Inactivation of Pectin methylesterase in carrot-kiwi juice during thermal processing

In this study, the mixture of carrot and kiwi juice is introduced as a new food product with high nutritional value. Turbidity is considered as a desirable property in carrot - kiwi juice that is influenced by the presence of released pectin into the extraction. Pectin methylesterase (PME) desterified the pectin that resulted in the turbidity loss in the juice. Thermal processing is known as the most common method to PME inactivation. The aim of this study is to investigate the PME inactivation during carrot - kiwi juice thermal processing at different temperature. To study the inactivation of pectin methylesterase, at first the carrot - kiwi juice was heated at 60, 70. 80 and 90 ℃ in different time (proportional to the temperature). Using a thermocople, the changes in the temperature of the sample were recorded and reported in a second. During thermal processing, the activity of the enzyme was measured according to Kimball (1991) method. D-value, Z-value and also thermodynamic parameters of the thermal destruction of PME were calculated. Based on the obtained results, Z-value of heat labile and stable isoforms of PME in the juice were estimated 17.85℃ and 22.27℃, respectively. The required activation energy to inactivate the heat labile and stable isoforms of PME in the carrot - kiwi juice were calculated 356.83 kJ/mol and 257.17 kJ/mol, respectively. Enthalpy for inactivation of the heat labile (354.1 to 353.8 kJ.mol-1) and stable (254.4 to 254.1 kJ.mol-1) isoforms were estimated. During the carrot - kiwi juice processing, the entropy of the inactivation of the heat labile and stable isoforms were calculated 0.79 to 0.73 K-1.kJ.mol-1 and 0.47 to 0.43 K-1.kJ.mol-1, respectively. Also, the free energy related to the inactivation of the heat labile (91.1 to 87.3 kJ.mol-1) and stable (97.9 to 97.0 kJ.mol-1) isoforms of PME was measured. These results represent the effect of temperature on the protein structure of the enzyme. In addition, the effectiveness percentage of the come up time (CUT) on inactivation of the heat labile (55-22%) and stable (66-22%) isoforms of PME in the carrot - kiwi juice were computed. The overheating of the product and also loss of nutritional value, organoleptic and qualitative properties and also energy wasting could be prevented by reducing the required thermal processing temperature of the carrot juice by its combination with the kiwi fruit that resulted in pH adjustment and also study the effect of CUT on the PME inactivation.