Contaminated homebrew is almost never dangerous to drink. The alcohol environment of fermented beer makes it inhospitable to harmful pathogens like E. coli, salmonella, or botulism — none of which can establish in beer. What contamination typically produces is sour, vinegary, or off-flavour beer. Bad-tasting, not bad-for-you. The two exceptions are covered below.
"My homebrew tastes funny, is it contaminated?" is the second-most-common worry behind "is it stuck?". This article walks through how to tell — visually, by smell, and by taste — whether something has gone wrong, what specifically caused it, and what to do about it. It also addresses the genuine safety question (which is rarely the issue) and the equipment question (which is more important than most brewers realise).
What "contamination" actually means in brewing
Brewing yeast is one specific organism (Saccharomyces cerevisiae for ales, S. pastorianus for lagers). The world is full of other microorganisms — wild yeasts like Brettanomyces, lactic acid bacteria like Lactobacillus and Pediococcus, and acetic acid bacteria like Acetobacter. They're on your skin, on every surface in your kitchen, in the air, on the outside of the fresh wort pouch.
Most of these organisms are harmless to humans. Many of them are even used deliberately in brewing — sour beers, lambics, and some Belgian styles depend on them. The issue isn't that they're dangerous; the issue is that they're not what you intended to brew. If Lactobacillus establishes in your Hazy Pale Ale, you're going to get a sour Hazy Pale Ale, which isn't really a Hazy Pale Ale anymore.
Brewing yeast normally outcompetes wild organisms when you pitch enough fresh, viable cells onto clean wort. Contamination happens when wild organisms got a head start — equipment that wasn't properly sanitised, wort exposed to air for too long, or hands that touched the yeast packet.
Visual signs in the fermenter
If you can see your wort through a translucent fermenter, several visual signs point to specific contamination types.
Krausen (the foam layer during active fermentation) is white, soapy-looking, and forms across the surface in the first 1-3 days. It's also normal for it to leave a brown ring on the inside of the fermenter as it falls. This is healthy fermentation, not contamination.
No action required
After days 7-10, if a thin grey or white film forms on the wort surface that doesn't look like krausen (more oily, less foamy), this is usually a wild yeast or bacterial film. Distinct from the healthy krausen, which falls back into the beer rather than persisting as a separate layer.
Likely contamination
A "pellicle" is a wrinkled, sometimes bubbly, leathery-looking layer that forms on the wort surface. It can be cream-coloured or grey. This is the unmistakable signature of wild yeast contamination, usually Brettanomyces. The pellicle is the yeast forming a protective biofilm.
Confirmed contamination
Distinct from the healthy uniform layer of krausen, coloured fuzzy patches on the wort or fermenter walls are mould. Different colours indicate different mould species — all are bad news.
Mould — dispose of the batch
Wort that was clearing nicely as fermentation finished but then suddenly becomes hazy again — without you doing anything — can indicate secondary bacterial activity. Less definitive than the other signs, but worth investigating with smell and taste.
Investigate further
What contamination smells like
Smell is often the most reliable early indicator, because contamination products are volatile and you can detect them well before they ruin the flavour.
Active fermentation smells like sweet wort with a yeasty edge. As fermentation finishes, the sweetness diminishes and a clean malt and hop character emerges. This is what your beer should smell like at every stage.
No issue
A distinct sourness or vinegar bite in the smell, especially late in fermentation, indicates bacterial contamination. Lactobacillus produces a lactic, yoghurt-like sourness; Acetobacter produces a sharp vinegar note. The smell tells you which one before the taste does.
Bacterial contamination
A distinctive "barnyard" or "horse blanket" smell — described almost universally that way by experienced brewers — is the signature of Brettanomyces. Funky, leathery, sometimes slightly tropical fruit alongside it. Not unpleasant in some sour and wild styles, decidedly unwelcome in a clean ale.
Wild yeast contamination
Truly bad smells — rotten food, putrid, sewage-like — are rare but indicate a serious problem. This isn't normal bacterial contamination; this is something that shouldn't be in beer at all.
Do not drink, dispose
A sulphurous smell during active fermentation is usually not contamination — it's a normal yeast by-product, especially during lager fermentation or with stressed yeast. The smell usually dissipates as fermentation completes. Persistent sulphur in finished beer is a separate issue worth investigating; transient sulphur during days 1-5 is fine.
Usually normal
What contamination tastes like
Once you can taste it, contamination has been there for a while. Visual and smell signs usually appear first. But if you've taken a hydrometer sample and it tastes wrong, here's the diagnostic.
The unmistakable signature of Lactobacillus. Tartness across the whole sip, sometimes resembling a Berliner Weisse or sour beer. Mild at first, more pronounced as fermentation continues. Permanent — sour doesn't fade with time.
Lactobacillus contamination
A distinctive vinegar character (acetic acid) means Acetobacter got in, which only happens when there's been ongoing oxygen exposure plus initial bacterial contamination. Worse than sour because the acetic character is harsher and harder to enjoy as an accidental sour.
Acetobacter contamination
An exaggerated diacetyl character (buttered popcorn, slick mouthfeel) that persists despite proper conditioning. Normal yeast cleans up diacetyl at the end of fermentation; Pediococcus contamination produces continuing diacetyl that healthy yeast can't keep up with.
