Schlunder's model of a surface, is the only fundamental attempt to explain the constant drying rate phenomena, for materials with pore sizes of a few microns. Here, a direct comparison between this approach and the conventional mass transfer (CMT) theory has been made, by extending the approach to incorporate cell sizes of food, plant materials and to low water contents. Contrast to CMT, the extended Schlunder's model analysis suggests that the boundary layer thickness have an effect on the relative drying rate. Some new experimental results are also presented to support one viewpoint regarding the characteristic drying rate method.