In the presence of food fibers, proteolysis occurs with the formation of sorption complexes. At the initial stages of hydrolysis, the adsorption of both casein and pepsin is observed, the adsorption influencing the initial rate of hydrolysis. The composition of the sorption layer changes during hydrolysis in favor of intermediates. The initial and stationary rates of hydrolysis depend on the protein/fiber ratio. The catalytic activity of the system decreases both with a little amount and with an excess of fibers. The highest rate of hydrolysis is observed at a protein/fiber ratio of 1/2.