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Why it’s important to know off flavours in beer

Why it’s important to know off flavours in beer, and how to identify them

As well as being incredibly delicious, beer is a fragile, highly perishable product. It’s why you’ll often pick up a can that says “drink fresh, store cold” on the label—instructions you absolutely should follow if you want it to stay at its best. 

It’s because of beer’s inherent fragility that brewers and bartenders go to great lengths to ensure the beer you spend your hard-earned cash on is served as fresh and tasty as possible, because without proper care it can easily be spoiled, and ruin that experience. The bad news is that there is a lot that can go wrong in terms of said spoilage. From stressed yeast and fermentation faults, to bacterial contamination, taint from dirty lines, or a glass that hasn’t been cleaned properly, we refer to a negative impact on a beer’s taste as “off flavours”. 

The good news, however, is that the vast majority of beer you drink will be fault-free, and with a little practice it’s easy enough to teach yourself how to identify them. The simplest way to do this is to pick up an off flavour kit from a manufacturer such as Aroxa or Lallemand, which gives you the ability to dose beer with off flavours so that you can learn how they impact a beer’s taste. It can be tempting to use these on a very neutral beer so that they can be identified more easily, but it will be far more rewarding to use them in a beer style you’re more familiar with—if you want to know how diacetyl affects hazy pales for example, that’s what you should be dosing.

Understanding how off flavours impact a beer is only part of the equation however. It can be immensely challenging, or even uncomfortable returning a beer in which you think there is something wrong. But it shouldn’t be, and it’s a case of a little good etiquette going a long way. Learning off flavours and then lording your newfound knowledge over the person who sold you your faulty beer will not do you any favours. Remember when returning a beer to be polite, which can be as simple as asking for a different beer because “something doesn’t taste right with this one.” If you’re unsure and in a bar with a great beer selection, don’t be shy about asking for a taster of the beer you’re about to buy either, a good bartender will always let you taste it first!

Here’s a few of the most common off flavours in beer, what causes them, how they present themselves, and how to identify them correctly.

Diacetyl (2,3-butanedione)

Perhaps the easiest way to describe the dreaded diacetyl, one of beers most common off flavours, is that it tastes like buttered popcorn. It doesn’t just affect a beer’s taste either, with high amounts leaving an almost butter-like, slick mouthfeel. What’s perhaps most interesting about diacetyl is that it is produced naturally by yeast during fermentation, but the yeast will clean it up given time (during a stage of the brewing process known as the “diacetyl rest”).

That it’s a byproduct of fermentation can indicate if a beer has been packaged and released too soon. It can also appear in beers that undergo secondary fermentation, such as cask ale, if not correctly managed, or it could be a sign of bacterial contamination in the beer. It’s also important to know, however, that trace amounts of diacetyl (its name often shortened to “diac” by brewers) are normal in several beer styles, most notably Czech pilsners, but also in some cask beers, stouts and porters. It’s often undetectable though, thanks to fine-tuned balance from proper hop additions. 

Lightstrike (3-methyl-2-butene-1-thiol)

Also known as “skunking” (in that it smells and tastes similar to the spray produced by a skunk, and another naturally occurring product that shares the use of the word) lightstrike is caused by UV light breaking down isomerized hop compounds in beer. It can be easily avoided by keeping beer out of direct sunlight, or using brown glass, which filters out the UV light and prevents the chemical reaction from taking place. Lightstrike is common in commercially produced beer that use green or clear glass bottles, to the point where many consumers believe that is actually how these beers are meant to taste.

Isoamyl Acetate

Isoamyl Acetate is an ester; a natural byproduct produced by yeast during fermentation. This particular ester commonly manifests itself as flavours of banana (specifically those foam banana sweets you used to enjoy when you were a kid) or pear drops/boiled sweets. It is a desirable flavour in certain styles such as hefeweizen, or witbier, but if you discover it in a pale ale or lager, it means the yeast was stressed during fermentation, and produced esters due to being worked too hard, or for too many generations. Fresh, healthy yeast used in the proper quantities is the best way to avoid Isoamyl Acetate in beer. 

Ferrous Sulphate

When beer smells or tastes metallic—which could range from the aroma of old copper coins, to the taste of blood when you lick a wound—it means it is tainted with ferrous sulphate. It’s caused by brewing with water containing high levels of metallic ions, or by direct contact with metallic equipment, especially if that equipment has not been adequately cleaned. It can occur at any stage of the brewing process, right the way up to being served at the bar. Thankfully it’s perhaps one of the easiest off flavours to identify, meaning your exposure to it should be minimal. (Although if you taste it regularly at the same venue, it’s a sign they are not adequately cleaning their lines, and that you should perhaps take your custom elsewhere).

DMS (Dimethyl Sulphide)

Dimethyl Sulphide (usually referred to as DMS) is produced by a naturally occurring compound that is present in lighter malts such as lager, or pilsner malt, as well as corn if used as an adjunct. It presents itself as a stale, vegetal flavour that can range from boiled cabbage to tinned sweetcorn, or tomato soup. 

