An extruded composite food packaging film containing pectin, polylactic acids (PLAs) and nisin can inhibit Listeria monocytogenes, according to scientists based at the Agricultural Research Service (ARS) of the US Department of Agriculture.
According to the ARS team, bacteria like Listeria are a concern in food with extended shelf life because they tolerate salt, pH changes, inadequate thermal pasteurisation and refrigerated temperatures.
The team explained that they selected PLA (purchased from Cargill Dow) as the packaging material in their study due to the fact that it can be derived from renewable resources, coupled with the growing interest in the future replacement of fossil fuel-derived polymers with alternatives.
Pectin, said the ARS team, is a water soluble hygroscopic polymer, and has been used as a thickening, coating and encapsulating material; the scientists claim that relatively few studies have reported on the use of pectin, alone or in combination with PLA as a base packaging material and as a carrier for nisin for antimicrobial food packaging.
The pectin used here was purchased from Danisco, as was the nisin used in the project.
Nisin was employed, said the researchers, as it is increasingly used in a variety of foods including dairy, eggs, vegetables, meat, fish, beverages and cereal-based products to inhibit growth of foodborne pathogens including L. monocytogenes; it is non-toxic, heat stable and does not contribute to off-flavours, said the team.
The scientists added that they chose brain heart infusion (BHI), orange juice and liquid egg white as the foods for the study as they serve as representatives for neutral, high acid and low acid foods respectively.
While no outbreaks involving L. monocytogenes in fruit juices have been reported, continued the ARS team, the fact that this pathogen may survive well beyond the normal shelf-life of nonsterile acidic fruit juices suggests that these products are potential vehicles of infection.
The film, found the researchers, was effective in reducing L. monocytogenes by 2.1, 4.5 and 3.7 log units mL-1 in the BHI, as well as in the orange juice and liquid egg after 48 hours at 24°C.
The ARS scientists concluded that this composite film has great potential to reduce post-process growth of food pathogens, and they added that they will further explore the use of nisin containing films as packaging materials with antimicrobial activity against L. monocytogenes for solid foods such as meat products.
Source: International Journal of Food Science and Technology 2009, 44 322-329. Antimicrobial activity of nisin in films against L. monocytogenes. Authors: T. Jin et al.