Fault Diagnosis — The Clinic
skill-metadata
- skill-id: sommelier/tasting-evaluation/fault-diagnosis
- skill-type: knowledge
- parent: sommelier/tasting-evaluation
- version: 1.0.0
description
Provides systematic identification and verdict for every recognized wine fault and flaw, from TCA cork taint to mousiness. Covers compound chemistry, sensory thresholds, cause, and the "send it back?" decision. Distinguishes flaws (minor, drinkable) from faults (major, unacceptable) and addresses the natural wine style ambiguity zone.
When This Applies
Use this skill when the user:
- Detects an off aroma or taste and wants to identify it
- Is unsure whether to return a bottle in a restaurant
- Is studying fault recognition for CMS, WSET, or sommelier exams
- Wants to understand what causes wine faults at a technical level
- Encounters a fault in a wine they are evaluating on the deductive grid (nose condition: flawed)
- Is a professional evaluating a wine and needs to make a confident fault verdict
The Flaw vs Fault Distinction
This distinction is not semantic — it determines your action.
Flaw: A minor defect present at low levels. The wine remains drinkable and the flaw may even contribute complexity in some contexts (low-level Brett in a traditional Bordeaux; slight reduction in a natural wine that blows off with air). A flaw does not automatically warrant returning a bottle.
Fault: A major defect that renders the wine unacceptable for service. A faulty wine should be replaced in restaurant service without question. At home, there is no recourse except to discard the bottle. A fault is not a matter of preference — it is a technical failure.
Critical caveat before diagnosing: Never blame the wine until you have ruled out:
- The glass — Dirty, soapy, or odor-contaminated glasses transmit false aromas. Rinse with the wine itself (wine conditioning) if suspected.
- Temperature — Wine served too cold suppresses aromas and can mask faults; wine served too warm exaggerates alcohol and tannin, and can mimic certain faults. Reassess at correct serving temperature.
- The taster's palate — Certain aromas have dramatically variable detection thresholds between individuals (TCA sensitivity can vary 1000-fold). If a taster suspects a fault but others do not detect it, assess the wine with more tasters before condemning it. Equally, some tasters are blind to Brett or DMS.
- Context of style — Natural wines, orange wines, and some traditional regional styles will show aromas (reduction, oxidation, Brett) that are intentional stylistic choices. Know the wine's context before diagnosing a "fault."
The Fault Taxonomy
| # |
Fault |
Compound |
Detection Threshold |
Sensory Descriptors |
Cause |
Verdict |
| 1 |
Cork Taint |
2,4,6-Trichloroanisole (TCA) |
1.4–6 ng/L (extremely low) |
Wet cardboard, damp basement, musty newspaper, wet dog, suppression of fruit |
Mould (Armillaria, Trichoderma) metabolizing chlorine-bleached cork or winery surfaces |
FAULT — always replace |
| 2 |
Oxidation |
Acetaldehyde (primary) + multiple oxidation products |
100–125 mg/L for acetaldehyde |
Roasted nuts (walnut, hazelnut), dried straw, bruised apple, sherry-like (if intentional), flat, lifeless fruit |
Oxygen exposure — faulty closure, poor handling, warm storage, damaged cork |
FAULT if unintentional; INTENTIONAL in Sherry, Vin Jaune, Madeira, Tawny Port |
| 3 |
Volatile Acidity (VA) |
Acetic acid (primary); also ethyl acetate |
Acetic acid: ~700 mg/L (perception); fault > 1.2 g/L in dry red |
Vinegar, pickles, acetic sharpness on back palate |
Acetobacter bacteria converting alcohol to acetic acid; poor sulfite management; oxygen exposure |
FLAW at low levels; FAULT above 1.2 g/L (EU legal max); return if dominant |
| 4 |
Ethyl Acetate |
Ethyl acetate |
~150–200 mg/L |
Nail polish remover, paint thinner, model airplane glue, solvent note |
Acetic acid + ethanol esterification; spoilage yeasts (Pichia, Candida); VAB |
