A multicomponent diffusion model has been developed to describe the moisture sorption onto a spray-dried amorphous skim milk particle, with the accompanying processes of moisture-induced crystallisation and phase change, moisture redistribution within new components and moisture transport between different internal layers. The model has simulated the moisture sorption for a series of different temperatures and humidities in a single particle with a diameter of 30 microns, which has been divided into six layers with the same thickness. Moisture profiles and crystallisation rates show trends in the changes of the moisture diffusivities and permeabilities of the layers within the particles as the amount of crystallinity in each layer changes. The results show the amount of amorphous lactose decreased during the process, while the amount of crystalline material increased. The fraction of amorphous material started at 0.5 and decreased to 0.05 at the end of the processing simulation. Results from several case studies have demonstrated that the proposed model has predicted the processes of sorption, phase change and moisture redistribution inside a particle well. (C) 2013 Elsevier Ltd. All rights reserved.