A British term for a yeast leaven In brewing, the term "barm" refers to the foamy yeast residue from the fermentation of ale, then used to leaven bread (different strains of S cerevisiae are used to ferment both bread and alcohol) Today some Americans (including some San Francisco bread bakers and instructors) use the term "barm" to describe a natural leaven started with whole wheat flour or grains A barm started from whole wheat grains or flour is a mix of natural or "wild" yeast and lactobacilli originating from the grains As a by-product from brewing yeast, barm makes a fairly bitter tasting bread unless the bittering agents are distilled out As a natural leaven from whole wheat flour and/or grains, barm produces a mild fruity buttery flavored bread lactic acid contributions from the lactobacilli and is not necessarily sour