Effects of domestic cooking on flavonoids in broccoli and calculation of retention factors

The flavonoid contents in vegetables are strongly influenced by domestic cooking. The objective of this study was to evaluate the effects of domestic cooking on the structurally complex flavonoids in broccoli. Raw broccoli was cooked by boiling, steaming and microwaving. Seven kaempferol (Km) glycosides and one quercetin (Qn) glycoside were identified and quantified in raw and cooked broccoli by HPLC-MS. Boiling resulted in significant loss of all flavonoids, while steaming and microwaving led to minor loses or even increases of the flavonoids. Apparent retention factors (AR) and true retention factors (TR) were calculated for individual flavonoids. AR ranged from 35.6% to 147.5% and TR ranged from 30.4% to 174.1%, respectively, depending on the cooking method and chemical structures of flavonoids. Two different ways to calculate total retention factors, Retention Factor by Glycoside and Retention Factor by Aglycone, were also calculated. In conclusion, domestic cooking significantly altered the flavonoid contents in broccoli, with cooking method and chemical nature being key influential factors. Acylated Km tri- or tetra-glycosides appeared to be more resistant to domestic cooking.