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'''Butyric Acid''' (chemical name '''butanoic acid''' <ref name="wikipedia"></ref>) is a carboxylic acid that is produced by anaerobic bacteria such as ''Clostridium butyricum'', ''C. kluyveri'', and ''Megasphaera spp'' <ref name="wikipedia">[http://en.wikipedia.org/wiki/Butyric_acid#Production Wikipedia description of Butyric Acid production]</ref> during glucose fermentation, and is generally considered an off flavor in sour beer. If not done right, [[Sour Mashing]] and [[Wort Souring]] can be a big producers of butyric acid. Butyric acid is produced by anaerobic bacteria. The aroma and flavor is often described as a vomit, bile, or rancid cheese. The odor threshold is quite low (240 ppb) while the flavor threshold is much higher (2000-6800 ppb), with the flavor being more easily detected the lower the pH of the beer <ref>[http://www.flavoractiv.com/products/butyric-beer-flavour-standards/ Butyric (Butyric Acid). FlavorActIV Website. Retrieved 10/24/2016.]</ref><ref>[http://www.leffingwell.com/odorthre.htm Odor Thresholds Odor & Flavor Detection Thresholds in Water (In Parts per Billion). Leffingwell & Associates website. Retrieved 10/24/2016.]</ref>. 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. ''[[Brettanomyces]] '' can convert butyric acid into [[Ethyl Butyrate]] at low levels, which has a pineapple, tropical fruit aroma and flavor, but at moderate to high levels not all butyric acid will be converted into ethyl butyrate. Ethyl butyrate production occurred in only half of the ''Brettanomyces'' strains studied by Yakobson, with this conversion being less with the addition of lactic acid, indicating that ethyl butyrate conversion from butyric acid is strain dependent and slows in the presence of lactic acid <ref>[http://www.brettanomycesproject.com/dissertation/pure-culture-fermentation/pure-culture-fermentation-discussion/ "Pure Culture Fermentation Discussion." The Brettanomyces Project. Chad Yakobson. 2011. Retrieved 06/06/2016.]</ref><ref>[http://brettanomycesproject.com/dissertation/pure-culture-fermentation/impact-of-pitching-rate/ "Impact of Pitching Rate." The Brettanomyces Project. Chad Yakobson. 2011. Retrieved 06/06/2016.] </ref><ref>[http://brettanomycesproject.com/dissertation/pure-culture-fermentation/impact-of-initial-concentration-of-lactic-acid/ "Impact of the Initial Concentration of Lactic Acid on Pure Culture Fermentation." The Brettanomyces Project. Chad Yakobson. 2011. Retrieved 06/06/2016.]</ref>. ''Saccharomyces'' species are not known to esterify butyric acid into ethyl butyrate. Butyric acid has shown to have some health benefits in medical research <ref>[http://jmm.sgmjournals.org/content/59/2/141.full Van Immerseel F, Ducatelle R, De Vos M, Boon N, Van De Wiele T, Verbeke K, Rutgeerts P, Sas B, Louis P, Flint HJ. Butyric acid-producing anaerobic bacteria as a novel probiotic treatment approach for inflammatory bowel disease. J Med Microbiol 2010;59:141–3.]</ref><ref>[http://iai.asm.org/content/8/1/30.full.pdf Effect of Colon Flora and Short-Chain Fatty Acids on Growth In Vitro of Pseudomonas aeruginosa and Enterobacteriaceae. MATTHEW E. LEVISON. 1973.]</ref>, although the presence of butyric acid during fermentation is an indicator that there could be a pathogen present (such as some species of ''Clostridium'').

''Clostridium'' is a Gram-positive anaerobic genus of bacteria that is found in plants, wounds, soil, and feces. It has also been found in spent grains. Several species of the genus ''Clostridium'' produce butyric acid, and is produced in at least 3 days. ''Clostridium'' are generally obligatory anaerobes <ref>[https://microbewiki.kenyon.edu/index.php/Clostridium Chlostridium. MicrobeWiki. Retrieved 09/29/2015.]</ref>, but some can be active in aerobic conditions (aerotolerant), such as ''TC. tertium''. Species of ''Clostridium'' that can produce butyric acid from glucose include ''C. butyricum'', ''C. fallax'', ''C. paraputrificum'', ''C. sartagoformum'', ''C. tertium'', ''C. thermosaccharolyticum'', and to a lesser degree ''C. pasteurianum''. <ref name="Hawthorne">[http://www.asbcnet.org/publications/journal/vol/abstracts/49-02.htm Butyric Acid Off-Flavors in Beer: Origins and Control. D. B. Hawthorne, R. D. Shaw, D. F. Davine, T. E. Kavanagh, and B. J. Clarke. 1991.]</ref>

''Clostridium'' contamination in brewing methods such as [[Sour Mashing|sour mashing]] or [[Wort_Souring#Souring_in_the_Boiler_.28Kettle_Sour.29|kettle souring]] can be controlled by [[Sour_Worting#How_to_Pre-Acidify|lowering the pH of the wort/mash]]. However, some species of ''Clostridium'' are acid tolerant. For example, ''C. butyricum'' and ''C. tertium'' were found to survive a pH as low as 4.0 <ref>[http://www.hindawi.com/journals/isrn/2013/731430/ Growth Limiting pH, Water Activity, and Temperature for Neurotoxigenic Strains of Clostridium butyricum. Hamid B. Ghoddusi, Richard E. Sherburn, and Olusimbo O. Aboaba. 2013.]</ref><ref name="Hawthorne"></ref>. Therefore, pre-acidifying during the [[Sour Mashing]] process to a pH of 3.9 or lower may produce the best results (brewers have long reported good results with acidifying their wort/mash to a pH of 4.4; this may be a function of greatly suppressing butyric acid bacteria, rather than eliminating it completely <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1097057266989153/?comment_id=1097064400321773&offset=0&total_comments=6&comment_tracking=%7B%22tn%22%3A%22R2%22%7D Conversation with Derek Springer and Malcom Frazer on MTF. 6/22/2015.]</ref>). Fermentation by yeast also inhibits growth, probably due to out-competing the bacteria <ref name="Hawthorne"></ref>.