Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen, via an electron transport chain. Fermentation is important in anaerobic conditions when there is no oxidative phosphorylation to maintain the production of ATP (adenosine triphosphate) by glycolysis. During fermentation, pyruvate is metabolised to various compounds. Homolactic fermentation is the production of lactic acid from pyruvate; alcoholic fermentation is the conversion of pyruvate into ethanol and carbon dioxide; and heterolactic fermentation is the production of lactic acid as well as other acids and alcohols. Fermentation does not necessarily have to be carried out in an anaerobic environment. For example, even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to oxidative phosphorylation, as long as sugars are readily available for consumption (a phenomenon known as the Crabtree effect). The antibiotic activity of Hops also inhibits aerobic metabolism in Yeast.
Sugars are the most common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, lactose, and hydrogen. However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone. Yeast carries out fermentation in the production of ethanol in beers, wines, and other alcoholic drinks, along with the production of large quantities of carbon dioxide. Fermentation occurs in mammalian muscle during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of lactic acid.
Fermentation products contain chemical energy (they are not fully oxidized), but are considered waste products, since they cannot be metabolized further without the use of oxygen.
The chemical equation below shows the alcoholic fermentation of glucose, whose chemical formula is C6H12O6. One glucose molecule is converted into two ethanol molecules and two carbon dioxide molecules:
- C6H12O6 → 2 C2H5OH + 2 CO2
 Lactic acid fermentation
Lactic acid fermentation is the simplest type of fermentation. In essence, it is a redox reaction. In anaerobic conditions, the cell’s primary mechanism of ATP production is glycolysis. Glycolysis reduces (i.e. transfers electrons to) nicotinamide adenine dinucleotide (NAD+), forming NADH. However there is a limited supply of NAD+ available in any given cell. For glycolysis to continue, NADH must be oxidized (i.e. have electrons taken away) to regenerate the NAD+ that is used in glycolysis. In an aerobic environment, where oxygen is available, oxidation of NADH is usually done through an electron transport chain in a process called oxidative phosphorylation, but oxidative phosphorylation cannot occur in anaerobic environments because oxygen is absent due to the pathway's dependence on the terminal electron acceptor of oxygen. Instead, the NADH donates its extra electrons to the pyruvate molecules formed during glycolysis. Since the NADH has lost electrons, NAD+ regenerates and is again available for glycolysis. Lactic acid, for which this process is named, is formed by the reduction of pyruvate.
In heterolactic acid fermentation, one molecule of pyruvate is converted to lactate; the other is converted to ethanol and carbon dioxide. In homolactic acid fermentation, both molecules of pyruvate are converted to lactate. Homolactic acid fermentation is unique because it is one of the only respiration processes to not produce a gas as a byproduct.
Homolactic fermentation breaks down the pyruvate into lactate. It occurs in the muscles of animals when they need energy faster than the blood can supply oxygen. It also occurs in some kinds of bacteria (such as lactobacilli) and some fungi. It is this type of bacteria that converts lactose into lactic acid in yogurt, giving it its sour taste. These lactic acid bacteria can be classed as homofermentative, where the end-product is mostly lactate, or heterofermentative, where some lactate is further metabolized and results in carbon dioxide, acetate, or other metabolic products.
The process of lactic acid fermentation using glucose is summarized below. In homolactic fermentation, one molecule of glucose is converted to two molecules of lactic acid:
- C6H12O6 → 2 CH3CHOHCOOH.
or one molecule of lactose and one molecule of water make four molecules of lactate (as in some yogurts and cheeses):
- C12H22O11 + H2O → 4 CH3CHOHCOOH.
In heterolactic fermentation, the reaction proceeds as follows, with one molecule of glucose being converted to one molecule of lactic acid, one molecule of ethanol, and one molecule of carbon dioxide:
- C6H12O6 → CH3CHOHCOOH + C2H5OH + CO2
To extract chemical energy from glucose, the glucose molecule must be split into two molecules of pyruvate. This process generates two molecules of NADH and also four molecules of adenosine triphosphate (ATP), yet there is only net gain of two ATP molecules considering the two initially consumed.
- C6H12O6 + 2 ADP + 2 Pi + 2 NAD+ → 2 CH3COCOO− + 2 ATP + 2 NADH + 2 H2O + 2H+
The chemical formula of pyruvate is CH3COCOO−. Pi stands for the inorganic phosphate. As shown by the reaction equation, glycolysis causes the reduction of two molecules of NAD+ to NADH. Two ADP molecules are also converted to two ATP and two water molecules via substrate-level phosphorylation.
 Aerobic respiration
In aerobic respiration, the pyruvate produced by glycolysis is oxidized completely, generating additional ATP and NADH in the citric acid cycle and by oxidative phosphorylation. However, this can occur only in the presence of oxygen. Oxygen is toxic to organisms that are obligate anaerobes, and are not required by facultative anaerobic organisms. In the absence of oxygen, one of the fermentation pathways occurs in order to regenerate NAD+; lactic acid fermentation is one of these pathways.
 Hydrogen gas production in fermentation
Hydrogen gas is produced in many types of fermentation (mixed acid fermentation, butyric acid fermentation, caproate fermentation, butanol fermentation, glyoxylate fermentation), as a way to regenerate NAD+ from NADH. Electrons are transferred to ferredoxin, which in turn is oxidized by hydrogenase, producing H2. Hydrogen gas is a substrate for methanogens and sulfate reducers, which keep the concentration of hydrogen low and favor the production of such an energy-rich compound, but hydrogen gas at a fairly high concentration can nevertheless be formed, as in flatus.
As an example of mixed acid fermentation, bacteria such as Clostridium pasteurianum ferment glucose producing butyrate, acetate, carbon dioxide and hydrogen gas: The reaction leading to acetate is:
- C6H12O6 + 4 H2O → 2 CH3COO- + 2 HCO3- + 4 H+ + 4 H2
Glucose could theoretically be converted into just CO2 and H2, but the global reaction releases little energy.
 Methane gas production in fermentation
- CH3COO– + H+ → CH4 + CO2 ΔG° = -36 kJ/reaction