Comparison between conventional and ultrasound-assisted extractions of natural antioxidants from walnut green husk

Agricultural industries produce substantial quantities of phenolic-rich by-products, which have gained much attention due to their antioxidant properties. Ultrasonic technology was applied for extraction of antioxidants from the walnut green husk using ethanol as a food grade solvent. Response surface methodology (RSM) was used to optimize experimental conditions. The responses were total phenolic content (TPC), ferric reducing antioxidant power (FRAP), scavenging activity of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical and yield. TPC varied from 6.28 to 7.23mg GA g^?1 dry sample. FRAP and DPPH values varied from 0.33 to 0.46 mmol Fe²⁺ g^?1 of dry sample and 33.98% to 56.31% inhibition, respectively. Extraction yields ranged from 33.04% to 38.72%. The optimal conditions were 60% ethanolwater mixture as solvent, temperature of 60 °C and extraction time of 30 min. Comparison of ultrasonic-assisted extraction (UAE) and conventional extraction was shown that TPC, FRAP, DPPH and yield obtained by UAE during 30 min were significantly higher than by conventional extraction during 16 hours. The extract can be used as substitute of synthetic antioxidants for food products, color and oxidative stabilization.