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PHYSICAL PROPERTIES OF MILK
Milk has a pH of about 6.5 to 6.7. The salts of the minerals - calcium, phosphorus, sodium and potassium help to maintain this pH level.
The viscosity of milk is affected by temperature, amount and nature of dispersion of protein and fat, acidity and the effects of various enzymes and bacteria. Homogenization increases the viscosity of milk.
Freezing Point :
The freezing point of milk is -0.55 o C.
Boiling Point :
Milk boils at 100.2 o C.
PASTEURISATION OF MILK
Milk is a favourable medium for bacterial growth. Pasteurisation destroys all pathogenic bacteria, including those causing typhoid, tuberculosis, diphtheria as well as yeasts and moulds.
Pasteurization is a process which consists of heating milk to a certain temperature for a definite time to ensure destruction of harmful bacteria. There are three methods of pasteurisation.
a) Holding method or Batch process :
In this method, milk is held at 62.8 o C for 30 minutes and then rapidly cooled to prevent multiplication of surviving bacteria.
b) High temperature short time (HTST) method or continuous process :
Milk is heated to 71.7 o C for not less than 15 seconds.
c) Ultra High temperature method :
Milk is heated to a temperature of 93.4 o C for 3 seconds.
Advantages of Pasteurization
· During pasteurization the nutritive value of milk is not altered to a great extent because the temperature employed is not high and cooking time is short. However, there is a slight decrease in heat labile vitamin such as thiamine and ascorbic acid. Proteins are denatured only slightly and minerals are not appreciably precipitated.
· It does not produce an unpleasant cooked flavour.
· Shelf life of milk is increased due to a marked decrease in the total bacterial count.
Harmful pathogens especially TB bacteria are destroyed.
· It inactivates enzymes such as phosphatase and lipase in milk which adversely affect the quality of milk.
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