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Plasma processing: a sustainable technology in agri-food processing

Anbarasan Rajan a, Bhavadharini Boopathy b, Mahendran Radhakrishnan *a, Lakshminarayana Rao b, Oliver K. Schlüter c and Brijesh K. Tiwari d
aCentre of Excellence in Non-Thermal Processing, National Institute of Food Technology, Entrepreneurship and Management, Thanjavur-613005, Tamil Nadu, India. E-mail:
bCenter for Sustainable Technologies, India Indian Institute of Science, Bangalore-560012, Karnataka, India
cQuality and Safety of Food and Feed, Leibniz Institute for Agricultural Engineering and Bioeconomy (ATB), Potsdam, Germany
dTeagasc Food Research Centre, Ashtown, Ireland

Received 11th September 2022 , Accepted 25th November 2022

First published on 3rd January 2023


Globalization and rapid urbanization have led to tremendous improvement in the agriculture and food processing sectors to fulfil the food demands. In this context, managing food product safety and quality throughout the agri-food chain (pre-harvest to post-harvest) becomes vital to avoid food spoilage and increase production. Numerous innovative interventions have been investigated to achieve these goals; however, no single technology can be applied at all processing stages and may require different technologies. Nevertheless, cold plasma is a multifaceted solution for most pre and post-harvest issues, including soil/water contamination, microbial spoilage, insect infestation, and prolonged seed dormancy. In addition, the recent applications of plasma to food shape transformation is an evidence of the versatility of this technique in agri-food processing. Advantages, such as on-site production, residue, and toxic-free treatment, make the plasma process more sustainable. Reactive species, UV photons, and electrons are this plasma treatment's major compounds, giving them the peculiar and unique property to tackle most pre and post-harvest challenges. This review provides comprehensive possibilities for utilizing plasma technology throughout the agri-food chain. Various plasma systems have been developed, but their potential is limited to the lab scale. Research on large-scale applications can utilize cold plasma in future.

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Anbarasan Rajan

Anbarasan is pursuing his PhD in Food Process Engineering and also working as a Senior Research Fellow at the National Institute of Food Technology, Entrepreneurship and Management – Thanjavur. Presently, he is working on projects titled “cold plasma applications in the post-harvest processing of soybean seeds” and “development of process for production of coconut sap (neera) sugar granules and crystals (Govt. aided project)”. He has authored and co-authored 13 peer-reviewed international journal articles.

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Bhavadharini Boopathy

Bhavadharini has completed her master's degree in Food Process Engineering from National Institute of Food Technology, Entrepreneurship and Management, Thanjavur. Currently, she is working on the generation, characterization, and applications of plasma activated water at the Centre for Sustainable Technologies, Indian Institute of Science, Bangalore, as Project Assistant.

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Mahendran Radhakrishnan

Dr R. Mahendran completed his PhD in Food Process Engineering from Tamil Nadu Agricultural University, Coimbatore, India. Presently, he is working as an Associate Professor at NIFTEM-Thanjavur and heading the Centre of Excellence in Non-Thermal Processing. Mahendran's research focus includes cold plasma applications on agri-food, 4D foods and novel nonthermal processing on food systems. He is also a fellow of the Institute of Food Science and Technology and a Fellow of the Institution of Engineers (India). Mahendran has over 100 publications and filed 3 patents. He serves as a guest editor in the Journal of Food Processing and Preservation, Frontiers in SFP, and Journal of Food Process Engineering.

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Lakshminarayana Rao

Lakshminarayana Rao received his PhD degree from the Department of Chemical Engineering, McGill University, Canada for his thesis on the development of superior plasma torch electrodes. He is currently working as an Associate Professor with the Centre for Sustainable Technologies, Indian Institute of Science, Bengaluru, India. Over the past 13 years, he has worked in various areas of plasma, developing technologies in both thermal and non-thermal plasma. His current research is on the generation and characterization of plasma-activated water and its applications. He has 6 granted U.S. patents, authored over 40 peer-reviewed articles, 35 conference proceedings, and 2 book chapters.

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Oliver K. Schlüter

Professor Oliver Schlüter is a spokesman of the ATB program area “healthy foods”, vice-head of the Department of Horticultural Engineering, and head of the ATB microbiology lab. His research work focuses on emerging technologies in primary food production (including edible insects), minimal processing of food, and non-destructive quality and safety monitoring. He has over 150 publications and 350 conference papers. He is the chair of the EFFoST Standing Committee on career development and education, chair of the Technical Board of CIGR Section VI: bioprocesses, and editorial board member of Innovative Food Science and Emerging Technologies and the Journal of Insects as Food and Feed.

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Brijesh K. Tiwari

Professor Brijesh K. Tiwari is a Principal Research Officer at Teagasc and Professor (Adjunct) at the University College Dublin. He obtained his PhD from the UCD School of Biosystems and Food Engineering. His current research investigates the application of ultrasonic and plasma technologies on biological materials and the role they play in modulating cellular activities. Further, he also analyses the impact of process control parameters and physicochemical characteristics of these technologies on biological materials. He has over 250 peer-reviewed research publications. Dr Tiwari is also a fellow of the Institute of Food Science & Technology (UK) and Royal Society of Chemistry (UK).

1. Introduction

Agriculture and food processing sectors need progressive growth in production and value addition to feed the increasing world population. Nevertheless, the required food production has to be achieved with the available resources without compromising the product quality. Thus, we need a technology that could bring progressive growth in food supply and address food security and quality-related issues. In agri-food processing, irrigation and soil preparation are the two significant steps; any issues associated with these steps affect the entire supply chain and reduce product quality and yield. However, due to industrialization and urbanization, effluents are continuously introduced into environment and contaminate soil and water with heavy metals and cause phytotoxicity.1,2 Further, medical and related waste containing antimicrobial contaminants also affect plant growth based on their type and nature.3 Therefore, removing these contaminants from soil and water before the farming stage could increase agricultural yield and food safety.4–7 Nevertheless, the seed germination rate and adaptability to different environments also affect crop yield. In this regard, selecting a suitable seed treatment would also increase seed tolerance against salinity and drought stress.8,9 However, the real challenge arises at the post-harvesting stage, where the harvested fresh edible agricultural produce needs to be treated to avoid quality deterioration concerning microbes,10–12 enzymes,13 insects, and their secondary toxic contaminants.14 Furthermore, in later stages, most agricultural products reach industries where they will undergo various unit operations before they are stored and distributed to consumers.15 However, these product rheological properties, microbial/pest safety, functional properties, extract quality, and quantity would be low until these products are treated with suitable processing methods.16–18 Hence, while selecting the processing method/treatment, we should focus on its ability to address all these issues, irrespective of the product physical nature and intended final use.

