Experiments were conducted to determine the effect of pretreatment osmotic–ultrasonic dehydration on the desorption isotherms of quince by static gravimetric method at 30, 45 and 60C temperatures. The isotherms exhibited type II behavior, with the sorption capacity decreased with increasing temperature. Out of the six equilibrium sorption models that were evaluated, the Peleg equation gave the best fit to the sorption data. The Hasley equation was further used to estimate the thermodynamic functions (such as net isosteric heat, enthalpy–entropy compensation and spreading pressure) 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 are higher than that pretreated sample quince. PRACTICAL APPLICATIONS The desorption isotherms are important in the analysis and design of various food processes, such as preservation, drying, packing and mixing. The net isosteric heat of sorption can be used to estimate the energy requirements of dehydration processes. Combined methods involving osmotic–ultrasonic dehydration and convective air-drying are usually used to obtain a final product with a better quality (i.e., better preserved from undesirable changes) and in this sense, applying an osmotic–ultrasonic dehydration step prior to the dehydration process could be interesting. Nevertheless, when osmotic–ultrasonic dehydration is employed, together with water removal, an acquisition of osmotic solute takes place. In this way, the composition of the final product is changed and the characteristics of the sorption isotherm may be modified. One of the aims of this work was to determine the effect of osmotic–ultrasonic dehydration of quince on its desorption isotherm.

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