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Dielectric barrier discharge-atmospheric cold plasma (DBD-ACP) is a promising non-thermal technology effective against a wide range of microorganisms. These studies were performed using a custom built DBD-ACP system. The inactivation efficacy was found to be governed by a series of critical control parameters, including treatment time, mode of exposure (Direct/Indirect exposure), applied voltage, applied gas content but was also very dependent on the characteristics of treatment targets. In this study, these parameters were investigated using in-package design along with a post-treatment storage procedure to align with industrial processing times as well as mitigation of post-process contamination. A range of food borne pathogens were used as targets to optimize system and process parameters and further reveal the inactivation mechanisms.
The inactivation efficacy of ACP against all applied strains was dependent on treatment time, applied voltage and oxygen percentage. Reactive species, especially reactive oxygen species (ROS) are the main microbicidal agents of plasma. By increasing the value of the above processing parameters, the detected amount of ROS increased and enhanced the bactericidal effect. Due to the in-package design of this study, post-treatment storage time investigated (0, 1 and 24 h) also had positive effect on inactivation level, which provided a retention time for reaction and effect with longer live or recombined reactive species. Additionally, the mode of exposure resulted in different inactivation levels, where the chemistry associated with the mode of exposure to the discharge was altered with or without samples in discharging area.
Inactivation mechanisms were different for the Gram negative and positive bacteria studied. The obvious loss of cell membrane integrity was shown for Gram negative bacteria, while the damage on Gram positive bacteria was more significant on intracellular components (DNA in this study). This effect was indicated as a result of their different cell envelope structures, which dominated ROS action/inactivation patterns. Using five knockout mutants and parent strain of E. coli as target cells, cellular responses were observed as general regulation, short-term and long-term actions with regard to ROS scavenging and cellular repair.
Furthermore, the optimized ACP parameters were applied to meat product models to ascertain the impact of the technology to model food surface contamination and to challenge the processing parameters previously determined as critical in the preceding studies. Increasing the microbial challenge to ACP from liquid suspensions through to colonised surfaces did require greater treatment duration at a higher voltage level. Comprehensive antimicrobial actions of ACP generated reactive species were based on the food matrix complexity and the structure of the target cells exposed. Subsequently, the additional protection effect of nutritive composition against reactive species was observed. Nonetheless, the results indicate that ACP can be employed to address a range of microbiological safety challenges pertinent to the meat industry, particularly where the processing advantage of in package treatment is available. However, careful consideration of additional hurdles is required along the product chain.
Lu Han (2016). Microbiological control and mechanisms of action of high voltage atmospheric cold plasma. Doctoral Thesis. Technological University Dublin, Ireland. doi:10.21427/D7988S