Optimization of the Microwave-Assisted Extraction of Polyphenols from Red Pitaya Peel using Response Surface Methodology

In order to maximizing extraction yield of polyphenols from red pitaya (Hylocereus undatus) peel, an agro-industrial byproduct in pitaya juice processing, the present work firstly focused on optimization of the microwave-assisted extraction (MAE) using response surface methodology (RSM). A Box-Behnken design (BBD) was used to monitor effects of five MAE factors on the polyphenols yield, including extraction time, ratio of solvent to raw material, microwave power, extraction temperature, and ethanol concentration. The optimal conditions were extraction time of 20.3 min, ratio of solvent to raw material 33.4:1 (mL:g), microwave power 497 W, extraction temperature 43.3 degrees C and ethanol concentration 64.9%. Validation tests suggested that the actual yield of polyphenols was (463.8 +/- 1.1) mg gallic acid equivalents (GAE) per 100 g dry pitaya peel with the relative standard deviation (RSD) of 2.15% (n = 5) under the optimized conditions, which was in good agreement with the predicated yield. Antioxidant assays suggested that the ethanol extract from red pitaya peel had stronger DPPH, hydroxyl and superoxide free-radical scavenging capabilities than vitamin C at 1.0 mg/mL. These results provide an alternative way to make good use of red pitaya peel to produce natural antioxidant.