Osmotic dehydration of apples in sugar/salt solutions: Concentration profiles and effective diffusion coefficients

Apples (Fuji variety) were treated in aqueous sucrose (50% w/w) and salt (NaCl, 10% w/w) solutions for 2, 4 and 8 h (27 °C). Concentration profiles were determined as a function of the distance, considering the unidirectional and normal diffusion to the exposed face of the immersed fruit. The density, water, sugar and salt contents were determined for each piece of apple. A mathematical model was fitted to the experimental data for the water, sucrose and salt contents. A numerical method of finite differences allowed for the calculation of the effective diffusion coefficients as a function of concentration, using the material coordinates to consider tissue shrinkage. The coefficients were obtained by simultaneously integrating the three differential equations (for water, sucrose and salt). The behavior of the apple cellular tissue was also studied using light microscopy techniques to obtain images of the osmotically treated pieces.