UF fractionation of fish protein hydrolysate

A commercial fish protein hydrolysate was fractionated using polymeric spiral wound ultrafiltration (UF) membranes. As protein hydrolysates are electrolytes, the effects of pH and ionic strength were investigated. Three different polyethersulfones (PES) membranes with different nominal molecular weight cut-offs (10, 5, and 3 kDa) and a polyamide-thin film composite (PA) membrane with a nominal molecular weight cut-off of 3 kDa were used. The membrane flux and retention profile, and the selectivity for <4 kDa peptides were analyzed and discussed. The PES membranes were suitable for fractionation of the hydrolysate, while the PA membrane showed high retention for all components because of the adsorption of solutes to the membrane surface. The results showed that with charged solutes, the nominal molecular weight cut-off as supplied by manufacturers was not the decisive parameter for choosing a membrane. The permeate flux at pH 8 was higher than that at pH 5 for all PES membranes. The presence of salt in the hydrolysate significantly improved the flux and selectivity. The overall retention mainly was dependent on the retention of small peptides (<4 kDa). The mass transfer mechanism of charged peptides within the concentration polarization during UF separation was simplified and presented regarding size and charge exclusions.