Brettanomyces / Dekkera — Barnyard, Horse, Band-Aid, Leather Character

Brettanomyces is the non-spore-forming (anamorphic) genus, while Dekkera describes the spore-forming (teleomorphic) counterpart. In 1904, Niels Hjelte Claussen isolated the yeast at the Carlsberg Research Laboratory from an English stock ale and named it Brettanomyces, meaning 'British fungus.' Ascospore formation was first observed by van der Walt and van Kerken in 1960, leading to the introduction of the genus Dekkera in 1964. Today, five species are recognised, of which B. bruxellensis is confirmed as the primary wine spoilage organism. Modern molecular DNA techniques have not detected systematic differences between the anamorph and teleomorph states, and under current one-fungus-one-name conventions, Brettanomyces is likely to be retained as the preferred name. The yeast is a member of the family Pichiaceae and is closely related to Saccharomyces cerevisiae, sharing roughly 49% amino acid identity across gene orthologs.

• B. bruxellensis is the dominant wine spoilage species; B. anomalus is occasionally isolated but far less significant
• Brettanomyces is facultatively anaerobic, highly ethanol-tolerant (surviving at up to 15% abv), and capable of growth at very low residual sugar concentrations
• The yeast colonises oak barrel interiors particularly effectively, aided by its ability to ferment cellobiose, a disaccharide from cellulose hydrolysis that Saccharomyces cannot metabolise

Brett produces 4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG) through a two-step enzymatic pathway acting on hydroxycinnamic acids naturally present in grapes. First, phenolic acid decarboxylase (PAD) converts p-coumaric acid into 4-vinylphenol and ferulic acid into 4-vinylguaiacol. Second, vinylphenol reductase (VPR), an enzyme particularly active in B. bruxellensis, reduces these vinyl intermediates to 4-ethylphenol and 4-ethylguaiacol respectively. Brett typically produces 4-EP and 4-EG in approximately a 10:1 ratio, reflecting the relative abundance of their precursors in wine. Critically, only the free (unbound) forms of p-coumaric and ferulic acids are available to Brett; malolactic bacteria that express cinnamyl esterase can release additional free precursor, increasing spoilage risk. A relatively low concentration of 275 mg/L combined fermentable sugars is sufficient for Brett to grow and exceed sensory thresholds.

• 4-ethylphenol (4-EP): horsy, leather, barnyard, medicinal, sweaty saddle; the dominant volatile phenol in Brett-affected wines
• 4-ethylguaiacol (4-EG): smoky, spicy, clove-like; contributes to secondary 'good Brett' complexity at sub-threshold levels
• Phenolic acid metabolism continues post-fermentation during barrel aging, making Brett a time-delayed problem often discovered six to ten months after barrelling begins

The sensory impact of Brett exists on a continuum and is deeply contested. The aggregate detection threshold for ethylphenols is approximately 400 μg/L, though individual thresholds for 4-EP alone range from 230 to 650 μg/L depending on the study. At sub-threshold concentrations, some producers in regions such as Bordeaux, Burgundy, and the Barossa Valley view low-level Brett as contributing earthy, leathery complexity and the impression of bottle age. Château Musar in Lebanon is one of the most widely cited examples of a wine that relies on Brettanomyces character as part of its signature style. Above the sensory threshold, Brett becomes an unambiguous fault: wines smell of horse stall, wet wool, Band-Aid, and burnt plastic. Individual taster sensitivity varies considerably, and the same wine may be praised for complexity by one judge and penalised as faulty by another. Red wines aged in oak barrels are most susceptible, though Brett has also been isolated from Chardonnay and Sauvignon Blanc.