Regarding processing, non-thermal technologies are preferred over thermal technologies to avoid undesirable product quality changes. In food processing, non-thermal treatments such as pulsed light, ultrasound, UV, ozone, pulsed electric field, and high-pressure processing technologies are considered alternatives for thermal treatments. As the name suggests, these non-thermal food processing techniques are performed nearly at room temperature to produce safe, nutritive products and retain freshness without inducing thermal damage in the product,19–21 and cold plasma technology is also one among them. Plasma is created by supplying high energy to gas molecules. The supplied energy dissociates molecules into atoms, and a further increase in the energy breaks those gas atoms into wholly or partially charged ions. Irving Langmuir coined the name plasma for this ionized gas in the early 20th century.22 It is the fourth state of matter and can be generated under atmospheric or vacuum conditions.23 Photons, electrons, ions, atoms, free radicals, and excited or unexcited molecules are present in this state. Based on energy given to the plasma system and energy transferred to the molecules in plasma, the plasma is classified as equilibrium (thermal) and non-equilibrium (low temperature) plasma.24 One of the critical advantages of this novel technology is that it can be used for both constructive and destructive applications. Constructive approaches include functional modification,25 seed germination, plant growth,26 and quality improvement of extracts,27 while destructive applications include decontamination,28 disinfestation,29 and pesticide degradation.30 This review explains up-to-date research findings of plasma-assisted agri-food processing studies and the mechanisms involved in each process. Further, the article focuses on pre and post-harvest plasma applications specific to the commodities obtained from different plant parts (leaves, roots, fruits, vegetables, nuts, and spices).

2. Mechanism of plasma in various applications of agri-food processing

2.1. Microbial destruction and inactivation mechanism

Though various plasma-chemical reactions are initiated during plasma generation, O3, O+, OH, and H2O2 are the primary species responsible for microbial inactivation.31 These RS breaks peptidoglycan (PG) bonds of the cell wall and causes destruction wherein the PG structural bonds like C–O, C–N, and C–C are destroyed by reactive oxygen species.32 It is also suggested that the intense bombardment of the radicals causes surface lesions leading to etching, severely affecting Gram-negative bacteria.33

Firstly, the RS oxidizes lipids, enzymes, and cytoskeletal proteins in the cell membrane and damages the cell wall. In comparison, DNA damage is caused by high-energy UV photons released from excited atoms or molecules at 220 to 280 nm. They form thymine dimers and break plasmid DNA's single and double strands that affect cell replication and other functions.34,35 In addition, RS in plasma increases the solution's acidity and further inactivates microbes.36

2.2. Chemical and toxin removal

Interaction of pesticides with plasma RS results in the oxidation of these chemical compounds and degrades them. Apart from oxidation, other intermediate processes also occur during the degradation and finally end in mineralization.30,37 Pesticide and other organic or inorganic pollutant removal follows different pathways, where pollutants are converted into simple non-toxic components such as CO2, H2O, inorganic carbons, and other organic or inorganic components based on treatment time and reactive species nature. Oxidation, isomerization, and H2O/CO2 removal are some essential reactions that take place during this conversion.38

2.3. Insect mortality

The nerve toxin effect caused by plasma ROS on the neuromuscular systems of insects induces mortality. These species enter through the respiratory pathway of insects and affect the insect behaviour and immune and circulatory systems.39–41 Mechanisms for insect mortality vary concerning the insect life stages. Ramanan et al.29 reported the different mechanisms involved in the insect's mortality of egg, larva, and adult stages.
2.3.1. Eggs. Mechanisms, such as disruption of embryogenesis due to ROS, eggshell cell membrane disruption due to electrostatic accumulation, and delay or lack of hatching due to anoxic or hypoxic conditions, are the significant causes of mortality in eggs.
2.3.2. Larva. The ROS creates severe oxidative stress in the larva and damages the cuticle and epidermis of the insect. In addition, oxidation also leads to the melanization of hemolymph and deformation of the body, causing the death of larvae.
2.3.3. Adult. Electrostatic excitation on insects' membranes due to high voltage discharge affects the nerves and neuromuscular system, which causes anoxia conditions (primary mode) and leads to insect death. Another mode of action (secondary mode) is the destruction of C–H bonds present in the cuticular lipid layer of the insect's surface by ROS. Damages in the insect surface layer cause dehydration and death.

2.4. Seed germination

Many factors influence the germination rate of plasma-treated seeds. However, the exact reasons are not clear. However, a few acceptable reasons are etching, the opening of seed coats, increased seed wettability, deposition of small bioactive molecules on the seed coat, and decontamination of seed surface microbes.42,43

2.5. Functionality modification

Plasma treatment alters the nature of food constituents and their property. ROS reacts with sulfur and aromatic amino acids in the protein, resulting in oxidation. Sometimes oxidation (carbonylation) also assists enzyme inactivation. In addition, the treatment also changes the secondary structure of proteins and alters their functionality. Further, ROS attacks methyl groups near the double bond regions and oxidizes them in lipids. Meanwhile, carbohydrates, aldose, and ketose form formic acids during oxidation.38

This review discusses the applications of plasma treatment on agri-food processing in various stages to enhance the agricultural production rate, reduce spoilage & waste, and improve the quantity and quality of the final product.

3. Plasma applications in pre-harvest stages

Plasma is used in different stages of pre and post-harvest processing of the agri-food chain (Fig. 1), where the pre-harvest operation includes soil irrigation, soil preparation, sowing, planting, etc., and continues till harvesting (Fig. 2). Each stage has a noticeable impact on final product quality and yield. Hence, plasma technology has been used to obtain the desirable change during pre and post-harvest operations.
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Fig. 1 Application of plasma in different stages of agri-food chain.

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Fig. 2 Plasma application in pre-harvest stages of agri-food chain.

3.1. Plasma treatment of water

Water is one of the vital sources for agricultural practices and is usually contaminated by microbes, heavy metals, and pharmaceutical compounds.44–46 Removing these contaminants can improve agricultural product quality and overall production. The reactive species present in plasma can oxidize and remove the contaminants from water or effluent.47 Furthermore, plasma-activated water (PAW) can also be used to suppress microbial activity48 and to remove chemical contaminants from water.49 The interaction of reactive species with water during plasma treatment and PAW production varies based on the nature of RS (Fig. 3). The degradation of organic and inorganic compounds is shown in eqn (1).
Chemical compounds + RS → simple carbon/nitrogen compound + CO2 + H2O(1)

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Fig. 3 Effect of plasma treatment on seeds.

Though degradation follows different pathways, the end product will likely fall under any of those components mentioned in eqn (1). For instance, pesticide degradation (diuron) by OH species oxidation releases CO2 and H2O after producing organic acids.50 Similarly, after producing different intermediate components, dimethoate degraded as non-toxic PO43− during plasma treatment.51 In addition to oxidation, mineralization, removal of alkyl groups, CO2, H2O, halogens, and isomerization could take place during the degradation process. This degradation mechanism is often similar to colourant and off-odour-producing compounds.38

