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''Brettanomyces '', also known as brett or bretta, is a yeast that was originally thought of as a spoilage yeast. Known for it's barnyard, fecal, horsey, metallic or Band-Aid flavors, brett was unwelcome in most breweries. However, in some styles like Saison and Lambic these flavors add a layer of complexity to the beer. =''Brettanomyces'' Metabolism= ==Carbohydrate Metabolism==''Brettanomyces'' strains may possess both alpha and beta glucosidases. These enzymes allow ''Brettanomyces'' strains to break down longer chain carbohydrate molecules and to liberate glycosidically bound sugars which are unfermentable to ''Saccharomyces'' yeasts. The cleavage of glycosides, which are sugar molecules connected to other compounds, allows ''Brettanomyces'' to liberate aroma and flavor compounds into their aroma-active and flavor-active states. Through this activity ''Brettanomyces'' strains are able to produce novel flavors and aromas from hops fruits, and fruit pits. In addition, the liberated aroma and flavor active compounds may be further processed by ''Brettanomyces'' in ester production or destruction pathways. ==Ester Production and Destruction== ==Phenol Production==Phenols such as 4-vinylphenol (4VP, barnyard, medicinal) and 4-vinylguaiacol (4-VG, clove) can be produced in beer by the decarboxylation of hydoxycinnamic acids, which are found in malt. While both ''Saccharomyces'' and ''Brettanomyces'' strains are capable of converting hydroxycinnamic acids to their vinyl derivatives, ''Brettanomyces'' is also able to reduce these vinyl derivatives to ethyl derivatives. These vinyl derivatives have similar tastes to the ethyl derivatives but have lower flavor thresholds. ==Acid Production==In the presence of oxygen, ''Brettanomyces'' strains are capable of producing acetic acid. Depending on the brewer's palate and the degree of acetic production, this can be a desirable or undesirable trait. The degree of acetic acid production varies among different brett strains. Acetic acid produced by ''Brettanomyces'' may also be used in the synthesis of [[Secondary metabolites|acetate esters]] such as ethyl acetate.