Fermentation typically refers to the conversion of sugar to acids, gases and/or alcohol using yeast or bacteria. In its strictest sense, fermentation is the absence of the electron transport chain and takes a reduced carbon source, such as glucose, and makes products like lactic acid or acetate. No oxidative phosphorylation is used, only substrate level phosphorylation, which yields a much lower amount of ATP. Fermentation is also used much more broadly to refer to the bulk growth of microorganisms on a growth medium. The science of fermentation is known as zymology. The process is often used to produce wine and beer, but fermentation is also employed in preservation to create lactic acid in sour foods such as pickled cucumbers, kimchi and yogurt. Fermentation is a form of anaerobic digestion that generates the enzyme adenosine triphosphate (ATP) by the process of substrate-level phosphorylation. The energy for generating ATP comes from the oxidation of organic compounds, such as carbohydrates. In contrast, during 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. 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 aerobic respiration, 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. Fermentation uses an endogenous, organic electron acceptor. A widely used endogenous electron acceptor is pyruvate. During fermentation, pyruvate is metabolized 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. Sugars are the most common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, lactose, and hydrogen gas (H2). 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.
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