Identifying Wine Faults: Off-Flavors and What Causes Them

Why Every Wine Lover Should Understand Faults

A wine fault is any undesirable characteristic in a wine's aroma, flavor, or appearance that detracts from its quality and enjoyment. Faults can originate in the vineyard, the winery, during storage, or at the point of service. Some are glaringly obvious, like the smell of vinegar in a bottle that should be dry and fruity. Others are subtle, lurking beneath the surface and slowly diminishing a wine's appeal without the taster quite understanding why.

For home winemakers, identifying wine faults is not merely an academic exercise. It is a survival skill. Detecting a problem at its earliest stage often means the difference between an easy correction and an unsalvageable batch. For consumers and enthusiasts, recognizing faults prevents you from blaming your own palate when a wine simply isn't right. Understanding faults sharpens your ability to distinguish between a wine you don't prefer and a wine that is genuinely flawed.

Cork Taint (TCA Contamination)

What It Smells Like

Cork taint is caused primarily by a chemical compound called 2,4,6-trichloroanisole, or TCA. At detectable concentrations, TCA produces aromas of wet cardboard, damp basement, musty newspaper, and moldy cellar. At lower concentrations that fall below the conscious detection threshold, TCA strips the wine of its fruit character and makes it taste dull and flat without an obvious off-odor. This "sub-threshold" impact is one of the most insidious effects of cork taint.

What Causes It

TCA forms when naturally occurring fungi in cork bark interact with chlorine-containing compounds. It can also develop on other cellar surfaces, including wooden barrels, drains, and even cardboard packaging. Industry estimates suggest that between two and seven percent of all wines sealed with natural cork are affected to some degree.

Prevention and Response

For home winemakers, sourcing high-quality cork from reputable suppliers reduces risk significantly. Switching to synthetic closures or screw caps eliminates TCA from the closure entirely, though ambient TCA in your winemaking environment remains a possibility. If you detect cork taint in a commercial bottle, the wine should be returned or replaced. There is no way to reverse TCA contamination once it has occurred.

Oxidation

What It Smells and Tastes Like

Oxidation is the most common fault in both homemade and commercial wines. Oxidized white wines turn from pale straw to deep amber or brown and develop aromas of bruised apple, sherry, stale nuts, and caramel. Oxidized red wines lose their vibrant fruit, shift toward brown tones, and take on flat, dried-out, or sherried flavors. The wine's freshness and vitality disappear.

What Causes It

Oxygen is wine's most persistent enemy after fermentation is complete. Excessive exposure during racking, bulk aging with too much headspace, faulty closures, and inadequate sulfite levels all invite oxidation. Temperature swings during storage accelerate the process. Even micro-amounts of oxygen ingress through a poorly sealed barrel or an aging carboy with a compromised airlock can cause cumulative damage over months of aging.

Prevention and Response

Maintain appropriate free sulfur dioxide levels throughout aging and at bottling. Minimize headspace in all storage vessels. Ensure closures are airtight. Handle wine gently during racking and transfers to limit oxygen pickup. Once oxidation has progressed significantly, it cannot be reversed. Mildly oxidized wines can sometimes be blended with fresher wine to mask the damage, but this is a salvage operation rather than a cure.

Volatile Acidity (VA)

What It Smells and Tastes Like

Volatile acidity at elevated levels gives wine an unmistakable vinegar-like sharpness. The primary culprits are acetic acid and its ester, ethyl acetate. At low concentrations, ethyl acetate contributes a fruity, nail polish-like aroma that some tasters find initially pleasant but that quickly becomes cloying. At higher concentrations, the vinegar character becomes dominant and the wine is effectively ruined for most palates.

What Causes It

Acetic acid bacteria, particularly strains of Acetobacter, convert ethanol into acetic acid in the presence of oxygen. These bacteria are ubiquitous in winemaking environments and will colonize any wine left exposed to air. Fruit flies are notorious vectors, carrying acetobacter directly to uncovered fermentations. Stuck fermentations, where residual sugar sits unprotected by active yeast, are also vulnerable.

Prevention and Response

Strict sanitation, minimal oxygen exposure, and adequate sulfite management prevent volatile acidity. Cover all fermentations and aging vessels. Eliminate fruit fly access to your winemaking space. If you detect early-stage VA, increasing sulfite levels can arrest bacterial activity, but the acetic acid already produced remains in the wine. Blending with a clean, low-VA wine is the most practical remediation for moderate cases.

Reduction and Sulfur Compounds

What It Smells Like

Reduction occurs when wine develops sulfur-containing compounds in the absence of sufficient oxygen. The mildest form smells like struck match or rubber. More severe reduction produces aromas of rotten egg (hydrogen sulfide), cooked cabbage, garlic, burnt rubber, or even sewage (mercaptans and disulfides). These compounds are among the most offensive faults in wine.

What Causes It

Sulfur compounds form when yeast cells are stressed, typically from nutrient deficiency during fermentation. Yeast strains vary in their tendency to produce hydrogen sulfide. Low temperatures, excessive sulfite additions, and prolonged contact with spent yeast lees without stirring can also trigger reduction. Some grape varieties, particularly Syrah and Chardonnay, are more prone to reductive aromas.

