Change in Flavor Components of Onion by Roasting

Flavor Components of Roasted Onion (Part 1)

Abstract

Chopped-onions roasted with butter, salad oil or silicone oil gave rise to good-flavor. We attempted to elucidate the components responsible for good-flavor caused from roasted onion.
Onions were roasted with silicone oil at 140°C for 25 min and steam-distilled for 4 hr. Roastedonion flavor and fresh-onion flavor were identified by GC and GC-MS. Major components of roastedonion flavor were 2, 4-dimethylthiophene, methyl propyl trisulfide and propyl propenyl trisulfide (cis and trans). Those of fresh onion flavor were 2, 3-dimethylthiophene, propyl propenyl disulfide (cis and trans), dipropyl disulfide and dipropyl trisulfide. Trisulfides being stable components were increased by roasting.
Roasted-onion flavor components were separated in the 7 fractions by chromatography on a column of silica gel-celite 545 (1: 1, w/w) with n-hexane-ether gradients. Most of sulfides in volatiles were eluted in the first fraction with n-hexane and they smelled fresh onion flavor. This fraction was further separated by chromatography on a column of 10% AgNO₃-silica gel with n-pentane-n-hexane gradients. But sweet-smelling flavor components were not eluted in a single fraction. The flavor of other 6 fractions separated by silica gel-celite 545 column chromatography was not so good-flavor. However, mixing of a few fractions in the first fraction revealed the roasted onion flavor again.