Food Protection Trends - October 2010 - 591

FIGURE 3. Thermal inactivation of L. innocua on cast aluminum guard of a deli meat slicer in a dry oven at 80oC for up to 15 h. a-dvalues with different superscripts are significantly different by t test (P < 0.05). Each value is the mean of duplicate samples from three experiments.

cleaned and sanitized prior to heating. Dry thermal heating overnight could provide an extra hurdle for Listeria contamination in the worst case scenario of an inadequately cleaned and sanitized slicer.

ACKNOWLEDGMENTS
Research support from the American Meat Institute Foundation and USDA Food Safety Consortium is gratefully acknowledged. REFERENCES
1. Anonymous. 2007. Washington state Salmonella cases connected to Arby’s. Available at http://foodsafety. k-state.edu/articles/1096/FSNinfosheet-8-29-07.pdf Accessed 09 March 2010. 2. Bremer, P. J., I. Monk, and C. M. Osborne. 2001. Survival of Listeria monocytogenes attached to stainless steel surfaces in the presence or absence of Flavobacterium spp. J. Food Prot. 64:1169–1176. 3. CDC. 2009. Preliminary FoodNet data on the incidence of infection with pathogens commonly transmitted through food. MMWR 58:333–337. 4. Darmady, E. M., K. E. A. Hughes, J. D. Jones, D. Prince, and W. Tuke. 1961. Sterilization by dry heat. J. Clin. Pathol. 14:38–44. 5. Donganay, M. 2003. Listeriosis: Clinical presentation. FEMS Immunol. Med. Microbiol. 35:173–175. 6. Fairchild, T. M., and P. M. Foegeding. 1993. A proposed nonpathogenic biological indicator for thermal inactivation of Listeria monocytogenes. Appl. Environ. Microbiol. 59:1247–50. 7. Foegeding, P. M., and N. W. Stanley. 1991. Listeria innocua transformed with an antibiotic resistance plasmid as a thermal-resistance indicator for Listeria monocytogenes. J. Food Prot. 54:519–23. 8. Friedly, E.C., P.G. Crandall, S.C. Ricke, C.A. O’Bryan, E.M. Martin, and L.M. Boyd. 2008. Identification of Listeria innocua surrogates for Listeria monocytogenes in hamburger patties. J. Food Sci. M174–M178. 9. FSIS/USDA. 2003. FSIS risk assessment for Listeria monocytogenes in deli meats. Available at http:// www.fsis.usda.gov/OPPDE/rdad/ FRPubs/97-013F/ListeriaReport.pdf Accessed 09 March 2010.

nificant differences in survivors as time progressed, levels of survivors never declined below 4 logs. Temperature of the oven was increased to 80oC and the experiment was repeated. Results of the second experiment are shown in Fig. 3, where it can be seen that there was recovery of over 3 log CFU/cm2 of L. innocua survivors. The decrease at 80ºC was over 4 log, but the thermal treatment time required to achieve this level of reduction (15 h) is most likely not practical for use in a working delicatessen. However, it should also be noted that the thermal resistance of this L. innocua is 1.3 times greater than that of L. monocytogenes, so it is possible that this temperature overnight would be sufficient to cause a 5-log reduction of L. monocytogenes. Because of the nonlinear nature of the curves obtained, it was not possible to calculate D- and z-values for the slicer materials. We chose a 5-log reduction target because food regulations routinely require that treatments reduce pathogens by this amount in food products. In the case of the deli slicer, it should be noted that we did achieve a reduction of the L. innocua in the first three hours of heating that could be adequate, given that levels of residual L. monocytogenes on equipment are likely not as high as the levels in the inocula we used.

Rodriguez and others (22) found that as Listeria biofilms dried on stainless steel, they were more able to transfer Listeria to food. Although we did not study L. innocua biofilms on these slicers, results of our tests indicate that contaminated slicers subjected to dry heat might be more likely to transfer residual Listeria to foods. Although dry heat is economical and could easily be used with typical dry heat ovens found in most delis, it appears that it is not well suited to sterilization of deli slicers. An alternative avenue of exploration would be the use of moist heat applied on the contaminated slicer at similar temperatures as those used in the current study. With moist heat, it is critical for the moisture to penetrate to the contaminated areas, but sterilization is usually achieved over shorter time periods at lower temperatures.

CONCLUSION
Dry thermal treatment at 80oC and times up to 15 h are not sufficient to achieve a 5-log reduction of residual L. innocua that may have survived improper cleaning and sanitizing of the deli slicer. However, a three-hour treatment at 80oC produced a 2 or 3 log reduction, which would likely be adequate for a machine that had been

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http://foodsafety.k-state.edu/articles/1096/FSN-infosheet-8-29-07.pdf http://foodsafety.k-state.edu/articles/1096/FSN-infosheet-8-29-07.pdf http://foodsafety.k-state.edu/articles/1096/FSN-infosheet-8-29-07.pdf http://www.fsis.usda.gov/OPPDE/rdad/FRPubs/97-013F/ListeriaReport.pdf http://www.fsis.usda.gov/OPPDE/rdad/FRPubs/97-013F/ListeriaReport.pdf http://www.fsis.usda.gov/OPPDE/rdad/FRPubs/97-013F/ListeriaReport.pdf

Food Protection Trends - October 2010

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