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Forms of '''Tetrahydropyridines''' (abbreviated and often referred to generically by brewers as '''THP'''), specifically 2-acetyl-3,4,5,6-tetrahydropyridine and 2-acetyl-1,4,5,6-tetrahydropyridine (abbreviated ATHP or ACTPY), 2-ethyltetrahydropyridine (abbreviated ETHP or ETPY), and 2-acetylpyrroline (abbreviated ACPY or APY) <ref name="Snowdon"></ref>, which are classified as a [https://en.wikipedia.org/wiki/Ketone ketone] and a cyclic [https://en.wikipedia.org/wiki/Imine imine] <ref>[https://en.wikipedia.org/wiki/6-Acetyl-2,3,4,5-tetrahydropyridine "6-Acetyl-2,3,4,5-tetrahydropyridine". Wikipedia. Retrieved 07/22/2016. ]</ref>, are commonly attributed to the "mousy", "urine" (in high amounts) Cheerios® or Cap'n Crunch® (in low amounts), "breakfast cereal", or more generically, "cracker biscuit" flavor in sour beers. The flavor is detected towards the end of the swallow, and the aftertaste can remain for a few minutes. Not all people are able to detect the flavor of THP. The low pH of sour beer or wine makes it harder to detect the flavor and often impossible to detect the aroma. An increase in pH is needed in order to detect the off-flavor, and the mouth's salivary pH serves that purpose when tasting beer or wine with THP. This effect on sensory detection by low pH might also explain why some people are better at detecting it since people have different pH's on the surface of their tongues and saliva <ref name="Snowdon">[http://pubs.acs.org/doi/abs/10.1021/jf0528613 Mousy Off-Flavor: A Review. Eleanor M. Snowdon, Michael C. Bowyer, Paul R. Grbin, and Paul K. Bowyer. 2006.]</ref>. Diacetyl is sometimes mistakenly indicated as a potential cause of this flavor in sour beers. However, Tetrahydropyridines are the accepted cause. The flavor tends to age out of sour beers after 2-6 months in the fermenter, kegs, or bottles (although aging periods as long as possibly 8-12 months have been reported on MTF <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1447796001915276/?comment_id=1447826811912195&comment_tracking=%7B%22tn%22%3A%22R%22%7D MTF thread about how long THP takes to age out with comment by Mark Trent. 10/24/2016.]</ref>). The exact mechanism for how THP ages out of beer is not fully understood, and it is unknown whether cold or room temperature storage speeds up the breakdown process (it is more likely that room temperature storage will result in faster breakdown of THP) <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1451857101509166/ MTF discussion regarding THP degredation under room temperature versus refrigeration temperatures. 10/28/2016.]</ref><ref>[http://www.homebrewtalk.com/f127/cheerios-character-after-bottling-504468/#post6516169 Tonsmeire, Michael. Homebrewtalk.com post 1. 11/21/2014. Retrieved 3/10/2015.]</ref>. Many brewers have noticed that pitching rehydrated wine yeast at packaging reduces the amount/duration of this flavor in kegs and bottles <ref>[http://www.homebrewtalk.com/f127/cheerios-character-after-bottling-504468/#post6522207 Tonsmeire, Michael. Homebrewtalk.com post 2. 11/21/2014. Retrieved 3/10/2015.]</ref>.

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). 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 exceedlingly 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 packged 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. 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''.