Special issue on plasmas in agriculture and the food cycle

This study applied the combination of plasma-activated water (PAW) and mild heat (40 °C and 55 °C) for the decontamination of Bacillus cereus spores in rice (Oryza sativa L. ssp. japonica). In this study, PAW combined with 40 °C and 55 °C (PAW-40 and PAW-55) achieved 1.54 and 2.12 log₁₀ CFU g⁻¹ reductions of B. cereus spores in rice after 60 min exposure, which was significantly higher than the inactivation level (0.72 log₁₀ CFU g⁻¹) induced by PAW treatment alone. The assays for the leakage of intracellular contents (nucleic acids, proteins, dipicolinic acid-DPA) and the propidium iodide (PI) fluorescence probe confirmed that the combination of PAW and mild heat caused significant damages on the intact multilayered structure of B. cereus spores than the individual PAW did. The application of scanning electron microscope (SEM) and transmission electron microscope (TEM) provided a direct observation for the visible disruption of external structure and leakage of intracellular components caused by the combination of PAW and mild heat. Besides, the combination of PAW and mild heat caused no adverse effect on the texture and sensory qualities of the rice after cooking. These findings provide some knowledge for the application of PAW combined with mild heat for the decontamination of spores in rice.

Basil (Ocimum basilicum) seeds were treated for different exposure times with a non-equilibrium plasma produced by a volume dielectric barrier discharge in humid air at atmospheric pressure.

Plasma treatment did not change the seed structure and morphology, as visualized by high-resolution computed x-ray microtomography. A faster and higher germination rate was observed with plasma treatment of 1 and 3 min. Plantlets grown in sand, after both 2 and 3 weeks, showed a more developed root apparatus and better biometric parameters, compared to plants developing from non-treated seeds.

After the plasma treatment, internal redistribution of macro and micronutrients was observed by using micro x-ray fluorescence spectroscopy. In particular P, K and Mg concentrated in the radicle, moving from the endosperm and cotyledons, while Zn, initially concentrated in specific tissues of the cotyledon, appeared more homogeneously distributed inside the whole seed after the plasma treatment. Significant variations in electrical impedance spectra were also observed after plasma treatment.

This element redistribution in the seed was caused by the intense electrical field generated by the volume dielectric barrier discharge plasma, causing a movement of important micro and macronutrients from the storage regions of the seed towards the radicle tissues. This ion movement could explain the observed faster germination of the plasma-treated seeds. Indeed, such movement of ions is similar to what is generally observed in seed tissues during germination. The plasma treatment therefore somehow anticipates and implements the mobilization of key nutrients towards the radicle, resulting in faster and higher germination of the seeds as well as improved characteristics of the basil plantlet, especially at the root level.

In this study, we investigated the antimicrobial effects of plasma activated fine droplet (PAD) produced from arc discharge plasma on planktonic Listeria monocytogenes and Escherichia coli O157:H7. NaCl (0.9%, w/v) was used as the feeding solution for the plasma discharge. The inactivation mechanism of the PAD treatment was also investigated. PAD mainly contains H₂O₂ and OCl⁻, which play a significant role in the inactivation process against L. monocytogenes and E. coli O157:H7. The population of L. monocytogenes and E. coli O157:H7 was significantly reduced by approximately 3 and 4 log units, respectively, within 5 min of exposure to PAD. However, the bactericidal effects of PAD against L. monocytogenes and E. coli O157:H7 showed different trends by showing 0.58 and 4.13 log reductions, respectively, after 1 min of PAD exposure time. The change of membrane integrity was evaluated using two DNA-binding fluorescence dyes, SYTO 9 and propidium iodide (PI). The breakage of the cell wall and membrane of both microorganisms was evidenced by the uptake of PI by cells after 5 min of exposure to PAD, but the effect was less in L. monocytogenes compared to E. coli O157:H7 after 1 min of PAD exposure time. The transmission electron microscopy results clearly showed morphological changes in both microorganisms, including denaturation or leakage of intracellular materials as a consequence of PAD treatment. These findings suggest that PAD-induced chemical species can eventually affect the intracellular materials of bacterial cells by passing through or attacking the cell envelope. In addition, L. monocytogenes are less susceptible to PAD compared with E. coli O157:H7.

