Modeling transfer of Listeria monocytogenes from slicer to deli meat during mechanical slicing.

Listeria monocytogenes has been implicated in several listeriosis outbreaks linked to the consumption of presliced ready-to-eat (RTE) deli meats. The possible contamination of sliced RTE meats by L. monocytogenes during the slicing process has become a public health concern. The objectives of this study were to investigate the transfer phenomena of L. monocytogenes between a meat slicer and ham slices, and to develop empirical models to describe the transfer during slicing. A six-strain cocktail of L. monocytogenes was inoculated onto a slicer blade to an initial level of approximately 3, 6, or 9 log(10) colony-forming units (CFU)/blade (2, 5, or 8 log CFU/cm(2) of the blade edge area), and then the ham was sliced to a thickness of 1 to 2 mm (Case I). As a second cross-contamination scenario (Case II), a clean blade was used to slice ham previously inoculated with L. monocytogenes (3, 6, or 9 log(10) CFU per meat surface of ca. 100 cm(2)) prior to slicing uninoculated ham. The ham slicing rate was maintained at an average of three to four slices per minute for both Case I and II. Although the overall recovery ratio, including slicer surfaces and collected ham slices, was less than 100%, more ham slices were contaminated with L. monocytogenes when the blade was contaminated with higher initial levels of L. monocytogenes. Empirical models were developed to describe the transfer of L. monocytogenes between blade and ham slices. The models may be applied to predict the number of ham slices that may be contaminated by a L. monocytogenes-contaminated slicer during ham slicing operation. However, the models are both microbial load and contamination route dependent, which might limit their applications to certain conditions. This study showed the initial step for the development of surface transfer model and discussed the factors that might need to be considered and included in future study to expand the model applications.