The inactivation kinetics of endogenous pectin methylesterase (PME) in freshly squeezed orange juice under high hydrostatic pressure (100-800 MPa) combined with moderate temperature (30-60 degreesC) was investigated. Atmospheric pressure conditions in a temperature range of 50-80 degreesC were also tested. PME inactivation followed first order kinetics with a residual PME activity (5-20%) at all pressure-temperature combinations used. The values of activation energy and activation volume were estimated as 109 kJ mol(-1) and -19.76 ml mol(-1) at a reference pressure of 600 MPa and reference temperature of 50 degreesC respectively. Pressure and temperature were found to act synergistically, except in the high temperature-low pressure region where an antagonistic effect was found. The PME inactivation rate constant was expressed as a function of the temperature and pressure process conditions used. This function incorporates the observed exponential dependence of activation energy on the pressure conditions, as well as the linear dependence of activation volume on process temperature. (C) 2003 Elsevier Ltd. All rights reserved.