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Julak, J. Non-thermal plasma for decontamination of cereals: An overview. Encyclopedia. Available online: https://encyclopedia.pub/entry/17265 (accessed on 01 December 2023).
Julak J. Non-thermal plasma for decontamination of cereals: An overview. Encyclopedia. Available at: https://encyclopedia.pub/entry/17265. Accessed December 01, 2023.
Julak, Jaroslav. "Non-thermal plasma for decontamination of cereals: An overview" Encyclopedia, https://encyclopedia.pub/entry/17265 (accessed December 01, 2023).
Julak, J.(2021, December 17). Non-thermal plasma for decontamination of cereals: An overview. In Encyclopedia. https://encyclopedia.pub/entry/17265
Julak, Jaroslav. "Non-thermal plasma for decontamination of cereals: An overview." Encyclopedia. Web. 17 December, 2021.
Non-thermal plasma for decontamination of cereals: An overview
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This entry is adapted from the peer-reviewed paper 10.3390/foods10122927

Cereals may carry microbial contamination that is undesirable to the consumer or to the next generation of plants. Currently, non-thermal plasma (NTP) is considered a new and safe microbicidal agent without adverse side effects. Here, we summarise the impact of NTP on various cereals and related products. The beneficial effects of NTP have been indisputably demonstrated and promise potential practical applications in decontamination and disinfection.

food contamination cereal microbicidal effects

1. Wheat

The largest share of studies deal with this crop. Wheat is an important, tradable commodity. Its use is versatile, from direct feeding to animals, through the production of flour, to the production of ethanol. Microbiological protection of wheat grain is important both in the field and in grain processing.
Thomas-Popo et al. [1] reported the inactivation of both artificial and natural contamination of wheat grains. For artificial contamination by E. coli and Salmonella enterica, the total cfu decreased for the initial cca 7 log10 by 3–4 log10 after 20 min of plasma treatment. For natural contamination, the decrease in total cfu of mesophiles, psychrotrophs and Enterobacteriaceae after 20 min of treatment was almost 1, more than 2 and 1.4 log10, respectively. On the contrary, the yeast and molds were completely destroyed after only 10 min.
The following two related works [2][3] reported the inactivation of bacterial endospores of Bacillus amyloliquefaciens and Geobacillus stearothermophilus in wheat grains. While in the first case, the total cfu of B. amyloliquefaciens was reduced by 2 log10 from initial 106 cfu/g after 30 s, using the other source of NTP in the second case led to the 0.8 log10 and 3 log10 after 5 min and 60 min, respectively.
According to Zahoranova et al. [4], the concentration of epiphytic bacteria decreased from the initial cca 5 × 104 cfu/g by more than 1 log10 after 600 s. Epiphytic yeast was not detected and filamentous fungi were completely inactivated from the initial 600 cfu after 120 s of treatment. For artificial contaminations, the less resistant Fusarium nivale and F. culmorum were completely inhibited after 90 s, Trichothecium roseum after 180 s and Aspergillus flavus after 240 s; however, the most resistant, A. clavatus, was not totally inhibited after 300 s.
Selcuk et al. [5] used Aspergillus paraciticus and Penicillium spp. isolated from foods for artificial contamination in 5 × 106 cfu/g of grains and reported a reduction of more than 2 log10 after 30 min of treatment.
Hoppanova et al. [6] treated the grains inoculated with Fusarium culmorum spores in a concentration of 105 g grain–1 with plasma or in combination with 10% of Vitavax2000 fungicide. Complete inactivation occurred after 180 s and 60 s of plasma exposure alone and plasma exposure with fungicide, respectively.
Filatova et al. [7] used artificial contamination with Fusarium culmorum and natural contamination with Alternaria spp.; the infection levels decreased from 40% to 7% and from 4% to 2%, respectively. Inactivation of these fungi led to better germination, growth and grain yield.
In [8], the authors did not report the inactivation of fungal spores, but the resistant behavior of the treated samples to fungus attack, which decreased from 40% to 20% after 2 or 4 min of treatment.
In the work of Los et al. [9], the authors inactivated the natural microflora of mesophilic bacteria, yeasts and molds of 104–105 cfu/g. Maximal reductions of 1.5 log10 CFU/g for bacteria and 2.5 log10 CFU/g for fungi were achieved after 20 min of treatment. The following study [10] demonstrated that direct plasma exposure for 20 min significantly reduced the concentration of all pathogens. The reduction levels for the vegetative cells of Bacillus atrophaeus were higher than for all the fungal species tested, while the spores of B. atrophaeus were the most resistant. Repeating sublethal plasma treatment did not induce resistance to ACP in either B. atrophaeus or A. flavus spores.
Kordas et al., 2015 [11], reported the decrease in fungal contamination on grains from an initial cca 250 cfu per 100 grains to cca 25 cfu per 100 grains after 10 s of treatment.
Works related to wheat are the most numerous and show the possible applications of plasma in the widest range; as for the inactivation of the microorganism, so for increasing the resistance of crops. So far, all works are on a more or less laboratory scale.
Insects may also cause serious problems. The following four papers described the possible inactivation of Tribolium and other species in wheat by NTP.
Shahrzad et al. [12] reported the killing of Tribolium confusum and Ephestia kuehniella larvae in wheat from cca 300 to 0 in 20 s. Ratish Ramanan et al. [13] achieved the total elimination of eggs, larvae and adults of T. castaneum in wheat flour containing 10 eggs, 5 larvae or 5 adults in 15 min. In [14], 25 insects of T. castaneum and T. confusum per 30 g of wheat showed 100% mortality after 15 min of exposure. On the other hand, a very low mortality of T. castaneum of approximately 5% in wheat grains was reported in an otherwise chaotic paper [15].

