Brettanomyces, also known as Brett, is a yeast that imparts plastic or animal aromas, such as sticking plasters, smoke, leather, or sweaty horses, to wine. To put it differently, Brettanomyces could cause spoilage in wines via the production of volatile phenol compounds. At first glance, these characters may seem unpleasant. However, many wine enthusiasts enjoy them and do not consider low levels of Brett in wine a fault.

Brettanomyces belongs to a family of nine different naturally occurring yeast species (B. lambicus, D. bruxulensis, B. bruxellensis, B. intermidious, among others). Like its cousin, Saccharomyces, the principal agent of alcoholic fermentation, Brett feeds on sugars and converts them into alcohol, carbon dioxide, and diverse compounds that influence the wine aroma, taste, and texture. Unlike the compounds created by Saccharomyces, however, the ones produced by Brettanomyces are not so much appreciated. Some common descriptions could be barnyard, animal sweat, sewage, vomit, Band-Aid, and wet dog.

Different Growth

Apart from bestowing different aromas to the wine, the two yeasts differentiate in how they grow, too. For example, Saccharomyces multiplies in a must, feasting on all available fructose and glucose. It only dies when the food runs out, the alcohol content gets high, or the winemaker freezes the wine. On the other hand, Brett has steady but slow growth, and for this reason, it appears only months after the fermentation is over. Additionally, it feeds on a range of substrates. Fructose and glucose are favorites, sure, but Brett eats unfermentable sugars, as also oak sugars. Consequently, second-hand oak barrels can be a source of Brettanomyces infection.

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Some other useful links on the topic
https://beerandbrewing.com/dictionary/sZ3rBkmAXZ/
https://www.internationalwinechallenge.com/Canopy-Articles/brettanomyces-the-most-interesting-of-all-wine-faults.html
https://www.awri.com.au/industry_support/winemaking_resources/frequently_asked_questions/brettanomyces-faq/

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