DIMETHYL SULFIDE (DMS) DMS ( Dimethyl Sulfide) 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 or creamed corn.
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.
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.
The most common causes of excessive DMS in finished beer are:
- Covered boil
- Covering the brew kettle during the boil prevents DMS from evaporating, and results in high levels of DMS in the finished beer. Leave the boil uncovered to reduce the levels of DMS.
- 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. Cool the wort faster to reduce the levels of DMS.
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.2
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
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.