STUDY OF POSSIBLE MECHANISMS OF INACTIVATION OF LISTERIA INNOCUA IN THERMO-SONICATED MILK USING SCANNING ELECTRON MICROSCOPY AND TRANSMISSION ELECTRON MICROSCOPY

摘要

Listeria innocua cells were studied under batch pasteurization (63C) and thermo-sonication treatments (63C, 24 kHz, 129 mW/mL) in milk. Heat-treated cells were collected after 30 min and thermo-sonicated cells after 10 and 30 min. Environmental scanning electron microscopy (ESEM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to evaluate the effects of cavitation in cells. Two different sample preparation techniques, hexamethyldisilazane (HMDS) dehydration and freeze-drying (FD), were evaluated for SEM. Heat-treated cells showed thinning in cell walls; thermo-sonicated cells showed the formation of pores. After 10 min of treatment, some cells showed a breakage line, but after 30 min of ultrasound, cells were broken. Differences between the HMDS and FD techniques in the images were not noticeable, but the FD technique was faster. ESEM gave the highest magnification, showing cells with roughness on cell wall. TEM images showed clumping of cytoplasm content and disruption of cell membrane.