Evaluation of sterilization efficiency in cold plasma treatment of packing-sheets

The subject for plasma sterilization was the bacteria Mycobacterium sp. B-5 (Gram-positive) seeded onto the surface of the packing-sheets like paper and polymer. Our standard dose of a liquid suspension with microorganisms deposited onto the aseptic coupons of 10×10 mm² in sizes was equal to 10 μl. We used white writing paper with density of 80 g/m² and 20 μm in thickness, brown craft-paper “A” with density of 52 g/m² and 20 μm in thickness, white filter paper of 20 μm in thickness. As for polymer materials we used white food polystyrene (PS) of 10 μm in thickness, transparence polyethylene (LDPE) of 20 μm in thickness, brown and transparence food polyethylene-terephthalate (PET) of 20 μm in thickness, and transparence food polypropylene (PE) of 250 μm in thickness. Before plasma treatment, the coupons contaminated with the microorganisms were dried over 2-3 hours at room temperature. Non-thermal plasma was generated by plane-to-plane DBD in ambient air at atmospheric pressure. Metallic electrodes were covered by glass of 1 mm in thickness. The length of gas gap between dielectric barriers was varied from 1 to 3 mm. Barrier discharge was activated by sinusoidal voltage with a frequency of 35 kHz and pick-to-pick amplitude varied from 10 to 26 kV. The contaminated coupons were placed in the gap between plane electrodes and exposed to non-thermal plasma over 1-60 s.