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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 or , tomato sauce, and celery. 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>[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, hypothesized to help 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 While malted barley contains small but not insignificant amounts of DMS (more than 10 ppm) <ref name="Scheuren2016"></ref>, the primary source of DMS that is not from microbial contamination are the precursors S-methyl methionine (SMM) and dimethyl sulphoxide (DMSO), both of which are present in malted barley <ref name="Anness"></ref>. When listed on malt analysis sheets they are usually listed as a combined value as "DMS-P" or "DMSP". This value should be between 5-15 ppm for pilsner malts, and less for fully modified malt <ref>[http://morebeer.com/brewingtechniques/bmg/noonan.html Understanding Malt Analysis Sheets -- How to Become Fluent in Malt Analysis Interpretation. Greg Noonan. Retrieved 03/02/2016.]</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 quickly driven off by the high temperatures of kilning due to it's 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>.