plotter (Fig.7.9).

Fig.7.8. Flow diversion valve (FDV)

The FDV is in diverted flow position if the milk temperature is <72oC. If heat treatment is
not adequate for pasteurization and milk temperature is <72oC, milk returns to the balance
tank through divert line. The FDV switches to forward flow position if the milk temperature
is more than preset cut-in temperature (>72°C).

v. Regenerative Cooling
Milk flows to the regeneration section where it is cooled at the temperature between
32 to 10oC by the incoming cold milk.

vi. Chilling
The warm milk is cooled to 4oC or below by chilled water (flow rate: 1:2 times of milk).

68 Fluid Milk Processing (Student Handbook for Class XI)

 Fig.7.9. Chart recorder for temperature

vii. Vacuum Breaker

After chilling, pasteurized milk passes through a vacuum breaker which is at least 30 cm
above the highest milk line in the HTST system and then to Pasteurized Milk Storage
Tank (PMST).

viii. Pressure Differential Switch

The pressure difference between the incoming raw milk line and pasteurized milk line
(at regeneration exit) is continuously monitored by pressure differential switch. A chart
recorder is provided to record pressure difference. Pasteurized milk is kept at a pressure
of 1 psi (pound per square inch) more than raw milk in regeneration section. This is done
to prevent contamination of pasteurized milk with raw milk if any pin hole leak develops
in thin plates. The pressure difference is maintained by:

a. For simple system: using timing pump

b. For complex system: differential pressure controllers and back pressure flow regulators
at the chilling section outlet.

Regeneration Efficiency
The heating and the cooling regeneration features of the HTST unit offer important

Fluid Milk Processing (Student Handbook for Class XI) 69

 efficiencies by conserving heat, refrigeration, space, and time requirements. In regeneration
section of the pasteurizer the incoming chilled raw milk at 4oC is heated by the outgoing
pasteurized milk at 72oC. The advantages of regeneration are:

zz Requirement of heat energy or steam gets reduced for heating of raw chilled milk.

zz After pasteurization milk has to be chilled to 4oC for packaging. Pasteurized hot milk
at 72oC is cooled first by raw milk and is further cooled to 4oC by chilled water. Thus
the process of regeneration reduces the required of chilled water.

zz Regeneration is done using plate heat exchanger (PHE). Another major advantage
of using PHE for pasteurization is that heating, cooling and regeneration section are
formed by segmenting plates of the PHE. Thus, such an arrangement eliminates the
requirement of different heat exchangers and saves space.

Regeneration efficiency is a measure of heat recovered from the pasteurized hot milk by
the incoming raw cold milk. It is calculated as:

Regeneration efficiency

Tor = Temperature of heated raw milk at regeneration outlet

Th = Temperature of hot pasteurized milk

Tc = Temperature of incoming raw chilled milk

Numerical: According to the data provided in fig 7.5, calculate the regeneration efficiency
of the pasteurizer.

70 Fluid Milk Processing (Student Handbook for Class XI)

Phosphatase Test
Phosphatase test is used to determine the effectiveness of the pasteurization process.
Alkaline phosphatase is a heat sensitive enzyme present in raw milk and is inactivated
by pasteurization. The destruction kinetics of phosphatase enzyme and Mycobacteriun
tuberculosis are almost the same. If any time-temperature combination results in inactivation
of phosphate enzyme, it also indicates that the most heat resistant pathogen Mycobacteriun
tuberculosis present in milk has been destroyed. Active phosphatase will be present in
larger amounts than found in properly pasteurized milk if:
zz Milk is contaminated with raw product.
zz Milk is not held at the proper temperature (72oC) for sufficient time (15 s).

If the concentration of phophatase enzyme is more than 4 ìg/litres in pasteurized milk, it

means that milk is not pasteurized properly. The possible causes for positive phophatase
test are:
1. Faulty operation of FDV
2. Pasteurization temperature is not set at 71.7oC or above.
3. Cracks, pinholes on the plates of the heat exchanger.
4. Leaks in the valve seat of FDV.
5. Heated milk is not held for 15 s in the holding tubes.

REVIEW QUESTIONS

1. What is the purpose of pasteurization?

2. Briefly describe principle of pasteurization.

3. Briefly explain time temperature combination for pasteurization.

4. What is regeneration efficiency?

5. What is the purpose of holding tubes?

6. What is the function of flow diversion valve?

7. Write about phosphatise test.

Fluid Milk Processing (Student Handbook for Class XI) 71

 Chapter 8

Milk Homogenization

Objective
This chapter gives an overview of milk homogenization, principles and
equipments.

Introduction
Homogenization is a mechanical process of disintegration of particles into small size.
Milk homogenization is the process of breaking of larger fat globule into smaller size to
prevent fat separation and making a permanent emulsion of milk fat and serum. During
homogenization, breakup of the fat globules is done to such extent that after 48 hours of
quiescent storage no visible cream separation occurs. The average fat globule size is in
the range of 2 to 6 µm and few may be of 10 µm. The critical diameter for fat globule
agglomeration is 0.7 µm. Fat globule-milk serum emulsion is stable when the size of fat
globule is less than 0.7 µm. Advantages of homogenization are:

a. Small fat globule size leading to no cream line formation

b. Homogenized milk is more whiter in colour

c. Prevents fat oxidation

d. Produces better quality powder

e. Better mouth feel, milk has full bodied flavour

f. Improves digestion. Homogenized milk produces a soft curd which is easily digested
by infants

72 Fluid Milk Processing (Student Handbook for Class XI)

Homogenization may be required for dairy products and beverages such as: cream,
pasteurized milk, flavoured milk, milk for yogurt and cheese, dressings, ice cream mix,
yogurt drinks, and for other products like fruit juice, soya milk etc. Homogenization is
required for production of recombined milk to prevent fat separation. A milk homogenizer
is shown in fig.8.1.

Fig.8.1. Milk homogenizer

Principle
Homogenization is accomplished by forcing the milk or cream or any product under
pressure through narrow gap. Due to high turbulence, shear and compression larger fat

Fluid Milk Processing (Student Handbook for Class XI) 73

 globules are disintegrated into smaller globules. Homogenized milk or cream forms a stable
emulsion, if the size of the fat globules is sufficiently reduced. The size of fat globules
after homogenization depends upon:

a. Temperature of milk

b. Source of the milk (season of the year)

c. Flow rate of milk

d. Homogenization pressure

e. Geometry of the homogenizing valve

f. Stages of homogenization (single or two)

g. Percent and viscosity of milk fat

Methods
Milk homogenization requires high pressure. Piston type reciprocating pump is used to
produce such high pressure. The construction of a single stage piston pump is shown
in fig.8.2. Crank transforms the rotating motion of motor shaft into reciprocating motion.
When the piston moves back, milk is sucked into the cylinder through the inlet valve. With
forward movement of the piston, inlet valve closes and outlet valve opens up. Generally
three stage piston pump is used to produce high pressure for milk homogenization. The
pressurized milk flows under pressure towards the homogenization valve. Disruption of
larger size fat globule takes place in the homogenization valve. The opening for milk flow
can be adjusted and maintained using homogenization valve system (Fig.8.3).

