Moisture sorption thermodynamics of Moringa oleifera leaf powder: isothermal modeling, activation energy, and mechanistic insights for storage stability

In this paper, the hygroscopic characteristics of powdered leaves of Moringa oleifera were explored experimentally by conducting adsorption isotherm experiments at 25, 40, and 55 °C within water activity (a_w) levels ranging from 0.0016–0.9562. In this case, the equilibrium moisture content values exhibited Type II sigmoidal shapes. For the data analysis, four equations including the Guggenheim-Anderson-de Boer (GAB), Brunauer–Emmett–Teller (BET), Langmuir, and Freundlich models were considered. The GAB model gave the highest accuracy level (R² ≥ 0.997 and RMSE ≤ 0.22). Monolayer moisture content (X_m) decreased from 6.45 g/100g (at 25 °C) to 5.01 g/100g (at 55 °C). Thus, the monolayer moisture content decreased by 22.3% from lower to higher temperatures. The value of the net isosteric heat of sorption (Q_st) was calculated from 51.2 kJ/mol to 44.5 kJ/mol, as moisture content increased. The value of Q_st approached the latent heat of vaporization of pure water. According to the Arrhenius plot, the value of activation energy (E_a) equals to 6.9 kJ/mol. The results obtained from the FTIR spectra revealed that the water adsorption took place through interactions between OH, NH2, and COOH groups and water molecules. The proposed methodology involving isotherm modeling, thermodynamic parameters, activation energies, and FTIR spectra gives a greater insight into the hygroscopic nature of M. oleifera leaf powder and thus offers a basis for the development of drying and storage techniques.