Factors affecting nature of milk

November 15, 2020

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Factors affecting nature of milk

Normal whole milk contains a balanced proportion of milk fat (4 per cent), lactose (4.8 per cent), proteins (3.5 per cent), minerals (0.7 per cent), vitamins and other minor constituents such as enzymes and hormones. The pH of normal raw milk is about neutral (pH 6.7) with a corresponding titratable acidity of 0.16-0.17 per cent due to the natural buffering capacity of milk proteins and salts. Wholesome milk should contain only a few bacteria and no extraneous matter, if it has been produced hygienically.

Factors affecting nature of milk

Depending on how milk is handled during and after milking, the natural composition and physico-chemical properties of raw milk may change.

Effect of milking practices : Incomplete milking results in low milk yield and low fat content because the last milk (strippings) contains more fat than the foremilk.
Stage of lactation: Immediately after calving, a cow produces colostrum during the ﬁrst ﬁve days, after which the milk reverts to its normal composition. Colostrum is also more alkaline (pH 6.8–6.9) than normal milk.
Effect of mastitis : On farms practicing good husbandry, 20 to 30 per cent of lactating cows have one or more quarters infected with sub-clinical mastitis. With poor hygiene, up to 70–80 per cent of the cows may be affected. The composition of mastitis milk approaches that of blood. It has more whey proteins, less casein and less water-soluble vitamins. It also tends to be more alkaline, has a higher chloride content than normal milk, and tastes salty like the milk of very old cows (more than six lactations) or milk of cows in late lactation (near drying off).
Effect of feeding : Cows have to be fed properly. If cows are fed a diet low in forages and high in starch, the butterfat content of the milk may fall below 2.5 per cent. A good forage-to-concentrate ratio is important to enable cows produce good quality milk to their potential.
Effect of cold storage : On cooling milk, the multiplication of bacteria delays, except for a few cold-tolerant bacteria (psychrotrophs) which can even grow at refrigeration temperatures. If milk is kept chilled at 4°C for more than 72 hours, the cold-tolerant bacteria will multiply and produce lipase and protease enzymes that, respectively, break down milk fat and proteins. These enzymes are also heat resistant, and can cause spoilage of pasteurised milk and other processed dairy products. Extended cooling also makes calcium in the milk less soluble and unavailable during coagulation of milk by rennet in cheese making.
Effect of heating : Pasteurisation of milk involves heating it to 63°C for 30 minutes or 72°C for 15 seconds in order to destroy harmful microorganisms. Pasteurisation kills more than 90 per cent of bacteria and causes minor denaturation of proteins and loss of some water-soluble vitamins.
Effect of treatment of cows with antibiotics : When cows suffer from mastitis they are treated with antibiotics by intramammary or intramuscular injection. The antibiotics circulate in the blood and are secreted in the milk for up to 72 hours. Longer acting (slow release) antibiotics, such as are used in dry cow therapy against mastitis, remain in the blood longer. Drug residues in milk are undesirable because they can trigger allergies and drug resistance in humans, and inhibit the lactic acid starter cultures used in the manufacture of fermented milk products. For this reason, milk processors routinely screen raw milk for antibiotic.