Fig.8.2. Single stage piston pump

74 Fluid Milk Processing (Student Handbook for Class XI)

 Fig.8.3. Homogenizer valve system

Single and Double Stage Homogenization

Requirement to reduce the fat globule size also depends on the desired shelf life of the
milk product. Smaller globule size is needed for longer storage period to prevent creaming.
If the fat globules has a tendency to agglomerate after the first homogenizing stage
(2100-3000 psi), second stage (300-600 psi) is required to re-disperse them. Ultra high
temperature (UHT) milk requires higher pressure for longer shelf life (Table 8.1). When a
two-stage homogenizing valve is used, the second-stage pressure should be 10 to 15%
of the total homogenizing pressure.

Table 8.1. Homogenizing pressure for UHT milk

Shelf life

10 days 14 days 60 days 180 days

Homogenizing pressure (PSI) 1200-2000 1400-2200 2100-3500 2500-4000

Average fat globule size 0.80 µm 0.75 µm 0.55 µm 0.40 µm

Fluid Milk Processing (Student Handbook for Class XI) 75

 Equipment
The construction of milk homogenizer is shown in fig.8.4. Milk at high pressure is forced
through first stage and second stage homogenization value. Pressure gauge 1 and 2
displays the homogenization pressure in the first and second homogenization stage
respectively (Fig.8.5).

Fig.8.4. Milk homogenizer

Fig.8.5. Pressure gauge

76 Fluid Milk Processing (Student Handbook for Class XI)

Homogenization Efficiency
Milk homogenized at 42oC is tested for homogenization efficiency by laser light scattering
to determine the fat globule size distribution. Operating parameters which affects the
efficiency of high-pressure homogenizers are as follows:

zz Pressure

zz Temperature

zz Valve design

zz Flow rate

REVIEW QUESTIONS

1. What is homogenization?

2. What are the advantages of milk homogenization?

3. Differentiate between single and two stage homogenization?

4. List parameters affecting homogenization efficiency.

5. What is the pressure during homogenization at 1st and 2nd stage valve?

Fluid Milk Processing (Student Handbook for Class XI) 77

 Chapter 9

Sterilization

Objective
The purpose of this chapter is to understand sterilization process and
equipments required for it.

Introduction
Sterilization is a severe heat treatment designed to destroy all microorganism in milk.
Sterilization of milk on commercial scale was started long back in 1894. Milk can be sterilized
either in glass bottles, cans etc or by using ultra-high temperature (UHT) processing, which
involves continuous sterilization followed by aseptic packaging.

Principle
The sterilization effect is quantified using D, Z and Fo values defined as follows:

D Value
At a given temperature, D is the time required for 90% destruction of a given microbial
load. The heat resistance of bacteria and spores at a constant temperature is characterized
by their decimal reduction time (D value). D value is the time required to reduce the
microbial population of 90% or one decimal reduction (one log cycle). The number of
decimal reductions (log No/N) can be evaluated from:

log (No/N) = heating time/D

78 Fluid Milk Processing (Student Handbook for Class XI)

Where,
No = the initial microbial population,
N = final microbial population

For example, if initial microbial count was 100,000 and heat treatment of product at a
temperature of 116°C for 4 minutes reduced the count to 10000, the D value 116°C would
be 4 minutes.

Z value
Z is the temperature variation giving a 10 times variation of D.

Sterilizing value Fo
At a given point in a bottle where the temperature has a constant value T for the time t,
the sterilizing value Fo will be :

Fo = (10(T-121)/Z) x t/60

Z value = 10 (for many heat resistant spores)

Methods
Sterilization can be done by any of the following methods:

i. Direct heating: Steam is used for direct heating by injection or infusion followed by
vacuum flash cooling to remove undesirable odours and added moisture.

a. Injection: Steam is sprayed in to the milk through steam injector in a mixing

chamber.

b. Infusion: milk is sprayed into a chamber where steam environment is

maintained.

ii. Indirect heating: Indirect method employs heat exchangers for heat treatment. The
systems used for indirect heating are plate heat exchangers, concentric tube and
multiple tube-in-tube systems.

Fluid Milk Processing (Student Handbook for Class XI) 79

 Equipments

In Bottle Sterilization
The original form of sterilization, still used, is in-container or bottle sterilization, usually at
110-120°C for some 20-40 minutes. As the thermal conductivity of glass is low, temperature
of milk rises slowly specially when bottles are not agitated. The bottles cannot withstand
extreme or sudden temperature change, therefore milk sterilization is done by low
temperature long time process. This causes development of cooked flavour and brown
colour. In bottle sterilization could be done using:

a. Autoclaves

b. Batch retort/sterilizer (Horizontal or vertical)

c. Hydrostatic vertical steriliser

d. Horizontal steriliser

Ultra High Temperature (UHT) processing

Milk starts to undergo browning reaction under lower sterilization temperatures. Therefore
sterilization is done at very high temperature (135 to 150oC) for short time (6-2 s) to
prevent browning and development of cooked flavour (Table 9.1). The process is commonly
known as Ultra High Temperature (UHT) and operation is followed by aseptic packaging.
Advantages of UHT processing are:

zz Higher D and Z valves for better quality and food safety

zz Reduction in process time due to higher temperature

zz Longer shelf life. (> 6 months)

zz Product can be stored at ambient temperature

zz Product can be marketed to far off distances

80 Fluid Milk Processing (Student Handbook for Class XI)

 Table 9.1. Effect of temperature on bacteria destruction rate
and relative rate of browning

Sterilization Bacteria Relative rate

Temperature°C destruction rate of browning

100 1 1

110 10 2.5

120 100 6.2

130 1000 15.6

140 10000 39.0

150 100000 97.5

REVIEW QUESTIONS

1. What is the difference between pasteurization and sterilization?

2. Differentiate between injection and infusion methods of sterilization.

3. Define D value

4. Define Z value

5. Define F value

6. What do you understand by UHT sterilization?

Fluid Milk Processing (Student Handbook for Class XI) 81

 Chapter 10

Cleaning and Sanitation of Dairy

Equipments, Clean-in-Place (CIP)

Objective
This chapter discusses the principles of cleaning and sanitation of dairy
equipment. Cleaning in place (CIP) system is an integral part of any
dairy or food processing plant. CIP system consists of equipment, pipes
and automation systems that manage the circulation of cleaning solutions
through the process equipment and piping network.

Introduction
Cleaning and sanitisation of pipes, fittings, dairy equipment, tanks etc is necessary to
prevent:
zz Accumulation and growth microorganisms in the equipment. Product will be contaminated
by pathogens during processing.
zz Development of foul smell in the equipment which will adversely affect the product
quality.
zz Corrosion of metal surface due to lactic acid.

