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''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 lower 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. Fermentation also inhibits growth <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>.