Drying kinetics and thermodynamic properties of boldo (Plectranthus barbatus Andrews) leaves

Abstract

Boldo leaves are used as medicinal plants by the Brazilian population. Among several factors that contribute to the proper storage of this product, we highlight the drying process, which is fundamental to reduce its moisture content, aiming to maintain phytotherapic characteristics. Different works involving the drying of leaves were car­ried out in the last years. However, there is not much information related to the drying process related to boldo leaves. This work evaluates drying kinetics at temperatures of 20, 40 and 60 °C, as well as determines thermody­namic properties: enthalpy, entropy and Gibbs free energy. Forced air circulation ovens were used for thin layer drying of leaves. The leaves, uniformly cut rectangular, were weighed periodically until hygroscopic equilibrium. Several mathematical models were fitted to the experimental data to characterize the drying processes, using the following statistical criteria: coefficient of determination (R²), mean standard error (SE), chi-square (χ²), and resi­due distribution. According to the criteria used, Modified Henderson and Pabis was the model that best repre­sented the drying kinetics of boldo leaves. The activation energy for the drying of boldo leaves was 17.969 kJ mol^-1. Both enthalpy and entropy decreased with increasing drying air temperature, while Gibbs free energy increased.