Mathematical modeling the cross-contamination of Escherichia coli O157:H7 on the surface of ready-to-eat meat product while slicing☆
Introduction
Surface cross-contamination of foodborne pathogens in food products during processing or preparation is a major concern to consumers and food manufacturers. Although risk assessment and analyses have been applied to monitor and to reduce the hazards, some knowledge gaps, including surface cross-contamination in different processing stages, need to be addressed for the risk assessments (Gallager et al., 2003). Researches that describe, simulate and model the potential pathogen contaminations may provide significant insight to enhance risk analyses, to ascertain the importance of equipment sanitation and to improve equipment design. den Aantrekker et al. (2002) reviewed the models for cross-contamination and recontamination of food products via three routes of contamination, i.e., equipment, air and hands. These authors concluded that transfer rates for different recontamination scenarios and routes were needed to properly quantify the risk in a quantitative microbial risk assessment.
Surface foodborne pathogen transfer during slicing is one of the important factors to impact the food safety in preparing sliced RTE meat products such as ham, salami, bologna and other restructured meat. A slicer is commonly used and most likely to be the last preparation step before packaging or wrapping of the RTE foods. These products are available in the supermarket refrigeration section, either produced by brand-named manufacturers or in store preparation, including made-to-order. Sliced RTE products are also commonly sold in delicatessen and fast food restaurants, where a retail-scale slicer is often used on site for meal or sandwich preparations. The slicing equipment, if not properly cleaned and sanitized, can cause microbial cross-contamination.
Escherichia coli O157:H7 was linked to the illness outbreaks of dry fermented salami in Washington and California in 1994 (Tilden et al., 1996). Studies on cross-contamination of foodborne pathogens, such as Listeria monocytogenes, and E. coli O157:H7, between processing equipment and deli meats found that the slicing played an important role on surface microbial transfer between equipment and sliced meats (Vorst et al., 2006, Pérez-Rodríguez et al., 2007, Sheen and Hwang, 2008, Sheen, 2008). The degree of surface microbial transfer to foods correlated with the low numbers (1–3 log CFU/g) of Listeria inoculated onto the slicer blade was reported by Lin et al. (2006). Sheen (2008) applied the models derived by using relatively high Listeria inoculum levels (5–9 log CFU/test) to describe the transfer trend of low Listeria cross-contamination (≤4 log CFU/test). Flores et al. (2006) published the transfer coefficient models for E. coli O157:H7 on contact surfaces between beef and high-density polyethylene. Pérez-Rodríguez et al. (2007) determined the surface transfer coefficients for E. coli O157:H7 and Staphylococcus aureus between a contaminated domestic slicing machine and a cooked meat product during slicing. They also developed transfer models (log-linear and Weibull) for both microorganisms by fitting the microbial concentration data of 20 continuous slices (initially at 6 and 8 log CFU/blade level) and obtained a good fit (r2 ≥ 0.73) for those models.
Mathematical models to predict the transfer of food pathogens between RTE meat and the slicer blade assist in assessing the risk of cross-contamination during slicing. The surface transfer models for L. monocytogenes during slicing were published by Sheen and Hwang (2008). Since E. coli O157:H7 is another potential contaminant in the environment especially when the slicing equipment/operation involves multiple products, the cross-contamination of E. coli O157:H7 in RTE meats becomes highly possible including ham. Also, understanding the surface transfer patterns of different pathogens may further improve the food safety. In this study, the transfer of E. coli O157:H7 from one contact surface to another for RTE deli meats and a delicatessen restaurant type slicer was investigated. The objective was to develop mathematical models to describe the surface cross-contamination during slicing operation. Two cross-contamination routes, which were similar to those used by Vorst et al. (2006), and Sheen and Hwang (2008) were used in this study.
Section snippets
E. coli O157:H7 strains
A cocktail of five strains was used for the surface cross-contamination and transfer studies. The five E. coli O157:H7 strains were isolates obtained from the Food Safety and Inspection Service (FSIS) from pork sandwich (strain OB90520A), bottom round (strain OB1525C), beef patty (strain OB 1423C), ground beef (strain OB1680G) and salami outbreak (strain 380-94). A loopful of each strain was transferred from a stock culture stored at −80 °C into 10 ml of Brian Heart Infusion broth (BHI, Becton,
Surface transfer of E. coli O157:H7 from inoculated blade to ham
The surface transfer was initially performed using the ham unwrapped from the package. The counts of background microorganisms were high and impede the correct count of E. coli O157:H7 colonies. A simple surface hot-water wash eliminated the problem. The ham surface temperature was near 50 °C after wash but decreased to room temperature (21 °C) in around 30 min and maintained at this temperature during slicing process. Since the ratio of peripheral to entire surface of a sliced ham was very
Conclusion
The surface transfer of E. coli O157:H7 cells from slicer to ham (RTE Deli meat) during slicing operation was simulated and modeled for cross-contamination predictions of E. coli O157:H7. Since the slicer in RTE food service usage may involve different food slicing operation, the E. coli O157:H7 cross-contamination risk still exists. The transfer was found significantly affected by the microbial inoculation level and contamination route. In general, the higher the initial contamination levels
Acknowledgement
The author recognizes the valuable and dedicated laboratory work of Jenelle May of the Microbial Food Safety Research Unit, ERRC/ARS/USDA Wyndmoor, PA.
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