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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 andCand C. W. Bamforth. 1982.]</ref>. The flavor and aroma of DMS have been characterized as being like cooked sweetcorn, tomato sauce, 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>.

The primary source for DMS in beer (as well as cooked vegetables) is caused by the decomposition of SMM into DMS. This decomposition is caused by heat above ~80°C. Levels of SMM in raw barley are initially low, but as the barley is malted the SMM precursor is formed inside the malt. Many factors influence the amount of SMM found in malted barley. SMM amounts are correlated with nitrogen amounts. The longer the barley is stored before malting, the more SMM will be produced. The majority of SMM in malted barley, however, is determined by how the malt is kilned. During kilning temperatures above 70°C the SMM is partially broken down into DMS and homoserine (isothreonine). Some of the DMS is driven off by the high temperatures of kilning due to it's its high volatility, but the DMS present during kilning can also be oxidized into [[Dimethyl_Sulfide#DMSO_Precursor_and_Effects_of_Fermentation|DMSO]]. The lower the temperatures are during kilning (such as for pilsner malt), the more the SMM precursor is retained in the malted barley <ref name="Anness"></ref><ref name="Scheuren2016"></ref>.

The largest contribution of DMS from SMM is after boiling the wort, and during the chilling process. SMM continues to breakdown break down 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 andCand C. 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>.

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, with wort made from higher kilned malts containing more DMSO <ref name="Anness"></ref><ref name="bamforth"></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>.

Many types of microbes are capable of producing DMS from DMSO as a secondary metabolite of fermentation. Microbes that can produce high amounts of DMS include gram-negative, facultative anaerobes in the [https://en.wikipedia.org/wiki/Enterobacteriaceae ''Enterobacteriaceae''] family, which includes species of ''Klebsiella'', ''Citrobacter'', ''Enterobacter'', ''Obesumbacterium'', ''Proteus'', ''Salmonella'', and ''Escherichia'', as well as gram-negative aerobic bacteria such as ''Pseudomonas aeruginosa'' <ref>[http://mmbr.asm.org/content/77/2/157.short The Microbiology of Malting and Brewing. Nicholas A. Bokulicha and Charles W. Bamforth. 2013.]</ref><ref name="zinder">[http://www.ncbi.nlm.nih.gov/pubmed/347031 Dimethyl sulphoxide reduction by micro-organisms. Zinder S.H., Brock T.D. 1978.]</ref>. Gram-positive bacteria can also produce high amounts of DMS, such as ''Bacillus subtilis'' <ref name="zinder"></ref>. These bacteria species can convert 17-37% of DMSO into DMS, whereas ''S. cerevisiae'' converts around 5% of DMSO into DMS. Many other bacteria such as species of ''Clostridium'', ''Streptococcus'', and ''Staphylococcus'' produce only small amounts of DMS (less than 1% of DMSO converted into DMS) <ref name="zinder"></ref>. All bacteria that can produce DMS from DMSO do so using with a different enzyme than yeast, which might account for the ability of some bacteria to convert a higher percentage of DMSO to DMS than ''S. cerevisiae''. The DMS production by facultative anaerobic bacteria is encouraged by the lack of oxygen <ref name="Anness"></ref>.

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 long boil of the wort due to the [[Turbid Mash|turbid mash]]. After two weeks of fermentation, 450 ppb of DMS were was 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 CO₂. 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>.

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 the laws of thermodynamics. They presented a somewhat counter intuitive counterintuitive 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 gravity of the wort, and that it is instead effected affected by temperature, atmospheric pressure, and the concentration of DMS (higher concentration of DMS slightly raises the volatility of DMS). For non-wort solutions, 10% sucrose in water greatly increased the volatility of DMS, possibly due to a [http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Thermodynamics/Non-Ideal_Systems/Salting_Out salting-out effect] of sucrose which leads to a higher evaporation rate for DMS molecules <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 <ref name="Anness"></ref>. Higher fermentation temperatures (18°C versus 9-12°C, for example) can lead to higher rates of DMS volatilization <ref name="narziss2008">[http://www.wiley.com/WileyCDA/WileyTitle/productCd-3527659889.html Die Bierbrauerei: Band 2: Die Technologie der Würzebereitung, 8 Auflage. Ludwig Narziss. 2008. Section 5.6.4.3.]</ref>. DMS can spike towards the end of fermentation from yeast metabolizing DMSO into DMS (see [[Dimethyl_Sulfide#DMSO_Precursor|DMSO Precursor]]) <ref name="Anness"></ref>.

[http://www.garshol.priv.no/blog/331.html Raw ale], also referred to as "no-boil", is a method of wort production that involves not boiling the wort, or perhaps by some definitions, very short boils <ref name="larsblog">[http://www.garshol.priv.no/blog/331.html Raw ale. Lars Marius Garshol. Larsblog. 06/05/2016. Retrieved 03/02/2016.]</ref>. Although mainly a historical method of brewing, this style of brewing has recently become popular in the production of [[Berliner Weissbier]] and other styles of beer using [[Sour_WortingWort_Souring|sour worting wort souring or kettle souring]] methods. Many recipes for these styles of beer call for pilsner malts to be used, which can contain higher amounts of SMM precursor. An often asked question about no-boil/raw ales and wort boiled for 15 minutes or less is: are there concerns about DMS production?