Fermentation is the decomposition of carbohydrates by microorganisms in the absence of air. Louis Pasteur first demonstrated that fermentation is a biochemical process, each type being caused by one species.
Fermentation is an aspect of bacterial and fungal metabolism, in which glucose and other sugars are oxidized by enzyme catalysis to pyruvic acid (see citric acid cycle). Pyruvic acid is then reduced to lactic acid or degraded to carbon dioxide and ethanol. Considerable energy is released in this process: some is stored as the high-energy compound ATP, and the rest is given off as heat.
Fermentation by yeast has been used for centuries in brewing and making bread and wine; fermentation by lactic acid bacteria is used to make cheese. Special fermentations are used industrially for the manufacture of acetone, butanol (see alcohols), glycerol, citric acid, glutamic acid, and many other compounds. Modern biotechnology employs fermentation methods to mass produce genetic materials.