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'''Butyric Acid''' (chemical name '''butyrate''' <ref>[https://en.wikipedia.org/wiki/Butyrate Butyrate. Wikipedia. Retrieved 6/20/2015.]</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]] can be a big producer of ''butyric acid''. ''Butyric acid'' is produced by anaerobic bacteria. 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. [[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>. ''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>.

''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 anaerobic, but some can be active in aerobic conditions (aerotolerant), such as ''T. 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]] 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>.

''Megasphaera'' species produce large amounts of ''butyric acid'', as well as some acetic, isovaleric, valeric, and caproic acids. 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>.

''F. nucleatum'' is a Gram-netagive 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.

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>.

''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>.