The freeze drying characteristics of planar and slab-shaped food products were numerically studied using a simulation program that considered the conjugate heat and mass transfer, sublimation of ice, and motion of sublimation interface. The fixed grid method, which was adopted in the numerical simulation, was able to handle the complex sublimation interface during the multi-dimensional freeze drying of slab-shaped products appropriately. The results showed that the lateral permeable surface of the slab-shaped products significantly altered the freeze drying characteristics by reducing the diffusion length for the water vapor transport as well as by increasing the interfacial area for sublimation. These two effects were found to cooperatively enhance the freeze drying rate while decreasing the average sublimation temperature. (c) 2007 Elsevier Ltd. All rights reserved.