Hygroscopic proprieties of fig (Ficus carica L.): Mathematical modelling of moisture sorption isotherms and isosteric heat kinetics

Abstract

Moisture sorption isotherms of two fig varieties most grown in Moroccan orchards were assessed simulating the conditions of storage using the gravimetric static method at 30, 40, and 50°C. The equilibrium was reached within 8 to 10 days for desorption and adsorption, respectively over a stepwise increase of relative humidity ranging from 5% to 90%. Experimental data of fig sorption were fitted to three of most used models: GAB, Enderby and Peleg. High-accuracy prediction of the sorption isotherms for “Sarilop” and “Kadota” was obtained with Enderby and Peleg, respectively, since they presented the best adjustment (generally, R² ≥ 0.997 and SE ≤ 1.85). Thermodynamic proprieties revealed a remarkable varietal effect regarding the net isosteric heat, the differential entropy. These differences were probably attributed to chemical composition, structure of the tissues and the intermolecular forces between the sorption sites, water vapor, chemical composition, structure of the tissues and the intermolecular forces. The net isosteric heat of fig samples sorption decreases as the moisture content increased, and was found to be a polynomial function of moisture content for adsorption and desorption. The hysteresis loop was observed among all experiments and its magnitude was remarkable in water activity range of 0.4–0.8. The applied enthalpy-entropy compensation theory to sorption isotherms and plots net isosteric of sorption versus differential entropy provided the isokinetic temperatures, which suggested an enthalpy-controlled sorption process. As a first study of fig thermodynamic proprieties in Morocco, this work aimed to contribute to the fig drying and packaging design and optimization. It also intends to predict moisture changes that might occur during heating process and determine energy requirement for fig drying and shelf life stability.