Dec. 18, 2002

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COLLEGE STATION – A new product called acidified calcium sulfate is showing promise as a way to kill Listeria monocytogenes and keep lunch meats and frankfurters safer for consumers.

"Our goal was to look at different treatments that might be used to decontaminate the surface of cooked products to ensure that Listeria was killed and it had very little opportunity to grow after that," said Dr. Jimmy Keeton, professor with the department of animal science at Texas A&M University. "Listeria grows at refrigerator temperatures."

Foodborne listeriosis is most commonly associated with ready-to-eat products such as frankfurters and hot dogs, lunch meat, smoked fish and certain types of soft cheeses, Keeton said.

L. monocytogenes is considered a serious threat because -- even though it doesn't affect that much of the population -- when it does strike, it can be deadly. In humans, listeriosis can cause flu-like symptoms, meningitis, spontaneous abortions and prenatal septicemia, said Keeton. About 20 percent of listeriosis cases are fatal.

"There's a real concern about from the time ready-to-eat products are cooked until the time they are packaged that they not become contaminated with pathogens, specifically L. monocytogenes," he said.

When these products are cooked, they are pasteurized and the Listeria is killed. "Assuming the product is cooked adequately, the risk of contamination comes from the surface," he said.

If the product is contaminated after cooking, there is a risk in eating that product without proper reheating. Some luncheon meats, such as bologna, are routinely not cooked before eating.

Research had already shown that adding substances such as lactic acid and sodium lactate created microbiological "hurdles" to organisms such as Listeria, Keeton said. But still, these were not considered entirely effective against the regrowth of the organism.

However, acidified calcium sulfate – an organic acid, calcium sulfate combination – is showing potential as a product that not only kills the Listeria on the surface of products, but also keeps it from coming back. Even though it is acidic, Keeton said, it is safe enough to hold in the hand and has Generally Recognized As Safe status from the Food and Drug Administration.

The Texas A&M researchers inoculated frankfurters manufactured under commercial processing with a four-strain L. monocytogenes "cocktail," which contained 10 million microorganisms per gram.

"You wouldn't expect to find levels that high. It's a worst-case scenario, so if you're going to get protection, you should get it at this point," Keeton said.

Each group was then treated with either with a saline solution (the control group); with acidified calcium sulfate; potassium lactate; or lactic acid.

The frankfurters were then vacuum-packaged much like they would have been processed commercially, stored under refrigeration 40 F for 12 weeks, and evaluated at 2-week intervals.

What researchers found was the acidified calcium sulfate killed the Listeria on the surface and also had a residual effect on the surface. "The organism didn't come back," Keeton said.

The lactic acid initially reduced the number of organisms, but it didn't kill all of them. Also, the Listeria started growing on the frankfurter again during refrigerated storage.

The potassium lactate was not effective at all, he said.

Researchers also tested the "sensory" and physical properties and found the acidified calcium sulfate changed the product very little. It had the same taste, even though the pH was reduced. It also slightly increased the calcium content of the frankfurters, Keeton said. Researchers also noticed a slight amount of moisture released in the packaging.

"We attributed this to the fact the material was acidic, and it remained acidic throughout the storage period," he said.

Listeria can be introduced from the environment or from personnel in a meat-processing plant. Some of the outbreaks often have involved factors such as remodeling in a plant that made the organism air-borne. Keeton noted meat-processing plants are constantly keeping sanitation programs in place to prevent cross-contamination.

The acidified calcium sulfate could give meat processors another method of intervention to increase the safety of their products, Keeton said, and several already want to test acidified calcium sulfate on their own products to see how effective it is.

"Companies don't want to produce an unsafe product, because their reputation is on the line," he said, "so if they can find intervention procedures through research and new technologies, it's very important."

A summary of the research also has been sent to the U.S. Department of Agriculture Food Safety Inspection Service as a validation study to gain support for the use of this material. USDA is presently considering policies on how to control L. monocytogenes in processing plants.

In November, the USDA released an administrative directive outlining additional steps to be taken by its inspectors to ensure that establishments producing ready-to-eat meat and poultry products are taking the necessary steps to prevent contamination with Listeria.

Under the directive, plants producing high and medium risk ready-to-eat products such as deli meats and hot dogs that do not have an evaluated environmental testing regime designed to find and take necessary actions to eliminate Listeria, will be placed under an intensified testing program by USDA's Food Safety and Inspection Service. The program will consist of increased testing of the final product, and testing of food contact surfaces and plant environment.

The Texas A&M study was funded by the American Meat Institute Foundation, whose members are from the meat industry.