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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>, which generally equates to about 5-10 µg/L of DMS <ref name="narziss2008"></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. 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 <ref name="Anness"></ref>.