A
leavening agent (also
leavening or
leaven) 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—biological, chemical, or even mechanical—reacts with moisture, heat, acidity, or other triggers to produce gas (usually
carbon dioxide and sometimes
ethanol) that becomes trapped as
bubbles within the dough. 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.
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 (i.e. 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 sourdough breads, the flavor is further enhanced by various lactic acid bacteria (lactobacilli) or acetic acid bacteria (acetobacilli).
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 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.
