Cooking science

Gas expansion

Whatever put the bubbles into your batter, what makes it rise is heat: gases expand as they warm, water turns to steam and expands enormously, and the bubbles already present push outwards. The rise you see in the oven is almost entirely expansion of air that was already there.

Three things expanding at once

When a batter or dough goes into a hot oven, three things happen more or less together. The air already trapped in it warms and expands. Any carbon dioxide dissolved in the liquid comes out of solution as the temperature rises, joining the existing bubbles. And water in the mixture begins to turn to steam, which occupies vastly more space than the liquid did.

Steam is by far the biggest contributor in most baking, which surprises people who assume it is the baking powder. Baking powder creates the bubbles; steam inflates them.

A bubble can only expand into a bubble that exists. This is why you cannot rescue an under-creamed cake by adding more baking powder — there are too few bubbles to inflate, and the extra gas simply escapes and leaves a soapy taste.

Why the first ten minutes decide it

Rising is a race. The gases are expanding and pushing outwards; simultaneously the proteins and starch are setting into a permanent structure. If the structure sets too early, the rise stops short. If it sets too late, the bubbles reach the surface, burst, and the whole thing sinks.

That is why oven temperature at the start matters more than at any other point, and why opening the door early is so damaging: the temperature drops, expansion stops, and a structure that has not yet set falls back.

In bread the same event is called oven spring — the sudden surge in the first few minutes, before the crust sets and the yeast is killed. Steam in the oven delays the crust setting, which is why bakers steam the oven: a softer surface for longer means more spring.

Where it dominates

Choux pastry

Almost pure steam. There is no raising agent at all — the hollow is where the steam was.

Yorkshire pudding

Same. Smoking-hot fat sets the outside instantly and the steam inflates it.

Puff pastry

Steam from water in the butter layers, forcing the pastry apart into leaves.

Bread

Oven spring — expansion of existing fermentation gas plus steam, before the crust sets.

Cake

A mixture, with steam larger than most people assume.

Popcorn

The extreme case. Water inside the kernel turns to steam until the hull fails.

Practical consequences

• Preheat properly. An oven that reads 200°C may not have heated its walls and shelves yet, and radiant heat from those is a large part of the initial burst.
• Do not open the door for the first two-thirds of the bake. This is the most-broken rule in baking.
• Fill tins no more than two-thirds full. The expansion is larger than it looks.
• Get raised batters into the oven promptly. Gas that escaped before the heat arrived cannot be expanded.
• Steam helps bread and harms biscuits. A softer surface for longer is good where you want a rise and bad where you want crispness.
• A fan oven runs effectively hotter and sets surfaces faster. Drop the temperature about 20°C for anything that needs to rise.

Common questions