Cooking science

Evaporative cooling

Turning water into steam takes a great deal of energy, and that energy comes out of whatever the water was sitting on. As long as there is water evaporating from a surface, that surface is being actively cooled — which is why wet food cannot brown.

Evaporation takes energy out of the food

It takes far more energy to turn water at boiling point into steam than it does to heat the same water from cold to boiling. That energy has to come from somewhere, and it comes from the surface the water is sitting on.

So a wet surface in a hot pan is being heated by the pan and cooled by its own evaporation at the same time, and the two roughly balance at around the boiling point of water. It cannot go higher until the water has gone.

This is the mechanism behind a surprising number of kitchen facts: why food steams instead of browning, why a covered pan cannot brown anything, and why the inside of a piece of meat cannot exceed boiling point while it still contains water.

Where it works against you

• Anything that will not brown. The surface is being held at boiling point by its own evaporation.
• A crowded pan, where released moisture keeps every surface wet.
• Roasting under a lid or foil. The moisture cannot leave, so it condenses back and the cooling continues.
• Vegetables straight from washing. The water on them has to be paid for before anything else happens.
• Frozen food in a hot pan. Melting and then evaporating takes an enormous amount of energy.
• A wet oven — several trays of wet food raise the humidity and slow browning across all of them.

Where cooks use it deliberately

• Steam in a bread oven. Keeping the crust cool and flexible for the first ten minutes lets the loaf expand fully before the crust sets.
• A water bath for a custard or a cheesecake. The water cannot exceed boiling point, so the custard sets gently and does not curdle.
• A splash of water in a pan to stop something browning further — the classic way to arrest caramelising onions.
• Covering vegetables to steam them, where you want them cooked and not coloured.
• Spritzing a barbecue to slow the surface and keep it receptive to smoke.
• Blanching, where the food never exceeds boiling point and so cannot brown.

The related ideas

Surface drying

What has to happen before browning can start.

Crowding suppresses evaporation

Why quantity in the pan decides the outcome.

Internal steam generation

The same physics working inside the food.

Condensation

The reverse process, and why it makes crisp things soggy.

Evaporation

The parent process.

The stall, and why a big joint stops getting hotter

Anyone who has cooked a large piece of meat slowly has watched its temperature climb steadily and then simply stop for a couple of hours, well short of done, with the oven or smoker running exactly as before.

That plateau is evaporative cooling reaching equilibrium. The surface is losing heat to evaporation at very nearly the same rate the oven is supplying it, so the net energy going into the meat approaches zero and the temperature stops rising. It resumes only when the surface has dried enough for evaporation to slow.

The remedies all reduce evaporation rather than adding heat: wrapping the meat, raising the humidity, or simply waiting it out. Turning the oven up mostly increases evaporation too, which is why it helps far less than it should.

Common questions