Beer Resource: Off Flavors in Beer

Off Flavors in Beer
One of the biggest challenges to brewing is being able to identify off flavors and why these exist. We’ve compiled a list of the common off flavors in the list below. You should note that some of these off flavors are expected in some styles but are flaws in others. The BJCP offers subsidized off-flavor kits to its ranked members that can be found here: Guidelines for Doctoring Beers

Alcoholic
This flavor may be detected as a spicy, vinous character in the aroma and taste and is often accompanied by a warm or prickly mouthfeel. The simplest and most prevalent alcohol in beer is ethanol, which is produced by the fermentation of glucose and other reducing sugars. Higher, or fusel, alcohols are usually present at sub-threshold concentrations, but elevated levels are associated with underpitching, low levels of dissolved oxygen prior to pitching or low levels of free available nitrogen (FAN). These deficiencies force the yeast to metabolize fatty acids in the trub as a source of oxygen and carbon, producing a greater fraction of long chain alcohols. High gravity worts and high fermentation temperatures also tend to increase the concentration of these higher alcohols through increased yeast activity. Alcoholic characteristics are desired in strong ales and lagers as long as they are not coupled with the solvent notes associated with elevated ester or fusel alcohol levels.

Astringency
This flavor is a mouthpuckering sensation that is comparable to chewing on grape skins or grape seeds. It is often produced by the extraction of tannins from grain husks due to overcrushing oversparging, or sparging with sparging with water with a pH over 6.0 and a temperature higher than 170 °F. Astringency may also be produced by polyphenols that result from spoilage by acetobacter or wild yeast. Another possible source is oxidation, in which case the responsible compounds are polyphenols and aldehydes. Finally, spices such as coriander, orange peel and cinnamon also contribute astringent flavors, but these tend to mellow with age. Note that over-attenuation and low dextrin levels can increase the perception of astringency.

Diacetyl
This compound is responsible for an artificial butter, butterscotch or toffee-like aroma and taste. At low levels, it may also produce a slickness on the palate. A significant number of tasters cannot perceive diacetyl at any concentration, so every judge should be aware of his or her limitations. Diacetyl is a fermentation by-product which is normally absorbed by the yeast and reduced to more innocuous diols. High levels can result from prematurely separating the beer from the yeast or by exposure to oxygen during the fermentation. Low FAN levels or mutation may also inhibit the ability of yeast to reduce diacetyl. Note that high fermentation temperatures promote both the formation and elimination of diacetyl, but the latter is more effective. For that reason, lager breweries often employ a diacetyl rest, which involves holding the beer in the 50-55 F range for a few days after racking to the conditioning tank. Diacetyl is also produced by some strains of lactic acid bacteria, notably Pediococcus damnosus. Low levels of diacetyl are permissible in nearly all ales, particularly those brewed in the United Kingdom, and even some lagers, notably Czech pilseners.

Estery/Fruity
This is an aroma and taste that recalls bananas, strawberries, pears, apples, plums, papaya and/or other fruits. The responsible compounds are esters, which are formed from the combination of an alcohol and an organic acid. High ester levels are a product of the yeast strain, fermentation temperature, high gravity worts and the metabolism of fatty acids in the trub. These flavors are desirable in most ales, particularly Belgian and British styles, and the signature banana notes in Bavarian wheat beers are primary due to the ester isoamyl acetate. Note that esters often have solvent notes at very high concentrations.

Lightstruck/Skunky
This aroma and taste is due to the presence of the same mercaptans that are found in the scent glands of skunks. These compounds are formed when ultraviolet light cleaves an isohumulone molecule, and the resulting radical combines with a sulfur compound. Beer stored in clear or green glass bottles is more susceptible to this reaction, which is why brown glass offers more protection. Lightstruck flavors are not desirable in any style, but many European imports possess this quality. Note that Miller Brewing is able to use clear glass bottles because they use a chemically modified form of isohumulone that does not interact with light.

Phenolic (Medicinal)
This is an aroma and taste often compared to Band-aids ™, medicine chest or disinfectant. Chlorophenols are particularly offensive members of this family with bleach-like flavors in addition to the ones listed above. High levels of phenols are generally produced by bacteria or wild yeast, both of which indicate a sanitation problem. Phenols may also be extracted from grain husks by overcrushing, oversparging or sparging with hot or alkaline water. Chlorinated water or sanitizer residue are possible sources of chlorophenols.