Cold plasma is a promising nonthermal technology for the decontamination and preservation of fresh and fresh-cut produce. The rapid action of a cocktail of reactive oxygen and nitrogen species in plasma against a variety of food spoilage and pathogenic organisms make it a suitable decontamination intervention. This study evaluates the effectiveness of high voltage atmospheric cold plasma in reducing the natural microflora of carrots treated at 60, 80, and 100 kV (60 Hz frequency) for 1, 2, 3, 4, and 5 min. About 2 log₁₀ CFU/g reduction in the population of both, total aerobic mesophiles, and yeast and mold was observed in carrot discs treated at 100 kV for 5 min (~250 W power). The changes in pH, colour, texture, and total carotenoids of cold plasma treated carrots were minor. For shelf-life evaluation of plasma treated cut-carrots voltages between 80 to 100 kV and treatment times between 4 to 5 min is recommended, such that reduction in spoilage microflora and quality retention is maximized. Voltage was found to have a significant effect on the total carotenoids and colour of the carrot discs. Further optimization of the process parameters, preferably using modified gases, is necessary to balance the desired reduction in microflora versus the quality retention.

Atmospheric pressure cold plasma technology has great application potential in the field of food safety. A novel atmospheric air dielectric barrier discharge device has been designed to generate plasma directly on the surface of spherical fruit. This device consists of the quartz tube, the high-voltage electrode attached to the outer wall of the quartz tube, and the fruit placed onto the inner surface of the quartz tube, acting as the ground electrode. The intensified charge-coupled detector images show that a uniformly scanning discharge channel is formed between the surface of treated substance and the inner wall of quartz tube. When the treated substance is rolled in the device, the uniform plasma is directly formed over the fruit surface. The bactericidal efficiency of plasma against pathogenic microorganisms on rolling fruit surface has been studied. The number of Staphylococcus aureus (S. aureus) on the surface of fruit treated by plasma for 15 s in rolling state is 1.57 log₁₀ CFU less than that of the static state. Compared with the initial bacterial concentration, the number of S. aureus on the surface of 1 cm² decreases by 5.9 log₁₀ CFU after plasma treatment of rolling fruit for 15 s. In conclusion, the fruit surface is uniformly treated by the novel plasma device and S. aureus on the fruit surface was sterilized without causing thermal damage to the fruit surface.

Sustainable and cost effective food preservation techniques are of industrial, environmental and public health significance globally. A promising means for the gentle but efficient sanitation of foods is the application of cold atmospheric plasma. Here, the preservation of fresh-cut apples was investigated using a gas phase surface discharge plasma (SDP) reactor within an exposure chamber. Results show that the microbial load reduction of the fresh-cut apples was found to be strongly dependent on the storage time and preservation method, e.g. refrigeration (control), SDP-room temperature and SDP-refrigeration (SDP-RF). After 6 d of storage, the microbe load on the apple pieces for the SDF-RF treated groups was found to be significantly lower compared to the refrigeration-stored (4 °C) and the SDP only-processed groups, with the lowest bacterial load on the 120 s SDP-RF stored apple pieces (1.76 CFU g⁻¹). Furthermore, the effects of the preservation method on the quality attributes (weight loss, firmness, and physical appearance), and the surface chemistry directly after cutting and SDP processing, as well as the activities of polyphenol oxidase and peroxidase after the different duration of storage were evaluated. This study successfully demonstrates the feasibility of SDP for the effective preservation of fresh-cut apples and contributes to the fundamental understanding of surface plasma-induced effects on the microbial inactivation and postharvest quality of fresh-cut fruits.

