Based on both our research online and with brewers and our experiences at the tasting, here are 15 common off flavours in beer, how you might be able to identify them and what the causes for each of them might be:
Off Flavour: Diacetyl
Chemical Name: 2,3-butanedione
How to Identify: This one smells like butter, butter popcorn or butterscotch and can present with a slickness or creaminess on the tongue and in the mouth.
What it is: Referred to as brewing’s ‘original sin’, Diacetyl is present in most beers at some concentration and is a natural part of the brewing process. It can be a desirable flavour in small amounts in stouts and ales but is generally regarded as a flaw in most lagers.
How it is caused: Diacetyl is naturally produced by all yeast during fermentation and is then ‘reabsorbed’ by yeast cells. Any Diacetyl that is not reabsorbed may be a result of either high flocculating yeast, weak or mutated yeast, issues with oxygenating, low fermentation temperatures or short boils.
BJCP:
How it is caused:
| Low fermentation Temperature | Increase temperature in primary fermentation |
| Premature racking | Allow fermentation to complete |
| Mutant Yeast | Use pure yeast |
| Lactic acid bacteria | Practice good sanitation |
| Cooling wort takes too long | |
| Underpitching of yeast | Use yeast starter with enough cells |
| Too long acid rest in mash | decrease acid rest |
Off Flavour: DMS
Chemical Name: Dimethyl Sulphide
How to Identify: This one can smell like sweetcorn, cooked cabbage, tomato sauce and even shellfish or oysters.
What it is: DMS is a desirable flavour in some pale lager beers and ales and an off flavour in other beers.
How it is caused: Compounds are created during the malting process of grain which are later converted to DMS when heated. It is therefore primarily formed during wort production and – to a lesser extent – during fermentation. This makes DMS is naturally more prevalent in pale ales and lagers and can also be produced from bacteria that have managed to contaminate the beer. It can be driven off through evaporation when boiling wort.
BJCP:
How it is caused:
| Poor sanitation | |
| Covered Boil | Open, rolling boil for an hour or more |
| not boiling for at least an hour | |
| contaminated yeast | Use Pure Yeast |
| Over sparging with low temp water below 160 degrees | |
| Slow cooling of wort | |
| Under pitching | Use pure yeast Use starter |
Off Flavour: Metallic
Chemical Name: Ferrous Sulphate
How to Identify: This one can smell of iron (or other metals), pennies, ink or blood. While the smell can be quite clear, we felt that it was useful to taste this one as you can feel it in your mouth and on your teeth. We also felt that rubbing a little of the beer onto the back of our hands delivered a very strong metallic odour and made it a lot easier to identify.
What it is: Ferrous sulphate is a taint and occasionally an off flavour in beer. It primarily affects beer mouthfeel but sometimes beer odour can also be affected.
How it is caused: This taint occurs when beer or raw materials come into contact with poor quality metal pipework or machinery, particularly by wort being boiled in unprocessed metals (excluding stainless steel). It can also be imparted via packaging – such as metal cans, bottle caps or kegs. Improperly stored grains can also cause metallic off flavours.
Off Flavour: Sulfidic
Chemical Name: Hydrogen Sulfide (H2S)
How to Identify: This one can smell of boiled or rotten eggs, a burning match or raw sewage.
What it is: H2S is present in all beers and concentrations vary considerably from beer to beer. H2S is an off flavour in most beer styles but it is a signature flavour character in Burton ale.
How it is caused: Hydrogen sulfide is produced naturally by all yeast during fermentation. Lager yeasts tend to create greater sulfur aromas than ale strains. At low levels, it can impart a ‘fresh’ flavour to beers but at high concentrations it becomes an off flavour. CO2 will carry most of the hydrogen sulfide away and so conditioning or lagering after primary fermentation can ensure any left over sulfur smells or tastes fade over time.
Off Flavour: Estery
Chemical Name: Isoamyl acetate
How to Identify: This one can smell of banana or pear drops and to a lesser extent, strawberry, raspberry and grapefruit.
