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==Microbes and Metabolism==
===Clostridium spp===
''Clostridium'' is an anaerobic genus of bacteria that is found in plants, wounds, soil, and feces. Several species of the genus ''Clostridium'' produce ''butyric acid'', and is produced in less than 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 [[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>.