Gastric digestion is the result of a physical and enzymatic reactions leading to the release of nutrients from the food matrix to be absorbed in the upper intestinal tract. Recent works have shown on a mini-pig model that the digestion process may be modulated by the initial structure of the dairy matrix and suggested that the gastric phase is the critical step. There is a lack of information concerning the microscopic scale of the digestion process but it is considered that proteolysis would be limited by the diffusion of gastric fluid, especially pepsin, into the food. So far, most of the studies conducted to better understand gastric digestion have only focused on acid diffusion from the gastric fluid, but no data are currently available for pepsin diffusion. Using fluorescence recovery after photobleaching technique, diffusion coefficients of inactive isothiocyanate (FITC)-pepsin were measured in rennet gels at different casein concentrations, i.e. different structures of protein networks. To investigate the microstructural characteristics of native gels, electron microscopy image analysis was performed, and qualitatively related to the diffusion behavior of FITC-pepsin in these gels. The diffusion coefficient of FITC-pepsin was reduced by a factor 2.4 as the casein concentration of the rennet gels increased from 32.5 to 130 g/kg. Models from polymer science were successfully applied, allowing the reliable prediction of diffusion coefficients calculated from diffusing solutes of known size and physicochemical properties of the food product, e.g. volume fraction, particle size of the gel network. In a more holistic view of food behavior in the human stomach, the diffusion of pepsin should now be confronted with its activity in acidic environment, in order to be able to define more precisely the food particle disintegration mechanisms.