CIP System
As the name suggest, cleaning is done in-place. It is a method of cleaning pipes, fittings,
tanks, vessels, process equipments, etc without need for dismantling or disassembling.

82 Fluid Milk Processing (Student Handbook for Class XI)

After any processing operation the piping systems and process equipments must be properly
cleaned. The deposits left after processing due to milk and milk product is known as “soil”.
Soiled equipment surface allow bacteria to grow and cause contamination of milk. This
can result in quality defects and the product will have limited shelf life. In some cases it
may cause food poisoning. Therefore cleaning of equipments becomes necessary after
use. Cleaning can be done in two ways:

a. Manual Cleaning: Requires dismantling of equipment for cleaning.

b. Cleaning in-place (CIP): CIP systems are designed for automatic cleaning without
major disassembly and assembly work.

Advantages of CIP
zz Reduces and eliminates the possibility of cross contamination.

zz Good reproducibility of the cleaning process because the cleaning parameters are
defined. Repeatable, validateable and controllable cleaning results.

zz Does not require dismounting of equipment which saves time and reduces downtime
of process equipment.

zz Reduces labour requirement.

zz The cleaning system can be automated and can be programmed for different cleaning
cycles.

zz Lower operating cost. Savings in energy and chemical costs.

zz The cleaning solution can be reused.

zz Reduce operator hazards associated with handling cleaning chemicals.

zz Reduce wear-and-tear on equipment.

zz Cleaning cycle reports/logs can be recorded.

zz Uniform cleaning over all surfaces and conformity to cleaning requirements cleaning
requirements.

zz Reduction of potential operator interface error.

zz Simple and easy operation through automatic controls and human machine
interface.

Fluid Milk Processing (Student Handbook for Class XI) 83

 Factors Affecting CIP Cleaning
The cleaning principle for CIP relies on application of suitable detergent or solvent at a
suitable flow, pressure, temperature and concentration for sufficient period of time.

a. Flow velocity: For adequate cleaning of pipes and surfaces the velocity of the
cleaning solution should be in the range of 1.5-3.0 m/s.

b. Pressure: Adequate pressure in CIP fluid circulation systems is necessary for reliable
performance of spray devices. Pressure and fluid velocity are interdependent. CIP
solution pressure is proportional to the square of its velocity.

c. Temperature: Cleaning efficiency greatly depends on the solution temperature. Very

high temperature hardens the soil making it difficult to remove. Low temperature may
reduce the cleaning efficiency. The operating temperature range is determined by
process requirements, type of solution and its chemical activity.

d. Cleaning Solution chemical concentration: The concentration/strength of the cleaning

solution and sanitizer must be maintained to ensure adequate cleaning. Too high
concentration will be wastage of chemicals and add to cost. A lower concentration
may not clean equipments and piping networks effectively.

e. Cleaning time: Cleaning time is the time in which the cleaning solution is in contact
with the equipment or cleaning surface. Time of cleaning depends on the concentration
of solution, solution quantity, temperature and nature of the milk soil (Hard/soft).

Sequence Operations of CIP Systems

A general sequence for CIP cleaning is as follows:

Water pre-rinse : Flushing to eliminate residues

Alkaline rinse : Alkaline detergents dissolve fats and proteins. Aids in

cleaning where harder deposits have occurred.

Intermediate water rinse : To flush alkali solution.

Acid rinse: : Required for neutralizing the caustic remaining on

the surfaces of the plant. Acidic detergents remove
mineral deposits on the equipment surface.

Final water rinse : Cold water to flush residual acid solution

84 Fluid Milk Processing (Student Handbook for Class XI)

Methods
CIP systems can be classified in the following ways:

a. Single use: Cleaning solution is used once and it is drained after cleaning (Fig.10.1).
Cleaning solution is not reused. Such systems are used for small units and
equipments.

b. Re-use: If the equipment is not heavily soiled, the cleaning solution is re-used by
adding more chemical concentrate as required (Fig.10.2).

Fig.10.1. Single use CIP system

Fig.10.2. Re-use CIP system

Fluid Milk Processing (Student Handbook for Class XI) 85

 Equipments
A CIP system for 1 lac litre per day dairy processing unit is shown in fig.10.3. A CIP
Systems comprise of following components (Fig.10.4):

zz Stainless steel tanks

zz Plate heat exchanger for heating water and cleaning solutions

zz Temperature sensors and temperature control system

zz Chemical metering and dosing pumps

zz Conductivity meter for chemical strength control

zz CIP supply pump

zz Supply pipeline

zz Cleaning heads, spray devices for equipment cleaning

zz CIP return pump

zz Return pipeline

zz Flow valves and routing valves

zz Programmable Logical Control (PLC) and control system

Fig.10.3. CIP section

86 Fluid Milk Processing (Student Handbook for Class XI)

 Fig.10.5. Components of a CIP system

Fig.10.5. Lye tank

Fluid Milk Processing (Student Handbook for Class XI) 87

 Fig.10.6. Acid tank

Fig.10.7. Hot water tank

88 Fluid Milk Processing (Student Handbook for Class XI)

 REVIEW QUESTIONS

1. Why cleaning and sanitation is required in a milk processing plant?

2. What are the differences between manual and CIP cleaning?

3. What are the advantages of CIP cleaning?

4. Briefly describe the factors affection CIP cleaning.

5. With a neat diagram differentiate single and re-use CIP systems.

Fluid Milk Processing (Student Handbook for Class XI) 89

 Chapter 11

Effluent Treatment and Dairy Plant

Waste Disposal

Objective
The purpose of this chapter is to provide knowledge about effluent treatment
plant

Introduction
Conventionally, the dairy and food processing industry has been a large water user.
Water is used in the milk processing plant for processing, cleaning etc. The waste water
generated from any industry is known as effluent. Environmental pollution by food or
related industries via effluent discharge has become a threat to plants and animals. It
pollutes ground water and water resources. Therefore effluent has to be treated before
it leaves the plant.

Effluent: The waste water generated from any industry is known

as effluent.
Sewage: Sewage is a water-carried waste, in solution or suspension
and is also known as wastewater

Principle
Effluent is the wastewater and sewage that flows out from any processing plant. Effluent
refers to waste which may consist one or more of the following:

90 Fluid Milk Processing (Student Handbook for Class XI)

zz Wastewater from milk processing plant due to washing of equipments, cleaning of
floors etc.

zz Biological waste from milk, cream, product spillage etc

zz Chemicals, cleaning solutions etc.

zz Lubrication oil, grease etc

zz Other organic waste

Dairy processing units handle large volumes of milk, and the major waste material from
processing is water. The volume of effluent may vary from 1.5-10 litres of milk processed
depending upon the product manufactured. Untreated effluent will cause pollution if it
is discharged into the drainage system, wastewater or river. Environment and pollution
control agencies have issued mandatory guidelines for treatment of effluent before it is
discharged from the plant site.

