Evaluation of six drying treatments with respect to essential oil yield, composition and color characteristics of Thymys daenensis subsp. daenensis. Celak leaves

Thymus daenensis subsp. daenensis. Celak is known as an endemic and valuable spice plant. The effect of six different drying treatments (sun, shade, oven 50 °C, oven 70 °C, microwave and freeze–drying) on the essential oil yield, composition and color characteristics of leaves was assessed. The essential oils from fresh and dried samples were isolated by hydrodistillation in a Clevenger apparatus and analyzed using gas chromatography–mass spectrometry (GC–MS). The highest essential oil yields were obtained by freeze–drying (1.7%) followed by oven 50 °C (1.46%), sun drying (1.42%), oven 70 °C (1.01%), shade drying (0.91%) and microwave (0.89%). Results showed that air drying at ambient temperature increased significantly the essential oil yield. Higher temperatures (microwave and oven 70 °C) lead to increase in thymol/carvacrol and β-caryophyllene content. In this study, Lightness (L*), greenness (a*), and yellowness (b*) of dried leaves were also compared with fresh one. Oven drying at higher temperature resulted in a considerable decrease in the color quality of the leaves. Air drying and oven 50 °C and 70 °C had the highest yellowness in comparison to other treatments. In spite of relatively low essential oil yield in Microwave drying, this method had many advantages such as shortening of drying time, high color quality and increased major compounds of thyme leaves.

► The effects of six drying treatments on the essential oil content, composition and color characteristics of Thymus daenensis subsp. daenensis were studied.
► Oven drying at higher temperatures resulted in a considerable decrease in color quality and essential oil content.
► Concentration of major compounds such as thymol/carvacrol and β-Caryophyllene were increased in the case of microwave and oven 70 °C.
► Microwave drying showed the appreciable characteristics such as shortening of drying time, high color quality and high thymol and carvacrol content.