Barm

Barm is the foam, or scum, formed on the top of liquor (i.e. 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 wrongly used in 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.

In parts of the North of England a barm or barm cake is a common term for a soft, floury bread roll.

"Barmy" is also British slang for "crazy", comparing the foamy texture of barm to the perceived emptiness of such a person's head.

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