Optimization of Ultrasound-assisted Enzymatic Extraction of Walnut Diaphragm Polyphenols and Its Antioxidant Activity

Purpose To optimize the extraction process of walnut diaphragm polyphenols (WDP) and to study its antioxidant activity.

Methods Walnut diaphragm from Yangbi, Yunnan Province, was used as material, the effects of cellulase addition amount, solid-liquid ratio, ultrasonic power, ultrasonic time and ethanol volume fraction on the extraction rate of WDP were explored by single-factor experiment. The polyphenol extraction process was then optimized by response surface method. The crude extract was initially purified by D101 macroporous resin, getting WDP, which were then analyzed for purity. The antioxidant properties were evaluated by the scavenging ability of DPPH· radicals, the scavenging ability of ABTS⁺ radicals, and the iron reducing ability.

Results The optimal extraction process for WDP by ultrasound-assisted cellulase method included: enzyme addition was 0.6%, solid-liquid ratio was 1∶80 (g∶mL), ultrasound power was 600 W, ultrasound time was 60 min, and volume fraction of ethanol was 60%. The rate of polyphenol extraction was 7.26%, and the polyphenol purity obtained from the preliminary purification was 89%. IC₅₀ values of the scavenging ability of DPPH· radicals and ABTS⁺ radicals was 11.11 and 183.12 µg/mL, respectively. Furthermore, the iron reducing ability reached 59.62% of that of the same mass concentration of ascorbic acid, which indicated that the WDP had strong antioxidant activity.

Conclusion The optimal extraction process of WDP is identified, providing a foundation for the development of the walnut industry and the utilisation of active ingredients of walnut diaphragm.