Since ancient times, the anomalous occurrence of mushrooms has been observed in and around locations struck by lightning. Based on this phenomenon, various cultivation techniques using a high voltage to cultivate shiitake mushroom in bed log (wood) have been proposed. Undoubtedly, the fruiting capacity of shiitake mushrooms is significantly promoted by direct application of a high voltage to the cultivation bed logs. However, mushroom bed logs are rarely struck directly by lightning in the natural environment. Thus, it is important to devise a stimulation method that matches conditions in the natural environment. In this study, we performed experiments involving indirect lightning strikes using a new method that mimics the natural environment as closely as possible. Specifically, we used the electric discharge created by an impulse voltage generator to create lightning but did not apply it directly to the cultivation bed log. Instead, the cultivation bed logs were placed a few meters away from the lightning rod. Subsequently, the fruiting capacity of the shiitake mushrooms with and without the indirect lightning strikes was compared. The experimental results indicated that, with the indirect lightning strikes, the extent of shiitake fruiting almost doubled compared to the case without the indirect lightning strikes. Furthermore, the results confirm that an increased shiitake mushroom production is possible using lightning strikes, even if the associated high voltages are not applied directly to the bed logs.

The objective of this study was to assess the antimicrobial effects of atmospheric cold plasma decontamination treatment on foodborne pathogens on various foods. The study employed a 30 l chamber using surface dielectric barrier discharge as a plasma source applying an atmospheric pressure and ambient gases. The inactivation rates of three foodborne pathogens, E. coli O157:H7, S. Typhimurium, and L. monocytogenes, were examined in fresh vegetables, fruits, nuts, and powdered food samples in response to cold plasma treatment; several strains of each pathogen were evaluated. The hydrophobicity and surface roughness of selected samples were examined using the water contact angle and non-contact three-dimensional surface profiling measurements, respectively. Samples were then inoculated with the food pathogens and treated in cold atmospheric plasma for up to 20 min. As the treatment time increased, different levels of microbial reduction were observed among the samples and pathogens. Surface roughness was negatively correlated with the inactivation rate. Even surfaces showed higher microbial reduction. Taken together, these results indicate that surface roughness is an important factor for the antimicrobial efficacy of cold plasma treatment.

The efficacy of seed disinfection utilizing atmospheric pressure non-thermal plasma is still a subject of intensive research. Previously, we found that rice seeds infected by Fusarium fujikuroi (fungus causing bakanae disease) could be disinfected via underwater arc discharge plasma. In this study, we further investigated the mechanism of disinfection and the effect on disease severity. In addition, we evaluated disinfection of rice seeds in air by surface DBD plasma. Disease severity was significantly reduced in seedlings germinated from rice seeds treated with underwater arc plasma, compared to non-treated seeds (44%–62% plasma treated compared to 92% non-treated). A shockwave of 11 atm pressure was generated during arc discharge, which likely caused fungal detachment from the seed surface. Moreover, reactive oxygen species such as atomic oxygen (O) was emitted from the underwater arc discharge plasma and could have contributed to the degeneration of the chemical composition on the seed surface and the inactivation of the fungal spores. Rice seeds treated with surface DBD plasma in the presence of H₂O₂ at a lower pressure (0.6 atm) for 30 min showed the highest seed disinfection efficiency (about 93% seed disinfection) and a significant reduction in disease severity in the germinated seedlings (23% plasma-treated compared to 100% non-treated). Taken together, our results suggested that underwater arc discharge and surface DBD plasma could be usefully applied to develop control strategies for seed-borne fungal diseases via seed decontamination during dry storage or water imbibition before sowing.

A plasma-ozonation system was investigated as a possible method for the reclamation of pesticide contaminated water. A multi-wire to plate corona discharge in oxygen was used, operated in pulsed regime (110 ns full width at half maximum). The synthetic contaminated water contained 25 mg l⁻¹ 2,4-dichlorophenoxyacetic acid (2,4-D) in tap water. Complete degradation of 2,4-D and over 90% mineralization were achieved during treatment. The toxicity of the reclaimed water was assessed using seeds of Solanum lycopersicum, known to be sensitive to 2,4-D. It was found that plant growth was inhibited by the presence of the pesticide, however, the treatment reduced the toxicity of the contaminated water. The plasma-ozonation method has proved thus far a good candidate for water reclamation for agriculture.

