Fermentation
Fermentation is a metabolic process that produces chemical changes in organic substances through the action of enzymes. In biochemistry, fermentation is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen, while 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.[1] The science of fermentation is known as zymology.
For other uses, see Fermentation (disambiguation).
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. See Fermentation in food processing. Fermentation also occurs within the gastrointestinal tracts of all animals, including humans.[2]
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.
Biological role[edit]
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 environments – before plant life on Earth, that is, before oxygen in the atmosphere.[4]: 389 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. In addition, he notes that the differences between the fermentation processes in archaea and bacteria indicate that fermentation likely evolved later on, developing independently in both types of primitive life.[5]
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.[6][7]
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.[8]: 63 In invertebrates, fermentation also produces succinate and alanine.[9]: 141
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.[10]