Leavening agents add lightness to baked goods by "raising" them. Common leavens include yeast, baking powder, and baking soda.
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Baking powder and soda: Baking powder and baking soda are chemical leavening agents that produce carbon dioxide just as yeast does. Double-acting baking powder produces gases in two stages: first, when liquids are added and, second, during baking. Baking soda creates carbon dioxide bubbles instantly when it's mixed with acidic ingredients such as buttermilk, sour cream, or fruit juices. Any recipe that uses only baking soda as leaven should be baked immediately, before all those bubbles deflate.
Yeast: Yeast is a one-celled organism that wakes up and goes into action when it's combined with a warm liquid and sugar or starch. It produces little bubbles of carbon dioxide gas that get trapped in your dough and make it rise. There are three forms of yeast available: active dry, quick-rising, and compressed. Active dry yeast is the most popular form. These tiny, dehydrated granules are mixed with flour or dissolved in warm water before they're used. Quick-rising yeast (sometimes called fast-rising or instant yeast) is a more active strain of yeast. It's usually mixed with the dry ingredients before the warm liquids are added. Quick-rising yeast cuts rising time by about one-third. The first rising will take about 10 to 15 minutes less; the second rising will be shortened, too. Quick-rising yeast can be substituted for active dry yeast, except in recipes requiring the dough to rise in the refrigerator and in doughs using sourdough starter. Compressed yeast, also called fresh yeast, comes in small, foil-wrapped square cakes. Soften it in warm water according to the package directions.