Cooking science

Aeration

A great deal of good cooking is air. Whipped cream, meringue, a risen cake, mousse, bread and batter are all mostly about bubbles — how they get in, and what stops them escaping before the structure sets.

Three ways air gets in

Mechanical

Whisking, creaming, folding, kneading. You physically beat air in and something holds it. Meringue, whipped cream, creamed butter.

Chemical

Baking powder or bicarbonate of soda reacting to produce carbon dioxide. Fast, predictable, and only as good as the batter holding it.

Biological

Yeast fermenting sugars to carbon dioxide. Slow, and it develops flavour as well as gas — which is the whole argument for a long rise.

Steam

Water in the mixture turning to vapour in the oven. The fourth and most-overlooked. Choux, Yorkshire pudding and puff pastry rise on steam alone.

Why a bubble survives, or does not

A bubble in food is a pocket of gas with a thin film of liquid around it. Left alone that film drains downwards under gravity, thins, and bursts. So every stable foam is a foam where something is slowing the drainage or strengthening the film.

In meringue, sugar makes the liquid syrupy so it drains slowly, and egg protein forms a skin around each bubble. In whipped cream, partly crystallised fat globules clump around the bubbles and hold them. In bread, gluten forms an elastic film that stretches without tearing. In a cake batter, starch and egg set into a permanent solid before the bubbles have time to escape.

That is the whole game: create bubbles, then get the structure to set before they leave.

What destroys a foam

• Fat, in a protein foam. Fat molecules insert into the protein film and stop it forming. A trace of yolk in egg whites is enough.
• Over-whipping. Past a point the protein or fat network becomes too rigid, tears, and the foam collapses or separates. Over-whipped cream becomes butter.
• Waiting. A chemically raised batter starts losing gas the moment it is mixed. Get it into the oven.
• Stirring rather than folding. Stirring shears bubbles; folding moves the mixture around them. The difference in finished volume is roughly double.
• Heat arriving too slowly. If the structure sets after the gas has escaped, the food is dense. This is why most raised things want a hot oven at first.
• Sugar added too early to egg whites. Sugar interferes with the proteins unfolding, so the foam builds slowly and to less volume. Add it once soft peaks are there.

Aeration is not the same as lightness

A common mistake is to treat more air as always better. It is not: the bubbles have to be the right size and evenly distributed. A batter with a few large bubbles bakes into a coarse, tunnelled crumb, while the same amount of air in many small bubbles gives a fine, even one.

This is why bread is knocked back and re-shaped — not to remove the gas, but to break up the large bubbles and redistribute them. It is why you rap a tray of macarons on the worktop, and why a cake batter is tapped down before baking. Some of the work of aeration is undoing your own excess.

Common questions