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[[File:THP Pathway.JPG|thumb|400|Proposed pathway for THP production by ''Brettanomyces'' <ref name="Elsevier">[https://books.google.com/books?hl=en&lr=&id=KJJwAgAAQBAJ&oi=fnd&pg=PA346&dq=brettanomyces+Tetrahydropyridine&ots=ktbn8PR_fF&sig=r3lkcV-gBa-pK86HSOgFDVIJVDk#v=onepage&q=brettanomyces%20Tetrahydropyridine&f=false Managing Wine Quality: Oenology and Wine Quality. A Reynolds Elsevier, Sep 30, 2010. Pg 359.]</ref>]]
It is thought that THP in mousy wines/beers is mostly produced by microorganisms. All species of ''[[Brettanomyces]]'' can produce forms of tetrahydropyridine in varying amounts, although some below threshold. Additionally, Lactic Acid Bacteria (LAB) including ''[[Lactobacillus]]'' and ''[[Pediococcus]]'' can produce forms of THP. Acetic Acid Bactera (AAB) has also been demonstrated to produce forms of THP <ref name="Snowdon"></ref><ref name="Grbin_2000" />.
Moulis et al. (2023) studied THP production by 22 strains of ''Brettanomyces bruxellensis'', 20 strains of ''Oenococcus oeni'' and 10 strains of ''Lentilactobacillus hilgardii'' (formerly classified as ''Lactobacillus hilgardii''), all of which have been reported to produce THP compounds. They found that all strains could produce ATHP, but not all strains could produce ETHP or APY. This variability was determined mostly by species, but also by strain. for example, all of the 22 ''B. bruxellensis'' strains only produced ATHP and ETHP and not APY. Variability between strains was less pronounced for the species ''L. hilgardii'' compared to the ''B. bruxellensis'' and ''O. oeni'' strains (different strains of ''B. bruxellensis'', for example, produced much different levels of ATHP/ETHP, where as every strain of ''L. hilgardii'' produced relatively the same amount of APY). The researchers also noted that repeatability of THP levels was difficult to achieve, and they owed this to unknown variables such as the physiological state of the cells at time of inoculation into the test media. Interestingly, there was no correlation between strain genealogy and how much THP they produced . The researchers also isolated other species from 25 French wines with mouse taint, including 'S. cerevisiae'', ''Pichia manshurica'', ''Priceomyces carsonii'', ''Pediococcus parvulus'', but none of these strains produced THP in the test growth media <ref name="Moulis_2023" />.
===''Brettanomyces''===
The production of ATHP is not efficient, meaning that the amount of ATHP produced is not proportional to the amount of L-lysine consumed. Therefore, the production of ATHP appears to be a byproduct (secondary metabolite) of L-lysine catabolism <ref name="Snowdon"></ref>. ATHP is further metabolized into ETHP by ''Brettanomyces'', although not much is known about this metabolic process <ref>[http://ucce.ucdavis.edu/files/repositoryfiles/Joseph_5_Aromatic_Diverswity_of_Brettanomyces-82350.ppt Joseph, C.M. Lucy. ''Aromatic Diversity of Brettanomyces''. U.C. Davis. Retrieved 3/10/2015.]</ref><ref name="Snowdon"></ref>. ETHP has a significantly higher taste threshold, and is often not detected in contaminated wine <ref name="Oelofse"></ref>.
Although ''Brettanomyces'' is capable of producing APY from L-ornithine <ref name="Grbin_2007" />, the amount produced is much less than that of LAB and high amounts of L-ornithine are required. In wine, there isn't enough L-ornithine present to production significant amounts of APY from L-ornithine. Therefore, the presence of APY (which is much easier to detect aromatically than ATHP) indicates a bacterial contamination in wine (it is unknown if this applies to beer) <ref name="Snowdon"></ref>. Additionally, Moulis et al. (2023) found that out of 25 French wines with THP, only 20% of them had ''B. bruxellensis'' in them, indicating that THP is mostly produced by bacteria or chemically in wine <ref name="Moulis_2023" />.
The presence of the "mousy off-flavor" caused by forms of THP appears to be temporary in beer. Although not much is known about the degradation or metabolic breakdown of ATHP/ETHP, it tends to age out of beer after 2-6 months. Since the odor/taste threshold for ETHP is much higher than ATHP, and ATHP appears to be metabolized into ETHP by ''Brettanomyces'' over time, this may be one of the mechanisms by which the mousy off-flavor ages out of beer. The possibility of ETHP breakdown is not mentioned in any studies that we know of, although Moulis et al. (2023) reported that for organisms that produced ETHP, there was always a 1:10 ratio between ETHP/ATHP or ETHP/APY, suggesting that this ratio might be governed by the chemistry of the media used and/or the [https://en.wikipedia.org/wiki/Reduction_potential reduction potential] <ref name="Moulis_2023" />. Another unknown is why does ''Brettanomyces'' produce ATHP shortly after kegging and force carbonating a beer that has reached final gravity. The most likely cause is oxygen pick up during the kegging process. 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''.