Brettanomyces: Detection, Prevention, and Barrel Hygiene

Problem Definition

Brettanomyces bruxellensis (Brett) is a spoilage yeast that produces volatile phenols—primarily 4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG)—that impart “barnyard,” “medicinal,” and “Band-Aid” aromas to wine. At low concentrations, these compounds may add complexity (debated); at elevated levels, they are unambiguous faults.

Brett is particularly problematic in:

• Barrel-aged red wines with residual sugar or high pH
• Cellars with poor hygiene practices
• Wines with low free SO₂
• Extended aging programs (Barolo DOCG, Rioja DOCa Gran Reserva)

Technical Context

Microbiology

Brettanomyces bruxellensis is a slow-growing, highly persistent yeast that:

• Tolerates high ethanol (up to 14-15% v/v)
• Thrives at low pH (3.0-4.0)
• Utilizes residual sugars and cellobiose from oak
• Tolerates moderate SO₂ (though inhibited by molecular SO₂)
• Forms biofilms in barrel pores; difficult to eradicate

Metabolic pathway:

1. Hydroxycinnamic acids (p-coumaric acid, ferulic acid) are released from grape skins or oak
2. Brettanomyces cinnamate decarboxylase converts these to vinyl phenols (4-vinylphenol, 4-vinylguaiacol)
3. Vinyl phenol reductase converts vinyl phenols to ethyl phenols (4-EP, 4-EG)

Sensory Impact

Compound	Sensory Descriptor	Detection Threshold	Recognition Threshold
4-Ethylphenol (4-EP)	Band-Aid, barnyard, horse stable	230-440 μg/L	600-700 μg/L
4-Ethylguaiacol (4-EG)	Smoky, spicy, clove	33-47 μg/L	110-140 μg/L

Ratio consideration:

• 4-EP:4-EG ratio typically 8:1 to 10:1
• Lower ratios (more 4-EG) shift perception toward smoke/spice rather than barnyard
• Total volatile phenol (4-EP + 4-EG) >600 μg/L generally considered faulty

Perception variation:

• Matrix effects influence threshold (oak, tannin, fruit mask Brett)
• Consumer sensitivity varies 10-fold between individuals
• Market preferences vary (some tolerance in traditional regions vs. zero-tolerance in New World)

Risk Factors

Wine conditions:

• pH >3.5: Reduced SO₂ efficacy
• Residual sugar >0.5 g/L: Metabolic substrate
• Alcohol <13% v/v: Less inhibitory
• Low free SO₂ (<25 mg/L): Insufficient inhibition

Cellar conditions:

• Old barrels with deep-stave contamination
• Poor sanitation of equipment (pumps, hoses, valves)
• Cross-contamination from infected wines
• Extended aging without monitoring

Options and Interventions

Prevention

SO₂ management:

• Maintain molecular SO₂ at 0.5-0.8 mg/L continuously
• At pH 3.5, this requires ~30-35 mg/L free SO₂
• At pH 3.7, requires ~50+ mg/L free SO₂
• Check SO₂ monthly during barrel aging

Barrel hygiene:

• Hot water rinse (80°C for 5 minutes) after emptying
• Steam treatment (10-15 minutes at 100°C)
• Ozone treatment (as complementary method)
• SO₂ wicks or tablets during storage
• Discard barrels after 3-4 fills (Brett colonization in stave depth)

Wine management:

• Complete fermentation to dryness (<2 g/L RS)
• Avoid leaving wine on high-residual-sugar lees
• Maintain cool cellar temperatures (14-16°C)

Equipment hygiene:

• Hot water and sanitizer rinse of pumps, hoses after each use
• Dedicated equipment for barrel work vs. tank work
• Quarterly sanitation audits

Detection

Sensory screening:

• Train staff to recognize 4-EP/4-EG aromas
• Blind inclusion of spiked samples in QC tastings
• Early detection prevents widespread contamination

Analytical methods:

1. GC-MS: Gold standard; quantifies 4-EP and 4-EG separately
2. HPLC: Alternative quantitative method
3. Plating: Culture on selective media (DBDM); slow but confirms viability
4. PCR/qPCR: Rapid detection of Brett DNA; quantifies cell counts

