Extraction of α-lactalbumin from whey protein concentrate with modified inorganic membranes

The aim was to extract α-lactalbumin (α-LA) selectively from acid casein whey protein concentrate (WPC) at pH 7 by limiting β-lactoglobulin (β-LG) transmission. In order to achieve a high selectivity (ratio of α-LA transmission/β-LG transmission), inorganic membranes were chemically modified by a polyethyleneimine coating bearing positive charges. In-depth study by anion-exchange chromatography with a similar polymer coating permits to expect a more selective ion-exchange process with β-LG by the membrane at low or moderate ionic strength. Accordingly, transmission was investigated versus ionic strength (NaCl added): transmission of β-lactoglobulin was lowered with the modified membrane (α-LA transmission about 10%) and selectivities close to 10, were achieved at low ionic strength ( I≤0.02 mol l −1) when unmodified membrane selectivities were about 3 whatever the molecular-weight cut-off be. High selectivity of the tailor-made membrane was due to the adjustment of molecular sieving combined with anion-exchange interactions between negatively charged β-LG and the membrane, the reversible fouling of which was enhanced. Modification of the net charge of protein by specific adsorption of divalent ion such as calcium or phosphate increased or decreased the transmission of protein, respectively, but the membrane selectivity was similar because the adsorption of divalent ion occurred on the two proteins.