Steam leavening

When a batter or dough goes into a hot oven, some of its water turns to steam. Steam takes up many times the volume of the water it came from, so it pushes outward on the gluten or other protein structures and lifts the product.

Steam is the main lifting force in popovers, pie crusts and pastries. In puff pastry, for example, the moisture in thin layers of dough and fat turns to vapor and pushes the layers apart. High-moisture batters and a hot oven give the strongest steam rise.

The lift only lasts while the structure can still stretch. As the proteins set and a crust forms, the product firms up, which is why a popover that is pulled out too early can collapse.