Sorptions isotherms and isosteric heats of sorption of olive leaves (Chemlali variety): Experimental and mathematical investigations

Knowledge of sorption isotherms of agricultural products is necessary to control rehydration/dehydration and storage processes. The aim of this work is to determine moisture adsorption and desorption isotherms of Tunisian olive leaves of Chemlali variety (Olea europea L.). The static gravimetric method was used to determine sorption isotherms of olive leaves at three temperatures (40, 50 and 60 °C) and in the range of water activity varying from 0.0572 to 0.898. Eight models available in the literature were used to describe the experimental data. The agreement between experimental and calculated sorption isotherms was satisfactory (correlation coefficients and standard deviation values ranging, respectively, from 0.91 to 0.99 and from 5.6 × 10−5 to 0.23). GAB and Peleg models were found to be the best for describing the relationship between the equilibrium moisture content, the water activity and temperature. The isosteric heats of desorption and adsorption were determined from sorption isotherms and then correlated with corresponding equilibrium moisture contents. The isosteric heat of desorption was higher than the isosteric heat of adsorption and both decreased continuously with increasing of the equilibrium moisture content.