Bean moisture diffusivity and drying kinetics: a comparison of the liquid diffusion model when taking into account and neglecting grain shrinkage

Abstract

The aim of this work was to compare the results of the liquid diffusion model with respect to bean (Phaseolus vulgaris L.) drying when taking into account or neglecting grain shrinkage. Bean grains were harvested with a moisture content of 0.92 kg water/kg dry matter and dried at air temperatures of 25-55 deg C and relative humidities of 20-75%. The volume of each grain, understood as a sphere, was determined several times over the drying process, taking the diameter to be the mean length of the three orthogonal axes. Grain shrinkage was determined by examining the relationships between the volume associated with each moisture content and the initial volume. The results show that the liquid diffusion model describes the drying kinetics of beans satisfactorily, and that grain shrinkage can be ignored. The diffusion coefficient increases with air temperature, with values ranging between 10.8 x 10-10 and 67.0 x 10-10 square m sE-1. This is described by the Arrhenius equation, with an activation energy of 40.08 kJ molE-1.