TISSUE AND SPATIAL-DISTRIBUTION OF FLAVONOL AND PEROXIDASE IN ONION BULBS AND STABILITY OF FLAVONOL GLUCOSIDES DURING BOILING OF THE SCALES

Two major flavonol glycosides [quercetin 3,4'-diglucoside (F1) and que rcetin 4'-monoglucoside (F2)] and a flavonol aglycon quercetin were ma inly localized in the abaxial epidermis of scales. Their contents incr eased on aging. Peroxidase in scales oxidized flavonols in the order q uercetin >> F2 > isoquercetin >> F1, and the activity was higher in th e outer than in the inner scales. These results suggest that the enzym e can participate in the formation of defense substances against infec tion and brown compounds in the dry skin from quercetin. Contents of F 1 and F2 in scales were decreased by cooking by boiling. This decrease was due to the release of F1 and F2 into cooking water and their oxid ation. F2 was oxidized more rapidly than F1 during cooking. The differ ence in the stability between F1 and F2 was due to the presence or abs ence of a hydroxyl group at the C-3 position of the glucosides.