Leavening agent, substance causing expansion of doughs and batters by the release of gases within such mixtures, producing baked products with porous structure. Such agents include air, steam, yeast, baking powder, and baking soda.

Leavening of baked foods with air is achieved by vigorous mixing that incorporates air bubbles, producing foam. Egg white is well suited to this purpose because it produces voluminous and strong foams that retain their expanded structure when dried by the baking process. Egg white is used in such baked products as angel food and chiffon cakes and sponge cakes. Gluten, the elastic protein of flour, may also be whipped to produce a foam, as in beaten biscuits.

Puff pastes, which are used for light, flaky pastries, are expanded by water-vapour (steam) pressure. During baking, as the interior of the product nears the boiling point, the vapour exerts pressure within bubbles that have been incorporated earlier by other means, producing swelling.

More Info: www.britannica.com