In this work, experimental data on the rejection coefficient and the specific output flux are obtained. Based on the studies, graphical dependencies of the rejection coefficient on the specific output flux were constructed and analyzed. During the analysis, it was noted that with an increase in the output specific flow of the solvent, the rejection coefficient decreases. The reason for this is the boundary layers of fat and protein formed in the near-membrane layers, which prevents the passage of protein molecules through the pores of the membrane. For the theoretical calculation of the rejection coefficient and specific output flux, mathematical expressions are developed and numerical values of the values of empirical coefficients are obtained. The developed mathematical expressions describe the experimental data with good confidence. The obtained experimental and calculated data can be used with great reliability in the calculations of mass-transported flows of substances through semipermeable membranes, as well as in engineering methods for calculating and predicting the effectiveness of the use of membrane processes for the concentration of whey.