There's a really simple way to tell if your beer is contaminated
Ready for this life changer?
Drink it.
If it tastes like the scummiest thing you've ever put in your mouth, it's infected. If it makes you vomit, it's infected. If it smells like someone set off a sulfur bomb, it's infected. Rotten eggs... well, we'll come back to that one, because it isn't always the smoking gun people think it is.
If you open the cap and the beer explodes like it has been shaken up a thousand times, it's probably infected. This happens as rogue yeast or bacteria have over-carbonated your beer, chewing through sugars your own yeast had already finished with and pumping out extra CO2 until the pressure has nowhere to go. Such an explosion should not be confused with a beer bomb caused by adding too much sugar when you primed the beer — same mess, different cause.
Basically, a good rule of thumb is that if you really have to ask whether your beer is infected, then the chances are it probably is.
You can, of course, do a visual inspection before you bottle as well. What you are looking for at the top of the wort is the formation of a 'pellicle' (not to be confused with a harmless yeast raft) — a collection of microbes hanging out on top of your beer. This may not happen with every infection, however.
The pellicle formation can look a bit like this:
or even this:
Which is a real shame, because it's not just that your beer has been ruined by bacteria or wild yeast (commonly a strain of brettanomyces). It's that you've lost your time — it doesn't matter if you used a kit or did a diligent boil, you've lost those precious minutes. You've also lost a bit of cash, which can hurt, especially if you sourced that special wheat yeast from the brew shop or drove 45 minutes for homegrown hops from a mate who swears they're the best he's ever grown.
So what did you get out of this? Experience. It's quite likely that user error caused the infection, so maybe there's a lesson here you can learn.
First, the good news: beer fights back
Before we scare you off brewing forever, here's a reassuring bit of science. Finished beer is a genuinely hostile place for most microbes. It has four built-in defences working at once:
🔬 Why your beer is harder to spoil than milk
Alcohol is mildly antiseptic. Low pH (beer finishes around 4.0–4.5, tart and acidic) blocks most spoilage organisms. Hops are the secret weapon: iso-alpha acids from the boil are directly antimicrobial and are especially good at stopping Gram-positive bacteria like Lactobacillus. And a blanket of CO2 plus very low oxygen starves the bugs that need air to grow.
The upshot: there is no known human pathogen that can survive and thrive in beer. An "infection" almost always spoils the flavour rather than making you ill. That's why the honest test really is to taste it.
An infection, then, is simply an unwanted microbe that got past those defences — usually because something gave it a foothold before the alcohol, acid and hops could take hold. Understanding the handful of usual suspects tells you exactly where those footholds are.
Meet the usual suspects (the microbiology)
"Infection" is a catch-all word for a few very different organisms, and they leave very different fingerprints in the glass. Knowing which one you've got tells you both what went wrong and whether the beer is a drain-pour or a happy accident.
Wild yeast (Brettanomyces & friends)
Brett is a yeast, not a bacterium, and it plays a long game. It slowly munches the complex sugars (dextrins) that your brewing yeast leaves behind, which is why a Brett infection often shows up as a beer that keeps drying out, thinning and over-carbonating for months — and eventually gushes. The tell-tale aromas come from real chemistry: Brett converts the wort's hydroxycinnamic acids into 4-ethylphenol (barnyard, horse-blanket, sticking-plaster) and 4-ethylguaiacol (smoky, clove-like). Here's the twist — it's completely safe to drink, and sour and wild-ale brewers deliberately pitch it. If your accidental Brett beer tastes interesting rather than awful, you might have stumbled into a funky farmhouse ale.
Lactic acid bacteria (Lactobacillus & Pediococcus)
These are the souring bugs. They ferment sugars into lactic acid, dropping the pH and turning your beer tart and sour. Pediococcus adds two extra party tricks: diacetyl (a slick, buttery, butterscotch note) and, in bad cases, long chains of sugar called exopolysaccharides that make the beer turn thick and ropy — brewers charmingly call this "sick beer." The good news from the science above: both are Gram-positive and hop-sensitive, so a decently hopped beer is naturally resistant. That's also why deliberate sour brewers use very low-hop worts.
