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The anaerobic process by which yeast convert sugars into alcohol and carbon dioxide.

In brewing, the term 'fermentation' usually encompasses an entire process, starting when yeast is pitched into wort and ending when final gravity has been reached and yeast has reduced undesirable flavors to acceptable levels.

Fermentation more generally refers to the physiological state in which yeast gain metabolic energy in the absence of oxygen, by reducing simple sugars mainly to carbon dioxide and ethanol. Fermentation is much less efficient than respiration (which occurs in the presence of oxygen) for producing cellular energy.