Sterilising effect of high power pulse microwave on Listeria monocytogenes

In the present work, Listeria monocytogenes was used as the target strain to investigate the sterilising potential and mechanism of high power pulse microwave (HPPM). Results showed that the inactivation was positively correlated with the pulse frequencies and operating times. The count of Listeria monocytogenes was decreased by 5.09 log CFU/mL under 200 Hz for 9 min, which was used as the optimised condition to further explore the sterilisation mechanism. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the L. monocytogenes cells of untreated group presented intact surfaces, clear boundary, and its intracellular contents distributed uniformly in the cytoplasm. Following treatment, the cell wall surfaces began to deform in small areas, and cell membranes were severely ruptured, thus resulting in the appearance of electron transmission areas. Extracellular protein and nucleic acid contents, represented by OD260 nm and OD280 nm, increased with the increase in operating time significantly. After treatment, SDS-PAGE profiles of whole-cell proteins displayed that the protein bands became lighter or even disappeared. Na+ K+-ATPase activities and intracellular ATP content decreased by 72.97 and 79.09%, respectively. This was consistent with the cell viability of L. monocytogenes observed by confocal laser scanning microscopy. Overall, the sterilisation mechanism of HPPM on L. monocytogenes may be caused by membrane damage, intracellular component leakage, and energy metabolism hindrance.