alpha-acetyl-N-heterocycles in the Maillard reaction

alpha-Acetyl-N-heterocycles are a special class of Maillard reaction products with typical roasty flavor characteristic and generally low odor thresholds. A total of 12 alpha-acetyl-N-heterocycles have so far been reported to occur in heated foodstuffs or process flavorings. The partially unsaturated alpha-acetyl-N-heterocycles 2-acetyltetrahydropyridine, 2-acetyl-1-pyrroline, 2-acetyl-2-thiazoline, and 5-acetyl-2,3-dihydro-1,4-thiazine are reported to have very low odor thresholds (in water) between 0.1 and 1.6 ppb, whereas the odor thresholds for the corresponding aromatic alpha-acetyl-N-heterocycles as well as for acetylpyrazines range from 10 ppb to 170 ppm. Seven of the 12 alpha-acetyl-N-heterocycles, especially those with partially unsaturated monocyclic ring systems have been described as important aroma compounds, and therefore their formation has been studied extensively. In particular, the amino acids proline, hydroxyproline, ornithine, citrulline and cysteine have been considered efficient precursors for these alpha-acetyl-N-heterocycles. The mechanism of their formation in the Maillard reaction is reviewed here.