Optimization of ultrasound-assisted extraction conditions of natural antioxidants from Stachys lavandulifolia

In this study, ultrasound-assisted extraction (UAE) conditions of natural antioxidants from Stachys lavandulifolia, with the separate and simultaneous maximization of total phenolic (TP), total flavonoid (TF) and antioxidant activity (AA) by response surface methodology, were optimized. A central composite design, with four independent variables including power of ultrasound, (W), time (min), liquid to solid ratio, and solvent composition (%), with 30 runs at five levels with six replicates at the central point, was used. Results exhibited that the second-order polynomial model could be successfully used for optimizing the extraction parameters. Correlation coefficients (R2) of the models for TP, TF and AA were 0.83, 0.90 and 0.92, respectively. The results of the optimization process of extraction showed that the highest concentration of TP (19.8 mg gallic acid equivalents (GAE)/g of dry matter (DM)) in the conditions were power of ultrasound 297.8 W, liquid to solid ratio 56.3, time 14 min and solvent composition 43.2% methanol; the highest concentration of TF (5.54 mg quercetin equivalents (QE)/g DM) in the conditions were power of ultrasound 300 W, liquid to solid ratio 60, time 14 min and solvent composition 69.4% methanol, and the optimum conditions in terms of AA 78.4% under conditions were obtained at power of ultrasound 300 W, liquid to solid ratio 40, time 14 min and solvent composition 61.7% methanol.