Does boiling affect calcium, proteins and vitamins present in milk?



Milk is heat treated (Pasteurized) at the Dairy to kill any pathogenic micro-organisms that may be present without affecting its nutritional quality. But milk boiling (100 degrees C) in houses causes changes in its constituents. The higher the temperature and the longer the exposure to heat the greater the changes.

The Solubility of calcium phosphate is very temperature-dependent. Unlike most compounds, the solubility of calcium phosphate decreases with temperature. This means that heating 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 temperatures, the reversibility is more sluggish and incomplete.

The changes at high temperature imply that the milk becomes more acid and pH drops.

Major protein, Casein (2.8 per cent) is not considered denaturable by heat within normal ranges of pH, salt and protein content.

Whey proteins (0.7 per cent), on the other hand, particularly Beeta-Lactoglobulin which makes up about 50 per cent of the whey proteins, are fairly heat sensitive. Denaturation begins at 65 degrees C and is almost total when whey proteins are heated to 90 degrees C for five minutes.

For whey proteins heat denaturation is an irreversible reaction. The randomly coiled proteins “open up”, and Beta-Lactoglobulin in particular is bound to the alpha casein fraction by sulphur bridges.

Milk is an important source of A, D and group B vitamins. The fat soluble vitamins are very thermo stable and their level is not lowered by heat treatment. However, when milk is fortified with vitamin A, the relative loss seems to increases.

Losses of vitamins 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 B12 and protects folic acid from oxidation.