Comparative study on the effect of extraction solvent on total phenol, flavonoid content, antioxidant and antimicrobial properties of red onion (Allium cepa)

Red onion extract is rich of phenol and flavonoid component that has many applications in food, cosmetic and pharmaceutical industries. Effect of various solvents on total phenolic content (TPC), total flavonoid content (TFC), antioxidant activity and antimicrobial properties of red onion skin extract was evaluated. After drying of the first outermost layer of onion peel, extraction was performed in a series of solvents with increasing polarity (ethyl acetate, n-butanol ethanol, methanol, and water) using maceration method. Extraction in highly polar solvents led to high extract yield but low phenolic and flavonoid content as compared to ethyl acetate and n-butanol. The highest yield of extraction was obtained using methanol (5.6 +/- 0.07% w/w) followed by water (4.8 +/- 0.06% w/w). Results depicted that the extracts acquired by ethyl acetate had the highest TPC (423.95 +/- 11.39 mg GAE/g of extract), TFC (29.59 +/- 1.71 mg QE/g of extract) and antioxidant and antimicrobial activity. High correlation was also found between DPPH radical scavenging activity and total flavonoid content (R = 0.9468). Moreover, the extracts were more active against Staphylococcus aureus as compared to Escherichia coli and Salmonella Typhimurium. Using intermediate-polar solvents in contrast to polar solvents resulted to extract the components with higher antioxidant and antimicrobial properties. In conclusion, red onion skin can be used as antibacterial agents against foodborne pathogens and in dietary applications with a potential to reduce oxidative stress.