Acetic acid bacteria (Acetobacter)
This is the one that turns beer into vinegar, and it needs one thing above all: oxygen. Acetobacter oxidises the alcohol in your beer into acetic acid (sharp vinegar) and ethyl acetate (nail-polish, solvent). It can only do this where beer meets air — a slack airlock, a half-empty fermenter, splashy transfers, or a long stay in an under-filled vessel. Control oxygen after fermentation and you starve it out completely.
| The culprit | What it is | Tell-tale sign | Needs air? |
|---|---|---|---|
| Brettanomyces | Wild yeast | Barnyard/horsey funk, creeping dryness, gushing | Tolerates it |
| Lactobacillus | Bacterium | Clean tart sourness | No (dislikes it) |
| Pediococcus | Bacterium | Sour + buttery, sometimes ropy/thick | No |
| Acetobacter | Bacterium | Vinegar & solvent | Yes — must have it |
What a pellicle actually is
That skin, film or clump of alien-looking growth on top isn't the microbes themselves so much as their fort. When aerobic organisms — Brett, acetic bacteria, film yeasts — colonise the surface where beer meets air, they secrete a matrix of sticky polymers (the science name is extracellular polymeric substances) and knit themselves into a raft. In other words, a pellicle is a biofilm: a self-made shield that holds the colony at the oxygen-rich surface and protects it from the beer's defences underneath.
Two things follow from that. First, a pellicle usually points to an air-loving organism, which is a clue to how the bug got in. Second, and importantly, not every infection builds one — plenty of sour or off batches look perfectly normal. And a raft on top is not automatically a death sentence: it can be a harmless yeast raft, or the tail end of a healthy krausen. The only reliable referee is your nose and your tongue.
So let's talk about the causes of infection
The most likely cause, as you've probably worked out by now, is simple uncleanliness. By giving bacteria something to feed on or hide in, you hand them the foothold they need before the alcohol, acid and hops can slam the door. And here's the part that catches people out: the danger isn't loose dirt you can see. It's the biofilm.
Every scratch in a plastic fermenter, every ring of dried krausen, every skerrick of yeast left in a tap houses a microscopic layer of microbes bonded to the surface and wrapped in that same protective slime. A quick splash of sanitiser rolls straight over the top of a biofilm without touching the bugs living underneath it. That is the single most misunderstood point in the whole hobby, and it's exactly why sanitation quietly fails so many brewers.
💡 Cleaning is not sanitising — and it's not superstition
Cleaning removes the soil (the organic gunk microbes feed and hide in). Sanitising then reduces the microbes on an already-clean surface to a safe level. Sterilising kills everything — and is rarely achievable, or necessary, in a home. You cannot sanitise a dirty surface, because the soil physically shields the bugs and chemically burns through the sanitiser first.
The order is non-negotiable: clean first, sanitise second, every single time. More on that debate in do I really have to sterilise all my gear?
The lesson, then, deserves to be shouted:
ALWAYS CLEAN & SANITIZE
YOUR BREWING EQUIPMENT
I learned from my screw-up and have never had an infected batch since — and that was years ago. Sure, it can be a pain to do the job right, but if you want a beer that's right to drink, you gotta clean.
The chemistry of getting gear clean
So clean your fermenter, brewing spoons, pipes, spigots, taps, mash tuns and whatever else you touch on brewing day. Threads and fittings are where the worst infections hide, so take them apart — a cleaner can't reach a surface it can't touch. There are plenty of cleaning agents, and it's worth knowing what each is actually doing.
The famous Powdered Brewery Wash (PBW) is a buffered alkaline cleaner: the high pH saponifies fats and loosens proteins while an oxygen-releasing agent and surfactants lift the soil away, all without you scrubbing scratches into your plastic. You can even mix your own PBW substitute. For everyday pre-cleaning, a soft sponge and warm water — and yes, a little dishwashing soap is fine as long as you rinse well — knocks off the bulk before you soak. A bit of elbow grease with damn hot water will do you justice too.
Then, sanitisation is key. We've promoted sodium percarbonate many times, and the chemistry is lovely: dissolved in warm water it splits into washing soda (alkaline, lifts soil) and hydrogen peroxide, which decomposes into water and active oxygen. Those bubbles oxidise organic residue and physically float it off the surface — no chlorine, no off-flavours, no pitting. The best part? You've probably already got some, because it's the active ingredient in ordinary oxygen laundry soak.
🔬 Cleaner vs sanitiser, in one line
Percarbonate and PBW are primarily cleaners — they strip the surface. A no-rinse acid sanitiser like Star San (phosphoric acid at a very low pH, plus a surfactant that ruptures microbial cell walls) is what you use as the final step, right before you fill. If you're unsure what to buy, here's a run-down of the best homebrew sanitiser and how to use it.
So on brewing day — clean and sanitise everything. Don't be lazy or your beer will be hazy!
The second danger zone: bottling and kegging day
The next time you'll want to think about bacteria is bottling or kegging day. Yep, it's almost a case of literally rinsing and repeating. Your keg and bottles must be free of any gunk and residue yeast — give them a damn good clean and then use your sanitiser of choice.
