α‐Acetyl‐N‐heterocycles in the Maillard reaction

α‐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 α‐acetyl‐N‐heterocycles have so far been reported to occur in heated foodstuffs or process flavorings. The partially unsaturated α‐acetyl‐N‐heterocycles 2‐acetyltetrahydropyri‐dine, 2‐acetyl‐1‐pyrroline, 2‐acetyl‐2‐thiazoline, and 5‐acetyl‐2,3‐di‐hydro‐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 α‐acetyl‐N‐heterocycles as well as for ace‐tylpyrazines range from 10 ppb to 170 ppm. Seven of the 12 α‐acetyl‐N‐heterocycles, especially those with partially unsaturated monocy‐clic 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 α‐acetyl‐N‐heterocycles. The mechanism of their formation in the Maillard reaction is reviewed here.