Question Corner: Boiling milk

The boiling point of milk is 100.5°C. Photo: S. Siva Saravanan

Boiling milk

Why does milk boil over after heating but water does not?


Water is a simple liquid which does not contain any solids (if it is, in ppm level only) and the boiling temperature is 100°C.

Milk is a compound liquid which contains fat in emulsion form, protein in colloidal state and lactose as true solution and the boiling point of milk is 100.5°C. When milk is heated, the fat which is lighter than water is collected on the surface along with certain protein in the form of a layer called cream.

During heating, the water vapour being lighter than all other ingredients in the milk will rise up. Since the surface of the milk is covered with a thin layer of cream, the water vapour gets trapped below the thin layer and layer prevents the water vapour from escaping.

When milk is heated further, the water vapour expands, pressure builds up and lifts the creamy layer up and thus the milk spills out.

But in case of water, the water vapour escapes very easily on boiling since it does not have any layer on the surface to interrupt till complete evaporation of the liquid.


Dy. Gen. Manager (QC)

Tamil Nadu co-op milk producers’ federation Ltd


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Printable version | Apr 25, 2022 6:28:59 am |