Quantitative analysis of the flavonoid content of commercial tomatoes, onions, lettuce, and celery

Quantitative estimates of conjugated flavonoid content were obtained by using HPLC to analyze the level of free flavonoids present in acid-hydrolyzed extracts from commercial fruits and vegetables. Cherry tomatoes contained 17-203 mu g of quercetin g(-1) fresh weight compared to 2.2-11 mu g g(-1) detected in normal-sized Scottish, Spanish, and Dutch beef tomatoes. The quercetin levels in onions ranged from 185 to 634 mu g of quercetin g(-1) fresh weight. ''Round'' lettuce contained 11 mu g of quercetin g(-1) fresh weight compared to 911 mu g g(-1) in the outer leaves and 450 mg g(-1) in the inner leaves of 'Lollo Rosso'' lettuce. The conjugated flavonoid content of celery was very variable, ranging from undetectable to 40 mu g of luteolin and 191 mu g of apigenin g(-1) fresh weight. Cooking lowered the quercetin content of both tomatoes and onions with greater reductions being detected following microwaving and boiling than after frying.