Cooking science

Evaporation

Evaporation explains more everyday cooking failures than any other single process. Food will not brown, the pan is not hot enough, the sauce is too thin, the vegetables are soggy — all four are the same fact about water leaving, or not.

Water caps the temperature

Liquid water in an open pan cannot exceed 100°C. Add energy and it does not get hotter — it turns to steam and leaves, taking the energy with it. That is why a pan of boiling water sits at the same temperature however high you turn the hob.

The same is true of a wet food surface. As long as there is liquid water on it, that surface is stuck at around 100°C, and the browning reactions that create flavour do not begin until well above 140°C. So nothing browns until the water has gone.

This is the whole reason for a long list of otherwise unrelated instructions: pat the meat dry, do not crowd the pan, steam the potatoes dry before roasting, use a wide tin, let the mushrooms release their water first. Every one of them is about getting water out of the way.

What controls how fast it happens

Surface area

A wide pan evaporates several times faster than a tall narrow one holding the same liquid. This is why reductions are done in a wide pan.

Airflow

Moving air removes the saturated layer sitting against the surface. This is exactly what a fan oven and an air fryer do, and why they brown better.

Crowding

Food releases steam. If it cannot escape, the air around the food stays saturated, no further water can leave, and the food steams in its own moisture.

A lid

Traps steam and returns it. Prevents evaporation entirely, which is right for braising and wrong for reducing.

Temperature

Higher heat evaporates faster, up to the point where the surface scorches before the interior water reaches it.

Humidity

A steamy kitchen genuinely does slow crisping. Open a window.

Evaporative cooling

Turning water into steam takes a great deal of energy, and that energy comes from the food itself. So while water is leaving, the surface is actively being cooled — which is why a wet piece of meat in a hot pan can sit there hissing for a minute without colouring at all.

The same mechanism is why a large joint takes so much longer than its weight suggests, why a fan oven can cook at a lower temperature and get the same result, and why food kept in a very humid oven, deliberately, stays moist: with no evaporation, there is no cooling and no drying.

It is also why steam is so much more dangerous than boiling water. The energy released when steam condenses on your skin is far larger than the heat in the same weight of water at 100°C.

Where it decides the outcome

• Browning anything. Dry surface first, always.
• Roasting vegetables. Space and heat, so the water leaves rather than pooling.
• Mushrooms. They release a lot of water; the whole technique is getting past that phase.
• Reducing a sauce. Wide pan, no lid, and the concentration of everything left behind.
• Braising. The opposite — a lid, so nothing evaporates and the liquid stays put.
• Slow cookers and pressure cookers. Sealed, so the liquid level rises rather than falls. This is why their recipes need far less liquid.
• Crisping and staling. Surface drying makes a crust; moisture migrating back out of the crumb softens it again.

Common questions