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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 and winemakers 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 a few 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 off-flavorof 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>.

In foodTraditionally, Tetrahydropyridines are associated with the aroma 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 baked goods such as white breadTHP 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, popcornTHP 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 sulphite usage and tortillasbetter controlled fermentations. However, and is formed in recent years there has been an increase in the popularity of natural wine which has a higher chance of being affected by Maillard reactions during heatingTHP. 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>.

TraditionallyIn food, Tetrahydropyridines are associated with the mousy/Cheerios® flavor from THP aroma of baked goods such as white bread, popcorn, and tortillas, and is considered an off flavor in both wine and sour beerformed by Maillard reactions during heating. There is some debate and differing opinions as to whether or not a small amount These versions of THP flavor is allowable (or even enjoyable) in sour beers have a different chemical form than the forms produced by microbes, 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-but they can have a similar flavor. 10/27/2017.]</ref>.

THP in wine ("mouse taint") was first described in wine by Müller-Thurgau and Osterwalder in 1913, although mention of an off-flavor in wine that 'is closely resembling the smell of a residence of mice' dates to 1894 in "A Treatise on wines" by J.L.W. Thudichum. Müller-Thurgau and Osterwalder attributed it to be produced by bacteria. They also established that the ability to detect THP varies from person to person <ref name="Grbin_1996" />. A 1908 text by the Germans Nessler and Windish linked THP off-flavors to wine that aged longer on lees, perhaps due to yeast autolysis <ref name="Snowdon" />. In the 1940's, some scientists proposed that THP was purely a chemical reaction, however, their evidence was inconclusive. In 1977, Tucknott et al. was able to identify that THP production was attributed to both ''Brettanomyces'' yeast and lactic acid bacteria, and that ethanol and L-lysine played a role in its production, and this was confirmed by Heresztyn et al. in 1986 <ref name="Grbin_1996">[httphttps://iwrdbdigital.sonomalibrary.org/cgi-bindigital/collection/p15763coll33/id/koha34976/opac-detail.pl?biblionumber=27016&shelfbrowse_itemnumber=19456 rec/1 Developments in the sensory, chemical and microbiological basis of mousy taint in wine. Grbin, P.R, Costello, P.J, Herderich, M. 1996.]</ref>.

In 2007, the Australian team of Grbin et al. confirmed that higher lysine levels increased the amount of ATHP produced (but not ETHP), although with diminishing increases of THP as the lysine level was increased. They also discovered that L-ornithine also functioned as a precursor for THP production in ''Brettanomyces''. The group proposed a biochemical pathway for the different forms of THP in ''Brettanomyces'' <ref name="Grbin_2007" />.

Due to the specialized GC/MS equipment needed for measuring forms of THP that most labs do not have, other human sensory-based methods have been developed in the winemaking world. Originally, a "palm & sniff" method was developed to detect THP where a small amount of wine is rubbed on the palm which increases the pH of the wine and then immediately sniffed to detect THP. Since then, some studies have used alkaline strips as a way to smell the aroma of THP, and while not as precise as the specialized GC/MS lab equipment, could still help further the understanding of THP. These strips were prepared in the following way: knowing Knowing that mousy off-flavor has a lingering sensory impact, the technique of alkaline paper strip assessment was adapted from Heresztyn (1986a). Paper strips (Whatman No. 1, 4–5 mm × 50 mm) were prepared by soaking in NaOH (0.1M) and drying overnight at room temperature. The alkaline paper strips were then briefly dipped into cell-free samples (centrifuged) and immediately assessed for the mouse-like odor by sniffing <ref>Private correspondence with Dr. Paul Grbin by Dan Pixley. 11/2/2017.</ref>.

