It’s misleading for a cook to speak of a successful soufflé as one which does not fall, because all soufflés fall. Sure, some fall faster and more dramatically than others, and perhaps at inopportune moments, but the fact is a soufflé is a foodstuff that’s never really at rest. It’s either rising or its falling, according to its temperature.
This is a result of its structure which is made up of thousands upon thousands of tiny bubbles. If you did your reading on egg foams since Friday, you have a pretty fair idea how those bubbles form. Each one contains a small amount of air, which when heated, expands. This tendency for gas to expand as its temperature rises is known as Charles’ Law, after Jacques Charles, a French inventor and balloonist. Gas expansion, however, only accounts for maybe 10% of a soufflé’s rise. The rest is created by steam. For as the bubble heats and expands, water evaporates from its interior surface, adding to the volume of the gas in the bubble and causing expansion. That expansion creates more interior surface area which means more evaporation, increased gas volume and still more expansion. The bubble grows and grows until it reaches its peak size, when the soufflé is removed from the oven.
At that point the soufflé begins to cool, and since the bubbles have very little support around them relative to a bread or a cake, they begin to collapse. Just how fast and dramatically they collapse is a factor of how fast they expanded in the first place — faster-growing bubbles tend to have thinner walls — and what ingredients (other than egg) there are in the soufflé batter. But make no mistake, there will always be some degree of collapse. This is actually an indicator of quality to some extent, as it means your soufflé is not so loaded up with starches, proteins and fats that it will be very delicate on the tongue. So the ideal soufflé is really one that rises high (but not too high) and falls (but not too far). It’s not as tricky a balance as it may seem.
One of the interesting things about a soufflé is that if it falls normally and not catastrophically, it can be re-inflated with the application of more heat. For the bubbles don’t disappear when the soufflé falls, the moisture inside them merely condenses. Blowing them back up again is as simple as putting the pan back in the oven. And ’round and ’round we go!