Changes

Oxygen plays a key role and has a stimulatory effect in ATHP and ETHP production (particularly ATHP), but its exact role is not understood. It has been speculated that since ATHP production is associated with ''Brettanomyces'' growth, and ''Brettanomyces'' grows better under aerobic conditions, that this is why more ATHP is produced under aerobic conditions <ref>[http://www.brettanomycesproject.com/dissertation/introduction/ Yakobson, Chad. The Brettanomyces Project; Introduction. Retrieved 3/10/2015.]</ref><ref name="Grbin_2007">[http://pubs.acs.org/doi/abs/10.1021/jf071243e The Role of Lysine Amino Nitrogen in the Biosynthesis of Mousy Off-Flavor Compounds by Dekkera anomala. Paul R. Grbin, Markus Herderich, Andrew Markides, Terry H. Lee, and Paul A. Henschke. J. Agric. Food Chem., 2007.]</ref><ref name="Oelofse">[http://scholar.sun.ac.za/handle/10019.1/8437 Significance of Brettanomyces and Dekkera during Winemaking: A Synoptic Review. A. Oelofse, I.S. Pretorius, and M. du Toit. 2008.]</ref>. It has also been hypothesized that oxygen may have a direct effect on the THP molecules themselves <ref name="Snowdon"></ref>. ATHP production was also shown to increase when anaerobically precultured cells were transferred to an aerobic environment, indicating that oxygen has a direct role on the production of ATHP, not just a byproduct of ''Brettanomyces'' growth <ref name="Snowdon"></ref>. Limiting oxygen exposure during kegging/force carbonating is recommended for helping to reduce ATHP production; even very small amounts can have an effect (although the exact threshold of how much oxygen is required has not been determined). For example, the purity of the CO² supply should thus be taken into consideration when force carbonating. At 0.5% impurity (the impurity is air, 1/5 of which is oxygen) and at 2 volumes of CO², ~1,420 ppb of O² would be added to the packaged beer, which is an exceedingly high amount of oxygen. The CO² supply should ideally be 99.990% pure or better (this would introduce 46 ppb of oxygen at 2 volumes of CO²). The method that the CO² is added can also determine how much oxygen is introduced into the packaged beer. Sparging CO² (bubbling it through the beer) dissolves significantly less oxygen due to Henry's Law (see reference), while injecting (flushing) dissolves significantly more oxygen <ref>[https://tapintohach.com/2014/01/27/how-the-purity-of-sparged-carbon-dioxide-affects-the-oxygen-concentration-of-beer/ How the Purity of Sparged Carbon Dioxide Affects the Oxygen Concentration of Beer. Tap Into Hach blog. 01/24/2014. Retrieved 06/29/2017.]</ref><ref>[https://tapintohach.com/2013/12/02/how-the-purity-of-injected-carbon-dioxide-affects-the-oxygen-concentration-of-beer/ How the Purity of Injected Carbon Dioxide Affects the Oxygen Concentration of Beer. Tap Into Hach blog. 12/02/2013. Retrieved 06/29/2017.]</ref>. Vessel purging methods with CO² are also less efficient than some might expect, and might still leave enough oxygen behind to stimulate THP production (see [http://www.homebrewtalk.com/showpost.php?p=8004741&postcount=3 this HomebrewTalk thread]). Pitching fresh ''Saccharomyces'' at bottling/kegging time and naturally carbonating the beer with sugar has reportedly reduced mousy off-flavor detection, perhaps because ''Saccharomyces'' metabolizes both the oxygen and sugar faster than ''Brettanomyces''. Different strains of ''S. cerevisiae'' might be more efficient than others at helping reduce THP. For example, Mitch Ermatinger from Speciation Artisan Ales anecdotally observed that switching from CBC1 conditioning yeast to EC1118 reduced THP off-flavors from 1 month to two weeks <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1839193092775563/?comment_id=1839266972768175&reply_comment_id=1839402709421268&comment_tracking=%7B%22tn%22%3A%22R7%22%7D Mitch Ermatinger. Milk The Funk Facebook group post on THP reduction using CBC1 and EC1118. 10/03/2017.]</ref> (see [[Packaging#Re-yeasting|Packaging]] for details on re-yeasting at packaging time). Just Amaral reported an anecdote that purging bottles with CO² reduced THP levels, although limiting oxygen in the bottle also had some negative effects on some conditioning yeast strains <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1917477671613771/?comment_id=1917629674931904&comment_tracking=%7B%22tn%22%3A%22R%22%7D Justin Amaral on purging bottles with CO2. Milk The Funk Facebook group. 12/13/2017.]</ref>.