ALPHA-ACETYL-N-HETEROCYCLES IN THE MAILLARD REACTION

alpha-Acetyl-N-heterocycles are a special class of Maillard reaction p roducts with typical roasty flavor characteristic and generally low od or thresholds. A total of 12 alpha-acetyl-N-heterocycles have so far b een reported to occur in heated foodstuffs or process flavorings. The partially unsaturated alpha-acetyl-N-heterocycles 2-acetyltetrahydropy ridine, 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 c orresponding aromatic alpha-acetyl-N-heterocycles as well as for acety lpyrazines 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 th erefore their formation has been studied extensively. In particular, t he amino acids proline, hydroxyproline, ornithine, citrulline and cyst eine have been considered efficient precursors for these alpha-acetyl- N-heterocycles. The mechanism of their formation in the Maillard react ion is reviewed here.