Changes

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-acetyl-1-pyrroline acetylpyrroline (abbreviated ACPY or APY) <ref name="Snowdon"></ref>, which are classified as ketones <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1034461653248715/?comment_id=1034611563233724&offset=0&total_comments=29 Humbard, Matt. Milk The funk Discussion. 3/10/2015.]</ref>, are commonly attributed to the "mousy", "urine" (in high amounts) "cheerios" or "Captain 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. The exact mechanism for this 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>[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>.

APY (2-acetylpyrroline) is a more volatile form of THP, and has a significantly stronger odor and much lower odor threshold in wine than ATHP. It can also be found in damp pearl millet, bread, and more aromatic rice such as Indian Basmati. APY is primarily produced by heterofermentative LAB. <ref name="Snowdon"></ref>.