A comparison of drying kinetics based on the degree of hydration and moisture ratio

The most widely used experimental parameter for modeling drying processes is the loss of sample mass, which is converted to the ratio between the water content at time t and the initial water content (moisture ratio, MR). Here we studied the drying of pineapple fruits and converted the experimental data for loss of mass to degree of hydration alpha(H) (t), which corresponds to the ratio between the water content and the sample mass. This approach allowed us to propose a thermodynamic model for the drying process. The results for the minimum local Gibbs free energy showed that for a degree of hydration of 0.1 the process was more spontaneous at 110 degrees C and a drying time of 90 min. However, the entropy of the process was maximal at 70 degrees C and a drying time of 163 min. The optimization of drying process and quality of the dried product should be better controlled throughout such this thermodynamic monitoring. The treatment of the data applying the MR approach in the structure transition model produced reasonable fitting, but with several limitations related to the thermodynamic derivation. (C) 2014 Elsevier Ltd. All rights reserved.