Marlene E. Janes and Richelle L. Beverly

Listeria monocytogenes is a bacterium that can cause illness when consumers eat refrigerated, ready-to-eat foods contaminated with this micro-organism. Eating foods contaminated with L. monocytogenes normally causes flu-like symptoms in healthy adults. This disease is more serious for elderly adults and adults with compromised immune systems and can cause menin-gitis. In pregnant women, the disease may cause spontaneous abortions or stillborn babies.

From July 1998 to January 1999, four U.S. companies recalled ready-to-eat meats after L. monocytogenes was found in their products. Two major recalls involved 30 million pounds each. According to the Centers for Disease Control, tainted meat traced to a Michigan plant was responsible for 16 deaths, six miscarriages and 100 illnesses. As a result of this outbreak, the U.S. Department of Agriculture’s Food Safety and Inspection Service (FSIS) addressed the problem of L. monocytogenes contamination in a directive published in the Federal Register in 2001. It requires new per-formance standards for producing ready-to-eat meat products.

By September 2003, FSIS introduced a final rule, which requires the ready-to-eat food industry to impose a post-lethality treatment and growth inhibitor for Listeria monocytogenes or employ sanitation measures for control-ling L. monocytogenes in the processing environment. To help Louisiana meat processors meet this new requirement, LSU AgCenter researchers have been investigating acidified sodium chlorite as a possible means for controlling Listeria monocytogenes on ready-to-eat meat products.

One of the main concerns of the food industry is that L. monocytogenes can grow at refrigerator temperatures. Contamination of ready-to-eat products with L. monocytogenes after cooking is a serious threat that many food processors are trying to combat. Listeria monocytogenes has been found in the environment of ready-to-eat processing plants and could be a source for contamination of these products.

From February through October 2004, 13 U.S. companies had to recall 460,317 pounds of ready-to-eat meat products as a result of routine FSIS microbiological testing that found L. monocytogenes in their products. These recalls clearly indicate the need for new control measures to help meat proces-sors in the continuing battle against L. monocytogenes contamination, and we are working with a Louisiana-based company to develop new ways to combat this pathogen in their products.

Our research has evaluated various concentrations of acidified sodium chlorite (sodium chlorite treated with citric acid) to find out how much is needed to effectively reduce Listeria monocytogenes counts on samples of ready-to-eat roast beef at refrigerator temperatures. Acidified sodium chlorite is available commercially and has been approved for use as a wash for controlling bacteria on the surface of chicken. FSIS has also approved acidified sodium chlorite for use on ready-to-eat meat products, but no research is avail-able proving how effective it is against L. monocytogenes. The FSIS rule states that ready-to-eat food companies have to show their antimicrobial treatments are effective against L. monocytogenes. Because Listeria is present in the environment, federal regulations require the effectiveness of a treatment be demonstrated by reducing the size of a bacteria colony on cooked roast beef within given parameters.

In laboratory tests, we inoculated cooked roast beef samples with L. monocytogenes, then treated them with acidified sodium chlorite. The purpose was to demonstrate the effectiveness of acidified sodium chlorite in controlling L. monocytogenes. At day 1, we found that compared with nontreated control samples, all the treated samples showed significantly decreased bacterial counts because of the rapid destructive action of acidified sodium chlorite. The study also showed acidified sodium chlorite was effective in inhibiting the growth of L. monocytogenes on the surface of cooked roast beef stored at refrigerator temperatures.

On day 7, the initial innoculation of 1.17 billion L. monocytogenes bacteria per pound was reduced to 146,512 per pound with 1000 ppm acidified sodium chlorite, 3.9 million per pound with 750 ppm or 500 ppm acidified sodium chlorite and 7.7 million per pound with 250 ppm acidified sodium chlorite on surface of the roast beef. These reduc-tions in bacterial counts remained constant until day 28 when the 500, 750 and 1000 ppm acidified sodium chlorite treated roast beef samples had reduced L. monocytogenes counts to about 143,337 per pound compared with the controls. Furthermore, acidified sodium chlorite had no effect on the color or taste of the cooked roast beef products. These reductions meet the federal regulations for showing the effectiveness of the treatment in controlling Listeria.

The recurring recalls of ready-to-eat meat products due to contamination by Listeria monocytogenes demonstrate a clear need to develop additional methods to prevent economic loss and possible deaths from foodborne diseases. Our research has shown that acidified sodium chlorite is effective in preventing the growth of L. monocyto-genes on the surface of ready-to-eat meat products stored at refrigerated temperatures. Based on these results, a Louisiana-based ready-to-eat meat processing company has already included the use of acidified sodium chlorite as a method to combat L. monocytogenes on their ready-to-eat meat products.

Marlene E. Janes, assistant professor, and Richelle L. Beverly, postdoctoral researcher, Department of Food Science, LSU AgCenter, Baton Rouge, La.

(This article appeared in the winter 2005 issue of Louisiana Agriculture.)