Why does milk get a skin, and why do bad eggs float?

We explore the curious questions that science can answer

When warm milk cools down, evaporation causes a skin to form (Getty/iStock)

Why does milk get a skin when it cools down?

The skin is a complex of casein, a milk protein, and calcium. It is the result of evaporation at the surface of the milk, which concentrates the proteins there. Skin formation can be minimised by covering the pan or by whipping up a little foam, both of which slow down the rate of evaporation.

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