Effect of processing on major flavonoids in processed onions, green beans, and peas

The contents of flavonoids in onions, green beans, and peas have been analysed in relation to the effect of different heat treatments. Two major flavonoids were studied, quercetin and kaempferol. The identification and quantification of the flavonoids were performed with high performance liquid chromatography and UV detection. The greatest loss of flavonoids in onion took place during the pre-processing step where the onion was peeled, trimmed, and chopped before blanching. Blanched onion contained 25 mg quercetin and 0.35 mg kaempferol per 100 g edible part. Blanched green beans contained 1.3 mg quercetin and 0.24 mg kaempferol per 100 g, and blanched peas only 0.15 mg quercetin per 100 g. No kaempferol was detected in peas. Further cooking, frying or warm-holding for up to 2 h of the blanched vegetables, did not influence the flavonoid content. Onions in ready-made dishes and home-cooked food as well as green beans may be good dietary sources of flavonoids.