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.

Milk fat (lipid): The bulk of the fat in milk exists in the form of small globules, which average approximately 2 to 5 microns in size (range 0.1 to 22 microns). This is an oil-in-water adsorbed layer of material commonly known as the fat globule membrane. This membrane contains phospholipids and proteins in the form of a complex, and stabilizes the fat emulsion. In other words, the membrance prevents the fat globules from coalescing but keeps separated from one another. The thickest cream layer is secured from milks which have a higher fat content and relatively large fat globules (such as buffalo’s milk when compared with cow’s milk).

Milk proteins: Proteins are among the most complex of organic substances. They are vital for living organisms, as they constitute an indispensable part of the individual body cell. Proteins are composed of a large number of amino-acids, some ‘essential’ and other ‘non-essential’. The essential amino-acids are necessary in the diet for the formation of body proteins. On hydrolysis, proteins furnish a mixture of amino-acids. The proteins of milk consist mainly of casein, b -lactoglobulin, a -lactalbumin, etc. Casein exists only in milk and is found in the form of a calcium caseinate-phosphate complex. It is present in the colloidal state. It forms more than 8 per cent of the total protein in milk. Casein itself is composed of a , b , g fractions. b -lactoglobulins and a -lactalbumin are also known as whey or serum-proteins. They are also present in the colloidal state and are easily coagulable by heat.

Milk sugar or lactose: On crystallization from water, it forms hard gritty crystals. It is one-sixth as sweet as sucrose. Lactose is responsible, under certain conditions, for the defect known as ‘sandiness’ in ice cream and sweetened condensed milk. Chemically, lactose is composed of one molecule each of glucose and galactose.

Mineral matter or ash: The mineral matter or salts of milk, although present in small quantities, exert considerable influence on the physico-chemical properties, exert nutritive value of milk. The major salt constituents, i.e., those presents in appreciable quantities, include potassium, sodium, magnesium, calcium, phosphate, citrate, chloride, sulphate and bicarbonate.

Phospholipids: In milk, there are three types of phospholipids, viz. Lecithin, cephalin and sphingomylin. Lecithin, which forms an important constituent of the fat globule membrane, contributes to the fat globule membrane, contributes to the ‘richness’ of flavour of milk. It is highly sensitive to oxidative changes, giving rise to oxidized/ metallic flavours. Phospholipids are excellent emulsifying agents, and no doubt serve to stabilize the milk fat emulsion.

Pigments: These are 1) fat soluble, such as carotene and xanthophyll, and 2) water soluble, such as riboflavin. Carotene is the colouring matter of all green leaves, where it is masked by chlorophyll. Carotene (the pure substance of which has a reddish-brown colour) is fat soluble and responsible for the yellow colour of milk, cream, butter, ghee and other fat-rich dairy products. Besides contributing to the colour of cow milk, carotene acts as an anti-oxidant and also as a precursor of vitamin A. Riboflavin, besides being a vitamin, is a greenish-yellow pigment, which gives the characteristic colour to whey.

Enzymes: These are ‘biological catalysts’, which can hasten or retard chemical changes without themselves participating in the reactions. The enzymes are protein-like, specific in their actions, and inactivated by heat, each enzyme has its own inactivation temperature. The important milk enzymes and their specific actions are as follows. 1) Analase (diastase) – starch splitting, 2) Lipase – fat splitting, leading to rancid flavour, 3) Phosphate – capable of splitting certain phosphoric acid esters (basis of phosphatase test for checking pasteurization efficiency); 4) Pro-tease – protein splitting; 5) Peroxidase and Catalase – decomposes hydrogen peroxide.

Vitamins: Those found in milk are; fat-soluble vitamins A, D, E and K; and water-soluble vitamins of the ‘B-complex’ group (such as thiamine or B₁, riboflavin or B₂, pantothenic acid, niacin, pyridoxine or B₆, biotin, B₁₂, folic acid, etc.) and vitamine C (ascorbic acid).

Acidity: Freshly drawn milk is amphoteric to litmus, i.e., it turns red litmus blue and blue litmus red. However, it shows a certain acidity as determined by titration with an alkali (sodium hydroxide) in the presence of an indicator (phenolphthalein). The titrable acidity is usually expressed as a ‘percentage of lactic acid’.

PH: The pH of normal, fresh, sweet milk usually varies from 6.4 to 6.6 for cow milk and 6.7 to 6.8 for buffalo milk. Higher pH values for fresh milk indicate udder infection (mastitis) and lower values, bacterial action.

Density and Specific Gravity

Density of a substance is its mass (weight) per unit volume, specific gravity is the ratio of density of the substance to density of a standard substance (water). Milk is heavier than water. The average specific gravity ranges (at 60⁰F) 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.

As milk fat is the lightest constituent, the more there is of it the lower the specific gravity will be, and vice versa.

Freezing point of milk

Milk freezes at temperatures slightly lower than water due to the presence of soluble constituents such as lactose, soluble salts, etc., point depression of Indian cow milk may be taken as 0.547⁰C (31.02⁰F) and buffalo milk 0.549⁰C (31.01⁰F). The freezing point test of milk is a highly sensitive one and even up to 3 per cent of watering can be detected. The fat and protein contents of milk have no direct effect on the freezing point of milk. The drawbacks of the freezing point test are; 1) it does not detect the addition of skim milk or removal of fat from the milk sample; and 2) watered milk, which has subsequently soured, may pass the test.

Colour of milk

The colour is a blend of the individual effects produced by; 1) the colloidal casein particles and the dispersed fat globules, both of which scatter light, and 2) the carotene (to some extent xanthophyll) which imparts a yellowish tint. Milk ranges in colour from yellowish creamy white (cow milk) to creamy white (buffalo milk). The intensity of the yellow colour of cow milk depends on various factors such as breed, feeds, size of fat globules, fat percentage of milk etc.

Flavour

This is composed of smell (odour) and taste. The flavour of milk is a blend of the sweet taste of lactose and salty taste of minerals, both of which are damped down by proteins. The phospholipids, fatty acids and fat of milk also contribute to the flavour. Changes in the flavour of milk occur due to type of feed, season, stage of lactation, condition of udder, sanitation during milking and subsequent handling of milk during storage. The sulfydryl compounds significantly contribute to the cooked flavour of heated milks.

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