Flux decline during ultrafiltration of protein-fatty acid mixtures

Proteins frequently co-exist with fatty acids and have specific binding interactions, forming complexes which can affect membrane performance. While specific and non-specific binding to proteins occurs at lower concentrations of fatty acid, at higher concentrations, excess fatty acids can produce additional interactions by adsorption in the protein cake or in the membrane pores. The effect of the fatty acid on ultrafiltration is multifold. The fatty acids can behave as a hydrophobic solute at low pH and a charged highly soluble solute at high pH. Using caprylic acid and bovine serum albumin (BSA) as a model fatty acid and protein, this study examines the flux decline caused by protein-fatty acid mixtures in the pH range of 3–10 and the potential effect of capillary condensation of fatty acids on membranes with proteins present as co-foulants. The flux and retention effects of increasing excess of fatty acids were particularly examined with respect to potential removal of residual fatty acids in the presence of proteins for food or environmental applications. Experiments on flux were carried out by diafiltration of solutions containing BSA and caprylic acid, using a 30 kDa polyethersulphone membrane at 50 kPa. BSA-fatty acid binding parameters were evaluated in bulk and compared with fatty acid flux and retention through BSA pre-fouled membranes. The fatty acid retention by the protein deposit exceeded specific and non-specific binding sites by several orders of magnitude, thus supporting a capillary condensation mechanism. Despite the high retention of fatty acid, the flux decline did not become significant until at high fatty acid concentrations relative to the saturation, suggesting that condensation occurred initially in small and dead end pores which did not carry the bulk of the flow. The possible effect of the protein conformation on this process is also discussed.