The application of osmotic dehydration as a pre-processing technique in food processing has not been fully utilised because of the lack of appropriate models that could be used for designing processing parameters and equipment. This study aimed to contribute to these needs by developing methods that describe the mass transfer behaviour of plant materials treated in osmotic solutions. Macroscopic and microscopic approaches were employed to describe the changes that occur in the tissues during the process. An in-depth study of apple tissues led to a method of classification that describes the rate of dehydration of various plant materials. This method took into account the initial states of the material and the various mass transfer resistances from the solution and the material itself. These behaviours were also verified with microscopy and image analysis. The measurements of the volumes of the cells are facilitated by the development of an image analysis program that automatically detects the cell's edges and measures the volume in three-dimension.