The effects of several thermal treatments and high intensity pulsed electric fields (HIPEF) processing on a protease from Bacillus subtilis, suspended in simulated milk ultrafiltrate (SMUF) or milk, were evaluated. This protease did not show high resistance to conventional thermal treatments since, in milk, a high reduction of protease activity was achieved with both LTLT (low temperature long time; 63 degrees C-30min) and HTST (high temperature short time, 75 degrees C-15s) pasteurization treatments. The enzyme was more heat resistant suspended in SMUF than in milk. HIPEF affected the activity of the studied protease in different ways when treatments up to 500kJ/l were applied using several types of equipment. Only when HIPEF treatment was applied with a continuous flow device and coaxial electrodes did the enzyme activity vary, changing from a slight inactivation when suspended in SMUF to some enhancement of activity when suspended in milk. (c) 2004 Elsevier Ltd. All rights reserved.