Off-flavours in beer

what	described as	origin	comment
acetaldehyde	green apple, cidery, latex paint	yeast activity, microbial contamination	increased by underpitching; fermentation too cool or incomplete; insufficient conditioning; bacterial infection (if accompanied by sulfur or acetic acid) more present in young beer; might increase with oxidation
acetic sourness	tart, vinegary	microbial contamination	increased by inoculation with bacteria or wild yeast; exposing green or packaged beer to air high levels are caused by oxidation of ethanol by bacteria
autolyzed	meaty, soy sauce, burned rubber, dirty diaper	yeast, aging	occurs when yeast cells die
butyric acid	rancid butter, baby vomit, cheese	microbes in the cellar or storage vessels	never an asset
cidery	green apple, apple	yeast, adjunct sugars	beer made with a high proportion (20% or more) of sugar often has a cidery flavor and aroma. Can also be due to weak or incomplete fermentation (see "acetaldehyde")
chlorophenol	band-aid, plastic, medicinal	process/equipment faults; contamination
diacetyl (Vicinal Diketones (VDK))	butter, popcorn, butterscotch, honey, milky	yeast; microbial contamination	natural byproducts of fermentation which should be reabsorbed by the yeast in final phases. Low levels ate acceptable in Bohemian Pilsner, English Pale and Brown Ales, some Porters and Stouts.
DMS (dimethyl sulfide)	cooked vegetable, celery, cabbage, corn, sulfury	malt, microbial contamination	darker malts have less DMS than paler malts. Since DMS is produced at temperatures below boiling, slow cooling of the wort means that DMS is formed which isn't boiled away. Wild yeast may produce high DMS levels but also produce other off-flavors (acetic acid, phenols, sulfur).
fusel alcohols	burning, alcoholic, harsh	yeast	avoid with proper fermentation temperature and/or proper yeast health
gushing	explosive release of carbonation at the opening of the bottle; splash and overflow	sugar; bacteria	too much sugar used for the second fermentation in bottle or development of a mushroom from the malt; can also be caused by a high temperature change
lactic sourness	citric, lactic, tart, yogurt	microbial contamination	caused by infection of bacteria - most commonly Lactobacillus. Medium to high levels are expected in Berliner Weisse and in sour ales.
lightstruck/skunky	aroma of gaz (thiol), catty, fecal, mercaptan, skunky, sulfury	mishandling	decomposition of the isohumulones (compounds of hops) by the light of sun or neons; this is the reason why bottles should be brown and not clear or green to better protect the beer against this action
metallic	blood-like, coppery, iron	contamination	caused by unprotected metals dissolving into the wort or pasteurization problems; to avoid: properly treat water to remove excessive metallic ions; don't use equipment likely to corrode
mineral	chalky, dusty, sulfate, eggs	water	properly treat brewing water; don't use excessive amounts of brewing salts
oxidation / maderisation	almond, leathery, papery, cardboard, musty	aging, process faults	due to wort exposed to oxygen at temperatures above 26 C; can also appear when the beer gets too old or the lagering temperature is too high