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==Introduction=='''Butyric Acid''' (chemical name '''butanoic acid''' <ref name="wikipedia"></ref>) is a carboxylic acid that is produced by aenarobic anaerobic bacteria during sour mashing production such as ''Clostridium butyricum'', ''C. kluyveri'', and Fusobacterium nucleatum ''Megasphaera spp'' <refname="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. It 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 and flavorof 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 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''). ==Microbes and Metabolism=====Clostridium spp===''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 ''C. 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>. Temperature resistance varies from species to species of ''Clostridium''. Most species have an optimal growth at around 104°F (37°C) <ref>[https://books.google.com/books?id=bA9tAwAAQBAJ&pg=PA232&lpg=PA232&dq=clostridium+butyricum++temperature+range&source=bl&ots=JQLbTobiNS&sig=qfc-2rsj3zOXHf4pGpvP6PW1A90&hl=en&sa=X&ei=-weGVbW-E8WeNtK4gfgP&ved=0CCQQ6AEwATgU#v=onepage&q=clostridium%20butyricum%20%20temperature%20range&f=false Textbook of Microbiology & Immunology. Subhash Chandra Parija. 2nd Edition. Feb 10, 2014.]</ref>. However, ''C. thermosaccharolyticum'' can survive temperatures as hot as 149°F (65°C). Additionally, spores of some species of ''Clostridium'' (such as ''C. tertium'') can survive boiling temperatures for longer than 1.5 hours <ref name="Hawthorne"></ref>. Carbon dioxide from both fermentation and artificial introduction has been shown to have a stimulatory effect on the growth of ''Clostridium butyricum'' (as well as other bacteria such as ''E. coli'') <ref>[http://eds.b.ebscohost.com/ehost/detail/detail?sid=7c2c7dba-294d-48c3-b771-56332446bf5e%40sessionmgr111&vid=1&hid=103&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#AN=77331781&db=a9h Gaseous CO2 signal initiates growth of butyric-acid-producing Clostridium butyricum in both pure culture and mixed cultures with Lactobacillus brevis. Hakalehto, Elias and Hänninen, Osmo. July 2012.]</ref>. If sanitation issues allow for ''Clostridium'' to enter the brewery, CO2 purging may encourage butyric acid formation. Even so, many brewers recommend purging with CO2 during the souring phase of kettle souring to prevent other off-flavors <ref>[http://www.stitcher.com/podcast/the-sour-hour/e/36488361?autoplay=true The Sour Hour on the Brewing Network. Interview with Khris Johnson. Nov 20, 2014.]</ref> (~15 mins in). This has the benefit of creating positive pressure inside the souring vessel so that dust cannot be sucked into the vessel during the souring phase of kettle souring. It might also have the benefit of discouraging unidentified contaminates from creating off-flavors in the wort. The true source of butyric acid formation in kettle sour beers has not been well explored, and needs further investigation.  Although ''C. botulinum'' and other identifiably pathogenic ''Clostridium'' species do not tend to grow in [[Sour Mashing]] conditions, it is advisable to not consume wort produced from a sour mash until after it is boiled. Seek specific food and safety advice from a qualified physician. ===Megasphaera spp===''Megasphaera'' species produce large amounts of butyric acid, as well as some acetic, isovaleric, valeric, caproic acids, and hydrogen sulfide. They are considered a true beer spoilage organism in beer. ''M. cerevisiae'' is a common species found in spoiled beer, and is a Gram-negative bacteria. They are obligate anaerobes, and die in the presence of oxygen. Growth occurs at a temperature range between 59°-98.6°F (15°-37°C), with an optimal growth rate at 82.4°F (28°C) <ref>[https://books.google.com/books?id=6JHxBwAAQBAJ&pg=PA238&lpg=PA238&dq=butyric+acid+bacteria+beer&source=bl&ots=knKFqJFgmA&sig=VmO6rhgzmwlqt_WXAlgm8I4Zgmg&hl=en&sa=X&ei=O8-FVcGQOYeggwTt9oPgAg&ved=0CDcQ6AEwAw#v=onepage&q=butyric%20acid%20bacteria%20beer&f=false Brewing Microbiology. Fergus Priest, Iain Campbell. Springer Science & Business Media, Jun 27, 2011.]</ref>. Growth of ''M. cerevisiae'' cannot occur above 5.3% abv, and slows significantly at 2.66% abv. Growth also stops at a pH of 4.0 or less <ref>[https://books.google.com/books?id=O0idBAAAQBAJ&pg=PA211&dq=Megasphaera+cerevisiae+ph&hl=en&sa=X&ei=CReGVf63PIunNt-Zo7gN&ved=0CB0Q6AEwAA#v=onepage&q=Megasphaera%20cerevisiae%20ph&f=false Brewing Microbiology: Managing Microbes, Ensuring Quality and Valorising Waste. Hill, Annie. Woodhead Publishing, May 26, 2015.]