We are looking for the best method for concentrating protein in cell culture media while not degrading the protein.

Answer

There are two main causes for protein degradation during ultrafiltration processes; unbalanced protein to pH/sample buffer conditions and high shear stresses. Certainly the more linear the protein shape the greater the adverse effects on structure caused by high g-forces. If the protein in question is linear we would recommend having a slower concentration period with lower g-forces than the maximum recommended (~50% the recommended is a good starting place). For more standard globular molecules it is good practice to use a device closer to the minimum volume, this ensures a higher surface area and reduces the potential for blockage and increasing stress on the samples. Although it should be noted that with higher surface areas it increases the rate of non-specific binding if you have a non-optimal protein-membrane combination or if your protein is considered to be “sticky”. Although the advent of vertical membranes took away most of the issues of blockages caused by dead-end filtration. Laboratory-based TFF cassettes take that one step further and offer a true parallel flow path for the sample, further ensuring minimal shear stress. It should be noted that typically stirred cell vessels put on relatively high amounts of shear stress.

For pH/buffer conditions, once the correct buffer ratio is selected it can be maintained using continuous buffer exchange techniques (diafiltration). These are offered both with the Vivaflow devices and the centrifugal Vivaspin 20 and ensure that even with increasing concentrations the correct buffer balance is maintained, or a new buffer added if needed.

One final point to ensure the best concentration method is to test and consider the ultrafiltration membrane used, as it’s not always a one-stop-shop. So test out what is best, usually either RC, PES or CTA (CTA is more commonly used when interested in the filtrate, as it has a lower internal non-specific binding).