Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 3, 2007
Publication Date: April 1, 2008
Citation: Sheen, S., Hwang, C. 2008. Modeling Transfer of Listeria monocytogenes on Deli Meat During Mechanical Slicing. Foodborne Pathogens and Disease. 5(2):135-146. 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 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 of L. monocytogenes between a meat slicer and ham slices, and to develop empirical models to describe the transfer. Two routes of blade contamination were examined: i) direct inoculation onto the blade of a retail-scale slicer, or ii) inoculation onto the blade through slicing of contaminated ham. Regardless of the route of blade contamination, uncontaminated ham was sliced using the contaminated slicer. The numbers of L. monocytogenes cells transferred from the blade to the ham slices were determined. The results indicated that more ham slices were contaminated with L. monocytogenes when the blade was contaminated with a high level (ca. 1,000,000,000 cells) compared to a low level (ca. 1,000 cells) of L. monocytogenes. The contamination scenarios also had a significant impact on the transfer of L. monocytogenes between the blade and ham slices. L. monocytogenes remained on the blade longer and contaminated more ham slices when it was introduced by slicing of contaminated ham rather than by direct inoculation of the blade. Models describing the transfer were developed that may be applied to predict the numbers of ham slices that may be contaminated by a L. monocytogenes-contaminated slicer during a ham slicing operation.
Technical Abstract: Listeria monocytogenes has been implicated in several listeriosis outbreaks linked to the consumption of pre-sliced 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 ca. 3, 6, or 9 log CFU/blade, and then the ham was sliced to a thickness of 1-2 mm (Case I). As a second (Case II) cross-contamination scenario, a clean blade was used to slice ham that was previously inoculated with L. monocytogenes (3, 6 ,or 9 log10 CFU per meat surface per test) followed by slicing of ham that was not inoculated. The ham slicing rate was maintained at an average of 3 slices per minute for both Case I and II. Although the recovery ratio was less than 100 percent, more ham slices were contaminated with L. monocytogenes when the blade was contaminated with higher rather than lower initial levels of L. monocytogenes. For Case I, the number of contaminated slices compared to the total number of ham slices collected was 5/40, 30/40 and 150/150 for inoculation levels of 3, 6, and 9 log CFU/blade, respectively. Results for Case II showed that L. monocytogenes stayed on the blade longer and contaminated more ham slices than results observed for Case I, that being 5/40, 110/120, and 150/150 for inoculation levels of 3, 6 or 9 log CFU/test, respectively. The microbial count on the 150th consecutive slice starting from an initial level of 9 log CFU level was about 1.0 log CFU higher for Case II than for Case I. The empirical models developed [F-statistic: P > F (< 0.000) and R-square > 0.7] predicted the trend of Listeria transfer between the 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.