A soufflé rises on steam and falls because the foam was never set

Two components with two entirely different roles

A soufflé is a flavoured base combined with beaten egg whites, and the division of labour between them is worth stating plainly. The whites contribute the air, in the form of thousands of bubbles held in a protein film. The base contributes flavour, viscosity and, in most sweet versions, the starch or the extra protein that will eventually turn a soft foam into something that holds a shape once the heat has done its work.

A soufflé made with whites alone would rise spectacularly and collapse into nothing within seconds of leaving the oven, because there would be no structure other than the foam. A soufflé made with too heavy a base will not rise at all, since the weight of the mixture exceeds what the bubbles can lift. The formula is a balance between those two failures, and most recipes sit closer to the heavy end than beginners expect.

The rise is water turning into vapour

The air whipped into the whites contributes almost nothing to the height. What lifts a soufflé is the water in the mixture converting to steam, which occupies vastly more volume than the liquid did, and that vapour has nowhere to go except into the existing bubbles. The air pockets are not the leavening. They are the pre-existing spaces into which the leavening expands, which is why a mixture with no foam in it simply boils rather than rising.

Gas expansion from heating contributes a share as well, since a gas at oven temperature occupies noticeably more volume than the same gas did in a cool bowl. But steam does the bulk of the work, and this explains a number of practical points at once: why a soufflé must go into a properly hot oven, why the mixture should not be prepared and left standing indefinitely, and why the dish is filled to the rim so the mixture has a wall to climb.

Collapse is what happens when the structure never set

Inside the oven the bubbles are inflated by steam pressure. Take the dish out and the vapour condenses back to liquid within moments, so the internal pressure drops and anything not rigid contracts. If the protein and starch in the walls have set, the structure holds and the soufflé sinks only a little. If the centre is still liquid batter, the whole thing deflates immediately, and no amount of care carrying it to the table will change that.

The folklore about noise and vibration is therefore mostly beside the point. A properly baked soufflé is not fragile in that way, and slamming a door will not fell one that is cooked. What will fell it is opening the oven halfway through, which drops the temperature and lets the expanded structure cool before it has set. The instruction survives for a sound reason and the explanation attached to it has drifted.

Which is why the base carries starch or a lot of yolk

A classic sweet soufflé is built on pastry cream or a similar starch-thickened base, and that starch is the insurance policy. It gelatinises in the oven and gives the walls something rigid to become, so the finished soufflé has a set outer region and a soft, almost saucy centre that is regarded as correct rather than underdone. Fruit soufflés made on a purée base often lean on egg white alone plus a great deal of sugar, and they are correspondingly more fragile.

Folding technique protects the same structure from the other direction. A third of the whites is usually stirred into the base first, thinning it enough that the remainder can be folded in without excessive working. Every extra stroke of the spatula deflates bubbles, and a mixture folded until perfectly uniform has usually lost more air than one left with a few streaks of white still showing.

The dish, the collar and the practical details

Buttering the dish and coating it with sugar gives the rising mixture a surface it can grip and climb, and the vertical straight sides of a soufflé dish exist so there is nothing to obstruct that climb. Running a thumb around the inside rim to clear a channel is a small habit with a visible payoff: it stops the top sticking at the edge and lets the soufflé rise evenly rather than tilting.

Some traditions add a paper collar to allow the rise to go higher than the dish, which works, and some kitchens now prepare the mixture ahead and freeze it before baking, which also works surprisingly well because the foam is immobilised before it can drain. Neither trick changes the underlying physics. The soufflé still rises on steam, still sets on protein and starch, and still has to be eaten fairly promptly once it reaches the table.