Beer Odor Guide: DMS

One of the most frequently mentioned odors in beer evaluation is dimethyl sulfide (DMS). DMS is a sulfur compound that is generally thought to give beer creamed corn or cooked vegetable characteristics. Most beer brewers know very well how to reduce the risk of DMS entering the beer, such as checking the boiling time and ensuring that the lid is 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 a high concentration of odor in beer. It is introduced into beer by thermal decomposition (wort heating) of S-methyl methionine (SMM) produced in barley germ during the germination process. Because SMM is easily dissolved from malt at all mash temperatures, SMM will be converted to DMS during the boiling process.

Although DMS is the natural result of malting and mashing, DMS is generally unpopular. But in most beer styles, DMS is not a serious problem. But because the perception threshold of DMS is very low (humans can detect it at a very low level), a small amount in time can also resist light-tasting beer.

How To Remove DMS In Wort?

Most brewers do not want detectable levels of DMS in their beer (especially ale). DMS is produced whenever wort is heated, so there is some DMS in any beer. When the DMS content is higher than the flavor threshold (30-60 µg/L), the beer will show obvious cream and corn flavors. DMS is produced in the wort, but most DMS will evaporate during boiling, so if you encounter DMS problems, you need to focus on boiling.

• Boiled

Most of the DMS in beer produced by SMM is lost (evaporated) during boiling. Specifically, it is determined that the half-life of SMM at the boiling point is 37 minutes, which means that half of the SMM evaporates from the boiling pot at 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. When the temperature decreases, the half-life of SMM increases. This half-life of SMM is increased when the temperature is decreased. For example, if the wort is heated but not boiled (like a no-boil Berliner Weisse) to around 190°F, it would have to be held at this temperature for approximately 130 minutes to evaporate half of the SMM precursor. The physical location of the brewery can change the evaporation of SMM because the wort boils at lower temperatures or elevated altitudes, which will result in less conversion of SMM during boiling For DMS. Also, the vitality of the boiling will also affect the final DMS level in the beer.

• Whirlpool/Hop Stands

After boiling, the wort continues to maintain a high temperature during the boiling period (now commonly referred to as hop stands). Any dedicated SMM in the wort that has not evaporated during the boiling period will continue to form more DMS. A study extended the shelf time of hot wort by one hour and found that during the extended shelf time, the DMS concentration increased from an average of approximately 49 µg/L to 104 µg/L. This means that if a shorter boiling time is combined with longer hop stands, more DMS may be formed in the final beer. In the commercial field, if a long-term Whirlpool is carried out in a well-insulated container or a long-term transfer occurs at a high temperature, this may cause the SMM that survived the boiling to continue to decompose.

• Foaming When Boiling

A recent study on DMS looked at the role of beer foam in the boiling process and the effect of DMS evaporation in the wort. The author found that the evaporation of DMS was greatly enhanced in the presence of beer foam compared to the experiment using defoamers. Although the direct cause of this situation is not the focus of the study, they speculated an explanation: It may be that DMS may actually be concentrated in the foam and then be stripped from the wort by the boiling rising steam.

• The Role Of Carbon Dioxide

During the fermentation process, DMS levels will be removed by the escape of CO2. Recent studies have shown that DMS levels decrease significantly during the first five days of vigorous fermentation, with the most significant decrease during the first 1-2 days. But after the 5th day, the DMS level in beer increased, which was produced by yeast from the DMSO precursor.

• Open/Closed Fermentation

Studies have shown that the DMS level of beer fermented in an open fermentation system (a fermentation tank with a partially open top) and a closed conical fermentation tank is different. The DMS level of beer in the open fermentation system is like the expected DMS level of malt potential. But, beer fermented in a closed conical fermentation tank contains more DMS levels than expected by malt potential.

• Fermentation Temperature

The Journal of the Brewing Research Institute published in 1980 studied the influence of wort’s original gravity and fermentation temperature on DMS levels in beer. Fermentation temperature also affects the level of DMS in beer. As the fermentation temperature increases, the DMS level decreases. Fermentation at 46°F reaches the maximum DMS level, which is five times that of fermentation at 77°F. This is because yeast produces more DMS at lower temperatures.

• The Specific Gravity Of The Wort

As mentioned above, the original gravity of the wort will affect the DMS level in beer. High specific gravity wort produces more DMS than low specific gravity wort. Generally speaking, the DMS of 1.060 wort fermentation is more than three times that of 1.033 wort (adjusted with glucose or fructose to achieve a higher specific gravity). This is because glucose or fructose will increase the volatility of DMS.

• Ungerminated Grains

Since barley malt is the main source of SMM (DMS precursor), the less it is used in grains, the less likely it is that DMS will be produced in the finished beer. One way to reduce the amount of barley is to replace some grains with ungerminated grains. These grains may also contain DMS precursors but are likely to be completely boiled like corn. Interestingly, grains with a high gelatinization temperature need to be boiled before use, and may actually contain fewer DMS precursors. Because this process decomposes and evaporates DMS, the total amount of DMS precursors in boiling is less than that of whole malt brewing.

• Yeast Storage/Health

The storage of harvested yeast may also affect the DMS content in beer. According to research, stirring may make the yeast unhealthy, resulting in weakened fermentation vigor. The content of DMS in beer stored without stirring is lower than that in beer stored under stirring. This discovery is a good reminder to ensure the right amount of healthy yeast is used. Because healthy fermentation helps control DMS levels.

• Hops

DMS can also be introduced into beer through hops. At the end of the usual 60-minute boiling, all sulfur volatiles have evaporated. But, when adding hops in the later period or Whirlpool, a small amount of DMS will be added to the wort. These DMS will not be reduced by CO2 during the fermentation process, but some will survive and stay in the beer until packaging.