Inactivation mechanism of Geobacillus stearothermophilus by microwave-induced ultra-high temperature thermal shock: nonlinear synergy of oxidative stress and membrane disruption

Abstract

Microwave ultra-high temperature (UHT) processing is promising for liquid food sterilization, yet its inactivation mechanisms against thermophilic bacteria remain inadequately understood. This study investigated the bactericidal efficacy and mechanisms of microwave UHT treatment against vegetative cells of Geobacillus stearothermophilus ATCC 7953. Using a single-mode microwave system at 2450 MHz, bacterial suspensions were subjected to various power-time combinations achieving 136 ± 1 °C followed by immediate cooling. A nonlinear relationship between power and inactivation efficacy was observed: optimal reductions of approximately 5 log CFU∙mL^–1 were achieved at 150 W∙mL^–1 for 40 s and 300 W∙mL^–1 for 20 s, while intermediate powers yielded inferior outcomes. Mechanistic analyses revealed that microwave treatment induced significant membrane damage, suppressed metabolic activity, and dramatically elevated intracellular reactive oxygen species and malondialdehyde levels, with the 300 W∙mL^–1 treatment generating the highest oxidative stress. Scanning electron microscopy confirmed distinct morphological alterations without electroporation. The similar trends observed between oxidative markers and bactericidal efficacy suggest that oxidative stress-mediated lipid peroxidation may constitute a primary inactivation mechanism, with low-power prolonged exposure promoting cumulative damage and high-power short-duration treatment triggering acute oxidative burst. These findings elucidate the power-time synergistic mechanism of microwave UHT inactivation and provide a theoretical foundation for process optimization.

Graphical abstract

Keywords

microwave sterilization / ultra-high temperature processing / Geobacillus stearothermophilus / oxidative stress / thermal shock

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