Influence of Extraction Techniques and Solvent Composition on Phenolic Recovery and Antioxidant Activity of Extracts from New Zealand-Grown Macadamia Husk

Macadamia husk, a major by-product of the macadamia industry, represents an underutilised source of phenolic compounds with significant potential for value-added applications. This study aimed to investigate the effects of extraction methods (conventional solvent extraction, accelerated solvent extraction and ultrasonic probe-assisted extraction) and solvent compositions (water, aqueous ethanol, and methanol) on the phenolic recovery and antioxidant activity of New Zealand-grown macadamia husk. Extraction performance was assessed by determining total phenolic content (TPC) and total flavonoid content (TFC), and antioxidant activities were evaluated by 1,1-diphenyl-2-picrylhydrazyl (DPPH), and 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging assays. Extraction efficiency varied significantly (p < 0.05) depending on both the solvent and extraction technique. Among the solvents evaluated, aqueous ethanol demonstrated superior performance, with 50% ethanol yielding higher total phenolic content (45.19 mg GAE/g dry sample) and the highest total flavonoid content (149.85 mg QE/g dry sample). Ethanol-rich extracts also exhibited the highest DPPH and ABTS radical scavenging activity. Ultrasonic probe-assisted extraction generally achieved higher phenolic recovery than conventional solvent extraction and accelerated solvent extraction under the conditions evaluated. Although UPAE using 90% methanol yielded the highest total phenolic content, 50% aqueous ethanol provided the most practical food-grade extraction system, achieving comparably high phenolic recovery together with strong antioxidant activity. Overall, the findings demonstrate that phenolic extraction efficiency and antioxidant capacity are strongly influenced by both solvent composition and extraction technique. The results support further investigation of New Zealand-grown macadamia husk as a potential source of natural antioxidant ingredients, particularly using food-grade aqueous ethanol extraction.