Pediococcus contamination
This is oxidation rather than microbial contamination, but worth noting because it presents similarly — "my beer tastes wrong." Develops after fermentation, gets worse over time. We cover it in detail in our off-flavours guide.
Oxidation (not infection)
Is it safe to drink?
Direct answer: almost always yes, with two exceptions.
Beer is a hostile environment for human pathogens. The combination of alcohol (even modest amounts), low pH, hop antibacterial compounds, and the absence of accessible sugars makes it impossible for organisms like E. coli, Salmonella, Listeria, or Clostridium botulinum to establish or produce toxins. There is no documented case of foodborne illness from drinking contaminated homebrew that was otherwise visually intact.
What contaminated homebrew typically delivers is unpleasant flavours, not danger. Sour, vinegary, funky, off-tasting — but safe to swallow. You can taste a small sample to confirm what you have without worry.
The two exceptions:
Visible mould growth. Distinct from healthy krausen — mould grows in patches with colour (green, black, blue, fuzzy). Mould species can produce mycotoxins. If you see mould, dispose of the batch.
Genuinely foul smells. If the beer smells of rot, garbage, or decomposition — not just sour or funky, but actually bad — don't drink it. Trust your nose.
Can the batch be saved?
Almost always no. Bacterial and wild yeast contamination is permanent — once Lactobacillus or Brettanomyces is in the beer, the character stays. There's no boiling or filtering process you can do at home that removes it. What you have is what you have.
The realistic options:
Finish the fermentation and bottle it anyway
If the contamination produces a tolerable flavour (mild sour, some funk), let fermentation complete, bottle as normal, and treat it as an "accidental sour" or "accidental Brett beer." Plenty of homebrewers have rescued their relationship with a contaminated batch this way. The beer isn't what you brewed, but it can still be enjoyable.
Bottle and forget about it for 6 months
Some wild yeast and bacterial contaminations mellow with extended ageing. The beer doesn't become a clean Pale Ale, but it may evolve into a more balanced sour or funky character that's more drinkable than the early-stage version.
Dump it and brew again
If the contamination is severe (strong vinegar bite, pellicle thick enough to disturb the beer), the honest path is to compost the batch and start over with stricter sanitation. Time spent trying to save an actively bad batch is usually time better spent on a fresh batch.
Most contaminated batches taste wrong, not dangerous. Drink it if you can stand it. Dump it if you can't. Either way, the equipment matters more than the batch.
Equipment recovery for next batch
This is the part most homebrewers underestimate. If you had contamination, your equipment likely has wild organisms living on it. Pitching fresh yeast into the same fermenter without proper cleaning means the same contamination shows up in your next batch.
Step 1: Strip everything down
Empty the contaminated batch, then disassemble the fermenter completely. Remove the lid, tap, airlock, grommet. Anything that can come apart, comes apart.
Step 2: Soak in hot soapy water
Fill the fermenter with hot water (as hot as the plastic can safely tolerate — usually 50-60°C) and a generous amount of unscented dish soap or oxygen-based brewing cleaner. Soak everything for 30-60 minutes. Scrub thoroughly with a clean cloth or non-abrasive sponge. Pay particular attention to the underside of the lid, the inside of the tap, and the area where the airlock grommet sits.
Step 3: Rinse extremely well
Rinse multiple times with clean hot water until you can't see or smell any soap residue.
Step 4: Sanitise generously
Mix a fresh batch of no-rinse sanitiser to the recommended strength. Fill the fermenter with it. Submerge every part you've cleaned. Leave for the full recommended contact time (usually 30 seconds minimum). Drain.
Step 5: Consider replacing soft components
The grommet, the airlock, and any soft plastic parts can hold microorganisms in micro-scratches that survive cleaning. After a serious contamination, replacing these inexpensive parts ($5-$10 worth) is cheap insurance.
Most contaminated batches are safe to drink — they just taste wrong. The batch itself usually can't be saved, but the equipment can be recovered with thorough cleaning and sanitation. The single most important next step is preventing the same contamination next time.
Preventing contamination next batch
Contamination comes from one of a small number of sources. Tightening these prevents almost all repeat incidents.
- Sanitiser solution for every step. Mix it before you do anything else on brew day. Every surface, every implement, every part of every implement gets 30 seconds of contact time minimum.
- Don't touch the inside of anything. The lid, the fermenter, the tap, the test jar, the bottle openings. Fingers carry organisms even when they look clean.
- Sanitise scissors before opening yeast. The blades of unwashed scissors are a common contamination entry point.
- Don't open the fermenter unnecessarily. Every lid lift introduces oxygen and possibly airborne organisms. Open the fermenter only to dry-hop, take a hydrometer sample, or transfer to bottling.
- Cover any opening with sanitised foil. If you have to leave the fermenter open briefly during transfer or sampling, cover the opening with sanitised aluminium foil.
- Replace soft parts annually. Airlocks, grommets, and tubing that's been in service for a year accumulates micro-damage that holds organisms.
For more on common homebrewing failure modes, see 10 common homebrew mistakes. For the specific flavour signatures of various off-flavours, see our off-flavours diagnostic guide. For the basic brewing process that prevents most contamination in the first place, see how to brew from a fresh wort kit.