Brettanomyces

Contamination by Brettanomyces/Dekkera yeast species, which thrive on trace sugars in wine. Brett produces 4-ethylphenol (band-aid, barnyard) and 4-ethylguaiacol (smoky, spicy). Can colonize barrels, making them permanent sources. Controversial: at low levels, some consider it a positive complexity factor.

Band-aid, barnyard, horse stable, smoky bacon, or sweaty saddle aromas. At low levels, adds a 'funky' or 'earthy' complexity that some drinkers enjoy. At high levels, overwhelms fruit character entirely. The perception threshold varies greatly between individuals.

Strict cellar hygiene, regular SO2 management, sterile filtration (0.45 microns), and monitoring with plating or PCR testing. Infected barrels should be discarded. Low pH and adequate SO2 levels inhibit growth.