Off-flavours in beer

Here are some informations to help you identifying unwanted flavours in beers and the causes for those off-flavours.

what symptoms caused by
autolysis a meaty flavour with sometimes a low carbonation the yeast died during fermentation
bacteria contamination notes of sulfur, cauliflower, boiled vegetables (DMS) or sewer; most of the time along with acetic acid presence of bacterias in the wort
cidery thin-bodied with a taste of fermented apple; typical of some homebrews often the result of adding too much cane or corn sugar in the wort or/and at bottling
diacetyl butter or butterscotch flavours, sometimes rancid; ok at low level in some ales fermentation problem (weak yeast or insufficient aeration)
DMS (dimethyl sulfide) boiled vegetables or corn problems during boiling or bacteria contamination
fusels harsh alcohol taste on the tongue often due to a too high fermentation temperature or excessive amounts of yeast
green dusty nose, unclean hop profile, notes of green fruits (acetaldehyde), astringency and sometimes diacetyl (butterscotch) the beer is too young and needs more time to condition or could be caused by the presence of an unwanted bacteria
gushing explosive release of carbonation at the opening of the bottle; splash and overflow 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
metallic metallic flavours caused by unprotected metals dissolving into the wort or pasteurization problems
oxidation / maderisation wet cardboard or sherry-like flavours 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
phenols medicinal flavours of band-aids, cloves and plastic; acceptable in low levels caused by wild yeasts
skunky aroma of gaz (thiol) remembering the one of the smelly compound found in the musk gland of a skunk 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
soapy soapy flavours can result from the breakdown of fatty acids after fermentation
solvent esters smelling like solvant (acetone) often result from a combination of high fermentation temperatures and oxidation; can also come from cheap plastic brewing equipment
sour/acidic lactic, vinegar-like acids; can be OK for some beer styles like lambics, sour ales or barrel aged caused by spoilage bacteria
sulphur flavours of rotten eggs or sulphur; can be ok in low levels in pale ales, IPAs or bitters due to the usage of hard, sulphuric water boiling problems