leaven

leaven , 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.

leaven Yeast, or a piece of dough kept to ferment the next batch.

leav·en / ˈlevən/ • n. a substance, typically yeast, that is added to dough to make it ferment and rise. ∎  dough that is reserved from an earlier batch in order to start a later one fermenting. ∎ fig. a pervasive influence that modifies something or transforms it for the better: they acted as an intellectual leaven to the warriors who dominated the city. • v. [tr.] 1. [usu. as adj.] (leavened) cause (dough or bread) to ferment and rise by adding leaven: leavened breads are forbidden during Passover. 2. permeate and modify or transform (something) for the better: the proceedings should be leavened by humor [as n.] (leavening) companies of militia volunteers with a leavening of regular soldiers.