Fermentation is defined as an energy-yielding metabolic pathway that involves no net change in oxidation state. Anaerobic glycolysis is a type of fermentation. The lactic acid fermentation (conversion of glucose to lactate) is important in the manufacture of cheese. Another important fermentation involves cleavage of pyruvate to acetaldehyde and CO2, with the acetaldehyde then reduced to ethanol by alcohol dehydrogenase in the reaction that follows:

Acetaldehyde + NADH + H⁺ <-> Ethanol + NAD⁺

As carried out by yeasts, this fermentation generates the alcohol in alcoholic beverages. (see also - alcoholic fermentation) Yeasts used in baking also carry out the alcoholic fermentation; the CO2 produced by pyruvate decarboxylation causes bread to rise, and the ethanol produced evaporates during baking. Among the dozens of other useful fermentations are those leading to acetic acid (manufacture of vinegar) and propionic acid (manufacture of Swiss cheese).