Experimental and Computational Analysis of Drying Characteristics and Quality Attributes of Indirect and Mixed-Mode Solar Dried Stevia rebaudiana Leaves

Abstract

This study comprehensively investigates the temperature dynamics, fluid flow patterns, and moisture distribution during solar drying of Stevia rebaudiana leaves. Drying experiments were conducted using an indirect mode solar dryer (ISD) and mixed-mode solar dryer (MSD), considering both natural convection mode (NCM) and forced convection mode (FCM). Results revealed that MSD dried the leaves 40.00%–44.44% faster than ISD. Additionally, the average effective moisture diffusivity was 26.73% higher in MSD under NCM, and 34.75%–36.67% higher under FCM compared to ISD. The average mass transfer coefficient in MSD was also significantly higher than in ISD (p < 0.05), ranging from 2.92 × 10⁻⁶ to 3.68 × 10⁻⁶ m/s, a 25.86%–37.31% increase. The experimental data were evaluated using nine thin-layer drying models, with the Page and Two-term exponential models emerging as the best fit based on Akaike increment criteria and Akaike weights. The total color change was significantly higher (p < 0.05) by 35.41% under NCM and 40.17%–76.10% under FCM in MSD compared to ISD, indicating substantial quality and microstructural alterations in stevia due to its interactions with high temperature airflow. The total phenolic content of leaves showed a significant (p < 0.05) increase, ranging from 63.65% to 71.99%, when dried using ISD compared to MSD. The 3D finite element modeling performed using COMSOL Multiphysics software showed strong agreement with the experimental data, offering valuable insights into stevia's drying kinetics and the airflow patterns inside the solar dryer. These observations strongly support the efficacy of the ISD for drying heat sensitive medicinal herb like stevia.