The effect of pre-sowing plasma seed treatment of maize (Zea mays L.), narrow-leaved lupine (Lupinus angustifolius L.) and winter wheat (Triticum aestivum L.) on seed germination, plant resistance to common diseases during vegetation and crop yield is studied in laboratory and field experiments. It is shown the efficiency of seed treatment by low-pressure radio frequency plasma in suppression of a number of fungal crop diseases such as boil smut of maize, root rot of lupine and winter wheat at different growth stages. At the stage of V9 (9th leaf visible) the infection level in maize plants from treated seeds was 3 times less than that in control. Root rot disease development of lupine at the first stages (3rd–4th leaves emerged) of growth did not exceed 6.9% in plants from the treated seeds while reached to 47.8% in control. Pre-sowing seed treatment led to suppress the anthracnose spreading on narrow-leaved lupine up to the flowering stage. It was revealed that, due to a decrease in the level of seed infection, stimulation of field germination, early seedling growth and plant resistance to pathogens during the vegetation period, the winter wheat grain yield increased by 2.3%, maize—by 1.7%, narrow–leaved lupine—by 26.8% compared to control plants. Increases in activity of non-enzymatic antioxidants (proline, anthocyanins as well as total phenolic content) in roots of maize seedling were observed which may indicate a significant role of plasma seed treatment in improving the plant resistance to biotic and abiotic stress during the vegetation.

Pre-sowing treatment of red clover seeds with cold plasma (CP) and electromagnetic field (EMF) was applied aiming to evaluate changes induced in germination and early seedling growth under controlled laboratory conditions, and to estimate treatment effects on plant growth, biomass production, nutritional value and the amounts of major leaf secondary metabolites under long-term field experimental conditions. The effects of seed treatment using radio-frequency EMF (5, 10, 15 min) and CP (2, 5, and 7 min) on seed germination kinetics, morphometric parameters of seedlings were assessed for two cultivars of red clover—'Vyčiai' and 'Sadūnai'. Long-term observations revealed that the effects of seed treatments persist for the entire vegetation season, while the observed effects on germination (CP increased germination rate by 4%–20%) do not provide sufficient information to predict the impact on plants on a longer time scale. The effects on morphometric parameters were dependent on plant cultivar, and were much stronger (up to 24% for 'Sadūnai' and 49% for 'Vyčiai') after growing the plants in the field for 5 months in comparison to the effects (absent or below 10%) observed in the early growth stages. A minor improvement of nutritional value per biomass unit was observed for 'Vyčiai'. Protein yield per plant increased substantially (up to 70%). Pre-sowing seed treatment with CP and EMF had an obvious impact on the amounts of major isoflavones in leaves, and the nature of these changes strongly depended on the vegetation stage of plants (before flowering or at flowering stage). CP treatment for 5 min was found to be the most effective in this respect in not flowering plants, inducing a strong increase in biochanin A/formononetin ratio in leaves of both cultivars. However, an opposite effect was observed in the flowering plants, where the CP and EMF treatments decreased biochanin A/formononetin ratio.

Cold atmospheric plasma is a promising new non-thermal technology for improving the microbiological safety and shelf-life of food products, particularly fresh produce and minimally processed fruit and vegetables. Limited research has been conducted on the safety of plasma-treated foods for human or animal consumption. This study focuses on basic safety studies by investigating lettuce broth treated with a di-electric barrier discharge plasma device as a fresh produce model in terms of in vitro cytotoxic and mutagenic effects on mammalian cells and its in vivo toxicity on Galleria mellonella larvae. Low cytotoxic effects were detected in vitro and mutagenic events were likely to be spontaneous mutations. However, a strong response of G. mellonella larvae to injection with plasma-treated lettuce broth was observed for 5-min-treated broth, with larvae survival of less than 10%. No significant effects on quality attributes such as colour were detected and only low concentrations of peroxide were generated in the broth. This study highlights the need for more detailed investigations into the impact of plasma treatment on food components and the subsequent in vitro and in vivo effects to ensure safe implementation of plasma technology for the processing of food products.

(−)-Epigallocatechin gallate (EGCG), which is the most abundant bitter polyphenol in green tea (Camellia sinensis) leaves, is generally unstable under oxidative conditions. In this study, the efficient dimerization of EGCG was successfully achieved without changes in stereochemistry by treatment with nonthermal dielectric barrier discharge (DBD) plasma. The stereochemically pure dimers 2 and 3 connected by a methylene linkage exhibited significantly enhanced melanogenesis inhibitory activities compared to the parent EGCG and the active compounds 2 and 3 suppressed cellular tyrosinase, tyrosinase-related protein (TRP)-1, and TRP-2 expression in B16F10 melanoma cells, which could be considered one of the mechanisms of action. These results indicate a new method for efficient dimerization of stereochemically active depigment agent induced by DBD plasma treatment.

