Changes

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 enteric bacteria 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.