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In lactic acid bacteria, diacetyl can be the byproduct of both homofermentative metabolism of sugars as well as the metabolism of citric acid, and it is a way for the cells to regenerate NADP<sup>+</sup>. In either of these two pathways, extra pyruvate is turned into alpha-acetolactate which then undergoes an oxidative decarboxylation reaction to produce diacetyl. Diacetyl is often reduced by yeast to acetoin and/or 2,3-butanediol, which have a higher threshold and less of an impact on the finished beer/wine <ref name="Wade_2018" />. In mixed fermentation sour beer, the breakdown of diacetyl into acetoin and 2,3-butanediol is often thought to be carried out by ''Brettanomyces''.
Several variables the affect diacetyl and acetoin production have been identified. First of all, some strains of ''Pediococcus'' species produce diacetyl, and others do not. During malolactic fermentation(conversion of citric acid to lactic acid), the temperature at which MLF is conducted can influence whether or not diacetyl is produced. For example, two studies reported that more diacetyl and acetoin were produced during the MLF in wine at 18°C compared at 25°C. The effect of temperature is not well understood, but it has been hypothesized that it could be that at lower temperature yeast is less active and thus cannot break the diacetyl down to acetoin and 2,3-butanediol (extended time exposed to active yeast or on lees can reduce diacetyl in wine and probably also beer). If SO<sub>2</sub> is used, it can bind to diacetyl in a form that cannot be tasted, although the SO<sub>2</sub> can become unbound and release the diacetyl again. An increase in citric acid can also lead to more diacetyl under semi-aerobic conditions but not anaerobic conditions. The presence of glucose has also been associated with higher levels of diacetyl production in wine <ref name="Wade_2018" />.
==="Ropy" or "Sick" Beer===