What Brettanomyces Does to Wine
Brettanomyces bruxellensis, the species most relevant to winemaking, metabolises compounds in wine to produce volatile phenols:
- 4-Ethylphenol (4-EP) — responsible for barnyard, horse stable, and medicinal aromas
- 4-Ethylguaiacol (4-EG) — creates smoky, spicy, clove-like notes
- Isovaleric acid — contributes sweaty, cheesy aromas at high concentrations
At low levels, these compounds can add complexity and a sense of terroir. At high levels, they overwhelm fruit character and are universally considered a fault.
Where Brett Thrives
Brettanomyces lives in the winery environment — in barrel pores, on equipment surfaces, and in the air. Risk factors include:
- Used barrels — Brett can survive deep within barrel staves and reinfect successive vintages
- Low SO₂ levels — sulfite-free or low-sulfite wines are more vulnerable
- Residual sugar — even trace amounts provide food for Brett metabolism
- Warm cellar temperatures — accelerate Brett growth
- Red wines — more susceptible than whites due to lower acidity and higher phenolic content
The Great Brett Debate
Opinions on Brett are sharply divided:
- Fault perspective — most trained tasters and the wine industry mainstream consider Brett above detection threshold a defect. Australian and New World producers are especially intolerant
- Complexity perspective — some traditional European producers (particularly in the southern Rhône, Burgundy, and Bordeaux) tolerate or even welcome low-level Brett as part of a wine's regional character
- Natural wine — many natural wines carry detectable Brett, which adherents view as an authentic expression of wild fermentation
Prevention and Control
Strict hygiene is the primary defence. Regular barrel testing (PCR analysis), appropriate sulfite levels, sterile filtration before bottling, and replacing old barrels all reduce Brett risk. Once established in a cellar, eradication is extremely difficult.