Brettanomyces
Brettanomyces is a spoilage yeast that loves to set up housekeeping in wooden barrels and tanks. Brett gives wine a variety of off flavors, ranging from bandaid or stinky tennis shoes to metallic flavors. There are nine different strains, all of which produce varying aromas. Brett populations also tend to clump in barrels or tanks, and are not evenly distributed throughout a wine. Once a barrel is infected, it is almost impossible to remove the bacteria, so winemakers are very careful when racking wines from barrel to barrel to check for Brett barrels which might infect the rest. When a Brett barrel is discovered, the wine is usually discarded, although, truth to tell, many famous French producers produce Brett wine and sell it for $70 a bottle or more. A small amount of some strains can give wine a characteristic odor that is distinctive but not necessarily unpleasant. In addition, rather than giving off a single odor, populations of Brett give off a range of aromas as they develop, beginning with barnyard, leather and clove given off by small populations, to bad infections with smells of bandaid or wet animal hide.
The yeast Brettanomyces produces an array of metabolites when growing in wine, some of which are volatile phenolic compounds. Together these compounds are often referred to as "Brettanomyces character", or simply "Brett". The main constituents are listed below, with their sensory threshold and common sensory descriptors:
• 4-Ethylphenol (>140 µg/L): Band-aids, barnyard, horse stable, antiseptic
• 4-ethylguaiacol (>600 µg/L): Bacon, spice, cloves, smoky
• isovaleric acid: Sweaty saddle, cheese, rancidity