Barm is the foam or scum formed on the top of a fermenting liquid, such as beer, wine, or feedstock for spirits or industrial ethanol distillation. It is 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 (sourdough). 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.
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- Chisholm, Hugh, ed. (1911). Encyclopædia Britannica. 3 (11th ed.). Cambridge University Press. p. 407. .
- Botham's of Whitby. "The story behind a loaf of bread".
- Reinhart, Peter (1998). Crust and Crumb. Berkeley, CA: Ten Speed Press. ISBN 1-58008-802-3. Reinhart derived the term from his training under Monica Spiller.