Ultrasound-assisted extraction, optimization, purification of gallic acid from walnut green husk and analysis of their antioxidant and antibacterial properties

The walnut green husk (WGH), a byproduct of food industry processing, is often discarded as waste. This study presents a comprehensive and efficient approach for the extraction, purification, and characterization of gallic acid (GA) from WGH. Using response surface methodology (RSM), optimal extraction conditions were determined, achieving a GA yield of 15.585 % under the following parameters: ethanol volume fraction of 73 %, solid-liquid ratio of 1:30 g/mL, ultrasonic temperature of 65 °C, and ultrasonic time of 43 min. GA was further purified through macroporous resin adsorption, with ADS-17 exhibiting the highest adsorption capacity. Under optimal conditions, the adsorption capacity reached 0.89 mg/g, desorption efficiency was 75 %, and GA purity was 83.07 %. Compared with the crude extract of gallic acid (CEGA) and a pure GA standard solution, the purified extract of gallic acid (PEGA) showed significantly increased antioxidant activity. The scavenging efficiency of DPPH· and ABTS·⁺ free radicals increased linearly with GA concentration. PEGA also demonstrated potent antibacterial activity against Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), and Candida albicans (C. albicans). These findings highlight the potential of GA extracted from WGH as a natural antioxidant and antimicrobial agent for applications in the food industry.