Estimation of water diffusivity parameters on grape dynamic drying

A computer program was developed, aiming at estimating water diffusivity parameters in a dynamic drying process with grapes, assessing the predictability of corresponding non-isothermal drying curves. It numerically solves Fick's second law for a sphere, by explicit finite differences, in a shrinking system, with anisotropic properties and changing boundary conditions. Experiments were performed in a pilot convective dryer, with simulated air conditions observed in a solar dryer, for modelling the process. The equivalent radius of grapes decreased 30% until the end of the process, stressing the need to include shrinkage in mass and heat transfer models. It was observed that macroscopic shrinkage reflects cellular shrinkage, if plotted versus the normalised water content. Diffusivity values ranged between 1 × 10^-16 and 1 × 10^-10 m²/s. The developed methodology yields very good prediction of dynamic drying curves.