US researchers have designed an edible coating for refrigerated meats that reduces the risk of contracting food poisoning. In tests the protein-based substance reduced the presence of listeria to undetectable levels in refrigerated samples of pre-cooked, chilled chicken for up to 24 days.
Listeria is present in water, silage and soil, and is passed to humans through the consumption of animal products. It is found in ready-to-eat foods such as pate and can survive vacuum packing and fridge temperatures.
In an outbreak in France in 1993, 63 people died after eating infected jellied pork tongues. Although the bacteria is killed by thorough cooking, pre-cooked food may be recontaminated between the cooking and packaging stages.
The protective coating is made from a combination of zein, a protein obtained from corn, and nisin, a natural biopreservative protein that can kill bacteria, along with additional calcium. Zein is insoluble in water, but can be brought into solution using propylene glycol or glycerol plus ethanol, then mixed with the nisin. Foods are then dipped into the solution.
The film works because it has both a killing and protective action, said Michael Johnson, professor of Food Microbiology at Bumpers College of Agricultural, Food and Life Sciences.
‘Nisin will kill less complex (Gram-positive) bacteria such as Listeria monocytogenes but not so-called Gram-negative bacteria such as E coli. We are now trying some other strategies to control another pathogen, Campylobacter, on raw poultry and meats.’
To test the coating’s effectiveness the research team irradiated chicken breast segments to destroy all bacteria before cooking and cooling them. They were then immersed in listeria cultures and dipped in zein solutions, with and without nisin.
A combination of nisin, zein and calcium proved most effective, and listeria levels treated with this remained below detectable levels for up to 24 days at a fridge temperature of 4°Centigrade.
But the coating’s effectiveness declines when temperatures move closer to the pathogen’s optimum growth temperature of 30°C, so it cannot control bacteria on food left outside a fridge.