Milk rises when boiled primarily due to the formation of a skin on the surface that traps steam produced by the boiling water content within the milk.
Here's a breakdown of the process:
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Milk Composition: Milk is an emulsion, meaning it's a mixture of fat, water, proteins, and sugars.
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Heating and Separation: As milk heats, the fat molecules start to separate and rise to the surface. Proteins also denature (unfold) and aggregate, contributing to surface film formation.
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Skin Formation: The fat and denatured proteins combine to form a skin or layer on the milk's surface.
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Steam Entrapment: When the water in the milk reaches its boiling point, it turns into steam. This steam is trapped beneath the skin layer that has formed on top.
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Rising and Overflowing: The trapped steam exerts pressure on the skin, causing it to rise. If the pressure becomes too great, the steam will eventually push the skin over the edge of the pot, resulting in the milk boiling over.
In contrast, water doesn't rise and overflow in the same way because it doesn't contain fat and proteins that form a skin. The steam can escape freely from the surface of boiling water.
