Evaluation of mass transfer coefficients and volumetric shrinkage during osmotic dehydration of apple using sucrose solutions in static and non-static conditions

Effects of temperature, solution concentration and solution flow rate on osmotic dehydration/impregnation rate observed during immersion of apple cylinders in sugar solutions at low temperature (less than or equal to25 degreesC) are presented. Water and sucrose mass transfer coefficients are calculated and correlated using Magee's model; an extension of the model is proposed to account for solution flowrate; in both cases average relative deviations of less than 3% are obtained. Analysis of results obtained suggests that solute gain by the sample be controlled by diffusion inside the material while water loss is governed by mixed internal-external flow. Additionally, volumetric shrinkage of apple cylinders treated by osmotic solutions is predicted by measuring the change of the moisture content or the net mass loss due to the treatment. Volume changes observed in samples were correlated linearly with moisture content (dry basis) and with the net change in sample weight. These results suggest that shrinkage be essentially due to water removal/solid gain and offer a simple way to predict such changes during industrial processing. (C) 2002 Elsevier Science Ltd. All rights reserved.