Fermentation

Fermentation, chemical changes in organic substances produced by the action of enzymes (see Enzyme). This general definition includes virtually all chemical reactions of physiological importance, and scientists today often restrict the term to the action of specific enzymes, called ferments, produced by minute organisms such as molds, bacteria, and yeasts. For example, lactase, a ferment produced by bacteria usually found in milk, causes the milk to sour by changing lactose (milk sugar) into lactic acid. Probably the most important type of fermentation is alcoholic fermentation, in which the action of zymase secreted by yeast converts simple sugars, such as glucose and fructose, into ethyl alcohol and carbon dioxide. Many other kinds of fermentation occur naturally, as in the formation of butyric acid when butter becomes rancid and of acetic acid when wine turns to vinegar.

Generally, fermentation results in the breakdown of complex organic substances into simpler ones through the action of catalysis. For example, by the action of diastase, zymase, and invertase, starch is broken down (hydrolyzed) into complex sugars, then simple sugars, and finally alcohol.

Glycerin, acetone, butyl alcohol, and butyric acid are now produced on a large commercial scale by special fermentation processes. Various fermentation productions of milk, such as acidophilus milk, Bulgarian milk, and yogurt, are widely consumed for their nutritive properties.

The action of certain bacteria on undigested carbohydrates causes fermentation in the human intestine. As a result, gases such as hydrogen sulfide and carbon dioxide may form in amounts large enough to cause distention and pain. Acids such as lactic acid and acetic acid may also form in the intestines of infants, causing diarrhea.

See Brewing; Zymology. See also articles on many of the enzymes, foods, and organic substances mentioned.