Milk is made up of water, fats, carbs and protein. When milk is boiled, soluble milk proteins are denatured and then coagulate with milk's fat and form a sticky film across the top of the liquid, which then dries by evaporation. The layer does not need to be discarded and can be consumed, as protein's nutritional value is unaffected by the denaturation process. When 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.
Bringing it to a boil quickly can burn the sugars and curdle the whey protein. The milk and cream contain casein proteins that gather near the surface as they heat. Once milk comes to a boil, steam bubbles rising from the bottom of the pot are forced through the protein rich layer at the top.The inclusion of the water vapor in the bubbles lowers the density of the mixture, causing it to expand and overflow the pot.
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