Optimisation of Ultrasonic Conditions as an Advanced Extraction Technique for Recovery of Phenolic Compounds and Antioxidant Activity from Macadamia (Macadamia tetraphylla) Skin Waste

Thousands of tons of macadamia skin waste are generated annually with very limited utilisation of this extensive by-product. The aim of this study was to develop optimal ultrasonic extraction conditions for maximized recovery of phenolic compounds and antioxidant properties from macadamia skin using Response Surface Methodology (RSM). Three ultrasonic parameters, including temperature (30–50 °C), time (10–50 min) and power (150–250 W), were tested for their impact on the extraction of total phenolic compounds (TPC), flavonoids, proanthocyanidins and antioxidant properties. The results showed that ultrasonic temperature, time and power had an impact on TPC and antioxidant capacity; however, the effects varied. The optimum ultrasonic conditions for the maximum recovery of phenolic compounds and antioxidant properties from macadamia skin were found to be a temperature of 40 °C, a time of 35 min and a power of 80%/200 W. Under these optimal conditions, approximately 168 mg of TPC, 135 mg of flavonoids and 188 mg of proanthocyanidins can be extracted from one gram of dried macadamia skin.