Ultrafiltration of protein mixtures: measurement of apparent critical flux, rejection performance, and identification of protein deposition

Crossflow ultrafiltration of binary protein solutions was carried out using flux-stepping and constant flux experiments to identify the apparent critical flux where fouling is rapid. The contributions of individual protein species to the apparent critical flux were evaluated as well as the separation performance. For mixtures of gðglobulin/lysozyme and BSA/lysozyme the larger retained protein tended to control the critical flux behaviour while the observed rejection of the smaller transmitted protein went through a minimum close to the apparent critical flux. Identification of the respective protein species deposited onto membrane surfaces was carried out using MatrixAssisted Laser Desorption Ionisation Mass Spectroscopy (MALDI-MS). Mass spectra showed that the transmitted proteins resulted in a higher incidence of peaks relative to the retained proteins. This was thought to be the result of desorption of proteins from the membrane surface, from inside pores and from the membrane substrate. It was shown that the MALDI-MS technique is a powerful tool for distinguishing between different proteins in fouling deposits and has potential for quantitative measurement of protein fouling on membrane surfaces.

In conclusion, the apparent critical flux measured for binary mixtures of proteins indicates that the formation of a dynamic layer of the retained species is the main contributor to the rapid increase in transmembrane pressure. The protein deposition measured using MALDI-MS indicates that much of it is the transmitted species, probably from the surface, the pores and substrate of the membrane. The use of MALDI-MS has shown very promising results in the quantitative analysis of mixtures of BSA and βLG standards using conalbumin and ovalbumin as internal standards. This shows potential for application in the analysis of proteinfouled membranes in the dairy industry, or if the standards and internal standards have their roles reversed, in the analysis of membranes fouled during the ultrafiltration of chicken egg white proteins.