Wine Faults: How to Identify and Handle Flawed Bottles

Wine Faults: How to Identify and Handle Flawed Bottles

Opening a bottle of wine is always an act of anticipation. But occasionally, what greets you in the glass is not pleasure but disappointment — a strange smell, an off taste, or a color that simply does not look right. Wine faults are more common than most drinkers realize, and knowing how to spot them is one of the most empowering skills you can develop as a wine enthusiast.

This guide walks through the most prevalent wine faults, explains their causes, and gives you the practical vocabulary and confidence to handle a flawed bottle correctly — whether at home or in a restaurant.

What Is a Wine Fault?

A wine fault is any characteristic that detracts from a wine's quality and is generally considered undesirable by sensory standards. Faults can arise at every stage of winemaking — in the vineyard, during Fermentation, in the cellar, or even during shipping and storage. Some faults are immediately obvious; others are subtle and require experience to detect.

Not every unusual characteristic is a fault. Some wines are intentionally oxidative, deliberately reductive, or purposely high in Volatile Acidity. Context matters enormously. A slight sherry-like quality in an aged white Burgundy Chardonnay may be appropriate; the same character in a young unoaked Sauvignon Blanc would be a defect. This is why developing a broad sensory reference library is so important — you need to know what is intentional before you can identify what is accidental.

Cork Taint (TCA)

Cork taint is perhaps the most notorious wine fault, and it is the reason the term Corked has entered everyday language. The culprit is a chemical compound called 2,4,6-trichloroanisole, or TCA. It forms when naturally occurring fungi in cork react with chlorine-based compounds used in sanitation.

TCA suppresses fruit aromas and replaces them with a distinctive musty, damp-cardboard, or wet-dog smell. Severely corked wines smell like a moldy basement; mildly corked bottles may simply seem flat and lifeless, which is more insidious because the fault is harder to pinpoint. The human threshold for detecting TCA is extraordinarily low — we can perceive it at parts per trillion.

TCA can affect wines sealed with natural cork, composite cork, or even wooden barrel staves — though it is most commonly associated with natural cork closures. Estimates suggest that somewhere between 1% and 5% of naturally corked bottles are affected to some degree. The wine industry has made progress in reducing cork taint through improved supplier standards and alternative closures, but the problem has not been eliminated.

TCA also has an unusual secondary effect: it temporarily impairs our ability to detect certain other aromas. This is why a mildly corked wine often seems unremarkable rather than obviously flawed — the corking has suppressed the wine's positive attributes along with its ability to signal wrongness.

What to do: If you suspect cork taint, pour a small amount into a plain white container to assess the aroma without oak or fruit distraction. If the musty quality is present, the wine is corked. In a restaurant, present the glass to your server or Sommelier without drama; a legitimate cork-tainted bottle will be replaced. At home, a retailer will usually exchange a corked bottle with receipt in hand. There is no remedy for a corked wine — the TCA cannot be removed.

Oxidation

Oxidation occurs when wine is exposed to too much oxygen. A small amount of oxygen is essential during winemaking and aging — controlled Aeration develops complexity and softens Tannin. But uncontrolled exposure degrades wine rapidly, breaking down aromatic compounds and altering the wine's chemical structure.

Oxidized wine loses its fresh fruit aromas and develops flat, nutty, or sherry-like characteristics. Whites turn deep yellow or amber; reds take on an orange or brick-brown rim. The Browning of color is one of the most reliable visual cues. Holding the glass at an angle against a white background makes rim color visible. On the Palate, oxidized wine tastes flat, bitter, and hollow, with none of the fruit freshness that should animate it.

Common causes include a faulty or dried-out cork, improper Ullage (too much empty space in the bottle), poor storage conditions with temperature fluctuations that allow air in and out through the cork, or a wine that has simply been open too long. Occasionally, winemaking errors during production — excessive exposure during racking or filtering — cause pre-bottling oxidation that no amount of careful storage can reverse.

It is worth distinguishing fault-level oxidation from intentional oxidative aging. Wines like Sherry, certain Riojas, and some white Burgundies are deliberately exposed to oxygen as part of their production. The difference lies in whether the oxidation is controlled and appropriate to the style.

