Changes in Essential Oil Content and Composition of Lemon Verbena (Lippia citriodora Kunth.) Under Various Drying Conditions

Abstract

Background: Aromatic herbs and spices are sensitive to drying processes; thus, optimum drying is an essential requirement for attaining a high-quality herbal product. Objectives: The study evaluated the effects of drying conditions on essential oil content and composition of Lippia citriodora. Methods: In this study, leaves of Lippia citriodora were divided into two sets. The first set included immediate drying after harvest using the given drying methods and the second set included sun drying for 3 hours after harvest and then drying according to the given drying methods. The drying methods included oven-drying at 35, 45, and 55°C, microwave-drying at 100, 500, and 1000 W, and shade-drying in the laboratory. The essential oils were extracted by hydrodistillation using a Clevenger apparatus and analyzed by GC and GC-MS. Results: The drying methods significantly affected the essential oil content and composition. The maximum content of essential oil was achieved in microwave-drying at 100 W with the pre-drying operation. The highest contents of monoterpenes including monoterpene hydrocarbons and oxygenated monoterpenes were observed in microwave-dried and fresh plants, respectively. In addition, the maximum amounts of sesquiterpenes including sesquiterpene hydrocarbons and oxygenated sesquiterpenes were observed in oven-drying at 35°C with no pre-drying operation. In both pre-drying and no pre-drying conditions, the amount of limonene as a major monoterpene hydrocarbon was increased by microwave-drying. Although oxygenated monoterpenes including neral and geranial were decreased, the content of 1,8-cineol was increased after drying, especially without pre-drying. The maximum amount of curcumene as a sesquiterpene hydrocarbon was obtained in shade-drying without pre-drying operation. In pre-drying condition, the essential oil content and its components were more in the microwave and oven-drying procedures than in shade-drying. Conclusions: The microwave-drying at 100 W following pre-drying was the best drying method to achieve the highest quantity and quality of essential oil.