The present work was undertaken in order to characterize the impact of fouling by a beta-LG tryptic hydrolysate on the retention characteristics of a "loose" NF membrane. The influence of some physico-chemical (feed concentration, pH) as well as operating parameters (transmembrane pressure, feed velocity) on the ability of the G- 10 membrane (Osmonics, Minnetonka, MN) with a molecular weight cut-off (MWCO) of 2500 g mol(-1) to fractionate acid, neutral and basic peptides was first evaluated using a screening NF procedure. Preliminary electrolyte retention (NaCl, Na2SO4, CaCl2) measurements had shown that this NF membrane seems to exhibit Dorman exclusion mechanism typical of a negatively charged membrane. Complete retention of acid peptides was observed with the G- 10 membrane at both pH 5 and 9. Separation between neutral and basic peptides was also achievable at pH 9 as compared to pH 5 due to the electrostatic repulsion of the neutral peptides that acquire a net negative at pH 9. The fractionation between neutral and basic peptides was optimal when the transmembrane pressure was raised to intermediate or higher values. The effect of fouling in these selected NF optimal conditions on the sieving and electrostatic properties of the G- 10 was further studied by test solute retention methods. It was shown that an important hydraulic resistance due to fouling developed during NF of a 0. 1 % beta-LG hydrolysate solution. At pH 5 and 9, the resulting sieving property of the G- 10 membrane clearly depend on this fouled peptidic layer. Furthermore, the net negative charge of the membrane seemed to be reduced only after fouling at pH 9. It was thus suggested that the composition of the peptide layer interacting with the G- 10 membrane and causing fouling is pH-dependent. (c) 2005 Elsevier B.V. All rights reserved.