Draft beer can be one of the freshest and most flavourful choices in a bar. There is one basic condition, however: the keg must be fresh and cold, and the entire system from keg to tap must be properly maintained.
When that does not happen, beer can reach the glass smelling of butter, vinegar, a drain or “rotten eggs”. It may foam excessively or taste completely different from the same label in a can or bottle.
The easy conclusion is “dirty lines”. Sometimes that is right — but not always.
The Brewers Association explains that yeast and bacteria can grow in draught systems, while organic and inorganic deposits build up in the lines. It therefore recommends a cleaning cycle of roughly every 7 to 14 days. Bacterial growth can increase significantly when maintenance is neglected.
So how can a customer tell that something is wrong?
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1. It smells of butter or popcorn
A lager or an ordinary pale ale should not suddenly bring melted butter or buttered popcorn to mind.
The compound most closely associated with this smell is diacetyl. It can bring a buttery or butterscotch aroma and sometimes a slightly oily mouthfeel.
The Cicerone Certification Program notes that inadequately cleaned lines are among the most common sources of draft-beer flavour deterioration, and that bacteria in the lines often cause buttery or sour flavours — or a combination of the two.
There is an important catch, however: diacetyl may already have been produced at the brewery. In some traditional British styles, a small amount is even considered an acceptable characteristic.
So “it smells of butter” is a sign, not proof of dirty lines.
2. A beer that is not normally sour tastes like vinegar
A sour or vinegary note is even more suspicious in a beer that we know does not normally have that character.
In its Draught Beer Quality Manual, the Brewers Association links the growth of aerobic bacteria in unclean lines to sour and vinegary off-flavours. Its material also shows that microbial growth increases as the interval without cleaning gets longer.
Of course, this does not apply to sour ales, lambics and other styles in which acidity is part of the beer.
But if a pilsner you drink often suddenly tastes of vinegar, something is worth checking.
3. It smells of “methane”, a drain or rotten eggs
This is an odour many consumers describe as “methane”.
Chemically, though, that is not correct. Methane is odourless. The unmistakably foul notes we associate with drains or rotten eggs usually come from sulphur compounds.
Hydrogen sulphide (H₂S) can resemble rotten eggs or a drain. In small quantities, it can occur naturally in some lagers because it is produced during fermentation.
There is also methanethiol (or methyl mercaptan). The Brewers Association describes its smell as a dirty drain, rubbish or boiled cabbage. It can form during fermentation, through the breakdown of yeast cells and, in some cases, after prolonged dry hopping.
So a smell that resembles a cesspit does not automatically mean dirty lines.
But if you have drunk the same beer elsewhere or from a can and it has none of that smell, while you encounter it only at one bar’s tap, then the dispensing system fairly belongs on the list of suspects.
4. The keg may simply have been on tap for too long
Here is a factor that is often overlooked.
A draft beer that does not “move” quickly enough can remain connected for weeks.
The Brewers Association does not give one universal expiry date for every beer. The freshness window varies by brewery and style.
It does, however, offer a particularly useful reference point: the flavour of beer from a keg that has been on tap for more than 45 days may have changed from the flavour the brewer intended.
That does not mean the keg “spoils” on day 46.
It does mean that, in a bar with 20 taps where two or three labels sell very little, the age of the open keg is an entirely reasonable question.
Especially aromatic and heavily hopped beers can lose a substantial part of their original character as time passes.
An “open” keg is not like an open bottle
A clarification is needed here.
When a modern keg is connected to a proper draft system, it does not mean that air is continuously entering it, as happens when we open a bottle.
The keg is kept under pressure with CO₂ or an appropriate gas blend. The Brewers Association explicitly states that compressed air should not be used to dispense draft beer.
So time alone does not tell the whole story. Storage, temperature, the gas system and, naturally, the style of beer all matter too.
5. The beer consistently pours with too much foam
“There is too much foam, so the lines must be dirty.”
Not necessarily.
Excessive foam can result from poor system balance, incorrect pressure, faulty equipment or — very often — a high temperature.
The Brewers Association recommends keeping draft beer at around 2–3°C (36–38°F), both in the cooler and at the point of dispense. Even a small rise in temperature can create serving problems and excessive foam.
