Invisible antimicrobial wrapper protects poultry

The food processing industry is always looking for new ways to protect its products from dangerous bacteria. Researchers at the University of Arkansas have found some protective coatings that are good enough to eat themselves.

The coatings are edible films that can be used on refrigerated and pre-cooked ready-to-eat chicken products to protect against pathogenic Listeria monocytogenes bacteria. One such film is zein, a commercially available coating used to coat candies, that Arkansas researchers found to be effective against the bacteria on ready-to-eat chicken when they added bacteriocins. Bacteriocins are small digestable proteins produced by safe bacteria, which grow and survive in part by producing these bacteriocins which can inhibit the growth of other harmful bacteria.

Nisin is a bacteriocin protein that has inherent antimicrobial qualities. Arkansas researchers added nisin to the zein edible film to enhance its effectiveness. Nisin has already been used to coat candy, dried fruits and nuts.

"You have chicken with a transparent colorless film, but the film is carrying a bacteriocin," said Michael Johnson, a Food Safety Consortium principal investigator and food science professor, in explaining the use of zein. "The model would be for preventing post-processing recontamination of cooked foods."

Johnson used a chicken breast with Listeria monocytogenes on it as an example. Processors put the zein film containing nisin on the breast. "The nisin molecule is within the film just like a plastic packaging that allows it to come in contact with the chicken surface. It kills the Listeria."

Marlene Janes, an Arkansas Food Safety Consortium researcher, said the tests she conducted showed that chicken cubes inoculated with Listeria experienced significant reductions of the bacteria upon the application of nisin, with no detectable levels by the end of a 28-day trial. The zein film prevented the Listeria from growing back.

By putting the food additive calcium propionate with the nisin, Janes found the levels of bacteria were reduced to non-detectable levels sooner. "I found the calcium propionate enhanced the activities of nisin," she said.

Johnson compares the effort to hurdles on a track. "Some people are good runners but they can't clear the hurdles," he said. "This is the same concept with bacteria. We're trying to set up a preservative system. Many bacteria that are pathogens don't grow well in refrigerators. But Listeria does. Refrigeration in itself is not an adequate hurdle. So what else can you put in there as a hurdle?" The answer is nisin.

Zein film can prevent recontamination of ready-to-eat meat between the in-plant cooking and the packaging, a potentially vulnerable point in processing. Chicken patties, for example, are cooked and then frozen or packaged. But before freezing, Janes explained, the patties are dipped in the film solution. "They have an antimicrobial wrapper on them, so to speak," Johnson said.

The importance of such protection is underscored by the federal government's regulatory stance on Listeria monocytogenes: zero tolerance. If any of this pathogen are found on a 25-to-50 gram sample of a cooked meat or poultry item for sale, the product has to be recalled.

"That's why there's so much attention to this organism," Johnson said. "It kills more people, although Salmonella and Campylobacter cause more cases of foodborne illness. Once it infects someone who is susceptible, there's a greater chance of death."

Industry has shown interest in the process being investigated by the Arkansas lab. Janes said one company has expressed interest in the process for application to cubed chicken. But additional regulatory approval is necessary before the process can be used. Zein film and calcium propionate may be applied to ready-to-eat meats, but the addition of nisin would have to be specifically approved by the government. Nisin is currently approved in the U.S. by the Food and Drug Administration only for use on soft cheese to control Clostridium botulism.

This article is reprinted with permission from The Food Safety Consortium Newsletter, Vol. 10, No. 3, Summer 2000.

For more information contact The Food Safety Consortium, 110 Agriculture Bldg., University of Arkansas, Fayetteville, AR 72701. Tel: 501-575-5647.