Tetrahydropyridine

Forms of Tetrahydropyridine, 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 to this flavor in sour beers. However, Tetrahydropyridines are the accepted cause. The flavor tends to age out of sour beers, although the exact mechanism for this is not fully understood . Michael Tonsmeire also 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

Editor's note: special thanks to ne0t0ky0 on Homebrewtalk for collecting some of these resources.

Brettanomyces, Lactobacillus, and Pediococcus can produce forms of Tetrahydropyridines. In Brettanomyces, 2-acetyl-3,4,5,6-tetrahydropyridine (ATHP) can be metabolized by the amino acid L-Lysine and ethanol with oxygen having a stimulatory effect in it's production, although little else is known about it's production in Brettanomyces . ATHP is further metabolized into 2-ethyltetrahydropyridine (ETHP/ETPY) by Brettanomyces, although not much is known about this metabolic process either .

Heterofermentative Lactobacillus spp. can also produce ATHP and ETHP from Lysine and ethanol .

Some species of Pediococcus have been associated with the production of ATHP. In particular, these include P. pentosaceus , and P. clausenii (note that commercial cultures of Pediococcus are normally P. damnosus).

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.
• 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.