Comparative analysis of drying kinetics, diffusivity, and thermodynamic properties of hybrid solar and oven dryers for Egyptian sweet marjoram

Egyptian sweet marjoram leaves (ESML) are aromatic herbs long valued for their ability to enhance flavor and extend shelf life by inhibiting the autoxidation of food lipids. Despite their widespread use, limited research exists on how various drying techniques, air temperatures, and thin-layer thicknesses affect the drying behavior, mathematical modeling, effective moisture diffusivity (EMD), activation energy, and thermodynamic properties of ESML. This study addresses this gap by examining the drying characteristics of ESML using a hybrid solar drying system (HSDS) at three air temperatures (50, 60, and 70 °C) and three layer thicknesses (1, 2, and 3 cm), comparing with a conventional oven drying (OD). The findings revealed that HSDS at 70 °C with a 3 cm layer thickness achieved the highest equilibrium moisture content (EMC), while HSDS at the same temperature with a 1 cm layer thickness resulted in the shortest drying time, highest drying rate, and moisture ratio. Additionally, the highest activation energy was observed using HSDS at 70 °C and a 2 cm layer thickness. Notably, drying at 70 °C with a 1 cm layer thickness reduced drying time by 66.67% compared to drying at 50 °C for the same thickness. Nonlinear regression analysis of eleven thin-layer drying models identified Weibullian (I) and Midilli as the best-fitting models for HSDS and OD, respectively. The HSDS demonstrated comparable performance to the OD while utilizing solar energy as a renewable heat. These findings advance the knowledge of drying systems by demonstrating the effectiveness and sustainability of hybrid solar drying for preserving the quality and functional properties of medicinal and aromatic herbs like marjoram.