What to do: Prevention is the best strategy. Store bottles horizontally to keep corks moist. Once opened, use a wine preservation system and consume within two to three days for most wines.

Volatile Acidity and Acetic Acid

All wines contain trace amounts of Volatile Acidity (VA), primarily in the form of Acetic Acid. At low concentrations, VA contributes complexity — some tasters appreciate a slight VA lift in aged Barolos or old-vine Zinfandels. But when VA exceeds threshold levels, wine tastes and smells like vinegar.

Acetic acid forms when acetic acid bacteria (Acetobacter) convert ethanol to acetic acid in the presence of oxygen. Winemakers who work with minimal Sulfites or leave open-top fermenters unattended risk VA spikes. The legal limits for VA vary by country: in the European Union, red wines may contain up to 1.2 grams per liter; in the United States, the limit is 1.4 g/L for red wines. Many high-quality wines stay well below these thresholds.

A related compound, ethyl acetate, forms alongside acetic acid and contributes a nail-polish-remover aroma that is often detected before the vinegar smell itself. The combination of acetic acid smell and ethyl acetate character is the classic VA signature.

What to do: Mild VA is not necessarily a dealbreaker, especially in powerful, age-worthy reds where some producers intentionally allow slightly elevated VA for complexity. But if a wine is aggressively vinegarish, it is faulty. Return it to the retailer and describe what you found. Keep a sample if possible — some retailers will want to taste the fault themselves.

Reduction and Sulfur Compounds

Reductive faults arise from a lack of oxygen during winemaking. Winemakers who protect wine aggressively from oxygen — particularly those working with natural or minimal-intervention techniques — risk reduction. The condition is particularly associated with certain winemaking approaches: early bottling, heavy use of inert gas blankets, and minimal racking.

Hydrogen Sulfide produces a rotten-egg smell that is immediately recognizable and deeply unpleasant. Mercaptan (thiols) smells like struck matches, garlic, rubber, or cooked cabbage. These compounds form when yeast are stressed during Fermentation, often due to nutrient deficiencies, excessive sulfur additions to the vineyard, or inadequate oxygen during fermentation.

The confusion for tasters is that reduction sits on a spectrum. Mild reduction is common in young wines and frequently dissipates within minutes of exposure to air. Many respected winemakers, particularly in natural and biodynamic traditions, intentionally maintain some reductive character in their wines, arguing that it protects freshness and fruit expression. A young Muscadet, a Grüner Veltliner from Alsace, or a mineral-driven Chablis might show a brief reductive note on opening that transforms into something compelling with air.

What to do: Swirl the wine vigorously, or pour it into a decanter. Light reduction often blows off within 10–20 minutes. If the smell persists after vigorous aeration, the wine is likely faulty at a problematic level. Some collectors drop a copper coin into a heavily reduced wine — copper reacts with hydrogen sulfide to form copper sulfide, removing the smell. This is a controversial remedy that works in mild cases.

Brettanomyces

Brettanomyces is a wild yeast strain that produces a range of aromatic compounds called phenols. At low concentrations, Brett adds complexity — experienced tasters describe earthy, leathery, smoky, or spicy notes that can genuinely enhance certain wine styles. But at high concentrations, Brett dominates wine with barnyard, horse saddle, or band-aid aromas that many tasters find deeply objectionable.

Brett is particularly associated with Bordeaux, certain Rhone Valley reds, and some Rioja producers who rely on older oak barrels that harbor yeast colonies. It thrives in wooden vessels that are difficult to sanitize thoroughly, and it can persist in the winery environment for decades. High Brett levels are generally considered a fault in modern winemaking, though opinion varies significantly among traditionalists who associate its character with regional typicity.

The Brett debate is one of wine's most contentious. Wines that win major competitions in Europe sometimes show significant Brett character that American tasters consider clearly faulty. This cultural divergence reflects genuinely different aesthetic standards, not a simple right and wrong.

