Butyric Acid
Butyric Acid is a carboxylic acid that is produced by aenarobic bacteria during glucose fermentation, and is generally considered an off flavor in sour beer. If not done right, Sour Mashing can be a big producer of butyric acid. Butyric acid is produced by aenarobic bacteria such as Clostridium butyricum, C. kluyveri, and Fusobacterium nucleatum [1]. The aroma and flavor is often described as a vomit, bile, or rancid cheese. It is also present in the human gut, and is the cause of the aroma of vomit [1]. This is not to be confused with Isovaleric Acid which has a more "feety" aroma and flavor, and is produced by Brettanomyces. Brettanomyces can convert butyric acid into Ethyl Butyrate at low levels, which has a pineapple, tropical fruit aroma and flavor [2].