Atmospheric Cold Plasma as a Tool for Microbiological Control

Outbreaks of foodborne human illnesses resulting from contaminated raw or minimally processed fruits and vegetables have been widely reported globally. The microbiological challenges associated with fresh produce are diverse and respond differently to minimal processing technologies. Atmospheric cold plasma is a relatively new technology and represents a potential alternative to traditional methods for decontamination of foods. The objective of this work was to determine the influence of extrinsic atmospheric cold plasma (ACP) treatment control parameters and to optimize treatment parameters for decontamination with respect to different forms of key safety challenges pertinent to fresh produce. The optimisation studies demonstrated that inactivation efficacy of treatment, when tested against high populations of E. coli suspended in liquid media, was governed by the processing parameter of mode of exposure, treatment time, post treatment storage time, voltage levels, working gas and media composition. Post treatment storage time emerged as a critical treatment parameter for consistency and efficiency of bacterial inactivation with the system. The effect of media complexity was evident with higher inactivation rates achieved in media with simpler composition. Antimicrobial efficacy of ACP increased when voltage level and gas mixture with higher oxygen content was utilised, nullifying the effect of mode of ACP exposure and media composition. High voltage in-package indirect ACP treatment with 24 h of post treatment storage time, selected as the more favourable treatment approach in terms of produce quality retention, was highly effective for decontamination of cherry tomatoes and strawberries inoculated with Salmonella, E. coli and L. monocytogenes monocultures and against background microflora of produce. However, the produce type and the contaminating pathogen influenced decontaminating effect of ACP with higher inactivation rates achieved for Gram-negative bacteria and bacteria associated with smooth surface of produce. The antimicrobial potential of high voltage either direct or indirect in-package atmospheric air ACP treatment with subsequent 24 h of storage was proven to be effective for inactivation of pathogens in the form of monoculture biofilms commonly implicated in foodborne and healthcare associated human infections, E. coli, L. monocytogenes, S. aureus, P. aeruginosa established during 48 h on abiotic surface. However, the efficiency of ACP treatment was again bacterial type dependant. Although complete inactivation of metabolic activity of Gram-negative bacteria could not be achieved, electron microscopy analyses confirmed the destructive action of ACP treatment. In-package high voltage indirect ACP treatment was effective against Salmonella, L. monocytogenes and E. coli biofilms developed on lettuce. This study also demonstrated that produce storage conditions, such as temperature, light and storage time had interactive effects on bacterial proliferation, internalisation, 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. Significant reductions of P. aeruginosa quorum sensing (QS)-regulated virulence factors, such as pyocyanin and elastase production, were achieved, suggesting that ACP technology could be a potential QS inhibitor and may play an important role in attenuation of virulence of pathogenic bacteria. Despite the varying parameters that influenced plasma bactericidal activity, high voltage in-package atmospheric air ACP decontamination approach showed an efficient reduction of high concentrations of bacteria in liquids, associated with produce and bacteria in their most resistant, biofilm form. These results represent significant technological advances in non-thermal bactericidal treatment with a key advantage of elimination of post-processing contamination of the product, thereby increasing microbiological safety and extension of produce shelf life