Wine Defects: Diagnosis and Prevention

Problem Definition

Wine defects range from subtle quality-reducing faults to severe contamination rendering wine undrinkable. Accurate diagnosis requires understanding the sensory characteristics, chemical causes, and analytical thresholds of each defect. Prevention strategies begin in the vineyard and continue through fermentation, aging, and bottling. Distinguishing true defects from stylistic choices (e.g., reduction as style vs. fault) requires contextual understanding and threshold awareness.

Technical Context

Defect Categories

Microbial Defects:

Brettanomyces contamination
Volatile acidity (acetic acid)
Lactic acid bacteria spoilage
Refermentation

Chemical/Oxidative Defects:

Oxidation
Reduction
Cork taint (TCA)
Light strike

Fermentation Defects:

Stuck fermentation
Excessive sulfur compounds
Off-yeast aromas

Options and Interventions

Brettanomyces (“Brett”)

Sensory Characteristics:

Barnyard, horse stable
Medicinal, band-aid
Smoky, clove
“Mouse” (in severe cases)

Compounds:

4-Ethylphenol (4-EP): >400 µg/L detectable
4-Ethylguaiacol (4-EG): Smoky, spicy

Prevention:

Hygiene (barrel, equipment sanitization)
SO₂ management (molecular SO₂ >0.5 mg/L)
pH control (lower pH inhibits growth)
Avoid residual sugar in dry wines
Barrel rotation/replacement

Detection: PCR, plating, sensory evaluation

Volatile Acidity (VA)

Sensory Characteristics:

Vinegar (acetic acid)
Nail polish remover (ethyl acetate)
Sharp, pungent

Thresholds:

Acetic acid perception: ~0.7 g/L
Legal limits: Typically <1.2 g/L (varies by region)
Desirable (lift): 0.3-0.5 g/L

Causes:

Acetic acid bacteria (Acetobacter)
Stressed/dying yeast
Warm temperatures
Oxygen exposure

Prevention: See Volatile Acidity Prevention

Oxidation

Sensory Characteristics:

Browning (visual)
Sherry-like (aldehydes)
Flat, tired, lacking freshness
Bruised apple (acetaldehyde)

Compounds:

Acetaldehyde: >20-30 mg/L problematic
Color shift (browning)
Loss of varietal aromatics

Prevention: See Oxidation Management

Reduction

Sensory Characteristics:

Rotten eggs (H₂S)
Struck match (SO₂)
Rubber, onion, garlic (mercaptans)
Cabbage, cooked vegetables

Compounds:

H₂S: >1-2 µg/L detectable
Mercaptans: Various thresholds
Disulfides: Often permanent

Prevention: See Reduction & Sulfide Management

Cork Taint (TCA/TBA)

Sensory Characteristics:

Musty, moldy
Wet cardboard
Damp cellar
Suppressed fruit (even at sub-threshold)

Compound:

2,4,6-Trichloroanisole (TCA)
Detection threshold: 1.5-3 ng/L (varies individually)
Some perceive at <1 ng/L

Causes:

Cork contamination
Winery environmental contamination
Chlorophenol precursors + microorganisms

Prevention:

Quality cork sourcing
Alternative closures (screwcap, synthetic)
Winery environmental testing
Avoid chlorine-based sanitizers

Light Strike (Goût de Lumière)

Sensory Characteristics:

Reduced sulfur (similar to reduction)
Wet wool, cabbage
Develops rapidly under light exposure

Compounds:

Dimethyl disulfide
Methanethiol
Photodegradation products

Susceptibility:

Champagne and sparkling wines (high risk)
White wines in clear glass
Riboflavin (B2) catalyzes reaction

Prevention:

Dark glass or UV-filtering bottles
Limit light exposure during storage
Minimize riboflavin content

Refermentation

Sensory Characteristics:

Spritzy, effervescent (unintended)
Cloudy (yeast cells)
Changed sweetness level
Possible off-odors

Causes:

Residual sugar + viable yeast
Insufficient sterile filtration
Warm storage temperatures

Prevention: See Residual Sugar Management

Trade-offs and Risks

Threshold Considerations

Sub-Threshold Effects:

Some compounds suppress fruit below detection
TCA particularly insidious
Combined effects (multiple low-level compounds)

Individual Variation:

Sensitivity varies by compound and person
Training improves detection
Anosmia (inability to smell) varies

Stylistic vs. Fault

Context-Dependent:

Oxidation: Fault in Chablis; style in Sherry
VA: Fault at high levels; “lift” at low levels
Brett: Controversial (some consider low levels acceptable)
Reduction: Fault or terroir expression (debated)

Practical Implications

Sensory Training

Detection Practice:

Spike training wines with known compounds
Calibrate thresholds
Regular panel calibration
Document individual sensitivities

Prevention Program

Critical Control Points:

Vineyard: Healthy fruit; no rot
Crush: Hygiene; SO₂ addition
Fermentation: Temperature; nutrition; completion
Aging: Oxygen management; barrel hygiene
Bottling: Sterile filtration; closure selection

Corrective Actions

Defect	Correction Possibility
Early reduction	Copper fining; aeration
Light VA	Blending with clean wine
Brett (early)	Sterile filtration
Oxidation	Cannot reverse
TCA	Cannot remove (wine affected)

References

Waterhouse, A.L., Sacks, G.L., & Jeffery, D.W. (2016). “Understanding Wine Chemistry.” Wiley. Publisher Link DOI: 10.1002/9781118730720
Fugelsang, K.C. & Edwards, C.G. (2007). “Wine Microbiology.” Springer. DOI: 10.1007/978-0-387-33349-6
Goode, J. & Harrop, S. (2011). “Authentic Wine: Toward Natural and Sustainable Winemaking.” University of California Press. Publisher Link
Polášková, P., et al. (2008). “Wine flavor: Chemistry.” American Journal of Enology and Viticulture, 59(3), 249-262. AJEV Link

Last Updated: January 6, 2026