FAYETTEVILLE, Ark. -- An edible film consisting of two protein-based substances can prevent growth of Listeria monocytogenes bacteria on ready-to-eat chicken, creating a safer product for consumers, according to a University of Arkansas study. Using this method, food scientists Marlene Janes and Mike Johnson were able to reduce bacterial counts below detectable levels for 24 days. Their research will be published in an upcoming issue of the Journal of Food Science.
"Food production occurs in several stages, each of which provides potential opportunities for bacterial contamination," says Johnson. Chickens grown for commercial food production live in crowded conditions that are ideal for the spread of bacteria. Thorough cooking will kill most dangerous bacteria that evade safety measures in the food production chain. But pre-cooked foods may become re-contaminated between the cooking and final packaging steps.
Ready-to-eat meals, kept in the refrigerator until needed, provide a niche for bacteria that thrive at low temperatures. Listeria bacteria, which can survive refrigeration and can contaminate foods such as deli meat and hot dogs, pose a special risk to children, the elderly, and pregnant women, often causing serious illness and miscarriage.
Along with Janes (now an assistant professor at Louisiana State University), Johnson explored a method that involves coating the food with an edible protein substance called zein, along with nisin, a natural biopreservative protein substance that kills bacteria. Johnson and Janes found that the resulting combination, which is harmless to humans, effectively kills Listeria bacteria that may re-contaminate foods between the cooking and packaging steps.
Johnson and Janes purchased chicken breast tenders from a local supermarket, cut them into 5-gram pieces, froze the pieces, and had them irradiated to eliminate bacteria. The researchers then cooked and cooled the chicken pieces, immersed them in Listeria cultures, and dipped them in solutions containing edible zein films with and without nisin.
The researchers refrigerated their samples and determined bacterial counts after 0, 4, 8, 16 and 24 days. They found that the samples treated with zein and nisin showed significantly reduced bacterial counts compared to non-treated samples. The combination of zein with nisin and calcium propionate was the most effective, resulting in non-detectable levels of Listeria within 24 days when refrigerated at 4 degrees Celsius (40 degrees Fahrenheit).
Johnson, coordinator for research programs at the Center for Food Safety and Quality in the University of Arkansas' Institute for Food Science and Engineering, sees food safety research as one of the primary purposes of a modern land-grant institution: to improve continuously the microbial safety of food production and processing practices from farm to fork.
Americans enjoy one of the safest and most abundant food supplies in the world. Food production is mostly automated and large-scale. This incredible system delivers the ample quantity, staggering variety, and year-round availability that we have come to expect. Problems are rare, but when they do occur, the nature of mass production means that repercussions are widespread or even national in scope. Bacterial contamination anywhere in the production chain can cause serious human disease outbreaks, often scattered over a large geographical area.
The CDC estimates that foodborne disease causes 76 million illnesses, over 300,000 hospitalizations, and 5,000 deaths each year in the United States alone. Although most victims suffer only minor inconvenience, some of these diseases can be quite dramatic and even fatal.
Changing consumer preferences for more fresh and ready-to-eat foods that are distributed refrigerated rather than frozen has coincided with an increase in cases of foodborne listeriosis. Between July 1998 and January 1999, Listeria outbreaks forced four companies to recall millions of pounds of ready-to-eat meat products. Innovative measures are needed to control this pathogen, minimizing the health risks and economic losses that can result from foodborne disease. Johnson and Janes' method should prove useful in reaching this goal.
What are some things consumers can do to protect themselves from the pathogenic bacteria that may inadvertently make their way through the food production process? Above all, raw poultry and raw ground meats should be thoroughly cooked, and utensils that have been in contact with raw meat should not be reused for raw salads, vegetables, or any other foods that are consumed without a cooking step. Refrigerated leftovers should be consumed within one or two days or frozen. Checking labels and storage instructions is a commonsense practice worth turning into a habit: Johnson and his family pay special attention to expiration dates, especially those on deli meats and other pre-cooked foods.
For Johnson and other food scientists, food safety is a matter of minimizing risks as much as possible, risks that will never completely go away. Cooperation and openness among food producers, scientists, and consumers are the best ways to achieve the balance that will help us reduce the risks as much as we can and respond quickly and effectively to any problems that may arise. Pathogenic bacteria, tiny but formidable adversaries, demand eternal vigilance. " Even with our continued best efforts, we likely will be able to keep up with them, but maybe never get ahead of them," Johnson says.
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