Methods
Effluent is normally treated by a combination of physical-chemical and biological operations.
However, it is possible to treat waste waters solely with physical-chemical methods.

a. Physical methods: Some of the physical methods used for effluent treatment are
screening, sedimentation, filteration etc

b. Chemical methods: Chemicals are added to change the properties of effluent. Lime
or ion exchange is used for softening. If the effluent is acidic or alkaline, it has is
treated to adjust the pH. Limestone is generally used for neutralization of acidic waste
water and sulphuric acid is commonly used for alkaline waste water.

c. Biological methods: Biological treatment is an important and integral part of any

wastewater treatment plant that treats wastewater having soluble organic impurities.
Biological methods can be further classified as:

i. Aerobic treatment: Microbial treatment of effluent takes place in the presence

of oxygen. In the aerobic treatment process, aerobic microorganisms use free
oxygen to assimilate organic impurities and convert them into carbon dioxide,
water and biomass.

Fluid Milk Processing (Student Handbook for Class XI) 91

 Aerobic microorganisms: An aerobic organism or aerobe is an
organism that can survive and grow
in an oxygenated environment.
Anaerobic microorganisms: An anaerobic organism or anaerobe
is any organism that does not require
oxygen for growth.

Fig.11.1. Difference between aerobic and anaerobic microorganism

ii. Anaerobic treatment: Microbial treatment of effluent takes place in the absence of
oxygen. In this treatment anaerobic microorganisms assimilate organic impurities in
the absence of oxygen. The end products of in this treatment are methane, carbon
dioxide gas and biomass.

92 Fluid Milk Processing (Student Handbook for Class XI)

Equipments
Effluent treatment plant (ETP) is designed on the basis of volume and nature of effluent
generated by the processing plant. A general design of ETP for dairy processing plant is
shown in fig.11.2.

Fig.11.2. General design of effluent treatment plant

Fig.11.3. Effluent treatment plant

Fluid Milk Processing (Student Handbook for Class XI) 93

 ETP comprises of the following components:

Screen: For removal of large size solids like packaging material, spare
parts etc from the effluent while it passes through the screen.

Grease and oil trap: Removes the grease and oil floating on the surface of the effluent
stream.

Equalization tank: Collects and equalizes the raw effluent. The flow of effluent from
the processing plant varies during the 24 hour operation period.
The quality and properties may varies. ETP is designed for a
particular flow rate and nature of incoming effluent. Therefore
equalization tank is required to mix the effluent and to maintain
a constant feed rate to treatment plant.

UASB reactor: Upflow anaerobic sludge blanket (UASB) reactor is a form of

anaerobic digester that is used in the treatment of wastewater.

Aeration tank: Air is pumped into the tank which provides oxygen and aids
in mixing. In presence of oxygen, aerobic bacteria oxidize and
degrade the organic matter present in dissolved or suspended
form.

Clarification tank: The suspended solids are separated in the clarification tank.
Separates suspended biological material. The wet sludge is
conveyed to the sludge drying bed.

Sludge drying bed: Drying bed consists of 20-30 cm bed of sand and has drainage
system beneath the sand layer. Sludge is distributed over the
sand bed and is left to dry under sun. Drying is accomplished
by evaporation and drainage of excess water over a period of
several weeks depending on climatic conditions. The dried sludge
is removed from the bed and may be used as land fill.

Disinfection unit: At disinfecting stage, chlorine or similar disinfectant is mixed with

the water stream to reduce the microbial load. The disinfecting
stage is optional, and is used where a sterile effluent is
required.

94 Fluid Milk Processing (Student Handbook for Class XI)

 REVIEW QUESTIONS

1. What is effluent?

2. Why effluent has to be treated before it is discharged from a processing plant.

3. What is the purpose of grit chamber.

4. Why oil/grease trap is used in a effluent treatment plant?

5. List differences between aerobic and anaerobic treatment.

6. What is the purpose of equalization tank?

Fluid Milk Processing (Student Handbook for Class XI) 95

 Chapter 12

Dairy Utilities-Steam, Water,

Refrigeration, Electricity

Objective
This chapter deals with the utilities required in milk processing plant for
processing and manufacture of milk products.

Introduction
Inputs such as steam, water, refrigeration, electricity, compressed air etc required to run
any processing plant is known as utilities or services.

Steam
Steam is used in heating operations like pasteurization, sterilization, CIP etc. Generally oil
fired boilers are used for generation of steam (Fig.12.1). Few dairy processing plants have
coal fired boiler. The steam requirement (kg/hr) depends upon the process operations.

Water
A milk processing plant ideally requires water in the ratio of 1.5-2:1 (1.5 to 2 liters of water
for 1 liter of milk processed) for processing, steam generation, cleaning of equipment, etc.
There may be 3-4 deep bore tubewells to meet the daily water requirement of the plant.
Water requirement in a dairy processing plant can be categorieed as:

zz Raw untreated water: cleaning of floor etc.

96 Fluid Milk Processing (Student Handbook for Class XI)

 Fig.12.1. Oil fired boiler

Fig.12.2. Oil tanks for boiler

Fluid Milk Processing (Student Handbook for Class XI) 97

 zz Potable water: Treated water which is used for drinking and in product formulation/
manufacturing.

zz Soft water: Used in boiler for steam generation.

zz Chilled water: Temperature of water is around 2-3oC and is used for cooling of milk
and milk products.

zz Hot water: Used in heating and CIP operations.

Refrigeration
The major refrigeration requirement in a milk plant may be listed as raw milk chilling,
pasteurized milk cooling, product cooling, cold store, deep freezer etc. Ammonia based
vapour compression refrigeration system are generally used in a processing plant. A
refrigeration system consists of following components:

zz Compressor

zz Condenser

zz Expansion valve

zz Evaporator

Fig.12.3. Compressor of the refrigeration system

98 Fluid Milk Processing (Student Handbook for Class XI)

 Fig.12.4. Ammonia reservoir/tank for the refrigeration system

Fig.12.5. Water cooled condensor

Fluid Milk Processing (Student Handbook for Class XI) 99

 Electricity
A three phase electricity supply is required to run heavy equipments in milk processing
plants. Diesel generator sets are required in for operating plant during power cuts.

Fig.12.6. Diesel generator set

Compressed Air
It will be required for various pneumatic operations flow control operations as well as for
cleaning purposes.

