Agents that are used to lighten the texture and increase the volume of baked goods such as breads, cakes and cookies. Baking powder, baking soda and yeast are the most common leaveners used today. When mixed with a liquid they form carbon dioxide gas bubbles, which cause a batter or dough to rise during (and sometimes before) the baking process. Some foods, such as angel food cake and are leavened by the air beaten into egg whites. When heated, the egg whites cook and set, trapping the air inside and creating a light, airy cake.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.