The factors of the characteristic and sweet flavor of cooked onion were studied.
Propanethiol (propyl mercaptane), which has been reported as the main component of the sweet taste of boiled onion, was ascertained to be not a sweet compound by sensory evaluation using 100% pure authentic sample.
Changes in the free sugar contents in the onions cooked by different methods were investigated by using HPLC. The contents of glucose, fructose, and sucrose decreased or scarcely changed by cooking. The increase of sugar concentration was confirmed to be caused by evaporation of water during cooking. This phenomenon seems to be the important factor for cooked onion flavor.
Aroma concentrate from the water extract of the baked onion was obtained by adsorbing on the porous polymer. A total of 35 components was identified by GC-MS. By GC-FID-FPD analysis, sulfur-containing compounds, which were the main components of the fresh onion volatiles, were found to decrease significantly by cooking. Oxygenated compounds which possessed sweet or caramel like aroma seemed to contribute to the characteristic sweet flavor of cooked onion.