Milk is heat treated (pasteurized) at the dairy to kill any pathogenic microorganisms that may be present without affecting its nutritional quality.
But milk boiling (100C) in houses for the preparation of tea causes changes in its constituents. The higher the temperature and longer the exposure to heat, the greater the changes. Milk is an important source of A, D and group B vitamins. The fat soluble vitamins (A & D) are very thermostable and their level is not lowered by heat treatment. Losses of water soluble vitamins (B & C) mainly concern vitamin C and some of the group B vitamins.
The loss of vitamin C as such is generally of minor importance, as milk is not an important source of this vitamin, but it may influence the nutritional value anyway. The breakdown of vitamin C is connected with that of Vitamin B 12 and protects folic acid from oxidation.
Milk is a major source of the important mineral calcium (1200 mg/lit). Solubility of calcium phosphate is very temperature dependent.Unlike most compounds, the solubility of calcium phosphate decreases with temperature. This means that heating the milk for the preparation of tea causes precipitation of Calcium Phosphate in the micelle, while cooling increases its concentration.
After cooling, the reaction is readily reversible but after heating to high temperature, the reversibility is more sluggish and incomplete. Hence for the preparation of tea instead of boiling the milk it can be warmed up to retain the minerals and vitamins.
Published in The Hindu on Sep 12, 2012.