Barm, or Yeast, is a substance which separates under the form of a froth, more or less viscid, from all the juices and infusions which experience the vinous fermentation. It is commonly procured from the beer manufactories, and is hence called the barm of beer. If left to itself for some days, in a close vessel, at a temperature of from 55° to 70°, it is decomposed, and undergoes the putrid fermentation. To prevent this decomposition, it is the custom, in Paris, to evaporate it to a solid form, in which state it is sold under the name of levure, which article, however, comprises not only yeast, but the bottoms of the beer in the working-tun and store-casks. This is purchased by the yeast merchants, the beer drained from it through sacks, and the remainder of the beer washed out by putting the sacks in a stream of water; the solid matter left in the sacks is then dried in the open air. The true yeast, or froth of the beer, is also dried for use in the same manner, as the bakers in Paris prefer it in a solid state. Dry levure ought to be yellow, brownish, or greyish white, by no means black or bitter. It should not yield to pressure by the fingers, and be equally dry throughout, so as to break with a smooth surface.

When dissolved in hot water, and a few drops of the solution are poured into boiling water, they should immediately rise to the surface. See Beer, and Bread.