Brett / Barnyard — Brettanomyces Yeast

Brettanomyces, usually shortened to 'Brett', is a type of yeast commonly found in wineries. Like Saccharomyces, the dominant yeast of wine fermentations, it feeds on sugars, but it grows slowly, which is part of the reason it rarely shows up until several months after fermentation. Its spore-forming form is known as Dekkera, though this is rarely seen in wine. Brett spoils wine by producing volatile phenols: it converts p-coumaric acid into 4-ethylphenol (4-EP) and ferulic acid into 4-ethylguaiacol (4-EG). The last step in making 4-EP depends on an enzyme, vinyl-phenol reductase, that Dekkera/Brettanomyces has and Saccharomyces lacks entirely.

• Brett can be found on the grapes, in the winery or in oak barrels, and is more common in wines fermented with ambient or wild yeast or matured in oak
• It is often found in oak barrels used for maturation and usually comes into contact with wine after the initial alcoholic fermentation is finished, when it starts consuming any leftover sugar
• It needs very little sugar: 0.3 g/L of sugar is enough for Brett to produce 1000 µg/L of 4-EP
• Brett is more common in red wines than in whites or rosés; Chardonnay is the white variety most often affected, and Brett has occasionally been observed in Riesling and in sparkling base wines
• Wine can be bottled with viable Brett cells, and 4-EP can then form in the bottle, so bottles of the same wine can differ

4-EP, the 'Band-Aid' aroma, is the main contributor to Brett character and the general marker for it. 4-EG adds smoky and spicy notes, described as cloves and smoky bacon, and a third compound, 4-ethylcatechol (4-EC), adds savoury, sweaty and barnyard or animal nuances. Brett also produces isovaleric acid, described as sweaty saddles. Together, the three phenols 4-EP, 4-EG and 4-EC are associated with 'Band-Aid', 'medicinal', 'horsey' and 'barnyard' characters. The palate can be affected too, with diminished fruit flavour intensity and a drying or metallic aftertaste, and at high levels the compounds mask a wine's fruit and can leave the finish feeling hard and dry. A trace is a different matter: many argue it can add layers of complexity, particularly toward the spicy, smoky bacon end of Brett's range, and leather is among its common descriptors.

• The ratio of 4-EP to 4-EG is related to the different levels of their precursors in different grape varieties: typical ratios are 3:1 in Pinot Noir, 9:1 in Cabernet Sauvignon and Nebbiolo, and 23:1 in Shiraz
• A Brett-affected Shiraz might smell like pure Band-Aid, while a Brett-affected Pinot Noir will possibly smell more animal, barnyard and spicy, with more leather
• In Pinot Noir, Brett is often confused with the grape's own savoury characters and with heavily toasted or spicy oak
• Other wine components can mask Brett, and as a wine ages and primary fruit fades, Brett that was hidden by fruit can be revealed
• Brett is sometimes blamed for other faults, including sulfidic characters, low levels of oxidation and smoke taint

Sensory thresholds for Brett are not fixed. Chatonnet and colleagues (1992) reported a 4-EP sensory perception threshold of 605 µg/L in a French Cabernet, while AWRI studies found a lower threshold of 368 µg/L in Australian Cabernet Sauvignon. The threshold depends very much on the style and structure of the wine: in a 'green' Cabernet Sauvignon and in a heavily oaked one it rose to 425 and 569 µg/L. 4-EG has lower aroma thresholds than 4-EP, 110 µg/L in French Bordeaux Cabernet Sauvignon and 158 µg/L in Australian Cabernet Sauvignon. The two compounds also have an enhanced effect together: with 37 µg/L of 4-EG present (a 9:1 ratio), the 4-EP threshold fell from 605 to 369 µg/L.

