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.