</ref><ref>[http://www.asbcnet.org/publications/journal/vol/2016/Pages/ASBCJ-2016-4895-01.aspx Monoclonal Antibodies Binding to Lipopolysaccharide from the Beer-Spoilage Bacterium Megasphaera cerevisiae Exhibit Panreactivity with the Strictly Anaerobic Gram-Negative Brewing-Related Bacteria. Barry Ziola. 2016.]</ref>. ===Fusobacterium nucleatum===''F. nucleatum'' is a Gram-negative bacteria that is commonly found in dental plaque. It is capable of fermenting glucose into butyric acid, and is tolerant of up to 6% oxygen <ref>[https://microbewiki.kenyon.edu/index.php/Fusobacterium_nucleatum Fusobacterium nucleatum. MicrobeWiki. Retrieved 6/21/2015.]</ref>. It has a optimal growth temperature at 98.6°F <ref>[https://www.atcc.org/~/ps/25586.ashx ATCC. Fusobacterium nucleatum subsp. nucleatum   (ATCC ® 25586™). Retrieved 6/21/2015.]</ref>. ''F. nucleatum'' is sensitive to a pH of 5.0 and lower, and temperatures above 140°F (60°C) <ref>[http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2672.2009.04250.x/full Ribeiro-Ribas, R.N., De Carvalho, M.A.R., Vieira, C.A., Apolônio, A.C.M., Magalhães, P.P., Mendes, E.N., Oliveira, J.S., Santoro, M.M. and De Macêdo Farias, L. (2009), Purification and partial characterization of a bacteriocin produced by an oral Fusobacterium nucleatum isolate. Journal of Applied Microbiology, 107: 699–705. doi: 10.1111/j.1365-2672.2009.04250.x]</ref>. Avoid contaminating wort with saliva, and this organism shouldn't be a problem. ===Lactic Acid Bacteria===''[[Lactobacillus]]'' and other LAB produce butyric acid by breaking down lipids to fats. The low level of lipids in brewer's wort most likely limits butyric acid to insignificant levels in beer production. Butyric acid production by ''Lactobacillus'' is more significant in milk based fermented products such as yogurt and cheese <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1182597671768445/?qa_ref=qd&comment_id=1182773928417486&reply_comment_id=1183242405037305&comment_tracking=%7B%22tn%22%3A%22R7%22%7D Conversation with Bryan of Sui Generis Blog on MTF regarding butyric acid production by Lactobacillus. 11/23/2015.]</ref>. For example, one study on a strain of ''L. plantarum'' that was fermented in three different substrates (oats, barley, and wheat) showed no production of butyric acid <ref>[http://www.sciencedirect.com/science/article/pii/S0308814609004373 Volatile compounds produced by the probiotic strain Lactobacillus plantarum NCIMB 8826 in cereal-based substrates. Ivan Salmeron, Pablo Fuciños, Dimitris Charalampopoulos, Severino S. Pandiella. 2009.]</ref>. Due to the low amount of lipids in wort, compounds from the reduction of lipids in grain fermentations by lactic acid bacteria have not been studied <ref name="peyer_review">[http://www.sciencedirect.com/science/article/pii/S0924224415300625 Lactic Acid Bacteria as Sensory Biomodulators for Fermented Cereal-Based Beverages. Lorenzo C. Peyer , Emanuele Zannini , Elke K. Arendt. 2016.]</ref>.  A common brewing myth is that ''Lactobacillus'' produces butyric acid in the presence oxygen, however, there is no biological basis or evidence for this claim (see [[Lactobacillus#Effects_of_Oxygen|effects of oxygen on ''Lactobacillus'']]). The lack of evidence for this claim is supported by Santeri Tenhovirta's masters thesis in Food Science from the University of Helsinki (2019). Tenhovirta soured wort with several species of ''Lactobacillus'', including ''L. delbruekii'', ''L. plantarum'', ''L. alimentarius'', ''L. brevis'', ''L. buchneri'' and ''L. rhamnosus''. During the two days of souring, the vessels were not purged of air, allowing the ''Lactobacillus'' to ferment under aerobic (containing oxygen) conditions. After two days of souring, US-05 was pitched to finish the fermentation (the soured wort was not boiled). A trained sensory panel reported low to no butyric acid flavor in the beers, except for the ''L. delbruekii'' sample which contained a moderate level of butyric acid flavor, but that sample never contained viable ''L. delbruekii'' (even at pitching time), and the butyric acid in that sample was hypothesized to be caused by some unknown contaminating microbe <ref name="Tenhovirta_masters">[https://helda.helsinki.fi/handle/10138/303018 The Effects of Lactic Acid Bacteria Species on Properties of Sour Beer. Santeri Tenhovirta; master thesis in Food Science from the University of Helsinki. 2019.