Abstract: Response surface methodology was employed to optimize the ultrasonic-assisted extraction of polyphenols from grape skin as a byproduct of wine making. On the basis of one-factor-at-a-time experiments, through which the optimal ultrasonic power and ethanol concentration were found to be 100 W and 40%, respectively, polyphenol yield was investigated by response surface methodology with respect to liquid-to-material ratio, extraction time and temperature at three levels. The optimal levels of liquid-to-material ratio, extraction time and temperature were determined to be 16:1 (mL/g), 57 min and 50 ℃. Under such conditions, the yield of grape skin polyphenols was (42.51 ± 1.21) mg/5 g. The antioxidant activity in vitro of the obtained extract was evaluated by DPPH free radical scavenging assay and potassium ferricyanide reduction assay using water-soluble VE as the control. Grape skin polyphenols were found to have powerful DPPH free radical scavenging capacity and reducing power in a linear dose-dependent manner.

Key words: grape skin, polyphenol, response surface methodology (RSM), ultrasonic-assisted extraction, antioxidant activity