Wine Faults

A wine fault is a sensory-associated characteristic of a wine that is unpleasant, arising from a chemical or microbiological origin, where specific off-odors or flavors may result from more than one underlying cause. Wine faults differ from legitimate stylistic variations because they represent quality failures rather than intentional winemaking decisions. Many compounds that cause faults are naturally present in wine at low concentrations and may even contribute positively to complexity; it is only when their concentration exceeds a sensory threshold that they become problematic. The systematic study of wine faults advanced significantly through 20th-century laboratory analysis, including the identification of TCA as a principal cause of cork taint in 1982 and the characterization of Brettanomyces spoilage compounds in the early 1990s.

• Faults arise from fermentation errors, microbial contamination, oxidation, or improper storage conditions
• The distinction between a fault and a stylistic feature often depends on concentration, context, and whether the character was intentional
• WSET Level 3 and Diploma curricula treat fault identification as a core quality assessment competency
• Laboratory methods including GC-MS allow winemakers to quantify specific fault compounds and diagnose root causes

The most prevalent wine faults include cork taint (TCA), oxidation, reduction, Brettanomyces contamination, volatile acidity, and hydrogen sulfide. Cork taint is caused primarily by 2,4,6-trichloroanisole (TCA), a compound formed through the interaction of plant phenols, chlorine, and mold; it produces musty, wet-cardboard aromas and, at lower concentrations, simply strips a wine of its fruit character. Oxidation develops from excessive oxygen contact, browning whites and dulling fruit in reds. Reduction occurs when volatile sulfur compounds accumulate, producing rotten egg, burnt match, or cooked cabbage aromas from hydrogen sulfide, mercaptans, or related compounds.

• Brettanomyces produces 4-ethylphenol and 4-ethylguaiacol, creating band-aid, medicinal, barnyard, and leather notes; 4-ethylguaiacol can also contribute clove or smoky character at its lower threshold of approximately 110-158 µg/L
• Volatile acidity becomes perceptibly faulty above approximately 700 mg/L acetic acid, with vinegary or nail-polish remover (ethyl acetate) aromas intensifying toward and beyond legal limits
• Hydrogen sulfide has a detection threshold of approximately 1-2 µg/L; if not removed during winemaking it can react with ethanol to form mercaptans, which are even harder to eliminate
• Premature oxidation (premox) affects white wines that oxidize far earlier than expected, showing browning color and bruised apple, honeyed, or waxy aromas before reaching their intended drinking window

Fault identification requires systematic sensory evaluation supported by chemical knowledge. Begin with appearance: oxidized whites turn deep gold or brown; prematurely oxidized whites show similar browning well before their expected peak. Assess the bouquet methodically for specific fault indicators. Cork taint presents as musty, damp cardboard, or mold-like aromas and can also simply mute a wine's fruit at lower concentrations. Hydrogen sulfide smells of rotten eggs; mercaptans present as rubber, onion, or garlic. Brettanomyces produces band-aid, medicinal, or horse stable aromas. Volatile acidity manifests as sharp vinegar or ethyl acetate. Cross-referencing sensory observations with bottle history, storage conditions, and closure type sharpens diagnosis.

• Cork taint: musty, damp cardboard, or mold aromas; at low levels, simply a muted, dull wine without obvious off-note
• Oxidation: browning color, flat or lifeless fruit, nutty or sherried character in wines not intentionally oxidized
• Reduction: rotten egg (H2S), burnt match (SO2), or cooked cabbage; often dissipates with aeration if H2S is the primary compound
• Brett: band-aid, medicinal, barnyard, or leather alongside earthy notes; distinguished from complexity by chemical intensity and context

Professional fault identification protects retailers, restaurants, and collectors from serving defective wines and allows appropriate remedies such as refunds or replacements. For sommeliers and educators, accurate fault recognition builds credibility and enables confident wine recommendations while correctly distinguishing faults from legitimate stylistic variation. Understanding faults also illuminates modern winemaking advances: improved TCA screening, inert gas systems, temperature-controlled storage, and alternative closures such as screw caps and DIAM technical corks have collectively reduced fault incidence compared to earlier decades. At a commercial scale, even a 3% TCA contamination rate across cork-sealed wines represents a staggering number of flawed bottles reaching consumers every year.

• Restaurant service: trained staff who identify faults before or during service maintain reputation and protect the guest experience
• Collector protection: fault knowledge helps investors identify at-risk bottles affecting cellar value and appropriate drinking windows
• Producer quality control: fault pattern analysis allows winemakers to trace systemic issues to specific vineyard, cellar, or closure protocols
• Consumer confidence: cork taint can cause consumers to perceive a fundamentally sound wine as simply not very good, harming both producer reputation and critic credibility

Cork taint's prevalence has historically been disputed, with estimates ranging from 1-2% (cork industry) to as high as 9.5% in some professional tasting contexts in the late 2000s; the Cork Quality Council consistently reports approximately 3% of corks contaminated by TCA. White Burgundy's premox phenomenon became widely recognized in the early 2000s when bottles from the 1995 to 2002 vintages were found to be prematurely oxidized. Research and expert opinion converged on a multifactorial explanation: a trend toward lower SO2 in pursuit of fresher wines, variable cork quality, changes in pressing technology, and potentially lower antioxidant levels in grapes. By the mid-2000s, many Burgundy producers had increased free SO2 at bottling in response, and the widespread adoption of technically consistent closures such as DIAM has further improved the situation.

• TCA was first identified as the principal cause of cork taint in 1982; by 2013 the Cork Quality Council reported an 81% reduction in TCA levels in natural cork shipments compared to eight years earlier
• White Burgundy premox most severely affected vintages from 1995 to 2002, with wines showing advanced oxidation at only 5-7 years of age
• Contributing factors to Burgundy premox included reduced SO2 use, variable cork oxygen permeability, and changes in pneumatic pressing technology adopted through the 1990s
• Natural wine movement trends toward minimal SO2 have increased the relevance of fault thresholds in professional evaluation contexts

Modern winemakers employ preventative strategies including rigorous sanitation, careful SO2 management, temperature monitoring, adequate yeast nutrient additions, and quality closure selection. The cork industry has improved TCA testing significantly, with 90% of natural cork shipments showing TCA values under 1.0 ppt in 2013 Cork Quality Council tests; technical closures such as DIAM and screw caps provide near-zero TCA contamination risk. Rectification options are limited. Reduction (H2S) may respond to aeration or copper sulfate fining if treated early, before conversion to mercaptans. Volatile acidity can be reduced by reverse osmosis or blending with low-VA wine. Cork taint, advanced oxidation, and mercaptan faults are largely irreversible. GC-MS laboratory analysis can quantify specific compounds, enabling producers to diagnose root causes systematically.

• Prevention: adequate SO2 management, temperature control, sanitation protocols, and technically consistent closures minimize the major fault categories
• H2S remediation: aeration or copper sulfate fining during winemaking can be effective if applied before the fault converts to mercaptans or disulfides
• VA reduction: reverse osmosis or blending with low-VA wine are the primary remediation options; prevention through oxygen exclusion and sanitation is far preferable
• Laboratory analysis: GC-MS identification of specific compounds allows producers to trace systemic problems and implement targeted corrections across future vintages