Degradation kinetics of fisetin and quercetin in solutions affected by medium pH, temperature and co-existed proteins

Abstract

Impacts of medium pH, temperature and coexisted proteins on the degradation of two flavonoids fisetin and quercetin were assessed by spectroscopic method in the present study. Based on the measured degradation rate constants (k), fisetin was more stable than quercetin in all cases. Increasing medium pH from 6.0 to 7.5 at 37?C enhanced respective k values of fisetin and quercetin from 8.30x10?3 and 2.81x10?2 to 0.202 and 0.375 h-1 (P<0.05). In comparison with their degradation at 37?C, fisetin and quercetin showed larger k values at higher temperature (0.124 and 0.245 h?1 at 50?C, or 0.490 and 1.42 h?1 at 65?C). Four protein products in medium could stabilize the two flavonoids (P<0.05), as these proteins at 0.10 g L-1 decreased respective k values of fisetin and quercetin to 2.28x10?2-2.98x10?2 and 4.37?10?2-5.97x10?2 h?1. Hydrophobic interaction between the proteins and the two flavonoids was evidenced responsible for the stabilization, as sodium dodecyl sulfate could destroy the stabilization significantly (P<0.05). Casein and soybean protein provided greater stabilization than whey protein isolate. It is thus concluded that higher temperature and alkaline pH can enhance flavonoid loss, whereas coexisted proteins as flavonoid stabilizers can inhibit flavonoid degradation.