DMS in beer, or Dimethyl Sulfide, is a volatile sulfur based compound which if noticed, will be perceived as cooked vegetables, most often corn, celery, cabbage or parsnips. According to the BJCP study guide, the flavor can even resemble shellfish, or the water that shellfish has been cooked in (hmmmm).
Dimethyl Sulfide is caused by thermal breakdown of a sulfur-containing amino acid in the malt called s-methylmethionine (SMM). This amino acid is present in all malts. Thus, all beers have some amount of DMS.
SMM in malt is the precursor for Dimethyl Sulfide. SMM is found in lightly kilned malts such as pilsner and pale ale. The SMM level in these malts is directly proportional to the Dimethyl Sulfide level in your wort.
The flavor threshold is 10-150 ppb. It is not until the levels becomes excessive that you will notice the typical canned or cream corn flavors and aromas in your beer. Below this level, DMS in beer is part of beer's flavor and aroma profile we are all used to.
In fact, DMS is part of the flavor profile in most lagers, particularly American light lagers and pre-prohibition pilsners, but DMS is not desirable in any ale style.
British ales are usually lowest in Dimethyl Sulfide at 10-20 ppb and German lagers and all-malt beers are highest at 50-175 ppb. The typical U.S. lager contains 40-100 ppb, falling pretty much in the middle of the range. Lighter beers with high adjunct ratios, or low-gravity beers, will have higher levels of DMS. Whereas in dark German beers, all-malt beers and any other flavorful beer, these cooked corn notes tend to be hidden.
Dimethyl Sulfide is created from the breakdown of SMM in heated wort
(above 140° F or 60° C). When the boil kettle is
uncovered, most of the DMS produced will be
Some of the remaining DMS in beer will be removed during a vigorous
fermentation, which is why warm fermented ales usually have lower concentrations of
these off flavors than lagers and cold-conditioned ales.
Some causes of excessive cooked corn notes (DMS) in your beers are covering the boil kettle (which will not allow the chemicals to escape) and cooling the wort too slowly (Dimethyl Sulfide is being produced as long as the wort is above 140 degrees F/60 degrees C).
It may not be obvious to beginning homebrewers, but you don't want to put a lid on the boil kettle. The main reason, as mentioned above, is that you will prevent the DMS in your wort from boiling off. The other reason, one which you will learn soon enough, is that when you cover your boiling wort, you are going to get a boil-over. Everyone has had one or more and they make a real mess of your equipment.
Getting your boiled wort cooled to below 140°F (60°C) as quickly as possible will reduce the amount of DMS that makes it into the fermenter. If you are brewing in the summer, you may need to pre-chill your cooling water prior to chilling the wort.
When using all pilsner malt or pale malt, it may be advisable to boil your wort for at least 90 minutes to reduce the Dimethyl Sulfide levels. Also, ensure that you have a vigorous boil with at least 8% evaporation.
These are some of the things you can do to help reduce DMS in your homebrew. A good vigorous fermentation will also help eliminate DMS in beer, so good yeast health is a must.
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