Calcium and temperature effect on structural damage of hot air dried apple slices: Nonlinear irreversible thermodynamic approach and rehydration analysis

• Mechanical changes in convective drying are hardly assessed by diffusional models.
• Energy used in deformations is estimated by nonlinear irreversible thermodynamics.
• Data confirm the coupling between diffusional and deformation-relaxation phenomena.
• Samples behaviour during rehydration confirms the extent of structural damage.
• Apples including calcium were the stiffest and the most damaged during air drying.

Mathematical models traditionally employed in fitting convective drying data do not use to report information about chemical and other physical changes different from the simple decrease in moisture content. In the present study, structural damage undergone by fresh and vacuum impregnated apple slices with different calcium lactate concentrations during convective drying at 30, 40 and 50 °C was analysed by applying equations derived from nonlinear irreversible thermodynamics to experimental data. According to the results obtained, vacuum impregnation with isotonic sucrose solution before drying at 30 °C provided maximum protection to cellular structure by promoting reversible deformations against irreversible breakages. On the contrary, cell walls strengthen with calcium had severe damaged during drying. Regarding air temperature, it was directly related both to the molar energy employed in deforming structures and the drying rate. These results were confirmed by analysing dried samples behaviour during further rehydration.