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A leavening agent (also leavening or leaven; pron.: // or //) is any one of a number of substances used in doughs and batters that causes a foaming action which lightens and softens the finished product.
The leavening agent incorporates gas bubbles into the dough—this may be air incorporated by mechanical means, but usually it is carbon dioxide produced by biological agents, or by chemical agents reacting with moisture, heat, acidity, or other triggers. When a dough or batter is mixed, the starch in the flour mixes with the water in the dough to form a matrix (often supported further by proteins like gluten or other polysaccharides like pentosans or xanthan gum), then gelatinizes and "sets"; the holes left by the gas bubbles remain.
- beer (unpasteurised—live yeast)
- ginger beer
- sourdough starter
- Whey protein concentrate
Chemical leaveners 
Chemical leaveners are chemical mixtures or compounds that release gases (usually carbon dioxide) when they react with moisture and heat; they are almost always based on a combination of acid (usually a low molecular weight organic acid) and an alkali; these leave behind a chemical salt. Chemical leaveners are used in quick breads and cakes, as well as cookies and numerous other applications where a long biological fermentation is impractical or undesirable.
Since chemical expertise is required to create a functional chemical leaven without leaving behind off-flavors from the chemical precursors involved, such substances are often mixed into premeasured combinations for maximum results. These are generally referred to as baking powders.
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, since the sugar crystals physically cut through the structure of the fat. Creamed mixtures are usually further leavened by a chemical leavener like baking soda. This is often used in cookies.
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 to be an effective way to deal with the soft wheat flours characteristic of British Isles agriculture, it is controversial 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.
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, Yorkshire puddings, and to a lesser extent in tempura.
Nitrous oxide is used as a propellant in aerosol whip cream cans. Large densities of N2O are dissolved in cream at high pressure. When expelled from the can, the nitrous oxide escapes emulsion instantly, creating a temporary foam in the butterfat matrix of the cream.
See also 
- Unleavened bread
- Aerated Bread Company, bakeries started in 1862 in the UK that made carbon dioxide leavened bread
- Matz, S (1972). "Bakery Technology and Engineering", AVI Publishing Co.