Fermentation is a metabolic process that produces chemical changes in organic substances through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as zymology.
In microorganisms, fermentation is the primary means of producing adenosine triphosphate (ATP) by the degradation of organic nutrients anaerobically.
Humans have used fermentation to produce foodstuffs and beverages since the Neolithic age. For example, fermentation is used for preservation in a process that produces lactic acid found in such sour foods as pickled cucumbers, kombucha, kimchi, and yogurt, as well as for producing alcoholic beverages such as wine and beer. Fermentation also occurs within the gastrointestinal tracts of all animals, including humans.
Industrial fermentation is a broader term used for the process of applying microbes for the large-scale production of chemicals, biofuels, enzymes, proteins and pharmaceuticals.
Definitions and etymology
Below are some definitions of fermentation ranging from informal, general usages to more scientific definitions. Preservation methods for food via microorganisms (general use). Any large-scale microbial process occurring with or without air (common definition used in industry, also known as industrial fermentation). Any process that produces alcoholic beverages or acidic dairy products (general use). Any energy-releasing metabolic process that takes place only under anaerobic conditions (somewhat scientific). Any metabolic process that releases energy from a sugar or other organic molecule, does not require oxygen or an electron transport system, and uses an organic molecule as the final electron acceptor (most scientific).
The word "ferment" is derived from the Latin verb fervere, which means to boil. It is thought to have been first used in the late 14th century in alchemy, but only in a broad sense. It was not used in the modern scientific sense until around 1600.
Biological role
Along with aerobic respiration, fermentation is a method to extract energy from molecules. This method is the only one common to all bacteria and eukaryotes. It is therefore considered the oldest metabolic pathway, suitable for primeval environmentsbefore plant life on Earth, that is, before oxygen in the atmosphere. Nick Lane criticizes this proposal as the amount of energy released by fermentation is small, which can't lead to a thermodynamic driving force of prebiotic chemistry. The enzymes involved in fermentations, which are encoded by genes, could not have existed during prebiotic chemistry.
Yeast, a form of fungus, occurs in almost any environment capable of supporting microbes, from the skins of fruits to the guts of insects and mammals to the deep ocean. Yeasts convert (break down) sugar-rich molecules to produce ethanol and carbon dioxide.
Basic mechanisms for fermentation remain present in all cells of higher organisms. Mammalian muscle carries out fermentation during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of lactic acid. In invertebrates, fermentation also produces succinate and alanine.
Fermentative bacteria play an essential role in the production of methane in habitats ranging from the rumens of cattle to sewage digesters and freshwater sediments. They produce hydrogen, carbon dioxide, formate and acetate and carboxylic acids. Then consortia of microbes convert the carbon dioxide and acetate to methane. Acetogenic bacteria oxidize the acids, obtaining more acetate and either hydrogen or formate. Finally, methanogens (in the domain Archea) convert acetate to methane.
Biochemical overview
Fermentation reacts the reduced form of nicotinamide adenine dinucleotide (NADH) with an endogenous, organic electron acceptor. Usually this is pyruvate formed from sugar through glycolysis. The reaction produces oxidized NAD+ and an organic product, typical examples being ethanol, lactic acid, and hydrogen gas (H2), and often also carbon dioxide. However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone. Fermentation products are considered waste products, since they cannot be metabolized further without the use of oxygen.