Effect of atmospheric non-thermal plasma against food-borne bacteria on food packaging film surfaces

Abstract

Over the past decade, non-thermal plasma has become established as a potential technology for microbial inactivation. As commonly known, plasma treatment can produce highly specific surface modifications, so it has been extensively used in packaging. Methods such as heating, surface washing with hydrogen peroxide, irradiation, or their combinations, widely used for sterilization in the packaging industry, are known to have negative drawbacks. For instance, using chemicals or preservatives may cause residue problems, or high thermal processes may cause the loss of the desired structure of the food or package. Despite that, sterilization by non thermal plasma has several advantages: A highly energy-efficient system, eco environmental nature, low cost, and versatility. Vegetative and especially spore forming bacteria exhibit strong resistance to external factors such as environmental stress, chemicals, and thermal inactivation due to their intrinsic resistance, outer layers, and low water content. These characteristics make spores more difficult to kill than vegetative forms. In this study, the effect of atmospheric non-thermal plasma application on Gram positive Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 39327 and Clostridium sporogenes ATCC 3584 spores, which are among the important food pathogens, was compared in different packaging materials. Within the scope of the study, microbial inactivation at different exposure times and application types (wet or dry application) was investigated using dielectric barrier discharge (DBD) plasma and corona discharge plasma devices. Low-density polyethylene (LDPE), biaxially oriented polypropylene (bOPP), and polyethylene terephthalate (PET) were used as packaging materials. Each film (3×3 cm2) was initially treated with DBD with an electrode gap set to 3 mm for 0.4 seconds. The plasma power was high, with 100% voltage and minimum frequency set for all treatments. The test microorganisms were inoculated into the center of the 3x3cm2 DBD-treated film surface before corona NTP treatment. The corona NTP power was set at 25-27% voltage, low power, and minimum frequency. The inoculated films were exposed to cold plasma (with the setting of 1.5 cm electrode gap, ~17 kHz wet or dry application) at different application periods depending on microorganism. Exposure periods for S. aureus and P. aeruginosa were 60, 120, and 180s and for C. sporogenes was 0, 360, 540, and 720s. The results showed that non-thermal plasma had an antimicrobial effect for all microorganisms, and wet application, by adding 10 µL of sterile distilled water before exposure to plasma, enhanced the microbial inactivation effect. There was also a direct relationship between exposure time and microbial inhibition. A significant antimicrobial effect was observed only after longer exposures. Considering the results of dry corona plasma application of all microorganisms, the highest D-value belongs to C. sporogenes inoculated on the PET film surface with 94.81±31.56 min. In addition, it was statistically observed that C. sporogenes was the most resistant bacteria to corona treatment for all films (p <0.05). For S. aureus, after dry and wet corona plasma application, all films showed statistical similarity and wet corona application was more effective than dry application (p<0.05). Dry corona application were not performed for P. aeruginosa due to the lack of vitality on the surface after drying. Among vegetative bacteria, the Gram-negative bacterium P. aeruginosa showed higher microbial inactivation than the Gram-positive S. aureus. The most effective result for P. aeruginosa after wet corona plasma application is 1.99±0.03 min and 1.96±0.02 min, with no statistical difference between LDPE and PET films (p>0.05). The results obtained in this study have provided a new perspective on the surface sterilization of packaging materials used in the food industry with cold plasma application of DBD or corona discharge non-thermal plasma systems. In addition, compared to the currently used packaging surface sterilization methods, its disadvantages have been reduced, and an environmentally friendly, affordable system has emerged without the need for complex high/low-pressure systems or gas systems. Thanks to the atmospheric cold plasma devices used in the study, the effective and innovative plasma system design can be easily integrated into the industry without damaging the film surface by using O2 in the atmosphere without needing a pressurized environment or additional gas systems.