Effect of PLA/ZnO Packaging and Gamma Radiation on the Content of Listeria innocua, Escherichia coli and Salmonella enterica on Ham during Storage at 4 °C

Abstract

The effect of ZnO and low γ-radiation on PLA based-films was investigated to be used for food packaging application. Ham slices were inoculated with E. coli, L. innocua and S. enterica and then covered with PLA and PLA/5% ZnO films. The samples were irradiated with a γ-radiation dose of 0.3 kGy at dose rate of 13.5 kGy/h. Microbiological analysis was performed at 0, 1 and 5 days on samples stored at 4 °C. Results showed that no onsistent reduction of bacteria was obtained, even at the fifth day of storage, when the ham was covered with PLA/5% ZnO film and no γ-radiation was performed. The use of γ-radiation results necessary to reduce the bacteria growth. In fact E. coli and S. enterica were not detected after 5 days of storage; whereas in the case of test with L. innocua a reduction of 1.3 log CFU/g was observed after 5 days of storage. The antibacterial results indicate that the presence of ZnOin PLA film is effective only for E. coli. The differences of the results obtained here with those reported in literature (where ZnO particles are reported to be very effective as antimicrobial material) are accounted for the different methodologies used. In conclusion considering the positive results, even if small, obtained here at least only for the E. Coli and considering that PLA/5% ZnO film shows, compared to plain PLA film, good tensile properties (especially Young’s modulus and stress at yielding) and good permeability (to O2 and CO2) induce to consider the PLA/5% ZnO composite film usable for food packaging when long shelf life and food safety are required, considering also that it is biodegradable and compostable.