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Dimethyl Sulphoxide (DMSO) is the second precursor to DMS, and is also present in malted barley. Conversion of DMSO to DMS in beer is a function of microbial activity. DMSO is formed in malted barley during kilning at temperatures above 60°C (ale wort can contain more DMSO than lager wort because of this <ref name="bamforth"></ref>). Drying the green malt before kilning also increases DMSO (and SMM). DMSO is readily dissolved into water during mashing, and with a boiling point of 189°C, it survives mashing and boiling temperatures. Wort generally contains 200-400 µg of DMSO per liter <ref name="Anness"></ref>.
''Saccharomyces'' species convert less than 25% of DMSO into DMS as a side effect of an enzyme whose primary function is to reduce methionine sulfoxide to [https://en.wikipedia.org/wiki/Methionine methionine] <ref name="bamforth"></ref>. In a lab setting with simple glucose-salts and DMSO added, ~13% of DMSO is converted to DMS. However, in wort only ~5% of DMSO is converted to DMS <ref name="bamforth"></ref>. The percentage of DMSO that is converted to DMS does not change as DMSO levels increase, so although low percentages are converted, high amounts of DMSO can still contribute significant DMS. DMS is a very volatile compound, and much of the DMS in wort from the SMM precursor is volatilized off during fermentation due to off-gassing of CO2. However, if high amounts of DMS survive the boil then off-gassing from fermentation may not be enough to volatilize all of the DMS. Additionally, with With high levels of DMSO in the wort, a slight increase in DMS from DMSO precursor can be observed towards the end of fermentation from yeast metabolism. This increase in DMS from yeast metabolism has been observed during the conditioning of fermented beer and surprisingly under cold temperatures (0°C in one report) <ref name="Anness"></ref>.
Yeast species/strain, temperature, pH, wort composition, and open/closed fermentation vessels contribute to how much DMSO gets converted into DMS. For example, ''S. uvarum'' (potentially reclassified to ''S. bayanus'') produces less DMS than ''S. cerevisiae'', as does ''S. pastorianus'' <ref name="bamforth"></ref>. DMSO is converted to DMS by yeast more readily at lower temperatures than warmer temperatures with five times as much at 8°C than at 25°C. Higher gravity worts (1.033 vs 1.060 in the linked reference) also produce more DMS from DMSO during fermentation. A higher pH of wort also leads to more DMS production; for example lager wort pH is typically 5.4-5.7, while ale wort pH is typically ~5.1. This might explain why DMS is present more in lager beers. Anderson et al. and Booer & Wilson showed that open fermentation leads to less DMS production compared to closed fermentation <ref name="Anness"></ref>.
====Spoilage Organisms and Spontaneous Fermentation====
In [[lambic]] production where the pH of the wort is not lowered to less than 4.5 before entering the [[coolship]] for [[Spontaneous_Fermentation|spontaneous fermentation]], ''Enterobacteriaceae'' are responsible for high amounts of DMS production. No DMS was found in the referenced study before the wort was cooled in the coolship, which might be due to the lengthy boil of the wort due to the [[Turbid Mash|turbid mash]]. After two weeks of fermentation, 450 ppb of DMS were found, far more than the 30 ppb taste threshold, and the vegetal aroma of DMS could be detected during the fermentation at this time. After two weeks the fermentation of ''Saccharomyces'' begins, and the DMS levels decline due to the formation and blow-off of CO<sub>2</sub>. At 6 months the DMS was down to 100 ppb, and a range of 25-75 ppb of DMS found in bottles of lambic (and at 16+ months), which is a typical amount for regular ales and lagers <ref>ORIGIN AND EVOLUTION OF DIMETHYL SULFIDE AND VICINAL DIKETONESDURING THE SPONTANEOUS FERMENTATION OF LAMBIC AND GUEUZE. D. Van Oevelen, P. Timmermans, L. Geens and H. Verachtert. 1978.</ref>.
===Volatility of DMS===
DMS is a very volatile compound. Scheuren et al. (2016) determined that there is not a significant difference in DMS evaporation in water versus wort, and came up with equations for determining the evaporation of DMS in water using laws of thermodynamics. They presented a somewhat counter intuitive result that DMS actually volatilizes more readily as temperatures drop, until about 50°C (volatilization of DMS drops significantly under 50°C). Their calculations state that 3.2% of the total wort volume needs to be evaporated for 90% of the DMS to volatilize at 100°C, whereas only 1.3% of the total wort volume needs to be evaporated for 90% of the DMS to volatilize at 80°C (keep in mind that the rate of evaporation at 80°C is much slower, and thus it takes more time to reach 1.3% evaporation). This indicates that some amount of DMS is evaporating off at temperatures below boiling until 50°C is reached <ref name="Scheuren2016mbaa"></ref><ref name="Scheuren2016"></ref>.
They also established that the volatility of DMS is the same regardless of the concentration of DMS, and that it is effected by temperature, atmospheric pressure, and the concentration of DMS (higher concentration of DMS slightly raises the volatility of DMS <ref name="Scheuren2016mbaa"></ref>). A larger top surface area will allow for faster evaporation of the total DMS present in the wort, but the total DMS present in the wort would eventually be evaporated off regardless of what the top surface area of the kettle is <ref name="Scheuren2016"></ref>. In order to limit DMS in the end product, it is advised to allow no more than 100 µg/L of DMS into the fermenter <ref name="Scheuren2016mbaa">Influence of Extract on Volatility of Flavor Components in Wort During Open and Closed Boil. Hans Scheuren Roland Feilner, Frank-Jürgen Methner, and Michael Dillenburger. MBAA website. 2016.</ref>.
Much of the DMS in wort from the SMM precursor is volatilized off during fermentation due to off-gassing of CO2. However, if high amounts of DMS survive the boil then off-gassing from fermentation may not be enough to volatilize all of the DMS. Shape and type of the fermenter also play a role in how much DMS is volatilized during fermentation, for example Anderson et al. and Booer & Wilson showed that open fermentation leads to less DMS production compared to closed fermentation. DMS can spike towards the end of fermentation from yeast metabolizing DMSO into DMS <ref name="Anness"></ref>.
==Short Boils and Raw Ale==