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'''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 <ref name="wikipedia">[http://en.wikipedia.org/wiki/Butyric_acid#Production Wikipedia description of Butyric Acid production]</ref>. 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 <ref name="wikipedia"></ref>. 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 <ref>[http://www.brettanomycesproject.com/dissertation/pure-culture-fermentation/pure-culture-fermentation-discussion/ Chad Yakobson's Brettanomyces Dissertation]</ref>.