What it is: Isoamyl acetate is a common ester flavour present in all beers. Concentrations vary considerably from beer to beer. It is a key flavour character in some lagers and ales and a signature flavour character in German-style wheat beer (Hefeweizens) and many Belgian ales.
How it is caused: This is a naturally occurring by-product of fermentation. Strong fruity flavours or fruity flavours that are inappropriate for the style of beer are sometimes a result of under pitching or high fermentation temperatures. As a general rule, the higher the fermentation temperature, the more esters the yeast will produce. Low oxygen levels can also help increase the production of esters.
Off Flavour: Lightstruck
Chemical Name: 3-methyl-2-butene-1-thiol / Mercaptan
How to Identify: This one can smell like a skunk or freshly-brewed coffee. It can also smell musty and can be similar to burned rubber or cat musk. We had one American in our group – Lana – who had been sprayed by a skunk (seemingly numerous times), and she identified this one immediately.
What it is: Lightstruck character is an off flavour associated with exposure to light of beer packaged in clear or green glass. Light-coloured beers and beers with a lot of hops are more prone to becoming skunky. Beers that have been bittered exclusively with chemically-modified hop bitter acids do not develop this flavour (think Heineken in its green bottle). Dark beers and beers that utilise isomerised hop extracts are less susceptible to becoming lightstruck.
How it is caused: When hops are exposed to Ultra Violet rays from sunlight or florescent lights, the alpha acids break down and react with the hydrogen sulfide produced by the yeast. This reaction creates mercaptan. Mercaptan is the same chemical skunks secrete when they spray which is why the smell of light-struck beer is so similar to that of a skunk.
Off Flavour: Oxidised
Chemical Name: trans-2-nonenal
How to Identify: This one can smell like stale or old wet cardboard or have a papery taste or smell.
What it is: Nonenal is an off flavour in beer associated with ageing. Formation of this off flavour is more pronounced when precautions have not been taken in relation to minimising the process of oxidation. An excessive level of oxygen being introduced to the beer, especially while wort is still warm or after fermentation is complete, can create cardboard or sherry-like flavours.
How it is caused: Oxidation occurs when oxygen negatively reacts with the molecules in the wort or beer. It can be difficult to avoid because aeration of wort before pitching yeast is necessary. It is almost always a result of unnecessary splashing of fermented beer (i.e. transferring beer from one vessel to the next). Too much headspace in bottles can also lead to oxidation.
Off Flavour: TCA
Chemical Name: 2,4,6-trichloroanisole
How to Identify: This one smells like mould, corked wine or a damp cellar. We all felt it was really like walking down into Franklin and Kim’s (impressive) beer cellar or sniffing the cork from a wine bottle.
What it is: TCA is a chemical created by mould which consumes chlorinated phenols and metabolises them and it can be detected by humans at extremely low levels.
How it is caused: Generally TCA is caused when beer is fermenting somewhere that is damp or where the grain has developed mould in storage but TCA is capable of migrating through most semi-porous packaging and thus cross contamination can occur across several areas of beer production, from raw materials to final packaging.
Off Flavour: Phenolic
Chemical Name: 4-vinyl guaiacol
How to Identify: This one smells and tastes of cloves, but can also assume some of the character of cough syrup, smoke and some other spices and herbs.
What it is: This is a key flavour in some ales and stouts but is generally regarded as an off flavour in lager beers. It can be a signature flavour in some beers, including German-style wheat beers.
How it is caused: Phenolic flavourings are normally due to the presence of wild yeasts during fermentation or down to the use of specialty yeasts. Additional sources of phenolic flavour occur due to bacterial action from poor hygiene and from contamination of raw materials to phenolic compounds.
Off Flavour: Acetaldehyde
How to Identify: This one tastes and smells like green apples, rotten apples, emulsion paint or freshly cut pumpkin.
What it is: Acetaldehyde is a naturally occurring chemical produced by yeast during fermentation in all beers. It is usually converted into Ethanol alcohol, although where not enough yeast has been pitched or when dealing with higher alcohol beers, this process may take longer. It is a characteristic flavour of some beer styles (e.g. Bière de Garde) but is considered an off flavour at high concentrations in beer.
