Tetrahydropyridine

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Introduction

Forms of Tetrahydropyridine (specifically 6-Acetyl-2,3,4,5-tetrahydropyridine (ATHP/ACTPY) and 2-acetyl-1-pyrroline (ACPY) [1], which are classified as ketones [2]) are commonly attributed to the mousy, urine (in high amounts) cheerios (in low amounts), breakfast cereal, or more generically, cracker biscuit flavor in sour beers. The flavor is detected towards the end of the swallow. The exact cause of this perceived flavor is not definitive. However, Tetrahydropyridine seems to be the most likely cause. Dimethyl Sulfide has also been indicated as a potential cause to this flavor in sour beers, probably due to the similar popcorn flavor that DMS can provide. The flavor tends to age out of sour beers, although the exact mechanism for this is not fully understood [3]. Michael Tonsmeire also noticed that pitching rehydrated wine yeast at bottling reduces the amount/duration of this flavor [4].

In food, Tetrahydropyridine is associated with the aroma of baked goods such as white bread, popcorn, and tortillas, and is formed by Maillard reactions during heating. ATHP and ACPY have an odor threshold of 0.06ng/l [1].

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 Tetrahydropyridine. In Brettanomyces, 2-acetyl-3,4,5,6-tetrahydropyridine (ATHP/ACTPY) 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 [5]. ATHP is further metabolized into 2-ethyltetrahydropyridine (ETHP/ETPY) by Brettanomyces, although not much is known about this metabolic process either [6].

Heterofermentative Lactobacillus spp. can also produce ATHP and ETHP from Lysine and ethanol [7][8][9].

Some species of Pediococcus have been associated with the production of ATHP. In particular, these include P. pentosaceus [10][11], and P. clausenii [12].

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 [13]

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.

References

  1. ↑ 1.0 1.1 6-Acetyl-2,3,4,5-tetrahydropyridine. Wikipedia. Retrieved 3/210/2015.
  2. ↑ Humbard, Matt. Milk The funk Discussion. 3/10/2015.
  3. ↑ Tonsmeire, Michael. Homebrewtalk.com post 1. 11/21/2014. Retrieved 3/10/2015.
  4. ↑ Tonsmeire, Michael. Homebrewtalk.com post 2. 11/21/2014. Retrieved 3/10/2015.
  5. ↑ Yakobson, Chad. The Brettanomyces Project; Introduction. Retrieved 3/10/2015.
  6. ↑ Joseph, C.M. Lucy. Aromatic Diversity of Brettanomyces. U.C. Davis. Retrieved 3/10/2015.
  7. ↑ Sparrows, Jeff. Wild Brews. Brewers Publications. 2005. Pg. 112.
  8. ↑ Lahtinen, Ouwehand, Salminen, von Wright. Lactic Acid Bacteria: Microbiological and Functional Aspects, Fourth Edition. Pg 348.
  9. ↑ Heresztyn, Tamila. Formation of Substituted Tetrahydropyridines by Species of Brettanomyces and Lactobacillus Isolated from Mousy Wines.
  10. ↑ UniProt article. Retrieved 3/10/2015.
  11. ↑ UniProt article. Retrieved 3/10/2015.
  12. ↑ UniProt article. Retrieved 3/10/2015.
  13. ↑ Malolactic Fermentation 2005. Geneva on the Lake. Feb 2005. Retrieved 3/10/2015.
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