Thin layer modeling and determination of thermodynamic properties of tomato slices during hot air drying

Abstract

The drying kinetics and determination of thermodynamic properties of sliced tomato samples during hot air drying in a hybrid solar-electric crop dryer were presented. One kilogram batch of freshly harvested tomato samples were sliced to 10 mm thickness and dried at varying air velocities (1.0, 1.5 and 2.0 ms-1) and temperatures (50, 55, 60, 65, and 70 o C) using the hybrid mode. In order to choose the best drying model, eight thin-layer mathematical models were fitted to the experimental data. The high values of coefficient of determination, R 2 and the low values of reduced sum of squares error (SSE) and root mean square error (RMSE) indicated that the Midilli et al. model adequately described the drying process of tomato slices, with its highest R 2 (0.9999), lowest SSE (0.1136) and lowest value of RMSE (0.0212) at 70 o C temperature and 2.0 ms-1 air velocity. Arrhenius model was used to represent the drying constant as a function of temperature. The effective moisture diffusivity increased with increase in temperature and air velocity; with the highest value obtained at air velocity of 2.0 ms-1 and temperature of 70 o C, whereas it was lowest at air velocity of 1.0 ms-1 and temperature of 50 o C. The mean activation energy required to dry 1kg batch of 10 mm sliced tomato samples was 39.34 kJmol-1. Enthalpy and Gibbs free energy values were found to decrease with increasing drying temperature as well as the entropy which also exothermically decreased with temperature. Recommendations for further studies were stated.