FLAW at low levels (may integrate); FAULT at high levels — harsh, persistent solvent |
| 5 |
Reduction |
Hydrogen sulfide (H2S) |
8–10 μg/L |
Rotten eggs, garlic, sewage, hard-boiled eggs |
Sulfur-limited or nitrogen-limited fermentation; yeast stress; lees contact; some varieties (Merlot, some Riesling) retain it stylistically |
FLAW — often correctable: aerate (decant, swirl) or copper contact (drop a clean copper coin or use commercial copper disc). If persists after aeration: fault |
| 6 |
Mercaptans / Thiols (elevated) |
Methanethiol, ethanethiol, and other thiols |
~1.5–5 μg/L depending on compound |
Onion, rubber, skunk, drains, putrid, garlic (more intense than H2S alone) |
H2S reacting with wine components; excessive lees contact; certain reductive winemaking |
FAULT — unlike H2S, mercaptans do not blow off with aeration alone |
| 7 |
Brettanomyces (4-EP) |
4-Ethylphenol |
>140 μg/L |
Bandaid, leather, barnyard, horse stable, animal, blood |
Brettanomyces yeast (esp. in barrel); high residual sugar, poor cellar hygiene |
FLAW at low levels (adds terroir complexity, especially in Old World reds); FAULT above ~600 μg/L when it suppresses fruit |
| 8 |
Brettanomyces (4-EG) |
4-Ethylguaiacol |
>600 μg/L |
Bacon, smoked meat, cloves, spice |
Brettanomyces yeast co-production with 4-EP |
Generally lower concentration than 4-EP; adds a smoky-spicy character at low levels; fault when dominant |
| 9 |
Excess SO₂ (Free) |
Sulfur dioxide |
Perception ~30–50 mg/L free SO₂ |
Matchstick, burnt rubber, striking a match, prickly sensation in nose |
Over-addition at bottling or late-stage addition; poor winemaking calibration |
FLAW — often dissipates with aeration; decant and reassess. If persistent: unacceptable |
| 10 |
Dimethyl Sulfide (DMS) |
Dimethyl sulfide |
>30 μg/L (white); >50 μg/L (red) |
Cooked cabbage, canned corn, asparagus, creamed corn |
Breakdown of sulfur-containing amino acids (S-methyl methionine) during aging; common in reductive aging |
FLAW — low levels can add complexity (some Riesling, some Burgundy); fault at dominant levels |
| 11 |
Heat Damage |
Multiple: increased acetaldehyde, ethyl acetate, HMF |
N/A — threshold varies by wine |
Cooked, jammy, stewed, brick-orange color in reds, seeping cork or pushed-up cork, flat finish |
Storage or transport above 18°C (especially > 25°C) |
FAULT — heat damage is irreversible. Check for seeping cork. Cannot be salvaged. |
| 12 |
Lightstrike |
Sulfur compounds (including methanethiol) from riboflavin photodegradation |
Variable |
Wet cardboard, wet wool, "freshly struck match with a wet quality," sometimes indistinguishable from reduction |
UV light exposure (wavelengths 325–450 nm) degrading riboflavin; Champagne in clear or very pale glass especially vulnerable |
FAULT — irreversible. Store Champagne and delicate whites away from UV light (dark/colored glass preferred). |
| 13 |
Geosmin |
Geosmin |
~10 parts per trillion (extremely low) |
Earthy, beetroot, musty earth, garden soil, wet earth after rain (petrichor-adjacent) |
Actinomycetes bacteria or botrytis cinerea on grapes; rare in wine, more common in poor-vintage fruit |
FLAW — very low levels may integrate as terroir character; at detectable levels in wine, indicates poor fruit selection |
| 14 |
Excess Diacetyl |
Diacetyl |
> 5 mg/L |
Excessive butter, butterscotch, artificial butter, popcorn |
Oenococcus oeni (MLF bacteria) metabolizing citric acid; excessive MLF or blocked secondary fermentation |
FLAW — low diacetyl (< 2 mg/L in reds) is acceptable and common in oaked whites; excessive amounts create an artificial, cloying quality |
| 15 |
Geranium Taint |
2-Ethoxyhexa-3,5-diene |
~1 ng/L |
Geranium leaf, crushed geranium, floral-chemical hybrid |
Lactic acid bacteria (LAB) metabolizing potassium sorbate (preservative) added to sweet wines |
