composition of milk, properties of milk.

1. Unit 1: MILK
By
Ms. Apoorva Udayashankar
Assistant Professor
Department of Life Sciences
Kristu Jayanti Auotonomous college
Bangalore

2. Introduction to Milk
• Milk is the secretion of mammillary glands in female mammals. It is the sole source
of food for newborns and plays a key role in their nourishment and in
immunological protection.
• Milk is a complex biological fluid consisting of seven main components: water, fat,
protein, sugar (lactose), minerals, vitamins and enzymes. It is a white opaque
fluid in which fat is present as an emulsion, protein, and some mineral matters in
colloidal suspension and lactose together with some minerals and soluble proteins in
true solution. The opacity of milk is due to its content of suspended particles of fat,
proteins and certain minerals. The colour varies from white to yellow depending on
the carotene content of the fat. Milk has a pleasant, slightly sweet taste, and pleasant
odour. It is an excellent source of calcium, phosphates and riboflavin.

3. Physical Properties of Milk
Colour and optical property
Milk appears turbid and opaque owing to light scattering by fat globules and casein micelles. Optical
properties are influenced by the manner of scattering of light by the molecules. Light scattering
occurs when the wavelength of light matches the magnitude of the particle. Thus, smaller particles
scatter light of shorter wavelengths and vice versa. Skim milk appears slightly blue because casein
micelles scatter the shorter wavelengths of visible light (blue) more than the red. Beta-carotene, the
carotenoid precursor of vitamin A, is responsible for the creamy colour of cow milk. The greenish
tinge in whey is due to the presence of riboflavin. Refractive index of milk is an optical property and
ranges from 1.3440 to 1.3485 at 20ºC. The relation between solids content of milk and refractive
index is linear, and the contributions of the several constituents is additive.

4. Flavour
The natural sweet flavour of milk is due to the combined effect of its components.
Off-flavours are very quickly developed in milk owing to several factors. The feed
consumed by animals may lead to some undesirable flavours. Bacterial growth in
milk causes fruity, barny, malty or acid flavours. Enzyme activities also may lead to
unnatural flavours, rancidity due to lipase action being a classic example. Oxidative
reactions may cause a cardboard flavour in milk. Processing of milk may produce
cooked flavours.

5.  Specific gravity and density
Milk is heavier than water. The specific gravity of cow milk varies from 1.018 to 1.036 and of buffalo milk
from 1.018 to 1.038. Though specific gravity varies with temperature, (lower at higher temperature and
vice versa), the rate of this variation is not uniform.
The density of milk varies within the range of 1.027 to 1.033 kg/cm3 at 20°C. The density of milk is used
to estimate the solids content, to convert volume into mass and vice versa and to calculate other physical
properties such as dynamic viscosity. It is dependant on temperature at the time of measurement,
temperature history of the sample, composition of the sample (particularly fat content) and inclusion of air.
Viscosity
Viscosity of milk depends on the temperature and the amount and state of dispersion of the solid
constituents, mainly casein and fat. Viscosity of the whole milk at 25°C is about 2.0 cP. Cooler
temperatures increase viscosity due to the increased voluminosity of casein micelles whereas temperatures
above 65°C increase viscosity due to the denaturation of whey proteins. An increase or decrease in pH of
milk also causes an increase in casein micelle voluminosity. The effect of agitation on viscosity is not
uniform. Sometimes, agitation causes partial coalescence of the fat globules, hence increasing the viscosity
and at other times, agitation may disperse fat globules that have undergone cold agglutination, leading to a
decrease in viscosity.

6. • Surface tension
The surface activity of milk is related to proteins, fat, phospholipids and
fresh fatty acids present in it. Homogenization and heat sterilization
increase the surface tension of milk. Milk has a surface tension of 50
dyne/cm at 20°C.
Freezing and boiling points of milk
The freezing points of cow and buffalo milk vary from -0.512 to -0.572ºC and
from -0.521 to -0.575ºC respectively. Freezing point of milk is mainly used to
determine added water. The boiling point of milk is 100.17ºC.

7. Acidity and pH
Freshly drawn milk has a pH value in the range of 6.5 to 6.7
 Heat stability of milk
Heat stability is defined as the length of time required to induce
coagulation at a given temperature or the temperature required to induce
coagulation in a given time. The stability of milk system at the high
processing temperatures to which milk is exposed for the manufacture of
certain products is very important. Caseins and salt balance of milk govern
its heat stability. Added citrates, phosphates and calcium have a great
impact on the heat stability.

8. Composition of milk
• In general, the gross composition of cow's milk in the U.S. is 87.7% water, 4.9% lactose
(carbohydrate), 3.4% fat, 3.3% protein, and 0.7% minerals (referred to as ash). Milk composition
varies depending on the species (cow, goat, sheep), breed (Holstein, Jersey), the animal's feed, and the
stage of lactation. Although there are minor variations in milk composition, the milk from different
cows is stored together in bulk tanks and provides a relatively consistent composition of milk year
round in the U.S.

9. Milk Microbiology
• In addition to being a nutritious food for humans, milk provides a
favorable environment for the growth of microorganisms. Yeasts, moulds
and a broad spectrum of bacteria can grow in milk, particularly at
temperatures above 16°C.
• Microbes can enter milk via the cow, air, feedstuffs, milk handling
equipment and the milker. Once microorganisms get into the milk their
numbers increase rapidly. It is more effective to exclude microorganisms
than to try to control microbial growth once they have entered the milk.
Milking equipment should be washed thoroughly before and after use.

