Optimized ultrasonic-assisted extraction of phenolic antioxidants from grape ( Vitis vinifera L.) skin using response surface methodology

Currently, the analysis of bioactive compounds is easily carried out by modern chromatographic and spectrometric techniques, but the success of the procedure still depends on the efficiency of the extraction methods. This work aimed at finding the optimal conditions for ultrasonic-assisted extraction (UAE) of phenolic antioxidants from grape skin using single-factor experimentation combined with response-surface methodology. Our extraction parameters were ethanol concentration, time and temperature. Total phenolic content (TPC and TPC-280), 2,2-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity and oxygen radical absorbance capacity (ORAC) were dependent variables. A second-order polynomial model satisfactorily described the experimental data. Optimized extraction conditions for UAE from grape skin were: 59.5% ethanol, 113.6 min and 66.8 °C for TPC extraction; 61.1% ethanol, 99.4 min and 66.1 °C for TPC-280 extraction; 57.1% ethanol, 99.5 min and 66.8 °C for DPPH radical-scavenging activity; 60.2% ethanol, 82.4 min and 66.8 °C for ORAC value. Our work provides detailed and useful information to optimize the extraction of grape skin phenols, which are widely used as dietary supplements and nutraceuticals.