Changes in the essential oil content and composition of Thymus daenensis Celak. under different drying methods

Some active ingredients of the medicinal plant are changed during a drying process. This study was aimed for the effect comparison of the various drying methods on quantity and quality of Thymus daenensis essential oil. The factorial experiment treatments included pre-drying under sun as the first factor and various drying methods (fresh plant, sun drying, shade drying, oven drying at 35, 45, and 55 °C, vacuum drying at 35, 45, and 55 °C, and microwave drying at 100, 500, and 1000 W) as the second factor. The results showed that various drying methods and their interaction with the pre-drying had a significant effect on the essential oil content and its components. In present study an adverse relation was found between the essential oil content and thymol or carvacrol amount in the dried plant. The highest essential oil amount was observed in the oven and vacuum oven-drying at 35 °C without the pre-drying operation. While, the greatest content of thymol as the predominant oxygenated monoterpenes was acquired in the vacuum oven-drying at 55 °C. Generally, the oven and vacuum oven-drying at 35 °C without the pre-drying are recommended for Thymus daenensis drying due to possessing the highest quantity of essential oil and appropriate quality.