Multiple stage microwave-assisted extraction of phenolic compounds from dark purple sweet potato (Ipomoea batatas (L.)Lam.) peels

Dark purple sweet potato (Ipomoea batatas (L.)Lam.) peel is considered as waste in food processing, but it is known to contain high concentration of phenolic compounds which are known to be useful in the prevention of many chronic diseases. The multiple-stage microwave-assisted extraction (MAE) is observed as an effective method to extract these compounds. Therefore, the objective of this study was to evaluate the multiple-stage microwave-assisted extraction (MAE) of phenolic compounds from dark purple sweet potato peels The optimization of parameters to get a high yield of total phenolics and anthocyanin were investigated at single-stage MAE by using RSM. Face-centered central composite design was used to investigate the effect of two independent variables namely, solvent-to-solid ratio (15-35ml/g) and ethanol concentration (60-100%). The results showed that the solvent-to-solid ratio had positive significant effect on total phenolics extracted but it had no significant effect on the yield of anthocyanin. The ethanol concentration had negative significant effect on the yield of total phenolics and anthocyanin. The optimal condition of MAE of phenolics were 33 ml/g and 67% of solvent-to-solid ratio and ethanol concentration, respectively. Moreover, the optimal condition of anthocyanin extracted were 29 ml/g of solvent-to-solid ratio and 63% of ethanol concentration. The optimal yield of total phenolics and anthocyanin were found to be 12.37 mg GAE/g and 7.96 mg CyE/g, respectively. The conventional solvent extraction and multiple-stage MAE was performed to compare the results of total phenolics and anthocyanin with that from single-stage MAE. The yield of total phenolics and anthocyanin extracted from conventional solvent extraction were 20% and 73.5% lower than that from single-stage MAE, respectively. However, the yield of total phenolics and anthocyanin obtained from multiple-stage MAE were 15.4% and 26.1% higher than that from single-stage MAE. Therefore, the multiple-stage MAE can improve the extraction yield of phenolic compounds from dark PSPP