Cooking science

Condensation

Everything hot and moist is giving off steam, and that steam turns back into liquid water wherever it meets something cooler. That is why crisp food goes soggy under foil, why a lid drips, and why chips in a closed container are a disappointment.

Steam has to go somewhere

Hot food releases water vapour. In open air that vapour disperses. Enclosed — under foil, under a lid, in a box, in a stacked pile — it accumulates until the air is saturated, and then it condenses onto whatever surface is coolest.

Usually that surface is the food itself, or the underside of whatever is covering it, from where it drips back on.

A crisp surface is crisp because it is dry. Condensation is water arriving on it. The crust softens within minutes, and once softened it does not crisp again without more heat.

Where it ruins things

• Roast chicken rested under foil. The commonest example. The skin is crisp coming out of the oven and leathery ten minutes later.
• Chips or fried food piled in a bowl. The bottom layer steams itself soft.
• Anything drained on kitchen paper. Paper holds moisture against the underside.
• Crackling put back on the joint under gravy.
• Pastry stored in a sealed container, which goes soft overnight.
• Bread in a plastic bag, where the crust softens even as the crumb stales.
• A takeaway container, which is why delivered fried food is never as good as it was in the kitchen.

Stopping it

1. Rest meat uncovered, or tented very loosely so vapour can escape.
2. Drain fried food on a rack, not on paper, so air reaches the underside.
3. Spread things out in a single layer rather than piling them.
4. Put crisp things in a warm oven with the door slightly ajar if you need to hold them, so vapour leaves.
5. Serve sauce and gravy alongside rather than over anything crisp.
6. For storage, let food cool completely before it goes in a container — warm food in a sealed box condenses onto itself.
7. Store bread cut side down on a board, or in a bag accepting a soft crust, but not both.

Where it is useful

A lid on a braise

Condensation returns liquid to the pot rather than letting it boil away.

Steaming

The whole method depends on vapour condensing on the food and giving up its heat.

A cartouche

Paper on the surface of a stew returns condensed liquid evenly.

Proving dough covered

Stops the surface drying and forming a skin.

Resting dough or pastry wrapped

Same reason.

A domed lid over rice

Returns moisture and steams the grains evenly.

Condensation in storage, where it costs most

Most of the damage condensation does happens after cooking, in containers and in the fridge, rather than in a pan.

• Warm food in a sealed container. Steam condenses on the lid and drips back, and the food sits in its own water. Cool it uncovered first.
• Bread in a plastic bag while still warm. The crust softens and stays soft, and the loaf moulds days earlier.
• Anything crisp in a container — biscuits, crackling, fried food, pastry — which goes soft as its own moisture redistributes.
• Salad in a bag in the fridge. The leaves transpire, the water condenses and they rot from the wet spots.
• A fridge door opened often, where warm room air condenses on cold surfaces and food.
• Cling film over a warm dish, which does exactly what a lid does.
• The remedies are the same in every case: cool before covering, and use paper or a cloth rather than plastic where the food should stay dry.

Common questions