Determination of the average coefficient of internal moisture transfer during the drying of a thin bed of potato slices

During the drying of farm-produced foodstuffs, the Combes transfer pattern implies two transfers in series: external mass transfer and internal mass t ransfer. Water vapour transfer within the product is a phenomenon that is d ue to the gradient of partial pressures of water vapour between its core an d its external surface. A coefficient of mass exchange characterises this t ransfer and cannot be calculated theoretically. We develop here a calculati on pattern stemming from the two main equations expressing the drying proce ss: the heat equation in a porous medium applied to the product arranged in a bed of particles and the equation of the drying rate using the Combes tr ansfer pattern. We take into account the product shrinkage phenomenon durin g the drying process, as well as the variation of the physical and thermal properties of the product as a function of its moisture content and tempera ture. The drying kinetics of the product that we are using are calculated f rom the experiment. This calculation pattern allows us to determine the ave rage coefficient of internal moisture transfer corresponding to a thin laye r of potato slices, which is traversed by a forced air flow. We obtain this coefficient as a function of the product's moisture content on a dry basis and of the drying temperature. Next, we study the influence of the air vel ocity and of the thickness of product slices on this coefficient. (C) 2001 Elsevier Science Ltd. All rights reserved.