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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 <ref name=Grbin> Grbin, PR; Hederich, M; Markides, A; Lee, TH; and Henschke, PA [http://pubs.acs.org/doi/abs/10.1021/jf071243e | The Role of Lysine Amino Nitrogen in the Biosynthesis of Mousy Off-Flavor Compounds by Dekkera anomala], 2007</ref>. Further, lysine usually remains in beer after fermentation, indicating that THP production is not limited by lysine availability, but rather some other unknown factor <ref name=Snowdon> Snowdon, EM; Bowyer, MC; Grbin, PR; and Bowyer, PK [http://pubs.acs.org/doi/pdf/10.1021/jf0528613 | Mousey Off-Flavor: A Review], 2006</ref>. 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 <ref name=Snowdon> Snowdon, EM; Bowyer, MC; Grbin, PR; and Bowyer, PK [http://pubs.acs.org/doi/pdf/10.1021/jf0528613 | Mousey Off-Flavor: A Review], 2006</ref>. 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 <ref name=Joseph> Joseph, CML [http://slideplayer.com/slide/4381124/ | Aromatic Diversity of Brettanomyces]</ref>. ETHP is not believed to be further metabolized by Brettanomyces <ref name=Snowdon> Snowdon, EM; Bowyer, MC; Grbin, PR; and Bowyer, PK [http://pubs.acs.org/doi/pdf/10.1021/jf0528613 | Mousey Off-Flavor: A Review], 2006</ref>.