Pathogenic Survivors

A study done in the food microbiology and safety labs at Louisiana State University AgCenter, Baton Rouge, La., investigated the effects that freezing had on the survival status of Listeria monocytogenes on ready-to-eat meat products that were under vacuum and non-vacuum conditions.

Laboratory test evaluated the recovery of Listeria monocytogenes on RTE products that were purchase at a local grocer. The products studied included roast beef, hot dogs, pork sausage, chicken breast, and deli style turkey breast. The meat samples were cut into 45 gram cubes and seeded with a known amount of Listeria monocytogenes culture. The culture was allowed to air dry on the cube meat samples for two minutes under a laminar flow hood and later bagged as vacuum packaged or non-vacuum packaged. The samples were frozen at -20ºC and bacterial counts determined weekly and monthly for three months.

The recovery of Listeria monocytogenes on the various meat products after 28 days of storage at -20ºC under vacuum and non-vacuum packaging conditions illustrated by Figure 1 shows that at day seven there was no significant difference in reduction of Listeria monocytogenes counts for all the ready-to-eat meats tested.However, by day 28, the RTE roast beef samples, vacuum and non-vacuum, Listeria monocytogenes counts had grown significantly more than the other meat samples. For all other RTE meat samples, except the vacuum-packaged turkey, Listeria monocytogenes counts had increase to greater than 0.5 Log CFU/g at day 28. After 28 days, the vacuum packaged turkey Listeria monocytogenes counts were at the same levels as on day zero.

By day 30, the Listeria monocytogenes counts had significantly increased to 0.8 Log CFU/g as compared to day 0 (figure 2). However, there was a significant decrease by days 60 and 90 for the vacuum and non-vacuum roast beef samples. There was no significant difference between the Listeria monocytogenes counts on day 90 when compared to counts on day zero for both vacuum and non-vacuum packaging roast beef.

Nevertheless, by day 90, the non-vacuum hotdogs had dropped to the initial inoculation levels and the vacuum hotdogs were only a non-significant 0.2 Log CFU/g higher from the initial inoculation levels. There was a significant difference between the vacuum packaged hotdogs and the non-vacuum packaged hotdogs on days 30 and 60.

In the vacuum and non-vacuum packaged sausage samples, the Listeria monocytogenes counts were significantly increased by 0.6 Log CFU/g on day 30 from initial inoculation levels, but by day 90 the vacuum packaged sausage counts had dropped below the initial inoculation level. From days 30 to 90, the vacuumed package sausage had significantly lower Listeria monocytogenes counts compared to the non-vacuumed packaged sausage. The non-vacuum packaged chicken, Listeria monocytogenes counts remained significantly at a lower level in comparison to the vacuum packaged chicken. The vacuumed packaged turkey was the only RTE meat product that did not have an increase in Listeria monocytogenes counts under frozen storage conditions. Listeria monocytogenes has a survival mechanism that is hardier then most bacteria. The reoccurrence of outbreaks due to Listeria monocytogenes contamination demonstrate a need for the development of additional measures to prevent economic loss and deaths from foodborne diseases.

References:

• Anonymous. 2003. Control of Listeria monocytogenes in ready-to-eat meat and poultry products. Code of Federal Regulation, 9 CFR 430. Office of the Federal Register, U.S. Government Printing Office, Washington, D.C.
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• Anonymous (b). 1999. Update: Multistate outbreak of listeriosis-United States, 1998- 1999. Morb. Mortal. Wkly. Rep. 47:1117-1118.
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• Anonymous (b). 1998. Secondary direct food additives permitted in food for human consumptioacidified sodium chlorite solutions. Code of Federal Regulation, 21CFR 173.325. Office of the Federal Register, U.S. Government Printing Office, Washington, D.C.
• El-Kest, S, and E. H. Marth. 1991. Injury and Death of Frozen Listeria monocytogenes as Affected by Glycerol and Milk Components. J. Dairy Sci. 74:1201-1208.
• El-Kest, S., A. E. Yousef, and E. H. Marth. 1991. Fate of Listeria monocytogenes during freezing and storage. J. Food Sci. 56:1068-1071.
• Ikeda, J. S., J. Samelis, P. A. Kendall, G.C. Smith, and J. N. Sofos. 2003. Acid Adaptation Does Not Promote Survival of Growth of Listeria monocytogenes on Fresh Beef Following Acid and Nonacid Decontamination Treatments. J. Food Prot. 66: 986-992.
• Lou, Y. and A. E. Yousef. 1999. Characteristics of Listeria monocytogenes Important to Food Processors. In: Listeria, Listeriosis, and Food Safety, E. T. Ryser and E. H. Marth (eds). Marcel Dekker, Inc, New York. pp. 131-224.

Richelle L. Beverly is a postdoctoral researcher and Marlene E. Janes is an assistant professor in the Department of Food Science, Louisiana State University AgCenter (Baton Rouge, La.) Reach Beverly at (225) 342-5812, ext. 122, or e-mail at [email protected]. Reach Janes at [email protected].