Wine Faults — Advanced Identification and Chemistry
In-depth guide to wine faults for Level 3 — causes, chemistry, threshold levels, and tasting note language
Learning Objectives
- Identify all major wine faults by their chemical cause and sensory characteristics
- Explain detection thresholds and why the same fault may be acceptable at low levels
- Describe the production conditions that increase the risk of each fault
- Write precise fault assessments using correct SAT vocabulary
Oxidation — Types and Detection
Oxidation results from uncontrolled exposure to oxygen. The primary compound is acetaldehyde, which gives a flat, bruised apple character. In white wines, oxidation causes browning (anthocyanins oxidise to golden-amber pigments), loss of fresh fruit, and development of sherry-like, nutty, flat aromas. In reds, oxidation leads to brick/tawny colour changes, flat, dried fruit character, and reduced freshness. Micro-oxygenation is a controlled winemaking technique that deliberately introduces small amounts of oxygen; this is not a fault. Premature oxidation (PO or premox) is a specific issue with some white Burgundies sealed under cork that develops oxidative characteristics far earlier than expected — still debated as to exact cause.
Brettanomyces — Threshold and Context
Brettanomyces (Brett) is a wild yeast that produces 4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG), responsible for barnyard, leather, horse, band-aid, and medicinal aromas. Detection thresholds vary by individual: most people detect Brett at 400–600 µg/L 4-EP, but some are much more sensitive. Below threshold, Brett may add complexity; above, it can overwhelm fruit character entirely. Brett is found in poorly maintained cellar equipment (oak barrels are a key reservoir), and warm temperatures during ageing encourage its growth. Some Old World wine regions (certain red Burgundies, old-style Rhône) have historically tolerated low levels as a regional character.
Volatile Acidity, Ethyl Acetate, and Other Chemical Faults
Volatile acidity (VA) is primarily acetic acid, produced by acetic acid bacteria under aerobic conditions. Small amounts are normal; high VA creates vinegary, sharp character. Ethyl acetate (the ester of ethanol and acetic acid) accompanies high VA and gives a nail polish/solvent aroma. TCA (2,4,6-trichloroanisole) causes cork taint — wet cardboard, mould, suppression of fruit. Geosmin creates earthy/beetroot aromas, often from contaminated equipment. Hydrogen sulphide (H₂S) causes rotten egg aromas from reductive winemaking. Mercaptans (thiols) produce onion, rubber, and other sulphurous notes. Geraniol degradation by lactic acid bacteria produces a geranium/rose note (geranium taint) — often associated with sorbic acid treatment.
Assessment in Tasting Notes
In Level 3 exam tasting notes, faults must be identified precisely: state the fault, its sensory character, the likely compound, and the impact on quality. "The wine shows evidence of cork taint (TCA) — a pronounced damp cardboard and mouldy aroma that suppresses the underlying fruit. This is a significant fault, reducing the wine to faulty quality" is a complete fault assessment. Do not guess if unsure — stating "I detect possible oxidation" acknowledges uncertainty appropriately. Faults that reduce quality below "acceptable" must be noted in the conclusions. Low-level Brett or VA that adds complexity without dominating can be noted as a character rather than a fault, depending on wine style.
Key Vocabulary
Exam Question Examples
Is Brett always a fault? How does this affect your tasting note assessment?
Approach
Brett is not always a fault — at sub-threshold concentrations, it may add complexity. The key question is whether it enhances or detracts from the wine's character and whether it is appropriate to the wine style. In your tasting note: if Brett is detectable but integrated (e.g. a savoury, leathery note complementing the fruit), note it as a characteristic and assess quality normally. If Brett dominates, suppresses fruit, and makes the wine unpleasant, assess as faulty or poor quality and state why. Always note the specific aroma descriptor (barnyard, leather, etc.) and link it to Brett.
Quick Summary
- 1.Oxidation: acetaldehyde → flat, nutty, browning — colour shift to golden (whites) or brick (reds)
- 2.Cork taint (TCA): damp cardboard/mould, fruit suppression — not fixable
- 3.Brett (4-EP): barnyard/leather — contextual fault, may be below detection threshold
- 4.VA: acetic acid (vinegar) + ethyl acetate (nail polish) — from aerobic bacteria exposure
- 5.H₂S: rotten eggs — often temporary reduction, may dissipate with aeration
- 6.In SAT: name fault → sensory character → compound → quality impact
Practice questions on this topic
Frequently Asked Questions
- How can you tell the difference between reduction and cork taint at a glance?
- The key diagnostic is aeration: reduction aromas (rotten egg, rubber, struck flint) often dissipate with swirling and aeration in the glass — improved or gone after 10 minutes open. Cork taint (TCA) does not improve with aeration; the damp cardboard character persists or worsens. Additionally, TCA suppresses fruit aromas entirely, while reduction may coexist with underlying fruit that becomes apparent after the sulphur notes dissipate.
- What is the legal limit for volatile acidity in wine?
- EU regulations set legal limits for VA in finished wine: for dry wines, typically 1.08g/L acetic acid for white/rosé and 1.2g/L for red wines. These limits exist to protect consumers from excessively faulty wine. Sensory detection of VA typically begins around 0.6–0.7g/L — well below the legal limit — so VA can be perceptible as a fault at levels still legal.
Consolidate your knowledge
Use Vinlecta to practise exam-style questions on wine faults — advanced identification and chemistry and related topics under timed conditions.
Related study guides: