Inactivation of <i>Salmonella</i> Typhimurium and <i>Listeria monocytogenes</i> on ham with nonthermal atmospheric pressure plasma

<div><p>The application of cold atmospheric pressure plasma (CAP) for decontamination of sliced ready-to-eat (RTE) meat products (in this case, rolled fillets of ham), inoculated with <i>Salmonella</i> (<i>S</i>.) Typhimurium and <i>Listeria (L</i>.<i>) monocytogenes</i> was investigated. Cold atmospheric plasma (CAP) is an ionised gas that includes highly reactive species and ozone, interacting with cell membranes and DNA of bacteria. The mode of action of CAPs includes penetration and disruption of the outer cell membrane or intracellular destruction of DNA located in the cytoplasm. Inoculated ham was treated for 10 and 20 min with CAP generated by a surface-micro-discharge-plasma source using cost-effective ambient air as working gas with different humidity levels of 45–50 and 90%. The chosen plasma modes had a peak-to-peak voltage of 6.4 or 10 kV and a frequency of 2 and 10 kHz. Under the tested conditions, the direct effectiveness of CAP on microbial inactivation was limited. Although all treated samples showed significant reductions in the microbial load subsequent to plasma treatment, the maximum inactivation of <i>S</i>. Typhimurium was 1.14 lg steps after 20 min of CAP-treatment (<i>p</i><0.05), and <i>L</i>. <i>monocytogenes</i> was reduced by 1.02 lg steps (<i>p</i><0.05) using high peak-to-peak voltage of 10 kV and a frequency of 2 kHz regardless of moisture content. However, effective inactivation was achieved by a combination of CAP-treatment and cold storage at 8°C ± 0.5°C for 7 and 14 days after packaging under sealed high nitrogen gas flush (70% N₂, 30% CO₂). Synergistic effects of CAP and cold storage for 14 days led to a clearer decrease in the microbial load of 1.84 lg steps for <i>S</i>. Typhimurium (<i>p</i><0.05) and 2.55 lg steps for <i>L</i>. <i>monocytogenes</i> (<i>p</i><0.05). In the case of <i>L</i>. <i>monocytogenes</i>, subsequent to CAP-treatment (10 kV, 2 kHz) and cold storage, microbial counts were predominantly below the detection limit. Measurement showed that after CAP-treatment, surface temperature of ham did not exceed the room temperature of 22°C ± 2°C. With the application of humidity levels of 45–50%, the colour distance ΔE increased in CAP treated samples due to a decrease in L* values. In conclusion, effectiveness of CAP-treatment was limited. However, the combination of CAP-treatment and cold storage of samples under modified-atmospheric-conditions up to 14 days could significantly reduce microorganisms on RTE ham. Further investigations are required to improve effectiveness of CAP-treatment.</p>