Use edible zein films to control recontamination.

The consumption of refrigerated, ready-to-eat foods can cause illness if the prepared food is undercooked or becomes cross-contaminated with L. monocytogenes. Scientists at the University of Arkansas have attempted to determine the inhibitory activities of nisin that was directly added to zein edible films and coated onto the surfaces of refrigerated, ready-to-eat chicken. The goal here was to protect against the growth of the bacteria after the product is processed.

Cooked 5-g chicken samples were immersed in broth cultures of L. monocytogenes strain V7 for 30 sec and then allowed to drip free of excess inoculum. Investigators then dipped the samples into an edible zein film dissolved in either propylene glycol (ZP) or ethanol (ZE), with and without nisin or 1% calcium propionate (CP).

The scientists placed the samples in sterile bags and stored them at 4 C or 8 C for up to 24 days. Bacterial counts were examined at 0, 4, 8, 16 and 24 days by spread-plating the samples on an agar medium and incubating them at 37 C for 24 h. With an initial inoculation of 6.5 log CFU/g, the L. monocytogenes grew to 8 log CFU/g by 4 days at 8 C, or within 8 days at 4 C. Under these conditions, the bacteria's growth declined using films of zein-ethanol-nisin, zein-propylene glycol-nisin-calcium proponate or zein-ethanol-nisin-calcium proponate by 16 days at 4 C.

Investigators found that a zein-propylene glycol-nisin-calcium proponate film was the most effective treatment. It reduced levels of L. monocytogenes by 5.0 log CFU/g by 24 days. The antimicrobial effect of nisin was lower at 8 C, causing a 3 log CFU/g reduction for zein-propylene glycol-nisin film by 16 days. A challenge level of 2.7 log CFU/g of Listeria was reduced to nondetectable levels using some of the zein-nisin films by day 24 at 4 C.

The results indicate that zein film with or without nisin and calcium propionate coated on refrigerated, ready-to-eat meat products can provide the additional safeguards needed to prevent possible growth of L. monocytogenes. The purpose would be to stop any recontamination of meat or poultry products between the cooking and packaging stages.

Further information. Michael Johnson, University of Arkansas, Department of Food Science, 272 Young Ave., Fayetteville, AR 72704; phone: [phone]; fax: [phone]; URL: www.uark.edu.