We investigated the fractionation of bio-functional peptides from micellar casein hydrolysate by long-term cross-flow ultrafiltration (CFUF) and long-term cross-flow electro membrane filtration (CFEMF) in order to characterize the impact of fouling on fractionation. Electrolytic parameters, flux, mass flow of protein, protein content and the peptide concentration were monitored to follow the evolution of fouling. The mass flow of protein of 9.8 +/- 0.9 g h(-1) m(-2) observed for CFUF increased significantly to up to 36.0 +/- 1.6 g h(-1) m(-2) on application of an electrical field. A higher total migration rate of 40% of ACE-inhibitory peptides was found by CFEMF compared to conventional filtration (23%). Once the composition of fouling was characterized via atomic force microscopy (AFM) and HPLC, the fouling layer was removed after CFUF by the application of an electric field and by the reversal of the electric field after CFEMF. The results indicate that electrostatic interactions are the main driving forces within the process. Additionally, the ACE-inhibitory activity of the micellar casein hydrolysate (217 mu g mL(-1)) and the permeate fractions (CFUF 109 mu g mL(-1) and CFEMF 36 mu g mL(-1)) revealed a significant increase in ACE-inhibitory activity of CFEMF permeate. (C) 2013 Elsevier B.V. All rights reserved.