100 Fluid Milk Processing (Student Handbook for Class XI)

 Fig.12.7. Equipment for producing compressed air

REVIEW QUESTIONS

1. Briefly describe type of services/utilities are required in a milk processing plant.

2. What is the ideal water requirement in a dairy processing unit?

3. Why refrigeration system is required?

4. List various water requirement in a milk processing plant.

5. List various section in a milk plant where steam is used.

Fluid Milk Processing (Student Handbook for Class XI) 101

 Chapter 13

Packaging – Packaging Materials

and Specifications, Machines,
Systems and Operational Aspects

Learning objectives
- To know about the need of packaging material for fluid milk,
- various formats of fluid milk packaging,
- fluid milk packaging machinery and its operation

Introduction
Milk contains very important nutrients which is important for the growth and development
of human beings: proteins, carbohydrates, lipids, minerals and vitamins. Also, milk is a
highly perishable commodity and serves a very good media for the growth of many spoilage
causing as well as and pathogenic microorganisms. These pathogenic organisms may
cause serious illness to the consumer. Milk is also susceptible to chemical changes such
as oxidation of fat when exposed to sunlight and metallic surfaces. Milk is also sensitive
to absorb the prevailing odour of surrounding environment. Hence, it is very important
for us to prevent the contamination of milk by such microorganisms and to prevent the
chemical deterioration of milk and also to protect the milk from objectionable odours of the
surrounding. Packaging like any other packaging is an external means of preservation of

102 Fluid Milk Processing (Student Handbook for Class XI)

milk during storage, transportation and distribution. The word “packing” can bedefined as
the enclosing of an individual product or item or several items in a package or container.
In this chapter you would learn about the functions of packaging, levels of packaging,
types of packaging materials and packaging machines and systems used in fluid milk
processing industry.

Functions of Packaging
Food packaging performs four major functions such as containment, protection,
convenience and communication. The main function of packaging is containment. It
means to contain the product. Packaging protects milk from external environmental factors
and contaminants during transportation and storage, keeping them safe until they are
consumed. Air and light cause chemical deterioration of milk particularly oxidative changes
in milk. Packaging also provides security from theft and tampering and communicates
essential information about products on the labels. Packaging also plays an important role
in marketing and establishing brand awareness in an intensely competitive marketplace.

Levels of Packaging
Packaging can be classified into primary, secondary, tertiary and quaternary levels. Primary
packaging refers to the one in which the productis in direct contactwith the packaging
material, for example a bottle. Secondary packaging refers to a packaging which contains
anumber of primary packages, for example, a plastic wrap containing a small number of
bottles. The main function of secondary packaging is to provide mechanical strength and
protection to the product. Tertiary packaging also called as distribution packaging refers
to packaging of a product for efficient handling during distribution, for example, packaging
of a large number of bottlesin corrugated boxes. Sometimes, corrugated boxes are also
used as secondary packages. Quaternary level of packaging is also called as unit load or
transport packaging and involves assembling of multiple containers into a single combined
bundle suitable for materials handling equipment. For transport, such packaging is frequently
stabilized and unitized through the use of pallets,strapping, shrink-wrapping, or similar
means to form a single unit. For pasteurized milk, pouch serves as primary packaging
and plastic crates are used as secondary packaging material to keep pouches intact from
the stress and abrasion during transportation.

Fluid Milk Processing (Student Handbook for Class XI) 103

 Packaging Materials
These packaging materials for fluid milk are flexible, semi-rigid or rigid. Commonly used
materials include low density polyethylene (LDPE) film, paper and aluminum based
laminates, multi-layered cartons and glass bottles.

Folding cartons: Folding cartons are containers made from sheets of paperboard (typically
with thickness between 300 and 1100 mm). Such cartons are fed to the filling machine
in a flat or collapsed form where they are erected, filled and sealed. Folding cartons,
for example, are used for packing bulk/family-sized ice cream packs. The most common
packaging material used for UHT milk is the paperboard laminate carton.

Corrugated fibreboard: Corrugated board is manufactured from three basic sheets –

two liner boards and a central corrugated sheet or medium (flute). These materials can
be varied as to weight, type and number and/or height of the corrugations in the fluting
medium. The properties of a corrugated board depend largely on the type, number and
position of the corrugations. Mostly corrugated fibreboards are used as secondary or tertiary
packaging in the distribution and marketing of dairy products. Corrugated fibreboard box
is manufactured in a very large and specialized machine called corrugator. Corrugated
fibreboardswith different types of corrugations are shown in Fig.13.1. The simplest is referred
to as single-face board and consists of one liner and one medium. It is usually used as
a cushioning material for protecting objects in other containers during shipping. When a
second liner is applied to the other side of single-face board, it is called as single-walled.
The addition of further single-face combinations to single wall board results in double-wall
and triple-wall constructions.

Fig.13.1. Corrugated fibreboardbox with different types of corrugations.

104 Fluid Milk Processing (Student Handbook for Class XI)

Composite containers: These containers usually consist of cylindrical bodies of paperboard
or fibreboard with metal or plastic ends. Small containers with less than 200 mm in
diameter are referred to as tubes or cans and are used for foods such as salt, pepper,
spices, custard powders, chocolate beverages and frozen fruit juices. Large containers,
known as fibreboard drums, are used as alternatives to paper or plastic sacks or metal
drums for products such as milk powder, emulsifying agents and cooking fats.

Glass
Glass is one of the oldest packaging materials and thought to be discovered in eastern
Mediterranean region of about 3000 B.C. Glass was used to contain cosmetics, perfumes
and foods by ancient Egyptians, Hebrews, Romans and Persians.Glass is a hard substance
that is made from limestone (about 10%), soda (about 15%) and silica (about 75%).
Lesser percentages of aluminum, potassium and magnesium oxides may be included.
Because of its outstanding properties, glass remains a most
important material for food packaging. These properties are
(a) chemical inertness, (b) non-permeability to gases and
vapours,(c) strength, (d) resistance to high internal pressures,
(e) excellent optical properties, and (f) surface smoothness.
Glassin its natural state is usually transparent butcan have
desired colours when incorporated with colouring agents during
manufacturing process.Its main disadvantages are its fragility
and heavy weight. Glass containers such as jars, bottles, etc.
are made by various methods and different types of closures
such as plug,crown, twist-off crown, wraps, heat seals, threaded
screw closures, etc. are used for closing the containers.Milk
for retail sale was traditionally packaged in refillable glass
bottles. However, single-serve paperboard cartons and plastic
pouches of various compositions and constructions dominate
current market. Presently, sterilized flavoured dairy drinks are
marketed in glass bottles (Fig.13.2) by some reputed dairy Fig.13.2. A typical glass bottle
for sterilized flavoured milks
industries.

Metals
Four metals are commonly used for the packaging of food materials viz. steel, aluminium,
tin and chromium. Tin and steel, and chromium and steel are used as composite materials
in the form of tin-plate and electrolytically chromium-coated steel (also called as tin-free

Fluid Milk Processing (Student Handbook for Class XI) 105

 steel). The term tin-plate refers to low carbon mild steel sheet coatedwith tin on both
sides of the material. The coating of tin-plate consists of four compartments such as
tin-iron compound layer, the free or unalloyed tin layer, the oxide layer and the oil layer.
The combination of tin and steel produces a material that has good strength, combined
with excellent fabrication qualities and a corrosion-resistant surface of bright appearance
suitable for packaging processed foods. Tin-plate is used for the manufacture of two-piece
and three-piece cans (Fig.13.3). The traditional method for packaging milk powders for
retail purposes uses three-piece tinplate cans where the atmospheric air is withdrawn
from the powder and replaced with an inert gas such as nitrogen prior to seaming the
base onto the can. The main advantages of metal cans are (a) their high strength, (b)
providing impermeability to gases and vapours,(c) resistance to high temperatures and
(d) protection against light. The disadvantages of metal containers are (a) heavy weight,
(b) high cost and (c) tendency to interact with contents and environment (internal and
external corrosion).