FAULT — distinctive and unmistakable; indicates improper use of sorbate in the presence of LAB |
| 16 |
Mousiness |
Tetrahydropyridines (2-acetyl-1-pyrroline and related) |
Non-volatile at wine pH; perceived only after warming on the back palate |
Mouse cage, mouse urine, popcorn-urine hybrid — detected on the back palate 10–15 seconds after swallowing, NOT on the nose |
Brettanomyces and/or Lactobacillus; common in natural wines with no SO₂ management |
FAULT — the distinctive back-palate activation is pathognomonic. Not detectable on nose alone; must be tasted and assessed on the back palate. Cannot be corrected. |
| 17 |
Ladybug Taint (IPMP) |
2-Isopropyl-3-methoxypyrazine (IPMP) |
A few parts per billion |
Rancid peanut butter, green pepper at high intensity, asparagus, bean |
Asian multicolored lady beetle (Harmonia axyridis) contaminating harvested fruit; a few beetles per ton of fruit can taint a wine |
FAULT — pyrazine-based; persistent and irreversible; a harvest and sorting-table problem |
Diagnostic Protocol
When a wine shows an off character, proceed systematically:
Step 1 — Assess at Correct Temperature
Faults are best detected at or slightly above ideal service temperature (reds at 16–18°C, whites at 10–12°C). A wine too cold will mask TCA and volatile aromas. A wine too warm will exaggerate alcohol and can mimic certain faults.
Step 2 — Check the Glass
Pour a small amount of the wine into a different, clean glass. If the off character disappears or diminishes significantly, the glass was contaminated (soap residue, storage odors). Never condemn a bottle on a single glass.
Step 3 — Nose Before Tasting
Most faults are detectable on the nose before the palate. Work through the fault taxonomy by aroma category:
- Musty/cardboard/wet → TCA (fault)
- Vinegar/solvent → VA/ethyl acetate (flaw to fault)
- Egg/garlic/sulfur → Reduction/H2S or mercaptans (flaw to fault)
- Barnyard/bandaid → Brett (flaw to fault by level)
- Matchstick/burnt rubber → Excess SO₂ (flaw)
- Cooked/stewed/flat → Heat damage (fault)
- Geranium → Geranium taint (fault)
Step 4 — Taste
Some faults are only confirmed on the palate:
- Mousiness — Only detectable on the back palate 10–15 seconds post-swallow; not perceptible on the nose at wine pH
- VA level — The intensity of acetic acid bite is better assessed on the finish than the nose
- Excess alcohol heat vs reduction — Both create a "prickly" sensation but in different locations (throat = alcohol; nose/front palate = SO₂)
Step 5 — Apply the Threshold Test
Identify the compound. Check: is the sensory intensity above or below the flaw-to-fault threshold? For Brett: is it below 140 μg/L (4-EP sensory threshold)? At low levels, consider whether it integrates with the wine's character. For VA: is it below 1.2 g/L? Above that threshold in a dry red, it is legally a fault in most jurisdictions.
Step 6 — Aerate and Reassess
For reduction (H2S), excess SO₂, and some DMS: vigorous decanting or swirling may resolve the issue. Reassess after 10–15 minutes of air exposure. If the fault persists after aeration, the compound is likely mercaptan-class (not aeration-responsive) or the level is too high to resolve.
"Send It Back?" Decision Guide
Always Replace
- TCA / Cork Taint — No threshold at which TCA is acceptable in a restaurant context. Return immediately. The bottle cannot be corrected.
- Heat Damage — Irreversible. A cooked, flat, maderized wine with a pushed-up cork has suffered during storage or transport. Not the consumer's fault; the bottle should be replaced.
- Mousiness — Unmistakable, irreversible, and unacceptable above trace levels.
- Geranium Taint — Distinctive, irreversible; a production fault.
- VA above fault threshold (> 1.2 g/L dry red) — When the vinegar note is dominant and persistent.
- Oxidation (unintentional) — A "fresh" wine that has lost all primary fruit and smells like sherry is faulty unless the style is intentionally oxidative.
