Formation and Expansion of Gases

The gases primarily responsible for leavening baked goods are carbon dioxide, which is released by the action of yeast and by baking powder and baking soda; air, which is incorporated into doughs and batters during mixing; and steam, which is formed during baking.

Some gases—such as carbon dioxide in proofed bread dough and air in sponge cake batters—are already present in the dough. As they are heated, the gases expand and leaven the product.

Some gases are not formed until heat is applied. Yeast and baking powder form gases rapidly when first placed in the oven. Steam is also formed as the moisture of the dough is heated.