Drying kinetics, thermodynamic properties and physicochemical characteristics of Rue leaves

Generally, medicinal plants are harvested with high amount of water, so it is essential to subject the product to drying as soon as possible to prevent degradation before application. Most compounds from medicinal plants are sensitive to drying processes, so it is important to adjust the drying conditions. The objective of this study was to describe the drying of Rue (Ruta chalepensis L.) leaves, select the models that best fit each drying condition, determine the activation energy and thermodynamic properties of the leaves, and evaluate their quality after drying. Leaves were harvested with moisture content of 3.55 ± 0.05 kg _water kg^-1_{dry matter} and subjected to drying at temperatures of 40, 50, 60 and 70 °C. Valcam model showed the best fit to represent the drying kinetics of Rue leaves at temperatures of 40 and 70 °C, and Midilli model proved to be better for the temperatures of 50 and 60 °C. Effective diffusion coefficient increased linearly with the increase in drying air temperature, and the activation energy was 60.58 kJ mol^-1. Enthalpy, entropy and Gibbs free energy values ranged from 57.973 to 57.723 kJ mol^-1, from - 0.28538 to - 0.28614 kJ mol^-1 K^-1 and from 147.34 to 155.91 kJ mol^-1, respectively, for the temperature range of 40-70 °C. Drying air temperature promoted darkening or tendency to loss of green color; increase in drying air temperature leads to greater discoloration, as well as a higher concentration of total phenolic compounds (about 221.10 mg GAE mL^-1 g^-1 dm), with a peak at temperature of 60 °C.