Why does milk boiling in a milk cooker not overflow?
The major constituent of milk is water i.e. 85-87 per cent and the rest is distributed among fat, protein, sugar and salts. The boiling point of milk (100.17 degree Celsius), therefore, is slightly higher than that of water (i.e. 100 degree Celsius). However before milk is brought to boiling relatively less dense constituents like the fat and the proteins partially segregate and mostly float up to form the layer of cream. This is a covering layer and does not allow water vapour to pass through it normally.
When milk is boiled in an ordinary vessel, the temperature of the liquid rises, even above the boiling point and a large quantity of water vapour is generated below the cream cover. This makes the cream layer bulge out.
Owing to its lower mechanical strength when the steam pressure builds up on the lower side of the membrane cream layer, it tears, releases some amount of the water vapour and the bulge collapses. In the place of the rupture soon the membrane gets repaired by fresh supply of the tiny protein and fat particles, abundantly present in the nearby medium.
Thus, the area of the top covering layer gradually increases and the surface appears bulged out, in order to accumulate a large amount of the water vapour. Ant at some point of time, the bulged out sheet of cream out grows the available space in the vessel, resulting in the overflow of the milk.
On the other hand when milk is heated in a milk cooker all these above processes do not occur. The milk cooker is a double-walled vessel with the annular gap containing water, which boils at a lower temperature than the boiling point of milk. As the boiling of water continues, the supplied heat is used in transformation of liquid water into water vapour at 100 degree C and the temperature of the water and the vessel remains constant at 100 degree Celsius.
The cream layer forms on the top of the mass of milk, like the earlier case. However, the temperature being lower than the boiling point of milk, it does not cause the vapour bubbles to form and bulge out of the cream layer. Thus milk does not overflow.
DR. K. ANBARASU
Quality Control Wing
Tamil Nadu Milk Producers Federation Limited