Papain and ficin are general non-specific plant thiol proteases applied in food processing. Inactivation kinetics of papain and ficin were studied for thermal (50-80 degrees C, ambient pressure) and high pressure/temperature (500-900 MPa at 50-80 degrees C) process conditions. The effect of temperature at each pressure and the effect of pressure at each temperature were expressed by the values of activation energy. E(a), and activation volume, V(a) The two enzymes showed the same inactivation under thermal process. At high pressures, at each process temperature, increase of pressure increases the inactivation rate. Up to 60 degrees C for ficin and 70 degrees C for papain, inactivation rates at high pressures are higher than the corresponding thermal rates at ambient pressure. At higher temperatures up to certain pressures an antagonistic effect was observed. At 80 degrees C thermal inactivation rates at all high pressures are lower than the ones at ambient pressure. E(a) increased for papain (from 50 to 88 kJ/mol) and decreased for ficin (from 139 to 43 kJ/mol) as process pressure increased, a different effect of pressure on temperature sensitivity. V(a) depended on process temperature showing increasing effect of pressure at higher temperatures for papain and decreasing for ficin. The enzymes inactivation rate constant was modeled as a function of both temperature and pressure conditions by a multi-parameter equation. Overall, papain and ficin showed a high thermal and pressure stability requiring intense process conditions for adequate inactivation. (C) 2008 Elsevier Ltd. All rights reserved.