Mathematical Description of Changes of Dried Apple Characteristics during Their Rehydration

The mathematical description of changes of dried apples characteristics (mass gain, volume increase, dry matter loss, rehydration indices, and colour) during their rehydration was performed. The effect of conditions of both processes on model parameters were also considered. Apple slices (3 and 10 mm) and cubes (10 mm) were dried in natural convection (drying air velocity 0.01 m/s), forced convection (0.5 and 2 m/s), and fluidisation (6 m/s). Drying air temperatures (T-d) were equal to 50, 60, and 70 degrees C. The rehydration process was carried out in distilled water at the temperatures (T-r) of 20, 45, 70, and 95 degrees C. Mass gain, volume increase, and dry matter loss were modelled using the following empirical models: Peleg, Pilosof-Boquet-Batholomai, Singh and Kulshrestha, Lewis (Newton), Henderson-Pabis, Page, and modified Page. Colour changes were described through applying the first-order model. Artificial neural networks (feedforward multilayer perceptron) were applied to make the rehydration indices and colour variations (Delta E) dependent on characteristic dimension, T-d, drying air velocity, and T-r. The Page and the modified Page models can be considered to be the most appropriate in order to characterise the mass gain (RMSE = 0.0143-0.0619) and the volume increase (RMSE = 0.0142-0.1130), whereas the Peleg, Pilosof-Bouquet-Batholomai, and Singh and Kulshrestha models were found to be the most appropriate to characterise dry matter loss (RMSE = 0.0116-0.0454). The ANNs described rehydration indices and Delta E satisfactorily (RMSE = 0.0567-0.0802). Both considered process conditions influenced (although in different degree) the changes of the considered dried apple characteristics during their rehydration.