Tetrahydropyridines

← Microbiology and Biochemistry

Tetrahydropyridines (THPs) are a class of keto-cyclic imines found in lambic that are responsible for odors described as "mousey", "horsey", or like cat pee in low concentrations and like tortilla chips or Cheerios in higher concentrations. Formed both by the Maillard reaction and by microbial metabolism, THPs are primary odor components in many familiar baked foods, such as breads, tortillas, and crackers, in addition to lambic and other beers.

Chemical and physical properties

There are three primary forms of THP found in lambic. In increasing odor thresholds, they are: ETHP, ATHP, and APY, each of which may themselves be composed of several closely-related compounds.

ETHP

2-ethyltetrahydropyridine has the highest odor thresholds of the forms of THP found in lambic at around 150 ug/L and has a flavor threshold around 3 ug/L in "mousey wines", though it may be higher in other contexts. Lactic acid bacteria may produce it in detectable concentrations.

ETHP may also be formed by the metabolism of ATHP by Brettanomyces over long time periods, which may explain why THP odors and flavors often drop off in beers where Brettanomyces is present due to higher odor and flavor thresholds of ETHP compared to ATHP. However, this is controversial.

ATHP

2-acetyltetrahydropyridines are much easier to smell (odor threshold ~1.6 ug/L ) than ETHP, though its flavor threshold is similar at ~5 ug/L (minimum). However the low pH of the beer may prevent the odor of ATHP from being detected. The flavor, however, remains present.

APY

2-acetylpyrroline has the lowest odor and flavor threshold of the three primary forms of THP found in lambic (odor ~100 pg/L, flavor down to "trace amounts" ), and is primarily produced by lactic acid bacteria.

In Lambic

THP forms an important class of flavoring and odor compounds of lambic, and are responsible for the classic "wet horse blanket" character in lambic. Varying concentrations of THP are also likely responsible for variations in horsey characteristics between different blends and manufacturers .

There are several sources of THP in lambic, not all of which are fully understood. Some THP may come from the Maillard reaction during the roasting of the grain or during the boil, with the latter likely being a more significant source due to the low roast used for most lambic grain and the extended time the wort is boiled. The combination of both pre-fermentation sources above is still likely only a small contribution to the total THP in lambic, as the wort is rarely described as excessively "bready" nor "mousey".

There are also several biological sources, which are likely the most important sources of THP in lambic. In the presence of oxygen, Brettanomyces will produce both ETHP and ATHP from lysine and glucose or sucrose, though the exact metabolic pathway remains unknown . Further, lysine usually remains in beer after fermentation, indicating that THP production is not limited by lysine availability, but rather some other unknown factor . Strangely, Brettanomyces cultures that have been exposed to oxygen continue to produce elevated amounts of ATHP after the oxygen is removed, indicating that the presence of oxygen may predispose Brettanomyces to THP production . The early growth of other aerobic yeasts in lambic, such as saccharomyces, as well as the formation of the pellicle, prevents much exposure to oxygen during the primary growth of Brettanomyces from month ~8 onward, which should reduce total THP production in lambic compared to all-brett beers.

Brettanomyces also appears to metabolize ATHP into ETHP, the latter of which has a much higher taste and odor threshold than the former. This may explain how THP flavors and odors may "age out" of lambic over time . ETHP is not believed to be further metabolized by Brettanomyces .

The second primary source of THP during fermentation are heterofermentive lactic acid bacteria (LAB), which various strains can produce all three major forms of THP found in lambic. While nearly every genus of LAB contains at least one strain known to produce THP, Lactobacillus appears to be the primary source of bacterial THP in beer. It is not known if oxygen plays a role in the production of THP by LAB as it does with Brettanomyces.