Barm

Barm is the foam or scum that forms on the surface of fermenting liquids, most commonly beer. This foamy layer contains active yeast and other particulates and has historically been collected and used as a natural leavening agent for bread and other baked goods.

In Britain and Ireland, barm was skimmed from fermenting ale or wort and used to leaven dough. Barm-based breads, sometimes called barm cakes or simply barm bread, were common before the widespread adoption of commercial baker’s yeast. The resulting loaves tended to be soft with a mild tang, and the practice gave rise to regional names for baked goods associated with barm.

Modern baking largely relies on commercial yeast, which provides more consistent and predictable leavening. As a result, the use of barm declined in everyday production. Nevertheless, some traditional or artisanal bakers, as well as certain sourdough and natural-leavening practices, may still employ barm or barm-derived cultures. The term can also refer more generally to the living yeast obtained from beer fermentation, used similarly to yeast cultures in baking.

In addition to bread, barm appears in regional foods such as barm brack, an Irish fruit loaf historically leavened with barm. The word “barm” has thus carried dual meanings: the foamy surface of fermenting liquid and the leavening yeast derived from it.