Barley sprouts are 5 to 20 day-old grown whole plants that are widely consumed as a functional vegetable due to their health-promoting compounds such as saponarin, γ-Aminobutyric acid (GABA), and policosanols. Recent plasma applications have been a promising approach in the agricultural and food industries to produce bioactive phytochemicals in sprouted crops. Therefore, this study investigated the plasma effects on the whole plant and its bioactive phytochemicals compared with those of an untreated control in barley sprouts. The optimal plasma exposure during seed germination can change physical and chemical properties of fully hydrated seeds of barley, accelerate the early growth of its sprouts and enhance bioactive phytochemicals in the sprouts. Thus, a single 6 min exposure of the plasma enhanced the fresh weight of whole barley seedlings by 137.5% of the untreated control at 9 d after the exposure and enhanced the contents of the primary metabolites including soluble sugars and free amino acids, and secondary metabolites including special phytochemicals such as saponarin, GABA, and policosanols. Therefore, our results suggest that the optimal plasma exposure can be used as a promising seed priming technique for the whole plant and the bioactive phytochemicals of barley sprouts.

Ethylene removal was investigated using a reactor system combining plasma and a catalyst (Pd/ZSM-5). The catalyst could adsorb ethylene effectively even under harsh conditions of 100% humidity and 10 000 ppm CO₂. The air in a 1 m³ container, which was prepared to imitate agricultural storage, was circulated through the reactor. The process of ethylene treatment consists of one cycle comprising adsorption, plasma-catalyst oxidation, and desorption, and this cycle is repeated until the ethylene is completely removed. Among the variables such as initial concentration, amount of catalyst, voltage, adsorption time, and flow rate of the circulating gas, the most influential were the flow rate and the catalyst amount. The lower the initial ethylene concentration is, the less time required for complete removal, although the percent removal was not largely affected by the initial concentration (20–200 ppm). The ethylene removal was improved by injecting 20 ppm ozone into the container once per cycle. In the absence of ozone, using only the plasma-catalytic reactor, it took 20 h for complete removal, whereas all ethylene was removed in about 8 h in the presence of ozone. A mass balance model could provide a good prediction of the temporal variations of ethylene concentration. The long-term storage stability of a Fuji apple in the container was tested for 40 d. A comparison of the control group with the group subjected to ethylene processing revealed that the ethylene concentrations were significantly different from each other, indicating the efficiency of the plasma-catalyst system. After 40 d of storage, the hardness and sugar content were higher in the group from which ethylene was removed than in the control group, and the acidity was higher in the control group. Furthermore, after 40 d, the control group showed a decay rate of 10%, whereas that of the group that underwent ethylene processing was only 1%.

Liquid–phase transport of plasma-generated reactive species toward large-scale plasma treatment for agricultural applications is experimentally studied. The liquid-phase reactive species in this study are generated by the contact of water solution with air–plasma effluent gas containing mainly low-solubility reactive species under elevated pressure, followed by spraying the plasma effluent gas dissolved solution into a target pathogenic conidium suspension. Low-solubility ozone in the liquid phase at the target is found to dominate the observed germination suppression effect, which also correlates with the gas-phase ozone density. The measured ozone concentration at the target is found to be ten times lower than the ozone concentration at saturation, estimated from Henry's law with the measured gas-phase ozone density. The liquid-phase ozone loss mechanism during the transport of the sprayed liquid is interpreted as volatilization and reactions with co-dissolved species. The deduced control parameters for precise ozone transport are the droplet diameter of the sprayed solution, sprayed jet flow, and co-dissolved reactant with the liquid-phase ozone such as nitrite.

