A cyclic batch membrane reactor (CBMR) for the production of reduced-antigenicity whey protein hydrolysates with subtilisin at 60 degrees C and pH 8.5, is presented in this paper. The operation of the CBMR comprises three steps, namely, hydrolysis, ultrafiltration and enzyme recycling, which are repeated a number of times. As a case study, a reduction of antigenicity of 1000 times was required. To this end, a hydrolysate with a degree of hydrolysis of 0.15 was needed. A 8000 Da polyethersulphone membrane was employed in order to retain the enzyme and let the permeation of the peptides generated. A theoretical model was developed, which was validated by experiments. The optimisation of the CBMR was performed, taking the total amount of enzyme as objective function. The optimum number of enzyme reuses was calculated and resulted to be up to 5. Compared to the traditional single batch operation mode, a save of enzyme up to 59% can be achieved. (c) 2005 Elsevier Ltd. All rights reserved.