Inactivation effect of microwave heating on pectin methylesterase in orange juice

Introduction. Enzyme inactivation is a major objective in orange juice production. Conventional heating in elevated temperature causes adverse effects on the final products such as color alterations, flavor damages, ascorbic acid losses. For this, microwave heating (MW) was used in this study as thermal treatment on orange juice production for pectin methylesterase (PME) inactivation, and response surface methodology (RSM) was used for optimization of MW conditions. Materials and methods. Oranges (Citrus sinensis Osb.) of Navel variety were used as raw material in this study. The effects of flow rate and power on PME activity were investigated. After optimization orange juice was produced with using optimized conditions and compared with untreated control juices and conventional thermally heated juices on the aspect of PME inactivation and some quality characteristics. Results and discussion. The linear effects of flow rate (x(1)) and power (x(2)), as well as the quadratic effect of flow rate (x(1)(2)), power (x(2)(2)) and the interaction effect of flow rate-power (x(1),x(2)) were significant for PME inactivation by MW. Lack of fit of experimental data was not significant (P>0.05) for model. The coefficient of variation (C.V.) was 6.27%. The model showed an adequate precision of 6.788x10(-3). The determination coefficient (R-2) was 0.9793, while the adjusted determination coefficient (adjusted R-2) value was 0.9645. R-2 and adjusted R values for the models did not differ dramatically indicating non-significant terms have not been included in the model. Reduction of PME activities was found approximately 93-95% in MW groups. The PME inactivation rate was described satisfactorily as a function of microwave heating conditions. The PME can be inactivating in moderate temperatures by MW (40 mL/min-900W-83 degrees C) and MW (50 mL/min-900W-75 degrees C). D values were calculated for two optimum conditions of MW and CH treatments and found as 39.24 sec for MW (40 mL/min-900W), 38.76 sec for MW (50 mL/min-900W) and 70 sec for CH (95 degrees C-60 sec). Total pectin content was increased 17.2% after MW application. And the loss of ascorbic acid content for MW sample was found lower than other applications. Conclusions. A synergistic effect of microwave energy and temperature on orange juice PME inactivation was found under microwave heating conditions. It was determined that MW (50 mL/min-900W) can be applied as a thermal treatment on orange juice production in moderate temperatures (75 degrees C) for PME inactivation and may improve functional properties of orange juice. And this result is extremely important in terms of cloud stability of orange juices.