Posted: March 19th, 2010 - 5:28pm
Source: Food Microbiology

Abstract
This study examined the growth characteristics of L. monocytogenes as affected by a native microflora in cooked ham at refrigerated and abuse temperatures. A five-strain mixture of L. monocytogenes and a native microflora, consisting of Brochothrix spp., isolated from cooked meat were inoculated alone (monocultured) or co-inoculated (co-cultured) onto cooked ham slices. The growth characteristics, lag phase duration (LPD, h), growth rate (GR, log10 cfu/h), and maximum population density (MPD, log10 cfu/g), of L. monocytogenes and the native microflora in vacuum-packed ham slices stored at 4, 6, 8, 10, and 12 °C for up to 5 weeks were determined. At 4-12 °C, the LPDs of co-cultured L. monocytogenes were not significantly different from those of monocultured L. monocytogenes in ham, indicating the LPDs of L. monocytogenes at 4-12 °C were not influenced by the presence of the native microflora. At 4-8 °C, the GRs of co-cultured L. monocytogenes (0.0114-0.0130 log10 cfu/h) were statistically but marginally lower than those of monocultured L. monocytogenes (0.0132-0.0145 log10 cfu/h), indicating the GRs of L. monocytogenes at 4-8 °C were reduced by the presence of the native microflora. The GRs of L. monocytogenes were reduced by 8-7% with the presence of the native microflora at 4-8 °C, whereas there was less influence of the native microflora on the GRs of L. monocytogenes at 10 and 12 °C. The MPDs of L. monocytogenes at 4-8 °C were also reduced by the presence of the native microflora. Data from this study provide additional information regarding the growth suppression of L. monocytogenes by the native microflora for assessing the survival and growth of L. monocytogenes in ready-to-eat meat products.
Refrigerated ready-to-eat (RTE) meat products contaminated with Listeria monocytogenes have been linked to several listeriosis outbreaks (CDC, 1998; CDC 1999; CDC 2002). L. monocytogenes is frequently isolated from RTE meat products. In 2,300 sliced luncheon meat samples collected from federally-inspected establishments between 1990 and 1999 in the U.S., L. monocytogenes was present in 4.2 to 8.0% of the samples (Levine et al., 2001). L. monocytogenes was present in 0.89% (82/9,199) RTE luncheon meat samples collected in California and Maryland in 2000-2001 (Gombas et al., 2003). In a study examining the prevalence of L. monocytogenes in packages of frankfurters obtained from several commercial manufacturers over a 2-year period, Wallace et al. (2003) reported that 1.6% (532/32,800) of the packages were positive for L. monocytogenes. The possible contamination of L. monocytogenes in RTE meat, beef and chicken products, such as corned beef, pastrami, and frankfurters, is one of the main reasons for Class I type food recalls in the U.S. (FSIS, 2005). A survey conducted in Belgium between 1997 and 1998 showed that 4.9% (167/3,405) of retail cooked meat products were contaminated with L. monocytogenes, and a higher incidence rate was found in cooked meat products after slicing (6.65%) compared to meat before slicing (1.56%) (Uyttendaele et al., 1999).In the U.S., a listeriosis outbreak that occurred in 1998 caused 40 illnesses in 10 states and was linked to the consumption of L. monocytogenes-contaminated frankfurters (CDC, 1998). Another outbreak in 2002 that caused seven deaths, three stillbirths or miscarriages, and a recall of 12.4 million kg of implicated products was linked to the consumption of turkey deli meat (CDC, 2002). Based on a risk assessment report (U.S. FDA/USDA/CDC, 2003), deli meats had the highest estimated per annum risk of illness and death from L. monocytogenes among 20 RTE food categories. While heat processing during the manufacturing of RTE meat products is sufficient to eliminate microorganisms in the meat products, L. monocytogenes and other microorganisms may re-contaminate the products during post-heat processing operations such as slicing, packaging or handling by the manufacturer, or at retail, or by the consumer. L. monocytogenes is a psychrotrophic microorganism; therefore it is able to grow at refrigeration temperature. Since RTE meat products are normally consumed without prior cooking, the ability of L. monocytogenes to grow at refrigeration temperature is a particular concern for refrigerated RTE meat (Amezquita and Brashears, 2002). Studies have been conducted to examine the survival and growth of L. monocytogenes in RTE meat as affected by food additives, product formulations, and storage conditions (Samelis et al., 2001; Stekelenburg and Kant-Muermans, 2001; Glass et al., 2002; Mbandi and Shelef, 2002; Stekelenburg, 2003; Uhart et al., 2004; Samelis et al., 2005; Luchansky et al., 2006,). In addition, mathematical models have been developed to describe the behavior of L. monocytogenes in RTE meat products with intrinsic and extrinsic parameters such as pH/acids, aw/salt, nitrite, polyphosphate, lactate, and diacetate, package atmosphere, and storage temperature (Devlieghere et al., 2001; Seman et al., 2002; Legan et al., 2004; Hwang and Tamplin, 2007). While data on the inactivation, growth, or survival of L. monocytogenes in RTE meat products as affected by food additives, product formulation, and storage conditions are readily available, data on the behavior of L. monocytogenes as affected by the native microflora of RTE meat products are limited. Examining the behavior of L. monocytogenes in RTE meat products with the presence of native microorganisms is important since these products generally contain native microorganisms. The objectives of this study were to examine and describe the growth characteristics of L. monocytogenes as affected by a native microflora in cooked ham at refrigerated and abuse temperatures.
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