Effect of extraction solvents on recovery of bioactive compounds and antioxidant properties from macadamia (Macadamia tetraphylla) skin waste

Macadamia is well known for its high-quality kernel; however, its skin and husk, known as waste, have had limited uses as by-products. This study aimed to determine the characteristics of the macadamia nut and to examine the impact of different solvents on the recovery of phenolic compounds (TPC), flavonoids, proanthocyanidins and antioxidant properties of the macadamia skin. The results showed that the kernel, the commercial part, only accounts for about 20% of the nut weight; whereas, the skin or the husk, the waste, account for approximately 40%, revealing that macadamia skin is an abundant starting material for the utilisation of by-products. The type of solvents was found to significantly affect the recovery yields of bioactive compounds from the macadamia skin. The combination of organic solvents, methanol, ethanol, acetonitrile and acetone with water (50%, v/v) had the highest recovery yields for TPC, flavonoids and proanthocyanidins as well as the most potent antioxidant properties, followed by absolute methanol, then water. Absolute ethanol, acetonitrile and acetone had the lowest recovery yields. The current study recommended that 50% acetone should be used for the recovery of TPC and flavonoids and 50% ethanol should be used for the extraction of proanthocyanidins from the macadamia skin.