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Fermentation
Article by: Cornelis B. Van Niel, Hopkins Marine Station, Pacific Grove, California. AccessScience, McGraw Hill, Nov. 2021.
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An enzymatic transformation of or-
ganic substrates, especially carbohydrates,
generally accompanied by the evolution
of a gas. Fermentation is a chemical trans-
formation of organic substrates, especially
carbohydrates, to alcohol or organic acids,
typically involving the production of a gas,
such as carbon dioxide (CO2) or hydrogen gas.
It is a physiological counterpart of oxidation,
permitting certain organisms to live and grow
in the absence of air. Fermentation is used in
various industrial processes for the manufac-
ture of products such as alcohols, acids, and
cheese by the action of yeasts, molds, and
bacteria. The best-known example is alcoholic
fermentation, in which sugar is converted into
ethyl alcohol (ethanol) and carbon dioxide
(Fig. 1). This conversion, chemically repre-
sented by C6H12O6 —> 2C2H5OH + 2CO2, was
established by J. L. Gay-Lussac in 1815.
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Key Concepts
• Fermentation is a chemical breakdown of organic substrates, especially carbohydrates, to ethyl alcohol or organic acids.
• Fermentation is performed primarily by yeasts, molds, and bacteria, and is usually accompanied by the production of a gas,
such as carbon dioxide or hydrogen gas.
• Fermentation occurs in the absence of oxygen, allowing microorganisms to survive anaerobically.
• Two well-known examples are alcoholic fermentation and lactic acid fermentation.
• Industrially, alcoholic fermentation is used to manufacture alcohols, acids, and certain foods, such as cheese.
Fig. 1: Alcoholic fermentation occurs in yeasts. This process converts pyruvate—the end product of glycolysis—into
ethyl alcohol (ethanol), yielding two adenosine triphosphate (ATP) molecules and reducing nicotinamide adenine
dinucleotide (NAD+ to NADH). (Copyright © McGraw-Hill Education)
