A novel concept of enzymatic membrane reactor (EMR) is presented, aiming at the production of cheese whey protein hydrolysates with low contents of phenylalanine (Phe) for the diet of phenylketonuria patients. Whey proteins were first hydrolyzed by chimotrypsin, followed by the action of carboxypeptidase A (CPA), immobilized on agarose gel particles, which were retained inside the reactor using a filter. The liquid medium passes through hollow fiber ultrafiltration unit (1 kDa cut-off), and the retentate is recycled to the reactor. The innovation here is that the membrane is not used to retain the enzyme neither physically nor as an immobilization support. The EMIR provided a higher performance than the classical, sequential approach: batch reaction followed by ultra filtration. It was confirmed that the removal of products promoted by the EMR enhances reaction rates, due to the reduction of inhibition effects. A mathematical model of the EMIR is also presented and validated. (C) 2008 Elsevier Ltd. All rights reserved.