Beer’s oldest new darling: Brett the British Fungus

By Amy Tindell

Watch out hops: yeast is about to become beer’s new darling. More commercial breweries, and home brewers, are experimenting with this living part of beer. Yeast wranglers are making a name for their trade, and inspiring scientific creativity in producing signature flavors in beers.

One yeast that seems to be drawing renewed attention is Brettanomyces. Brettanomyces, nicknamed Brett by those who work alongside it, is a non-spore forming genus of yeast in the family Saccharomycetaceae. In the wild, Brett lives on the skins of fruit, and when grown in the lab with large amounts of glucose, can produce acetic acid. Some know it as “wild yeast;” I know it as the “Belgiany” part of Belgian beers.

Brett was discovered in 1904 at the Carlsberg brewery. An employee identified the yeast as a potential cause of spoilage in English ales, forever relegating it to be known by the Greek term for “British fungus.” Indeed, brewers traditionally avoid Brettanomyces as a contaminant and source of unwelcome flavors. In particular, Brettanomyces produces three compounds with high sensory profiles – 4-ethyl phenol, 4-ethyl guaiacol and isovaleric acid– as it eats the sugars in the brew. 4-ethyl phenol is the culprit behind those “band-aid” and barnyard aromas that some beers feature, and 4-ethyl guaiacol gets credit for the bacon, smoky, spicy, and burnt wood smells. Isovaleric acid and its esters produce a fruity quality in a beer at best, but in certain quantities can lend sweaty, barnyard, smoky, or cheesy/rancid qualities to the brew.

Brettanomyces may be used alone or in combination with other microorganisms in the fermentation process. Lambics and Flanders red ales, for example, are fermented with a traditional brewer’s yeast like Saccharomyces along with Brett. Saccharomyces performs most of the heavy lifting, eating most of the sugar, and finishes its work early. In contrast, the wild yeast Brett evolved to eat wood sugars, and thus can process more complex sugars, working slowly. Thus, Brett does not require great quantities of sugar, and in a controlled environment where it will not dry out the beer over time, can produce some unique and desirable flavors.

100% Brett fermentation continues to be an experimental practice with less predictable results than fermenting with traditional brewing yeasts. Because Brett will consume almost all the sugars in a beer, surviving for months or years in the fermenter or in a bottle, the brew has the potential to become very dry. Brewers also use Brett, a capable CO2 producer, to bottle condition beers. However, brewers must use caution because that continued activity of Brett can increase carbonation over time, risking the creation of “bottle bombs.”

In addition to the more traditional styles of Cantillon Rosé de Gambrinus, Liefmans Brown Ale, and Duchesse de Bourgogne, all brewed in Belgian breweries, American breweries including Port Brewing Company, Russian River Brewing Company, Sierra Nevada Brewing Company, and New Belgium Brewing Company have all fermented beers with Brettanomyces bruxellensis. Those who appreciate Brett’s work describe these beers as slightly sour and earthy in smell and taste; others may highlight the “barnyard” or “wet horse blanket” characteristics. To find out where you fit in, try one of these beers: you, too, may embrace the “British fungus.”

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