Tetrahydropyridine

Forms of Tetrahydropyridines (THP), specifically 6-Acetyl-2,3,4,5-tetrahydropyridine (ATHP or ACTPY), 2-ethyltetrahydropyridine (ETHP), and 2-acetyl-1-pyrroline (ACPY or APY) , which are classified as ketones , 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. 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 (it is unknown whether cold or room temperature storage speeds this up), although the exact mechanism for this is not fully understood . Many brewers have noticed that pitching rehydrated wine yeast at bottling reduces the amount/duration of this flavor .

In food, Tetrahydropyridines are associated with the aroma of baked goods such as white bread, popcorn, and tortillas, and is formed by Maillard reactions during heating. ATHP and APY have an odor threshold of 0.06ng/l .

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.

Production

Proposed pathway for THP production by Brett

Brettanomyces, Lactobacillus, and Pediococcus can produce forms of Tetrahydropyridine. In Brettanomyces, ATHP and ETHP are produced by metabolizing the amino acids L-Lysine and D-Lysine, along with ethanol and a glucose or fructose molecule. Although Lysine can be produced by strains of S. cerevisae that express the associated genes , and S. cerevisae also consumes Lysine in wort during fermentation , and fluctuations of Lysine levels occur during fermentation, Lysine levels have been found to be the same in unfermented wort as they are in finished beer that is fermented with S. cerevisae . Oxygen has a stimulatory effect in it's production, but this is probably because Brett has a higher biomass formation under aerobic conditions . Therefore, limiting oxygen exposure during kegging/force carbonating is recommended for helping to reduce THP production. The level of THP production varies widely between species and strains of Brett, with some strains not producing it at all and some producing very high amounts above taste threshold. Additionally, THP production appears to require glucose or fructose, which explains why THP may be seen more often in stuck wine fermentations rather than wine that has finished fermenting. ATHP production by Brett was observed in wine with glucose or fructose added, along with synthetic growth media, suggesting that the type of growth substrate does not effect production .

ATHP is further metabolized into 2-ethyltetrahydropyridine (ETHP/ETPY) by Brettanomyces, although not much is known about this metabolic process . ETHP has a significantly higher taste threshold, and is often not detected in contaminated wine .

The presence of the "mousy off-flavor" caused by THP appears to be temporary in beer. Although not much is known about the degradation or metabolic break down of ATHP/ETHP, it tends to age out of beer after 2-6 months. Another unknown is why Brett produces THP shortly after kegging and force carbonating a beer that has reached final gravity. Pitching fresh Saccharomyces for bottle conditioning a beer with Brett in it has reportedly reduced THP production, perhaps through the quicker metabolism of both the oxygen and sugar that is introduced during packaging time.

Heterofermentative Lactobacillus spp., particularly L. hilgardii and L. brevis, can also produce ATHP and ETHP from Lysine and ethanol . Most species of Pediococcus do not create THP, although a few species do. In particular, these include P. pentosaceus , and P. clausenii (note that commercial cultures of Pediococcus are normally P. damnosus). Oenococcus oeni and Leuconostoc mesenteroides have also been associated with creating THP .

Thresholds

Editor's note: the following thresholds are from a study on wine, and may not hold true for beer.

• 2-ethyltetrahydropyridine (ETHP/ETPY)
  • Taste threshold (wine): 150 µg/L
  • Concentration reported in wines exhibiting mousy off-flavour: 2.7-18.7 µg/L
• 2-acetyltetrahydropyridine (/ATHP/ACTPY) -
  • Odor threshold (water): 1.6 µg/L
  • Concentration reported in wines exhibiting mousy off-flavour: 4.8-106 µg/L
• 2-acetyl-1-pyrroline (ACPY)
  • Odor threshold (water): 0.1 µg/L
  • Concentration reported in wines exhibiting mousy off-flavour: Tr-7.8 µg/L

Discussions

Below is a list of discussions on internet forum threads that may shed light on specific strains and individual experiences. Keep in mind that many of the opinions and experiences are anecdotal, although commonalities and shared experiences may prove to be useful and accurate.

• Milk The Funk Thread on March 10, 2015.
• milk The Funk Thread on Aug 25, 2015.
• Homebrewtalk thread started by "ne0t0ky0".
• Homebrewtalk thread started by "loctones", comments by Michael Tonsmeire.
• Babblebelt thread with comments by Chad Yakobson.
• Michael Tonsmeire observes that adding fresh Saccharomyces at bottling time seems to reduce THP production, versus force carbonating.
• Richie Preiss suggests that there is no difference in how long it takes to age out THP between storing the beer at room temperature or refrigeration temperature based on a side by side test that he did.
• Chad Yakobson on The Sour Hour, 3/11/2015 (around 50 minute mark) - indicates that he has been researching THP for 2 years, and is continuing this research. Little else is mentioned.