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Skatole metabolites have been considered as putative contributors to boar taint. Recently, 2-aminoacetophenone, a volatile phase I skatole metabolite, was identified in back fat samples from boars of Pietrain × Baden-Württemberg hybrid type. This paper addresses the question of the physiological origin of the observed 2-aminoacetophenone in these pigs. Microsomal fractions from nine boars were isolated, and formation of skatole metabolites was subsequently analyzed by stable-isotope dilution analysis (SIDA) using headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME/GC-MS). Significant breed-related differences in phase I skatole metabolism were observed, explaining the high levels of 2-aminoacetophenone in Pietrain × Baden-Württemberg hybrid type boars.

The aroma profile of porcine fat from tainted boars, female pigs, and castrated male pigs was investigated by application of comparative aroma extract dilution analysis (cAEDA) on a SAFE distillate of volatiles prepared from porcine back fat samples. The AEDA resulted in a total of 16 aroma active compounds for boar fat with flavor dilution (FD) factors ranging from 2 to 2048, whereas 12 aroma active compounds were found in fat of female pigs and 14 in fat of castrated male pigs, both with FD factors ranging from 2 to 32. Odor activity values (OAVs) of key components for each fat were identified: In boar fat androstenone, skatole, indole, and 2-aminoacetophenone showed highest OAVs, whereas 2,5-dimethylpyrazine, 2,4-decadienal, and δ-decalactone showed highest OAVs in fat of female pigs. Fat of castrated male pigs showed highest OAVs for skatole, indole, 1-octen-3-ol and methional. Finally, the off-flavor attributes of boar fat were successfully simulated by a recombinant of all odorants at their natural concentration level in deodorized sunflower oil.

2-Aminoacetophenone, a product of the hepatic 3-methylindole (skatole) clearance in pigs, contributes to the specific aroma of fat from tainted boar meat. Surprisingly, high formation rates for 2-aminoacetophenone from skatole in microsomal preparations from Pietrain × Baden-Württemberg hybrid type boars have been previously demonstrated, but the mechanism of this cytochrome P450-mediated reaction remained unknown. Therefore, microsomal fractions from boars were incubated with deuterium-labeled skatole and with possible reaction intermediates. 3-Methyloxindole and 3-hydroxy-3-methyloxindole were identified as degradation products of skatole en route to 2-aminoacetophenone. Additionally, the labeling studies provided further evidence for a cytochrome P450-mediated lyase reaction that leads to the cleavage of the indole heterocyclic ring system in 3-hydroxy-3-methyloxindole and demonstrated the involvement of several cytochrome P450-isoforms by employing isotopically sensitive branching experiments.

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