Often, these processes are performed in series, microfiltration followed by ultrafiltration, to recover and concentrate the protein. There are three different modes by which membranes foul. The first step in attempting to understand the mechanism by which proteins foul membranes which is different than that for other types of particles is to understand the structure of the protein molecule . Protein Aggregation Proteins in solution have the ability to interact with one another. When proteins interact, they tend to mass together to form large particles called protein aggregates.
Protein aggregation is similar to coagulation of colloidal particles in solution.
It is this aggregation of proteins, the formation of particles much larger than individual protein molecules which is one of the reasons protein solutions tend to foul filtration membranes to an extent greater than non- protein containing solutions. Often, during the filtration of suspensions, a cake layer is formed at the surface of the membrane which is non-compressible. Increasing the transmembrane pressure will increase the flux across the membrane for these systems. However, proteins tend to form compressible cakes at the surface of the membrane. As the transmembrane pressure is increased, the particle cake formed at the surface of the membrane compresses, thereby decreasing the flux across the membrane and increasing the sieving of the proteins.