Dimethyl Sulfide (Dms) is an organic sulfur-containing molecule with the formula (CH3)2S. It has a low boiling point (98.6°F or 37°C) and an odor that is generally described as “cooked sweet corn.” DMS contributes to the aroma of many foodstuffs, including cooked vegetables (beet, cabbage), tomato ketchup, milk, and seafood, as well as beer, especially lagers. Most people are able to detect it in very low concentrations, typically above 30 parts per billion. It plays a major environmental role in being the principal vehicle by which sulfur is cycled in nature, arising from the breakdown of chemical species within algae, including seaweed, and then evaporating to the atmosphere. There, it is oxidized to dimethyl sulfoxide (DMSO), a moisture-absorbing, high-boiling-point material that returns to earth in precipitation and is subsequently reduced back to DMS by microorganisms.
In beer, DMS can arise from two precursors. The principal one is S-methylmethionine (SMM; also known as DMS precursor, or DMSP), a molecule that develops in the embryo of barley during germination. It is heat-sensitive and is lost to a great extent during malt kilning. Therefore, there tends to be a high survival of SMM in more gently dried lager malts, this SMM being broken down to DMS during wort boiling and in the hot wort stand (the period post-boiling but pre-cooling). It is at this latter stage that the DMS is not purged and survives into the fermenter. Partly for this reason most brewers seek to minimize the hot wort stand and get the wort chilled and into the fermenter as quickly as possible. There is further loss of DMS with the carbon dioxide evolved during fermentation, but the compound is replenished by reduction by yeast of DMSO that is produced during curing of the malt. Wort spoilage bacteria such as Obesumbacterium proteus are especially capable of converting DMSO to DMS.
Most brewers regard DMS as an off-flavor, but in moderation it does make a significant contribution to the aroma of many lager beers and indeed has been identified as a key feature of German-style lagers.