SCIENCE: Steam Leavens the Dough
When it comes to pâte à choux, it's the steam that makes the puff.
Steam, the vapor produced when water within dough reaches 212 degrees, is an essential leavening agent that helps give rise to all baked goods, even those that use chemical leaveners like baking soda or baking powder (think cookies and quick breads) or biological leaveners like yeast (think breads and rolls). No matter what additional leavener is in your baked goods, steam will also play a role in giving lift because of the water in the dough. But can steam handle the job on its own? Given the right conditions, it sure can!
Classic recipes like souffles, genoise cake, puff pastry, and pâte à choux rely on steam alone for leavening. Before baking, choux paste takes a unique path with the execution of its dough. This fairly stiff, sticky dough is started on the stove by cooking the flour, butter, and water until the starch in the flour has gelatinized (this reduces the flour's elasticity); off heat, eggs are then beaten into the mixture, resulting in a dough that is thick enough to pipe into shapes.
The consistency of the dough belies the high moisture content, an important factor for the build-up of steam. It is piped into mounds and baked initially at a high temperature. The intense heat causes the moisture in the dough to expand rapidly, filling the interior with large, irregular cells. At the same time, the outside browns and congeals to form a firm exterior. Once the size and shape of each puff is set, the oven temperature is lowered to further cook and dry out the puffs, leaving them with a burnished exterior and a mostly hollow, custardy interior.
