Barm, also called ale yeast, 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.
A barm cake is a soft, round, flattish bread roll from North West England, traditionally leavened with barm. 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.
...the original method of making yeast bread in Britain was a by-product of ale-making. When traditional ale is made, a yeasty froth appears on top of the fermenting liquid, the wort. This used to be scooped off, washed and added to bread dough in order to leaven it. Bread made this way is sweeter tasting than sourdough, and the leavening yeast used to be called 'barm'. Its unpredictability created the word 'barmy'. In the 19th century, the process was refined and industrialized, manufacturing it on a large scale with what is known today as 'baker's yeast', and used worldwide as the primary method of leavening bread. The barm method appears to be an ancient method developed by Gaelic peoples, and was quite different from that used in Europe, which is to leaven bread with a sourdough or leaven (the French call a similar product 'levain'). When the Romans first conquered Gaul, modern day France, they were astonished by the light sweet bread made by the Celtic inhabitants. Barm bread survived with the Celtic peoples in Britain, Scotland and Ireland, but was not common in Europe, being condemned during the Enlightenment as 'unwholesome'. In England, noblemen's bread, manchet, was always made with the barm method, whereas the commoners' bread, maslin, was a sourdough. Barm bread survived until World War Two, and even later in the North of England, largely as barm cakes. Curiously, the old method of making a sponge, or thick batter of flour and water with the barm was still used with the new industrially produced yeast, and was re-introduced to Europe from Vienna where the first yeast factories were established. This became popular in France as a 'poolish', the favoured method of making crusty bread such as a baguette.