"I found the following post while investigating ester production. Although it's primarily geared towards home brewing, it contains a lot of very good information. And since so many off-flavors are related to fermentation, I thought this was an appropriate place to post.

Due to the length of the original post, it will appear as two separate posts with a few sections removed. Please refer to the supplied URL to view the original text in its entirety.

--JB




Introduction

This section is intended to give an overview of the more important flavors and flaws that may be
encountered while judging. Some of these flavors may be appropriate in some styles, but not in others,
and the desirability will depend on the concentration. For this reason, not all of these characteristics are
considered to be off-flavors. There are several references that offer a more detailed description of
potential flavor and appearance flaws in beer. Most homebrewing handbooks discuss them in
appendices, and although it is somewhat outdated, the 1987 Zymurgy Special Issue on Troubleshooting
is worth reading. The more technically inclined reader should consult George Fix's Principles of
Brewing Science and George and Laurie Fixs' Analysis of Brewing Techniques. Finally, Brewing
Techniques is running a Focus on Flavors column through 1998 that describes the flavors that appear
on the Beer Flavor Wheel.

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.

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 alkaline or boiling water. 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.

Bitterness

Bitterness, or rather excessive bitterness, is perceived as a harsh dry taste mostly on the back of the
tongue. It is usually due to over-hopping, especially when high alpha hops are used. Roasted malts and
high concentrations of magnesium and sulfate ions also contribute to the overall bitterness. Bitter
compounds may also be produced by oxidation or contamination by wild yeast, in which case there are
usually other off-flavors. High levels of hop bitterness are appropriate in IPAs and barleywines, while
bitterness due to roasted barley/malt is appropriate in robust porters and dry stouts.

Body

The body of a beer is characterized as the fullness of the flavor and mouthfeel, and descriptors range
from watery or characterless to satiating or thick. Body is technically separate from mouthfeel, which
encompasses physical sensations such as astringency, alcoholic warmth and carbonation, but the
combination determines how the beer stimulates the palate. The body is determined by the levels of
dextrins and medium-length proteins. Lack of dextrins is caused by low saccharification temperatures,
excessive use of adjuncts or by highly attenuative yeast strains. A low protein level may be caused by
excessively long protein rests, excessive fining or the addition of large amounts of fermentable sugars.
Light body is appropriate in American light lagers and lambics, but not in malt-accented styles such as
barleywines and doppelbocks.

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 Scotland, and even some lagers, including Czech pilsners and Vienna-style beers.

DMS

DMS, or dimethyl-sulfide 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,
but most of this evaporates during an open, rolling boil. A 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. Low levels of DMS are appropriate in most lagers, particularly American
light lagers and pre-prohibition pilsners, but are not desirable in any ale style.

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.

Grassy

This is the flavor and aroma of freshly cut grass or green leaves. Responsible compounds include the
aldehydes hexanal and heptanal, which are produced by the oxidation of alcohols in the finished beer or
the deterioration of improperly stored malt or hops. Some English and American hop varieties produce
grassy notes if used in large quantities, but this flavor should not be a significant part of the profile."