Submitted to: Journal of Food Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 23, 2008
Publication Date: June 1, 2008
Citation: Sheen, S. 2008. Modeling surface transfer of L. monocytogenes on salami during slicing. Journal of Food Science. Vol 73. (6). pg. 304-311. Interpretive Summary: Listeria monocytogenes can survive at refrigeration temperatures and has been implicated in several cases of food poisoning linked to the consumption of pre-sliced ready-to-eat (RTE) deli meats. The possible contamination of RTE meats by L. monocytogenes during the slicing process has become a public health concern. The objectives of this study were to investigate the L. monocytogenes cross-contamination, and to develop mathematical models which may predict and enhance food safety of sliced salami. Salami with 15% fat was used in this study. A six-strain cocktail of L. monocytogenes was inoculated onto a slicer blade (carbon steel) to an initial level of about one thousand to one billion microbial counts per blade, and then the salami was sliced to a thickness of 1-2 mm (Case I). In another cross-contamination scenario, a clean blade was used to slice salami that was previously inoculated with L. monocytogenes followed by slicing of salami that was not inoculated (Case II). The experimental results (positive L. monocytogenes) from low microbial counts per test (one thousand to ten thousand counts) were random on first 100 sliced salami, similar to those found previously in 2% fat ham slicing. In general, the cross-contamination may last longer in Case II than in Case I. The models, therefore, developed (using 100000 counts and higher) are microbial load, slicing salami number and contamination route dependent. However, the models may be applied to predict the low level L. monocytogenes cross-contamination (10000 counts and lower) during salami slicing process and serve as a building block in microbial risk assessment.
Technical Abstract: Listeria monocytogenes has been implicated in several listeriosis outbreaks linked to the consumption of pre-sliced ready-to-eat (RTE) deli meats, which has drawn considerable attention in regard to possible cross-contamination during slicing operation at retail and food service environment. Salami with 15% fat (a moderate fat content deli item) was used to investigate the transfer phenomena of L. monocytogenes between a meat slicer and salami slices and to understand the potential impact on food safety. A six-strain cocktail of L. monocytogenes was inoculated onto a slicer blade to an initial level of ca. 3, 5, 6, 7 or 9 log CFU/blade (or ca. 2, 4, 5, 6 or 8 log CFU/cm-squared of the blade edge area), and then the salami was sliced to a thickness of 1-2 mm (Case I). Another cross-contamination scenario, a clean blade was used to slice salami that was previously inoculated with L. monocytogenes on four rectangular side surfaces (ca. 3, 5, 6, 7, 8 or 9 log CFU/100 cm-squared area) followed by slicing of salami that was not inoculated (Case II). The salami slicing rate was maintained at an average of 3-4 slices per minute for both Case I and II. The empirical models simulated the trend/pattern of L. monocytogenes transfer reasonably well between the blade and salami slices for contamination level of 5 log CFU and higher. With 3 or 4 log CFU, the experimental transfer results showed random on sliced salami (first 100 slices) which was similar to those found previously in 2% fat ham slicing. The currently developed models are microbial load (n), sequential slice index (X) and contamination route dependent which might limit their applications to certain conditions. However, the models may be further applied to predict the 3 or 4 log CFU level (and below) cross-contamination of salami slicing process. Considering only few data available in the literature regarding food pathogen surface transfer, the empirical models may provide a useful tool in building the risk assessment procedures.