Effect of Drying Methods on Essential Oil Content and Composition of Mentha longifolia (L.) Hudson

In this study, the effect of six methods (sun-drying, shade-drying, oven-drying at 40oC, oven-drying at 80oC, microwave-drying at 600 and 1000 W) on essential oil (EO) content and composition of Mentha longifolia (L.) Hudson was evaluated. EO samples were extracted using hydrodistillation and were analyzed by GC and GC-MS systems. The highest EO contents (w/w) were obtained by oven-drying at 40oC (0.94%), followed by the shade-drying (0.93%), microwave-drying at 600 W (0.83%), microwave-drying at 1000 W (0.77%), oven-drying at 80oC (0.72%) and sun-drying (0.63%). Thirty-five components were identified in EO samples of M. longifolia mostly consisted of oxygenated monoterpenes and hydrocarbons monoterpenes. The main components were pulegone (29.77-55.79%), piperitenone oxide (14.29-40.03%), carvacrol (5.28-12.08%), menthone (1.52-7.15%) and 1,8-cineol (0.62-4.79%); the drying methods showed marked impact on the ingredients of EO samples. The percentage of pulegone in EO was decreased as plant material either being dried using oven at 80 degrees C and microwave or exposed to direct sunlight. In addition, oven-drying at 80 degrees C or microwave methods caused piperitenone oxide, as the second major compound to be significantly increased.