leaven [key], agent used to raise bread or other flour foods. Physical leavens include water vapor, which is released as steam at high temperatures (as in popovers), and air, which is incorporated by beating. Chemical leaven (baking powder and baking soda) and biological leavens (yeasts and certain bacteria) raise the mixture by the formation of carbon dioxide gas, which is expanded by heat. Some of the earliest leavens were barm, a yeast of fermenting malt liquor, and sourdough, a portion saved from a mass of dough as a starter for the next batch.
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