Tempère et al. (2019) developed what they suggest is a better way to test for THP mousy off-flavor in wine via oral sensory, specifically as a way to enable panelists who are not as sensitive to THP to detect it during sensory testing. They compared the alkaline strips method to a method where the wine's pH is increased by adding sodium bicarbonate to a pH of 5 and a pH of 7. This mild base is contained in human saliva. At a pH of 5, sensory panelists were more easily able to detect APY and to correctly order the intensity of APY in wine than when they used alkaline strips. For example, the range of detection level for all panelists went from a range of 15 - 300 µg/L to a range of 0.3 - 30 µg/L. At a pH of 7, panelists were not as easily able to detect the aroma of THP APY. Keep in mind that this test does not reflect the real world tasting of wine since the pH would never be raised during normal consumption, but it could be used by a sensory program as a way to more reliably detect smaller amounts of APY in wine <refname="Tempère_2019">[https://oeno-one.eu/article/view/2350 Comparison between standardized sensory methods used to evaluate the mousy off-flavor in red wine. Tempère, S., Chatelet, B., de Revel, G., Dufoir, M., Denat, M., Ramonet, P.-Y., Marchand, S., Sadoudi, M., Richard, N., Lucas, P., Miot-Sertier, C., Claisse, O., Riquier, L., Perello, M.-C., & Ballestra, P. 2019. DOI: https://doi.org/10.20870/oeno-one.2019.53.2.2350.]</ref>.

Kiyomichi et al. (2023) developed a simple method to accurately detect ATHP, ETHP, and APY using gas chromatography-mass spectrometry with stir bar sorptive extraction (SBSE-GC–MS instrumentation). Thermal desorption and injection were performed using a Twister thermal desorption unit (TDU) and a Gerstel CIS 4 cooled injection system with a programmable temperature vaporization (PTV) inlet, installed on an Agilent 6890 gas chromatograph combined with an Agilent 5975 Mass Selective Detector (Agilent Technologies, Massy, France), equipped with a Gerstel MPS 2 autosampler (Gerstel, Mülheim an der Ruhr, Germany). An HP-5MS fused-silica capillary column (30 m × 0.25 mm, 0.25 µm, film thickness, SGE, Courtaboeuf, France) was used, with helium as carrier gas (Messer France S.A.S, Suresnes, France) at a constant pressure of 70 kPa, corresponding to an initial flow of 1.3 mL.min−1 <ref>[https://www.sciencedirect.com/science/article/abs/pii/S0308814623000705 Daiki Kiyomichi, Céline Franc, Pierre Moulis, Laurent Riquier, Patricia Ballestra, Stéphanie Marchand, Sophie Tempère, Gilles de Revel. Investigation into mousy off-flavor in wine using gas chromatography-mass spectrometry with stir bar sorptive extraction. Food Chemistry, Volume 411, 2023, 135454, ISSN 0308-8146, https://doi.org/10.1016/j.foodchem.2023.135454.]</ref>. See also [https://ives-technicalreviews.eu/article/view/7867 "Simultaneous assay of mousy off-flavor markers in wine," Ives Technical Review]. Other techniques for detected THP in food have been developed, which might be applicable to wine or beer. For example, Grimm et al. (2001) developed a technique for detecting 2-acetyl pyrroline (APY or 2AP) in rice. The rice samples had to be heated to 80-85°C in order to extract the volatile APY, and then APY levels in the headspace of the rice container could be detected using solid phase microextraction (SPME) with fibers that operate at the higher temperatures <ref>[https://www.ncbi.nlm.nih.gov/pubmed/11170584 Screening for 2-acetyl-1-pyrroline in the headspace of rice using SPME/GC-MS. Grimm CC, Bergman C, Delgado JT, Bryant R. 2001.]</ref>. It isn't known if such methods would also work for measuring THP compounds in beer or wine, but they could provide a potential option for beer and wine researchers. Cider makers have used a baking soda in water solution to help detect THP. Dissolve a small amount of baking soda in water, swish the solution in your mouth for a few seconds, and then spit it out. While the pH of the saliva in your mouth is raised from the baking soda solution, taste the beer/wine/cider to more easily detect THP <ref>[https://groups.google.com/g/cider-workshop/c/a9JcCERQTYk?pli=1 "testing for mouse". Testing for Mouse. Retrieved 04/06/2021.]</ref>.  The effectiveness of these methods in beer has not been reported to our knowledge (please report any research or anecdotes in the [https://www.facebook.com/groups/MilkTheFunk/ MTF Facebook group]). ===University of Southern Maine Methods===Liesel Krout, Lucille Benedict, Meg Hausman, and Zach Bodah from University of Southern Maine and Allagash Brewing Company have begun developing a new way to produce an ATHP flavor spike and detection method. See [https://digitalcommons.usm.maine.edu/cgi/viewcontent.cgi?article=1180&context=thinking_matters the poster] for more details. This method is still in development.

