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From: Food Quality & Safety magazine, October/November 2005

Pathogenic Survivors

by Richelle L. Beverly and Marlene E. Janes

This study investigated the survival of Listeria monocytogenes at freezer temperatures (-20ºC) on the surface of ready-to-eat (RTE) meat products. An 18-hour culture of the pathogen was decimally diluted and inoculated (5.8 Log CFU/g) onto 45g cubes of RTE roast beef, hot dog, sausage, chicken breast and turkey.

The samples (vacuumed or non-vacuumed packaged) were stored at -20ºC and bacterial counts determined at day zero, seven, 14, 21, 28, 30, 60 and 90 by spread-plating onto modified Oxford media and then incubating plates at 37ºC for 48 hours and determining CFU/g.

At day seven, the results showed no significant difference in Lm counts between the different RTE meats tested. However, by day 28, Lm counts were significantly higher for the vacuum and non-vacuum RTE roast beef samples (6.7 Log CFU/g) from other meat samples (6.3 Log CFU/g). Furthermore, at day 28 all the RTE meat samples, except the vacuum-packaged turkey, had Lm counts greater than 0.5 Log CFU/g compared to day 0. Conversely, by day 90, the Lm counts had reached or were lower than the initial day 0 bacterial levels for all RTE meats. Throughout the study the vacuumed packaged roast beef, chicken and hot dogs Lm counts were significantly higher then the non-vacuumed packaged whereas the non-vacuumed packaged turkey Lm counts were significantly higher then the vacuumed packaged. Our study has shown that the ability of Lm to survive at freezer temperatures on RTE meats was dependent on the meat product and packaging condition.

Listeria monocytogenes, a gram-positive rod, has the capability to grow at refrigerator temperatures. Foods that are contaminated and consumed with Listeria have the ability to cause flu-like symptoms in healthy individuals. The disease, listeriosis, can cause more severe health effects for the elderly and immunocompromised persons. In the case of pregnant women, they can suffer from miscarriages or stillborn births.

Listeria monocytogenes has been associated with food recalls for a number of different products. However, in late 1998 to early 1999, there was a multistate outbreak of listeriosis identified in 22 states. The outbreak was traced to tainted hot dogs and deli meats. Two major recalls involved 30 million pounds each. According to the Centers for Disease Control and Prevention, tainted meat traced to a Michigan plant was responsible for 16 deaths, 6 miscarriages and 100 illnesses. Another outbreak in 10 states in 2000 resulted in 29 cases of listeriosis with 4 deaths and 3 miscarriages /stillbirths (Anonymous, 2000). As a result of reoccurring Listeria monocytogenes outbreaks, the USDA’s Food Safety and Inspection Service (FSIS) addressed the problem in a directive published in the Federal Register in 2001 and amended in 2003. It requires new performance standards for producing ready-to-eat meat products.

According to USDA, the FSIS, in 1999, Listeria monocytogenes was responsible for 31 of the 62 (50 percent) recalls of cooked meat. Various studies have shown that Listeria monocytogenes survives and may increase by 0.5 to 3.0 log units in raw meat during refrigerated storage.

Freezing is a preservation technique that extends a product's shelf-life. This process lowers the temperature to levels in which metabolic processes are stop and the rate of chemical and biochemical reactions are reduced. During the freezing process, intracellular and extracellular ice crystals form which causes death and injury to microbial cells. Freezing has been observed to have lethal and sublethal effects onListeria monocytogenes based on the speed of freezing, temperature of frozen storage, cycles of freezing and thawing and the presence of nutrients in the freezing media. Freezing increases the sensitivity of Listeria monocytogenes to enzymes, lysozymes and lipases, which occur naturally in some foods.

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.


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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 Reach Janes at



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