Quantity and chemical composition of essential oil of peppermint (Mentha x piperita L.) leaves under different drying methods

This work was aimed at quantitative and qualitative analyses of the essential oil of peppermint leaves under different drying methods. Thin layer drying experiments of the leaves were performed in shade, hot air dryer (at temperatures of 50 degrees C, 60 degrees C, and 70 degrees C), and microwave oven (at power levels of 200, 400, and 800W). Essential oils of the fresh and dried samples were extracted by hydrodistillation and analyzed using gas chromatography-mass spectrometry (GC/MS). The highest (22.24g/kg (dry matter)) and the lowest (1.33g/kg (dry matter)) oil yields were obtained from the hot air-dried leaves at temperature of 50 degrees C and microwave-dried leaves at power of 800W, respectively. In general, increasing drying temperature decreased the essential oil content. The GC/MS analysis of essential oils showed that the chemical compounds belonged mostly to oxygenated monoterpenes class (72.34-86.41%). The chemical compounds group was significantly (p<0.01) decreased by microwave drying at power levels of 200 and 400W. The assessed drying methods caused significant (p<0.05 and/or p<0.01) variations in the main constituents of the peppermint leaves essential oil including menthol, menthone, menthofuran, 1,8-cineole, and menthyl acetate. The minimum (35.01%) and maximum (47.50%) concentrations of menthol, as the major compound of the oil, were found in hot air-dried leaves at temperature of 50 degrees C and microwave-dried leaves at power of 400W, respectively. The percentage of menthone, as the second constituent in the essential oil, was significantly lost (p<0.01) under microwave drying.