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Leaven. (a) A substance, as yeast (q.v.), barm (q.v.) or a portion of fermenting sponge (q.v.), that is used to initiate fermentation (q.v.) and expansion of dough (q.v.). Also, any material, such as baking soda (q.v.) with an acid-reacting substance, as buttermilk (q.v.) or leavening acids (q.v.), or baking powder (q.v.), that causes carbon dioxide (q.v.) to evolve and expand the volume of a batter (q.v.). (b) The act of adding any type of leaven to dough or batter for the purpose of causing it to aerate (q.v.) and expand.