2. Rice

Rice grains are often attacked by various microbiological pathogens [16][17]. Rice, as one of the most consumed cereals in the world, was the focus of the several following papers.
The first attempts were reported by Kang et al. [18], who treated with NTP rice grains infected by Fusarium fujikuroi mold spores that cause bakanae disease. They sprayed the spore suspension of 106 cfu/mL on rice plants. The harvested grains were then exposed to NTP, which caused the number of infected grains to decrease from 100% of the control set to 20% in grains exposed for 30 min.
The follow-up study [19] reported the successful effect of NTP on the control of two rice seed-borne diseases. It also examines the bakanae disease caused by Fusarium fujikuroi mold and the blight disease caused by Burkholderia plantarii bacteria. The bakanae disease severity index and the percentage of plants with symptoms were reduced to 18% and 8% after 10 min of exposure. The index of blight disease was reduced to 39%.
Natural rice contamination was also studied in the following two papers. Park et al. [20] reported the decontamination of natural contamination of brown rice grains by bacteria, yeasts and molds and reported a reduction of more than 1.5 log10 after 10 min of exposure. In [21], complete inactivation of natural contaminants (pathogenic fungi and other microorganisms) in a rice grain husk after 1 min of exposure was reported.
Inactivation of artificial contamination by Aspergillus oryzae, Penicillium digitatum spores and E. coli (initial concentrations of contaminants are not given) was reported in [22]. The surface of rice and lemons was sterilized after 20 min of irradiation with a combination of plasma and UV light.
Finally, an attempt to industrial application was reported in [23], where the development of a large-scale NTP generator followed by a UV-C treatment was described. To evaluate the efficacy of rice natural microorganisms decontamination, the number of natural bacteria was reduced from initial 5.6 log10 to 1 log10 cfu/g; for yeasts and molds, the reduction was from 3.7 log10 to 2 log10 cfu/g after 7 min of treatment.
Works related to rice present similar results as those for wheat; however, the attempt to industrially up-scale gives hope for further development and usage.

3. Maize

Maize is currently grown all over the world, with the United States being one of the world’s largest producers. Several papers devoted to corn decontamination start with Selcuk et al. [5], who used the Aspergillus parasiticus and Penicillium spp. food isolated for artificial contamination of 5 × 106 cfu/g of grains. They reported an approximate 70% reduction after 30 min of treatment. The paper [24] focused mainly on grain germination but also reported that, after 4 min of grain treatment, the inhibition of artificial contamination of grains by Fusarium verticillioides and F. graminearum was achieved so that all grains, contrary to the control, germinated without visible mold growth occurrence.
In [7], the authors used artificial contamination of Fusarium culmorum and natural contamination of Alternaria spp. The infection level decreased slightly from 76% to 66% and from 30% to 10%, respectively. This inactivation of fungi caused by grain treatment led to better germination, growth and grain yield.
In [25], the authors investigated the inhibition of the native microbiota and potentially dangerous pathogens (Aspergillus flavus, Alternaria alternata and Fusarium culmorum) in grains. Complete devitalization of the native microbiota was observed after 60 s of treatment for bacteria and 180 s for filamentous fungi. For artificial contaminations, total elimination from the initial 3–4 log10 (CFU/g) was observed after 60 s for F. culmorum and after 300 s for A. flavus and A. alternata.
In [26], the decrease in artificial infection with A. flavus and A. parasiticus from the initial 107 cfu/g by 5 log10 in 5 min was reported. The natural contamination of the fungi of the initial almost 104 cfu/g and of the aerobic mesophilic bacteria of the initial 103 cfu/g was totally inactivated after 3 min. Much lower inhibition was reported in [27], where the initial number of more than 200 fungi per 100 grains was reduced to 30% after 20 min of treatment.
Although all cited works are devoted to the fungi only, it can be assumed that, for other microorganisms, the decontamination efficiency will be comparable to previous crops.