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2724807184214145/?comment_id=2725477224147141&reply_comment_id=2725508117477385&comment_tracking=%7B%22tn%22%3A%22R%22%7D Santeri Tenhovirta. Milk The Funk Facebook group thread on his masters thesis and the lack of butyric acid production under O2 conditions. 06/14/2019.]</ref>. Note that much research has been done on the production of γ-amino butyric acid (also called "gamma-aminobutyric acid" or "GABA" for short) by various bacteria species due to its health benefits. ''Lactobacillus brevis'', ''L. plantarum'', and a few other species of bacteria are able to produce GABA from the non-protein amino acid glutamate via the enzyme glutamate decarboxylase. GABA (C₄H₉NO₂) has a different chemical formula and is a different compound than butyric acid (C₃H₇COOH) <ref>[https://en.wikipedia.org/wiki/Gamma-Aminobutyric_acid "Gamma-Aminobutyric Acid". Wikipedia. Retrieved 06/11/2018.]</ref><ref>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3769009/ Production of gaba (γ – Aminobutyric acid) by microorganisms: a review. Radhika Dhakal, Vivek K. Bajpai, and Kwang-Hyun Baek. 2012.]</ref><ref name="wikipedia"></ref>. GABA reportedly smells "savory" or "meaty" <ref>[http://www.thegoodscentscompany.com/data/rw1198861.html "gamma-aminobutyric acid". The Good Scents Company. Retrieved 06/11/2018.]</ref>. Therefore, the information regarding the production of GABA by ''Lactobacillus'' species should not be confused with that of butyric acid. ===Other Microbes===Other microbes that are butyric acid producers, probably won't be found in the brewhouse. These include bacteria that are commonly found in the guts of animals such as ''Butyrivibrio fibrisolvens'' <ref>[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC203391/ Cotta, M A, and R B Hespell. “Proteolytic Activity of the Ruminal Bacterium Butyrivibrio Fibrisolvens.” Applied and Environmental Microbiology 52.1 (1986): 51–58. Print.]</ref> and ''Eubacterium limosum'' <ref>[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3372111/ Lee, Meng-Rui et al. “Clinical and Microbiological Characteristics of Bacteremia Caused by Eggerthella, Paraeggerthella, and Eubacterium Species at a University Hospital in Taiwan from 2001 to 2010.” Journal of Clinical Microbiology 50.6 (2012): 2053–2055. PMC. Web. 22 June 2015.]</ref>. A few strains of ''Brettanomyces bruxellensis'' have been shown to produce butyric acid in growth media when supplemented with the amino acids phenylalanine and tyrosine <ref name="Lucy_2015">[http://www.ajevonline.org/content/66/3/379 Brettanomyces Dissertationbruxellensis Aroma-Active Compounds Determined by SPME GC-MS Olfactory Analysis. C.M. Lucy Joseph, Elizabeth A. Albino, Susan E. Ebeler, Linda F. Bisson. 2015.]</ref>. ===Metabolism===[[File:Butyrate pathway.jpeg|thumb|100px|Butyric acid pathway <ref name="Effects on Food Properties">[https://books.google.com/books?id=6SLB1j0rX78C&pg=PA76&lpg=PA76&dq=what+is+obligatory+facultatively+heterofermentative&source=bl&ots=6E4DB8Px_X&sig=PXZTNO4fB3ifCF_qAl5_I4euKmk&hl=en&sa=X&ei=Z-10VZqvFIWyggTInIHYBw&ved=0CCsQ6AEwAw#v=onepage&q=what%20is%20obligatory%20facultatively%20heterofermentative&f=false Fermentation: Effects on Food Properties. Bhavbhuti M. Mehta, Afaf Kamal-Eldin, Robert Z. Iwanski. CRC Press, Apr 12, 2012. Pg 76,77.]</ref>]] ''Clostridium'' species such as ''C. butyicum'', ''C. lactoacetophilum'', ''C. pasteurianum'', etc. produce butyric acid and acetic acid via the butyric acid fermentation pathway. Some species of ''Clostridium'', such as ''C. acetobutylicum'' also produce isopropanol or acetone. Carbon dioxide and ethanol are also produced during butyric acid fermentation, as well as butunol. <ref>[https://books.google.com/books?id=GjRwl1D0tiYC&pg=PA533&lpg=PA533&dq=butyric+acid+pathway&source=bl&ots=e68dMW_N1T&sig=C4v9NYkpuFC9Tq1AADTgJ71EJpw&hl=en&sa=X&ei=tI2HVYW1BLWQsQSngr_ICw&ved=0CG8Q6AEwDg#v=onepage&q=butyric%20acid%20pathway&f=false Principles of Microbiology. R S. Mehrotra. ata McGraw-Hill Education, 2009. Pg 53.]</ref>. Butanol is a solventy smelling alcohol <ref>[http://pubchem.ncbi.nlm.nih.gov/compound/1-butanol#section=Top PubMed. 1-Butanol. Retrieved 6/21/2015.]</ref>. ==See Also=====Additional Articles on MTF Wiki===* [[Sour Mashing]]* [[Wort Souring]]* [[Isovaleric Acid]]* [[Brettanomyces]] ===External Resources===* [http://www.flavoractiv.com/products/butyric-beer-flavour-standards/ FlavorActIV]* [http://www.aroxa.com/multi-purpose/multi-purpose-flavour-standards/butyric-acid Aroxa]* [http://www.bjcp.org/cep/kits.php BJCP Off Flavor Kits]