Fig.13.3. A typical two-piece and three-piece cans for food packaging

Destruction or deterioration of a metal due to chemical or electrochemical reaction with its

environment is called as corrosion. In food cans, internal corrosion is due to the reaction
of the contents with the container walls. When food products especially high acid products
like fruits and vegetables are packed in metal (steel) containers, they interact with the
container leading to corrosion of the metal. Due to this, hydrogen is liberated and when
sufficient quantity of it accumulates, it causes the ends of the can to bulge. This bulging
is known as hydrogen swell. The best remedy is to coat internal surfaces with tin or other
protective polymeric substances called as lacquers.

106 Fluid Milk Processing (Student Handbook for Class XI)

Aluminum is the earth’s most abundant metallic constituent, comprising 8.8% of the earth’s
crust.Purified alloys of aluminum with small amounts of magnesium and manganesein the
form of foil, tubes or retort pouches for packagingin the food industry. Aluminum foil is a
thin-rolled sheet of alloyed aluminum varying in thickness from about 4 to 150 mm. It is
essentially impermeable to gases and water vapour when it is thicker than 25.4 mm. It
can be converted into a wide range of shapes and products including semi-rigid containers
with formed foil lids, caps and cap liners, composite cans, laminates containing plastic and
sometimes paper or paperboard, etc. The advantages and disadvantages of aluminum
are similar to those of metal cans, except that aluminum is significantly lighter but also
more expensive.It is also used for metallization of plastics and paper to provide improved
barrier properties. Aluminum foil has an important role as light and gas barrier in the 5 or
6 layered laminate used for packaging of UHT milk.

Plastics
Plastic is derived from the Greek plastikos, meaning easily shaped or deformed. Food
industry is a major user of plastic packaging materials. Plastics are usually divided into two
broad categories: thermosetting and thermoplastic. Polyethylene, polystyrene, polypropylene,
polyvinyl chloride, etc. are thermoplastic type of plastics. Mostly thermoplastics are being
used for packaging of dairy products.

Polyolefins: Polyolefins form an important class of thermoplastics and include low, linear
and high density polyethylenes (PE) and polypropylene (PP). Industry commonly divides
polyethylenes into two broad categories based on their densities: high density polyethylene
(HDPE) and low-density polyethylene (LDPE). The properties that make polyethylene film
a popular packaging medium in dairy industry are its low price, nontoxic, excellent heat
sealing property, flexibility, pleasing appearance and softness, chemical inertness. Low-
density polyethylene (LDPE) accounts for the biggest proportion of the plastics used in
food packaging due to its versatility. LDPE film accounts for a little over 50% of total PE
in this sector because it provides strength, gloss, flexibility, and the good clarity. It can
be extruded into film, blown into bottles; injection molded into closures, extruded as a
coating on paper, Al foil or cellulose film, and made into large tanks and other containers.
It is easily heat sealable. LDPE film is used for packaging of milk, oil, salt etc. For an
equal wall thickness, HDPE gives grater rigidity to bottles than LDPE. HDPE films have
a cloudy appearance, occasionally utilized as a liner for the bulk packaging of skim milk
powder due to their high strength and barrier properties. Furthermore, HDPE is the most
commonly used film as carton liner. However, polyethylene is not suitable for packing foods
with strong aromas or the products, which have to be packed under vacuum. Chemically,

Fluid Milk Processing (Student Handbook for Class XI) 107

 the polypropylene (PP) is similar to LDPE and HDPE. It is harder than either and has a
less waxy feel. It can be converted into film and sheet or can be thermoformed to give
thin – walled trays of excellent stiffness. PP has an excellent grease resistance. It has
better mechanical strength and less prone to stress cracking than PE. It has poor low
temperature performance and impact resistance. The properties of PP can be altered by
orienting the film. The oriented PP (OPP) has better tensile strength, low permeability
to water vapour, and oxygen gas. Polypropylene is non-toxic, and extensively used as
packaging material for various food products. The advantages of PP are: low cost, good
impact strength, processable by all thermoplastic equipment, low coefficient of friction, and
good moisture and chemical resistance.

Polystyrene: Polystyrene (PS) is amorphous and in pure form, it is very brittle but when
blended with chemicals like butadiene and copolymerized with acrylonitrile, it becomes tough.
It is relatively cheap and has high water vapour and gas transmission rate, hence also
called as “breathing film”. PS is crystal clear, sparkling and is also used for thermoformed
cups, trays and glasses for yoghurt, ice cream, meat, soft drinks, etc. However, it is
unsuitable for heat sterilization, and deteriorates on exposure to sunlight.

Polyvinyl chloride: Polyvinyl chloride (PVC) has several overall balanced properties
required by the food packaging systems: glass like clarity, good mechanical strength,
resistance to water vapour, gases and chemicals, retention of flavour, excellent printability,
and lower weight/ volume ratio. Due to these properties, PVC is commonly used for food
packaging in various forms like films and sheets, bags/liners, shrinkable tubes/films, skin/
blister packs, film laminates, bottles and sachets.

Polyvinylidene chloride: Polyvinylidene chloride (PVDC) is a polymer based on the

vinylidene chloride monomer. The film has excellent water vapour and gas barrier properties,
besides outstanding characteristics like retention of odours and flavours, resistance to oils
and fats, heat sealing characteristics, non-toxicity, abrasion, and chemical resistance. The
laminates comprising of PVDC are widely used for the packaging of baby foods, snacks,
chips, powdered soups and sauces, powdered coffee, coffee beans, sweets, biscuits
and crackers, and powdered sugar. Saran, a copolymer of vinyl chloride and vinylidene
chloride, is extensively used for food packaging. Saran is suitable for the packaging of
several traditional dairy products such as sweet meats.

Nylon: The unique properties of nylon film are that it has high mechanical strength, high
elongation capability, excellent resistance to cutting, perforation, abrasion and bursting,
high chemical resistance to oils and fats, outstanding impermeability to gases and vapours,
easy printability, easy metallizing. The film can be biaxially oriented, and its properties

108 Fluid Milk Processing (Student Handbook for Class XI)

remain unchanged between –30º to 120º C. The film is used in food packaging, especially
where aroma retention is required.

Polyester: Polyethylene terephthalate (PET) is important polyester used in food industry.

The main advantages of polyester films are that they are tough, sterilizable, very clear,
chemical resistant, has low water absorption, low moisture vapour permeability, low gas
permeability and are used as a laminate base. Its main applications include in metallized
films, vacuum and gas packaging, shrink packaging, cured meat, and in boil-in-the-bag
applications. This film is of great interest to the food packagers as it contains no plasticizers
and is non-toxic.

