Brettanomyces, often called Brett for short, is a genus of yeast consisting of multiple species found naturally in wood. Brettanomyces contributes distinctive flavors to the beverage it grows in. It gernerally considered an undesirable, spoiling infection by home brewers; however, its extreme, distinctive flavor and aroma is considered desirable in some sour beer styles, and at low levels it is depended on to add complexity to many styles of wine.
The flavor contributed by Brettanomyces is often called barnyard, but has also been described as gamy, or as smelling like damp wool, leather, wet fur, a sweaty saddle or horse blanket, or a butcher shop.
Brewing with brettanomyces
Brettanomyces is found naturally in wood, and some brettanomyces character will be found in many wood-aged beers. It is also prominent in several traditional Belgian beer styles, notably Lambic, Gueuze, and Flanders Red Ale, where it was probably first introduced in the barrels used for fermentation and aging.
Brettanomyces requires oxygen to create its distinctive flavor-containing acids, but it prefers a lower level of oxygen. In wooden vessels, brettanomyces feeds on the oxygen that soaks through the walls of the vessel. In order to regulate the amount of oxygen available to it, brett will form a gelatinous film called a pellicle on the top of the fermenter. This pellicle should be left in place during fermentation.
Unlike saccharomyces, brettanomyces is a voracious organism that can digest just about any kind of sugar. Because its natural habitat is trees and lumber, brett has evolved the ability to digest the cellobiose in wood, and can survive by feeding on the barrel itself. It can also ferment sugars unfermentable by other strains of yeasts, such as dextrin. As a result, beers fermented with brett will have very low final gravity, often 1.000 or below.
Once Brettanomyces has been introduced to a brewery, it can be difficult to eliminate if rigorous cleaning and sanitation practices are not adhered to. Brettanomyces can take up residence in micro-scratches in plastic fermentation vessels that have been carelessly handled, escaping alkaline cleaners and acid sanitizers commonly used by homebrewers. It is no more difficult to kill than S. cerevisae if equipment is properly maintained and replaced as mandated by use and age. Homebrewers who follow accepted and provent cleansing and sanitation protocols needn't worry about infecting future batches after a Brett-infused beer has touched their equipment post-ferment.
Brettanomyces in wine
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In most styles of beer, brettanomyces is undesirable, and beer containing it is considered spoiled.
The key to avoiding brettanomyces infection is sanitation. Carefully clean your equipment, and avoid letting any wooden implements touch your wort after boiling is complete. Replace plastic equipment and clean glass and metal thoroughly.