What to do: This is partly a matter of taste. If Brett ruins your enjoyment of a wine, consider it a fault and seek a refund or exchange. If the character is mild and you find it interesting, enjoy it as a stylistic feature.

Mousiness

Mousy is one of the most challenging faults to detect because it is not apparent on the nose — it only manifests on the palate and lingers in the aftertaste. The fault is caused by specific compounds (2-acetyltetrahydropyridine and related chemicals) produced by lactic acid bacteria or certain wild yeasts under low-sulfite conditions.

The characteristic is exactly as it sounds: a stale, grain-like, rodent-cage aftertaste that can persist for minutes after spitting. The fault is pH-dependent — it becomes apparent only in the alkaline environment of the mouth, which is why sniffing a mousy wine gives no warning. Some tasters are more sensitive to mousy compounds than others, which creates genuine disagreement about whether a wine is faulty.

Winemakers working with no added Sulfites are most vulnerable because sulfur dioxide inhibits the bacteria responsible. The rise of natural winemaking has unfortunately coincided with an increase in mousy wines appearing in the market.

What to do: There is no fix for mousiness. If a wine is mousy, it is faulty and should be returned to the retailer. Be specific in your description — many retailers have encountered mousiness and will recognize the description.

Heat Damage and Cooked Wine

Wine is fragile in heat. Sustained exposure to temperatures above 25°C (77°F) accelerates chemical reactions and can produce a cooked or stewed character. In severe cases, the cork is pushed upward by expanding liquid, creating visible seepage or a raised cork.

Heat-damaged wine often smells flat and jammy, with excessive sweetness but no freshness or lift. The Finish is dull. The color may be more advanced than expected for the Vintage — whites browning prematurely, reds showing premature brick tones at the rim. The damage is permanent and irreversible.

Heat damage most commonly occurs during transportation — wines sitting in shipping containers in summer, delivery trucks without climate control, or bottles displayed in store windows in sunlight. It can also occur in restaurant cellars or home storage areas where temperature is not properly managed.

What to do: Inspect bottles carefully before purchase, particularly wines from retailers without climate-controlled storage. A protruding cork, sticky capsule, or visible seepage is a clear warning sign. Ask questions when buying from unfamiliar sources.

How to Send a Bottle Back at a Restaurant

Etiquette around returning wine is frequently misunderstood, and many diners are intimidated by the prospect of challenging a selection. But you are entirely entitled to return a bottle with a genuine technical fault.

When the bottle is presented, examine the cork briefly — a severely compromised cork may show signs of damage, unusual color, or excessive compression. When the wine is poured for your inspection, swirl, sniff, and take a small sip. If you detect a clear fault — cork taint, severe oxidation, extreme VA — quietly inform the server or Sommelier that you believe the wine has a problem. Be specific: "This wine smells musty and damp, which makes me think it may be corked" gives them something to work with.

A reputable establishment will not argue with a genuinely faulty bottle. They may taste it themselves to confirm, but this is professional practice, not challenge. They will offer a replacement bottle of the same wine (if available) or suggest an alternative.

You are not entitled to return a wine because you changed your mind about the style, because it does not pair as well as you expected, or because you simply do not like it. Ordering is a contract; faults are the exception that justifies return.

Developing Your Fault-Detection Palate

The best way to learn wine faults is direct exposure. Some wine schools and trade educators offer fault-identification workshops using deliberately compromised samples — wines dosed with TCA, acetic acid, or hydrogen sulfide at controlled concentrations. Reading about faults is useful, but smelling them in isolated form is transformative. Once you know what a corked wine smells like, you will recognize it instantly and reliably for the rest of your life.

Keep notes when you encounter faults in real bottles. Over time, you will develop a sensory library that makes identification almost automatic. This skill serves you at a casual dinner party, a professional Blind Tasting event, or a wine competition judging panel.

Understanding faults also deepens your appreciation of faultless wines. When you know everything that can go wrong — the bacterial contaminations, the chemical reactions, the physical damage possible between harvest and glass — the clean, vibrant expression of Terroir in an impeccable bottle of Pinot Noir from Burgundy feels like a minor miracle. Which, in many ways, it is.