Thermal vs. Microwave Inactivation Kinetics of Pectin Methylesterase in Orange Juice Under Batch Mode Heating Conditions

Abstract

Thermal and microwave inactivation kinetics of pectin methylesterase (PME) in single-strength orange juice were evaluated under batch mode heating conditions. For thermal inactivation, aliquots of orange juice sealed in glass ampoules were subjected to conventional heat treatments in a well agitated water-bath for various times at selected temperatures (60, 70, 80, 85 and 90 °C). For microwave inactivation, different amounts of orange juice in a glass beaker (100 mL capacity) were heated in a microwave oven for preselected time intervals to yield the desired final temperatures (50, 55, 60 and 65 °C). Heating times under both modes were corrected to accommodate only their effective portions for kinetic data handling. Based on residual PME activities and first order rate of inactivation kinetics, kinetic parameters were evaluated. Results indicated the enzyme inactivation data to be characterized by the following: microwave: D 60= 7·37 s (z = 13·4 °C); thermal: D 60= 154 s (z = 17·6 °C). Thus, the PME inactivation in orange juice was significantly faster in the microwave heating mode than in conventional thermal heating mode indicating some contributory nonthermal effects under the microwave heating conditions. Thermal inactivation also indicated the presence of two fractions, one more heat sensitive than the other, while only one fraction was apparent with microwave heating. Inactivation kinetics were also dependent on pH and soluble solids content of orange juice. PME was more sensitive to thermal inactivation at lower pH and higher soluble solids concentration.