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added new information regarding uneven boiling, and reference to new prediction model to determine uneven boiling.
During mashing, small spikes of DMS have been reported. This has been proposed to be due to the volatility of DMS existing in the malt rather than being converted from SMM (mash infusion temperatures are too low to convert significant amounts of SMM into DMS). When mashing in a closed system, evaporated DMS condenses and falls back into the mash. The small amount of DMS that is produced during the mash is volatilized by the early stages of boiling. Decoction mashing also introduces DMS due to the boiling of the mash and the resulting conversion of SMM into DMS. SMM from the malt is easily dissolved into the wort during mashing <ref>[http://onlinelibrary.wiley.com/doi/10.1002/jib.234/abstract Explanation for the increase in free dimethyl sulphide during mashing. H. Scheuren, K. Sommer and, Dillenburger. 2015.]</ref><ref name="Anness"></ref>.
Boiling and cooling have the most effect on levels of DMS in beer. At boiling temperatures, SMM is decomposed into DMS. Wilson & Booer showed that SMM's [https://en.wikipedia.org/wiki/Half-life half-life] is about 35 minutes at a pH of 5.4, meaning that it takes ~35 minutes to reduce half of the SMM present into DMS <ref name="Anness"></ref>. pH plays a role in the reduction of SMM to DMS, with a higher pH reducing the half-life of SMM. Dickenson showed that at a wort pH of 5.2, SMM had a half-life of 38 minutes, but at a pH of 5.5 the SMM has a half-life of 32.5 minutes <ref>[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1979.tb03914.x/abstract THE RELATIONSHIP OF DIMETHYL SULPHIDE LEVELS IN MALT, WORT AND BEER. C. J. Dickenson. 1979.]</ref>. It has long been reported that the half-life of SMM doubles for every 6°C cooler, meaning that at 95°C the half-life is ~70 minutes (see the table below) <ref name="bamforth"></ref>. If the wort is held at a perfectly uniform temperature (which may not reflect real brewery conditions) then the half-life doubles more quickly as the wort cools <ref name="Scheuren2014"></ref>. During the boil, the converted DMS is evaporated off due to its low boiling temperature of 37.3°C <ref name="pubchem">[https://pubchem.ncbi.nlm.nih.gov/compound/dimethyl_sulfide#section=Odor Dimethyl Sulfide. PubChem. Retrieved 03/02/2016.]</ref> and the convection currents of the boil. On larger commercial systems, unevenly boiling wort can be a cause of DMS (calculations have been proposed to determine if this is a problem; see reference) <ref>[http://www.mbaa.com/publications/tq/tqPastIssues/2016/Pages/TQ-53-3-0817-01.aspx Quantification of Wort Homogeneity for Projecting the Evaporation of Dimethyl Sulfide in an Open, Discontinuous Boiling Process by Means of Direct Heating of the Wort Kettle. Benjamin Kloos and Hans Scheuren. 2016.]</ref>.
The largest contribution of DMS from SMM is after boiling the wort, and during the chilling process. SMM continues to breakdown into DMS after boiling and before the wort is completely chilled. DMS formed during this time is mostly retained in the wort due to the wort being still, especially in a closed cooling system where evaporation is prevented completely. Once the wort reaches a temperature of 80-85°C, the decomposition of SMM into DMS is greatly reduced <ref name="Anness"></ref>. It has been shown that a longer boil will help decompose the SMM and drive off DMS <ref>[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1979.tb06845.x/abstract CONTROL OF THE DIMETHYL SULPHIDE CONTENT OF BEER BY REGULATION OF THE COPPER BOIL. R. J. H. Wilson andC. D. Booer. 1979.]</ref>, however if the level of SMM in the malt is high (3-8 µg DMS equivalents/g malt) and more than 50 µg DMS equivalents/liter of SMM survives the boil, then reducing the time in the whirlpool where the wort sits above 80°C can help reduce the amount of DMS in the finished beer. SMM that is not decomposed into DMS during the boil/whirlpool and survives going into the fermenter is not metabolized by yeast, but is also not decomposed into DMS (typical brewing conditions result in little SMM going into the fermenter) <ref name="Anness"></ref><ref name="bamforth"></ref>.