Prevention and Response

Ensure adequate yeast nutrient additions during fermentation, particularly diammonium phosphate (DAP) and organic nitrogen sources. Choose yeast strains known for low hydrogen sulfide production. If you detect struck match or rotten egg aromas early, rack the wine off its lees and aerate it briefly. Copper sulfate additions at very low concentrations (a fraction of a part per million) can bind hydrogen sulfide, but this requires careful measurement. Advanced sulfur compounds like mercaptans are far more difficult to remove and may require professional intervention.

Brettanomyces

What It Smells Like

Brettanomyces, commonly called "brett," is a wild yeast that produces aromas described as Band-Aid, barnyard, horse stable, sweaty saddle leather, and smoky or medicinal phenolic notes. At low levels, some tasters and winemakers consider brett part of a wine's complexity, particularly in traditional Old World styles. At higher levels, the aromas become unmistakably foul and mask the wine's fruit entirely.

What Causes It

Brettanomyces thrives in environments with low sulfite levels, residual sugar, and oxygen. It colonizes porous surfaces like wooden barrels and is notoriously difficult to eradicate once established. Brett is more common in red wines than whites, as the lower acidity and higher pH typical of reds provide a more hospitable environment.

Prevention and Response

Maintain adequate free sulfur dioxide levels, especially during barrel aging. Keep pH below 3.6 when possible. Sanitize barrels thoroughly between uses. Once brett is established in a cellar, it can become a persistent contaminant. Sterile filtration at bottling removes brett cells and prevents further development in the bottle.

Other Common Faults

Refermentation

Refermentation in bottle occurs when residual sugar or malic acid is consumed by yeast or bacteria after bottling, producing carbon dioxide. The wine becomes unexpectedly fizzy and may push corks or crack bottles under pressure. Ensuring fermentation is complete before bottling and using appropriate sulfite levels and sterile filtration prevents this dangerous fault.

Light Strike

Light strike affects wines, particularly those in clear glass bottles, that are exposed to ultraviolet light. The light triggers a reaction with riboflavin and amino acids that produces sulfur-containing compounds smelling of cooked cabbage, wet wool, or struck flint. Champagne and light white wines are most susceptible. Store wines in dark conditions and use colored glass for bottling.

Heat Damage

Wines stored at temperatures above 80 degrees Fahrenheit for extended periods suffer heat damage, which manifests as stewed, cooked fruit flavors, premature browning, and a general flattening of the wine's complexity. In severe cases, the wine may have pushed its cork due to thermal expansion. Always store wine between 50-65 degrees Fahrenheit in a stable environment.

Excess Sulfur Dioxide

While sulfite is essential for preservation, excessive sulfur dioxide produces a pungent, burnt match aroma and a sharp, irritating sensation in the nose and throat. This fault is more common in recently bottled wines and often dissipates with aeration. Careful measurement and adherence to recommended sulfite levels prevent this issue.

How to Train Yourself to Detect Faults

The best way to learn fault identification is through deliberate sensory training. Some wine education programs offer fault-spiked wines, where measured amounts of TCA, acetic acid, acetaldehyde, hydrogen sulfide, and other compounds are added to clean wine samples. Tasting these alongside a clean control wine calibrates your sensitivity.

At home, you can simulate some faults by adding a tiny piece of wet cardboard to wine (simulating cork taint), exposing wine to air for several days (simulating oxidation), or adding a drop of vinegar to a clean glass of wine (simulating volatile acidity). While imprecise, these exercises build awareness.

Frequently Asked Questions

Is it safe to drink wine with a fault?

Most wine faults affect flavor and aroma but are not harmful to health. Oxidized wine, corked wine, and wine with brett are all safe to consume, though they may not be pleasant. The exception is wine with dangerously high volatile acidity or methanol levels, though the latter is extremely rare in properly made wine. If a wine smells or tastes wrong, you can safely taste it to diagnose the issue, but you don't need to force yourself to drink it.

How can I tell if a wine is faulty or just a style I don't prefer?

This is one of the most common questions in wine education. A fault is an objective defect that detracts from quality, while style is a matter of preference. If a wine smells of wet cardboard, it is corked regardless of your personal taste. If a wine is more tannic or more acidic than you prefer, that is a style difference. When in doubt, compare the wine against other bottles from the same producer, vintage, and variety. If the anomaly is unique to one bottle, it is likely a fault.

What percentage of wine bottles are actually faulty?

Estimates vary, but industry research suggests that three to eight percent of wines sealed with natural cork are affected by TCA to some degree. Oxidation, the second most common fault, affects a smaller but significant percentage of bottles, particularly those that have been poorly stored. Other faults like brettanomyces and volatile acidity occur less frequently but are more common in certain wine regions and styles.

Can faults develop after I buy a bottle?

Yes. Improper storage conditions can introduce or accelerate faults after purchase. Heat, light, vibration, and dramatic temperature swings all degrade wine quality over time. Even a perfectly sound bottle can develop oxidation if stored upright for years, causing the cork to dry out and allow air ingress. Store all cork-sealed wines on their sides at 50-60 degrees Fahrenheit in a dark, stable environment.

Should I return a faulty bottle at a restaurant?

Absolutely. Sending back a corked or faulty bottle is perfectly acceptable and expected. Describe what you detect to the server or sommelier. A professional will verify the fault and replace the bottle without question. You are not being difficult; you are being knowledgeable. Restaurants build spoilage into their wine program costs for exactly this reason.