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'''Dimethyl sulfide (or DMSfor short)''', 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 and 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 the malt-derived precursors, S-methyl methionine (SMM) and dimethyl sulphoxide (DMSO). SMM is converted to DMS during boiling wort, and to a lesser extent DMS can be formed from DMSO 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>. DMS can be [https://beersensoryscience.wordpress.com/2011/06/02/corn/ confused with a malt-based compound called 2-acetylpyridine (2AP)] which is formed during Maillard reactions and is described as tasting like "corn tortilla"; 2AP has also been confused with [[Tetrahydropyridine#Similar_Compounds|tetrahydropyridines]].

Boiling and cooling have the most effect impact 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. Unhomogenized boiling of the wort can be a cause of DMS (e.g. dead-spots where the wort doesn't mix throughout the boil kettle). Calculations have been proposed to determine if this is a problem for a given kettle (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>.

Much of the DMS in wort that is formed during the wort production process is volatilized off during fermentation due to off-gassing of CO2CO². However, if high amounts of DMS survive the boil, then the off-gassing from fermentation may not be enough to volatilize all of the DMS. The 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 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>.

Anecdotal reports of no issues with DMS in these types of beers seem to far outweigh the reports of DMS problems <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2655174547844076/ Multple MTF members. Milk The Funk Facebook group thread on experiences with DMS in np-boil beer. 05/09/2019.]</ref><ref>[http://brulosophy.com/2015/10/08/update-lab-data-on-pils-malt-boil-length-exbeeriment/ "Update: Lab Data on Pils Malt Boil Length Exbeeriment" on Brulosophy. Retrieved 03/08/2016.]</ref><ref>[http://beerandwinejournal.com/30-min-boil/ "All Grain Pale Ale 30-Minute Boil Experiments" by James Spencer on Beer & Wine Journal. 06/24/2015. Retrieved 03/08/2016.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1243791188982426/ Discussion on MTF regarding DMS in raw ale/no boil/short boils. 03/01/2016.]</ref><ref name="larsblog"></ref>. The specific nature of (or lack of) DMS detection in no-boil/raw ale has not been widely explored by science. There are, however, some explanations that have been proposed. For example, when boiling smaller volumes of wort such as on the homebrew scale there is a larger surface area to volume ratio. This larger surface area to volume ratio allows for more evaporation and volatilization of DMS to occur <ref name="beersmith">[https://www.youtube.com/watch?v=YDBKUCkg8cM Boiling Home Brewed Beer with Dr Charlie Bamforth - BeerSmith Podcast #121.]</ref> (~30 minutes in). Smaller fermenters would also benefit from a larger surface area to volume ratio since CO2 CO² from fermentation volatilize DMS. This may account for the general lack of DMS reported in homebrewed and small-scale farmhouse beer.

* Use more highly kilned malts such as 2 row pale malt instead of pilsner malt.* If the pH must be lowered, for example , when [[Sour_WortingWort_Souring#How_to_Pre-Acidify|pre-acidifying the wort before kettle souring]], lower the pH near the end of the boil but before the cooling or whirlpooling process. A higher pH will increase the rate that SMM converts to DMS during the boil, and lowering the pH after the boil but before cooling will slow the rate at which SMM converts to DMS <ref>[http://onlinelibrary.wiley.com/doi/10.1002/jib.49/full#jib49-sec-0018 125th Anniversary Review: Bacteria in brewing: The good, the bad and the ugly. Frank Vriesekoop, Moritz Krahl, Barry Hucker and Garry Menz. 2013.] </ref>.

SMM left over = 1,000 µg/L x 2^{-(35 min/300 min)}> = 1,000 x 2^-0.12> = 1,000 x 0.92 = 920 µg/L

SMM left over after cooling = 920 µg/L x 2^-0.765> = 920 µg/L x 0.588 = 541 µg/L

===Kettle Souring and Effects Impacts of pH===

The normal procedure for [[Wort_Souring|kettle souring]] techniques is to boil the wort a second time after the pH has been lowered in order to kill the ''Lactobacillus'' souring culture. The lactic acid fermentation by the ''Lactobacillus'' generally results in a wort pH of around 3.0 - 3.6, at which time the wort is boiled to kill the ''Lactobacillus''. In this case, yet another consideration is the effect impact of low pH on the decomposition of SMM into DMS. The [[Dimethyl_Sulfide#Mashing_and_Boiling|half-life table above]] demonstrates that a decrease of -0.3 pH increases the half-life by 5.5 minutes at 100°C. Unfortunately, we do not have data to show how the much lower pH values that are achieved after souring wort will affect the half-life of SMM <ref name="Scheuren2014"></ref>, but assuming the effect impact is linear then the conversion of SMM into DMS during the second boil would be greatly reduced. Assuming the effect of pH on SMM half-life is linear, and that every -0.3 pH increases the half-life by 5.5 minutes at 100°C, the half-life table could hypothetically be updated to include data that looks something like what we present in the first column below. This indicates that wort boiled after souring probably does not create significant amounts of DMS from SMM.

! Temp°C !! SMM half-life at ph 3.4 (min - minutes; may not be reflect reality) !! SMM half-life at ph 5.2 (minminutes) !! SMM half-life at ph 5.5 (minminutes) <ref name="bamforth"></ref>

* [[Sour WortingWort Souring]]