Assess and Explain — May Not Need Replacing
- Low-level Brett (below fault threshold) — In many Old World reds, a barnyard/leather note is part of the wine's character. Explain this to the guest; assess whether it is at a complexity-enhancing or fault level. If the guest finds it unpleasant, replace it.
- Reduction (H2S) in a natural wine — Many natural wines show some reduction on opening. Pour, swirl, and aerate. Reassess in 10 minutes. If it blows off, proceed. If it persists at unpleasant levels, replace.
- Excess SO₂ — Decant and aerate. Assess after 15 minutes. If it resolves, proceed. If not, replace.
- Slight VA — Low-level volatile acidity can add complexity. If the guest notices it and is bothered, offer to replace. If it is within acceptable limits and the guest is enjoying the wine, note it without alarm.
Style vs Fault: The Natural Wine Context
Natural wines often deliberately show characters that trained tasters are taught to associate with faults:
- Reduction — Many natural winemakers ferment and age without SO₂, and some reduction is expected on opening. It blows off. It is not a fault in this context.
- Brettanomyces — Some natural winemakers accept low-level Brett as part of terroir expression and minimal intervention. At low levels in this context, it may be intentional.
- Slight oxidation — Orange wines and some natural whites are exposed to oxygen intentionally. An amber color and walnut/sherry note in a natural white may be a feature.
- Cloudiness — Unfiltered, unfined wines are intentionally hazy. This is not a fault; it is a stylistic and philosophical choice.
Protocol: Before diagnosing a natural wine as faulty, confirm: (1) Is the level of the compound above the fault threshold? (2) Is the wine still balanced and pleasurable? (3) Does the producer's documented style include this characteristic? If uncertain, present the wine with context and allow the guest to decide.
Learn Block
After substantive responses using this skill:
Learn ─── Quality Assessment
Next: Once you can identify when a wine is flawed, the natural next step is learning how to assess quality in wines that are clean — BLIC, scoring systems, and readiness windows.
1---2name: fault-diagnosis3description: Identify and diagnose wine faults and flaws using a complete taxonomy of 17+ compounds with detection thresholds and sensory descriptors. Use when a wine smells or tastes wrong, when deciding whether to send a bottle back, or when distinguishing intentional natural wine style from genuine faults.4---56# Fault Diagnosis — The Clinic78## skill-metadata9- skill-id: sommelier/tasting-evaluation/fault-diagnosis10- skill-type: knowledge11- parent: sommelier/tasting-evaluation12- version: 1.0.01314## description15Provides systematic identification and verdict for every recognized wine fault and flaw, from TCA cork taint to mousiness. Covers compound chemistry, sensory thresholds, cause, and the "send it back?" decision. Distinguishes flaws (minor, drinkable) from faults (major, unacceptable) and addresses the natural wine style ambiguity zone.1617---1819## When This Applies2021Use this skill when the user:22- Detects an off aroma or taste and wants to identify it23- Is unsure whether to return a bottle in a restaurant24- Is studying fault recognition for CMS, WSET, or sommelier exams25- Wants to understand what causes wine faults at a technical level26- Encounters a fault in a wine they are evaluating on the deductive grid (nose condition: flawed)27- Is a professional evaluating a wine and needs to make a confident fault verdict2829---3031## The Flaw vs Fault Distinction3233This distinction is not semantic — it determines your action.3435**Flaw**: A minor defect present at low levels. The wine remains drinkable and the flaw may even contribute complexity in some contexts (low-level Brett in a traditional Bordeaux; slight reduction in a natural wine that blows off with air). A flaw does not automatically warrant returning a bottle.3637**Fault**: A major defect that renders the wine unacceptable for service. A faulty wine should be replaced in restaurant service without question. At home, there is no recourse except to discard the bottle. A fault is not a matter of preference — it is a