In this paper, a process for the stable production of low allergenicity hydrolysates is presented. Whey protein was hydrolysed at 50 degrees C and pH 8.5 using a bacterial protease in a continuous stirred tank membrane reactor including a polyethersulfone plate and frame ultrafiltration module with a molecular weight cut-off of 3 kDa. The reactor was maintained in operation for 16 h. Conversion reached a steady value around 80% after 10 h of operation, while a steady permeate flow was achieved after 13 h. A slight, first order enzyme thermal inactivation was detected. A hydrolysate with an average peptide chain length around 4 amino acids was obtained. The antigenic whey protein in the product was reduced 99.97%, which suggests that it can be incorporated as nitrogen source in infant formula and enteral nutrition. (c) 2005 Elsevier L.td. All rights reserved.