The thin skin that forms on milk as it heats comes from protein and fat concentrating on the surface as water evaporates. As the temperature rises, water evaporates from the milk's surface and the remaining protein molecules bond together into a thin layer, which becomes more noticeable as it cools.
Is this skin the same as clotted cream?
This skin is different from traditional clotted cream: clotted cream forms when milk is left to simmer gently over a long time, concentrating the fat layer, while the thin skin that forms on boiling milk is mostly protein-based and forms much faster, within a few minutes. Both collect on the surface of the milk, but the way they form and their texture are different; clotted cream is soft and layered, while the boiling skin is thin and lifts off in one piece.
How thick the skin gets also depends on how fatty the milk is; full-fat milk leaves a noticeably thicker skin than semi-skimmed milk, because the fat molecules help the proteins bond together more easily. Milk heated in a small pot with a narrow surface can leave a thicker skin than milk heated in a wide pot, since the evaporating water concentrates over the same area.
How can skin formation be reduced?
Stirring the milk often while it heats keeps the surface proteins from settling into a single layer, which noticeably cuts down skin formation. Heating the milk in a covered pot over medium rather than high heat also slows evaporation and has the same effect.
The skin that does form can also be put to good use; in dishes like rice pudding or milk custard, stirring it back into the rest of the milk gives a smoother final texture. Anyone who prefers can simply lift it off with a spoon instead, which does not affect the quality of the remaining milk.
Some kitchens loosely rest a plate over the pot while heating milk; letting the steam condense on the plate instead of the milk's surface can help keep less protein concentrating directly on top, leaving a thinner skin.
The temperature range the milk is held at also matters; milk kept close to boiling for a long time leaves a thicker skin, while milk warmed gently and taken off the heat before it boils leaves a much thinner, almost unnoticeable layer.
Plant-based milks such as almond or oat milk either do not form this skin at all or form a very different kind, because the type and amount of protein in them differs from cow's milk. Milk that has been given a long shelf life through UHT processing can also form a slightly weaker skin than freshly pasteurised milk, since its proteins have already been exposed to high heat once and their structure has partly changed.
The pot's material affects how quickly the skin forms too; a thin metal pot that conducts heat quickly heats the milk's surface faster and starts evaporation sooner, while a heavy-bottomed pot spreads heat more evenly, so the skin forms a little later and thinner. That means the same milk heated for the same length of time can leave a different thickness of skin depending on the pot.
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