In cooking, a leavening agent (/ˈlɛvənɪŋ/) or raising agent, also called a leaven (/ˈlɛvən/) or leavener, is any one of a number of substances used in doughs and batters that cause a foaming action (gas bubbles) that lightens and softens the mixture. An alternative or supplement to leavening agents is mechanical action by which air is incorporated (i.e. kneading). Leavening agents can be biological or synthetic chemical compounds. The gas produced is often carbon dioxide, or occasionally hydrogen.^[1]

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When a dough or batter is mixed, the starch in the flour and the water in the dough form a matrix^[2] (often supported further by proteins like gluten or polysaccharides, such as pentosans or xanthan gum). The starch then gelatinizes and sets, leaving gas bubbles that remain.

• Saccharomyces cerevisiae producing carbon dioxide found in:
  • baker's yeast
  • Beer barm (unpasteurised—live yeast)
  • ginger beer
  • kefir
  • sourdough starter
• Clostridium perfringens producing hydrogen found in salt-rising bread

Chemical leavens are mixtures or compounds that release gases when they react with each other, with moisture, or with heat. Most are based on a combination of acid (usually a low molecular weight organic acid) and a salt of bicarbonate (HCO₃⁻). After they act, these compounds leave behind a chemical salt. Chemical leavens are used in quick breads and cakes, as well as cookies and numerous other applications where a long biological fermentation is impractical or undesirable.

Steam and air are used as leavening agents when they expand upon heating.^[6] 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 products having one large cavity, such as popovers, Yorkshire puddings, pita, and most preparations made from choux pastry. The effect is also seen to a lesser extent in tempura.

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Using a whisk on certain liquids, notably cream or egg whites, can also create foams through mechanical action. This is the method employed in the making of sponge cakes, where an egg protein matrix produced by vigorous whipping provides almost all the structure of the finished product.

The Chorleywood bread process uses a mix of biological and mechanical leavening to produce bread; while it is considered by food processors^[who?] to be an effective way to deal with the soft wheat flours characteristic of British Isles agriculture, it is controversial^{[according to whom?]} due to a perceived lack of quality in the final product. The process has nevertheless been adapted by industrial bakers in other parts of the world.

• Aerated Bread Company, bakeries started in 1862 in the UK that made carbon dioxide leavened bread
• Baking powder
• Chametz
• Parable of the Leaven
• Passover
• Unleavened bread