A soufflé is made by whipping egg whites and then folding them into egg yolks. When you whip the egg whites you form bubbles of air encased in a protein skin. Then, when you put the mixture in the oven, the bubbles expand and the protein stiffens, causing your soufflé to rise.
The tricky part is preventing these fragile bubbles of air from bursting. Over-beating the egg whites, folding them into the yolks too vigorously and cooking at the wrong temperature or for the wrong amount of time can all cause the air to escape, resulting in a collapsed dessert.
Answered by Joanna Stass for Brain Dump in How It Works issue 122.
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