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'''Dimethyl sulfide (DMS)''', sometimes spelled "dimethyl sulphide" <ref name="pubchem"></ref>, is the simplest type of [https://en.wikipedia.org/wiki/Thioether thioether] <ref name="Scheuren2016">[http://onlinelibrary.wiley.com/doi/10.1002/jib.301/full Evaporation behaviour of DMS in an aqueous solution at infinite dilution – a review. H. Scheuren, M. Baldus, F.-J. Methner and M. Dillenburger. 2016]</ref>, which are sulfur containing oils that are generally considered off-putting in beer <ref>[https://en.wikipedia.org/wiki/Thioether Wikipedia. Thioether. Retrieved 03/01/2016.]</ref><ref name="Anness">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1982.tb04101.x/abstract DIMETHYL SULPHIDE—A REVIEW. B. J. Anness andC. W. Bamforth. 1982.]</ref>. The flavor and aroma of DMS have been characterized as being like cooked sweetcorn, tomato sauce, and celery, or sauerkraut. In beer, it is sometimes confused with methyl thiocetate, ethanethiol, and dimethyl trisulphide. DMS in beer originates from malt-derived precursors, S-methyl methionine (SMM) and dimethyl sulphoxide (DMSO), and to a lesser extent can be formed during fermentation by certain microbes <ref name="aroxa">[http://www.aroxa.com/beer/beer-flavour-standard/dimethyl-sulphide/ Aroxa. Dimethyl sulphide. Retrieved 03/01/2016.]</ref>. Small amounts of DMS have also been found in hops, which is volatilized during boiling <ref name="Scheuren2014">[http://onlinelibrary.wiley.com/doi/10.1002/jib.156/abstract Decomposition kinetics of dimethyl sulfide. H. Scheuren, J. Tippmann, F.-J. Methner, and, K. Sommer. 2014.]</ref>. The flavor threshold of DMS is 30-50 µg/liter. Low levels above threshold between 30-100 µg/liter are considered acceptable and even beneficial to some lagers in the United Kingdom (but not in Germany) <ref name="beersmith"></ref> (~25 minutes in), and maybe some traditional farmhouse ales that are not boiled (see [[Dimethyl_Sulfide#Considerations_for_Historical_Examples_of_Raw_Ale|Considerations for Historical Examples of Raw Ale]]). However, amounts above 100 µg/liter are generally considered offensive for any beer. Ales typically have below the flavor threshold of DMS <ref name="Anness"></ref>. The basis of the understanding of DMS and it's creation in beer was uncovered in the late 70's and early 80's. DMS is a common compound found throughout nature, including having an importance in cycling sulfur in ecosystems involving algae and other microbes, helping the navigation of seabirds, and is found in many foods such as corn, cabbage, parsley, asparagus, potatoes, beef, Camembert cheese, fish (carp), tea, cocoa, milk, wine, rum, beetroot, black truffles, and seafood <ref name="Scheuren2016"></ref><ref name="bamforth">[http://www.asbcnet.org/publications/journal/vol/abstracts/ASBCJ-2014-0610-01.htm Dimethyl Sulfide – Significance, Origins, and Control. Charles W. Bamforth. 2014.]</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 and in 1.060 gravity wort, 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>. 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). 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.

DMS is very volatile, and evaporates easily at temperatures below boiling (80°C, for example), and assuming the wort is not in a closed system, will eventually evaporate off even if the wort is not boiling. Scheuren et al. (2016) determined that there is not a significant difference in DMS evaporation in water versus wort or different gravities of wort, and came up with equations for determining the evaporation of DMS in water using laws of thermodynamics. They also established that the volatility of DMS is the same regardless of the concentration of DMS, and that it is effected by temperature and atmospheric pressure. 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>.