Effects of extraction techniques on antioxidant and antibacterial activity of stevia (Stevia rebaudiana Bertoni) leaf extracts

The effects of extraction techniques on the antioxidant and antibacterial activity of stevia (Stevia rebaudiana Bertoni) leaf extracts were assessed. The stevia leaf extracts contained appreciable amounts of total phenolic contents (7.8–21.63 mg GAE/g of dry extract), and total flavonoid contents (31.37–70.41 mg QE/g of dry extract). The EC50 values of DPPH and ABTS radical scavenging activity were 0.41–1.10 and 0.82–2.96 mg/ml, respectively. The ECA0.50 values of cupric reducing antioxidant capacity ranged from 0.01 to 0.21 mg/ml. Extracts obtained by shaker showed comparatively higher phytochemical contents and antioxidant activity than the extracts obtained by soxhlet extraction technique, regardless of the solvent used. An aqueous ethanolic (80 %) extract obtained by shaker exhibited a higher zone of inhibition against Staphylococcus aureus, Escherichia coli, Enterococcus faecalis, and Pseudomonas aeruginosa than other solvent extracts. The phytochemical contents of stevia leaf extracts were correlated with antioxidant activity. Phytochemical constituent profile of 80 % aqueous ethanolic extracts obtained by shaker was assessed by HPLC-ESI-Q-TOF-MS, and several phenolic acids, flavonoids, and steviol glycosides were identified. The results of this study demonstrated that 80 % aqueous ethanolic extracts of stevia leaf obtained by shaker showed significantly the highest antioxidant and antibacterial activity, which could be applied in food preservation and pharmaceutical use