Etude expérimentale et modélisation des isothermes de sorption d’humidité et propriétés thermodynamique des variétés des maïs cultivées dans la région de Ghardaïa

Abstract

This thesis analyzes the hygroscopic and thermodynamic properties of three maize varieties (Colonia, Kalassioul, Oasis) grown in the El Menia region, by studying their sorption isotherms to optimize drying and storage. The objective is to characterize their behavior at different temperatures (30 °C, 45 °C, 60 °C) and to determine the most suitable mathematical models to predict their sorption properties. The study uses a gravimetric method with sulfuric acid solutions to control water activity (aw). Sorption isotherms are obtained by measuring mass variations until equilibrium, then fitted to models (GAB, BET, etc.). The isosteric heat of sorption is also calculated to analyze water-matter interactions. The isotherms exhibit a type II shape, typical of cereal products, with three distinct zones: monomolecular adsorption (aw < 0.3), multilayer (0.3 < aw < 0.7), and capillary (aw > 0.7). A marked hysteresis is observed between the adsorption and desorption curves, especially for aw > 0.6, indicating higher water retention after desorption. Increasing the temperature reduces the

equilibrium water content (Xeq) for the same aw, reflecting a decrease in water- matter affinity at high temperatures. The Peleg and GAB models are found to

be the best performers in describing the isotherms, with high coefficients of determination (R2) (> 0.99) and low errors (RMSE). The Peleg model is particularly suitable for predicting complex behaviors, while the GAB model provides key thermodynamic information, such as the monolayer water content (Xm). The isosteric heat of sorption is high at low water contents, reflecting strong interactions between water and the active sites of corn. It decreases with increasing humidity, approaching the latent heat of vaporization of pure water, indicating less energetic adsorption in the upper layers. The results demonstrate the importance of sorption isotherms for optimizing corn drying and storage conditions, particularly in arid regions such as southern Algeria. The Peleg and GAB models provide valuable tools for predicting hygroscopic behavior and minimizing spoilage risks. Thermodynamic analyses highlight the energy required for the early stages of sorption, which is crucial for designing efficient postharvest processes.