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added information on oxygen levels in impure CO2 sources
Although the exact pathway is not known in ''Brettanomyces'' (several are proposed), the conditions for THP production are well documented. ATHP is produced by metabolizing the amino acid L-lysine, along with ethanol and a glucose or fructose molecule. Iron is also needed for THP production, although its exact role in biosynthesis is not known <ref name="Snowdon"></ref>. As with other amino acids, lysine is taken up by ''Saccharomyces'' during fermentation, and then released after fermentation. Levels of lysine fluctuate slightly throughout fermentation, but are generally high throughout a beer's lifetime (including after fermentation) <ref>[http://link.springer.com/article/10.1385/CBB:46:1:43 The α-aminoadipate pathway for lysine biosynthesis in fungi. Hengyu Xu, Babak Andi, Jinghua Qian, Ann H. West , Paul F. Cook. Sept 2006.]</ref><ref>[http://pubs.acs.org/doi/abs/10.1021/bi9829940 Lysine Biosynthesis in Saccharomyces cerevisiae: Mechanism of α-Aminoadipate Reductase (Lys2) Involves Posttranslational Phosphopantetheinylation by Lys5. David E. Ehmann , Amy M. Gehring , and Christopher T. Walsh. 1999.]</ref><ref>[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2007.tb00249.x/abstract Elucidation of the Role of Nitrogenous Wort Components in Yeast Fermentation. C. Lekkas, G.G. Stewart, A.E. Hill, B. Taidi and J. Hodgson. May 2012.]</ref><ref>[http://www.sciencedirect.com/science/article/pii/S0308814699000710 Proteins and amino acids in beers, their contents and relationships with other analytical data. S. Gorinstein, M. Zemsera, F. Vargas-Albores, J-L. Ochoa, O. Paredes-Lopez, Ch. Scheler, J. Salnikow, O. Martin-Belloso, S. Trakhtenberg. 1999.]</ref>. Wheat generally has a slightly lower amount of lysine than barley, and oats have a slightly higher amount of lysine than barley <ref>[http://www.aaccnet.org/publications/cc/backissues/1983/Documents/chem60_461.pdf Amino Acid Composition of Six Grains and Winter Wheat Forage. Morey, D.D. 1983.]</ref><ref>[https://diy.soylent.com/ingredients/oats "Oats". DIY Soylent website. Retrieved 02/07/2017.]</ref><ref>[https://diy.soylent.com/ingredients/barley-malt-flour "Barley malt flour". DIY Soylent website. Retrieved 02/07/2017.]</ref><ref>[https://diy.soylent.com/ingredients/wheat-flour-wholegrain "Wheat flour, whole-grain". DIY Soylent website. Retrieved 02/07/2017.]</ref>.
Oxygen 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>[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). The purity of the CO<sup>2</sup> 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 CO<sup>2</sup>, ~1,420 ppb of O<sup>2</sup> would be added to the packaged beer, which is an exceedlingly high amount of oxygen. The CO<sup>2</sup> supply should ideally be 99.990% pure or better (this would introduce 46 ppb of oxygen at 2 volumes of CO<sup>2</sup>). The method that the CO<sup>2</sup> is added can also determine how much oxygen is introduced into the packged beer. Sparging CO<sup>2</sup> (bubbling it through the beer) dissolves less oxygen, while injecting (flushing) dissolves 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. Retreived 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. Retreived 06/29/2017.]</ref>. 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''.
Interestingly, for unknown reasons ''Brettanomyces'' cells grown under aerobic conditions and then transferred to an anaerobic environment still produced significant amounts of ATHP in the anaerobic environment. It has been suggested that the aerobic conditions made the ''Brettanomyces'' cells predisposed to creating ATHP <ref name="Snowdon"></ref>. Oxygen exposure during ''Brettanomyces'' starters could potentially stimulate ATHP production later on down the road, even if the beer is not exposed to oxygen, although anecdotal evidence shows that this may not be a concern for brewers. It is still advised to use an aerobic or semi-aerobic starter for ''Brettanomyces'' unless the brewer believes this might be the direct cause of ATHP problems in their beer. Any other oxygen pick up after the beer has finished fermentation is the more likely cause and the brewer's post-fermentation processes should be examined first.