Assessing the antimicrobial potential of aerosolised electrochemically activated solutions (ECAS) for reducing the microbial bio-burden on fresh food produce held under cooled or cold storage conditions
Introduction
In post-harvest fresh produce processing and manufacturing, effective microbiological management is essential for the control of spoilage organisms, environmental pathogens and foodborne diseases (Hammond et al., 2015). To maintain produce quality and reduce waste, intervention at all stages of the supply chain is important and must involve a multifaceted, integrated approach. Consumption of fresh food produce has increased substantially over the last few decades, but this has been linked with an associated increase in foodborne disease outbreaks (Olaimat and Holley, 2012). This contamination event can occur at any point during the food chain, whereby potentially pathogenic organisms can persist for long periods, both within soil environments and on the fresh food produce itself (Olaimat and Holley, 2012). This is likely the result of biofilm formation (Olmez and Temur, 2010, Niemira and Cooke, 2010), and is of serious concern, given that biofilm structures are known to be more resistant to post-harvest treatments (Aruscavage et al., 2006), including the chlorine washes which are widely used within the food processing industry (Houdt and Michiels, 2010).
The use of antimicrobials is essential in the control of food spoilage organisms and potentially pathogenic foodborne organisms (Holah et al., 2002), and the importance of antimicrobial inclusion in post-harvest washing solutions has been demonstrated using quantitative modelling (Danyluk and Schaffner, 2011). Post-harvest produce is often washed in chlorine to reduce the microbial bioburden, both to improve shelf-life (Meireles et al., 2016) and to target and inactivate potential pathogens (Warriner et al., 2009). Industry typically utilises chlorine solutions of sodium hypochlorite (pH 6.5) applied at concentrations between 50 and 200 ppm free chlorine, for a contact time of between 1 and 5 min (Goodburn and Wallace, 2013). However, high levels of organic loading have been shown to reduce the efficacy of 30–50 ppm chlorine when used as a produce washing solution (Zhang et al., 2009), and the presence of organic loading itself can lead to the production of harmful bi-products (Bull et al., 2011), including trihalomethanes and haloacetic acids (Shen et al., 2016). Hence, alternative chemical approaches (e.g. quaternary ammonium compounds, ozone and hydrogen peroxide), biological methods (e.g. bacteriophage, bacteriocins and enzymes), physical-interventions (e.g. UV-light, temperature and ionising radiation) or combinatorial approaches (Meireles et al., 2016, Warriner et al., 2009, Olmez and Kretzschmar, 2009, Goodburn and Wallace, 2013) are now being developed.
One emerging technology within the food industry are electrochemically activated solution(s) (ECAS; variously named electrolysed oxidising water or electrolysed water). ECAS are generated through the electrolysis of a low salt solution within an electrochemical cell which can be configured to produce solutions with a variety of physicochemical properties (Thorn et al., 2012). These solutions have been shown to be extremely fast acting (Robinson et al., 2011) with broad spectrum antimicrobial activity (including bacterial spores; Robinson et al., 2010). Additional benefits of the use of ECAS include in situ generation from inexpensive raw materials coupled with environmental compatibility (Thorn et al., 2012). Numerous studies have demonstrated the potential use of ECAS within the fresh food produce industry for controlling foodborne pathogens on onions (Park et al., 2008), lettuce (Park et al., 2001, Abadias et al., 2008, Guentzel et al., 2008, Keskinen et al., 2009), tomatoes (Bari et al., 2003, Deza et al., 2003), pears (Al-Haq et al., 2002), peaches (Al-Haq et al., 2001), apples (Okull and Laborde, 2004), strawberries and cucumbers (Koseki et al., 2004a).
Effective antimicrobial action is impacted by concentration, contact time, contact surface and organic loading (Russell, 2004). Aerosol delivery technologies, whereby solid particles or liquid droplets are suspended in a gas, have been shown to be an effective delivery mechanism for a range of antimicrobials, including ECAS (Thorn et al., 2013). Within the food industry application of antimicrobials is mainly via liquid or spray delivery systems; however, over wetting of produce can result in deterioration of the produce and potentially expedient the spoilage process. The effectiveness of aerosol delivery of hydrogen peroxide, sodium hypochlorite, citric acid or ethanol for reducing postharvest diseases has been shown in strawberries (Vardar et al., 2012). This biocidal delivery mechanism has also been successfully utilised to reduce the microbial bioburden of figs (Karabulut et al., 2009), and the presence of Penicillium digitatum within a citrus degreening room (Smilanick et al., 2014). However, aerosol delivery of ECAS has not been previously investigated within the food industry.
The main aim of this study was to assess the antimicrobial potential of aerosolised ECAS for reducing the microbial bio-burden on fresh produce held under cooled (cucumber and tomatoes) or cold (rocket and broccoli) storage conditions for up to 5 days. This technology platform is currently being developed as part of an integrated microbial management system to improve shelf-life by controlling spoilage and pathogenic organisms within post-harvest produce. Ultimately, such approaches will contribute to food security and food safety.
Section snippets
Growth and maintenance of target micro-organisms
Pseudomonas syringae (Pseudomonas syringae pv. Phaseolicola), Escherichia coli (ATCC 10536) and Pencillium expansum (IMB 11203/DSM 62841) were stored at −80 °C until required. P. syringae was recovered onto King’s B medium (Sigma-Aldrich Ltd., Dorset, UK) at 25 °C, Escherichia coli was recovered onto nutrient agar (CM0003; Oxoid, Basingstoke, UK) at 37 °C and P. expansum was recovered on potato dextrose agar (CM0139; Oxoid, Basingstoke, UK) at 25 °C. P. expansum spores were prepared by
Results and discussion
An integral part of this research was the development of a piezoelectric transducer based fogging technology platform, such that any ECAF treatment regimen could be configured and left to operate autonomously (shown in Fig. 1). This enabled automated delivery of electrochemically activated fog (ECAF) or RO water fog for pre-defined treatment durations. The piezoelectric transducer (resonating frequency ∼1.6 MHz) utilises ultrasonic waves that are focused on ECAS, generating a relatively dry fog
Conclusions
Sanitation procedures play a critical role in fresh produce safety (Olaimat and Holley, 2012) and the potential use of ECAF is only one part of the whole food production control system that could be implemented to control food spoilage and food safety organisms. This study has demonstrated that 1100 mV ECAF is capable of reducing the microbial load of both potential food spoilage and pathogenic microbes, on various fresh food produce types under both cooled and cold storage conditions for a 5
Funding disclosure
This work was supported by the Department for Environment Food and Rural Affairs (DEFRA) through an Innovate UK Research and Development Competition [grant number 23237-161166, 2012].
Acknowledgements
The authors would like to acknowledge Bridge Biotechnology Ltd. for their help and assistance in developing the novel ECAF technology platform. The authors would also like to thank Mack Multiples Ltd., Manor Fresh Ltd., Laurence J Betts Ltd. and Thanet Earth Ltd. for providing all the fresh food produce used within this study, as well as their insight and expertise during experimental design.
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