barm

Barm is the foam, or scum, formed on the top of liquor – fermented alcoholic beverages such as beer or wine, or feedstock for hard liquor or industrial ethanol distillation – when fermenting. It was used to leaven bread, or set up fermentation in a new batch of liquor. Barm, as a leaven, has also been made from ground millet combined with must out of wine-tubs and is sometimes used in English baking as a synonym for a natural leaven. Various cultures derived from barm, usually Saccharomyces cerevisiae, became ancestral to most forms of brewer's yeast and baker's yeast currently on the market.