Microwave-Ultrasound Co-assisted Extraction and Identification and Antioxidant Activity Evaluation of Walnut Meal Polyphenols
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Abstract:
Using hydraulic walnut meal as the raw material, the polyphenols in walnut meal were extracted using a microwave-ultrasound co-assisted extraction process optimized by single-factor experiments and response surface design. Ultra-high-performance liquid chromatography-tandem mass spectrometry-time of flight mass spectrometry (UPLC Q-TOF/MS) was 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 to walnut oil to verify their impact on the oxidative stability. The results indicated that the solid-liquid ratio and the volume fraction of ethanol significantly affected the extraction of polyphenols from walnut meal (P<0.05). The optimal extraction conditions were the solid-liquid ratio of 1:57.33 g/mL, the ethanol volume fraction of 69.13%, ultrasonic power of 100 W, and microwave power of 109.92 W, the polyphenol extraction yield was of 40.59 mg GAE/g from walnut meal. A total of 24 polyphenolic compounds were identified, with higher contents of 3,4-dihydroxybenzoic acid (8233.93 μg/g), kaempferol-3-O-rutinoside (4504.20 μg/g), and ellagic acid (1882.50 μg/g). Walnut meal polyphenols exhibited good DPPH and ABTS radical scavenging effects (over 95%) at a mass concentration of 0.80 mg/mL and maintained a hydroxyl radical scavenging rate of 60%-70%. At a mass concentration of 0.08%, walnut meal polyphenols could increase the oxidative stability of walnut oil by more than 30%. As natural antioxidants, walnut meal polyphenols had high application value in the food industry after further separation and purification.