technical failure.3839**Critical caveat before diagnosing**: Never blame the wine until you have ruled out:401. **The glass** — Dirty, soapy, or odor-contaminated glasses transmit false aromas. Rinse with the wine itself (wine conditioning) if suspected.412. **Temperature** — Wine served too cold suppresses aromas and can mask faults; wine served too warm exaggerates alcohol and tannin, and can mimic certain faults. Reassess at correct serving temperature.423. **The taster's palate** — Certain aromas have dramatically variable detection thresholds between individuals (TCA sensitivity can vary 1000-fold). If a taster suspects a fault but others do not detect it, assess the wine with more tasters before condemning it. Equally, some tasters are blind to Brett or DMS.434. **Context of style** — Natural wines, orange wines, and some traditional regional styles will show aromas (reduction, oxidation, Brett) that are intentional stylistic choices. Know the wine's context before diagnosing a "fault."4445---4647## The Fault Taxonomy4849| # | Fault | Compound | Detection Threshold | Sensory Descriptors | Cause | Verdict |50|---|---|---|---|---|---|---|51| 1 | Cork Taint | 2,4,6-Trichloroanisole (TCA) | 1.4–6 ng/L (extremely low) | Wet cardboard, damp basement, musty newspaper, wet dog, suppression of fruit | Mould (Armillaria, Trichoderma) metabolizing chlorine-bleached cork or winery surfaces | FAULT — always replace |52| 2 | Oxidation | Acetaldehyde (primary) + multiple oxidation products | 100–125 mg/L for acetaldehyde | Roasted nuts (walnut, hazelnut), dried straw, bruised apple, sherry-like (if intentional), flat, lifeless fruit | Oxygen exposure — faulty closure, poor handling, warm storage, damaged cork | FAULT if unintentional; INTENTIONAL in Sherry, Vin Jaune, Madeira, Tawny Port |53| 3 | Volatile Acidity (VA) | Acetic acid (primary); also ethyl acetate | Acetic acid: ~700 mg/L (perception); fault > 1.2 g/L in dry red | Vinegar, pickles, acetic sharpness on back palate | Acetobacter bacteria converting alcohol to acetic acid; poor sulfite management; oxygen exposure | FLAW at low levels; FAULT above 1.2 g/L (EU legal max); return if dominant |54| 4 | Ethyl Acetate | Ethyl acetate | ~150–200 mg/L | Nail polish remover, paint thinner, model airplane glue, solvent note | Acetic acid + ethanol esterification; spoilage yeasts (Pichia, Candida); VAB | FLAW at low levels (may integrate); FAULT at high levels — harsh, persistent solvent |55| 5 | Reduction | Hydrogen sulfide (H2S) | 8–10 μg/L | Rotten eggs, garlic, sewage, hard-boiled eggs | Sulfur-limited or nitrogen-limited fermentation; yeast stress; lees contact; some varieties (Merlot, some Riesling) retain it stylistically | FLAW — often correctable: aerate (decant, swirl) or copper contact (drop a clean copper coin or use commercial copper disc). If persists after aeration: fault |56| 6 | Mercaptans / Thiols (elevated) | Methanethiol, ethanethiol, and other thiols | ~1.5–5 μg/L depending on compound | Onion, rubber, skunk, drains, putrid, garlic (more intense than H2S alone) | H2S reacting with wine components; excessive lees contact; certain reductive winemaking | FAULT — unlike H2S, mercaptans do not blow off with aeration alone |57| 7 | Brettanomyces (4-EP) | 4-Ethylphenol | >140 μg/L | Bandaid, leather, barnyard, horse stable, animal, blood | Brettanomyces yeast (esp. in barrel); high residual sugar, poor cellar hygiene | FLAW at low levels (adds terroir complexity, especially in Old World reds); FAULT above ~600 μg/L when it suppresses fruit |58| 8 | Brettanomyces (4-EG) | 4-Ethylguaiacol | >600 μg/L | Bacon, smoked meat, cloves, spice | Brettanomyces yeast co-production with 4-EP | Generally lower concentration than 4-EP; adds a smoky-spicy character at low levels; fault when dominant |59| 9 | Excess SO₂ (Free) | Sulfur dioxide | Perception ~30–50 mg/L free SO₂ | Matchstick, burnt rubber, striking a match, prickly sensation in nose | Over-addition at bottling or late-stage addition; poor winemaking calibration | FLAW — often dissipates with aeration; decant and reassess. If persistent: unacceptable |60| 10 | Dimethyl Sulfide (DMS) | Dimethyl sulfide | >30 μg/L (white); >50 μg/L (red) | Cooked cabbage, canned