This work examines the use of wide-pore negatively charged ultrafiltration membranes for whey protein concentration. The hypothesis is that by placing a negative charge on the surface of an ultrafiltration membrane, negatively charged proteins are rejected by electrostatic repulsion and not simply sized based sieving. This allows using wide-pore membranes that have a higher flux without suffering a loss in protein recovery. It was found that negatively charged 100 kDa ultrafiltration membranes had the same protein recovery as 10 kDa unmodified membranes used in the dairy industry, but offered a flux that was at least two-fold higher. The new membranes were used for a 40-fold concentration of whey with subsequent diafiltration to mimic the industrial process for making whey protein concentrate. Mass balance models of concentration and diafiltration were developed and each agreed well with the experimental results. The experimental methods and mathematical models developed in this work can be used to design, simulate and optimize different process flow sheets, and explore the effect of various operating conditions on the membrane processing of whey. (C) 2014 Elsevier B.V. All rights reserved.