Generalization of temperature and thickness effects in kinetic studies of turmeric (Curcuma longa) slices drying

The effects of temperature and slice thickness on the drying kinetics of turmeric (Curcuma longa) slices are modeled to obtain a generalized master curve equation. Drying data obtained from experiments carried out in a laboratory scale tray dryer at different drying temperatures in the range of 40-70°C for different turmeric slice thicknesses (3-10 mm), are fitted to Midilli-Kucuk model (R² 0.992), the best fitted model among the common semi-theoretical models. To incorporate the temperature and slice thickness effects, temperature-thickness superposition technique was applied in two stages. In the first stage, at a given drying temperatures moisture ratio is expressed as a function of ‘reduced time’ (t.ah) which is the product of drying time (t) and a thickness shifting factor (ah). Thickness reduced master curves obtained at each temperature are again shifted by temperature shift factor (aT) to generate a single master curve expressing moisture ratio as a function of another reduced time, t′′, product of drying time (t), thickness shifting factor (ah), and temperature shift factor (aT), to obtain the generalized Midilli-Kucuk model (MR(t.ah.aT,)). Temperature dependence of ah could be regressed into the Arrhenius - type equation, and aT could be regressed into a linear equation. The developed model was compared to the generalized drying model based on generalized drying rate constant, where upon both the models were found to yield prediction of turmeric slice drying with high accuracy (R² value 0.99).