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 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.

In parts of the North of England a barm or barm cake is a common term for a soft, floury bread roll. This is a regional term: other areas describe an identical roll as a "bap", "bread bun", "bread cake", "blaa" (Ireland) or many other variants. It is applied equally to yeast-leavened bread, without implying the use of sourdough or barm leavens.

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