Fractionation of whey protein hydrolysates using charged UF NF membranes

The possibility to separate peptides from a tryptic hydrolysates of whey proteins with charged UF/NF membranes has been investigated. A total hydrolysate (TH) was prepared by tryptic hydrolysis of a commercial whey protein isolate followed by UF-treatment using a 10 kDa MWCO in order to remove the enzyme and non-hydrolyzed material from the reaction mixture. Firstly, five different membrane materials were evaluated for the fractionation of TH and the membrane SG13 with a molecular weight cut-off (MWCO) between 1000 and 5000 g mol(-1) showed the best separation properties accompanied with high fluxes. The separation behavior of the TH by SG13 membranes at pH 5 and 9, with or without 0.5 M NaCl additions was further investigated. It was observed that pH increase from 5 to 9 not only raised the flux and nitrogen transmission values but also decreased the tendency to fouling. However, NaCl additions increased permeability but also increased the fouling. A detailed examination of the peptide separation revealed that negatively charged peptides were in lower proportion in the permeates whereas the opposite trend was observed for neutral and positively charged peptides. The results are explained by charge effects related to the rising of a Donnan potential at the membrane surface during the fractionation of hydrolysates. (C) 1999 Elsevier Science B.V. All rights reserved.