How it is caused: This off flavour is caused when the yeast does not have enough time to convert the Acetaldehyde into Ethanol due to either a lack of fermentation time, not enough yeast for the wort or low-quality yeast. It can also be as a result of packaging problems in bottles.
Off Flavour: Butyric Acid
How to Identify: This one smells putrid, rancid, like baby sick.
What it is: Butyric off flavour is a highly noticeable flavour that stinks of baby sick. Unsurprisingly, it is considered an off flavour in all beers.
How it is caused: Butyric Acid is caused mainly by bacteria during the wort production phase but it can also be produced by bacteria when beer becomes spoiled once it’s been packaged.
Off Flavour: Acetic
How to Identify: This one smells like vinegar, acidic or spoiled beer and can be tasted on the sides of the tongue towards the back of the mouth.
What it is: Acetic is a vinegar or acid flavour that is found in all beers at some levels of concentration. It is present in all lagers, ales, stouts and wheat beers as a normal component of a balanced flavour – think of lambic beer which has been purposefully exposed to specific types of wild yeast and bacteria. However, it becomes an off flavour in most beers when present at high concentrations.
How it is caused: Acetic is produced by yeast in fermentation and is a natural part of the brewing process. It can also be imparted by wild yeasts which produce far more acetic acid than other yeasts and will shift the flavour profile of the beer. Extremely sour or vinegary flavours are almost always the result of a bacterial or wild yeast infection. Acetic acid can also be produced by bacterial consuming sugars and indicates spoilage of the beer or more problematic issues.
Off Flavour: Cheesy
Chemical Name: Isovaleric
How to Identify: This one smells like stale cheese or sweaty socks.
What it is: Isovaleric flavour notes are characteristic of some beer styles, e.g. India Pale Ale. Typical ‘cheesy’ characters are often associated with beers of very high bitterness. In pale lager beers, isovaleric character is regarded as an off flavour.
How it is caused: Isovaleric off flavours are generally introduced by old hops. The compound arises as hops age and lose their bitter alpha acids, producing a flavour typified by flavours of cheese, sweat or must.
Off Flavour: Chlorophenol
Chemical Name: 2,6-dichlorophenol
How to Identify: This one smells like mouthwash, band-aid, antiseptic, disinfectant, and has a medicinal or hospital characteristic.
What it is: It is imparted to beer by the external contamination of either brewing raw materials or packaging materials with chlorophenols – for example, where chlorinated water has been used to brew or rinse equipment that has come into contact with beer.
How it is caused: It can be caused by poor rinsing processes within the brewery during the cleaning phase meaning that raw materials of beer become introduced to chlorophenols. It can also occur in beer delivery lines if they haven’t been rinsed correctly.
Off flavour: Astringency
How to Identify: This one is a dry, grainy, mouth-puckering, tannic sensation. Think of sucking on a wet tea bag or a grape skin.
What it is: Astringency gives a puckering sensation, almost powdery or metallic in the mouth. It can also be tart, vinegary, tannin-like and drying.
How it is caused: Polyphenols or tannins are the number one cause of such flavours. Tannins are found in the skins or husks of the grain as well as in the skin of fruit. Steeping grain for too long or grain that has been excessively milled or crushed can release tannins. Over-hopping can also lend a hand in creating astringent qualities.
(source Belgian Smaak)
How it is caused:
| Chill Haze: - Insufficent conversion time |
- Longer Mash - Use Protein rest - Use Clearing Agents - Use filtration |
| Permanent Haze - excessive or high temp sparge |
- Reduce Sparge temperature - Use Clearing Agents |
| Bacterial or wild Yeast contamination | Use pure yeast Use starter Practice good sanitation |
| Poor, Wrong, Weak or mutated yeast strains | Use pure yeast Use starter Use well-flocculating yeast strain |
| Wheat malt | Use clearing agents |