Comparative study of two preservation methods (freeze drying and gamma irradiation) on the phenolic profile, antioxidant and antimicrobial activities of the essential oils of stevia

Stevia is known as a valuable spice plant. Freeze drying and gamma irradiation processing have been valuably used for microbial decontamination. In this study, the influence of freeze drying and gamma irradiation at the doses of 0.5, 1 and 2 kGy on the extraction yield, total polyphenol content, DPPH radical scavenging activity, and antimicrobial activity of stevia essential oils are investigated. The essential oils were obtained from dried stevia leaves by maceration with hexane. The results showed that the highest essential oils yield, polyphenol compounds, and antioxidant activity evaluated by scavenging of radicals DPPH were obtained by freeze drying. Moreover, the yield, total phenolic compounds, and antioxidants of volatile oils from commercial and irradiated stevia showed no significant changes with the irradiation doses. In addition, the extracts were tested in vitro for antimicrobial activity by the disc diffusion method, and minimal inhibitory concentration was determined. The antimicrobial tests showed that essential oils strongly inhibited the growth of both Gram-positive ( Listeria innocua and Staphylococcus aureus ) and Gram-negative ( Escherichia coli and Pseudomonas aeruginosa ) strains. Therefore, the freeze drying and γ-irradiation processes can be suggested for the decontamination of stevia without changes in its phenolic profile, and antioxidant and antimicrobial properties.