Cold atmospheric pressure plasma has potential as a non-thermal processing technology to decontaminate food and food contact surfaces due to its ability to generate an abundance of reactive oxygen and nitrogen species with antimicrobial attributes at ambient conditions. In this study, we present a comparison on the effectiveness of surface decontamination against feline calicivirus (FCV) and Salmonella spp using four different plasma sources, a dielectric barrier discharge (DBD) in direct contact with the substrate and three remote plasma treatment sources: a 2D DBD, a volumetric DBD and a gliding arc discharge. The plasma sources were all operated in air at atmospheric pressure. The decontamination efficacy was enhanced by the presence of humidity on the sample surface and only direct contact between plasma and samples allowed the inactivation of pathogens on dry substrates. Across all sources, FCV was seen to be more susceptible to the plasma-generated species than Salmonella spp. The diminished effectiveness of the gliding arc discharge compared to the DBDs operating at the same power is most likely due to the low Henry's law constant of NO, the dominant reactive species generated by the gliding arc. Control experiments illustrate that the co-existence of O₃ and NO₂, as in the afterglow of the remote DBDs enhances the inactivation compared to the inactivation by O₃ or NO₂ only. A chemical kinetics model of the plasma effluent and the plasma treatments show a strong correlation between the gas-phase concentration of N₂O₅ and inactivation of the virus. We experimentally show that the production of N₂O₅ coincides with the enhanced generation of reactive nitrogen species in the liquid phase.

We confirmed that micro dielectric barrier discharge plasma improved the growth and immunity of Panax ginseng C. A. Meyer seedlings. The growth rate of the ginseng seedlings was compared by height and weight monitoring. The length and weight of the aerial part of the 3 min. Plasma treated ginseng increased by 25% and 54%, respectively, compared to the untreated ginseng. The difference between the initial root weight before planting in the soil and the root weight 5 weeks after planting in the soil increased by 28% and 22% for the 3 and 10 min. Plasma treated ginseng, respectively. Chlorophyll and total phenolic contents were increased 62% and 56%, respectively in the aerial part of the plasma-treated ginseng seedlings compared to the untreated controls. Radical scavenging activity was also increased by 18% in the 3 min plasma treated ginseng seedlings. The plasma treatment increased the expression level of chlorophyll metabolism-related and auxin response factor genes in ginseng and might be the result of ginseng growth development by enhancing plant growth hormone. Increasing disease resistance through induced systemic resistance can ultimately improve crop productivity by strengthening the immune system. We demonstrated this by comparing the transcription patterns of genes involved in the pathogenic defense mechanisms of ginseng. The expression of the pathogen-associated PgPR2, PgPR5, PgPR10 and PgCAT genes improved significantly and rapidly in the plasma treated ginseng seedlings. In particular, mRNA expression levels of the PgPR2 and PgPR5 genes were significantly increased 5- and 3-fold, respectively, in the 10 min plasma treated seedlings. An increased expression level of abscisic acid signaling related genes (PgABF and PgPYL) means an increased resistance to stress, which in turn, could affect the enhanced antioxidant activity. Our results suggest that plasma can enhance the growth and increase the immunity of ginseng seedlings especially by improving its resistance to pathogens. Effective control of diseases using plasma technology could be one of the biggest factors that increase the yield of perennial medicinal plants such as ginseng.

The possibility of the activation of beneficial microorganisms using atmospheric plasma has rarely been examined. The application of plant growth-promoting bacteria (PGPB) as a microbial fertilizer to the field requires consistent efficiency. To overcome the limitations of PGPB as a microbial fertilizer, we investigated the potential of μ-dielectric barrier discharge plasma to increase the bacterial activity of a PGPB, Klebsiella pneumoniae, KW7-S06. Bacterial proliferation and vitality increased after plasma treatment. The infrared band of the observed Fourier transform infrared spectroscopy spectrum showed no change in the plasma-treated bacteria and untreated bacterial cell membranes. These results demonstrate that cells are not damaged by oxidative stress caused by plasma treatment. The density of bacteria adhering to the surface of rice seed inoculated with plasma-treated bacteria was higher than untreated bacteria. The germination rates of rice and barley seeds inoculated were significantly increased, and the growth of rice plants was also improved. The level of salicylic acid hormone was higher in rice plants infected with plasma-treated than with untreated bacteria. Our results demonstrated that plasma can accelerate bacterial growth and vitality, and the increased bacteria improved the adhesion of plant seed surfaces and elevated the level of phytohormones, leading to the enhancement of plant growth and tolerance to disease.