The interactions between the plasma generated RONS and water molecules have been found to enrich water with biochemically active species that demonstrate significant disinfection (Lukes et al., 2014, Ikawa et al., 2016, Zhou et al., 2018) and seed germination enhancing properties (Sivachandiran and Khacef, 2017, Sarinont et al., 2017, Zhang et al., 2017), over a longer exposure time than the RONS generated during direct CAP treatment. The latter treatment plays a significant role in the surface modification of the seed coat, by increasing its surface wettability and increasing the absorption of water into the seed (Xiang et al., 2019). The acidic environment generated in PAW (air) (pH 3.34 ± 0.07) correlates with the higher levels of NO3− species quantified in PAW (air), as HNO3 is one of the leading agents causing acidification of PAW. Relatively higher pH values were observed in PAW (N2) (3.91 ± 0.05) and PAW (CO2) (3.64 ± 0.09) as the quantity of reactive nitrogen species decreased. Further analysis on the physicochemical properties of the PAWs produced include the quantification of hydrogen peroxide, that is another long lived reactive species with important functions in microbial deactivation and seed germination, and the characterisation of PAW exactly after its generation in order to compare the starting quantity of RONS with other literature.
Similar to the germination assessment outcomes of individual CAP and PAW treated mung bean seeds, the three combined CAP and PAW treatments showed no significant stimulatory effect on mung bean seed germination or performance, although as relevant is the lack of inhibitory effect. This outcome contradicts the body of research evidencing significant increases in germination rate following cold atmospheric pressure plasma treatment of seeds (Sera et al., 2010, Mitra et al., 2013, Tong et al., 2014, Meng et al., 2017, Puligundla et al., 2017). For instance, Zhou et al. (2016) found that air plasma produced through DBD microplasma array (4.5 kV; 25 W) offered the best efficiency in improving mung bean seed germination rate and seedling growth when compared to the control and O2, N2 and He microplasma arrays. However, contrary to the germination conditions used in this study (incubation of seeds in test-tubes at 25°C ± 1°C in the dark at 45% humidity level with water replacement after 48 h), Zhou et al. (2016) kept the seeds in Petri dishes, watered them daily and incubated them in light conditions. In another study, CAP produced using a DCSBD at 400 W with a plasma volume power density of 70 W/cm3 was employed to treat wheat seeds (Zahoranová et al., 2015). While these conditions are similar to those applied in this study, the wheat seeds were placed directly on the ceramic plate of the DCSBD device and fixed to a rotation device to homogeneously treat the seed surfaces (Zahoranová et al., 2015). Contrary to this, mung bean seeds assessed in this study were treated bi-directionally in a static position between two plasma plates kept at a distance of 1.5 cm each from the seeds. Furthermore, germination and growth of wheat seeds was assessed by sowing the seeds in pots containing soil substrate (a mixture of sand, peat and pearlite). The wheat seeds exhibited a greater uptake of water than their untreated counterparts. In fact, following 20–50 s CAP-air treatment of seeds, significant accession of germination rate, dry weight and vigour of wheat seedlings was observed (Zahoranová et al., 2015). In comparing studies, germination conditions using light and soil are not realistically associated with sprout production practices. However, while taking into account that the closed germination set-up used in this study (test tubes) facilitated the microbiological assessments performed to determine the decontamination efficiency of combined plasma treated sprouts, the same system is not realistically comparable to common sprout production practices that enable the regular irrigation of sprouts. Furthermore, in comparing the technical plasma treatment parameters between studies, the distance of the seeds to the plasma source and associated treatment time during CAP treatment of seeds along with the frequency of irrigation with PAW may be key factors in regulating the level of exposure of seeds to reactive species that in turn determines the efficacy of plasma treatment on seed germination and growth.
Few studies have looked into the combined effect of direct CAP treatment of seeds and irrigation with PAW on seed germination and growth. In one study, Sivachandiran and Khacef (2017) elucidated the short term effects of combined plasma treatment on seed germination and stem growth of radish sprouts, and the long term effects of combined plasma treatment on tomato and sweet pepper plant growth. The study made use of a plate-to-plate DBD (operated with a high voltage pulsed power of 40 kV and frequency of 1 kHz) for the CAP treatment of seeds in air for 10 min (P 10) and 20 min (P 20) and a cylindrical DBD (40 kV; 1 kHz) for the cold atmospheric plasma activation of water in air for 15 (PAW 15) and 30 minutes (PAW 30). Seeds of the P 10 treatment irrigated with PAW 15 displayed better seedling growth when compared to the untreated seeds and to the P 10 treated seeds irrigated with tap water. Furthermore, non-treated seeds irrigated with PAW 30 performed better than non-treated seeds irrigated with tap water. Through this outcome, it was observed that aside from the stimulatory effect of plasma discharge on seed surface, sustained plasma treatment through irrigation of treated seeds with PAW can significantly enhance seedling growth. As may be applied in this study, achieving enhanced seed germination and seedling growth would require optimization of combined plasma treatment for each seed type, taking into account both gas phase characterisation and the physicochemical properties of the PAW (Sivachandiran and Khacef, 2017).
