Simulations of fixed bed batch drying of foods with air plug flow and air complete mixing assumptions were developed in order to evaluate the effect of ideal behavior deviations. A general mathematical model for drying that takes into account heat and mass transfer in air, product and interface along with water thermodynamic equilibrium was implemented. Experimental drying of fixed bed of cassava slabs with 1 cm thickness was carried out at 60 and 63 degreesC input air for model validation. The results show that there is no significant difference between plug flow and complete mixing assumptions, when the drying is controlled by internal diffusion. Instead, some differences exist when the process is not controlled by internal diffusion. Simulation results showed that air outlet temperature measurement is a way of showing deviations from air plug flow behavior. (C) 2004 Elsevier Ltd. All rights reserved.