Mathematical modeling of thin layer drying kinetics of stone apple slices

This study was conducted to investigate the effect of temperature on hot-air drying kinetics of stone apple (Aegle marmelos correa) slices and to evaluate the best model predicting the drying kinetics along with the colour changes during drying. Stone apple slices were conditioned to remove the mucilaginous material followed by hot-air drying in single layer slices with thickness of 8 mm at different temperatures (40–70°C) in a forced convection dryer. In order to estimate and select the appropriate drying model, six different models which are semi-theoretical and/or empirical were applied to the experimental data and compared. The goodness of fit was determined using the coefficient of determination (R²), reduced chi square (χ²), root mean square error (RMSE) and mean bias error (MBE). Among the models proposed, the semi-empirical logarithmic model was found to best explain thin layer drying behavior of the stone apple slices as compared to the other models over the experimental temperature range. By increasing the drying air temperature, the effective moisture diffusivity values increased from 3.7317E-10 m²/s at 40°C to 6.675E-10 m²/s. The activation energy was calculated using an exponential expression based on Arrhenius equation. The relationship between the drying rate constant and drying air temperature was also established which gave a polynomial relationship. Samples dried at lower temperature had better lightness (higher L* values) compared to those dried at higher temperature. However, the samples dried at 60°C showed a significant overall deviation (ΔE*) in colour and may be considered as a limiting temperature for drying of stone apple slices.