Hydro-distillation and microwave-assisted distillation of Sideritis raeseri: Comparison of the composition of the essential oil,hydrolat and residual water extract

Sideritis raeseri is widely used as a tea in the traditional medicine of many Mediterranean, Balkan, and Middle East countries. In this study, microwave-assisted hydro-distillation (MAHD) at different power levels (180 W, 360 W, and 600 W) of Sideritis raeseri essential oil was compared in terms of extraction time, yield, qualitative analysis, and operational cost with the conventional method of hydro-distillation (HD). Obtained essential oil, as well as hydrolats and residual water extracts, were chemically characterized. An extraction time of 16–32 min with MAHD at different powers provided yields of 0.61–0.67%, while HD gave 0.01% after 141 min. The main constituents in all essential oils were oxygenated sesquiterpenes, and their content was higher in MAHD (32.93–51.42%) than in oil from HD (17.62%). HD oil was characterized by higher concentration of oxygenated monoterpenes (14.95%) in relation to MAHD oil (8.87–14.16%). The essential oil obtained by MAHD at 600 W has a higher content of diterpenes (13.26%) than HD oil (8.59%). All hydrolats displayed the highest content of oxygenated monoterpenes. The highest contents of 8-hydroxyflavone 7-allosylglucosides compounds in residual water extracts were obtained by applying higher (600 W) microwave power. MAHD could be selected as a more appropriate extraction technique for S. reaseri essential oil than HD with respect to the extraction time, quality, and energy savings above 99%.