Microbial decontamination of food packaging films using corona non-thermal plasma

Abstract

Foods are packaged during various processes, especially transportation and storage, during the "from farm to fork" period. Food packaging has four primary purposes: protection, convenience, containment, and communication. Apart from these, packaging has many functions such as extending the shelf life of food and reducing food waste, contributing to sustainability, ensuring the traceability of food, and protecting consumer health. Although foods can be packaged with many packaging materials such as glass, metal and paper, the most widely used material in food packaging today is polymers, in other words, plastics. Polymers are preferred due to their advantages of being easily shaped, have low cost, are light, transparent, and can be easily sealed with temperature. Although packaging materials are barriers that protect food from external factors, when adequate hygienic conditions are not provided, physical, chemical or biological cross-contamination from packaging material to food is possible. Although it varies according to the physicochemical properties of packaging materials and environmental conditions, it has been proven that various microorganisms, including pathogens, can be present on packaging surfaces. In fact, it has been reported that up to 6 log CFU/cm2 total mesophilic aerobic bacteria can be found on the packaging material. Since plastic packaging materials do not have antimicrobial properties, sterilization before contact with food is an obligation to protect consumer health. Today, various physical and chemical methods are applied for the disinfection or sterilization of packaging materials. Major examples of these methods include thermal treatments, irradiation, UV-C, pulsed light, ozone, hydrogen peroxide or the use of other disinfectants. Despite the disadvantages of existing technologies, non-thermal, or cold plasma stands out with its advantages. Plasma, known as the fourth state of matter, is a gas with reactive oxygen and nitrogen species (ROS: O, O2, O3, OH; RNS: NO, NO2 and NOx), UV, free radicals and charged particles. Plasma is obtained by ionizing the gas as a result of applying electrical energy to the gas between two electrodes with a high electrical potential difference. Plasma systems are classified according to the electrode used. Corona discharge is a type of discharge that causes a strong glow extending from the sharp point of the emitter electrode to the collector electrode. Corona discharge cold plasma systems can operate with lower power requirement, at atmospheric pressure, with ambient air, without causing an increase in temperature on the surface. Although cold plasma can also be used to improve the surface properties of packaging films, it is also widely used for microbial decontamination of packaging and surfaces. Although there are facilities that perform sterilization of packaging materials with cold plasma today, it is necessary to increase the application efficiency and reduce the cost in order for the technology to become widespread. In this study, the decontamination efficiency of the custom-made corona discharge non-thermal plasma (NTP) system on commonly used polymeric food packaging materials was investigated. In order to test the inactivation efficiency of the NTP system in a wide range of microorganisms, a test group of microorganisms was formed representing vegetative cells, spore-former bacteria and fungi. Unlike the cold plasma sterilization studies in the literature, how the NTP inactivation efficiency changed as a result of wetting the film surface was investigated. The main aim of wetting the film surface before plasma treatment is to convert H2O into ROS by the effect of plasma, and to obtain higher inactivation rates as a result of exposure of microorganisms to higher concentrations of ROS. In this context, the inactivation efficiency of corona discharge NTP system against Escherichia coli and Bacillus subtilis vegetative cells, B. subtilis spores, Aspergillus niger and Penicillium expansum spores was investigated on three different packaging materials, biaxially oriented polypropylene (bOPP), low density polyethylene (LDPE) and polyethylene terephthalate (PET). Microorganisms were inoculated on the surface of the packaging films by spot inoculation method and the inoculated films were dried to simulate a real contamination. In order to investigate the effect of wet application on the inactivation efficiency, the trials were repeated under the same application conditions, by additionally covering the film surface with a thin layer of water. Bacterial analyses were carried out by pour plate method, fungal analyzes were carried out by spreading plate method. Before the analysis of the inactivation efficiency of cold plasma, it was tested whether the selected microorganisms in the packaging materials remained viable. As a result of all analyzes, it was found that wetting the film surface and increasing the cold plasma exposure time statistically increased the microbial inactivation efficiency of the NTP system (p<0.05). Since inactivation on the surface of each packaging film was analyzed in experiments on different days, the film material was not considered as a variable, and the results were evaluated within themselves. Considering the maximum decontamination amounts determined for each specified application parameter, in dry applications, the detected inactivation rate by NTP system was 2.98 log CFU/film for E. coli in 3 min, 0.93 log CFU/film for vegetative cells of B. subtilis in 3 min, 0.33 log CFU/film for B. subtilis spores in 12 min, 1.34 log CFU/film for A. niger in 12 min and 0.67 log CFU/film for P. expansum in 3 min. Likewise, as a result of covering the surfaces of the contaminated packaging materials with a thin layer of water before plasma application, the inactivation rates were increased to 5.58 log CFU/film for E. coli in 3 min, 1.5 log CFU/film for vegetative cells of B. subtilis in 3 min, 1.05 log CFU/film for B. subtilis spores in 12 min, 1.46 log CFU/film for A. niger in 3 min and 3.20 log CFU/film for P. expansum in 3 min. Considering all the results together, it was observed that wetting the film surface before the cold plasma treatment increased the microbial inactivation efficiency of the corona discharge NTP system statistically in almost all conditions (p<0.05). It has been also noted that the treatment to the wet surface increases the effect up to 3 or even 4 times in some microorganisms compared to the plasma treatment on the dry surface. In this way, it has been seen that it is possible to reach the desired microbial inactivation times in shorter application times with the effect of wetting the surface. It was found out that E. coli was the most sensitive microorganism to the NTP system in both wet and dry applications, while the most resistant microorganism was B. subtilis spores. Considering the average values for all microorganisms, the resistance of microorganisms against the NTP system during dry application can be listed from lowest to highest as E. coli, vegetative B. subtilis cells, P. expansum, A. niger and B. subtilis spores, respectively. On the other hand, in the applications where the film surface was wetted, this order changed as E. coli, P. expansum, A. niger, vegetative B. subtilis cells and B. subtilis spores, respectively. This situation can be interpreted that the mechanism of inactivation of plasma is caused by two main effects; inactivation by ROS formation was more effective in microorganisms other than B. subtilis, while decontamination by UV radiation is more effective in B. subtilis cells. With the findings obtained as a result of the study, it can be said that the NTP system designed and produced within the scope of the project is effective on various microorganisms such as vegetative cells, spores and fungal spores. In addition, the fact that wetting the film surface before plasma application statistically increases the microbial decontamination efficiency of the NTP system under almost all conditions. The designed NTP system works with ambient air at atmospheric pressure. This will reduce the installation and operation cost in the facilities. When all the results are examined, the designed system is promising in terms of savings by increasing the desired effect in less energy and shorter time in polymeric packaging material sterilization.