Abstract:Walnut oil extraction residues are currently under-utilized; thus, hydraulically pressed walnut meal was used as raw material for the extraction of crude polyphenols using a microwave-ultrasound co-assisted extraction process optimized by singlefactor experiments and response surface design, and ultra-high-performance liquid chromatography-tandem mass spectrometry-time of flight mass spectrometry (UPLC Q-TOF/MS) employed to analyze and identify polyphenolic compounds. The antioxidant activity was evaluated through DPPH, ABTS, and hydroxyl radical scavenging assays, and the extracts were added back into the walnut oil to verify their impact on the oxidative stability. The results indicated that the solid-liquid ratio and volume fraction of ethanol significantly affected the extraction of polyphenols from walnut meal (P<0.05). The optimal extraction conditions of a solid-liquid ratio of 1:57.33 g•mL-1, an ethanol volume fraction of 69.13%, ultrasonic power of 100 W, and microwave power of 109.92 W resulted in a crude polyphenol extraction yield of 40.59 mg GAE (gallic acid equivalents)•g sample-1. A total of 24 polyphenolic compounds were identified, with higher contents of 3,4-dihydroxybenzoic acid (8 233.93 μg•g-1), kaempferol-3-O-rutinoside (4 504.20 μg•g-1), and ellagic acid (1 882.50 μg•g-1) observed. Walnut meal crude polyphenols exhibited good DPPH and ABTS radical scavenging effects (over 95%) at a mass concentration of 0.80 mg•mL-1 and maintained a hydroxyl radical scavenging rate of 60%~70%. The addition of walnut meal crude polyphenols at a mass concentration of 0.08 mg•g-1 increased the oxidative stability of walnut oil by more than 30%. As natural antioxidants, walnut meal polyphenols are expected to have high application value in the food industry after further separation and purification.
