Tetrahydropyridines

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

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[Mousy Off-Flavor:  A Review]. 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.