Fermentation is a specific type of heterotrophic metabolism that uses organic carbon instead of oxygen as a terminal electron acceptor.  This means that these organisms do not use an electron transport chain to oxidize NADH to  and therefore must have an alternative method of using this reducing power and maintaining a supply of  for the proper functioning of normal metabolic pathways (e.g. glycolysis).  As oxygen is not required, fermentative organisms are anaerobic.  Many organisms can use fermentation under anaerobic conditions and aerobic respiration when oxygen is present.  These organisms are facultative anaerobes.  To avoid the overproduction of NADH, obligately fermentative organisms usually do not have a complete citric acid cycle.  Instead of using an ATP synthase as in respiration, ATP in fermentative organisms is produced by substrate-level phosphorylation where a phosphate group is transferred from a high-energy organic compound to ADP to form ATP.  As a result of the need to produce high energy phosphate-containing organic compounds (generally in the form of Coenzyme A-esters) fermentative organisms use NADH and other cofactors to produce many different reduced metabolic by-products, often including hydrogen gas (). These reduced organic compounds are generally small organic acids and alcohols derived from pyruvate, the end product of glycolysis. Examples include ethanol, acetate, lactate, and butyrate. Fermentative organisms are very important industrially and are used to make many different types of food products. The different metabolic end products produced by each specific bacterial species are responsible for the different tastes and properties of each food.
Not all fermentative organisms use substrate-level phosphorylation. Instead, some organisms are able to couple the oxidation of low-energy organic compounds directly to the formation of a proton motive force or sodium-motive force and therefore ATP synthesis.  Examples of these unusual forms of fermentation include succinate fermentation by Propionigenium modestum and oxalate fermentation by Oxalobacter formigenes. These reactions are extremely low-energy yielding.  Humans and other higher animals also use fermentation to produce lactate from excess NADH, although this is not the major form of metabolism as it is in fermentative microorganisms.