Separation of biopolymers is one of increasingly important applications of ultrafiltration (UF) membranes. The efficiency of separation is often evaluated through pilot experiments and seldom predicted. Available predictions focus on correlations between UF pore width and biopolymer hydrodynamic diameter, while overlooking key operational parameters including transmembrane pressure (TMP). The role of TMP in the retention of biopolymers is revealed, i.e., magnification of TMP leads to decreased retention of the particulates. No full retention of the biopolymers without a rigid outer shell has been observed. Pressure-induced stretching of flexible biopolymers and expansion of membrane pores are the possible reasons of deteriorating selectivity. A division into flexible and rigid biopolymers provides a better prediction of membrane selectivity.