EFFECTIVE DIFFUSION COEFFICIENTS BEHAVIOR IN OSMOTIC DEHYDRATION OF APPLE SLICES CONSIDERING SHRINKING AND LOCAL CONCENTRATION DEPENDENCE

Abstract

ABSTRACT The mass transfer during osmotic dehydration of apple slices immersed in 40, 50 and 60% (w/w) aqueous sucrose solutions was investigated to evaluate the influence of solution concentration on diffusivities. In the mathematical model, the diffusion coefficients were functions of the local water and sucrose concentration. The mass transfer equations were, simultaneously, solved for water and sucrose using an implicit numerical method. Material coordinates following the shrinkage of the solid were used. The predicted concentration profiles were integrated and compared to experimental data, showing a reasonable agreement with the measured data. On average, the effective diffusion coefficients for water and sucrose decreased as the osmotic solution concentration increased; that is the behavior of the binary coefficients in water–sucrose solutions. However, the diffusivities expressed as a function of the local concentration in the slices varied between the treatments. Water diffusion coefficients showed a remarkable variation throughout the slice and unusual behavior, which was associated to the cellular structure changes observed in tissue immersed in osmotic solutions. Cell structure changes occurred in different ways: moderate plasmolysis at 40%, accentuated plasmolysis at 50% and generalized damage of the cells at 60%. Intact vacuoles were observed after a long time of exposure (30 h) to 40 and 50% solutions. Effects of the concentration on tissue changes make it difficult to generalize the behavior of diffusion coefficients. PRACTICAL APPLICATIONSThe main innovation introduced in this study is represented by the model formulation that allows simulation of the concentration profiles during osmotic dehydration with a reasonable degree of accuracy. The model is useful for the design and control of this process and of further operations such as convective drying, as nonhomogeneous initial conditions can be established. In addition, the diffusivities expressed as a function of the local concentration in the slices allows interpretation of some of the physical changes in plant tissues exposed to concentrated solutions.