Plasma treatment has the potential to degrade individual dyes and their mixtures (i.e., alizarin yellow + orange II + methylene blue) from water.52 Iervolino et al.53 investigated the effect of plasma on different water pollutants by varying treatment time, power, and gas and identified the lower resistance of methylene blue (MB) and ceftriaxone against plasma (degraded within 5 min). While phenol (15 min), paracetamol (15 min), and caffeine (25 min) took more time for degradation. During this process, MB was almost entirely converted into CO2. Instead of focusing on degradation alone, Tampieri et al.54 studied the degradation and mineralization efficiency of plasma-treated rhodamine B, phenol, and metolachlor. For low pollutant concentrations, the highest mineralization efficiency of 59% and 20% was obtained for phenol and metolachlor in 30 min, respectively. To understand the combined effect of plasma with any other non-thermal method, Bradu et al.47 combined ozone treatment with plasma to remove organic pollutants from water. Ozone combined with plasma reduced the treatment time by half to remove 50% chlorophenoxyacetic herbicide (2,4-D) with doubled removal reaction rate. This study explains the role of OH species on 2,4-D oxidation and the importance of catalytic ozonation for faster pollutant removal and mineralization. Similar to other organic pollutants, Sarangapani et al.55 reported pesticide removal concerning power and treatment time. Higher power and exposure time increased pesticide degradation into simple chemical groups due to the oxidization of active O3 and OH species. Other than organic pollutants and pesticides, microbes can also contaminate water. So, Pavlovich et al.56 reported the effect of plasma on Escherichia coli present in water and found that ozone concentration alone had a higher correlation with the antimicrobial activity of the treatment than pH and other species-generated acids. This study concluded that 5 s plasma treatment was enough to reduce E. coli to a not detectable level and suggested the importance of mixing on the antimicrobial effect of plasma species. In order to evaluate the effect of plasma on the removal of different pollutant mixes, Hijosa-Valsero et al.57 conducted a study where atrazine, chlorfenvinfos, 2,4-dibromophenol, and lindane pollutants were mixed with water and treated in DBD batch reactor (R1) and co-axial thin-film DBD reactor (R2). Results showed a decrease in the degradation efficiency of chemicals when their concentration increased, and even after pollutant degradation, byproduct removal took more treatment time. Similarly, pentoxifylline and its intermediates removal were analyzed by Magureanu et al.,45 and the author reported that removal was high at a lower concentration of pollutant (100 mg L−1), higher power, high pulse rate, and higher frequency. Intermediate components produced during this degradation were entirely removed after 120 min. Industrial wastewater with higher microbial contamination might also affect the water quality of agri-farms. So, industrial wastewater needs decontamination studies, and Rowan et al.46 conducted a study on poultry wash water to investigate microbial decontamination. In this study, other than RS and UV photons, nitric and carbonic acids formed during plasma treatment resulted in pH reduction (acidified) that helped in microbial destruction. These components resulted in the microbial reduction of 8 logs (less than or equal) to complete removal after 30 s of treatment, and the sensitivity of treatments varied for different microbes (Bacillus cereus endospores < Listeria monocytogenes < Salmonella enterica Typhimurium < Salmonella enterica Enteritidis < E. coli < Campylobacter jejuni < Campylobacter coli).

Apart from using plasma units for treating different water samples, it is also possible to carry the plasma species in water by which the treatments can be done. Ten Bosch et al.40 reported the effect of plasma-treated water on insect mortality (Mealybugs). Though Plasma Treated Water (PTW) and Classically Acidified Water (CAW) had no significant difference, the CAW mortality rate was higher than PTW at the same pH due to the complexity of RS present in PTW. This PTW can also be used as an alternative for sodium nitrite in sausage curing with similar sensory, microbial, and peroxide values as that of control.58 Even though plasma-activated water (PAW) possesses antimicrobial properties, its effectiveness reduces during the storage period. Traylor et al.59 reported that PAW prepared with 15 min and 3 h dosage intervals were found to have a similar antimicrobial effect of more than 5 logs. However, after 30 min, the effectiveness of PAW prepared with 15 min exposure time reduced from 5.6 logs to 2.4 logs, while the other was stable for 2 days. Similarly, the lethal effect of stable RS on Hafnia alvei was reported, and among NO, NO2, and H2O2 species, acidification of PAW by NO2 species acted as a primary factor in microbial reduction.60 Kamgang‐Youbi et al.48 reported the influence of pH reduction and substrate absence on saccharomyces destruction and found that due to its cell size, initial population, and adhesiveness of substrate surface, their resistance was higher against plasma treatment.

Table 1 shows the different plasma generation methods and significant results achieved in water treatments. However, further research must be explored to characterize the degradation process's end product. Since the future industrialization of PAW usage may produce a massive quantity of used PAW, there is a need for research on its recyclability and toxicity.

