Dimethyl Sulfide

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Dimethyl sulfide (DMS), sometimes spelled "dimethyl sulphide" [1], is a type of thioether, which are sulfur containing oils with off-putting aromas and flavors [2][3]. The flavor and aroma of DMS have been characterized as being like cooked sweetcorn or tomato sauce. 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 [4]. The flavor threshold of DMS is 30-50 µg/liter. Low levels between 30-100 µg/liter are considered acceptable and even beneficial to some lagers in the United Kingdom (but not in Germany) [5] (~25 minutes in), however amounts above 100 µg/liter in lagers are considered offensive even for UK brewers. Ales typically have below the flavor threshold of DMS [3]. The majority of scientific publication on DMS in beer was done 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 navigation of seabirds, and is found in many foods such as corn, cabbage, tea, cocoa, milk, wine, rum, beetroot, and seafood [6].

Production from Malt

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 [3]. These will be listed on malt analysis sheets as a combined value, usually referred to as "DMS-P" or "DMSP". This value should be between 5-15 ppm for pilsner malts, and less for fully modified malt [7].

SMM Precursor

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 the SMM is broken down into DMS, and the DMS is quickly driven off by the high temperatures of kilning due to it's high volatility. The lower the temperatures are during kilning (such as for pilsner malt), the more the SMM precursor is retained in the malted barley [3].

Mashing and Boiling

During mashing, little DMS is produced, and what is produced is volatilized by the mash temperatures, and then by the early stages of boiling. SMM from the malt, however, is easily dissolved into the wort during mashing. Very little of it is broken down into DMS at mash infusion temperatures, although slightly more amounts can be converted into DMS during decoction mashing [3].

Boiling has the most effect on levels of DMS in beer. At boiling temperatures, SMM is reduced to DMS. Wilson & Booer showed that SMM's 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 [3]. 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 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 [8]. The half-life of SMM doubles for every 6°C cooler, meaning that at 95°C the half-life is ~70 minutes. During the boil, the converted DMS is evaporated off due to its low boiling temperature of 37.3°C [1] and the convection currents of the boil. However, SMM continues to breakdown into DMS after boiling and before the wort is chilled. DMS formed during this time is retained in the wort. Once the wort reaches a temperature of 80-85°C, the decomposition of SMM into DMS is greatly reduced [3]. It has been shown that a longer boil will help decompose the SMM and drive off DMS [9], 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 (most brewery conditions result in little SMM going into the fermenter) [3][6].

DMSO Precursor and Effects of Fermentation

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 [6]). 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 [3].

Saccharomyces species convert less than 25% of DMSO into DMS as a side effect of an enzyme whose primary function is to reduce methionine sulfoxide to methionine [6]. In a lab setting with simple glucose-salts and DMSO added, ~13% of DMSO is converted to DMS. However, in wort only ~5% of DMSO is converted to DMS [6]. The percentage of DMSO that is converted to DMS does not change as DMSO levels increase, so although low percentages are converted, high amounts of DMSO can still contribute significant DMS. DMS is a very volatile compound, and 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. Additionally, with high levels of DMSO in the wort, a slight increase in DMS from DMSO precursor can be observed towards the end of fermentation from yeast metabolism. This increase in DMS from yeast metabolism has been observed during the conditioning of fermented beer and surprisingly under cold temperatures (0°C in one report) [3].

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 [6]. 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. Anderson et al. and Booer & Wilson showed that open fermentation leads to less DMS production compared to closed fermentation [3].

Spoilage Organisms and Spontaneous Fermentation

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 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 [10][11]. Gram-positive bacteria can also produce high amounts of DMS, such as Bacillus subtilis [11]. Many other bacteria such as species of Clostridium, Streptococcus, and Staphylococcus produce only small amounts of DMS (much less than S. cerevisiae even) [11]. All bacteria that can produce DMS from DMSO do so using 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 [3].

In lambic production where the pH of the wort is not lowered to less than 4.5 before entering the coolship for 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 boil of the wort due to the turbid mash. After two weeks of fermentation, 450 ppb of DMS were 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 [12].

Short Boils and Raw Ale

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, short boils [13]. 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 worting 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?

The specific nature of DMS (or lack of) formation in no-boil/raw ale has not been widely explored by science. When boiling smaller volumes of wort, for example on the homebrew scale versus a large commercial 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 [5] (~30 minutes in). Smaller fermenters would also benefit from a larger surface area to volume ratio since fermentation helps to volatilize DMS. This may account for the general lack of DMS reported in homebrewed and small-scale farmhouse beer [14][15][16][13].

Avoiding DMS

If the brewer is experiencing DMS in no-boil/raw ale/short boiled beers:

  • Avoid allowing wort to stand between 80-100°C (176-212°F).
  • If the wort is allowed to stand in the above mentioned temperature range, boil the wort rigorously for a few minutes afterwards, and then quickly cool it below 80°C (176°F).
  • Use open shallow fermenters [6].
  • Increase fermentation temperature [6].
  • Allow the beer to age longer.
  • Use more highly kilned malts such as 2 row instead of pilsner malt.

See Also

References

  1. ↑ 1.0 1.1 Dimethyl Sulfide. PubChem. Retrieved 03/02/2016.
  2. ↑ Wikipedia. Thioether. Retrieved 03/01/2016.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 DIMETHYL SULPHIDE—A REVIEW. B. J. Anness andC. W. Bamforth. 1982.
  4. ↑ Aroxa. Dimethyl sulphide. Retrieved 03/01/2016.
  5. ↑ 5.0 5.1 Boiling Home Brewed Beer with Dr Charlie Bamforth - BeerSmith Podcast #121.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Dimethyl Sulfide – Significance, Origins, and Control. Charles W. Bamforth. 2014.
  7. ↑ Understanding Malt Analysis Sheets -- How to Become Fluent in Malt Analysis Interpretation. Greg Noonan. Retrieved 03/02/2016.
  8. ↑ THE RELATIONSHIP OF DIMETHYL SULPHIDE LEVELS IN MALT, WORT AND BEER. C. J. Dickenson. 1979.
  9. ↑ CONTROL OF THE DIMETHYL SULPHIDE CONTENT OF BEER BY REGULATION OF THE COPPER BOIL. R. J. H. Wilson andC. D. Booer. 1979.
  10. ↑ The Microbiology of Malting and Brewing. Nicholas A. Bokulicha and Charles W. Bamforth. 2013.
  11. ↑ 11.0 11.1 11.2 Dimethyl sulphoxide reduction by micro-organisms. Zinder S.H., Brock T.D. 1978.
  12. ↑ 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.
  13. ↑ 13.0 13.1 Raw ale. Lars Marius Garshol. Larsblog. 06/05/2016. Retrieved 03/02/2016.
  14. ↑ "Update: Lab Data on Pils Malt Boil Length Exbeeriment" on Brulosophy. Retrieved 03/08/2016.
  15. ↑ "All Grain Pale Ale 30-Minute Boil Experiments" by James Spencer on Beer & Wine Journal. 06/24/2015. Retrieved 03/08/2016.
  16. ↑ Discussion on MTF regarding DMS in raw ale/no boil/short boils. 03/01/2016.
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