Packaging Machine for Fluid Milk

Packaging of pasteurized milk: In India, pasteurized milk is largest consuming fluid milk
in the form of pasteurized toned milk, double toned milk, standardized milk, full cream milk
etc. These processed milks are packaged in low density poly ethylene (LDPE) film. Film
has fixed width to suit machines of different manufacturers and it is around 325mm and
thickness of the film varies with the pouch capacity, for 500ml it is around 55µm and for
1000ml it is around 65µm. The machine used for packaging is vertical-form-fill-seal (VFFS)
machine and schematic diagram of functioning of this machine is illustrated in fig. 13.4.

Fig.13.4. Schematic diagram of vertical-form-fill machine

Fluid Milk Processing (Student Handbook for Class XI) 109

 Typically, VFFS machine has four parts. They are,
i) Pouch Forming Unit: This unit is responsible for unwinding film into the machine and
form pouch to fill milk into it. Here, roll-fed flexible packaging film is unwound from a
feeder roll. Then film is moved up through roller guides over a forming collar and then
down, forming inverted cone tube. In order to provide extra film for the vertical seal,
the width of the film will be greater than the circumference of the forming tube.
ii) Sealing unit: Continuous motion machines operate on the principle that both vertical
and horizontal pouch seals are made when the film is in motion. The long seal is
the static bar, which is a heated bar with heating element and thermocouple. Another
moving bar emboss against this bar to seal the pouch vertically. The horizontal seal
is responsible for three major functions—to seal the top of the previously filled bag,
to create the bottom seal for the soon to be filled bag and to cut or separate the
completed bag from the bottom of the new bag. Horizontal sealing unit consists of
two jaws, front and back jaws operate as a pair. Each jaw will have one or two
heating elements as well as a thermocouple for temperature control and either one
of these jaw is moveable (operate pneumatically) and has knife to cut the pouch
after sealing.
iii) Filling Unit: It consists of over head tank located above the top of the machine,
facilitate mixing of the milk for uniform composition and uninterrupted gravity feeding
to the machine. Filling of milk is based either on volume basis or weight basis. Based
on timer, milk is delivered into pouch after horizontal seal cuts the pouch.
iv) Control Unit: This facilitates the volume adjustment to a fixed level such as 500m
or 1000ml. Also, it controls the all logical circuit of machine involved in unwinding of
film, moving of pouch after forming tube, temperature control of sealing unit, water
temperature and pressure for sealing head.

Operational Aspects of VFFS Machine

Before start-up with milk, machine needs to run with water for couple of minutes to verify
sealing quality and quantity. Once the sealing is perfect with water, then switch to milk.

1. Turn on the manual mode and ensure water supply and water temperature are at
desired level.

2. Ensure the proper film is loaded to roll carrier

3. Switch on the UV lamp to destroy the microbes present on the packaging material.

4. Replace filter (provided at the end of filler tube) for ever batch and fix with new
filter

110 Fluid Milk Processing (Student Handbook for Class XI)

5. Ensure sealing rubber is intact and there is no wear and tear on it.

6. Set the volume of milk to be filled.

7. Press the fill and seal knob to fill the pouch and seal

8. Check the sealing intact and weight of the filled pouch

9. Switch to auto mode once the all the desired parameters are set.

Aseptic Packaging of Milk

Aseptic packaging is packaging process, wherein a sterile product (most commonly UHT
treated) milk is filled under sterile conditions into sterile packages in a sterile environment.
Paper board cartons and multi-layered laminates are used commonly for aseptic packaging
of fluid milk and other milk based beverages like flavoured milk, lassi, buttermilk etc. This
process enhances the shelf life of milk from 2 days at refrigeration temperature to 120 to
180 days at ambient temperature.

Packaging Material for Aseptic Packaging

The filling systems could be either of the following two types: those in which the carton
is formed within the machine from a continuous reel of material; and those in which the
cartons are supplied as preformed, folded flat, which are assembled into cartons in the
filling machine. The packaging material is mainly composed of printed-paper coated with
aluminium foil and several plastic layers (Polyethylene-paper board-polyethylene-aluminium
foil-polyethylene). The inner material side of the finished package is coated with a special
layer facilitating the sealing process. Each layer has a specific function:

zz The outer polyethylene layer: Protects the ink and enables the sealing process of
the package flaps.

zz The paperboard layer: Serves as a carrier of the decor and gives the required
mechanical strength to package.

zz The middle polyethylene layer: Binds the aluminium to the paper.

zz The aluminium foil: acts as a gas and light barrier.

zz The inner polyethylene layer: provides liquid barrier and prevent the entry of milk to
inside laminate

Fluid Milk Processing (Student Handbook for Class XI) 111

 Fig.13.5. Multi-layers of laminate used in aseptic packaging

Aseptic packaging environment: Creating aseptic environment is very much essential

during filling and packaging of UHT treated milk. Chemical sterilization processes for
the packaging film include treatment with hydrogen peroxide (H2O2). H2O2 shows poor
effectiveness at ambient temperatures, but at high sporicidal effect at 80°C makes it
useful for packaging sterilization. It is first applied on the material and then evaporated
by heating through hot air or infrared radiation.

Hot air at 330-350oC (for 30 min) may also be used for milk tube sterilization. Sterilized
air reduced to 180-200°C is used to evaporate H2O2 and when cooled to 50°C can be
employed for pressurizing the filling chamber. Air filtration by means of depth filters (mats
of compressed glass- or asbestos- fibre, or of sintered metal or ceramic) is effective in
freeing air from bacteria. The filters themselves may be sterilized by fumigation, hot air or
steam. UV radiation can also be used as a complementary treatment of already sterilized
packaging. Aseptic barriers in the form of steam or circulated liquid sterilant become
necessary with valves and fittings coming in contact with sterile milk. Detection of leaks
by using a dye test is essential to check recontamination of the packaged sterile milk.

Aseptic Packaging Systems

Filling of commercially sterile milk in sterilized packages/containers in a sterile environment,
and hermetically sealing the same to prevent recontamination of the milk can be achieved
in two major ways: (a) using pre-sterilized preformed containers such as bottle and cans,
and

112 Fluid Milk Processing (Student Handbook for Class XI)

(b) sterilizing the packaging material, forming it into suitable containers, filling the sterile
product and sealing the package on the so-called form-fill-and-seal (FFS) machines. The
latter employs a multi-layer laminate of polyethylene, polystyrene and/or polypropylene
films, paper and aluminium foil.

The most widely used FFS Tetra Pak systems using tetrahedron cartons, and Tetra-Briks
or hexahedron cartons are characterized by continuous formation of the package below
the milk level from a paper/PE/Al laminate strip which has been continuously sterilized
by H2O2 boiled off by radiant heat in the region immediately above the milk surface thus
giving a sterile atmosphere in the packaging zone as shown in figure 13.6 and 13.7. Tetra
Pak also introduced Pillow Pak’ to cut down the packaging cost of UHT milk.

