A leavening agent (also leavening or leaven; /ˈlɛvənɪŋ/ or /ˈlɛvən/) is any one of a number of substances used in doughs and batters that cause 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.
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Microorganisms that release carbon dioxide as part of their life cycle can be used to leaven products. Varieties of yeast are most often used, particularly Saccharomyces species (such as baker's yeast, Saccharomyces cerevisiae), though some recipes also rely on certain bacteria. Yeast leaves behind waste byproducts (particularly ethanol and some autolysis products) that contribute to the distinctive flavor of yeast breads. In the naturally leavened sourdough breads, the flavor is further enhanced by various lactic acid bacteria (Lactobacilli) or acetic acid bacteria (Acetobacter).
Leavening with yeast is a process based on fermentation, biologically changing the chemistry of the dough or batter as the yeast works. Unlike chemical leavening, which usually activates as soon as the water combines the acid and base chemicals, yeast leavening requires proofing, which allows the yeast time to reproduce and consume carbohydrates in the flour. Yeast can unhydrate itself and then rehydrate itself later.
Yeast can also be used to make alcoholic beverages like beer or wine. The resulting cast-off yeast, known as barm, can be used as a leavener and was probably ancestral to the use of modern pure-cultured yeast. Non-European cultures have used other by-products of making alcoholic beverages as leaveners, as in Ecuador: "In olden times when the sediment of chicha called concho was used as a ferment, we had good bread; and now with better mills good quality bread has disappeared entirely."[1]
While not as widely known, bacterial fermentation is sometimes used, occasionally providing a drastically changed flavor profile from a yeast fermentation; a well-known example is salt rising bread, which uses a culture of the Clostridium perfringens bacterium.
Some typical biological leaveners are:
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.
Chemical leavening was first publicized by Amelia Simmons in her American Cookery,[2] published in 1796, wherein she mentions the use of pearl ash as a leavening agent.
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.
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. 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.
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.
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