Comparing the drying of lemon verbena in a continuous flow dryer equipped with a Solar Pre-heating system to the traditional drying

Introduction One of the medical plants is lemon verbena with the scientific name of Lippia citriodora Kunth. It belongs to the family of Verbenaceae. The essential oil of lemon verbena has different compositions, which the most important of them are Geranial, Limonene, and Neral. The most important and commonly used physiological method after harvesting medicinal plants is the drying of plant organs which have high energy consumption. The temperature and the method of drying of these plants affect drug quality. The aim of this study was to investigate the effect of different drying methods on essential oil extraction efficiency and the drying time of lemon verbena leaves in a continuous flow dryer system and traditional drying method. Materials and Methods An amount of 150 g of lemon verbena leaves were dried in a continuous flow dryer equipped with the solar pre-heating system at three temperatures of 30, 40, and 50 °C, and three air velocities of 1, 1.5, and 2 m/s. To determine the significant difference between the effects of different air velocities and air temperatures in the dryer method, a factorial experiment was used in a completely randomized design with three replications for each treatment. The required heat for product drying was supplied by the combination of a gas water heater and a solar water heater. In the first stage, the gas water heater was off and the required thermal energy was provided only by the solar water heater. The radiation of solar energy heated the water in the solar water heater tank. After the water temperature reached the desired level, warm water was entered into the heat exchanger through an electric valve and a water pump. The air flow generated by the centrifugal blower was passed through the heat exchanger, and it was heated by the warm water. The warm air was then entered into the drying chamber through the air inlet pipe. If the temperature of the water in the solar water heater tank was lower than the desired level, the electric valve was actuated and made to direct the water flow to the inlet of the gas water heater before entering the water into the heat exchanger. At the same time, the gas water heater was turned on and provided the required thermal energy for warming up the water. In order to compare the quality of the dried product with the traditional drying method, 30 g of dried leaves from each treatment were submitted to hydrodistillation with a Clevenger-type apparatus and extracted with 450 ml of water for 3 h. A completely randomized design was used to determine the significant difference between the essential oil extraction efficiency of each method. Data were analyzed in SAS (Version 9.00) Software and the mean comparison was performed using Duncan's multiple range test. Results and Discussion The results showed that with increasing the air temperature and air velocity, the drying time of lemon verbena in the dryer significantly decreased. Also, with increasing airspeed to the dryer, moisture transfer from the product was performed faster. Finally, the drying time of the product was reduced. The highest essential oil extraction efficiency was obtained at 40 °C and 2 m/s (0.61%), while the lowest essential oil extraction efficiency was 0.23% in the sun drying method. The minimum drying time was obtained at 50 °C and the air velocity of 2 m/s in the dryer (110 min) and the maximum drying time was obtained in the (shade) traditional method (720 min). In this research, a new system was introduced in order to increase the uniformity and quality of the product in the process of drying the lemon verbena. In addition, by comparing the quality of dried product with traditional methods, it was found that the essential oil extraction efficiency in this dryer system was significantly (at 1% level) more than traditional drying methods. Also, the drying time in this system is much shorter than traditional drying methods. Conclusion In general, the results indicate that the drying methods in this study have a significant effect on the essential oil extraction efficiency and drying time of lemon verbena leaves. According to the results of this study, the use of this system for drying lemon verbena has advantages, such as increasing the quality of the product and reducing the drying time compared with the traditional methods. The most important advantages of using this system include: providing a considerable part of the required energy free, the use of warm water in a circulated circuit and the adaptation of the solar pre-heating system to the environmental objectives, the uniformity of product drying due to the continuous flow drying system, shorter drying time and the better quality of the product than traditional drying.