Dirty lines can make matters worse, but foam by itself does not prove poor cleanliness.
6. The same beer tastes very different only at this bar
This may be the most useful practical test for a customer.
Suppose you often drink a particular lager. You know what it tastes like from bottles, cans and other venues.
At one particular bar, though, the draft version:
- smells of butter,
- is oddly sour,
- resembles a drain,
- has a dull or “tired” aroma.
That does not prove exactly what is wrong. It does suggest that the problem may lie after the brewery: in the keg, its storage or that particular venue’s system.
If you have doubts, comparing it with a fresh packaged version of the same beer is far more useful than trying to diagnose a chemical compound yourself.
7. Look at the tap — and the glass
There are visible signs of poor practice, too.
The Brewers Association says taps should be clean, with no accumulated beer residue or mould. Taps should also be dismantled and cleaned along with the lines.
There is one more very simple sign.
The tap nozzle should not touch the glass and, above all, should not be immersed in the beer or foam.
The Brewers Association warns that a nozzle immersed in beer can transfer microorganisms and become a site where they grow.
Look at the glass, too.
If you see many bubbles sticking to the inside walls below the beer’s surface, or the head disappears unusually quickly, the cause may simply be a glass that is not truly beer-clean. Grease, detergent or other residues destroy foam. In a properly cleaned glass, the head leaves characteristic rings — known as lacing — as you drink.
That is a problem with the glass, not necessarily the lines.
How often should draft lines be cleaned?
The Brewers Association recommends a cleaning cycle every 7–14 days, and its wider guidance uses two weeks as a basic reference point for proper maintenance.
It is not enough to run a little water through the lines.
The system needs a suitable cleaning solution, the right concentration and contact time, while parts such as taps must be dismantled and cleaned.
Biofilms can form inside lines: layers of microorganisms that adhere to surfaces, together with yeast, proteins and inorganic residues. The Brewers Association notes that such deposits begin to form in even a brand-new system within a short time.
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The best question to ask the bartender
If your beer genuinely seems strange, rather than declaring that “the lines are dirty”, there are two much more useful questions:
“When was the keg changed?”
and
“When were the lines last cleaned?”
A well-run venue should keep track of both.
The second question also has an objective benchmark: if the answer is “a month ago”, that is clearly outside the Brewers Association’s 7–14-day recommendation.
Likewise, if the keg has been on tap for more than 45 days, there is a reasonable chance that its flavour has drifted from the profile the brewer intended.
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When should you send a draft beer back?
If the beer has a flavour you know should not be there — pronounced butter, vinegar, an unpleasant drain smell or an obviously stale character — it is entirely reasonable to mention it to staff.
You do not need to diagnose the cause.
Simply say that the beer does not seem to taste as it should and ask for it to be checked.
A good draft beer can indeed be exceptionally fresh. The Brewers Association considers that, when beer is handled properly from brewery to glass, draft can offer one of the best expressions of its flavour.
But when the keg is old, cooling is poor or the system is neglected, the advantage of draft beer disappears.
In that case, a properly stored can may be the much better choice.
There is no single number that applies to every beer. The Brewers Association notes, however, that for kegs on tap for more than 45 days, the flavour may have changed from what the brewer intended.
Not in the way an opened bottle does. In a proper draft system, the keg is kept under pressure with CO₂ or an appropriate gas blend; it is not filled with ordinary air each time a beer is poured.
Methane is odourless. The smell described this way may come from sulphur compounds such as hydrogen sulphide or methanethiol. Hydrogen sulphide resembles rotten eggs, while methanethiol can resemble a drain or rubbish.
Not necessarily. Sulphur compounds can form during beer production itself. But if the unpleasant smell appears only in the draft version at a particular venue, the dispensing system should also be examined.
The most characteristic off-flavours cited by Cicerone are buttery and sour flavours.
The Brewers Association recommends a cleaning cycle of approximately every 7 to 14 days.
No. It may result from temperature, pressure or poor system balance. It is a sign that something is not working properly, not proof of dirty lines.
Bubbles sticking to the inside walls below the beer’s surface, poor head retention and the absence of even lacing can indicate grease or other residues on the glass.
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