Mass transfer coefficients during osmotic dehydration of apple in single and combined aqueous solutions of sugar and salt

Apple cylinder samples were dehydrated by immersion in binary aqueous solutions of sucrose and sodium chloride with different concentrations and temperatures, as well as in ternary solutions of both solutes. Experimental data on sample moisture content and solids gain against time were fitted to a simplified diffusional model valid for short operation times. A net flow of solids towards the solution was observed during the initial contact with salt solutions, which changed later to an actual solids gain as with sugar solutions. Mass transfer coefficients calculated for binary solutions of a single solute were successfully used to estimate the coefficients for ternary solutions of both solutes, for the range of concentration and temperature studied, by means of a 'parallel flow' model. The ratio of water loss to solids gain (WL/SG) for each osmotic treatment was particularly high in the case of salt solutions, due to a low solids gain. In the case of ternary mixed solutions, intermediate values for WL/SG were obtained. (C) 2000 Elsevier Science Ltd. All rights reserved.