Competition Of L. Monocytogenes With Normal Flora - Brief Article

Nutrition Research Newsletter, Feb, 2001

It is a concern that irradiation of food may affect safety by decreasing the competition of indigenous microflora with foodborne pathogens, allowing the unrestricted multiplication of the pathogen without an associated indication of spoilage. Since irradiation is a frequently used method in food safety, it is important to determine whether there are any positive or negative effects associated with this technique.

Two researchers from the U.S. Department of Agriculture set out to test the hypothesis that irradiation pasteurization ([is less than or equal to] 2.5 kGy) will not significantly influence the ability of L. monocytogenes to compete with the residual indigenous microbial flora associated with ground turkey meat during storage. The researchers also investigated whether an atmosphere enriched with [CO.sub.2] alters the survival and/or multiplication of L. monocytogenes during storage.

Samples of ground turkey meat were irradiated to 0, 1.5, and 2.5 kGy and then were inoculated with 100 CFU/g of a mixture of L. monocytogenes isolates. The samples were then packaged in air-permeable pouches or under atmospheres containing 30 or 53% [CO.sub.2], 19% [O.sub.2] and 51 or 24% [N.sub.2] and stored at a mild abuse temperature of 7 [degrees] C for up to 28 days.

The initial population of L. monocytogenes was nearly identical in all three studies. Irradiating ground turkey did not decrease its safety when it was contaminated following processing with L. monocytogenes. The populations of lactic acid bacteria and anaerobic of facultative bacteria were reduced by irradiation. Following a dose of gamma irradiation of 2.5, the TPCs were [10.sup.6.42] and [10.sup.4.98] under 25% and 50% [CO.sub.2] atmospheres, respectively. The presence of either 30 or 53% [CO.sub.2] inhibited the multiplication of coliform bacteria and extended the lag phase.

The results show that only one of the initial hypotheses could be accepted. The hypothesis that an enriched atmosphere does not alter the survival and/or multiplication of L. monocytogenes is rejected. However, it does appear safe to use irradiation as a method of reducing foodborne pathogens, without altering the natural microflora of the ground turkey.

Donald W. Thayer and Glenn Boyd, Reduction of Normal Flora by Irradiation and Its Effect on the Ability of Listeria monocytogenes to Multiply on Ground Turkey Stored at 7 [degrees] C When Packaged Under a Modified Atmosphere, Journal of Food Protection 63(12): 1702-1806 (December 2000) [Correspondence to: Donald W. Thayer. Phone: 215-233-6582. Fax: 215-233-6406. E-mail: dthayer@arserrc.gov].