Learn how to recognise common off-aromas and what they reveal about brewing, storage and service.
When tasting beer, we usually focus on the aromas and flavours that make it enjoyable. Fresh bread from malt, citrus and tropical fruit from hops or spicy notes from yeast can all contribute to a beer’s character.
But sometimes the aromas in your glass are less appealing. Buttered popcorn, cooked sweetcorn or even something reminiscent of a skunk may greet your nose instead.
These are known as off-flavours. They can arise during brewing, ageing, storage or service, and recognising them can reveal a lot about how a beer was made and handled.
Where beer aromas come from
Before exploring common off-flavours, it helps to understand where beer aromas normally originate.
The Beer Lexicon within the WSET Systematic Approach to Tasting Beer®, used when evaluating beer, organises aromas into several broad categories based on their source.
Malt and grain aromas come from the grains themselves and from compounds formed during malt production. These can include comforting notes such as bread crumb, bread crust, caramel, chocolate or coffee.
Hop aromas come from essential oils within hops. These oils contain many aromatic compounds capable of producing a wide range of aromas reminiscent of citrus fruit, tropical fruit, flowers, herbs and spices.
Yeast and bacteria aromas develop during fermentation and maturation. Different yeast strains can produce fruity, spicy or neutral profiles, while bacteria can sometimes introduce distinctive aromas and acidity.
Other aromas may come from barrel ageing or flavour additions, including fruit, coffee, chocolate or spices.
However, faults during brewing, storage or service can create aromas that are considered undesirable. These are the off-flavours that trained tasters learn to recognise.
DMS: the cooked sweetcorn aroma
One of the most well-known beer off-flavours is DMS (dimethyl sulfide).
DMS is a malt-derived compound that naturally exists in wort (the sugary liquid extracted from grains during mashing, before fermentation) during brewing. During the boil it should evaporate and be removed from the beer.
If the boil is insufficient or the process is poorly managed, DMS may remain in the finished beer. When this happens it can produce aromas often described as cooked sweetcorn, cooked cabbage or tomato juice.
Low levels of DMS can be acceptable in some pale lager styles, but when the aroma becomes prominent it usually indicates a problem during brewing.
Acetaldehyde: green apple notes
Another compound that can appear in beer is acetaldehyde.
Yeast produce acetaldehyde during fermentation. As fermentation finishes and the beer matures, yeast normally reabsorb it.
If fermentation or maturation is interrupted or incomplete, acetaldehyde may remain in the beer. When present at noticeable levels it can produce aromas reminiscent of green apple, raw pumpkin or emulsion paint.
These flavours often make the beer seem unfinished or immature.
Diacetyl: butter and popcorn
Diacetyl is another by-product of fermentation that yeast usually reabsorb during maturation.
When maturation is incomplete, diacetyl may remain in the beer and produce a distinctive buttery or buttered popcorn aroma.
In most beer styles this is considered undesirable. However, low levels can sometimes be acceptable if they integrate well with other characteristics of the beer. Certain traditional styles, such as some Czech pilsners, English bitters, English-style porters and Irish red ales, may tolerate a small amount of diacetyl.
Understanding the context of a beer style is therefore important when judging whether this character is a fault.
Phenolic off-flavours
Phenolic compounds can produce aromas such as clove, peppercorn, medicinal or smoky notes.
In many cases these aromas arise from contamination by unwanted yeast strains and are therefore considered off-flavours.
However, phenolic notes are not always undesirable. Some beer styles intentionally use yeast strains that produce spicy phenolic characteristics. In these styles the aromas contribute to the beer’s identity rather than indicating a fault.
This highlights an important point in beer evaluation: the same aroma can be desirable in one style but considered a flaw in another.
Staling: cardboard and faded hops
Even a well-made beer can develop off-flavours over time.
Staling occurs due to oxidation reactions as beer ages. Oxygen slowly alters the beer’s aroma and flavour profile.
Stale beer often develops aromas described as paper or wet cardboard. At the same time, hop aromas tend to fade and bitterness may seem softer or duller.
This is why many beers, particularly hop-forward styles, are best consumed fresh.
Light strike: the skunk character
One of the most distinctive beer faults is light strike, sometimes called skunking.
When beer is exposed to light, chemical reactions involving hop compounds can produce sulphur compounds with very strong aromas.
These compounds can smell like skunks, cat urine or scorched hair.
Light strike is the reason many breweries package beer in dark brown bottles, which offer greater protection from light than clear or green bottles.
Bacterial infection
Off-flavours can also result from bacterial contamination.
If brewing equipment or dispensing systems are not properly cleaned, bacteria may grow and affect the beer.
Bacterial infection can produce a combination of unpleasant flavours, including rancid buttery notes or sour vinegar-like aromas.
In bars and pubs, poorly maintained beer lines are a common source of these problems.
Turning faults into learning
While off-flavours might sound unappealing, they are extremely useful for learning about beer.
Recognising them can help brewers diagnose issues during production. For hospitality professionals, identifying them can reveal problems with storage or service.
For enthusiasts and students, they offer insight into how ingredients, fermentation and handling shape beer’s flavour.
The more aromas you learn to recognise, the more confidently you can evaluate what is happening in the glass.
And sometimes, what first appears to be a fault turns out to be a deliberate feature of a particular beer style.
Beer is a complex drink shaped by ingredients and process. Understanding off-flavours simply adds another layer to appreciating it.
If exploring beer flavours and aromas has sparked your curiosity, learning to taste beer systematically can take your understanding even further.
WSET’s beer qualifications introduce the ingredients, brewing processes and styles that shape beer’s character. Along the way, you’ll build the skills to identify aromas and flavours more confidently and understand why they appear in the glass.
Explore WSET’s beer qualifications to start developing your tasting skills and deepen your understanding of one of the world’s most diverse drinks.
