Wine faults explained plainly, without the lab coat: this is the piece for the moment you’re standing over a glass wondering whether that odd smell is meant to be there — and, just as importantly, whether you’re within your rights to march the bottle back to whoever sold it to you. That second half is the bit most guides skip, and it’s the half a merchant actually deals with every week. A wine fault is a flaw picked up somewhere between the vineyard and your glass — in fermentation, at bottling, or through years sat badly in a cellar — that makes a wine smell or taste wrong rather than simply different from what you expected. By the end of this you’ll know the shape of the faults a trade taster checks for, the three you’re actually likely to meet, and when to open the next bottle instead of ringing us up.
If you want the basics of wine first, before the faults, our plain-English guide to wine is the place to start — this piece assumes you’ve got a glass in hand already.
What are the 12 wine faults?
Trade tasting guides commonly group wine faults into around a dozen recognised categories, running from the everyday to the rare. Most fall into four families, and that’s the version worth memorising: closure faults, oxygen faults, sulfur faults, and microbial faults.
Closure faults start with cork taint (TCA), the wet-cardboard smell that’s the one everybody’s heard of. Oxygen faults cover full oxidation (browning, flat, sherry-like) and something the trade calls random or sporadic post-bottling oxidation — where one bottle from a case goes brown early because its cork simply let more air through than its neighbours did. Sulfur faults run from reduction (onion to rotten egg) to excess sulfur dioxide itself, which can smell of a just-struck match if too much was added at bottling. Microbial faults include volatile acidity, where bacteria push a wine towards vinegar; ethyl acetate, its nail-polish-remover cousin; and Brettanomyces, which gives farmyard or medicinal notes some drinkers rather like in small doses and most reject in large ones.
Round out the list with lightstrike (a garlic, cabbage-y note that’s been turning up more in trade tastings of late), fungal must taint (mushroom, wet potato, from mould on the grapes themselves) and plain mouldy barrel taint, picked up from a badly kept cask rather than the cork at all. Twelve names, four families. That’s the whole shape of it — and knowing it turns the next odd glass into a question you can answer at the counter, rather than an evening quietly written off.
What are three common wine faults and list the markers for each fault?
The three you’ll meet in the real world are cork taint, oxidation and reduction — everything else is rarer, or found mostly by winemakers before the wine ever reaches you. Cork taint smells of wet cardboard and dulls the fruit. Oxidation browns the colour and pushes fruit towards bruised apple or dried fruit. Reduction smells of onion through to rotten egg, and it’s the one most likely to soften with a bit of air. If you only remember one line from this whole page, make it this: air fixes reduction; nothing fixes cork taint.
| Fault | Likely cause | What you’ll notice | What to do |
|---|---|---|---|
| Cork taint | TCA compound from a contaminated cork | Wet cardboard, damp cellar smell; fruit muted, finish drops away fast | Nothing fixes it — return the bottle |
| Oxidation | Too much oxygen exposure, faulty closure, or kept too long | Colour tips brown or gold; fruit turns to bruised apple, quince, prune; edges towards vinegar | Return it, unless it’s a style meant to be this way |
| Reduction | Sulfur compounds from low-oxygen conditions or stressed yeast | Struck match at best, onion or rotten egg at worst | Swirl hard, decant, give it ten minutes before deciding |
Cork taint is estimated by trade sources to affect somewhere around 3–5% of bottles sealed with natural cork — worth knowing before you assume every disappointing bottle is the winemaker’s fault. We’ve written more on spotting it specifically in Is my wine corked? How to tell — and what to do next, since it’s the one question we get asked most across the counter.