Table 1 Plasma applications in pre-harvest stages
Sample Plasma characteristics Significant results Reference
(1) Plasma treatment on water
(a) Effluent decontamination
Dimethyl phthalate Self-pulsing discharge (SPD) and multipin corona discharge (MCD) MCD was more effective in the degradation of dimethyl phthalate than SPD 184
Input – 30 kV, 12 mA
Treatment time – 5 to 30 min
Methylene blue, ceftriaxone, phenol, paracetamol and caffeine DBD reactor Degradation and mineralization were higher when O2 was used at 20 kV with a 0.18 nL min−1 flow rate 53
Gas – air or oxygen (0.09 to 0.36 nL min−1)
• Input power – 12 to 38 kV
Treatment time – 5 to 25 min
Rhodamine B, phenol and metolachlor Atmospheric plasma reactor In 5 × 10−4 M phenol solution, 10% residue remained after 30 min. Low concentrations were removed completely 54
Gas – air (flow rate – 100 mL min−1) Rhodamine B and metolachlor were removed in 15 and 20 min, respectively
Input – 5.9 ± 0.7 W
Treatment time – up to 30 min
Chlorophenoxyacetic herbicide (2,4-D) Pulsed corona discharge with zonation reactor Plasma with ozone provides >99.8% removal of 2,4-D in 30 min of treatment time 47
• Gas – oxygen (0.3 L min−1)
• Input power – 11 to 31 W
• Treatment time – up to 60 min
Pesticide (dichlorvos, malathion, endosulfan) DBD plasma reactor Degradation was maximum for dichlorvos (78.98%), malathion (69.62%), and endosulfan (57.71) at 80 kVRMS after 8 min 185
• Gas – atmospheric pressure plasma
• Input power – 60, 70, and 80 kVRMS
• Treatment time – 0, 2, 4, 6, and 8 min
Bacterial culture mixed water DBD plasma E. coli – vortex (5 s) increases the microbial reduction up to 5 log in 120 s at 0.2 W cm−2 56
• Gas – ambient air Increased power has less effect on microbial reduction
• Input power – 0.02 to 0.4 W cm−2
• Treatment time – 30 to 300 s
Polluted water (atrazine, chlorfenvinfos, 2,4-dibromophenol, and lindane) DBD batch reactor (R1) and co-axial thin-film DBD reactor (R2) R 2 was found to be efficient more efficient. For example, 15 min of treatment degraded the pollutant while the byproducts degradation was low 57
• Gas – helium at atmospheric pressure (5 L min−1)
• Input power – 30 W
• Treatment time – 0 to 15 min
Pentoxifylline (in water) DBD plasma with coaxial configuration 92.5% pentoxifylline reduction after 90 min treatment 45
• Gas – oxygen (flow rate – 600 SCCM) Chemical reduction increases with increased O3 consumption
• Input power – 1.2 W
• Treatment time – up to 120 min
Poultry wash water Pulsed-plasma gas-discharge Campylobacter and Salmonella contamination was entirely removed to not detectable level 46
• Gas – sulfur hexafluoride (SF6)/air (flow rate – 10 L min−1)
• Input power – 23.5 kV
• Treatment time – 30 s
Methylene blue (MB) water DBD plasma with coaxial configuration Plasma treatment of 30 min with O2 gas removed 95% MB dye 186
• Gas – oxygen and air (300, 600, and 900 SSCM) Ozone concentration influences the removal rate
• Input power – up to 1 W
• Treatment time – up to 90 min
(b) PAW treatment
Plasma treated water (PTW) DBD plasma Mortality rate of PTW, tap water, and reference sample after 24 h was around 85, 8 and 2% for mealybug 40
Gas – air
Input – 11 W
Electrode gap – 3 mm
Treatment time – 1 to 10 min
PTW Surface dielectric barrier discharge PTW used sausage with nitrite (782 ppm) that can provide a curing effect 58
• Gas – atmospheric air
• Input power – 3.14 W
• Treatment time – 120 min
Plasma-activated water (PAW) Indirect DBD plasma Anti-microbial declined during storage 59
• Gas – atmospheric air After seven days, 2.4 logs and no reduction were observed in 3 h and 15 min treated PAW, respectively
• Input power – 5 W
• Treatment time – 20 min
PAW Atmospheric non-thermal quenched plasma Acidification by nitrites in PAW produces a lethal effect on Hafnia alvei 60
• Gas – atmospheric air (flow rate – 550 L h−1)
• Treatment time – 5 min
PAW Atmospheric pressure gliding arc plasma S. cerevisiae – 3 log reduction after 30 min 48
• Gas – atmospheric air Without solid substrate H. alvei, Leuc. mesenteroides, Staph. Epidermidis were eradicated after 30 min
• Input power – 1.2 W
• Treatment time – 5 min
[thin space (1/6-em)]
(2) Plasma treatment on soil remediation
p-Nitrophenol contaminated soil Novel spray-type coaxial cylindrical dielectric barrier discharge 54.2% of PNP was degraded after 50 s discharge treatment 187
Input – 0–30 kV, 200 Hz, carrier gas – 20% O2 + 80% N2
Fluorene Needle-plate pulsed corona discharge plasma 78.7% fluorene degradation was achieved after 60 min 64
Input – 30 kV When washing is done to remove oxidation products, 99% degradation achieved in 45 min
Treatment time – 60 min
Electrode gap – 20 mm
Non-aqueous phase liquid (NAPL)-mixed soils Ex situ DBD plasma cylinder-to-plane reactor High energy density eradicates NAPL irrespective of its initial concentration of it 65
• Gas – atmospheric air (flow rate – 1 L min−1)
• Input power ∼25 W (energy density 675 to 10[thin space (1/6-em)]125 J g−1)
• Treatment time – 2.5 to 33 min
Dye-polluted soil (acid scarlet GR) Plane-to-plane DBD plasma Degradation efficiency was 93% at 19.6 kV and also at 300 Hz (17.6 kV) after 25 min treatment 66
• Gas – atmospheric air
• Input power – 15.6 to 19.6 kV (3.51 to 5.72 W)
• Treatment time – 0, 5, 10, 15, 20, and 25 min
p-Nitrophenol polluted soil (PNP) Pulsed discharge plasma-TiO2 catalytic (PDPTC) Soil depth increase reduces PNP degradation 70
• Gas – atmospheric air (flow rate – 0.5 L min−1) Quartz sand containing PNP degraded more than sand and sandy soil. Clay soil has the lowest degradation rate
• Input power – 23 kV
• Treatment time – 45 min
Contaminated soil Ozonizer combined DBD plasma with TiO2-based surface discharge The microbial load of ozone treated soil reduced from 5.7 × 106 to 1.7 × 102 72
• Gas – oxygen (flow rate – 0.5 dm3 min−1)
• Input power – 23 kV
• Treatment time – 45 min
Chloramphenicol polluted soil Atmospheric pressure DBD plasma The presence of oxygen improves the degradation compared to air 67
• Gas – oxygen, ozone, air, argon, nitrogen (flow rate – 0.15 to 1.5 L min−1) Higher input voltage and flow rate increase the degradation efficiency
• Input power – 16.4, 18.4, and 20.4 kV
• Treatment time – 0, 5, 10, 15, 20, and 25 min
p-Nitrophenol polluted soil (PNP) Pulsed discharge plasma – TiO2 catalytic (PDPTC) Plasma alone caused 78.1% of PNP degradation, while PDPTC caused 88.8% degradation at 20 kV (10 min) 68
• Gas – atmospheric air (flow rate – 0.5 L min−1)
• Input power – 20 kV
• Treatment time – 30 min
Pentachlorophenol (PCP) in soil Pulsed corona discharge plasma An increase in voltage from 12 to 18 kV increased PCP degradation from 64 to 90% (60 min) 69
• Gas – oxygen, air, argon, nitrogen (flow rate – up to 8 L min−1)
• Input power – 12 to 18 kV
• Treatment time – up to 60 min
Pentachlorophenol (PCP) Pulsed corona discharge plasma (PCDP) 11.6, 13.6, 15.6 kV, and 17.6 kV degraded PCP up to 62, 77, 83, and 87%, respectively, after 60 min 71
• Gas – air (flow rate – 3 L min−1)
• Input power – 11.6 to 17.6 kV
• Treatment time – up to 60 min
Kerosene polluted soil Cylinder-to-plane DBD plasma An energy level of 960 J g−1 degrades the kerosene by 90% 188
• Gas – air (flow rate – 1 L min−1)
• Input power – 15 to 20 kV
• Treatment time – up to 60 min
