Fault
Dirty dispense lines
Beer spoiled between keg and glass by microbial growth in the lines — a venue problem rather than a brewery one.
Also called line taint, dirty pipes.
What it tastes and smells like
The diagnostic is comparative: if one beer in a venue tastes off and others do not, the line for that beer is the likely cause. If everything tastes off, suspect the cellar.
Industry practice is to clean lines every seven days. Beer left standing in a line over a quiet period spoils quickly.
Styles this tends to affect
Not where it belongs — where it is most likely to appear when something has gone wrong.
- Best bitter — A step up from ordinary bitter in strength and depth, with more malt body while staying firmly sessionable.
- Dark mild — A low-strength dark British ale, gently sweet and nutty, built for drinking in quantity after physical work.
If you are drinking it
If one beer in a pub tastes sour, buttery or dull while others are fine, the line is the likely cause. It is worth mentioning — staff generally want to know, and it is harmless.
Not a safety issue. Beer’s alcohol, hop compounds and low pH make it inhospitable to organisms that cause illness.
What causes it
| Cause | Stage | How often | Who can act |
|---|---|---|---|
| Lines not cleaned to a regular schedule, allowing biofilm to establish. | dispense | most common | venue |
| Slow-moving lines where beer stands for days between pours. | dispense | common | venue |
| Perished seals, couplers and tap parts that cleaning solution cannot reach. | dispense | common | venue |
If you are brewing it
- Venue: Clean all lines at least every seven days, and more often on slow-moving products. (prevents)
- Venue: Replace seals, washers and couplers on a schedule rather than waiting for a taste problem. (prevents)
Where in the process this comes from
Steps that can produce this fault when they go wrong. Each links to what the step is for and how it is controlled.
- Cellar conditioning — Receive, stillage, vent, fine and temperature-manage cask beer in the pub so it reaches the drinker in condition.
Often confused with
- Diacetyl — butter, butterscotch
- Unintended acidity — sharp sourness, tart
- Infection — unexpected sourness, barnyard
Evidence
What BeerHQ knows about this, and how. Every statement carries the state of the evidence behind it rather than being presented flatly as fact.
Cleaning a beer line on schedule does not clear it. Resilient biofilms persist through routine cleaning and re-establish the spoilage community quickly, which is why line contamination recurs rather than being solved once.
Checked against the source
Source: Bose et al., "Microbial Communities in Retail Draft Beers and the Biofilms They Produce" (2021), discussion and Importance (Microbiol Spectr 9(3):e01404-21)
The organisms living in retail dispense lines are far more varied than the handful of named beer spoilers: one survey identified over a hundred contaminant bacteria and close to twenty wild yeasts across retail draft systems.
Checked against the source
Source: Bose et al., "Microbial Communities in Retail Draft Beers and the Biofilms They Produce" (2021), abstract and results (Microbiol Spectr 9(3):e01404-21)
Line contamination belongs to the venue rather than to the brewery: the same organisms turn up in beers brewed at different breweries and served through the same system, which places them in the dispense equipment rather than in the kegs.
Checked against the source
Source: Bose et al., "Microbial Communities in Retail Draft Beers and the Biofilms They Produce" (2021), discussion (Microbiol Spectr 9(3):e01404-21)
What line contamination does to the beer is chiefly acid and texture: the organisms found produce acetic and lactic acid, and some cause a slimy, thickened mouthfeel.
Checked against the source
Source: Bose et al., "Microbial Communities in Retail Draft Beers and the Biofilms They Produce" (2021), discussion (Microbiol Spectr 9(3):e01404-21)