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 quercetin 4'-monoglucoside (F2)] and a flavonol aglycon quercetin were mainly localized in the abaxial epidermis of scales. Their contents increased on aging. Peroxidase in scales oxidized flavonols in the order quercetin >> F2 > isoquercetin >> F1, and the activity was higher in the outer than in the inner scales. These results suggest that the enzyme can participate in the formation of defense substances against infection and brown compounds in the dry skin from quercetin. Contents of F1 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 oxidation. F2 was oxidized more rapidly than F1 during cooking. The difference in the stability between F1 and F2 was due to the presence or absence of a hydroxyl group at the C-3 position of the glucosides.