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A leavening agent (also leavening or leaven; // or //) is any one of a number of substances used in doughs and batters that causes a foaming action that lightens and softens the finished product. Formation of carbon dioxide is induced by chemical agents reacting with moisture, heat, acidity, or other triggers.
The leavening agent incorporates gas bubbles into the dough. The alternative or supplement to leavening agents is mechanical leavening by which air is incorporated by mechanical means. Most leavening agents are synthetic chemical compounds, but carbon dioxide can also be produced by biological agents. 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.
Types of biological leavening agents
Chemical leaveners are mixtures or compounds that release gases (again, usually carbon dioxide) 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 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 producing 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. Sour milk and carbonates were used in the 1800s. The breakthrough in chemical leavening agents occurred in the 1930s with the introduction of monocalcium phosphates (Ca(H2PO4)2). Other leavening agents developed include sodium aluminum sulfate (SAS), sodium acid pyrophosphate (SAPP), and sodium aluminum phosphates (SALP). These compounds combine with sodium bicarbonate to give carbon dioxide in a predictable manner.
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.
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.
- Unleavened bread
- Aerated Bread Company, bakeries started in 1862 in the UK that made carbon dioxide leavened bread
- Simmons, Amelia; Mary Tolford Wilson (1984) . The First American Cookbook (1984 reprint ed.). Mineola, NY: Dover. ISBN 0-486-24710-4.
- John Brodie, John Godber "Bakery Processes, Chemical Leavening Agents" in Kirk-Othmer Encyclopedia of Chemical Technology 2001, John Wiley & Sons. doi:10.1002/0471238961.0308051303082114.a01.pub2
- Matz, S (1972). "Bakery Technology and Engineering", AVI Publishing Co.