Document Type

Theses, Ph.D

Rights

This item is available under a Creative Commons License for non-commercial use only

Publication Details

Thesis submitted to Technological University Dublin, in accordance with the requirements for the degree of DOCTOR OF PHILOSOPHY, January, 2019.

Abstract

Atmospheric cold plasma (ACP) is a novel emerging non-thermal technology that has attracted attention as a decontamination tool in several industrial, food and healthcare sectors. This study investigated the anti-microbial efficacy of ACP against microbiological risks associated with fresh foods. Treatment was performed using in-package ‘dry’ ACP technology and plasma functionalised liquid to decontaminate microorganisms, exploring the responses to real and challenging microbiological risks pertinent to both fresh foods themselves as well as the effluents generated from food processing industry. A range of critical control process parameters were investigated with respect to key pathogenic and spoilage microorganisms commonly implicated in the food environment. The inactivation efficacy of ACP against all applied bacterial strains was depended on applied voltage, treatment time and post treatment storage time (PTST). Greater inactivation was obtained at 80 kV with 24 h of PTST providing greater interaction between the bacteria and the reactive species. Bacterial biofilms were significantly susceptible to ACP. Viable and metabolic active cells in mono and dual biofilms were inactivated within short treatment time. Different inactivation rate was observed, depending on physiological state of the bacteria (planktonic or biofilms, mono or mixed culture). An extended time was required to reduce the challenge mixed culture biofilm formed on lettuce at environmental stress conditions. The study demonstrated that produce storage conditions, such as temperature and storage time had interactive effects on bacterial proliferation, stress response and susceptibility to the ACP treatment, highlighting the importance of preventive measures as key factors for the assurance of microbiological safety of fresh produce. Further, to ascertain the effect of stress conditions on ACP’s bacterial inactivation efficacy, L. monocytogenes and its knockout mutants associated with stress were treated with sub-lethal stress conditions. The gene expression of stress associated genes were significantly increased after 1 min treatment, while long treatment time reduced the gene expression and some cases down-regulated prfA and gadD3 gene expression. By comparing the response of mutants under ACP exposure to key processing parameters, the experimental results presented here provide a baseline for understanding the bacterial genetic response and resistance to plasma stress and offers promising insights for optimizing ACP applications. The impact of the ACP technology on model food surface and wash-water generated from fresh produce processing was also investigated. The ACP treatment reduced microbial load showing similar efficacy as chlorine, providing further advantage of continuously treating the lettuce wash water. Micro-bubbling along with agitation assisted bacterial detachment and distribution of reactive species, thus increasing bacterial inactivation efficacy from fresh produce and wash water. Liquid media complexity was explored as a factor in cold plasma decontamination efficacy for microbiologically safe effluents from food processing. The high nutritive components in the model effluents exerted a protective effect during treatment, showing higher inactivation in phosphate buffer solution (PBS) than in nutrient rich wastewater effluents. ACP was effective to inactivate principle indicator bacteria (mono and mixed culture planktonic bacteria and spores) from model dairy and meat wastewaters. This study also investigated the eco-toxicological impact of cold plasma treatment of the model wastewater using a range of aquatic bioassays. Differing sensitivities were observed to ACP treated effluents across the different test bio-assays; with greater sensitivity retained to plasma treated meat effluent than dairy effluent. The toxic effects were dependent on concentration and treatment time of the ACP treated effluents. ACP shows potential as an efficient decontamination approach against bacteria in their most resistant, biofilm or spore form associated with complex and nutritious food products during food production to wastewaters generated by the food industries.

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