White grapefruit pectin methylesterase (PME) was successfully purified by affinity chromatography using a kiwi PME inhibitor as ligand. Electrophoretic analysis combined with isothermal and isobaric-isothermal inactivation treatments suggested the presence of a labile PME fraction and a stable PME fraction with molecular weights of 31.5 kDa and 23.7 kDa, respectively. Both isothermal and isobaric-isothermal PME inactivation could be described by fractional conversion models with about 20% of the initial activity corresponding to the stable fraction. Optimum pH-ionic strength conditions for grapefruit PME thermostability were determined. Purified heat-labile PME (in 20 mM Tris Buffer (pH 7.0)) was submitted to combined thermal and high-pressure inactivation experiments in the ranges 10-62 degrees C and 0.1-800 MPa. The combined pressure-temperature dependence of the inactivation rate constants could be accurately described by a third degree polynomial model, showing a clear antagonistic effect of pressure and temperature on PME inactivation at temperature >= 58 degrees C in a pressure range of 0.1-300 MPa. In the context of fruit-juice processing, the results obtained suggest that a combined high-pressure-(low/mild) heat treatment can eliminate up to 80% of the total PME activity therefore significantly limiting the cloud-loss defect in juices. (c) 2005 Elsevier Ltd. All rights reserved.