Cooking science

Internal steam

Water expands enormously when it turns to steam. Trapped inside food, that expansion has to go somewhere — and puff pastry, crackling, Yorkshire puddings, poppadoms and popcorn are all the same physics dressed differently.

A very large expansion

When liquid water becomes steam at atmospheric pressure it occupies vastly more space — on the order of a thousand times as much. Inside a closed structure that is an enormous force.

If the structure can stretch, it inflates. If it is rigid, it bursts. If it can stretch and then set, it stays inflated — and that is what nearly every puffed food relies on.

The timing matters as much as the steam. The structure has to be flexible while the steam is generated and then set before the steam escapes, which is why almost all of these foods need a hot oven or hot fat rather than a gentle one.

What each food does with it

Puff pastry

Water in the butter layers turns to steam and forces the dough layers apart. The dough then sets as separate leaves.

Yorkshire pudding and popovers

A thin batter in a very hot fat-filled tin. Steam inflates the batter into a hollow shell before it sets.

Crackling

Water in the skin turns to steam and puffs the collagen into a brittle honeycomb.

Bread oven spring

Steam plus expanding carbon dioxide plus alcohol vapour, all expanding before the crust sets.

Choux pastry

Steam inflates the paste and the egg sets around the cavity.

Poppadoms and prawn crackers

Dried starch containing a little water, dropped in hot fat, expanding almost instantly.

Popcorn

A hard hull holding moisture until the pressure bursts it and the starch inflates.

Why it fails

• Not hot enough. Gentle heat lets the steam escape gradually instead of generating it fast enough to inflate anything.
• The structure set too early. A crust that forms before the steam arrives cannot expand — this is why bread ovens are steamed at the start.
• The structure could not hold it. Torn pastry, an over-proved loaf, a batter too thin.
• Not enough water. Over-dried pastry or a poppadom stored badly will not puff.
• The oven door opened. The temperature drop stops the steam generation and everything collapses.
• Pricked or vented. Sometimes deliberate — a pie lid is vented on purpose so it does not balloon.

Getting things to puff

The same handful of rules apply to pastry, batter, bread and skin alike.

1. Start hot. Steam has to be generated faster than it can escape, and a gentle oven never manages it.
2. Preheat whatever the food sits on — the tin, the tray, the stone, the fat. Heat arriving from below is what starts the expansion.
3. Keep the structure flexible at the start. This is why bread ovens are steamed for the first ten minutes and why puff pastry is kept cold until it goes in.
4. Do not open the oven. The temperature drop stops the steam and the structure collapses before it has set.
5. Make sure there is water to work with. Over-dried pastry and stale poppadoms will not puff.
6. Let it set fully before moving it. A puffed structure that is still soft will sink.

Common questions