FERMENTATION

in its broadest sense, is the chemical change by which organic substances are decomposed and re-combined in new substances or compounds. Ferments are of two classes--"organized," or living, as yeast fungi, lactic bacteria, etc., and "unorganized," as diastase, pepsin, etc.

In its most widely used significance, fermentation is the chemical change produced in substances, more or less liquid, containing some sugary solution, by which the latter is converted into a liquid, alcohol, and a gas, carbon-dioxide. Wine and beer fermentation is called Vinous. Under favorable conditions of temperature, etc., fermentation continues until the growth of the yeast cells is stopped by the exhaustion of the particular chemical components adapted for their subsistence, or by the formation of other substances in quantities inimical to their growth. As already noted, alcohol is one of the chief results of vinous fermentation, but it is itself adverse to yeast growth, and will stop it entirely, and with it fermentation, if a sufficient quantity is added to the liquid, or formed in it.

Vinous fermentation is followed under certain conditions by acetous fermentation--a class of acetic bacteria oxidizing the alcohol and producing vinegar.

The "souring" of milk is lactic fermentation--the milk sugar being converted by the action of lactic bacteria into lactic acid. See article on BACTERIA.

Putrefaction of meat, etc., is Putrefactive Fermentation.