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Brettanomyces is the yeast that divides the brewing world: to some brewers it’s an unwanted contaminant that ruins clean beers, to others it’s a deliberate tool for creating some of the most complex and distinctive fermented beverages available. I’ve worked with Brettanomyces intentionally for years and accidentally encountered it in batches I didn’t want it in, both experiences taught me something. Understanding what Brett does, why it does it, and how to control it separates brewers who can use it as a tool from those who can only experience it as a problem.
What Brettanomyces actually does
Brettanomyces (commonly called “Brett”) is a genus of wild yeast that produces a range of flavor compounds depending on the strain, the fermentation conditions, and the substrate. The primary character compounds are 4-ethylphenol (4-EP, the “barnyard” or “horse blanket” compound), 4-ethylguaiacol (4-EG, spicy, smoky, clove-like), isovaleric acid (cheesy, feet-like at high concentrations), and various esters. The relative production of these compounds varies enormously between Brett strains and fermentation environments, a well-managed Brett fermentation at low oxygen, moderate temperature, and with Saccharomyces co-fermentation tastes fundamentally different from the same Brett in a high-oxygen, warm environment.
Brett strains and their character
| Strain | Primary character | Best use |
|---|---|---|
| B. bruxellensis | Classic barnyard, leather, earthy | Traditional lambic, Orval-style |
| B. anomalus | Fruity, pineapple, ester-forward; low funk | American wild ales, approachable funk |
| B. claussenii | Low barnyard, more tropical/pineapple | Session Brett ales, lighter styles |
| B. trois (actually S. cerevisiae var.) | Tropical, pineapple, mango esters | Tropical sours, fruit-forward applications |
| WLP648 (Brettanomyces brux. trois) | Moderate funk, fruit, spice | Balanced Brett character |
Fermentation approaches with Brett
100% Brett fermentation
Fermenting with Brett as the sole yeast produces the most Brett-forward character but can be unpredictable. Brett ferments more slowly than Saccharomyces, primary fermentation may take 3–6 weeks, and the flavor profile continues developing for months. 100% Brett beers pitched with B. anomalus or B. claussenii at warmer temperatures (72–76°F/22–24°C) produce fruit-forward, light-funk character. B. bruxellensis fermented cool produces more traditional barnyard-leather character. Nutrients are important for clean Brett fermentation, Brett is slow but produces less off-character sulfur and isovaleric acid when well-nourished.
Secondary Brett addition (Orval-style)
Adding Brett to a fully fermented and packaged beer is the method that produces Orval’s characteristic evolving Brett character. The primary Saccharomyces fermentation is complete; Brett is pitched at bottling or kegging and slowly develops over months. The beer is drinkable immediately (as a clean pale ale) and progressively develops more Brett character with age. This approach requires extremely low oxygen during packaging and sufficient priming for the additional Brett fermentation. It’s the most accessible way to experience how Brett transforms a finished beer.
Co-fermentation
Pitching Brett alongside Saccharomyces allows both yeasts to compete and interact throughout primary fermentation. The Saccharomyces quickly dominates primary while Brett slowly builds character. Co-fermentation typically produces more fruit-forward, less extreme Brett character than extended secondary Brett aging. Some commercial sour beer producers use co-fermentation as a way to accelerate Brett character development while maintaining some control over the outcome.
Common Questions
How do I prevent Brett contamination in clean beers?
Brett is remarkably persistent once it colonizes equipment, it tolerates temperature extremes and most sanitizers at standard concentrations. The key prevention principle: any equipment that contacts beer post-fermentation (hoses, auto-siphons, carboys, valves) in contact with Brett should be designated “Brett-only” and never used for clean beers again. Brett in plastic equipment is nearly impossible to eliminate through cleaning alone. For a first Brett experiment, use dedicated plastic equipment purchased solely for Brett use, keep it clearly marked, and store it separately. Glass, stainless, and copper can be cleaned with hot caustic or bleach solution effectively; plastic cannot be reliably decontaminated after Brett exposure.
