Kinetic study of subcritical water extraction of flavonoids from citrus unshiu peel

Citrus peels are the main source of many important flavonoids, flavanones (hesperidin and narirutin) and polymethoxyflavones (PMFs; sinensetin, nobiletin, and tangeretin), which have antioxidant, anti-inflammatory, anticancer, and cardioprotective properties. In this study, the mechanisms controlling the extraction rates of flavonoids from Citrus unshiu peel using subcritical water (SW) were studied at different temperatures (120-180 degrees C) and flow rates (1.0-2.0 mL/min). The extraction yields increased from 40.9, 69.0, and 67.4% at 120 degrees C to 79.6, 81.9, and 89.0% at 160 degrees C for hesperidin, narirutin, and PMFs, respectively, while decomposition occurred at 180 degrees C. The extraction rate curves at different flow rates were used to determine whether the extraction was best described by a thermodynamic partitioning or kinetic desorption model. The extraction rate curves showed that the initial extraction phase is fast, while the subsequent phase is slow. The thermodynamic partitioning model did not match with the experimental data for the latter part of the extraction period. The two-site kinetic desorption model fit the entire extraction period very well, suggesting that the extraction of citrus flavonoids was mainly controlled by intra-particle diffusion. Interestingly, this model fit well even at the pyrolysis temperature (180 degrees C). Therefore, the two-site kinetic model can well describe both the decomposition mechanism and extraction mechanism of citrus flavonoids when using SW. The diffusion coefficient of hesperidin increased about 9.8-fold at 160 degrees C and 2 mL/min relative to 120 degrees C and 1 mL/min. The activation energy of hesperidin (37.2-43.8 kJ/mol) was higher than those of narirutin and PMFs (8.2-36.8 kJ/mol). This study showed that the extraction mechanism is mainly affected by intra-particle diffusion, and that use of small amounts of SW, an environmentally friendly solvent, promotes good recovery of flavonoids from citrus peel in a short time.