Continuous-Flow Microwave Heating of Orange Juice: Evidence of Nonthermal Effects

Microwave heating characteristics oforange juice were evaluated under conditions suitable for continuous-flow heat-holdcool pasteurization process. Orangejuice was pumped through a helical glass coil located inside a domestic microwave oven and exit temperatures were determined as a function of flow rate and initial temperature underfidlpower (700W, 2450MHz) heating conditions. The average residence time of the fluid was computed by dividing the volumetric capacity of the helical tube inside the microwave oven by the volumetric flow rate under the test condition. Fluid temperature rise from the tube inlet to the outlet was found to be non-linear based on the steady state temperature profile of the fluid along the tube length as measured experimentally using a fiber optic temperature probe. The calculated average residence times were converted to equivalent thermal times (or effective heating times) at the exit temperature by numerically integrating the thermal effect ofthe measured temperature profilefrom which the inactivation rate of enzyme pectin methyl esterase (PME) was computed. Results indicated that microwave heating was considerably more effective (D60 ˜ 22s) than conventional heating (D60 ˜ 150s) for inactivation of PME.