Contaminated soil DBD plasma Ozone produced during the process reduced the microbial count and pH 62
• Gas – oxygen and air (flow rate – 1, 2 L min−1) Increased the mineral content of the soil
• Input power – up to 30 W
• Treatment time – 60 min
[thin space (1/6-em)]
(3) Plasma treatment of seeds/plants
Rice seedling Non-equilibrium atmospheric pressure plasma 15% increase in grain yield when the plant was treated in direct plasma during the early period and no change in the later stage 189
Plasma activated ringer's lactate solution
Treatment period of solution – twice a week
Input – 60 Hz, 9 kV
Maize grains Diffuse coplanar surface barrier Wettability increased – the amount of water imbibition was higher 190
Input – 400 W with air, oxygen, and nitrogen 90 and more had a negative effect on growth and production indices
Treatment time – 0, 30, 60, 90, 120, 180, and 300 s
Buckwheat seeds Low-pressure radiofrequency system A shorter exposure time of up to 45 s was safer for the application 191
Input – 30 W The longest exposure of 120 s affected the fungal colonisation
Treatment time – 15, 30, 45. 60, 90, and 120 s
Carrot seed DBD 3 min had a great quantity of flavonoids, phenolics, and plant extracts had high radical scavenging activity 192
Input – 12.5 kV and 50 Hz
Treatment time – 1 to 4 min
Pea seeds (Pisum sativum L.) DCSBD plasma unit with a silver electrode and Al2O3 as the dielectric material DNA damage – 240 s treatment reduces the damage by 28 to 30.5%, 3.5%, and 21% for zeocin, distilled water, and hydrogen peroxide treated sample, respectively 90
• Gas – atmospheric air (flow rate – 3 L min−1)
• Input power – 400 W
• Treatment time – 60 to 300 s
Wheat DBD plasma with 3 mm electrode distance Plasma treatment of 15 and 90 s increased HSFA4A by six and four folds after 3 h in the root 91
Gas – Ar However, for the shoot after 6 h, a ten-fold increase was observed in the 30 s treatment
Input – 80 W
Treatment time – 15, 30, 60, and 120 s
Wheat Atmospheric DBD plasma After treatment, GP and GR in drought stress increased from 39.3 to 50.0% and 62.7 to 80.0%, respectively 92
Gas – dry air
Input – 13 kV
Treatment time – 4 min
Radish (Raphanus sativus) Double DBD reactor with plate-to-plate configuration for seed treatment Seed GR was 40, 60, and 100% for control, PAW-15 and PAW-30, respectively 42
Tomato (Solanum Lycopersicum) Gas – air (flow rate – 1 L min−1) In tap water (TW), growth was higher for control, P10, and P20 tomato plant than PAW watered plants
Sweet pepper (Capsicum annum) Input – 57 mJ per pulse (max) More growth in PAW-30 watered plants than TW
Treatment time – 10 (P10) and 20 (P20) min
Double DBD reactor with cylindrical configuration – water treatment
Gas – synthetic air (flow rate – 1 L min−1)
Input – 7 mJ per pulse (max)
Treatment time – 15 (PAW-15) and 30 (PAW-30) min
Rapeseed (Brassica napus L.) CDPJ generation system Germination was 7.7% higher after 1 min of treatment 86
• Gas – dry atmospheric air (2.5 m s−1) The microbial load of the treated sample was maintained lesser than the control sample (1–2 log reduction)
• Input power – direct current of 20 kV (1.5 A)
• Treatment time – 0 to 3 min
Wheat seed DBD plasma GP, GR, and GI increased in 4 min treatment from 62.5 to 77.5%, 88.0 to 95.3% and 36.7 to 41.0% compared to control 74
Gas – air (flow rate 1.5 L min−1)
Input – 1.5 W
Treatment time – 0, 1, 4, 7, 10 and 13 min
Broccoli seed (Brassica oleracea var. kialica plen.) CDPJ generation system Plasma reduced microbes in seed and sprout 75
• Gas – atmospheric air Less exposure increased germination rate, sprout length, and weight
• Input power – 20 kV DC
• Treatment time – 0 to 3 min
Peanut (Arachis hypogaea L.) seed Cold helium plasma After 15 s treatment at 80, 100, and 120 W, germination potential increased by 128, 128, and 150%. Finally, at 120 W germination rate was found to be maximum 78
• Gas – helium (150 Pa pressure)
• Input power – 60, 80, 100, 120, and 140 W
• Treatment time – 15 s
Wheat seeds (Triticum aestivum L. cv. Eva) DCSBD – cold atmospheric pressure plasma (CAPP) Water uptake increased with soaking time and dosage 76
• Gas – atmospheric air Bacteria and fungi load on seeds reduced with treatment time
• Input power – up to 100 W cm−3
• Treatment time – 10 to 600 s
Wheat seeds (Triticum aestivum) Atmospheric pressure surface discharge plasma reactor with glass plate Germination rate was similar for plasma treated (98%) and control (95%) wheat seeds 77
• Atmospheric air (flow rate – 1 L min−1)
• Input power – 2.7 W
• Treatment time – 5, 15 and 30 min
Peas (Pisum sativum ‘Salamanca’) CAPP – surface dielectric-barrier air-discharge 5 and 10 min CAPP treatment increased germination to 42 and 50%, respectively, after 24 h 82
• Atmospheric ambient air Germination rate reached a maximum (5 min treatment) of 65% after 48 h
• Input power – 9 kVpp
• Treatment time – 1 to 10 min
Bean (Phaseolus vulgaris) Cold radio frequency air plasma Water absorption increased in plasma treatment. hence, germination time reduce in plasma-treated samples 85
• Atmospheric air (pressure – 6.7 × 10−2 Pa)
• Input power – 20 W
• Treatment time – plasma treatment (2 min), vacuum treatment (3 min)
Pea seeds Coplanar surface discharge – low-temperature plasma Seed germination increased from 77.5 to 95% after 120 s treatment 84
• Input – 370 W
• Time – 60 to 600 s
Pre germinated rice (Oryza sativa L.) Plasma jet with quartz tube covered inner electrode Control – 97% germination 87
• Argon and oxygen Treated – 93 and 91% after 5 s at 10 W power with 8, 5 mm distances, respectively
• Input power – 10–14 W High γ-aminobutyric acid (GABA) content achieved in 96 h pre-germination
• Treatment time – 5 to 10 s
• Distance – 5, 8 mm
Soybean (Glycine max (L.) Merr) CDPJ generation system Maximum germination (G) potential, G rate, G index, and vigour index were observed after 80 W treatment 81
• Dry atmospheric air (2.5 m s−1) Water uptake increased by 14.03%
• Input power – 0 to 120 W
• Treatment time – 15 s
Tomato (Solanum Lycopersicum L.) seed Capacitively coupled plasma (CCP) Germination potential and rate increased by 8 and 11% 80
• Helium (150 pa) Bacterial wilt severity was reduced by 25% in the treated sample
• Input power – 80 W
• Treatment time – 15 s
Wheat (Triticum spp.) Cold helium plasma Germination potential and the rate increased by 6, and 6.7%, respectively, after 80 W treatment 26
• Helium (150 Pa pressure)
• Input power – 60, 80, 100 W
• Treatment time – 15 s
Poppy seed (P. somniferum L.) Panasonic – microwave generating RF plasma Germination was 115% in 3 min treatment 89
Input – 500 W Plasma treated seedlings were longer than the control seedlings
• Time – 180 to 5400 s
• Gas – O2 (50 mL min−1) and Ar (50 ml min−1)
Maize seed (Zea mays L.) DCSBD plasma treatment Plasma treatment for 60 s increased the root length, root fresh, and dry weight by 21, 10, and 14%, respectively 83
• Atmospheric air Soluble protein content in the root increased after 60 s treatment
• Input power – 370 W
• Treatment time – 60 and 120 s
Paulownia tomentosa seeds Glow discharge plasma treatment Air plasma treatment at 50 W, 100 W and 200 W produced maximum germination at 15, 5, and 1 min, respectively 88
• Air and argon (200 mTorr) When glass plates covered the seeds, germination was reduced
• Input power – 50, 100 and 200 W
• Treatment time – 1 to 40 min

