"Barm is the foam, or scum, formed on the top of liquor \u2013 fermented alcoholic beverages such as beer or wine, or feedstock for hard liquor or industrial ethanol distillation \u2013 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."@en .