This is also where the oxygen lesson from earlier really bites. Once fermentation is done, your beer has used up its protective CO2 and lost the yeast's appetite for oxygen, so every splashy pour now does two bad things at once: it invites air-loving Acetobacter, and it triggers plain old oxidation (the stale, wet-cardboard flavour that comes from a compound called trans-2-nonenal). Transfer gently, fill from the bottom, and keep headspace sensible. Doing bottling day properly and understanding bottle conditioning protects all that careful work.
In the case of bottles, my favourite trick is to run them through the dishwasher on the heaviest setting. First I rinse them with water to shift the sediment, then in they go. On Heavy Duty the dishwasher uses the hottest water it can, and that heat kills the bugs. I then store them in a clean drum under a blanket. On bottling day, a quick soak in some sodium percarbonate solution makes things just right — and I hold each bottle to the light to check it's spotless (you'd be amazed what you can find hiding in the bottom — ask me about my weta-infused lager).
You can always tell if you haven't done this part properly: if one or two bottles in a batch taste off but the rest are fine, the problem was the individual bottle, not the batch as a whole. Get the whole batch wrong and, well…
 |
| This "mega pellicle" was from a brew that was found to be infected. |
That rotten-egg smell from beer (and the science behind it)
We mentioned that rotten eggs can be a sign of infected beer. That may well be true, but it isn't true in every case — and the chemistry explains why. That smell is hydrogen sulphide (H2S), and your own yeast makes it as a normal by-product of building the amino acids it needs. Healthy yeast makes a little; stressed yeast makes a lot — and yeast gets stressed by high fermentation temperatures, poor nutrition, or simply being a sulphur-happy lager strain.
The reassuring part is that H2S is volatile and light. During an active ferment the stream of CO2 literally scrubs it out of the beer, and given enough conditioning time the yeast mops up the rest. So if you've used a strain that throws sulphur, or you've cracked a bottle-conditioned beer too early, those eggy tones will usually fade on their own. If your beer's water is high in sulphate — like the famous supply at Burton-on-Trent, England — you may also get a whiff naturally, the so-called 'Burton Snatch'.
If, however, the smell is backed by a genuine bacterial contamination, then that odour is a sign your beer is ruined. How can you tell the difference? Do the taste test — that gives you the big indicator. Stressed-yeast sulphur blows off and the beer tastes clean underneath; a real infection tastes wrong all the way down.
Wine, cider and the risk hiding in fruit
If you make wine or cider, there's another risk vector, because fruit carries its own wild yeasts on its skin that can wreck a brew. This matters more for fruit-based drinks than beer, because they lack the hop defence beer enjoys. That's why many cider and wine makers use Campden tablets to kill off the wild yeast first, then pitch a cultured strain suited to the style they're after.
How to avoid an infected batch (the checklist that actually works)
Everything above boils down to a short list of habits. Get these right and infections become a rare freak event rather than a regular heartbreak. Each step links to the fuller guide on the site.
- Clean before you sanitise — always. Remove the soil and biofilm first so the sanitiser can actually reach the surface. Here's the full walk-through for cleaning and sanitising your fermenter, plus the case for PBW and sodium percarbonate.
- Finish with a real sanitiser. A no-rinse acid sanitiser as your last step, right before the gear touches wort or beer. Pick one with this guide to the best homebrew sanitiser.
- Kill the biofilm hiding spots. Bin scratched, cloudy plastic; strip down taps, valves and spigots to clean the threads. Understand why sanitation fails and you'll never cut this corner.
- Pitch enough healthy yeast, fast. A strong, quick-starting ferment lets your yeast dominate the wort and out-compete any stray bugs before they get established — so pitch (and hydrate) your yeast properly.
- Control your fermentation temperature. Keep the yeast in its happy range. Too warm and you stress it into throwing sulphur and harsh flavours, while also handing warmth-loving bacteria a cosy nursery.
- Guard against oxygen after fermentation. Gentle transfers, minimal splashing and a full vessel starve Acetobacter and prevent staling. Do bottling day and bottle conditioning the tidy way.
- Keep bottle and keg day spotless. Spotless, sanitised bottles, a good cap and seal, and a light-check on every bottle before you fill it.
- Store cool and dark. Heat and light age beer fast — here's how to store your bottled homebrew properly.
- If it still happens, diagnose and learn. Taste before you dump — you might have a happy sour. If not, work through the common brewing problems and the wider troubleshooting guides, and chalk it up to experience.
New to all this and want the whole picture? Start with the simple step-by-step guide to home brewing and browse the full sanitisation section. Do the cleaning right and the rest of brewing gets a whole lot more forgiving.
