Purification, characterization, thermal and high-pressure inactivation of a pectin methylesterase from white grapefruit ( Citrus paradisi)

Abstract

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 °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 °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. Industrial relevance The present works describes the combined thermal and high pressure inactivation of pectinmethylesterase (PME) from white grapefruit. White grapefruit contains two PME fractions with different process stability (both towards temperature and pressure), the stable fraction contributing to about 20% of the total PME activity. The results demonstrate that combined mild heat and high-pressure processing cannot be used to fully inactivate PME in grapefruit juice. However, by eliminating up to 80% of the PME activity (labile fraction), such treatment can probably contribute to a significant decay of the cloud loss defect observed in grapefruit juices, while allowing pasteurisation and good quality retention of the latter. Combining high pressure mild temperature processing with other non-thermal approaches (e.g. use of PME-inhibitor) could be of interest in creating juices with extended shelf-life.