Which yeast species is specifically known for its contribution to the characteristic 'funky' or 'barnyard' aroma profile in certain beer styles?

The yeast species specifically known for contributing the 'funky' or 'barnyard' aroma profile in certain beer styles is Brettanomyces. Brettanomyces, often abbreviated as 'Brett,' is a genus of yeast different from the more commonly used Saccharomyces cerevisiae (ale yeast) and Saccharomyces pastorianus (lager yeast). While Saccharomyces yeasts are primarily known for clean fermentation profiles, Brettanomyces produces a range of flavor compounds that can be perceived as funky, barnyard-like, horse blanket, earthy, or even fruity, depending on the specific strain and fermentation conditions. These characteristic aromas are mainly due to the production of volatile phenolic compounds, particularly 4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG). The formation of these compounds is influenced by the presence of precursors in the wort (unfermented beer) and the yeast's enzymatic activity. Some strains of Brettanomyces also produce acetic acid, which contributes a sour or vinegary note. The 'funky' characteristics imparted by Brettanomyces are desirable in certain beer styles like lambics, gueuzes, and other Belgian-style beers, where they contribute to the complexity and unique flavor profile. Brewers carefully control the use of Brettanomyces to achieve the desired level of funk, as excessive amounts can lead to an unbalanced and unpleasant flavor.