• Sub-threshold Brett (below ~400 μg/L total ethylphenols): potentially integrates as earthy, leathery secondary complexity in structured red wines
• Above-threshold Brett (above ~400 μg/L): leather, horse sweat, and burnt plastic dominate; regarded as an unacceptable fault in most contexts
• Taster sensitivity varies widely; ethylphenol concentrations and perceived 'Bretty' descriptors in wine are often poorly correlated, partly because other volatile compounds can mask or amplify Brett character

Brett most commonly proliferates during barrel maturation, typically six to ten months after barrelling, though post-bottling spoilage has also been documented. Oak cooperage is a primary reservoir: the irregular, porous surface of barrel interiors cannot be fully sanitised by standard cellar practices, and Brett can form biofilms in hard-to-reach valve fittings, hoses, and pump components. The yeast can also be introduced via insect vectors such as fruit flies, or transported on Botrytis-affected grapes. Red wines are disproportionately affected due to their lower acidity, higher polyphenol content, and prolonged wood contact. Once established, Brett can enter a viable-but-not-culturable (VBNC) state when exposed to sulfites, allowing it to resurface after SO2 levels drop. Natural and minimal-intervention winemakers who forgo sulfite additions during aging accept a higher risk of Brett development as part of working with indigenous microflora.

• Barrel aging: used oak barrels carry significantly higher inoculum risk than new barrels; biofilm formation in stave wood enables Brett to persist between uses
• High pH wines (above 3.5) and warm cellar temperatures amplify Brett proliferation and volatile phenol production
• Post-bottling spoilage is possible when residual Brett populations survive filtration or bottling, particularly in wines with residual sugar

Once Brett establishes in a barrel or winery environment, complete eradication is extremely difficult without measures that risk damaging the wine. Prevention is therefore the most practical strategy. Molecular SO2 between 0.5 and 0.8 mg/L is commonly cited as an effective inhibitory threshold, though some wine strains are tolerant or resistant to these concentrations and will continue to proliferate. Maintaining cellar temperatures at or below 15°C in combination with adequate SO2 significantly reduces Brett metabolic activity. Barrel sanitation options include sulfur burning, hot water treatment, and ozone; heavily infected barrels are often most cost-effective to discard. Dimethyl dicarbonate (DMDC, commercially known as Velcorin) can be used at bottling as an additional sterilant. Sorbic acid is not effective against Brett and poses an independent risk of geranium taint if lactic acid bacteria are present, so it should never be used as a Brett control measure. Sterile filtration before bottling physically removes viable cells and is a reliable final step.

• Molecular SO2 management: calculate target levels based on wine pH using a molecular SO2 calculator; free SO2 alone is not a reliable guide
• Temperature control: cellars at or below 15°C significantly slow Brett growth and ethylphenol production compared to warmer aging environments
• Avoid sorbic acid as a Brett control: it has little activity against Brett while creating lactic acid bacteria risk; sterile filtration and DMDC are more reliable bottling tools

Attitudes toward Brett divide broadly along Old World and New World lines, though the picture is nuanced. Some long-established producers in Bordeaux, Burgundy, and the Rhône have historically tolerated low-level Brett as part of their wine's character, viewing it as an expression of terroir and traditional winemaking. Château Musar of Lebanon is one of the most frequently cited examples of a wine producer that actively embraces Brettanomyces as integral to its house style, producing wines with recognisable barnyard and leathery complexity. In Belgian lambic brewing, Brett is deliberately cultivated as a defining stylistic element in beers such as gueuze and Flanders red ale, and Orval adds Brett before final bottle fermentation. By contrast, many modern New World producers, particularly in California and Australia, eliminate Brett entirely through rigorous sanitation and SO2 management, valuing purity of fruit expression over funky complexity. Natural wine producers who omit sulfites during aging often accept Brett as an unavoidable part of wild fermentation, viewing it through a different aesthetic lens entirely.

• Old World tolerance: certain Bordeaux, Burgundy, and Rhône producers historically accepted low-level Brett as a complexity marker and sign of bottle age
• Château Musar (Lebanon) is one of the clearest examples of a wine producer building a global reputation partly on Brettanomyces character
• New World trajectory: modern California and Australian producers generally eliminate Brett entirely, creating a pronounced stylistic divide with low-intervention European producers