Changes

ATHP /ACTPY (which has two forms: 2-acetyl-3,4,5,6-tetrahydropyridine and 2-acetyl-1,4,5,6-tetrahydropyridine) has a much lower flavor threshold than ETHP (see [[Tetrahydropyridine#Thresholds_and_Quantities_Found_in_Mousy_Wine|Thresholds]]), and has been historically cited as the cause of mousy off-flavors detected in wine. In wine, its aroma cannot be detected due to the low pH of wine (it can be detected if the pH is raised), only the flavor. It is easier to detect in higher pH wines. ATHP is the form of THP that is the major contributor to the aroma of freshly baked bread, corn tortilla chips, and crackers. How different foods/wines/beers interact with ATHP on the palate may explain the different flavors that are detected by people, as well differing concentrations and peoples' ability to detect ATHP <ref name="Snowdon"></ref>.

ETHP /ETPY (2-ethyltetrahydropyridineethyl tetrahydropyridine) was first identified in wine in 1973, but until recently further studies weren't able to confirm its presence in wine. Its odor threshold is quite high (see [[Tetrahydropyridine#Thresholds_and_Quantities_Found_in_Mousy_Wine|Thresholds]]), and so it was not considered a major source of mousy off-flavors in wine for some time. Consequently, research on ETHP has been limited. More recently, it was shown that Lactic Acid Bacteria (LAB) can produce above threshold levels of ETHP, making it recently important to wine researchers <ref name="Snowdon"></ref>.

APY /ACPY (2-acetylpyrrolineacetyl pyrroline) is a more volatile but more potent form of THP, and . It has a significantly stronger odor and much lower odor threshold in wine than ATHP. It can also be found in damp pearl millet, white bread, taco shells, tortilla chips, and more aromatic rice such as Indian Basmati. APY is primarily produced by heterofermentative LAB. <ref name="Snowdon"></ref><ref name="Ggrbin_1996" />.

Heterofermentative [[Lactobacillus]] spp., particularly ''L. hilgardii'' and ''L. brevis'', as well as ''Leuconostoc oeni'' <ref name="Grbin_1996" />, can also produce high levels of ATHP and APY from L-lysine/L-ornithine, ethanol(must be present), and iron. A strain of ''L. plantarum'' (L11a) was shown to produce relatively low amounts. L-lysine stimulates production of ATHP, and L-ornitine stimulates the production of APY <ref name="Costello">[http://pubs.acs.org/doi/abs/10.1021/jf020341r Mousy Off-Flavor of Wine:  Precursors and Biosynthesis of the Causative N-Heterocycles 2-Ethyltetrahydropyridine, 2-Acetyltetrahydropyridine, and 2-Acetyl-1-pyrroline by Lactobacillus hilgardii DSM 20176. Peter J. Costello and Paul A. Henschke. 2002.]</ref><ref>[http://www.ajevonline.org/content/37/2/127.abstract Formation of Substituted Tetrahydropyridines by Species of Brettanomyces and Lactobacillus Isolated from Mousy Wines. Tamila Heresztyn. 1986.]</ref><ref>[http://onlinelibrary.wiley.com/doi/10.1111/j.1755-0238.2001.tb00205.x/abstract Ability of lactic acid bacteria to produce N-heterocycles causing mousy off-flavour in wine. PETER J. COSTELLO1, TERRY H. LEE1, and PAULA. HENSCHKE. 2008.]</ref><ref>Sparrows, Jeff. ''Wild Brews''. Brewers Publications. 2005. Pg. 112.</ref><ref>[https://books.google.com/books?id=tFjsAuo5WocC&pg=PA348&lpg=PA348&dq=lactobacillus+Tetrahydropyridine&source=bl&ots=QUVyoFtIwK&sig=h1cdjB0r1pIRX2Bms8wVA0UiLk4&hl=en&sa=X&ei=4DX_VPz5CsH6oQSAzoGgBA&ved=0CEwQ6AEwCQ#v=onepage&q=lactobacillus%20Tetrahydropyridine&f=false Lahtinen, Ouwehand, Salminen, von Wright. Lactic Acid Bacteria: Microbiological and Functional Aspects, Fourth Edition. Pg 348.]</ref><ref>[http://ajevonline.org/content/37/2/127.short Heresztyn, Tamila. Formation of Substituted Tetrahydropyridines by Species of Brettanomyces and Lactobacillus Isolated from Mousy Wines.]</ref>. Acetaldehyde has a stimulatory effect on ATHP and APY production, but is not required. No studies have been done to show whether or not oxygen plays a role in ATHP/APY production in LAB <ref name="Snowdon"></ref>. Most species of [[Pediococcus]] do not create forms of THP, although a few species do produce relatively small amounts. In particular, these include ''P. pentosaceus'' <ref>[http://www.uniprot.org/uniprot/Q03HT0 UniProt article. Retrieved 3/10/2015.]</ref><ref>[http://www.uniprot.org/uniprot/U5ZF76 UniProt article. Retrieved 3/10/2015.]</ref>, and ''P. clausenii'' <ref>[http://www.uniprot.org/uniprot/G8PEU4 UniProt article. Retrieved 3/10/2015.]</ref> (note that commercial cultures of [[Pediococcus]] are normally ''P. damnosus''). ''Oenococcus oeni'' and ''Leuconostoc mesenteroides'' have also been associated with creating ATHP, APY, and ETHP all above threshold amounts. Since only heterofermentative species produce significant amounts of THP, it is thought that its production is linked to the heterolactic pathway, and thus the metabolism of sugars in LAB <ref name="Costello"></ref>.

Although research is limited, acetic acid bacteria (''Gluconobacter'' sp.) have been shown to occasionally produce forms of THP <ref name="Snowdon"></ref>.

::''Editor's note: the following thresholds are from studies on wine, and may not hold true for beer. As stated above, detection is influenced by pH, and so the low pH of sour beer may have the similar effect of repressing odor (more so) and taste (less so) detection, whereas non-sour Brett Brettanomyces beers may have a higher detection rate.''