Fermentation is a form of metabolism in which organic compounds are broken down and their derivatives accept electrons, allowing a cell to continue obtaining energy without relying on respiratory electron transport. Familiar products include lactate, ethanol, and organic acids. In food production and industry, the word has a broader meaning: the transformation of materials by microorganisms or their enzymes, including processes that require oxygen. The biochemical and technological meanings therefore overlap but are not identical. (openstax.org)
Biochemical basis
A central function of fermentation is to regenerate NAD⁺, the oxidized form of the electron-carrying coenzyme NAD. During glycolysis, glucose is converted into pyruvate, while NAD⁺ accepts electrons and becomes NADH. Unless NADH is oxidized back to NAD⁺, glycolysis cannot continue. Fermentative reactions accomplish this by transferring electrons to pyruvate or another organic compound derived from the substrate. (openstax.org)
In the familiar pathways that convert glucose into lactate or ethanol, glycolysis supplies a net yield of two molecules of adenosine triphosphate (ATP) per glucose molecule. ATP is formed by substrate-level phosphorylation: direct transfer of a phosphate group from a metabolic intermediate to ADP. The subsequent NAD⁺-regenerating reactions do not themselves produce additional ATP. Much of the original chemical energy remains in the fermentation products rather than being extracted by the cell. (openstax.org)
Fermentation differs from cellular respiration, including anaerobic respiration. Respiratory pathways use an electron transport chain and can generate ATP through oxidative phosphorylation. Anaerobic respiration uses terminal electron acceptors other than oxygen, such as sulfate. Thus, the absence of oxygen alone does not identify a process as fermentation. Some organisms can switch between respiration and fermentation as environmental conditions change. (openstax.org)
Principal pathways
Lactic acid fermentation reduces pyruvate to lactate while oxidizing NADH. It occurs in numerous bacteria and in animal cells. In food processing, lactic acid bacteria acidify milk and vegetables. Homolactic fermentation produces predominantly lactate; heterolactic fermentation also produces compounds such as ethanol, acetate, and carbon dioxide. These differences influence acidity, flavor, and gas formation. (openstax.org)
Alcoholic fermentation typically converts pyruvate first into acetaldehyde, releasing carbon dioxide, and then into ethanol. The second reaction regenerates NAD⁺. Yeasts, particularly Saccharomyces cerevisiae, use this pathway in bread, beer, and wine production. Carbon dioxide expands bread dough, whereas ethanol is the principal alcoholic product of brewing and winemaking. Fermentation of plant-derived sugars also supplies ethanol for fuel production. (openstax.org)
Other microbial pathways produce mixtures of acids, alcohols, and gases. Propionic acid fermentation contributes to the characteristic flavor of Swiss-type cheeses. Fermentation products also provide useful laboratory characteristics: changes in acidity or gas production when a microorganism receives a particular carbohydrate can help distinguish organisms. The term therefore encompasses a family of pathways rather than a single chemical reaction. (openstax.org)
Fermentation in food production
Food fermentation exploits microbial activity to alter raw ingredients. Organic acids, alcohols, and other metabolites change taste and aroma, while microbial enzymes transform components of the food. Fermentation can also increase storage stability by creating conditions less favorable to unwanted microorganisms. Its effects depend on the starting material, the organisms present, and the processing conditions; different fermented foods do not share one uniform composition. (fao.org)
Production may begin spontaneously with organisms already present on ingredients or equipment. Alternatively, producers introduce a starter culture containing selected microorganisms. Another method, backslopping, inoculates a new batch with material from an earlier fermentation. These approaches differ in how the microbial community is established and controlled. Selected cultures allow producers to favor particular transformations rather than depending entirely on environmental microorganisms. (fao.org)
The broader food-processing definition includes vinegar production. Acetic acid bacteria oxidize ethanol into acetic acid in the presence of oxygen, often following an earlier alcoholic fermentation. Although conventionally called “acetic acid fermentation,” this oxygen-dependent oxidation differs from fermentation in the narrow biochemical sense. Microbial communities can thus carry out successive stages with different oxygen requirements. (fao.org)
Industrial cultivation and control
Industrial fermentation extends beyond traditional foods to microbial cultures, enzymes, flavors, fragrances, additives, and other useful products. Here, “fermentation” often describes a cultivation process rather than a particular energy-yielding pathway. The objective may be to obtain microbial cells, a secreted metabolite, or an enzyme-mediated transformation of the starting material. (fao.org)
Modern production commonly uses a bioreactor equipped to regulate temperature, pH, nutrient availability, and oxygen levels. Selected microorganisms or enzymes are introduced under controlled conditions. These operations combine microbiology with chemical engineering: the biological activity must be maintained while the physical and chemical environment is managed. Oxygen control is especially important because a process described industrially as fermentation may require aeration rather than oxygen exclusion. (fao.org)
Historical development
Louis Pasteur began systematic investigations of fermentation in 1857. His work on lactic and alcoholic fermentations established the roles of particular microorganisms and emphasized the use of sterile media inoculated with a defined ferment. These investigations helped establish experimental methods for studying microbial transformations and connected fermentation research with the development of microbiology. (pasteur.fr)
In 1897, Eduard Buchner demonstrated that yeast extracts without living yeast cells could convert sugar into alcohol. The experiment showed that fermentation depended on substances produced by cells, rather than requiring intact living cells throughout the reaction. His discovery of cell-free fermentation helped establish enzyme-centered biochemistry and was recognized by the 1907 Nobel Prize in Chemistry. (nobelprize.org)