During diffusion the molecules move from the region of high concentration to that lower. During drying water diffuses from internal regions (with a high moisture content) toward its surface (with low moisture content) where it evaporates if sufficient heat is supplied and the second Fick’s law described its mechanism but Fick’s law have several theoretical limits in describing diffusion mechanism. Some researchers consider food as porous media and defined food as biological material volume, consisting of solid matrix with interconnected void. therefore Capillary forces flow must be considered as one of the most important mass transfer mechanism during drying. Capillary flow in food may be expressed by Darcy’s law (1).

[Should be noted, permeability is a kind of diffusion from polymer film or layer which is porous. As a Conclusion, water transfer from internal regions toward dry surfaces with Capillary flow, interconnected void, is named Permeability].

I think this is a good reference with additional information.

Reference:

1-Derossi A., Severini C. and Cassi D. (2011). Mass Transfer Mechanisms during Dehydration of Vegetable Food: Traditional and Innovative Approaches, Advanced Topics in Mass Transfer, Prof. Mohamed El-Amin(Ed.), ISBN: 978-953-307-333-0, InTech, Available from: http://www.intechopen.com/books/advanced-topicsin-mass-transfer/mass-transfer-mechanisms-during-dehydration-of-vegetable-food-traditional-and-innovativeapproaches

I look forward for your opinion.

As far as I think, water molecules of any food sample comes at the outside layer from middle of the food sample during drying which is the example of permeability. But, the water molecules transferred from food sample to the atmosphere which is the example of diffusion.

in a porous medium, the effective diffusivity is the molecular diffusivity divided by the tortuosity. The permeability does depend on the tortuosity, i.e. the effective length for a flow path for the same pressure drop, but also on many other geometric factors of the pore space such as effective cross section to flow. The underlying reason is that permeability is connected to fluid flow for which a no-slip boundary condition at the pore wall applies. For diffusion, the boundary condition at the solid is only "no-invasion" (which also applied to permeability).