3.2. Plasma treatment for soil remediation

Industrial and agricultural-related wastes such as heavy metals, pesticides, petroleum, and its products cause soil contamination, which could affect humans and the ecosystem.61 These contaminants' concentration in soil has to be reduced to avoid losses in production and cross-contamination in agricultural produces. Plasma technology has already gained popularity at the industrial level as an emerging technology for different treatments. Hence, this technology can be used in soil decontamination.62

In plasma assisted soil remediation, factors such as soil thickness and its type affect the treatment voltage, frequency, soil thickness, and air flow rate. The effectiveness of the method was identified against different types of soil (sandy soil and loam soil).63

Soil pollutant removal varies depending upon the nature of the soil as well as the pollutant. Zhan et al.64 studied the influence of treatment parameters such as time and washing, soil properties such as pH, moisture, and initial concentration of pollutants on fluorene removal from soil. The study showed a significant increase in fluorene degradation (from 33.8 to 57.9%) while reducing the soil moisture content from 20 to 0.6%. In addition, the efficiency increased from 60.6 to 71% when the pH of the soil rose from 3.0 to 9.0. Washing, increasing treatment time, and reducing initial pollutant concentration are reported to be critical factors for pollutant removal. Aggelopoulos et al.65 studied the non-aqueous phase liquid (NAPL)-mixed soil decontamination with high and low energy density plasma treatment to avoid this influence of initial pollutant concentration. When the ex situ DBD plasma treatment energy density was around 10[thin space (1/6-em)]000 J g per soil, NAPL was obliterated, irrespective of the initial concentration. However, low energy density was affected by its increased concentration. Lu et al.66 included the effect of frequency, gas flow, and input power in removing acid scarlet GR dye from the soil. It was found that an increase in DBD plasma input power (3.51 to 5.72 W), frequency (200–300 Hz), and gas flow rate (0.5–1.0 L min−1) improved dye degradation. At the same time, a higher air flow rate (1.5 L min−1) reduced efficiency and O3 and OH reactive species were reported as critical factors in this degradation. However, to find the effectiveness of different gas sources on degradation other than flow rate, Lou et al.67 used O2 and air for chloramphenicol degradation, where the efficiency of O2 (41%) was higher than air (26%). For better efficiency, optimum moisture content (≤10%) and Fe0 addition were needed, and that can increase reactive species and discharge channels, respectively. Similarly, Wang et al.68 used a pulsed discharge plasma-TiO2 catalytic (PDPTC) reactor to enhance the reactor performance for organic component removal. In this study, TDPTC (55.1%) removed higher organic components than the plasma reactor performed without TiO2 (42.9%).

For pentachlorophenol (PCP) degradation, Wang et al.69 used an optimum flow rate (3 L min−1) and a high O2 environment to increase the degradation rate. However, it was found that prolonged exposure of pollutants (4 to 96 h) to soil reduced the degradation by 13.4% as the pollutant gets into the soil granules deeply. So, apart from process parameters and other soil properties, the depth of the soil also decides the degradation efficiency. To understand this, Wang et al.70 studied p-nitrophenol (PNP) degradation at a different depth from the surface of the soil using plasma. From this study, it was found that an increase in soil depth reduces the degradation rate from 77.9 (0–2 mm depth) to 52.8% (10–12 mm depth), and the removal of PNP in moist clay soil (44.1%) was higher than dry soil (11.3%) with increased discharge voltage. Similarly, Wang et al.71 studied the effect of the size of the soil granule on the degradation of pentachlorophenol (PCP) and found higher degradation in 20 mesh size granular soil (87%) than in 10 mesh size granular soil (72%). In addition to granule size, the high pH of the soil also assists the PCP degradation. Unlike other studies, Stryczewska et al.72 focused on microbial decontamination and plasma-induced soil property change. Here, the soil pH and temperature changes occurred with changes in O3 concentration and treatment time, resulting in decontamination. A similar study was carried out with different electrode configurations, and this study reported the importance of screw or pyramid-shaped electrodes on higher O3 production. O3 and NO produced in this process altered the soil conductivity (34 to 79 mS m−1) and microbial population.62 Later, Redolfi et al. (2010) studied the RS oxidation effect on kerosene-mixed soil to analyze the pollutant residues in the plasma exhaust gas and its bioavailability. Results indicated that only a negligible amount of organic components came along with the gas outlet, while most of them were retained in the soil and did not convert from a solid to a gas state. Hence the bioavailability of kerosene byproducts increased in the soil.

Extended research is needed to understand the effect of plasma treatment on soil mineral content, residue toxicity, and their effects on plant growth and crop yield after treatment. Apart from the decontamination of soil, the effect of plasma on desirable organisms present in it also needs to be addressed.

3.3. Plasma treatment of seeds/plants

Improving germination rate and agricultural yield is the primary concern in increasing food production,73 and plasma treatment has the potential to improve these two aspects of agriculture processing by altering the germination rate (GR), germination potential (GP), vigor index (VI), water absorbance, contact angle, wettability, and growing tolerance of seed. At optimum process conditions, these changes can influence the overall production of food produce by favouring seed and plant growth.
3.3.1. Germination and plant growth. Since germination is a crucial factor in deciding seed quality, efforts were made to increase the germination behaviour of seeds through plasma technology. Li et al.74 reported an increasing trend in GP, GR, GI, and VI of wheat seeds up to the treatment time of 4 min, while the root and shoot lengths had a positive response during germination. Kim et al.75 treated broccoli seeds in a corona discharge plasma jet (CDPJ) to know the additional benefits of this treatment. They found microbial reduction on the seed surface along with increased germination rate and growth; meanwhile, the sensory and physicochemical parameters were unaffected. Different factors are involved in seed germination (Fig. 3), and water absorption is one among them, Zahoranová et al.76 highlighted this in a wheat seed germination study and found an increase in water absorption of each seed at 2 h (6.41 to 9.60 mg) and 8 h (12.53 to 16.07 mg) of soaking times with respect to the control. In addition to this, GR (21%), VI-I (28%), VI-II (36%), and surface microbial reduction (2 logs) also increased. Along with germination and water absorption, Dobrin et al.77 reported the average increase in root length (32.89 to 36.49 cm), sprout length (4.26 to 4.34 cm), root weight (0.78 to 1.06 g), and root to shoot ratio (0.88 to 1.2) of treated wheat seeds after 15 min treatment. Though most research shows a positive impact on germination and physical characteristics, morphological changes and yield of crops during the growth periods were unidentified. So, Ling et al.78 decided to work on peanut (Arachis hypogaea L.) seed with helium plasma and recorded the increase in yield, plant height, stem diameter, shoot and root dry weight, area, thickness, water, and nitrogen content of leaf at 120 W power level. Not only seeds but plants such as basil showed more growth in height, around 10 mm, than the control.27 Apart from plasma treatment, PAW also helped in the growth of seeds, and Sivachandiran42 reported about a 28% to 45% increase in the stem length of radish after PAW watering. For tomato and pepper plants, plasma and PAW combined treatment showed positive plant growth with healthy leaves. Similarly, a synergistic effect was observed in the germination of mung bean seeds when they were treated with plasma and PAW. The germination percentage reached up to 100% (from 94%) when plasma and PAW (CO2) were used; however, the PAW generated using O2 [PAW(O2)] inactivated more microorganisms than the other PAW.79

Apart from germination enhancement, plasma also has the ability to change the minerals, pigments, enzymes, and other nutritional compositions of the seeds. For example, in capacitively coupled plasma (CCP) treated tomato seeds, calcium (7.73%) and boron (11.53%) contents were increased;80 whereas in soybean, nutrient fractions such as soluble protein and soluble sugar and enzymes such as peroxidase (POD) and phenylalanine ammonia-lyase (PAL) activities increased after CDPJ treatment.81 In helium plasma-treated wheat (Triticum spp.), chlorophyll content increased by 9.8% more than that in the control.26 However, photosynthesis efficiency and flavonoid content in peas are reduced due to UV-C production.82 Similarly, time-dependent reduction of dehydrogenase (27%) and catalase activities (75%) were observed in the roots of germinated maize seeds.83 In comparison, structural changes were observed with increased indolyl acetic acid (13.7%) in peas.84 For beans, germination time (44 to 40 h) and exotesta contact angle were reduced after plasma treatment.85

