Food Protection Trends - October 2010 - 589

INTRODUCTION
Listeria monocytogenes causes a rare but potentially fatal disease, listeriosis. The mortality rate from this disease is approximately 28%, particularly in immunocompromised individuals, who are most at risk (3, 5). The economic burden for US citizens for a single case of listeriosis is estimated to be $1,659,143 (23). The prevalence of L. monocytogenes in ready-to-eat (RTE) deli meat and poultry products has steadily decreased from 4.61% positive in 1990 to only 0.42% positive in 2008 (10), but the incidence of L. monocytogenes infections has not changed substantially in the past 3 years (3). It has been apparent for many years that meat slicers in retail establishments can harbor large populations of bacteria (12) and that these bacteria have the potential to cross contaminate sliced, ready-to-eat foods (11). A few outbreaks of salmonellosis have also been linked to meat slicers (1, 17). Although no cases of listeriosis have been directly linked to meat slicers, slicers have been found to be contaminated with L. monocytogenes (16), and the 2008 listeriosis outbreak in Canada was ultimately traced to the meat slicers in the RTE meat plant (15). In addition, a large study by Gombas and others (13) found that prevalence of L. monocytogenes was approximately 7 times higher in RTE meats sliced in retail deli as compared to commercially manufactured, sliced and packaged RTE meats. The fact that nearly 75% of consumers purchase deli sliced meats rather than commercially packaged meats (9) implies that significant numbers of consumers are exposed to L. monocytogenes. Foegeding and Stanley (7) proposed the use of Listeria innocua ATCC 33091 as a thermal-processing indicator for L. monocytogenes in milk because this organism is 1.3 times more heat resistant than L. monocytogenes. Fairchild and Foegeding (6) generated a natural mutation of L. innocua 33091 that was resistant to rifampin and streptomycin and designated it strain M1. Resistance to these two antibiotics allows this strain to be counted directly on nonselec-

tive medium with these antibiotics added, without interference from the normal background microflora present. Murphy and others (19) extended this research to poultry and determined the D(67.5°C) in chicken breast meat was 0.35 min for L. innocua M1 and 0.29 min for L. monocytogenes. L. innocua also responds similarly to L. monocytogenes to thermal processing, ultraviolet-C light, flash pasteurization, and ionizing radiation on the surface of RTE meats (8, 24, 25, 26). Studies conducted in a food pilot plant setting, using flash pasteurization, have used L. innocua cocktails as a nonpathogenic surrogate in place of L. monocytogenes (18, 20, 21, 24). Dry heat has been used for many years as a means to sterilize materials, especially for such medical commodities as assembled non-disposable syringes, where dry heat can penetrate into the sealed container, in contrast to steam, which cannot be relied upon to reach the interior of the container (4). In 1972, NASA used dry heat sterilization on the Mars landing craft to prevent forward contamination of Mars (14). Commercial manufacturers of RTE meats sometimes place their cleaned and sanitized commercial scale size slicers in their smoke houses at 60 to 80oC overnight in an effort to inactivate any residual L. monocytogenes (personal communication, John Butts, May 2009). The objective of this study was to attempt to recreate this treatment, using the existing delicatessen ovens to provide a lethal thermal treatment to destroy L. innocua on food contact surfaces. The objective was to mimic the use of bread proofing ovens, in which temperatures do not reach the typical dry heat sterilization temperatures of 160 to 180oC. We also assessed the effectiveness of studying coupons cut from a slicer rather than the entire slicer, as well as the difference in survival of L. innocua on the slicer’s stainless steel blade versus the cast aluminum guard.

enrichment (6), was originally obtained from Dr. P. M. Foegeding (Department of Food Science, North Carolina State University, Raleigh, NC). Stock cultures were maintained frozen (−80ºC). The working culture was started from frozen stock inoculated into tryptic soy broth (Bacto, Becton Dickinson Co., Sparks, MD) supplemented with 0.6% yeast extract (TSBYE) and was incubated at 37°C overnight.

Preparing deli slicer coupons and components
Stainless steel components from the blade of a Hobart heavy duty slicer (Hobart Food Equipment, Australia) were cut into 2 × 2.5 cm coupons, using a Flow Waterjet Cutting System (Flow International Corporation, Kent, WA). This cutting system was used to prevent heat-induced stress, which could cause a change in the physical properties of the stainless steel. From the blade guard of the same slicer, cast aluminum coupons (2 × 2 × 0.5 cm) were cut, using a Milwaukee Heavy-Duty metal cold-cutting metal saw (Brookfield, WI) and a Wellsaw metal-cutting band saw (Wells Manufacturing Corporation, Three Rivers, MI). Coupons were washed thoroughly in Micro 90 cleaning solution (International Products Corp., Burlington, NJ) prepared as per directions of the manufacturer and then rinsed in sterile deionized water. Coupons were sterilized by autoclaving for 15 min at 121°C. In addition, from another slicer, disassembled stainless steel and cast aluminum deli slicer food contact surfaces were marked off into 2 × 2 cm grids, using permanent markers. Gridded areas were serially numbered in a random fashion. Slicer components were wrapped in aluminum foil and autoclaved at 121°C for 15 minutes.

Inoculation of coupons and components
Sterile coupons were laid individually on sterile glass microscope slides and forty microliters (40 µl) of the L. innocua culture was pipetted into the middle of each coupon and carefully spread over the area with a sterile inoculation loop. The inoculum was allowed

MATERIALS AND METHODS
Bacterial cultures
Listeria innocua M1 resistant to streptomycin (250 mg/L), and rifampicin (50 mg/L), both generated by selective

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Food Protection Trends - October 2010

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