Oxidation is worth a second look, because it isn’t always a mistake. Tawny Port, palo cortado and oloroso Sherry, and Madeira are all built on deliberate, controlled oxidation — it’s the whole point of the style, not a flaw in it. And if you find you actually enjoy that nutty, dried-fruit end of the spectrum, buy it on purpose rather than meeting it by accident: a Tawny Port or an oloroso Sherry is oxidation done beautifully, and rarely expensive for what it is — genuinely one of the better-value corners of the whole shop. If you’re unsure whether a fortified bottle in your rack has turned or is simply doing what it’s meant to do, our guide on keeping Port and Sherry open walks through the difference.
How will a faulty wine look?
A genuinely faulty white often tips towards brown or dull gold before its age suggests it should, and with premature oxidation this can show within a minute or two of the wine hitting the glass — watch the rim as you pour, not just the colour once it’s settled. Reds affected the same way lean towards prune and stewed fruit rather than bright cherry or plum. But colour alone won’t tell you everything; the nose usually confirms it faster than the eye does.
Here’s where I used to get it wrong myself, years back behind the counter: a customer would bring back a bottle with a scatter of small white crystals in the bottom, convinced the glass had cracked somehow. Those are tartrate crystals — a natural, harmless by-product of cold stabilisation, nothing to do with a fault at all. Same with tiny flecks of cork floating in the glass; that’s a service slip, not a wine problem, and contact with sound cork never harmed a bottle. Learning to say “that’s fine, drink it” without sounding like I was fobbing someone off took longer than it should have.
The real tell is always smell plus taste together — a real fault shows up on both; a false alarm usually shows up on neither. Crystals, cork bits, a wine that’s simply young or simply tight — none of those pass that two-part test, because there’s nothing to pass. And a bottle that passes both tests but still leaves you cold isn’t faulty — it’s just the wrong style for you, which is a buying lesson rather than a refund conversation. Bring that lesson with you next time you’re choosing, and it pays for the disappointing glass several times over.
What to do when you think a bottle’s gone wrong
If reduction is your suspect, try before you condemn: a vigorous swirl, or ten minutes in a decanter, often clears the onion note away entirely and leaves the fruit standing on its own. Oxidation and cork taint won’t budge with air — if anything, cork taint tends to get more obvious the longer the glass sits.
Most shops worth their salt — us included, and outfits like the bigger high-street chains or the Wine Society — will take a genuinely faulty bottle back without a fuss. Keep the cork, keep the receipt if you’ve got it, and don’t feel awkward about it; a faulty bottle is the trade’s cost to bear, not yours. If it was a special bottle you’d been saving, and you’d rather not risk another special one straight away, our piece on second wines from Bordeaux is a good way back in at gentler stakes.
Questions customers ask us
What are the most common wine faults? Cork taint, oxidation and reduction, in roughly that order of how often they turn up. Recent trade tastings covered in the wine press have found a handful of faulty bottles in every batch of thirty-odd samples poured — mostly these three, occasionally something rarer like lightstrike.
Can a reduced wine just be left to sit and sort itself out? Sometimes, yes — reduction is the one fault that genuinely can soften with air, so a swirl or a spell in a decanter is worth trying before you write the bottle off.
Are wine faults always obvious? Not to everyone. Sensitivity to TCA and to sulfur compounds varies hugely from person to person, so a bottle that seems fine to one taster can read as clearly faulty to another sitting right next to them.
Does a screwcap mean a wine can’t be corked? Correct — cork taint needs an actual cork to originate from, so screwcap and synthetic-closure wines are safe from TCA specifically, though they can still develop reduction. If taint has stung you more than once lately, there’s no shame in steering your everyday whites toward screwcap and saving natural cork for the bottles you’re cellaring.
The merchant’s last word
None of this is about turning you into a lab technician with a glass in hand — it’s about drinking with a bit more confidence, and knowing the difference between a bottle that’s let you down and one that’s simply doing what it’s supposed to do. I’ve sent bottles back for the wrong reasons and kept ones I shouldn’t have, so trust your nose, ask if you’re not sure — a faulty bottle costs us a replacement, but a customer too embarrassed to mention it costs us far more. When you’re ready for the next one, come and browse the Red cellar.