In contrast to other research works, Puligundla et al.86 analyzed the negative impact of plasma over dosage on rapeseed germination and suggested the optimum treatment time for better germination since 1 min treatment showed better results in this study. However, the study on pre-germinated brown rice by Sookwong et al.87 showed a reduction in germination rate after the treatment and explained the dependency of treatment time and distance. Along with other dependent variables, gas used in treatment also has a considerable impact on seed germination, and studies by Živković et al.88 on paulownia tomentosa seed proved that by showing maximum germination at 15 min for air plasma, unlike argon plasma. It also noted that the impact of plasma on seeds could not be carried out for a long time since poppy seeds treated at 500 W plasma showed higher GR on the first day of germination but reduced on the sixth day. i.e., 13% reduction was observed in 3 min treated sample.89

3.3.2. Plasma-induced seed tolerance. In recent times, plasma-induced seed tolerance has been getting researchers' attention, which led to experiments on different stress parameters such as drought, heavy metals, toxins, and salinity against plasma treatment. For example, Kyzek et al.90 reported the adaptive response of plasma-treated pea seeds (Pisum sativum L.) against zeocin and found an increased tolerance against drought and heavy metals due to the interaction of RNS with seeds. However, the most common problem of salinity-induced stress was investigated by Iranbakhsh et al.91 for wheat, and this study showed an increase in heat shock factor (HSF) A4A and salinity tolerance after plasma treatment. Further, it increased the wheat plant's immunity, shoot weight, and enzyme activity (peroxidase and phenylalanine ammonia-lyase). In addition to saline tolerance, drought tolerance also increased for wheat due to the production of abscisic acid. Subsequently, proline (12.7%) and soluble sugar (16.4%) contents also increased with the treatment.92

Seed treatment results and their effects vary based on the plasma characteristics used in the respective treatments (Table 1). Many studies have shown a significant increase in the germination of seeds, but detailed studies are required to identify the effect on the quality and yield of the final product. In addition, the negative impact of plasma treatment on seeds needs to be addressed with the evident mode of action, which could provide the limitations of plasma treatment for seeds (Table 2).

