Reference
Ultrafiltration
**Ultrafiltration** is a pressure-driven membrane separation process that concentrates protein by physically retaining large molecules while allowing water, salts, and smaller solutes to pass through a semi-permeable membrane.
How it works
Ultrafiltration relies on a membrane with pore sizes typically rated by molecular weight cut-off rather than a physical dimension. A protein-containing liquid is pushed against the membrane under modest pressure. Molecules larger than the cut-off — proteins and peptides — are held back and accumulate in the retentate, while water, minerals, lactose, and other small compounds cross into the permeate. The result is a liquid stream progressively richer in protein.
Because separation is driven by size and pressure rather than heat or harsh chemistry, ultrafiltration is described as a gentle method. Avoiding high temperatures matters because excessive heat can cause protein denaturation, altering solubility and functional behavior. The process is often paired with diafiltration, in which water is added to wash residual small molecules out of the retentate and raise the final protein fraction further.
Why it matters for protein purification
Ultrafiltration is one of the standard tools for turning a dilute protein source into a concentrated, low-additive product. It separates on a different principle than isoelectric precipitation, which collapses protein out of solution at its isoelectric pH. Membrane processing keeps the protein in solution throughout, which can help preserve its native structure and reduce the need for downstream neutralization or chemical recovery steps.
The distinction between concentrate and isolate is partly a matter of how far purification is carried; ultrafiltration and diafiltration are common routes to the higher protein fractions seen in an isolate. For background on how these fractions differ, see our guide to potato protein.
Relevance to potato protein
Native potato protein occurs in potato fruit water, the dilute liquid left after starch extraction. Recovering this protein for food use has been attempted with methods including expanded bed adsorption and ultrafiltration (Food and Bioprocess Technology, 2012). Membrane processing is attractive here because the heat-sensitive proteins in potato fruit water — such as patatin and protease inhibitors — retain more of their functional properties when they are not exposed to high temperatures during recovery.
Related terms