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Keeping quality of fresh orange juice is highly dependent on pectinolytic enzyme activity and the growth of spoilage microorganisms. The inactivation kinetics of indigenous pectin methylesterase (PME) and of the two more pressure resistant species of spoilage lactic acid bacteria (LAB) Lactobacillus plantarum and L. brevis in freshly squeezed Valencia orange juice under high hydrostatic pressure (100-500 MPa) combined with moderate temperature (20-40°C) was investigated. PME inactivation followed first order kinetics with a residual PME activity (15%) at all pressure-temperature combinations used. The values of activation energy and activation volume were estimated at each pressure and at each temperature, respectively. Values of 90 kJ ⁄ mol and )30 mL ⁄ mol at reference pressure of 300 MPa and reference temperature of 35°C were estimated respectively. The corresponding z T and z P values of LAB inactivation were also estimated at all conditions tested. Values of 19.5°C and 95 MPa at reference pressure of 300 MPa and reference temperature of 30°C were estimated respectively for L. plantarum, while the corresponding values for L. brevis were 40°C and 82 MPa, respectively, at the same reference conditions. Pressure and temperature were found to act synergistically both for PME and LAB inactivation. The PME and LAB inactivation rate constants were expressed as functions of the temperature and pressure process conditions. These functions allow the determination of the pressure ⁄ temperature conditions that achieve the target enzyme and microbial inactivation at a selected processing time. The process conditions of 350 MPa at 35°C for 2 min are proposed as effective for Valencia orange juice cold pasteurisation.

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