corn, asparagus, creamed corn | Breakdown of sulfur-containing amino acids (S-methyl methionine) during aging; common in reductive aging | FLAW — low levels can add complexity (some Riesling, some Burgundy); fault at dominant levels |61| 11 | Heat Damage | Multiple: increased acetaldehyde, ethyl acetate, HMF | N/A — threshold varies by wine | Cooked, jammy, stewed, brick-orange color in reds, seeping cork or pushed-up cork, flat finish | Storage or transport above 18°C (especially > 25°C) | FAULT — heat damage is irreversible. Check for seeping cork. Cannot be salvaged. |62| 12 | Lightstrike | Sulfur compounds (including methanethiol) from riboflavin photodegradation | Variable | Wet cardboard, wet wool, "freshly struck match with a wet quality," sometimes indistinguishable from reduction | UV light exposure (wavelengths 325–450 nm) degrading riboflavin; Champagne in clear or very pale glass especially vulnerable | FAULT — irreversible. Store Champagne and delicate whites away from UV light (dark/colored glass preferred). |63| 13 | Geosmin | Geosmin | ~10 parts per trillion (extremely low) | Earthy, beetroot, musty earth, garden soil, wet earth after rain (petrichor-adjacent) | Actinomycetes bacteria or botrytis cinerea on grapes; rare in wine, more common in poor-vintage fruit | FLAW — very low levels may integrate as terroir character; at detectable levels in wine, indicates poor fruit selection |64| 14 | Excess Diacetyl | Diacetyl | > 5 mg/L | Excessive butter, butterscotch, artificial butter, popcorn | Oenococcus oeni (MLF bacteria) metabolizing citric acid; excessive MLF or blocked secondary fermentation | FLAW — low diacetyl (< 2 mg/L in reds) is acceptable and common in oaked whites; excessive amounts create an artificial, cloying quality |65| 15 | Geranium Taint | 2-Ethoxyhexa-3,5-diene | ~1 ng/L | Geranium leaf, crushed geranium, floral-chemical hybrid | Lactic acid bacteria (LAB) metabolizing potassium sorbate (preservative) added to sweet wines | FAULT — distinctive and unmistakable; indicates improper use of sorbate in the presence of LAB |66| 16 | Mousiness | Tetrahydropyridines (2-acetyl-1-pyrroline and related) | Non-volatile at wine pH; perceived only after warming on the back palate | Mouse cage, mouse urine, popcorn-urine hybrid — detected on the back palate 10–15 seconds after swallowing, NOT on the nose | Brettanomyces and/or Lactobacillus; common in natural wines with no SO₂ management | FAULT — the distinctive back-palate activation is pathognomonic. Not detectable on nose alone; must be tasted and assessed on the back palate. Cannot be corrected. |67| 17 | Ladybug Taint (IPMP) | 2-Isopropyl-3-methoxypyrazine (IPMP) | A few parts per billion | Rancid peanut butter, green pepper at high intensity, asparagus, bean | Asian multicolored lady beetle (Harmonia axyridis) contaminating harvested fruit; a few beetles per ton of fruit can taint a wine | FAULT — pyrazine-based; persistent and irreversible; a harvest and sorting-table problem |6869---7071## Diagnostic Protocol7273When a wine shows an off character, proceed systematically:7475### Step 1 — Assess at Correct Temperature76Faults are best detected at or slightly above ideal service temperature (reds at 16–18°C, whites at 10–12°C). A wine too cold will mask TCA and volatile aromas. A wine too warm will exaggerate alcohol and can mimic certain faults.7778### Step 2 — Check the Glass79Pour a small amount of the wine into a different, clean glass. If the off character disappears or diminishes significantly, the glass was contaminated (soap residue, storage odors). Never condemn a bottle on a single glass.8081### Step 3 — Nose Before Tasting82Most faults are detectable on the nose before the palate. Work through the fault taxonomy by aroma category:83- **Musty/cardboard/wet** → TCA (fault)84- **Vinegar/solvent** → VA/ethyl acetate (flaw to fault)85- **Egg/garlic/sulfur** → Reduction/H2S or mercaptans (flaw to fault)86- **Barnyard/bandaid** → Brett (flaw to fault by level)87- **Matchstick/burnt rubber** → Excess SO₂ (flaw)88- **Cooked/stewed/flat** → Heat damage (fault)89- **Geranium** → Geranium taint (fault)9091### Step 4 — Taste92Some faults are only confirmed on the palate:93- **Mousiness** — Only detectable