DMS is typically formed during the brewing process, either by over sparging, or by not crash-cooling finished wort quickly enough post boil (as DMS is formed at warm, but not boiling temperatures.) Many modern breweries (especially those producing lagers, or other pilsner malt adjacent beers) will have an oversized kettle, allowing for a vigorous boil which will drive off the volatiles that cause DMS to form. 

It can also be caused by a yeast or bacterial contamination within finished beer. And as with diacetyl, DMS is a naturally occurring flavour in many commercially produced beers, in particular certain adjunct-heavy mass produced lagers from the United States. 

Hydrogen Sulphide

Although now somewhat antiquated, “Burton Snatch” was a term used to refer to the smell of hard boiled eggs present in beers produced in Burton-upon-Trent. Considered to be the traditional home of brewing in the UK, water in Burton is very soft—ideal for brewing—but also contains high levels of calcium sulphate, more commonly known as gypsum. When beer made with high levels of sulphate ions interacts with yeast during fermentation, it will produce this “eggy” smell (which, in extreme amounts, will smell like rotten eggs). It’s pretty common these days for brewers to use calcium sulphate to “Burtonise” their water (as it aids hop compound absorption) but in minimal amounts, so hydrogen sulphide won’t present itself in the finished beer. A few classics made in Burton, such as Bass and Marston’s Pedigree, still present a faint egg aroma, but allegedly in much smaller quantities than in days gone by.


Presenting itself as an unripe, “green” apple flavour, in larger amounts acetaldehyde can be a particularly nasty off flavour, as it is the chemical that also causes hangovers (as it’s what our livers convert alcohol into, before returning it to our bloodstream.) It’s formed by yeast before it’s converted into ethanol, and at very high levels it can have an almost paint/solvent taste and aroma. If proper brewing procedures are followed, such as pitching enough healthy yeast, and waiting a couple of days before racking off spent yeast after fermentation, then the formation of noticeable amounts of acetaldehyde can be avoided. It can also form in packaged beers that contain high levels of dissolved oxygen, as trace amounts of yeast present in the beer will begin converting ethanol back into acetaldehyde given the correct conditions.


Contact with oxygen is largely considered to be bad for beer, and while modern packaging systems will keep its presence in packaged beer to a minimum, it will still exist in trace amounts, and gradually break down volatile compounds (such as hop oils) in a finished beer. Oxidised beers will taste musty and stale, initially like wet paper or cardboard, but at very high levels it can taste acrid, like cigarette ash. Oxidation will also affect the colour of a beer, with pale beers turning to a darker, muddier copper shade. 

Oxidation can have some desirable effects too, however, especially in stronger, less hop-forward beers such as imperial stouts or barleywines, which is why these beers can be good for ageing. Beers that undergo ageing follow what’s known as an “oxidation curve” initially picking up the musty flavours mentioned earlier, but eventually developing into sweet, sherry or port like notes that can be desirable. It can also be beneficial in beers dosed with Brettanomyces, such as the Belgian classic, Orval, for similar reasons.

Proper ageing, however, requires a temperature controlled environment such as a cellar, as seasonal temperature fluctuations will likely ruin the majority of aged beers, unless they are properly stored. 

Mouse/THP (Tetrahydropyridine) 

More commonly found in natural wine and low intervention cider, THP—or “mouse” as it has been affectionately nicknamed—is becoming more common in beer. This is due to a rise in styles such as lower alcohol barrel-aged beers such as saison and farmhouse ale, along with kettle sours. It is formed by yeast such as Brettanomyces during slow fermentations, and given enough time it should eventually be cleaned up (albeit very slowly) as a natural part of maturation on live yeast. The name “mouse” comes from the fact it shares an aroma with that of mouse droppings.

Presenting itself as a stale cereal or corn chip flavour, it can be undetectable for a few sips either due to the beer being too cold, or our mouths being at the wrong pH. Once our palates adjust the off flavour will be detected at the very back of the throat. Luckily for some, 3 in 10 people cannot taste THP as the ability to perceive it is genetic (much in the same way that coriander leaf tastes like soap for some people). That doesn’t excuse the beers that do possess this, quite frankly, ghastly flavour. 

A few others to watch out for…

Although the off flavours listed above are among the most common, there are other, less common variations out there. Some of which are truly putrid. Isovaleric Acid will make a beer taste like blue cheese, and is often caused by brewers using hops that have gone stale, or oxidised. Two of the absolute worst off flavours are Butyric Acid and Mercaptan, which will present themselves as baby vomit and faeces, respectively.

Thankfully they are very rare, and if proper brewing, packaging and dispense methods are followed, so too should the ones listed above be. Perhaps the most important way to prevent off flavours is through proper cleaning of equipment, whether that being a regular CIP (clean-in-place) of brewing equipment, or regular line cleans in pubs and bars. 

If you do come across a faulty beer, try to find out the batch number (either from the can, bottle, or on the keg/cask) and politely let the brewery know you think something isn’t right. But remember to be polite, as most breweries are genuinely trying to produce something delicious for you to enjoy time and again… plus no one likes a know-it-all.