• In an AWRI consumer study (Curtin et al. 2008), 104 Sydney-based red wine consumers tasted wines spiked with Brett compounds; all spiked samples were significantly disliked compared with the unspiked base wine
• Even the lowest level tested, 4-EP at 600 µg/L, substantially affected consumer acceptance, although only 4% of consumers had heard of Brett
• With repeated exposure to the aroma, Brett is detected more easily
• Over a 10-year Brett survey of Australian Cabernet Sauvignon, the mean 4-EP level dropped by 90%, to 107 µg/L for the 2005 vintage

Is Brett good or bad? As WSET puts it, the answer is: it depends. If a winemaker does not intend any Brett influence, it is more likely to be bad; beyond that, it comes down to personal preference, and individual drinkers have different sensitivities to these aromas. Some people like the flavours and feel they give the wine complexity, so depending on the drinker and the intensity, Brett is not always seen as a fault; others consider it a fault no matter how subtle the influence is. Pascal Chatonnet of the University of Bordeaux regards Brett as a flaw: at the perception threshold, he says, "you are obviously losing immediately all the fresh, typical fruit and the originality of the wine."

• GuildSomm notes that Brett has been indelibly associated with certain wineries (Beaucastel), regions (Rioja) and eras in winemaking (pre-2000s Bordeaux)
• Decanter describes Château Musar's wines as made naturally, often with high levels of volatile acidity and/or brett, which can be seen as an imperfection for some or as a living element for others
• Decanter also observes that Brett is generally less acceptable in delicate Pinot Noir-based wines than it might once have been
• Brett is historically more associated with some traditional European styles, as the fruit-forward style of many non-European wines is not considered compatible
• Brett is slightly more likely in minimal-intervention styles and can contribute to the more 'funky' styles of natural wines

With Brett, prevention is better than cure. The 'Brett zone' is the critical time between the end of primary and secondary fermentation until the point before sulfur dioxide (SO2) is added, and leaving wines unsulfured for long periods, particularly during slow malolactic fermentations, is the riskiest time for Brett growth. A molecular SO2 concentration of 0.6 mg/L is required to prevent Brett growth, best achieved with one large SO2 addition after malolactic fermentation; about 80 mg/L is generally recommended for wines of pH 3.6 or below, and higher-pH wines need more. Brett cells can be removed from wine by sterile filtration.

• Brett often spreads around wineries through wine transfers, clean wine going into contaminated barrels or the reverse, and cross-contamination of barrels via topping, sampling or barrel stirring
• Affected barrels should be filled with hot water, ideally at 85°C for 15 minutes or until the outside of the barrel is hot to touch; disposing of a barrel is sometimes the safer option
• Brett is virtually impossible to eradicate from a barrel once it is in the wood
• SO2 can leave Brett cells viable but not culturable: weakened rather than killed, they may not grow on a culture plate and can spring back to life when molecular SO2 drops or heat or oxygen is introduced
• Other control measures include a low pH, minimal residual sugar, general cellar hygiene and low cellar temperatures
• Of the Brett isolates collected across Australian winemaking regions, 85% were related to one strain, Strain A, which has some tolerance to SO2
• Steps taken to control Brett are also likely to help avoid other microbiological spoilage, including volatile acidity (VA)

https://www.awri.com.au/industry_support/winemaking_resources/frequently_asked_questions/brettanomyces-faq/ https://www.awri.com.au/industry_support/winemaking_resources/sensory_assessment/recognition-of-wine-faults-and-taints/wine_faults/ https://www.guildsomm.com/public_content/features/articles/b/kelli-white/posts/brettanomyces https://www.wsetglobal.com/knowledge-centre/blog/2026/brett-in-beer-and-wine-is-the-farmyard-funk-desirable-or-not https://www.wsetglobal.com/knowledge-centre/blog/2023/august/24/common-wine-faults-and-how-to-spot-them https://www.decanter.com/features/wine-flaws-characterful-to-a-fault-245568/4/ https://www.decanter.com/premium/musar-defying-time-382557/ Last verified 2026-11-02