ATHP/ACTPY (which has two forms: 2-acetyl-3,4,5,6-tetrahydropyridine and 2-acetyl-1,4,5,6-tetrahydropyridine(abbreviated: ATHP or ACTPY) 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>. In food, ATHP has a slightly different form (6-Acetyl-1,2,3,4-tetrahydropyridin). Its formation in food is due to cooking and thought to be associated with Maillard reactions. This form has a toasty flavor of crackers and popcorn, and its presence disappears quickly in food due to its extreme volatility <ref name="Adams_2005" />.

ETHP/ETPY (2-ethyl tetrahydropyridine(abbreviated: ETHP or ETPY) 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>.

It has been speculated by scientists studying mousy off-flavors in wine that its production is the result of slow metabolism of ATHP into ETHP by ''Brettanomyces''. ETHP was observed to form much slower than ATHP, and coincided with a decrease in ATHP. This slow production of ETHP may be another reason it has been underestimated by researchers until recently <ref name="Snowdon"></ref>. It is not known whether or not ETHP has not been shown to break down can degrade into another byproduct.

APY/ACPY (2-acetyl pyrroline(abbreviated: APY, ACPY, or AP) is a more volatile but more potent form of THP. It has a significantly stronger odor and lower odor threshold in wine than ATHP. It can also be found in damp pearl millet, toasted bread, taco shells, tortilla chips, boiled potatoes, cooked sweet corn products, roasted sesame seeds, pan-fired green teas, cured tobacco leaves, peanut and pumpkin seed oils, honey, several soy-based products, and more aromatic rice such as Indian Basmati, as well as many other foods. APY has also been detected in a pale lager beer from Bavaria <ref>[https://link.springer.com/article/10.1007/BF01190873 Primary odorants of pale lager beer. Peter Schieberle. 1991.]</ref>. APY from microbial metabolism is primarily produced by heterofermentative LAB (see [[Tetrahydropyridine#Lactic_Acid_Bacteria|below]]). In food, APY formation is due to cooking and thought to be associated with Maillard reactions, and its presence ages out quickly in food. For example, Schieberle (1989) showed that heating up yeast and sucrose produced APY, simulating how APY could be produced during baking bread <ref><[https://pubs.acs.org/doi/10.1021/bk-1989-0409.ch025 Formation of 2-Acetyl-l-pyrroline and Other Important Flavor Compounds in Wheat Bread Crust. Peter Schieberle. 1989. DOI: 10.1021/bk-1989-0409.ch025.]</ref>. It is extremely volatile; so much so that the food industry has created powdered forms of APY to increase the flavor stability of some foods associated with it. Some plants such as rice crops, Pandan leaves (''Pandanus amaryllifolius''), "bread flowers" (''Vallaris glabra''), Myabi muskmelon fruit, chempedak fruit and jackfruit contain varying levels of APY naturally <ref name="Snowdon"></ref><ref name="Grbin_1996" /><ref name="Adams_2005">[http://pubs.acs.org/doi/abs/10.1021/cr040097y Chemistry of 2-Acetyl-1-pyrroline, 6-Acetyl-1,2,3,4-tetrahydropyridine, 2-Acetyl-2-thiazoline, and 5-Acetyl-2,3-dihydro-4H-thiazine:  Extraordinary Maillard Flavor Compounds. An Adams and Norbert De Kimpe. 2005.]</ref><ref>[https://www.sciencedirect.com/science/article/pii/S1672630823001105 Abiotic and Biotic Factors Controlling Grain Aroma along the Value Chain of Fragrant Rice: A Review. Ayut Kongpun, Tonapha Pusadee, Pennapa Jaksomsak, Kawiporn chinachanta, Patcharin Tuiwong, Phukjira Chan-In, Sawika Konsaeng, Wasu Pathom-Aree, Suchila Utasee, Benjamapohn Wangkaew, Chanakan Prom-U-Thai. Rice Science. 2023. https://doi.org/10.1016/j.rsci.2023.11.004.]</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''===

