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Forms of '''Tetrahydropyridines''' are a group of flavor active compounds found in some wines and sour beers. While harmless, they are often considered to be off-flavors in wine and beer. With the recent increase in spontaneously fermented wine and lower sulphite usage by certain wine producers, these flavors have become more prevalent in wine <ref name="Moulis_2023">[https://oeno-one.eu/article/view/7481 Moulis, P., Miot-Sertier, C., Cordazzo, L., Claisse, O., Franc, C., Riquier, L., Albertin, W., Marchand, S., De Revel, G., Rauhut, D., & Ballestra, P. (2023). Which microorganisms contribute to mousy off-flavour in our wines?. OENO One, 57(2), 177–187. https://doi.org/10.20870/oeno-one.2023.57.2.7481.]</ref>. 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>, . They 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 beersand wine. The flavor is detected towards the end of the swallow, and the aftertaste can remain for more than 10 minutes <ref name="Stenseels_2015_Essential">[https://www.academia.edu/19646963/Brettanomyces_Bruxellensis_Essential_Contributor_in_Spontaneous_Beer_Fermentations_Providing_Novel_Opportunities_for_the_Brewing_Industry Brettanomyces Bruxellensis, Essential Contributor in Spontaneous Beer Fermentations Providing Novel Opportunities for the Brewing Industry. Jan Steensels. BrewingScience, Sept/Oct 2015 (Vol. 68). 2015.]</ref>. 30% of winemakers are not able to detect the flavor of THP in wine (we do not know if this statistic is reflected in sour beer, but some people have reported not being able to taste THP in sour beer) <ref>[https://books.google.com/books?id=8yegCgAAQBAJ&pg=PT1784&lpg=PT1784&dq=oxford+companion+to+wine+Once+detected,+the+taint+renders+the+wine+undrinkable+and+worsens+in+the+glass,+but+as+many+as+30%25+of+winemakers+are+unable+to+detect+it.&source=bl&ots=-3o-uaZUgX&sig=ET7jldMp3Xdk61FknJ54k4P40CQ&hl=en&sa=X&ved=0ahUKEwjsiZ7bxNPaAhVO8WMKHQHaBgMQ6AEIKTAA#v=onepage&q=oxford%20companion%20to%20wine%20Once%20detected%2C%20the%20taint%20renders%20the%20wine%20undrinkable%20and%20worsens%20in%20the%20glass%2C%20but%20as%20many%20as%2030%25%20of%20winemakers%20are%20unable%20to%20detect%20it.&f=false Jancis Robinson. "The Oxford Companion to Wine". Oxford University Press. Sep 17, 2005. Pg 483.]</ref>. 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 flavor of THP, and the mouth's salivary pH serves that purpose when tasting beer or wine with THP. For example, as the mouth's pH adjusts back up after swallowing a sip of sour beer or wine, the THP becomes detectable in the aftertaste. 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 "mousy" flavors 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, and anecdotes from MTF members seem to support this <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><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1972251579469713/?comment_id=1972290999465771&comment_tracking=%7B%22tn%22%3A%22R%22%7D Tariq Ahmed and Shane Martin. Milk The Funk thread about THP aging out of cider at room temperature. 01/30/2018.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2069944559700414/?comment_id=2070067719688098&reply_comment_id=2070405472987656&comment_tracking=%7B%22tn%22%3A%22R3%22%7D Ryan Sandlin. Milk The Funk Facebook gropu group thread on THP aging out after 90 days of room temperature storage. 04/23/2018.]</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>.

Traditionally, the mousy/Cheerios® flavor from THP is considered an off flavor in both wine and sour beer. There is some debate and differing opinions as to whether or not a small amount of THP flavor is allowable (or even enjoyable) in sour beers, however most consider any level to be an off flavor <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1864484553579750/ MTF discussion on whether or not THP should always be considered an off-flavor. 10/27/2017.]</ref>. In wine, THP is also generally considered an off-flavor, although to some people a small amount of THP is acceptable in natural wine. It has greatly been eliminated as a problem in wine making due to sulfite sulphite usage and better controlled fermentations. However, in recent years there has been an increase in the popularity of natural wine which has a higher chance of being affected by THP. There has therefore also been a recent increased attention to THP in wine due to the increased popularity of natural wine <ref>[https://www.sfchronicle.com/wine/article/There-s-a-mouse-in-your-wine-14465005.php Esther Mobley. San Francisco Chronicle. 09/24/2019. Retrieved 09/25/2019.]</ref>.

# Do sulfites sulphites bind to THP molecules, rendering them unperceivable <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2069944559700414/?comment_id=2070023539692516&reply_comment_id=2074259392602264&comment_tracking=%7B%22tn%22%3A%22R%22%7D Otto Forsberg. Milk The Funk Post discussion on THP and sulfites. 04/27/2018.]</ref>?

* [https://www.facebook.com/groups/MilkTheFunk/permalink/4042481855779998/ MTF thread on removing THP from wine made with low or no sulfitesulphite.]