A leavening agent (sometimes called just leavening or leaven) is a substance used in doughs and batters that causes a foaming action. The leavening agent reacts with moisture, heat, acidity, or other triggers to produce gas that becomes trapped as bubbles within the dough. When a dough or batter is baked, it "sets" and the holes left by the gas bubbles remain, giving breads, cakes, and other baked goods their soft, sponge-like textures.

Chemical leaveners

Chemical leaveners are chemical mixtures or compounds that typically release carbon dioxide when they react with moisture, heat, and acidity. They usually leave behind a chemical salt. Chemical leaveners are used in quick breads and cakes. Chemical leavening agents include:

• baking powder
• baking soda (a.k.a., sodium bicarbonate)
• ammonium bicarbonate (a.k.a., hartshorn, horn salt, bakers ammonia)
• potassium bicarbonate (a.k.a., potash)
• potassium bitartrate (a.k.a., cream of tartar)
• potassium carbonate (a.k.a., pearlash)
• monocalcium phosphate

Biological leaveners

Main article: Yeast

Microorganisms that release carbon dioxide as part of their lifecycle can be used to leaven products. Varieties of yeast are most often used. Yeast leaves behind waste byproducts that contribute to the distinctive flavor of yeast breads. In sourdough breads, the flavor is further enhanced by various lactic or acetic acid bacteria.

Leavening with yeast is often a slower process, requiring a lengthy proofing.

Yeast can also be used to make carbonated beverages like beer, which can then be used as leavening.

Some typical biological leaveners are:

• (unpasteurised - live yeast)

Mechanical leavening

Creaming is the process of beating sugar crystals and solid fat (typically butter) together in a mixer. This integrates tiny air bubbles into the mixture. Creamed mixtures are usually further leavened by a chemical leavener. This is often used in cookies.

Using a whisk on certain liquids, notably cream or egg whites can also create foams through mechanical action.

Other leaveners

Steam and air are used as leavening agents when they expand upon heating. To take advantage of this style of leavening, the baking must be done at high enough temperatures to flash the water to steam, with a batter that is capable of holding the steam in until set. This effect is typically used in popovers and Yorkshire puddings, and to a lesser extent in Tempura.

Nitrous oxide is used as a propellant in aerosol whip cream cans. When the gas boils out of the cream, it also instantly creates a foam.