Phenolic (Clove)
This is an aroma and taste that is clove-like. It is acceptable and expected in Bavarian wheat beers and some Belgian ales.

• It is yeast derived flavor and aroma
• Remedy is to use a different yeast, control fermentation temps, and/or improve sanitation to eliminate wild yeast contamination

Sherry-like
This is the aroma and taste of dry sherry and is often accompanied by hazelnut or almond notes. The responsible compounds are oxidized members of the melanoidin family. This flavor is one of the few positive flavors attributed to oxidation and adds complexity to barleywines and old ales. Sherry-like flavors are considered a defect in most other styles, particularly low-gravity ales.

Sour/Acidic (Acetic)
This is one of the five basic tastes, and is often perceived the most on the sides of the tongue, towards the rear of the mouth. The two most common acids responsible for this flavor are lactic and acetic, which both have related esters that may be perceived in the aroma. Lactic acid is produced by Gram positive bacteria such as Lactobacillus and Pediococcus, which are present in dust and saliva. Acetic acid may be produced by several contaminants, including Acetobacter, Zymomonas, and yeast in the Kloeckera and Brettanomyces families. High levels of sour and acidic flavors generally indicate a sanitation problem, but they are an important part of the profile of the lambic, oud bruin and Berliner weiss styles, and to a lesser extent, Belgian white beers.

Sour/Acidic (Lactic)
This is one of the five basic tastes, and is often perceived the most on the sides of the tongue, towards the rear of the mouth. The two most common acids responsible for this flavor are lactic and acetic, which both have related esters that may be perceived in the aroma. Lactic acid is produced by Gram positive bacteria such as Lactobacillus and Pediococcus, which are present in dust and saliva. Acetic acid may be produced by several contaminants, including Acetobacter, Zymomonas, and yeast in the Kloeckera and Brettanomyces families. High levels of sour and acidic flavors generally indicate a sanitation problem, but they are an important part of the profile of the lambic, oud bruin and Berliner weiss styles, and to a lesser extent, Belgian white beers.

Sweet
Sweetness is a basic taste perceived most strongly at the tip of the tongue and is due to the presence of reducing sugars. High levels of residual sugars can result from a flocculent or low-attenuating yeast or poor yeast health linked to low FAN levels or low levels of dissolved oxygen prior to pitching. High gravity worts, high dextrin content and the addition of lactose also play a role in determining the sweetness of the finished beer. The appropriate level is style-dependent, with high levels desirable in most strong ales and lagers, and low levels in American light lagers and lambics.

Acetaldehyde
This compound has the taste and aroma of fresh-cut green apples, and has also been compared to grass, green leaves and latex paint. It is normally reduced to ethanol by yeast during the secondary fermentation, but oxidation of the finished beer may reverse this process, converting ethanol to acetaldehyde. Elevated levels are generally present in green beer or if the beer is prematurely removed from the yeast. It can also be a product of bacterial spoilage by Zymomonas or Acetobacter. Background levels of acetaldehyde can be tasted in Budweiser due to the use of beechwood chips to drop the yeast before it can be reduced to ethanol.

Dimethyl-Sulfide (DMS)
DMS produces the aroma and taste of cooked vegetables, notably corn, celery, cabbage or parsnips. In extreme cases, it may even be reminiscent of shellfish or water in which shrimp has been boiled. DMS is normally produced by the heat-induced conversion of S-methyl-methionine (SMM), but most of this evaporates during a long, open, rolling boil. A short, weak or closed boil, or slow cooling of the wort may therefore lead to abnormally high levels. Some DMS is also scrubbed out during a vigorous fermentation, which is why lagers and cold-conditioned ales may have slightly higher levels than warm-fermented ales. Wild yeast or Zymomonas bacteria may produce high enough levels of DMS to make the beer undrinkable. Pilsner malt contains as much as 8 times the SMM of pale malt, so Pils-based beers sometimes have a DMS character; this is a much more common cause in most beer than a covered boil. Low levels of DMS are appropriate in most Pils-based lagers, particularly American light lagers and Classic American Pilsners, but are not desirable in most ale styles (Cream Ale is a notable exception).