15 Common Off Flavours in Beer (and How To Identify Them)

15 Common Off Flavours in Beer (and How To Identify Them)

Your beer smells of what?

When there is something wrong with your beer – when it has an off flavour – you want to know what’s wrong with it. Nobody wants to drink beer that’s not quite right.

This is even more important in the brewing world, where brewers should be able to identify off flavours so that they know how to remedy any issues they are having with fermentation or within the brewery.

There are many off flavours in beer but only a handful of very common ones. The majority of these off flavours in beer are naturally occurring (often as normal reactions during fermentation). Some are even necessary in certain beers and a few are actually characteristic of specialist styles. They become off flavours only when present in high concentrations and when inappropriate for the style.

Some of these undesirable flavours can also be taints and there is a distinction between an off flavour (which is imparted through internal deteriorative change) and a taint (imparted through external sources), but for the purposes of this tasting, we’ve decided not to differentiate. The usefulness of their distinction is more a matter for determining the cause of the off flavour or taint so that it can be quickly remedied.


In order to experience these first hand and to discuss how easily (or not) such off flavours in beer can be identified, we ordered a flavour standards tasting kit from UK firm Aroxa as recommended by the Cicerone programme in the U.S.

The beer we chose for the tasting was Jupiler, an adjunct lager with a relatively neutral aroma and flavour. A running joke throughout the evening was how some of the ‘off flavours’ actually improved the taste of this beer and there were genuine calls for a ‘control’ sample so we could actually decide which we preferred.

The requisite amount of each of the chemicals was added to 500ml of beer, swirled and then topped-up to 1 litre which was then divided among our six participants. Elisa not only prepared some homemade chocolate brownies for the occasion, but she acted as administrator and judge for the evening.

We received each sample blind, although we did have the Aroxa information cards for reference. This facilitated discussion and forced us to explore what we were smelling and tasting. The degree of perception of a flavour varies enormously among individuals, sometimes extending over several orders of magnitude and it was interesting to see some people in the group who had extreme sensitivities to certain off flavours in beer pick those up immediately.

After trying each sample, we delivered our verdict to Elisa, who revealed the answer, and we were able to talk some more about whether we had encountered these off flavours in beer before and how they might be caused and indeed prevented.


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.


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.


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: Key flavours in some ales and stouts but generally regarded as off flavours in beers that are bottom fermented. 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 off flavours in beer. 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.


There are lots of useful resources online which you can use to help you decipher or identify off flavours in beer. We have listed some below which we found helpful in our preparation for this tasting as well as helpful resources for people of varying levels of beer knowledge:

→ ‘Serious Eats‘ offers an easy to understand explanation of several of the common off flavours in beer: http://drinks.seriouseats.com/2013/03/how-to-indentify-off-flavours-in-beer-skunked-beer-diacetyl-dirty-draft-lines.html

Jay R. Brooks skillfully describes what he considers to be the 10 most common off flavours in beer: http://www.mercurynews.com/ci_21785199/brooks-beer-10-most-common-beer-defects

→ ‘More Beer!’ set out a useful summary of off flavours in beer and their causes: https://morebeer.com/themes/morewinepro/mmpdfs/mb/off_flavour.pdf

→ The Beer Judging Certification Programme (BJCP) have put together flash cards on off flavours to help you identify and study their characteristics: http://www.bjcp.org/docs/OffFlavourFlash.pdf

→ John Palmer has a section on off flavours in beer in his online book, ‘How to Brew’: http://www.howtobrew.com/section4/chapter21-2.html

→ For those who can’t afford to buy an Aroxa kit or who would rather try a home-made version first, this post from ‘Barlow Brewing’ offers some great tips: http://barlowbrewing.com/2010/10/14/how-to-do-your-own-off-flavor-beer-tasting/

→ ‘My Life On Craft’ also offers great advice on putting together a home-made off flavours in beer kit with some useful links and resources included: http://mylifeoncraft.com/?p=985

Good luck with your tastings and let us know how they go!

Beer Writer of the Year 2015 (British Guild of Beer Writers). Best Beer Blogger 2017 (North American Guild of Beer Writers). Certified Cicerone®. Accredited beer sommelier (UK Institute of Brewing & Distilling). Certified 'Zytholoog' (CVO Panta Rhei, Ghent). Professional Brewer (Siphon Brewing, Damme, Belgium).