Monitoring schedule:

• Baseline: after malolactic fermentation
• Every 2-3 months during barrel aging
• Before any blending operation
• Before bottling (final QC)

Remediation

If Brett detected before significant 4-EP production:

1. Add SO₂ to achieve molecular SO₂ >0.6 mg/L
2. Rack off lees
3. Consider DMDC (dimethyl dicarbonate) if legal and wine still fermenting

If 4-EP >600 μg/L:

• Remediation options limited
• Blending with clean wine (dilution)
• Fining trials: PVPP, activated carbon may reduce volatile phenols
• Reverse osmosis: Removes 4-EP/4-EG but affects wine matrix

Preventive removal:

• Sterile filtration (0.45 μm) removes Brett cells
• Does not remove 4-EP already produced
• Recommended before bottling for at-risk wines

Trade-offs and Risks

High SO₂ strategy:

• Effective for Brett prevention
• May cause reductive issues in sensitive varieties
• Consumer/regulatory limits on total SO₂ (150-200 mg/L typical)
• H₂S formation under highly reductive conditions

Fining for volatile phenol removal:

• Activated carbon: Effective but strips color and desirable aromatics
• PVPP: Moderate efficacy; less stripping than carbon
• Neither eliminates problem—only reduces perception

Sterile filtration:

• Removes cells; prevents further production
• Does not reverse existing contamination
• May strip body and texture from red wines

Barrel replacement:

• New barrels reduce Brett risk but increase oak flavor and cost
• Old barrels may harbor Brett in stave depth
• Economic trade-off: barrel cost vs. contamination risk

Appellation constraints:

• Extended aging mandates (Barolo: 38 months; Rioja Gran Reserva: 5 years) increase exposure time
• Cellar hygiene must match aging ambition

Practical Implications

Variety-specific considerations:

• Cabernet Sauvignon: Dense structure can mask low-level Brett initially; typically detected late when remediation is difficult.

• Syrah: Some overlap between varietal “gamey” character and Brett; distinguishing may require analytical confirmation.

• Pinot Noir: Lighter structure makes Brett aromas more apparent at lower concentrations; lower tolerance for contamination.

• Sangiovese: Extended aging for Barolo and Brunello increases risk; Italian tradition has historically been more tolerant but modern standards are tightening.

Appellation-specific implications:

• Barolo DOCG: 38-month minimum aging (18 months in oak) creates extended risk window. Traditional botte (large oak) may harbor persistent contamination.

• Rioja DOCa: Gran Reserva (5-year aging, 2 years oak) requires rigorous barrel program. American oak (traditional) provides different precursor profile than French.

• Napa Valley AVA: No aging requirements; producer choice determines risk. Premium producers with extended barrique programs require vigilance.

• Bordeaux AOC: Traditional extended aging in château cellars has historical association with Brett. Modern quality standards are zero-tolerance.

References

• Chatonnet, P., Dubourdieau, D., Boidron, J.N., & Pons, M. (1995). “The Origin of Ethylphenols in Wines.” American Journal of Enology and Viticulture, 46(4), 463-468. AJEV Link

• Oelofse, A., Pretorius, I.S., & du Toit, M. (2008). “Significance of Brettanomyces and Dekkera During Winemaking: A Synoptic Review.” South African Journal of Enology and Viticulture, 29(2), 72-96. DOI: 10.21548/29-2-1445

• Suárez, R., Suárez-Lepe, J.A., Morata, A., & Calderón, F. (2007). “The Production of Ethylphenols in Wine by Yeasts of the Genera Brettanomyces and Dekkera.” International Journal of Food Microbiology, 115, 223-231. DOI: 10.1016/j.ijfoodmicro.2006.11.003

• Curtin, C., Kennedy, E., & Henschke, P.A. (2012). “Genotype-Dependent Sulphite Tolerance of Australian Dekkera (Brettanomyces) bruxellensis Wine Isolates.” Letters in Applied Microbiology, 55, 56-61. DOI: 10.1111/j.1472-765X.2012.03257.x