10. Bacterial types commonly associated with
milk.
Pseudomonas Spoilage
Brucella Pathogenic
Enterobacteriaceae Pathogenic and spoilage
Staphylococci
Staphylococcus aureus Pathogenic
Streptococcus
S. agalactiae Pathogenic
S. thermophilus Acid fermentation
S. lactis Acid fermentation
S. lactis-diacetyllatic Flavour production
S. cremoris Acid fermentation
Leuconostoc lactis Acid fermentation
Bacillus cereus Spoilage
Lactobacillus
L. lactis Acid production
L. bulgaricus Acid production
L. acidophilus Acid production
Propionibacterium Acid production
Mycobacterium tuberculosis Pathogenic
able 3. Bacterial types commonly associated with milk.

11. Factors affecting milk composition
• Genetic
Milk composition varies considerably among breeds of dairy cattle: Jersey and Guernsey breeds give milk of higher fat and
protein content than Shorthorns and Friesians. Zebu cows can give milk containing up to 7% fat.The potential fat content of
milk from an individual cow is determined genetically, as are protein and lactose levels. Thus, selective breeding can be used
to upgrade milk quality. Heredity also determines the potential milk production of the animal. However, environment and
various physiological factors greatly influence the amount and composition of milk that is actually produced. Herd recording
of total milk yields and fat and SNF percentages will indicate the most productive cows, and replacement stock should be
bred from these.
• Interval between milkings
The fat content of milk varies considerably between the morning and evening milking because there is usually a much shorter
interval between the morning and evening milking than between the evening and morning milking. If cows were milked at
12-hour intervals the variation in fat content between milkings would be negligible, but this is not practicable on most farms.
Normally, solids-not-fat content varies little even if the intervals between milkings vary.
• Stage of lactation
The fat, lactose and protein contents of milk vary according to stage of lactation. Solids-not-fat content is usually highest
during the first 2 to 3 weeks, after which it decreases slightly. Fat content is high immediately after calving but soon begins to
fall, and continues to do so for 10 to 12 weeks, after which it tends to rise again until the end of the lactation.

12. • Age
As cows grow older the fat content of their milk decreases by about 0.02 percentage units per
lactation.
Feeding regime
Underfeeding reduces both the fat and the solids-not-fat content of milk produced, although
solids-not-fat content is more sensitive to feeding level than fat content. Fat content and fat
composition are influenced more by roughage (fibre) intake. The solids-not-fat content can
fall if the cow is fed a low-energy diet, but is not greatly influenced by protein deficiency
unless the deficiency is acute.
• Disease
Both fat and solids-not-fat contents can be reduced by disease, particularly mastitis.
• Completeness of milking
The first milk drawn from the udder is low in fat while the last milk (or strippings) is always
quite high in fat. Thus it is essential to mix thoroughly all the milk removed, before taking a
sample for analysis.

13. Processing of milk
• Milk is a valuable nutritious food that has a short shelf-life and requires careful handling.
Milk is highly perishable because it is an excellent medium for the growth of
microorganisms – particularly bacterial pathogens – that can cause spoilage and diseases
in consumers. Milk processing allows the preservation of milk for days, weeks or months
and helps to reduce food-borne illness

14. Milk pasteurization
• Pasteurization is the process used to destroy bacteria in milk. In pasteurisation, the milk is heated
to a temperature sufficient to kill pathogenic bacteria, but well below its boiling point. This also
kills many non-pathogenic organisms and thereby extends the storage stability of the milk.
• Numerous time/temperature combinations are recommended but the most usual is 72°C for 15
seconds followed by rapid cooling to below 10°C. This is normally referred to as High
Temperature Short Time (HTST) treatment. It is carried out as a continuous process using a plate
heat-exchanger to heat the milk and a holding section to ensure that the milk is completely
pasteurised. Milk is normally pasteurised prior to sale as liquid milk. Pasteurisation is used to
reduce the microbial counts in milk for cheesemaking, and cream is pasteurised prior to
tempering for buttermaking in some factories.
• Batch pasteurization is used where milk quantities are too small to justify the use of a plate heat-
exchanger. In batch pasteurization, fixed quantities of milk are heated to 63°C and held at this
temperature for 30 minutes. The milk is then cooled to 5°C and packed.
• Another method of is ultra-heat treatment, or UHT. In this system, milk is heated under
pressure to about 140°C for 4 seconds. However, it retains more of the properties of fresh
milk than conventionally sterilised milk.

15. Effects of pasteurization on milk
• Pasteurization reduces the cream layer, since some of the fat globule
membrane constituents are denatured. This inhibits clustering of the fat
globules and consequently reduces the extent of creaming. However,
pasteurization does not reduce the fat content of milk.
• Pasteurization has little effect on the nutritive value of milk. The major
nutrients are not altered. There is some loss of vitamin C and B group
vitamins, but this is insignificant.
• The process kills many fermentative organisms as well as pathogens.
Microorganisms that survive pasteurization are putrefactive. Although
pasteurized milk has a storage stability of 2 to 3 days, subsequent
deterioration is cause by putrefactive organisms. Thus, pasteurized milk
will putrefy rather than develop acidity.

16. Milk sterilization
• In pasteurization, milk receives mild heat treatment to reduce the number
of bacteria present. In sterilization, milk is subjected to severe heat
treatment that ensures almost complete destruction of the microbial
population. The product is then said to be commercially sterile.
Time/temperature treatments of above 100°C for 15 to 40 minutes are
used. The product has a longer shelf life than pasteurised milk.