Ultrasound-assisted extraction of phenolic compounds from macadamia (Macadamia integrifolia) green peel: Purification, identification and antioxidant activities

China is one of the world's main producers of macadamia. The macadamia green peel (MGP), byproducts of industrial processing, are commonly discarded for use as fertilizer. In this study, the ultrasound-assisted extraction and purification process of phenolic compounds from MGP were optimized, and the identification and analysis of antioxidant activities of purified extracts were performed. Results indicated that the optimal extraction conditions were sonication power 140 W, solid-liquid ratio 1:45 g/mL, and extraction temperature 43 degrees C, and the total phenolic content (TPC) was 18.23 mg gallic acid equivalent (GAE)/g dry weight (DW). Among four purified fractions (PF1, PF2, PF3 and PF4) obtained by ADS17 resin, sixty phenolic compounds were identified and twenty-one phenolic compounds were quantified for the first time, of which PF2 showed the highest TPC (246.18 mg GAE/g DW), mainly including 3,4-dihydroxybenzaldehyde, vanillic acid, 4-hydroxyben-zoic acid and gallocatechin. In addition, PF2 demonstrated excellent total antioxidant activity (ABTS, DPPH and FRAP assays were 2.16, 2.59, 1.96 mM Trolox equivalent/g DW, respectively) and cellular antioxidant activity (0.97 mM quercetin equivalent/g DW). The antioxidant activities showed significant positive correlated with the content of catechin and procyanidin B1. The results provide guidance for the industrial production and application of MGP.