on the back palate 10–15 seconds post-swallow; not perceptible on the nose at wine pH94- **VA level** — The intensity of acetic acid bite is better assessed on the finish than the nose95- **Excess alcohol heat** vs **reduction** — Both create a "prickly" sensation but in different locations (throat = alcohol; nose/front palate = SO₂)9697### Step 5 — Apply the Threshold Test98Identify the compound. Check: is the sensory intensity above or below the flaw-to-fault threshold? For Brett: is it below 140 μg/L (4-EP sensory threshold)? At low levels, consider whether it integrates with the wine's character. For VA: is it below 1.2 g/L? Above that threshold in a dry red, it is legally a fault in most jurisdictions.99100### Step 6 — Aerate and Reassess101For reduction (H2S), excess SO₂, and some DMS: vigorous decanting or swirling may resolve the issue. Reassess after 10–15 minutes of air exposure. If the fault persists after aeration, the compound is likely mercaptan-class (not aeration-responsive) or the level is too high to resolve.102103---104105## "Send It Back?" Decision Guide106107### Always Replace108109- **TCA / Cork Taint** — No threshold at which TCA is acceptable in a restaurant context. Return immediately. The bottle cannot be corrected.110- **Heat Damage** — Irreversible. A cooked, flat, maderized wine with a pushed-up cork has suffered during storage or transport. Not the consumer's fault; the bottle should be replaced.111- **Mousiness** — Unmistakable, irreversible, and unacceptable above trace levels.112- **Geranium Taint** — Distinctive, irreversible; a production fault.113- **VA above fault threshold (> 1.2 g/L dry red)** — When the vinegar note is dominant and persistent.114- **Oxidation (unintentional)** — A "fresh" wine that has lost all primary fruit and smells like sherry is faulty unless the style is intentionally oxidative.115116### Assess and Explain — May Not Need Replacing117118- **Low-level Brett (below fault threshold)** — In many Old World reds, a barnyard/leather note is part of the wine's character. Explain this to the guest; assess whether it is at a complexity-enhancing or fault level. If the guest finds it unpleasant, replace it.119- **Reduction (H2S) in a natural wine** — Many natural wines show some reduction on opening. Pour, swirl, and aerate. Reassess in 10 minutes. If it blows off, proceed. If it persists at unpleasant levels, replace.120- **Excess SO₂** — Decant and aerate. Assess after 15 minutes. If it resolves, proceed. If not, replace.121- **Slight VA** — Low-level volatile acidity can add complexity. If the guest notices it and is bothered, offer to replace. If it is within acceptable limits and the guest is enjoying the wine, note it without alarm.122123### Style vs Fault: The Natural Wine Context124125Natural wines often deliberately show characters that trained tasters are taught to associate with faults:126- **Reduction** — Many natural winemakers ferment and age without SO₂, and some reduction is expected on opening. It blows off. It is not a fault in this context.127- **Brettanomyces** — Some natural winemakers accept low-level Brett as part of terroir expression and minimal intervention. At low levels in this context, it may be intentional.128- **Slight oxidation** — Orange wines and some natural whites are exposed to oxygen intentionally. An amber color and walnut/sherry note in a natural white may be a feature.129- **Cloudiness** — Unfiltered, unfined wines are intentionally hazy. This is not a fault; it is a stylistic and philosophical choice.130131**Protocol**: Before diagnosing a natural wine as faulty, confirm: (1) Is the level of the compound above the fault threshold? (2) Is the wine still balanced and pleasurable? (3) Does the producer's documented style include this characteristic? If uncertain, present the wine with context and allow the guest to decide.132133---134135## Learn Block136137After substantive responses using this skill:138139```learn140Learn ─── Quality Assessment141Next: Once you can identify when a wine is flawed, the natural next step is learning how to assess quality in wines that are clean — BLIC, scoring systems, and readiness windows.142```
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