Drying Kinetics and Moisture Diffusion Behaviour of Selected Medicinal Leaves Under Electrically Controlled and Ambient Atmospheric Conditions

Abstract

Drying remains one of the most important postharvest engineering operations required for the preservation of medicinal leaves through effective moisture reduction while maintaining product quality, storability, and process reliability. This study investigated the time–temperature dependent drying kinetics and moisture diffusion behaviour of guava (Psidium guajava), bitter (Vernonia amygdalina), and scent (Ocimum gratissimum) leaves under ambient sun drying and electrically controlled oven drying conditions. Emphasis was placed on moisture content reduction, moisture ratio (MR), drying rate, moisture retention capacity, and thin-layer kinetic modelling as influenced by leaf type and drying environment. Drying experiments were conducted over an 8-hour period with hourly measurements of mass, temperature, and relative humidity. Sun drying was carried out under fluctuating atmospheric conditions (32–37 °C; 59–80% RH), whereas oven drying was maintained at 60 ± 1 °C. Results showed that sun drying reduced moisture content to safe storage levels (<10%) but exhibited irregular drying patterns characterized by fluctuating MR profiles, lower drying constants, and unstable falling-rate behaviour. In contrast, oven drying achieved rapid and uniform moisture reduction within 6 hours, with smooth MR decay curves and higher drying constants, indicating enhanced internal moisture diffusivity. Among the leaves, bitter leaf exhibited the fastest drying response, while guava leaf showed greater resistance to moisture migration. Thin-layer model fitting revealed that the Page model provided the best predictive performance with coefficient of determination (R²) values up to 0.998. Overall, electrically controlled drying demonstrated superior efficiency, predictability, and engineering suitability for large-scale medicinal leaf preservation.