Milk used for drinking purposes should be pasteurised for
hygienic and other reasons. In the case of dairies, pasteurising
is done with the help of steam. In houses it is boiled for want
of a suitable alternative. The object of pasteurising is (a) to
destroy certain disease producing organisms and bacteria and to
retain others which are said to be helpful, (b) to prolong the
keeping quality of milk by about 60 to 72 hours and (c) to
retain its digestibility which is seriously affected by boiling. The
boiling point of milk is 100.2°C. Its specific gravity normally
ranges between 1.028 to 1.033 (g/ml) and free acidity between
12o to 25°. When the acidity rises to 26 and beyond, milk
coagulates, even at a temperature of 30°C. Milk is an emulsion
of fat and proteins, the other main ingredients being sugar, salts
of iron, calcium and phosphates, vitamins, ferments, etc. The
proteins are of three types, casein, albumin and globulin, the
former being the major constituent. |