Mass transfer during the drying of skim milk and whole milk films is examined using a combination of experiments and mathematical modeling. Microwave power is used rather than the traditional convective heating characteristic of spray drying to provide. greater heating uniformity. In all experiments a period of constant rate drying is observed, followed by a period of diffusion control that is characterised by a rapid increase in sample temperature. The onset of diffusion control is shown to be directly related to the amount of both fat and protein in whole milk samples and to the amount of protein in skim milk samples. These changes occur as a result of the lower diffusivity of protein and fat relative to other milk solids. Over time this difference in diffusivities causes these higher molecular weight solids to segregate from the remaining solids, forming a thickening milk skin. The skim milk data has been successfully fitted to a moisture transport model that incorporates such a milk skin region. The diffusion coefficient within the skin is proportionately lower than in the bulk. The simulated rate of skin thickening is observed to be a simple function of the sample protein content and the sample temperature. (C) 2004 Elsevier Ltd. All rights reserved.