Formed into a tube

2 Sterile product 1

Join sealing
Sterile air (exhausts out
tape
entry and exit)
sterilising
Scored for box solution Side seam heat sealed
length punched (hydrogen
for pourer peroxide)
3

Filling tube below liquid

surface

Roll of preprinted scored,

Ends squeezed
laminated board
together, heat
sealed, no air
present cut

1 Product sterilised and piped to the packaging machine

2 Packaging sterilised before entering the sterile area
3 Product and package combined within the sterile area

Aseptic packaging of UHT milk and other sterile foods saves

energy by not needing refrigeration during its storage life

Fig 13.6. The forming and filling process for aseptic containers

Fluid Milk Processing (Student Handbook for Class XI) 113

 Fig.13.7. Aseptic packaging unit-Tetra brik

Activities Suggested
1. Visit packaging material manufacturing industry near to you and observe

2. Collect the milk pouch of 500ml and 100ml, measure their dimensions and
thickness

3. What are the packaging materials used to ghee, butter and chocolates?

4. Collect aluminum foil and polyester film and check for pin hole using dye method

114 Fluid Milk Processing (Student Handbook for Class XI)

 REVIEW QUESTIONS

1. What are the functions of packaging material with respect milk packaging?

2. List various advantages and disadvantages of glass as a packaging material?

3. Explain the various functional unit of vertical-form-fill machine with schematic

diagram.

4. What is the principle of aseptic packaging?

5. Explain why we use multilayer laminates in aseptic packaging of fluid milk with the
role of each layer.

6. Explain the forming and filling process for aseptic containers with schematic
diagram

Fluid Milk Processing (Student Handbook for Class XI) 115

 Chapter 14

Fluid Milk Processing

Dispensing of Fluid Milk through Bulk
Vending, Bottles, Cartons and Pouches

Introduction
Fluid milk requires a storage place in every step of transfer from milch animal to consumers.
More production of milk and demand of milk irrespective to production sites and also for
the reasons of hygiene and economy, there are the development and establishment of
dairy processing industries worldwide, where milk is processed into various milk products
including the different varieties of fluid milk products such as pasteurized milk (double
tonned, tonned, standardized, full fat, skim milk) sterilized milk, flavoured milk etc. The
growing demand for milk in cities far from production sites directed to the development
of containers suitable for various stages of marketing and distribution. These are metal
cans, glass bottles, pouches, cartons, bulk vending machine etc and thus contributing
substantially in rapid growth of distribution of fluid milk.

Heat treatment is given in processing of fluid milk for enhancing the shelf life and safety.
After that the processed milk is immediately chilled to below 5°C and stored under hygienic
conditions and packaged and subsequently stored and despatched through refrigerated
vehicles to maintain the low temperature. Processing of fluid milk is as follows:

116 Fluid Milk Processing (Student Handbook for Class XI)

 Fig.14.1. Dispensing of fluid milk

Pasteurization (HTST - high-temperature short-time - heating at 72°C for 15 seconds)

followed by cooling to below 5°C and packing in different types of useable and single
used containers for dispensing of milk.

Sterilization is achieved partly by using a more severe heat treatment (about 110°C for
20–30 min) and partly by applying the treatment after the bottle is filled and sealed. The
milk packages are stored and distributed under ambient conditions.

UHT treatment is a process of high bactericidal effect, developed as a continuous flow

process in which the milk is heated at 135°C–150°C for about two seconds followed by
aseptic packaging in sterile containers.

Pasteurized milk in general, a shelf life of several days at a temperature below 10°C
can be assumed. In-bottle sterilized milk can normally be kept for weeks and UHT milk
aseptically packaged can be kept for several months under ambient conditions. Sterilized
and UHT milks are commodities which by their very nature must be distributed to the
consumer as a packaged product. Pasteurized milk, however, which is normally required
to have a shelf life, under refrigeration, of only a few days can be distributed in wholesale
quantities provided the necessary care is taken to prevent contamination.

Fluid Milk Processing (Student Handbook for Class XI) 117

 Returnable packaging system: Returnable packaging containers are collected after use
and washed and sterilized before re-filling. For pasteurized milk this must be refrigerated.
Basic operation involved is washing and sterilization of container, filling and capping, crating,
and storage in stacking. This is suitable for pasteurized milk (e.g. bottles, aluminium cans
etc.) and sterilized milk (e.g. in bottles).

Single-service packaging system: Single-service containers are discarded after use.

There is no collection and washing of the milk packages - only crates are collected and
washed. Intermediate storage of packing material and filled packages is required and this
must be provided in the plant. Two basic types of single-service containers are cartons
and plastic sachets. This packaging system is suitable for pasteurized and UHT milk.
Plastic sachets are usually pillow-shaped and made of low density polyethylene film. They
may be reeled single or double film or lay-flat tube, the latter avoiding the necessity of
making the longitudinal seam in the packaging machine. The material should be coloured
to reduce light transmission and the colour code of packaging is used for identifying the
milk type packed easily during dispensing.

Fig.14.2. Packaging of pasteurized milk in pouches

118 Fluid Milk Processing (Student Handbook for Class XI)

 Fig.14.3. Storage of packed milk in cold store

Tetrahedral cartons made from polyethylene laminated paper board and packed in hexagonal
plastic crates have been chosen as the model for analysis of a system of pasteurized milk
packaging. This is very flexible type packaging system for dispensing the liquid milk.

Liquid milk is most commonly sold to the consumer by general grocery stores, selected
dairy shops, etc. Distributors maintain the cold chain through refrigeration facilities
for marketing pasteurized milk and sterilized and UHT milk is sold by retailers under
conventional conditions.

Automatic Milk Vending Machines

This does not require packaging of milk and milk is directly dispensed into container
provided by consumers. Cooled pasteurized milk is delivered by tanker to the vending
station from where milk is directly purchased by consumer. This consists of a refrigerated
room housing a milk storage tank of the requisite capacity. The milk is pumped from the
tanker to the storage tank from which it is discharged as required through the coin- or
token-operated dispensers like a petrol pump.

Fluid Milk Processing (Student Handbook for Class XI) 119

 Activity
zz Visit a pasteurized milk processing plant where liquid milk is processed and packed
in retail packaging system.

zz Visit a bulk milk vending system nearby city (e.g. Bulk milk vending system is operated
in New Delhi by the National Dairy Development Board/ Mother Dairy, Delhi).

REVIEW QUESTIONS

1. Compare Bulk vending vs retail packaging of milk.

2. Single use and reuse packaging system of liquid milk.

3. What do you mean by in bottle sterilization of milk.

4. Differentiate UHT sterilization vs in bottle sterilization.

5. Give shelf life of pasteurized milk under refrigerated conditions.

120 Fluid Milk Processing (Student Handbook for Class XI)

CENTRAL BOARD OF SECONDARY EDUCATION, DELHI
Shiksha Kendra, 2 Community Centre, Preet Vihar, Delhi-110092, India