Effect of osmosis and ultrasound pretreatment on the moisture adsorption isotherms of quince

The effect of pretreatment osmotic-ultrasonic dehydration on the sorption isotherms of quince was determined by static gravimetric method at temperatures of 30 degrees C, 45 degrees C, and 60 degrees C. The curves obtained can be considered as type II according to the Brunauer-Emmett-Teller (BET) classification. Adsorption data were fitted into seven isotherm models. The best fit of the experimental data was obtained with Peleg for both fresh- and pretreated dried quince slices. Thermodynamic properties such as net isosteric heat, differential entropy, enthalpy-entropy compensation, and spreading pressure were determined from moisture adsorption isotherm data of quince. The net isosteric heat of sorption and differential entropy decreased with increasing moisture contents in an exponential function. A plot of differential heat versus entropy satisfied the enthalpy-entropy compensation theory. The spreading pressure increased with increasing water activity, and decreased with increasing temperature. The value of net isosteric heat, differential entropy and spreading pressure of untreated samples is higher than that of pretreated samples of quince. (C) 2011 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.