Investigation of a large gap cold plasma reactor for continuous in-package decontamination of fresh strawberries and spinach
Introduction
Fresh produce such as spinach, lettuce, radish, alfalfa sprouts, tomatoes, peppers, cantaloupe and strawberries have been implicated in human health outbreaks caused by contamination with Escherichia coli, Salmonella spp. and Listeria monocytogenes (EFSA & ECDC, 2018; Olaimat & Holley, 2012). E. coli is a Gram-negative, short rod-shaped bacteria, which is the most common facultative anaerobe found in the gastrointestinal tract of humans and other mammals and warm-blooded animals (McClure, 2005). Enterohemorrhagic E. coli O157:H7 is a major foodborne pathogen responsible for the severe illnesses in humans (Lim, Yoon, & Hovde, 2010). In 2017, in the EU, E. coli (STEC) resulted in 6073 confirmed cases of infections and in 20 deaths (EFSA & ECDC, 2018). L. monocytogenes is a Gram-positive rod-shaped, facultative anaerobe, which is the agent of the disease listeriosis (Stefanovic, Reid, Nadon, & Grant, 2010). Deterioration of fresh produce as a consequence of microbiological spoilage may also constitute a hazard for consumers through the possible presence of microbial (myco)toxins (Rawat, 2015). The causative agents of microbiological spoilage in fruits and vegetables are highly variable. A range of environmental factors, such as storage temperature and produce pH, will influence microbial community diversity and microbial resistance. These can impact the efficacy of decontamination procedures, and as a consequence the microbiological quality and stability of the produce with respect to shelf-life (Gallagher & Mahajan, 2011; Leff & Fierer, 2013). Other factors, which enhance the propagation of pathogenic and spoilage microorganisms on fresh produce include a high water content, damage during harvesting, transport and type of processing (Spadaro & Gullino, 2004). Current trends of reduced use of agrochemicals may also lead to an increase in the numbers of pathogenic fungi present on fresh produce, therefore, leading to increased mycotoxin production (Van Boxstael et al., 2013). Therefore, the use of optimised minimal processing technology is necessary in order to retain nutritional quality as well as to maintain microbiological safety of perishable high-value produce.
Atmospheric cold plasma (ACP) has demonstrated a high potential for the reduction of microbial loads on fresh fruits and vegetables with good retention of produce quality attributes. ACP has a non-uniform distribution of energy among the constituent species, where multiple chemical reactions and reactive species are generated (Niemira, 2012; Scholtz, Pazlarová, Soušková, Khun, & Julák, 2015). The major reactive agents that play a role in inactivation of microbial targets, independently or in synergy, include reactive oxygen species (ROS) (singlet oxygen, superoxide anion, ozone) and reactive nitrogen species (RNS) (atomic nitrogen, excited nitrogen, nitric oxide); if humidity is present, hydroxyl anions and radicals or hydrogen peroxide are also generated (Scholtz et al., 2015). The composition and abundance of chemical species is often determined by the source used for generation of the plasma. The commonly used forms of ACP in terms of a high potential for industrial applications are dielectric barrier discharge (DBD) and plasma jets. The major advantages of the DBDs include the ease of the discharge ignition and adaptability to suit various produce commodities and the possibility of treatment of produce inside sealed-packaging material where the elimination of post-processing produce contamination can be achieved. Our previous research generated cold plasma discharges inside food packages at bench-scale, leading to rapid inactivation (within 2 min) of foodborne pathogens on the surface of different types of produce (Ziuzina, Patil, Cullen, Keener, & Bourke, 2014) and against bacterial biofilms and associated cells internalised in fresh produce tissue (Ziuzina, Han, Cullen, & Bourke, 2015). Furthermore, sporicidal effects were demonstrated (Los et al., 2018; Patil et al., 2014). Misra et al. (2014) reported a 2-log10 CFU/g reduction of natural microbiota of produce, which was achieved while retaining produce quality. However, to date, a single package unit approach was used to characterise the efficacy of the post- and in-package plasma process and the evidence on the use of ACP for larger-scale operation remains limited.
The aim of this work was to evaluate the effects of the pilot-scale SAFEBAG in package plasma system under conditions representative of industrial practices. The system was tested under static operational mode – to represent packages of fresh produce at the post-sealing stage of a production line, as well as under continuous operational mode – to represent processing of larger numbers of packaged fresh produce. The target microorganisms selected were E. coli and L. innocua and the produce was strawberries and spinach leaves. The effects of treatment on the quality and nutritional profiles, including colour, firmness, pH, total soluble solids (TSS) and changes in the sample metabolites were evaluated. The aims of enhanced microbiological safety and extension of produce shelf-life without compromising product quality remained critical to the study design. Therefore, the impact of treatment on total microbiota and quality parameters of un-inoculated strawberry samples was studied for immediate and retained effects of treatment during a storage study.
Section snippets
Plasma system description
The prototype SAFEBAG system is depicted on Fig. 1. The system employed a DBD reactor operating in open air. It consists of two parallel 1 m-long electrodes providing space for an adjustable discharge gap (up to 4.5 cm) allowing for several flexible packages (depending on the package size) to be treated simultaneously in static or continuous mode (Fig. 2), i.e., when a conveyor belt carries the bags through the plasma discharge. The control panel allows for control of applied voltage (0–100 kV)
Electrical and optical measurements
The current waveforms typical evolution is shown in Fig. 3 The discharge had a glow-filamentary mode and their pattern can be used to appreciate the discharge operation during different treatments (use of different gaps, voltages and number of bags and their load). The discharge current and displacement current are superimposed as the transformer was a bipolar type. The two optical emission spectra (Fig. 4) correspond to average rms discharge currents of 1.4 A (blue) and 2.2 A (red),
Conclusion
The current work demonstrated that with the pilot-scale large gap ACP system for in package treatment, significant reductions of pathogenic as well as spoilage microorganisms could be achieved and that the in-package approach did not adversely affect product quality properties. Greater reductions were achieved for E. coli on strawberries and spinach with application of static treatment, whereas for L. innocua inoculated on strawberries, higher reductions were achieved with continuous treatment.
Acknowledgements
The research leading to these results received funding from the EC Seventh Framework Programme (FP7/2207-2013) under grant agreement number 285820.
Data availability statement
All data generated or analysed during this study are included in this published article and are available on request.
Declaration of competing interest
The authors declare that there is no conflict of interest.
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These authors contributed equally to this work.