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==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 ''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>.