Also known as brewers-thick or beer-yeast,
In Britain, prior to the availability of baker's yeast, most bakers, commercial and domestic, obtained yeast for baking from breweries in the form of barm, which was produced in large quantities by the brewing trade and had no other use. Because of its often sour taste and odour, barm was often mixed with water and the sediment allowed to settle to the bottom; the water was then poured off and the process might be repeated several times, a process known as washing. The slurry was often stored in bottles until it was needed. Barm is far less potent than baker's yeast, and bakers would normally build up its strength using a pre-ferment.
Some authors refer to pre-ferments or natural leavens as 'barm', although this usage obscures the historical connection between brewing and baking.
Want to find out more? Look at the chapter on pre-ferments in the book flour and water.
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