Common Off-Flavors
Acetaldehyde
A flavor of green apples, green leaves, latex paint or freshly cut
pumpkins; it is an intermediate compound in the formation of alcohol.
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. It can be a product of
bacterial spoilage by Zymomonas or Acetobacter. Some yeast strains
produce more than others, but generally its presence indicates that the
beer is too young and needs time to condition.
Alcoholic
A sharp flavor that can be mild and pleasant or hot and bothersome. The
simplest and most prevalent alcohol in beer is ethanol. Higher, or
fusel alcohols are detectable at a lower threshold. High fermentation
temperatures (>80F) increase the concentration of the higher weight
fusel alcohol. Under pitching, low level of oxygen prior to pitching
and when the yeast sits too long on the trub are other causes for fusel
alcohol.
Astringent
This flavor is differs from bitterness by having a mouth puckering
quality, like sucking on a tea bag or grape seeds. It is often produced
by the extraction of tannins from grain husks due to over crushing or
over sparging. Sparging with alkaline (>5.6pH) water and also by using
boiling water or water too hot for sparging are other causes for
astringency. Bacterial infections can also cause astringency, like
acetobacter. Finally, spices such as coriander, orange peel and
cinnamon also contribute to the astringent flavors, but they can mellow
with age.
Cidery
Cidery flavors can have causes but are primarily the result of adding
too much cane or corn sugar to a recipe. Cidery flavors are encouraged
by warmer than normal temperatures and can be decreased by lagering.
Diacetyl
Diacetyl is most described as a buttery or butterscotch flavor. The
smell of unpopped butter microwave popcorn is a good example. It maybe
desired in some ales to some degree, but not in others (mainly lagers).
Diacetyl is a fermentation by-product that is normally produced by the
yeast at the beginning of the fermentation and is absorbed by the yeast
at the end. High levels can be caused by a long lag time due to weak
yeast or lack of aeration, because there is more diacetyl than the
yeast can consume at the end of the fermentation. Also, removing the
beer from the yeast prematurely from the primary fermentation.
Dimethyl Sulfides (DMS)
DMS has an aroma and taste of cooked vegetables, notably corn, celery
or cabbage. DMS is common in many lagers and is considered to be part
of the character. DMS is produced in the wort during the boil, but most
is evaporates during an open rolling boil. A closed boil or slow
cooling of the wort may therefore produce high levels of DMS. So, do
not allow condensate to drip back into the wort from the lid. Some is
also scrubbed out during a vigorous fermentation, which is why there
are higher levels in lagers.
Estery/Fruity
This is an aroma and taste that recalls bananas, strawberries, pears,
apples, plumbs, and/or other fruit. Esters are formed from the
combination of an alcohol and an organic acid. Ester levels are a
product of the yeast strain, some produce more than others. In general
higher temperatures will produce higher levels of esters.
Grassy
This is a flavor of freshly cut grass or leaves. The primary cause of
this flavor is poorly stored malts or hops. Aldehydes can form in old
malts if not stored in a dry, cool place.
Medicinal
These flavors are described as mediciney, Band-Aid like. The cause are
various phenols which are initially produced by the yeast.
Chlorophenols result from the reaction of chlorine-based sanitizers
(bleach) with phenol compounds producing this off-flavor. The best way
to resolve this problem is to rinse with boiling water after sanitizing
with chlorine based sanitizers.
Metallic
Metallic flavors are usually caused by unprotected metals dissolving
into the wort. Iron and aluminum can cause metallic flavors leaching
into the wort during the boil.
Oxidized
This is perceived in the flavor and aroma and described as wet
cardboard. If the wort is exposed to oxygen at temperatures above 80F,
the beer will sooner or later develop oxidative characteristics.
Minimizing splashing of the hot wort or the fermented beer while
racking/bottling.
Skunky
Skunky aromas in beer are caused by photochemical reactions of the
isomerized hop compounds. The blue and ultraviolet wavelength causes
the skunky flavor. Brown glass effectively screens out these
wavelengths, but green glass will not.