Dimethyl sulfide (DMS) is an organic sulfur compound present above its flavor threshold in most beers. Because of its low flavor threshold, 10 - 150 ppb, it is a primary flavor and aroma compound that makes a significant contribution to beer character, especially in lager beers. It has a characteristic taste and aroma of cooked corn or creamed corn.
The level of S-Methyl methionine (SMM) in malt is responsible for the DMS level in wort. During mashing the SMM, DMS and very soluble dimethyl sulfoxide (DMSO) are brought into solution. SMM can be hydrolyzed to DMS during mashing however much of the DMS is driven off since it is very volatile. Wort will always have some concentration of SMM, DMS, and DMSO - different grains and mashing techniques can effect these concentrations. During fermentation little to no SMM is converted to DMS, however DMSO can be reduced to DMS by yeast during fermentation.
DMS in Beer
Some detectable level of DMS is characteristic of many lager styles, and is especially noticeable in light lagers. However, DMS is present in most beers at some level. It is excessive DMS that gives some home brewed ales a "cooked corn" character.
The amount of DMS found in beer is lowest in British ales, 10 - 20 ppb and highest in German lagers and all-malt beers, 50 -175 ppb, while the United States' lagers generally contain 40 - 100 ppb.
Beers with high adjunct ratios or low gravities allow the DMS taste or off-taste to be more detectable, while German beers, all-malt beers, flavorful beers, especially dark beers, make the taste of DMS less discernible even at higher levels.3
Causes of DMS
DMS is created whenever wort is heated, by the breakdown of precursors found in pale malts. Under ordinary circumstances, most of the DMS that is created by heat is then evaporated during the boil. Some DMS is also removed during vigorous ale fermentations, which is why higher levels are often found in lagers.
- Covered boil
- Covering the brew kettle during the boil prevents DMS from evaporating, and results in high levels of DMS in the finished beer.
- Slow cooling
- Because DMS is created at temperatures below boiling, cooling the wort too slowly means that excessive levels of DMS can be created which cannot be evaporated once the boil has stopped.
The DMS produced during the hot wort stand will stay in solution even if the hot wort tank is vented. For every extra hour of hot wort stand, a DMS increase of approximately 30% will result. The level of DMS in the wort determines the level of DMS in finished beer. In order to predict the level of DMS in finished beer Table V shows the relationship between SMM in malt and DMS in beer.
The major source of DMS in finished beer is derived from its precursor, S-Methylmethionine (SMM), an amino acid, which is formed during the germination and kilning process of malting barley. Barley does not contain DMS or SMM. However, both are formed by the biosynthesis occurring during germination. SMM, also known as DMS precursor (DMSP), is heat-labile and decomposes on heating to form DMS during kilning, wort boiling and hot wort storage.
The most effective way to reduce SMM during germination is by slightly under- modifying malt, specifically by reducing the moisture content of barley at steep-out to 40-42% and reducing the germination temperature to 55-60oF. It has been shown that a reduced airflow during germination resulted in a 50% lower SMM level in the finished malt.4
Alkaline steeping liquor and use of potassium bromate and other factors which reduce the metabolic growth rate during germination have been shown to significantly reduce SMM and insuring DMS levels in finished malt.
Two row barley which has a normally lower nitrogen content than six row barley, has been shown to produce significantly less SMM during the malting process. European malt has less SMM and DMS than North American and Canadian malt.1
Regardless of variety or growing conditions, the most important factor for reducing SMM and DMS occurs during kilning. The SMM formed during germination is converted by the heat of kilning and air flow to DMS. The DMS formed is either removed or volatilized in the kilning draft, oxidized to (Dimethyl sulfoxide) [DMSO], or remains in the finished malt. Since DMS is easily removed during the kettle boil, it is important that the ratio of SMM to DMS be as low as possible in the finished malt.
The conversion of SMM to DMS occurs at about 1401F. Therefore, by increasing the withering temperature, increasing the final kilning or curing temperature and extending the final curing time the level of SMM and DMS will significantly be reduced in the finished malt. The stability of SMM in malt is greater at higher moisture levels.
The level of SMM in malt is responsible for the DMS level in wort. During mashing the SMM, DMS and very soluble DMSO are brought into solution. No SMM is hydrolized to DMS at this time.
Kettle boiling hydrolizes SMM to DMS which is removed during evaporation. The half life or time needed to remove half of the DMS is 40 minutes so that three-fourths is removed in 90 minutes. Narssis recommends a 100 minute boil to reduce the level of SMM and DMS to acceptable levels in most beers.
The level of DMSO does not change during the kettle boil. A small amount of DMS, 0.4 ppb, may be contributed by hops, especially if added in large amounts late in the boil. As long as the wort is hot SMM will be converted to DMS. It is important to convert SMM to DMS in the kettle so that build up during the hot wort stand is minimized.
The following steps should insure low levels of DMS in the finished beer:
- Boil the entire wort 90 minutes or longer
- Ensure that the boil is vigorous - rolling
- Allow at least 8% evaporation
- Minimize the hot wort standing time
- Rapidly cool the wort
Fermentation and DMS
During fermentation, the evolution of CO2 removes and reduces the level of DMS. At moderate DMS levels of 30-60 ppb a 30-35% reduction can occur, while a 35-60% reduction can occur at higher initial DMS levels, 60-150 ppb.
The yeast's metabolism does not convert SMM to DMS but certain yeast can produce higher DMS levels by reducing DMSO to DMS, especially in lager beer production at cooler temperatures. Certain wild yeasts and Enterobacter agglomerans can produce DMS.5 Table VI shows the reactions that take place in malting and wort boiling.
DMS After fermentation
Purging and contamination occurs can can change the the DMS concentration in beer.Water dilution of high gravity beers may reduce the perceived threshold of DMS due to dilution of other mashing flavors. If DMS precursors,e.g. DMSO, reach the final product they are reduced to DMS,incresing the DMS concentration during the beer shelf life.
DMS is naturally present in relatively high levels in many beers. There is no easy way to add DMS character to a beer artificailly, but to increase levels during brewing, simply cover the wort for part of the boil, taking care to avoid boilovers.
Beer Styles and DMS
Types of beers in order of their perceived threshold of DMS.
Those with the lowest thresholds are most likely to have off tastes at excessive DMS levels.
- Low adjunct beers with low gravities or diluted flavor.
- High adjunct beers with corn grits
- High adjunct beers with other adjuncts
- Low to medium gravity (1.040-1.048) beers - all malt
- All malt German or higher gravity, light colored - flavored beers
- Amber - dark flavorful beers
- British light ales
- American or British amber or dark ales
- Stouts or strong flavored beers
PROPERTIES of DMS
Boiling Point: 99oF
Flavor Threshold (Beer):
Perceived Threshold: 10-150 ppb Depends on amount of flavor in beer
Flavor: Cooked corn, creamed corn
Aroma: Same - highly volatile
Source: Precursor in malt (SMM), hops (minor)