The froth or yeast that forms on top of fermenting beer or ale, used to make bread rise or to start new batches of beer.
From Old English 'beorma' (yeast or froth), related to Old Norse 'barmr.' The word originally meant the foam itself, but came to mean the living yeast cells within it.
Barm was how people leavened bread before commercial yeast—you'd save the froth from your ale-brewing and use it to make bread rise! Some British bakeries still sell 'barm cakes' using this ancient yeast source.
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