4. Barley

It is one of the oldest cereals in the world and is geographically widespread. Today, most barley grown, especially winter barley, is used for feed purposes. Barley is an important feed grain for many countries, especially for those that are not suitable for maize production. Barley also received attention for NTP decontamination.
In [28], the concentration of artificial contamination with Aspergillus niger and Penicillium verrucosum in the total mold count of more than 5 log spores/g grains was reduced by 2.5–3 log. Furthermore, the use of air plasma also resulted in a decrease in ochratoxin A concentration from 56 (untreated) to 20 ng/g after 3 min.
The two previously mentioned works also deal with barley. Selcuk et al. [5] used Aspergillus parasiticus and Penicillium spp. isolated from foods for artificial contamination at 5.006 cfu/g of grains and reported a reduction of more than 1 log10 after 30 min of treatment. Hoppanová et al. [6] treated grains inoculated with Fusarium culmorum spores in a concentration of 105 g grain−1 with plasma or in combination with 10% Vitavax2000 fungicide. Complete inactivation occurred after 120 s and 60 s of plasma exposure alone and plasma exposure with fungicide, respectively.
The paper [24] is focused mainly on the germination of grains. They reported inhibition of artificial contamination of grains by Fusarium verticillioides and F. graminearum after 4 min of treatment, insomuch as all grains germinated without visible mold growth as opposed to the control. In the work [9], the authors inactivated both native microflora and artificial contamination. For the natural microflora of mesophilic bacteria, yeasts and molds of 104–105 cfu/g, maximum reductions of 1.5 log10 CFU/g for bacteria and 2.5 log10 CFU/g for fungi were achieved after 20 min of treatment. For artificial contamination, a total reduction of more than 3 log10 was observed after 20 min of exposure for E. coli, Bacillus atrophaeus vegetative cells and Penicillium verrucosum spores, while the reduction for the endospores of B. atrophaeus reached only 2.4 log10 CFU/g.
Much weaker inhibition was reported in [27], where the initial number of more than 200 fungi per 100 grains was reduced by up to 20% after 20 min of treatment.
In the work [29], unusual plasma-processed air (PPA) was used for inactivation of B. atrophaeus (DSM 675) endospores on barley grains, where gas flows from the active plasma to the incubation bottles. The number of spores was reduced from the initial concentration of ~106 CFU/per 10 g by 3.00 ± 0.33 log10 after 3 min of exposure.
Obtained results are again comparable with other crops, but the last cited work suggests the possibility of using PPA, which could markedly simplify the whole operating process and the transformation to real processing.

5. Miscellaneous

The following paper [5], devoted to wheat, oat, corn, rye and other, used the Aspergillus parasiticus and Penicillium spp., isolated from food, for artificial contamination of seeds. The initial concentration of 5 × 106 cfu/g of grains was reduced by more than 1 log10 after 30 min of treatment for oat and approximately by 80% for rye.

References

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  2. Butscher, D.; Zimmermann, D.; Schuppler, M.; Von Rohr, P.R. Plasma inactivation of bacterial endospores on wheat grains and polymeric model substrates in a dielectric barrier discharge. Food Control 2016, 60, 636–645.
  3. Butscher, D.; Schlup, T.; Roth, C.; Müller-Fischer, N.; Gantenbein-Demarchi, C.; Von Rohr, P.R. Inactivation of microorganisms on granular materials: Reduction of Bacillus amyloliquefaciens endospores on wheat grains in a low pressure plasma circulating fluidized bed reactor. J. Food Eng. 2015, 159, 48–56.
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