| 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 |