Effect of Microwave Power Cycle on Temperature Uniformity, Microbial Inactivation, and Quality: Computational Approach for Milk Thermal Processing.

Microwave power control is commonly achieved either by magnetrons` on-and-off power cycling or inverter-based continuous control to obtain the desired power. Although previous studies have focused on the effect of power cycling on temperature uniformity, its implications on microbial safety and quality remain insufficiently understood. In this study, the effect of different on-and-off power cycle configurations at a fixed duty cycle (0.5) on temperature evolution and microbial and enzymatic inactivation kinetics was investigated. For this purpose, milk was chosen as the sample food product, and the required evaluations were carried out using a finite element-based multiphysics model. The results showed that different power cycle settings led to significantly different temperature increase rates despite the identical final temperature and absorbed power by the sample. While temperature uniformity was not significantly affected due to the natural convection effects triggered in the liquid sample, microbial inactivation kinetics varied substantially. Longer on-cycle durations resulted in rapid temperature increases and higher inactivation levels. Besides the microbial inactivation, enzymatic quality degradation, represented by γ-glutamyl transferase (GGT) inactivation, was strongly influenced by the applied power cycles, indicating the requirement to balance the microbial inactivation with quality preservation. These results demonstrate that power cycle-induced temperature profiles play a critical role in determining safety and quality outcomes in microwave processing. This study further highlights the requirement of reporting power cycle settings explicitly in microwave processing to ensure repeatability and comparability. PRACTICAL APPLICATIONS: This study demonstrates that the same microwave power level does not always yield the same results in the view of temperature dependent effects since different power cycle settings lead to various temperature histories. Hence, especially in industrial settings, safety and quality cannot be controlled by average power inputs only. Instead, the power cycle configuration must be carefully selected and monitored with its effects on the temperature change of the product. These findings might assist in designing microwave processes to ensure required microbial inactivation while minimizing quality losses.