Abstract: String beans were used as the raw material to extract and purify saponin by ultrasonic-assisted ethanol extraction. The scavenging capacity of string bean saponin on DPPH, hydroxyl and alkoxyl free radicals was determined and compared with that of vitamin C. The results showed that string bean saponin and vitamin C had scavenging capability to DPPH, hydroxyl and alkyl free radicals. The scavenging capacity of string bean saponin at the concentration higher than 2.5 mg/mL was stronger than that of vitamin C. In addition, string bean saponin had obvious inhibitory effect on Bacillus subtilis, Escherichia coli and Staphylococcus aureus, but did not exhibit inhibitory effect on Aspergillus niger. The minimum inhibitory concentrations (MIC) were 0.001 mg/mL for Bacillus subtilis and Staphylococcus aureus, and 0.0005 mg/mL for E. coli, which demonstrated that string bean saponin was more sensitive to E. coli.

Key words: string bean, saponin, anti-oxidant, Sbacteriostasis