Submitted to: Journal of Food Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/30/2010
Publication Date: 7/1/2010
Citation: Sheen, S., Costa, S., Cooke, P.H. 2010. Impact of mechanical shear on Listeria monocytogenes survival on surfaces. Journal of Food Science. 75(N6):387-393. Interpretive Summary: Thermal and high pressure processes can make a significant impact on microbial lethality, which is important for food safety. The mechanical shear on surfaces (including bacterial surfaces) may also contribute to food-borne pathogen count reductions. Since microbial death caused by shear involves many factors, including surface properties, operation conditions, and others, the lethality mechanism is difficult to verify instrumentally using currently available techniques. In this study, the shear factor effect on microbial death was demonstrated by determining the transfer of the pathogen, Listeria monocytogenes, from one surface to another during slicing. There was a reduction of approximately 99% of the total bacterial counts using a model food system and a commercially available slicer used to prepare ready-to-eat (RTE) meat. This finding may provide useful information for risk assessment for RTE meats.
Technical Abstract: Microbial inactivation using high temperatures is well known process and has contributed significantly toward food safety and shelf life extension for the food industry. Mechanical high pressure (hydrostatic) treatment is also gaining interest in food processing applications for achieving microbial count reductions as an alternative for heat sensitive foods. The effect of mechanical shear on microbial survival during processing has not been explored. In this study, the impact of mechanical shear on solid surfaces created during a slicing operation on the reduction in the levels of Listeria monocytogenes was investigated. There have been cases of cross-contamination of ready-to-eat (RTE) deli meat with L. monocytogenes during slicing/processing operations reported. Cell death using an agar model food system was demonstrated by confocal microscopy in which the live and dead cells appeared green and red in color, respectively. The images indicated that a large percentage of dead cells transferred onto the sliced agar surface, thus the surface shear resulted in a lethal effect on L. monocytogenes during the slicing operation. The surface transfer results further showed that the initial agar slices (slices 1-7) had about a 2-4 log CFU/slice reduction compared to the cell counts inoculated on the blade surface (8.4 log CFU/blade on a 10 cm-squared edge area). The microbial count decreased to about 3 log CFU/slice (slice 30 to 40), and it would likely have decreased further with continued slicing. The overall live cell recovery (survival) ratio was about 2-3% by mathematical integration of the cell counts on blade, slicer, and agar surfaces after slicing. The results demonstrated that food pathogens can be significantly eliminated by mechanical shear on surfaces. The overall lethality due to surface shear is about 2 logs (99%). This research may provide important information for surface cross-contamination modeling and risk assessment for RTE meats.