Qualitative changes of spearmint essential oil as affected by drying methods

Abstract This research aimed to investigate natural (shade and sun drying) and artificial drying methods (oven, vacuum, microwave, infrared and freeze-drying) on the spearmint essential oil (EO) quality and its trichomes. Fresh plant (control), shade dried, sun dried, freeze dried, oven dried at 40, 50, and 60 °C, vacuum dried at 40, 50, and 60 °C, microwave dried at 100, 200, 300, 400, 500 and 600 W and infrared dried at 125,132 and 225 W spearmint samples were evaluated. The EO of samples was isolated by hydro-distillation method and their components identified by gas chromatography (GC) and gas chromatography/mass spectrometry (GC–MS). The results revealed that shade drying required the highest drying time (28 h) whereas the lowest (0.2 h) was obtained by microwave drying at 500 and 600 W. The highest EO content (2.15 mL/100 g D.M.) was measured after sun drying while the highest carvone content was returned by oven-drying at 60 and 40 °C (53% and 50.3%), respectively. Glandular trichomes were damaged by artificial drying methods but shade drying well maintained them. Generally speaking, shade and oven-drying at 60 and 40 °C the most recommended methods for Mentha spicata processing as these preserve the necessary levels and qualities of EO.