SOMETIMES HOT milk can seem to be the only remedy for a sleepless night. But this often means having to remove an unpleasant skin from the top of your bed-time drink with a teaspoon (or enduring a novelty white moustache). This skin formation is due to the loss of solids that the milk undergoes as it is warmed up.
As heat is applied to the milk, the proteins casein and beta-lactoglobulin start to coagulate, and form a skin on the surface. After further heating, the skin dries because of evaporation, and forms a still firmer barrier. Steam produced under the skin builds up and causes the milk to boil over.
The action of whisking stops the skin from forming (by creating a foam over the surface) and the milk from boiling over. Skin will also form on milky drinks after heating, but adding toppings - marshmallows or whipped cream, say - prevents this.
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