How to identify: Diacetyl can be recognized as an artificial butter, butterscotch or toffee-like aroma and/or taste. It may also produce a slickness on the palate, and may taste rancid at higher levels.

Details: Diacetyl is a ketone that is normally produced near the beginning of the fermentation cycle (it comes from Lactoacetic Acid which comes from Acetaldehyde), but is typically reduced enzymatically by the yeast near the end of fermentation. A beer that takes a long time to start active fermentation may produce excessive Diacetyl - more than can be reabsorbed later. Exposure to oxygen during fermentation may also cause higher levels of Diacetyl - so be sure to aerate your wort - but ONLY before you pitch the yeast. Also, the yeast may not be able to reduce the Diacetyl into Butanediol (which is flavor-neutral in beer) if the FAN (Free Amino Nitrogen) is too low (for example, by using too much unmalted grain or sugar, particularly in low-gravity wort). Highly flocculent yeasts will also leave more Diacetyl in the beer since the yeast will precipitate out of the beer before it has had a chance to reduce most of the Diacetyl. Colder fermentation temperatures (for example, lagers) cause the Diacetyl to be reduced less - so often a Diacetyl Rest (bringing the beer to 60-68°F for a few days) is used during secondary fermentation to allow the yeast to eliminate more of the Diacetyl. Krausening also helps reduce Diacetyl because it introduces fresh, active yeast after most of the secondary fermentation has been completed. Diacetyl can also be produced by Pediococcus and some strains of Lactobacillus bacteria due to contamination, but these bacteria also produce lactic acid, so the beer will also have an acidic character.

Beer styles: Diacetyl is acceptable (at low levels) in most ales, especially Scottish Ales (BJCP styles 9A, 9B, and 9C in the 2004 guidelines), but is not expected in lagers - other than Bohemian Pilsner (2B). Note that carmelization may be mistaken for Diacetyl in Strong Scotch Ales (9E).

Note: The sensitivity to Diacetyl varies from taster to taster - some tasters are unable to perceive it all.

How to identify: DMS (Di Methyl Sulfide) can be recognized as a cooked vegetable (e.g., corn or cabbage) aroma and/or taste.

Details: DMS is produced by reduction of SMM (S-Methyl Methionine) from heat. SMM is produced during malting, but is eliminated during malting in roasted or toasted malts - so it is generally found in light base malts (e.g., lager). DMS evaporates during the boil - which is why you should have at least an hour of good rolling boil, and also why you don’t want to cover the wort during the boil. Slow cooling of the wort will also produce DMS while the wort is still hot, but the DMS will not be eliminated by evaporation. DMS can also be removed by the carbon dioxide during a vigorous fermentation, so warmer fermented ales will eliminate more DMS than lagers. DMS can also be produced by infection from wild yeast or Zymomonas bacteria, and will have more of a rancid character - like cooked cabbage.

Beer styles: DMS is acceptable (in low levels) in lighter lagers, but should not be found in ales.

How to identify: A sour or acidic character is detected on the back sides of the tongue, and can be recognized as a vinegary aroma and/or taste.

Details: These flavors typically come from esters related to lactic or acetic acid. Lactic acid is produced by bacteria (present in dust and saliva) like Lactobacillus and Pediococcus (see also Diacetyl). Acetic acid is produced by bacteria Acetobacter and Zymomonas (see also DMS), and also by Brettanomyces yeast. In most cases, these flavors are due to contamination, sanitation, or lack of sterilization of the wort.

Beer styles: These flavors are not appropriate for most styles of beer. However, Sour Ales (BJCP style 17 in the 2004 guidelines, including Lambics, Flanders ales, and Berliner Weisse) have these flavors intentionally. These flavors are produced by Brettanomyces yeast, as well as other natural occurring yeasts and bacteria. Belgian Specialty Ales (16E) may have sour characteristics, and Belgian Wit (16A) and Saison (16C) may have a light sour/tart character.