One of the most frequently mentioned aromas in beer evaluation is dimethyl sulfide (DMS). DMS is a sulfur compound that is generally believed to impart beer with creamed corn or cooked vegetable characteristics. Most beer brewers are well aware of how to reduce the risk of DMS entering the beer, such as controlling the boiling time and ensuring that the lid is kept away from the kettle. But what exactly is DMS and where does it come from?
What Is DMS?
Dimethyl sulfide (DMS) is a sulfur-containing compound that is generally considered to be highly odorous in beer. It is introduced into beer through the thermal decomposition (wort heating) of S-methyl methionine (SMM) produced in barley germ during the germination process. Since SMM readily dissolves from malt at all mash temperatures, it is converted to DMS during the boiling process.
Although DMS is a natural byproduct of malting and mashing, it is generally undesired. However, in most beer styles, DMS is not a significant problem. Due to its low perception threshold (detectable at very low levels by humans), even a small amount can be perceived in light-tasting beers.
How To Remove DMS In Wort?
Most brewers aim to have undetectable levels of DMS in their beer, especially ales. DMS is produced whenever wort is heated, so there will always be some DMS present in any beer. When the DMS content exceeds the flavor threshold (30-60 µg/L), the beer will exhibit noticeable creamed corn flavors. While DMS is produced during wort creation, most of it evaporates during the boiling process. Therefore, if you encounter DMS issues, you need to focus on the boiling stage.
During boiling, most of the DMS in beer produced by SMM is lost through evaporation. Specifically, it has been determined that the half-life of SMM at the boiling point is 37 minutes. This means that half of the SMM evaporates from the boiling pot within 37 minutes. A 90-minute boil will remove approximately 75% of the DMS content, and most of the SMM will be lost during a 120-minute boil. As the temperature decreases, the half-life of SMM increases. For example, if the wort is heated but not boiled (like in a no-boil Berliner Weisse) to around 190°F, it would need to be maintained at this temperature for approximately 130 minutes to evaporate half of the SMM precursor. The physical location of the brewery can also affect SMM evaporation because wort boils at lower temperatures or higher altitudes, resulting in less SMM conversion to DMS. Furthermore, the vigor of boiling can also impact the final DMS level in the beer.
After boiling, the wort maintains a high temperature during the so-called hop stands. Any residual SMM in the wort that did not evaporate during boiling will continue to convert to DMS. A study extended the shelf time of hot wort by one hour and found that the DMS concentration increased from an average of approximately 49 µg/L to 104 µg/L during this extended period. This suggests that combining shorter boiling times with longer hop stands may result in more DMS formation in the final beer. In commercial settings, if a long-duration whirlpool or a prolonged transfer occurs at high temperatures in well-insulated containers, this can lead to the continued decomposition of surviving SMM.
Foaming When Boiling
A recent study on DMS explored the role of beer foam during boiling and its impact on DMS evaporation in the wort. The author found that the presence of beer foam greatly enhances DMS evaporation compared to experiments using defoamers. Although the direct cause of this phenomenon was not the focus of the study, it was speculated that DMS may become concentrated in the foam and subsequently stripped from the wort by the rising steam during boiling.
The Role Of Carbon Dioxide
DMS levels are reduced during the fermentation process through the escape of CO2. Recent studies have shown a significant decrease in DMS levels during the first five days of vigorous fermentation, with the most substantial reduction occurring within the first 1-2 days. However, after the 5th day, the DMS level in beer increases, as it is produced by yeast from the DMSO precursor.
Studies have indicated that the DMS level in beer fermented using an open fermentation system (a fermenter with a partially open top) and a closed conical fermentation tank differs. Beer fermented in an open system exhibits DMS levels similar to those expected based on malt potential. On the other hand, beer fermented in a closed conical tank contains higher DMS levels than expected based on malt potential.
An article published in 1980 by the Brewing Research Institute studied the influence of wort’s original gravity and fermentation temperature on DMS levels in beer. Fermentation temperature also affects the DMS level. As the fermentation temperature increases, the DMS level decreases. Fermenting at 46°F reaches the maximum DMS level, which is five times that of fermentation at 77°F. This is because lower temperatures promote more DMS production by yeast.
The Specific Gravity Of The Wort
As mentioned earlier, the original gravity of the wort affects the DMS level in beer. Higher specific gravity wort produces more DMS than lower specific gravity wort. Generally, a wort with an original gravity of 1.060 has over three times the amount of DMS compared to a 1.033 wort (adjusted with glucose or fructose to achieve a higher specific gravity). This is because glucose or fructose increases the volatility of DMS.
Since barley malt is the primary source of SMM (DMS precursor), reducing its usage in the grain bill decreases the likelihood of DMS formation in the final beer. One method to reduce the amount of barley used is to replace some grains with ungerminated grains. These grains may also contain DMS precursors but are less likely to be fully boiled like corn. Interestingly, grains with high gelatinization temperatures need to be boiled before use and may actually contain fewer DMS precursors. This is because the boiling process decomposes and evaporates DMS, resulting in a lower total amount of DMS precursors compared to brewing with whole malt.
The storage of harvested yeast can also affect the DMS content in beer. Research suggests that agitation or stirring can result in unhealthy yeast and weaker fermentation activity. Beer stored without agitation has lower DMS levels compared to beer stored with agitation. This finding serves as a reminder to use the appropriate amount of healthy yeast, as healthy fermentation helps control DMS levels.
DMS can also be introduced into beer through hops. At the end of a typical 60-minute boil, all sulfur volatiles have evaporated. However, when hops are added late in the process or during whirlpooling, a small amount of DMS is added to the wort. These DMS compounds are not reduced by CO2 during fermentation and can survive in the beer until packaging.