Barm, also called ale yeast,[1] is the foam or scum formed on the top of a fermenting liquid, such as beer, wine,[2] 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[3] and is sometimes used in English baking as a synonym for a natural leaven (sourdough).[4] 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.
A barm cake is a soft, round, flattish bread roll from North West England, traditionally leavened with barm.[5][6][7][8] In Ireland, barm is used in the traditional production of barmbrack, a fruited bread.
Emptins, a homemade product similar to barm and usually made from hops or potatoes and the dregs of cider or ale casks, was a common leavener for those living in rural areas far from a brewery, distillery, or bakery from which they could source barm or yeast.[1]
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