Study on antioxidant and antimicrobial activities of ethanolic extracts of onions (allium cepa) and shallots (allium ascalonicum) cultivated in Vietnam

Abstract

From the ancient time herbs and spices have attended in traditional medicinal treatments against minor disease such as cough, flu, etc. Due to their natural origin, these plant materials are safe and can be exploited for many purposes. This study aimed to evaluate the antimicrobial and antioxidant capacity of four Allium species of Vietnam: purple onion, white onion, purple shallot and white shallot. Under ethanolic extraction, the highest total phenolic and total flavonoid content was detected in purple onion (22.66 ± 1.35 mg FAE/gDW and 11.00 ± 3.74 mgRE/gDW, respectively), which correlated to the strong antioxidant activity (84.70% for 50 mg/ml extract concentration) measured by DPPH-radical scavenging assay; white onion ranked second, neutralized 67.82% DPPH-radical; while purple shallot and white shallot ranked third for three mentioned criteria. The antimicrobial activities were tested against two gram-positive pathogenic bacteria: S. aureus and B. cereus (VTCCB-1005); and two gram-negative pathogenic bacteria: P. aeruginosa (ATCC-9027) and S. typhi - by agar well diffusion method and measuring the minimum inhibitory concentration (MIC). Although not being a prominent antioxidant in four test subjects, white shallot was appreciated for expressing the best antimicrobial activity, followed by purple onion and white onion. S. typhi resisted against all extracts while purple shallot showed no antimicrobial activity against reference strains. The distinction in growth suppression between gram negative and gram positive bacteria were not significant. Key words: Onion Shallot Antioxidant activity Antimicrobial activity Total phenolic compounds Total flavonoid compounds