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''.

Heterofermentative [[Lactobacillus]] spp., particularly ''L. hilgardii'' (reclassified as ''Lentilactobacillus hilgardii'') and ''L. brevis''(reclassified as ''Levilactobacillus brevis''), as well as ''Leuconostoc oeni'' <ref name="Grbin_1996" />, can also produce high levels of ATHP, and to a lesser extent APY and ETHP from L-lysine/L-ornithine, ethanol (must be present), and iron. Although many strains of heterofermentative lactic acid bacteria can produce THP, not all do. For example, Costello et al (2008) found that all strains tested of ''L. brevis'' (3 strains tested), ''L. bucherni'' (3 strains tested), and ''L.hilgardii'' (8 strains tested) produced THP, several heterofermentative species did not produce any detectable levels of THP in a grape-based media (one strain each of ''L. fermentum'' and ''L. cellobiosus''). Some strains within a species produce high amounts while others produce low amounts, for example Costell et al. (2008) found that some strains of ''O. oeni'' produced very high amounts between 50-150 µg/L while others produced very little between 5-20 µg/L in a grape-based media <ref name="Costello_2008">[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>. A strain of ''L. plantarum'' (L11a) was shown to produce relatively low amounts. L-lysine stimulates production of ATHP, and L-ornithine 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 name="Costello_2008" /><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>, although one study found no THP in two strains of ''P. pentocaseus'' and only transient/occasional THP production in one out of five strains of ''P. parvulus'' <ref name="Costello" />. ''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>. A pathway for APY and ATHP production in ''Lactobacillus hilgardii'' was proposed by Costello and Henschke, which involves the intake of lysine or ornithine, along with ethanol (which is broken down into acetaldehyde) to produce APY and ATHP <ref name="Costello" />. ''Lactobacillus pontis'' has been shown to break down proteins via proteolysis, yielding free amino acids such as ornithine which could serve as precursors to THP formation, and it might be [[Lactobacillus#Foam_Degradation|reasonable to presume]] that other species of ''Lactobacillus'' could also free up ornithine as a precursor to THP <ref name="Adams_2005" />.

Although research is limited, acetic acid bacteria (''Gluconobacter'' sp. and many strains of ''Acetobacter aceti'') have been shown to produce forms of THP <ref name="Snowdon"></ref>. Mediterranean dried sauses covered in [[Mold|mould]] have been characterized as having APY as a flavor contributor. The source of the APY was identified with a mould that grows on the surface called ''Peniciilium nalgiovense'' <ref name="Adams_2005" />. Moulis et al. (2023) identified several strains of acetic acid bacteria in 32% of selected French wines with mouse taint, but none of the strains produced THP when tested individually in model medium <ref name="Moulis_2023" />.

Since a low pH hinders the detection of THP, one detection used by wine makers winemakers is to rub some wine on one's palm and smelling for THP. A more reliable method is to dip an alkaline paper strip into the wine, and then smelling the strip to detect the aroma of THP <ref name="Grbin_1996" />.

# 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>?