Protein Ultrafiltration

Protein ultrafiltration is a pressure-driven membrane process used for the concentration and/or purification of protein solutions. Ultrafiltration membranes typically have mean pore size between 10 and 500 Å, which is intermediate between reverse osmosis and microfiltration. Although ultrafiltration has often been viewed as a purely size-based separation, with species larger than the membrane pores being fully retained while smaller species pass freely, separation in ultrafiltration actually occurs because of differences in the rate of filtration of different components across the membrane in response to a given pressure driving force. Solute filtration rates, and thus the membrane selectivity, are determined by both thermodynamic (e.g. partitioning) and hydrodynamic (e.g. transport) interactions. Ultrafiltration is currently used throughout downstream processing for protein concentration, buffer exchange and desalting, protein purification, virus clearance, and clarification. Buffer exchange and desalting are typically accomplished using a diafiltration mode in which the low molecular weight components are washed away from the protein by simultaneously adding fresh buffer (or solvent) to the feed during ultrafiltration. Protein purification is accomplished using high performance tangential flow filtration, with the product collected in either the retentate or filtrate depending on the relative filtration rates. Virus filtration uses larger pore ultrafiltration membranes, which are permeable to protein but provide significant virus removal.