The New International Encyclopædia/Yeast, or Barm

YEAST, or Barm. The mass of vegetable cells produced during the alcoholic fermentation of sugars. Yeast is produced in large quantities in beer brewing, beer yeast being constituted mainly by the minute cells of a species of fungi known as Saccharomyces cerevisiæ, or Torula cerevisiæ. This plant, which multiplies exclusively by budding, is capable of existing for a time in a pure aqueous solution of sugar. An excellent medium is such a solution with some albuminous matter, or phosphates and ammonia salts, added to it. But the best medium for its existence and growth is constituted by some liquid like wort or grape juice. In such a liquid the formation of yeast, accompanying fermentation, may be induced by exposure to the air in a brewery or some other place where alcoholic fermentation is going on, the air of such places containing the germs that develop into the yeast plant. The temperature at which the plant thrives best is from 15° to 20° C. (59° to 68° F.). Even very intense cold does not kill it, although its vegetation is arrested below 5° C. (41° F.). Heat, in the presence of moisture, kills it at temperatures above 75° C. (167° F.); if dry, the plant may be heated above 100° C. (212° F.) without losing its vitality.

Ordinary beer yeast, owing to its bitter taste, is not suitable for use in baking. Baker's yeast may be readily made by introducing a little yeast previously obtained into a wort made from barley, malt, and rye, skimming off the yeast produced during the violent stage of the fermentation and freeing it from liquid by pressure. It is such a product that, mixed with dry starch and pressed into cakes, is brought into the market as 'dry yeast' or 'German yeast.'

Yeast has been used with considerable success in diabetes, in which it causes a marked diminution of sugar in the urine. Yeast poultices, prepared by gradually warming a mixture of yeast, wheat flour, and water until it rises by reason of fermentation, form a highly useful remedy for various ulcers. Beer yeast is also used for preparing a substitute for koumiss, a beverage of high medicinal value extensively used by the Tartars. While the true koumiss fermentation is induced by a specific organism in mare's milk, the substitute is prepared, according to Professor Botkin, by adding water and milk sugar to cow's milk and inducing the alcoholic fermentation of this liquid by means of ordinary brewer's yeast. See Bread; Distilled Liquors; Saccharomycetes; etc.