Milk cooker
Why is it that milk when boiled in a milk cooker does not spill?
P. Surendranath
Kochi, Kerala
Most part of milk, more than 85 per cent, is water and the rest is distributed among fat, protein, sugar and salts. The boiling point of milk, therefore, is slightly higher than that of water (i.e. 100 degree Celcius); by about half a degree. However, before milk is brought to boiling the 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 membranous 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. And at some point of time, the bulged-out sheet of cream outgrows the available space in the vessel, resulting in the spillage 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{ring}C and the temperature of the water and the vessel remains constant at 100 degree Celcius.
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 the cream layer. Thus, milk does not spill out.
H. K. SAHU
Senior Scientific Officer
IGCAR, DAE, Kalpakkam
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