When assessing the inactivation of E. coli following exposure to PAW (air), PAW (N2) and PAW (CO2), it was found that although all three PAWs exhibited an acidic pH (3.34–3.91), only PAW (air) was found to inactivate E. coli. This suggests that an acidic environment is unlikely to be the sole sterilizing agent associated with PAW treatment, but that the type and concentration of reactive species within the PAWs also play a significant role in determining its biological effect. In fact, peroxynitrite, a nitrogen containing reactive species, was previously found to play a crucial role in the antibacterial application of PAW (air) due to its cytotoxic effects (Ikawa et al., 2016, Zhou et al., 2018). Peroxynitrite is mostly formed by the reaction between H2O2 and NO2 (Zhou et al., 2018). Theoretically, this reaction would not be possible in PAW (CO2) for lack of nitrogen in the process gases, nor in PAW (N2) for lack of reactive oxygen species such as H2O2 during plasma discharge. Further insight into the effect of the reactive species could be gained through independent inactivation assessments of E. coli following exposure to simulated aqueous concentrations of the reactive species identified and quantified in the PAW (e.g. H2O2, NO3−, NO2−, ONOO−, O3, and OH−). Another study aimed to elucidate the bacterial cell damage caused to Staphylococcus aureus by oxidative stress from PAW generated in a single electrode alternating current cold plasma set-up (Zhang et al., 2013). Among the techniques used, atomic absorption spectroscopy detected an increased leakage of potassium ions from bacterial cytoplasm and transmission electron microscopy revealed morphological impairments to bacterial cell wall and membrane following exposure to PAW. The same paper also indicated the effect of short-lived species within PAW, noting that a bacterial suspension of S. aureus exposed to PAW directly after production and the same PAW stored in a 4°C refrigerator for 24 h after production required 10 min and 40 min, respectively, to achieve a 6 log reduction in the population of S. aureus (Zhang et al., 2013). The aforementioned techniques may also be employed to better substantiate and characterise the antimicrobial properties of PAW.
The lifetime of reactive species is an important parameter in understanding the long-term decontamination effects of PAW treatment on seeds. An initial screening on sprouts following CAP and PAW treatment of mung bean seeds indicated that CAP (air) treated seeds irrigated with PAW (air) demonstrated the greatest control over the natural microbiota (4.92 log CFU/g) of the sprouts after 96h in the incubation chambers (25°C) when compared to the control and other combined plasma treated groups (> 6 log CFU/g). This outcome potentially enhances the relevance of combined plasma treatment as a means of inhibiting bacterial proliferation during sprout production, which is stimulated by the warm temperatures, water activity and high nutritive content in the sprouts. The increased decontamination efficiency of the combined plasma treatment in air may correspond to the antimicrobial effect of PAW (air) determined in respect to E. coli, however further characterisation, primarily through identification of microorganisms is required. To the best of our knowledge, no previous studies have monitored the microbial population within sprouts following CAP and PAW treatment of seeds. However, recent studies have assessed the use of PAW as a means of decontaminating harvested mung bean sprouts with ancillary evaluation of the physicochemical characteristics of the sprouts following treatment. Schnabel et al. (2015) assessed the inactivation rates of E. coli, Pseudomonas fluorescens, Pseudomonas marginalis and Pectobacterium carotovorum in sprouts after 5 minutes immersion in air plasma processed water. The highest inactivation rates were observed for E. coli and P. marginalis while minimal effect on texture and appearance of the sprouts was recorded. More recently, Xiang et al. (2019) found that the total aerobic bacteria and yeasts within mung bean sprouts decreased by 2.32 and 2.84 log CFU/g, respectively following 30 min immersion of sprouts in air PAW. The authors also established that the washing treatment had no significant effect on antioxidant potential of mung bean sprouts and no changes in the phenolic and flavonoid contents nor sensory characteristics of the sprouts.
In the mung bean sprouts, kaempferol-3-rutinosid, quercetin-3-rutinosid, genistein, biochanin A and ononin derivative were tentatively identified. A review on the phytochemical profile of mung bean sprouts mentions other phenolic compounds and flavonoids present in the sprouts such as catechin, syringic acid, gallic acid, vitexin, robinin, kaempferol-7-O-rhamnoside and isoquercitrin (Ganesan and Xu, 2017) which were not found here. A variation of flavonoids and phenolic acids is common in Vigna species. in a previous study on pea sprouts exposed to CAP resulted in highest concentrations of quercetin and kaempferol glycosides, whereas the treatment of seeds and seedlings was not as efficient or even decreased quercetin and kaempferol glycosides (Bußler et al., 2015). Here all combined plasma treatments had no significant impact on the concentrations of secondary metabolites in the sprouts that gives a positive outcome on the application of cold plasma treatment in the agro-food industry.
Overall, this research demonstrated that air plasma activated water formed by 1 minute surface treatment with cold atmospheric plasma generated at 350 W (at a frequency of 15 kHz) in the DCSBD was able to reduce the population of E. coli DSM1116 by 7.43 log CFU/mL within 6 h of exposure. The combined seed treatment with direct air CAP (5 minutes, 350 W) and air PAW had no negative impact on mung bean seed germination and growth when compared to the control, nor was the concentration of secondary metabolites within the sprouts (kaempferol-3-rutinosid, quercetin-3-rutinosid, benzoic acid + malonic acid, ferulic acid + malonic acid and caffeic acid + malonic acid) reduced. The combined air CAP and air PAW treatment reduced the total microbial population in sprouts by 2.5 log CFU/g lower than the population of the control within 4 days, although further characterisation of the natural microbiota on the seeds prior to and after cold plasma treatment would allow for more accurate assessments on the effect of prolonged plasma treatment on the sprouts. The concentration of long-lived reactive nitrogen species, such as NO3−, NO2- are crucial in elucidating the complex mechanisms of action of PAW in disinfection and germination, along with the further characterisation of other reactive species such as hydrogen peroxide.