Abstract
After sautéing or frying onions as a first procedure in making onion soup, major onion antioxidants (quercetin 4′-glucoside and quercetin 3,4′-diglucoside) remained mostly intact. In the onion soup prepared with sautéed onions, the quantity of flavonoids remained high, but in the onion soup prepared with fried onions, a part of them was changed during cooking. The heating stability of these flavonoids was investigated through simple boiling and oven heating of these compounds. Major onion antioxidants were quite stable in a simple cooking model of boiling and oven heating at 100°C but considerably degraded in that of oven heating at 200°C. DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging activity of sautéed onions was similar to that of raw onions, and fried onions exhibited much higher activity based on equivalent amounts of raw onion. Fried onions had much higher browning degree than sautéed onions, and the browning substance was found to contribute to the radical scavenging activity. The onion soup prepared with sautéed onions had DPPH radical scavenging activity similar to plain sautéed onions, but the onion soup prepared with fried onions had much lower activity than fried onions alone.
