Nonlinear thermodynamic approach to analyze long time osmotic dehydration of parenchymatic apple tissue

Osmotic dehydration experiments of Granny Smith apple were carried out in order to model the operation by using a nonlinear irreversible thermodynamic approach. Samples were immersed into 65% (w/w) sucrose aqueous solution at 30 °C during 180, 360, 480, 720, 1463, 1577, 1722, 3375, 4320, 7200, 8540, 10,270 min. Some physical–chemical parameters were measured in fresh, treated and reposed (24 h at 30 °C) samples. The results allow identifying three differentiated steps in the osmotic operation process. At the beginning of the treatment, the cells were turgid and the wall-membrane system stored high level of mechanical energy. In this step, the highest velocity of shrinkage occurred. In the second step, the main driving force was diffusional, with important concentration profiles in the tissue; in this step, the deformation of the tissue reached the maximum shrinkage level of approximately 60%. It produces an important structural change in the tissue removing the concentration profiles and changing the mechanism driving forces. Last step shows internal relaxation of the tissue, increasing the mechanical transport behaviours.