Your nose is the fastest fault detector you own. Before the first sip, a flawed wine will usually announce itself, damp basement, nail-polish remover, or vinegar are not subtle. The trickier cases are the ones that just seem muted or "off" without a dramatic smell, and those are worth knowing too. Here's how to read the signals clearly, without sending back a perfectly good bottle by mistake.
The Big Three Faults to Know
Cork taint, caused by a compound called TCA (2,4,6-trichloroanisole), is a very common reason a wine smells like a wet dog, a damp cellar, or soggy cardboard. It strips fruit from the wine and leaves it smelling musty and grey. A faintly corked wine can be easy to miss on the first sniff, which is why sommeliers swirl and sniff before pouring: TCA can seem more obvious with a little air, as fruit aromas fade and the musty note stands out more, but the compound itself is not increasing in concentration.
Oxidation happens when too much oxygen gets to the wine, a loose cork, a bottle stored upright for too long, or a wine left open on the counter for several days. A white wine turns deep gold and smells of sherry or bruised apple; a red loses its brightness and takes on a flat, stewed-fruit quality. A small amount of oxygen is intentional in many aged wines. Separate from oxidation, sharp vinegar or nail-polish-remover aromas usually indicate excessive volatile acidity (acetic acid and ethyl acetate).
Heat damage, sometimes called 'cooked' or 'maderized' wine, occurs when a bottle is stored or transported in excessive heat. The fruit tastes flat and jammy in an unpleasant way, the wine may smell faintly of raisins or dried fruit where you'd expect fresh, and the cork may have pushed slightly out of the bottle, a visual clue worth checking before you even open it.
What Your Nose Is Actually Detecting
Wine faults have specific chemical signatures, which is why the smells are so distinctive. TCA (cork taint) suppresses your ability to detect other aromas, so the wine doesn't just smell bad, it smells like almost nothing, except for that musty background note. Think of it like static drowning out a radio signal.
Reduction goes the other direction: the wine has had limited oxygen during winemaking and may smell of struck match, rubber, or rotten egg. Mild reduction isn't necessarily a fault, but strong reductive aromas can be. It sounds alarming but it's often fixable, swirling vigorously can help mild cases dissipate within minutes; in stubborn cases, specialized copper fining tools used by professionals can bind with the sulfur compounds.
Brett, short for Brettanomyces, is a wild yeast that can produce leathery, barnyard, or band-aid aromas. At low levels some consider it complex; at higher levels it's generally regarded as a fault. It's common in some traditional Rhône reds and older Bordeaux. At high levels, it overwhelms the fruit entirely. Whether it reads as 'rustic charm' or 'fault' is contested among wine professionals.
- Musty, wet cardboard, damp cellar → cork taint (TCA)
- Vinegar, nail polish remover → excessive volatile acidity (acetic acid / ethyl acetate)
- Raisins, flat jammy fruit, pushed cork → heat damage
- Struck match, rubber, rotten egg → reduction (sulfur compounds)
- Barnyard, leather, band-aid → Brettanomyces
Signs Wine Has Gone Bad After Opening
Once a bottle is open, the clock starts. Most reds hold reasonably well for two to three days with a simple stopper in a cool spot; lighter reds and whites tend to fade faster, and sparkling wine goes flat within a day or two without a proper sparkling wine stopper. These aren't faults in the traditional sense, it's just oxygen doing its slow work.
A wine that has turned after opening will smell flat, vinegary, or hollow, the fruit has gone, and what's left is sharp or watery. The color shifts too: a white that was pale straw turns deeper amber, and a red loses its ruby brightness for a brownish rim. The taste is the final confirmation: if it reminds you more of wine vinegar than wine, it has gone past the point of enjoyment.
One useful habit: smell the glass before you pour from a newly opened bottle, then smell it again after a five-minute rest. Cork taint may become more apparent with a little air, while mild reduction often lessens after swirling. If the wine smells progressively worse, not better, you have a fault.
What's Not a Fault (Even If It Surprises You)
Sediment in an older red wine is not a sign it has gone bad, it's a natural byproduct of aging, as pigments and tannins polymerize and drop out of solution. Decanting resolves it. Similarly, tartrate crystals (small glassy deposits, sometimes on the cork or at the bottom of a white wine bottle) look alarming but are precipitated tartaric acid, harmless and tasteless.
High tannin, high acidity, strong oak, or earthy mineral flavors are not faults. They're stylistic choices, and whether you enjoy them is a matter of preference, not quality. A wine being 'too dry,' 'too tannic,' or 'too funky' for your palate means it's not the right wine for you, not that it's flawed.
Cloudiness is worth a closer look. In a natural or unfiltered wine, slight haze is normal and intentional. In a wine that was previously clear and has developed cloudiness over time, it can occasionally signal refermentation in the bottle, usually harmless in still wines but worth noting.
When to Send a Bottle Back
At a restaurant, it is reasonable to raise a genuine fault, such as cork taint, obvious oxidation, or clear heat damage, with the sommelier. Whether a bottle can be replaced for a style preference depends on the restaurant's policy. Sommeliers understand this distinction well, and most will not challenge you if you describe a specific fault rather than a vague disappointment.
The practical test is to smell the wine in the glass, then reassess it after a short rest. A musty cork can support suspicion, but it is not a reliable diagnosis on its own. If the wine smells actively unpleasant rather than just unfamiliar, say so plainly. 'This smells corked, do you agree?' is a perfectly reasonable thing to say to a sommelier, and a good one will smell it alongside you.
At home, a faulty bottle from a retailer is usually worth a call or email, many shops will replace or refund a corked bottle, especially if you've kept the cork and remaining wine as evidence. Cork taint in particular is a closure-related defect that is difficult to eliminate completely, though improved cork processing and quality control can substantially reduce its incidence.