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

The impacts of medium pH, temperature and co-existing proteins on the degradation of two flavonoids, fisetin and quercetin, were assessed in the present study using a spectroscopic method. Based on the measured degradation rate constants (k), fisetin was more stable than quercetin in all cases. Increasing the medium pH from 6.0 to 7.5 at 37 degrees C enhanced the respective k values of fisetin and quercetin from 8.30x10(-3) and 2.81x10(-2) to 0.202 and 0.375 h(-1), respectively (p < 0.05). In comparison with their degradation at 37 degrees C, fisetin and quercetin showed larger k values at higher temperature (0.124 and 0.245 h(-1) at 50 degrees C, or 0.490 and 1.42 h(-1) at 65 degrees C). Four protein products in the medium at 0.10 g L-1 stabilized the two flavonoids (p < 0.05), as they decreased the respective k values of fisetin and quercetin to 2.28x10(-2)-2.98x10(-2) and 4.37x10(-2)-5.97x10(-2) h(-1). Hydrophobic interactions between the proteins and the two flavonoids were 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 was thus concluded that higher temperature and alkaline pH could enhance flavonoid loss, whereas the coexistence of proteins as flavonoid stabilizers could inhibit flavonoid degradation.