Why a Soufflé Actually Rises

A soufflé starts with the humblest of ingredients: beaten egg whites and a flavored base. It rises to twice or three times its starting volume in the oven. If you cut into it a minute too late, it collapses. All of this is protein chemistry, not air pressure.

Egg whites are about 10 percent protein and 90 percent water. When you beat them, two things happen at once. The mechanical shear unfolds the native protein structures — albumin, ovotransferrin — exposing hydrophobic patches normally tucked inside. Those patches pin themselves to the surface of the air bubbles you are whipping in, forming a stable foam. Each bubble is now wrapped in a skin of partially unfolded protein.

Fold this foam into your base — pastry cream, cheese mornay, melted chocolate — gently, so the bubbles survive. Then into a hot oven. The temperature does two jobs. It expands the trapped air, which contributes a small amount of rise. Much more importantly, it vaporizes the water surrounding each bubble. Steam has more than a thousand times the volume of the liquid water it came from. The foam has to stretch to contain it.

Meanwhile the egg proteins on the bubble walls are denaturing further under heat and cross-linking to each other. By around 70°C they have set into a semi-rigid lattice. If the lattice sets while the steam is still pushing outward, the soufflé holds its shape. If the bubbles were under-beaten or the mixture folded too hard, the skin cannot hold the steam; the structure deflates.

That is why a soufflé must be served the instant it leaves the oven. The lattice is rigid but brittle. Once the steam cools and condenses, the scaffolding has nothing left to hold up.