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 wa… (More on Leavening)