It was previously demonstrated that peptide-peptide interactions occur in a beta-lactoglobulin (beta-LG) tryptic hydrolysate, mainly driven by hydrophobic interactions, and that some peptides aggregated at acidic pH. The objective of this work was to study the effect of removing peptide aggregates on hydrolysate fractionation by nanofiltration (NF). The beta-LG tryptic hydrolysate (reference) was adjusted to pH 4 to induce hydrophobic peptide precipitation (treated hydrolysate). Afterwards, both solutions were fractionated at pH 5 and 9 using an NF G-10 membrane (MWCO of 2500 Da). It was shown that the treatment temporarily increased the permeation flux at pH 9. Also, only < 1000 g mol(-1) peptides, and especially the positive ones, were transmitted on this membrane. The acid pre-treatment did not affect the individual peptide transmission after 60 min of filtration, but a significant decrease in the transmission of most peptides was observed after 120 min. The effect of peptide aggregates in the bulk of the retentate in relation to the polarization layer is discussed. (C) 2004 Elsevier B.V. All rights reserved.