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Why Does Chai Get a Skin on Top?

Quick answer: A thin skin forms when hot milk proteins and fat concentrate at the surface while chai cools and water evaporates. It is a normal milk behaviour, not proof that the tea has spoiled.

Why the surface behaves differently

The top of a cup is exposed to air, so it cools and loses moisture faster than the liquid underneath. Milk proteins gather at that interface and form a flexible film, often trapping a little fat. Full-fat milk tends to make the skin more noticeable.

How to reduce it

• Serve chai soon after straining rather than letting it sit uncovered.
• Stir the cup once or twice as it cools if you do not mind disturbing the surface.
• Covering the cup reduces evaporation, although condensation can drip back into the drink.
• Use a thermos or insulated pot when holding chai for several people.

Do not keep boiling to prevent skin

Longer boiling does not permanently solve the surface-film problem. It can instead over-extract tea, concentrate milk solids and make the chai harsher. Brew to the flavour you want, strain and serve.

If reheating chai

Stir the skin back in before gentle reheating if you are comfortable with it, or lift it off. Warm the chai only until hot; repeated hard boiling changes flavour and can form another skin as it cools again.

Skin versus spoilage

A normal milk skin is a thin surface film on otherwise fresh-smelling chai. Sour smell, curdling throughout the cup, unusual separation or chai left warm for an unsafe length of time are different issues and should not be treated as harmless surface skin.