Optimization of Ultrasound-Assisted Extraction of Phenolic Compounds from Walnut Shells and Characterization of Their Antioxidant Activities

Million tons of walnut shells, the waste by-product from walnut industries, are produced annually worldwide and are under-utilized in kernel manufacturing. Walnut shells are rich in phenolic compounds thus are valuable sources for antioxidants. In this study, the optimization of ultrasound-assisted extraction of phenolic compounds from walnut shells was performed with response surface methodology (RSM). The maximum yield of total phenolic compounds from walnut shells was 92.96±1.47 mg GAE/g DW under the optimum extracting conditions of 10 min at 0°C with 43.71% ethanol and 53.93% ultrasound amplitude. Analysis of antioxidant activities of the extracts found that the radical scavenging capacity of DPPH from the walnut shell extracts (10 - 500 μg/mL) was 4.05 – 88.59%, while the ferric reducing antioxidant power (FRAP) (10 - 100 μg/mL) was 119.64-1278.95 μM Trolox equivalent (TE). The results indicate that phenolic extracts from walnut shells are a good natural source of antioxidants, making this process a strong contender for future applications in the food and agricultural industries.