Fermentation is a metabolic process that consumes sugar in the absence of oxygen. The products are organic acids, gases, or alcohol. It occurs in yeast and bacteria , and also in oxygen-starved muscle cells, as in the case of lactic acid fermentation . The science of fermentation is known as zymology . In microorganisms, fermentation is the primary means of producing ATP by the degradation of organic nutrients anaerobically . Humans have used fermentation to produce drinks and beverages since the Neolithic age . For example, fermentation is used for preservation in a process that produces lactic acid as found in such sour foods as pickled cucumbers , kimchi and yogurt (see fermentation in food processing ), as well as for producing alcoholic beverages such as wine (see fermentation in winemaking ) and beer . Fermentation occurs within the gastrointe...