Barm

Barm is the foam or scum formed on the top of a fermenting liquid, such as beer, wine, 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 and is sometimes used in English baking as a synonym for a natural leaven (sourdough). 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 Ireland, barm is used in the traditional production of barmbrack, a fruited bread. Despite the name, the barm cakes of northern England are not necessarily made with barm.