A kinetic study of the inactivation of endogenous pectin methylesterase (PME) in Greek commercial peach pulp under high hydrostatic pressure (HHP; 100-800 MPa) combined with moderate temperature (30-70°C) was conducted. Thermal inactivation of the enzyme at ambient pressure conditions was also studied. PME inactivation was modeled by first order kinetics at all conditions tested. High pressure and temperature acted synergistically on PME inactivation, except at the high temperature of 70°C at the middle pressure range (100-600 MPa), where an antagonistic effect of pressure and temperature was observed. At this specific middle pressure range, an increase of pressure processing led to increased inactivation rate constants of peach PME. A multiparameter model was developed to express the PME inactivation rate constant as a function of temperature and pressure process conditions, taking into account the dependence of both activation energy and activation volume on pressure and temperature, respectively. A good correlation between experimental and predicted values of inactivation rate constants was established. This modeling approach enables the quantitative estimation of the HHP-temperature conditions needed to achieve a targeted PME inactivation in the peach pulp.

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The objective of this work was to study the application of High Hydrostatic Pressure (HHP) on quality attributes of tomato products. Textural properties of tomato fruit play an important role to the overall quality in both fresh market and processed tomatoes and can be attributed to the activity of the pectinolytic enzymes such as pectin methylestarase (PME) and polygalactouronase (PG). The effect of high hydrostatic pressure processing on the endogenous tomato enzymes, PME and PG, was kinetically studied. The enzyme inactivation kinetic study was performed using three different tomato varieties: Cherry tomatoes (Lycopersicon esculentum variety Cherrelino) and two industrial hybrids namely Oval Red and Red Sea.The inactivation of PME and PG in freshly squeezed Cherry tomato juice was studied in the pressure range of 0,1-800 MPa combined with temperatures ranging from 40 to 80°C. The decrease of enzyme activity during thermal and pressure inactivation for both PME and PG followed first-order kinetics. PME in Cherry xii tomato juice was found to be pressure-stable and an antagonistic effect of pressure and temperature was observed. On the other hand, PG was more sensitive to pressure than PME. The PG inactivation rate constant increased with increasing pressure at all temperatures tested indicating a synergistic effect of pressure and temperature. The effect of HP processing on cherry tomato juice quality attributes were studied at selected processing conditions. The HP processing conditions did not significantly affect the color and lycopene content of tomato juice. Furthermore HP processing allowed total PG inactivation with concurrent remaining PME activity leading to high quality products in terms of viscosity and textural properties.The inactivation of PME and PG in tomato juice from Oval Red and Red Sea hybrids, was kinetically studied in the pressure range of 0,1-400 MPa combined with temperatures ranging from 40 to 80°C. PG inactivation in both thermal and high pressure processing deviated from 1 st order kinetics and was described by an n th order model for both tomato hybrids. PG was found to be extremely thermotolerant, 60% Oval Red PG inactivation was observed only when tomato juice was thermally treated at 80°C for one hour. Red Sea PG was found to be more temperature sensitive than Oval Red PG.On the other hand PG activity was extremely sensitive when thermal treatment was combined with pressure. HP treatment at 400 MPa combined with temperature of 70°C led to total PG inactivation. In the case of Oval Red juice a synergistic effect of pressure and temperature was observed on PG inactivation. However, according to the results an antagonistic effect of pressure and temperature was observed in pressures lower than 200 MPa combined with high temperatures (70°C) in Red Sea tomato juice. PME inactivation was kinetically studied in thermally and HP treated OvalRed and Red Sea tomato juice. Tomato juice was thermally treated in the temperature range of 50-80°C. High pressure processing from 100 to 800 MPa wa...