Table 2 Plasma applications in post-harvest stages
Sample Plasma chamber Purpose of study Research goal Reference
(1) Plasma treatment of different agricultural produces
(a) Plasma treatment on leaves
Baby spinach High voltage atmospheric cold plasma Inactivation of Salmonella enterica and E. coli O157:H7 Significant inactivation after 24 h through 14 days post-treatment 193
Input – 30–130 kV After 7 days of refrigerated storage – 2.6 log CFU per g – 2 min
Treatment time – 2 min and 5 min 3.5 log CFU per g – 5.0 min
Date palm leave Radiofrequency plasma Surface modification Wettability of leaf surface increased 194
Input – 80, 100, 120 W, gas pressure – 0.95 torr Surface roughness was increased due to the removal of the waxy layer and impurities
Treatment time – 1, 5, and 8 min
Green tea leaves DBD Antioxidant activity 15 W and 15 min, the TPC and antioxidant activity increased by 41.14% and 41.06% 195
Input – 5, 10 and 15 W Catechin increased by 103.12%
Treatment time – 5, 10, 15 min
Radicchio (Cichorium intybus L.) Dielectric barrier discharge (DBD) Microbial inactivation L. monocytogenes – 2.20 log CFU per cm2 reduction after 30 min 94
• Atmospheric air E. coli O157:H7 – 1.35 log MPN per cm2 reduction after 15 min
• Input power – 15 kV
Treatment time – 15 to 30 min
Black and green tea Plasma jet with the copper electrode and Pyrex tube Microbial inactivation E. coli – complete removal in black and green tea after 3 and 4 min, respectively 95
• 99.99% argon (flow rate – 1 L min−1) Coliform – inactivated after 5 min in both samples
• Input power – 10 kV Yeast and mould – inactivated after 7 min in both the samples
• Distance – 1.5 cm
• Treatment time – 0 to 7 min
Romaine lettuce (Lactuca sativa L. var. longifolia) Atmospheric pressure plasma with nickel coated steel needle array Microbial inactivation E. coli – 10 min treatment at 12.83 kV reduces the microbial count to 1.5 logs 93
• Argon (flow rate – 455.33 standard cm3 min−1)
• Treatment time – 30 s to 10 min
• Input power −3.95 to 12.83 kV
Corn salad leaves Atmospheric pressure plasma jet Microbial inactivation The effect of plasma on microbes was higher (3.6 log reduction) at lower level surface contamination (104 CFU per mL) after 15 s treatment at 20 W 97
• Argon (flow rate – 20 L min−1)
• Input power – 10, 20, 30, and 40 W
• Treatment time – up to 5 min
Lamb's lettuce (Valerianella locusta) Atmospheric pressure plasma jet Bioactive compounds Chlorogenic acid, caffeic acid luteolin diosmetin content were reduced as time increased at 30 W plasma treatment 98
• Argon (20 cm3)
• Input power – up to 35 W
• Treatment time – 0 to 120 s (overall)
Lamb's lettuce (Valerianella locusta) Low-pressure oxygen glow discharge plasma Flavonoid content Flavonoid content increased in freeze-dried leaves after 120 s exposure 99
• Oxygen (0.5 mbar)
• Input power – 75 and 150 W
• Treatment time – 20 to 300 s
(b) Plasma treatment on root
Baby carrots (Daucus carota L.) Atmospheric pressure plasma with nickel coated steel needle array Microbial inactivation E. coli – inactivation was less than 0.5 log 93
• Argon (flow rate – 455.33 standard cm3 min−1)
• Treatment time – 30 s to 10 min
• Input power – 3.95 to 12.83 kV
(c) Plasma treatment on fruits
Strawberry DBD Decontamination 14% duty cycle and 20 min effective 196
Input – 7% and 14% duty cycle 1.46 log CFU per g – total aerobic mesophilic bacteria
Treatment time – 5, 10, and 20 min 2.75 log CFU per g – yeast and mould
Litchi DBD Enzymatic browning Residual activity of litchi peroxidase decreased to 47.16% on 10 min treatment 197
Input – 50 kV, 1.5 A
Treatment time – 0, 2, 4, 6, 8, and 10 min
Blueberry Atmospheric cold plasma Improving antioxidant activity and microbial inactivation Reduced the decay rate of blueberries 198
Input – 12 KV, 5 kHz Antioxidant activity increased but maintained at a low ROS level
Treatment time – 0, 30, 60, 90 s 60 s is the best exposure time
Cavendish banana DBD Post-harvest crown rot 0.5 min optimum duration 199
Input – 15 kV, compressed air Colletotrichum musae inhibition – 51.89% for 0.5 min
Treatment time – 0.5, 1, 2, and 3 min
Tomatoes DDP with aluminium-coated electrodes Pesticide degradation Chlorpyrifos reduced up to 89.18% at 5 W in 6 min 37
• Gas – atmospheric air (50–75 mmHg) Lycopene (84.86 to 15.38 mg kg−1), beta carotene (22.82 to 6.61 mg kg−1) and firmness were reduced
• Input power – 2 to 5 W
Treatment time – 4 to 6 min
Blueberries Circular aluminium electrodes with 2 and 10 mm thick perspex dielectric material at ambient temperature (25 ± 2 °C) Pesticide degradation Plasma treatment of 5 min at 80 kV degraded pesticides (75% of boscalid and 80% of imidacloprid) 104
• Atmospheric air
• Input power – 60 and 80 kV
• Treatment time – 0, 2, and 5 min
Strawberries DBD atmospheric CP with Al electrode and polypropylene dielectric material (in pack treatment) Microbial inactivation G 1 and G2 produced 3.1, 3.4 and 3.7, 3.3 log reduction of mesophiles, yeast/mould, respectively 100
G1 – O2[thin space (1/6-em)]:[thin space (1/6-em)]N2[thin space (1/6-em)]:[thin space (1/6-em)]CO2 of 65[thin space (1/6-em)]:[thin space (1/6-em)]16[thin space (1/6-em)]:[thin space (1/6-em)]19 ratio
G2 – N2[thin space (1/6-em)]:[thin space (1/6-em)]O2 of 90[thin space (1/6-em)]:[thin space (1/6-em)]10 ratio
• Treatment time – 5 min
• Input power supply 60 kVrms voltage (50 Hz)
Cocktail tomatoes (Lycopersicon lycopersicum) Atmospheric pressure plasma with nickel coated steel needle array Microbial inactivation E. coli – 12.83 kV input reduced microbial count to 1.7 log after 10 min 93
• Argon (flow rate – 455.33 standard cm3 min−1)
• Treatment time – 30 s to 10 min
• Input power – 3.95 to 12.83 kV
(d) Plasma treatment on nuts
Peanut Novel CAP with rotary jet system Inactivation of fungi Aspergillus flavus – not detected at 180 W for 7.5 min and 200 W for 5 min 200
Input – 180 W, 200 W Aspergillus niger – not detected at 180 W for 10 min and 200 W for 5 min
Treatment time – 5, 7.5, 10 min Aflatoxin concentration after 29 days of storage – 16.5 ppb
Pistachio nut DBD Decontamination of aflatoxin 4 log reduction after 180 s of treatment 201
• Air plasma system Maximum reduction of 52.42% of AFB 1 after 180 s
• Input – 130 W, 20 kHz, and 15 KV
• Treatment time – 15, 30, 60, 90, 120, 150, 180 s
Pistachio nut Cold plasma jet Pest management Plasma exposure time – 14.04 min, voltage – 19.99 kV and Ar/air ratio – 51.65% caused increased mortality of Plodia interpunctella 202
Input – 10, 15 and 20 kV and agon/gas ratio – 0, 50 and 100
Dried walnut kernels Radiofrequency low-pressure cold plasma Microbial load reduction The highest log reduction occurred at 50 W 203
• Air plasma 1.09 log CFU per g – total viable count, 0.97 log CFU per g – coliform
• Input – 20, 30, 40 and 50 W 0.89 log CFU per g – mold
• Treatment time – 10, 15 and 20 min
Cashew nut Low-pressure plasma-glow discharge Allergenicity Did not affect the allergenicity 204
Air plasma Improved lipid extractability
Input – 80 W and 20 kHz Anacardic acid content was higher
Almond slices Atmospheric argon plasma Surface disinfection 20 min treatment was effective 205
Input – 17 V, 2.26 A current 2.29 log CFU per g – total count
Treatment time – 5, 10, 15, and 20 min 1.81 log CFU per g – yeast and mould
2.72 log CFU per g – S. aureus
Whole peanut (WP) and defatted peanut flour (DPF) DBD plasma Allergen reduction The protein intensity of the Ara h 1 band was unaffected by plasma treatment 115
Input – 80 kV With increased time, Ara h 1 band intensity for IgE binding decreased for WP and DPF
Gas – air Antigenicity was reduced by 44 and 9.3% for DPF and WP after 60 min
Treatment time – 0 to 60 min
Tiger nut milk DBD Microbial inactivation Undetectable microflora in 12 min pH reduction after 8 and 12 min protein decreased beyond 4 min 206
Input – 1.22 A and 30 V
Treatment time – 2, 4, 6, 8, and 12 min
Peanut (Arachis hypogaea L.) Coplanar – multi hollow surface DBD unit Quality characteristics Oleic and linoleic acid content was reduced from 43.47 to 35.74% and 32.56 to 24.49% for fresh and treated peanuts 114
• Atmospheric air with 20 to 30% humidity (flow rate – 0.5 to 20 L min−1) Input power, treatment time, and gas mixer used in the study directly affected peanut quality
• Input power – 10 to 40 W
Treatment time – 1 to 15 min
Unpeeled almonds DCSBD plasma Microbial decontamination Air, O2, and N2 plasma reduced microbial count by 5, 4.8 and 2 logs after 15 min 107
Input – 350 W
Gas – dry air, O2, N2, CO2 and CO2/Ar mix (90% + 10%)
Gas flow rate – 0.8 L min−1
Treatment time – 15 min
Hazelnuts Atmospheric pressure fluidized bed plasma (APFBP) reactor Aflatoxin removal 1st reactor: A. flavus and A. parasiticus count reduced by more than 4 log in 5 min at 655 W 108
Input – 460 to 655 W 2nd reactor: both microbes count reduced by more than 3 log
Gas – dry air and N2 (flow rate – 3000 L h−1) A similar trend was followed in both reactors when N2 gas was used
Treatment time – 1 to 5 min
1st reactor – 49 mm diameter, 2nd reactor – 65 mm diameter
Walnut fruits (J. Regia L.) Plasma jet with a copper electrode and Pyrex tube Microbial decontamination The fresh and dried walnut microbial load was removed after 10 and 11 min treatment, respectively, for all walnut types 109
• Argon (1 L min−1)
• Input power – 15 kV DC supply
• Treatment time – 3 to 11 min
Hazelnuts DBD plasma Aflatoxin removal Pure N2 or N2 with O2 (0.1%) provided higher toxin removal 116
Input – 0.4 to 2 kW After 12 min exposure, residual AFB1 and AFs in nuts were 29.1 and 30.4% at 1150 W
Gas – pure N2 or N2 and air mix (flow rate – 120 L min−1 with 7 bar)
Treatment time – 1 to 12 min
Distance – 50 mm
Hazelnuts APFBP reactor Microbial decontamination At 655 W (25 kHz) within 1 min exposure, 2 log reduction was achieved, and the natural flora was reduced by 3.45 log in 2 min 110
Input – 460 to 655 W
Gas – dry air (flow rate – 3000 L h−1)
Treatment time – 1 to 5 min
Almonds Gliding arc plasma Microbial decontamination Plasma reduced 1.34 log CFU per mL of E. coli O157:H7 C9490 after 20 s treatment with 6 cm distance 111
• Input – 590 W
• Gas – air and nitrogen (60 psi)
• Time – 0, 10, and 20 s
• Distance – 2, 4, 6 cm
Hazelnut, peanut, and pistachio nut Low-pressure CP (LPCP) sterilization unit Microbial decontamination In hazelnuts, SF6 plasma reduces D-value (1.1 min) compared to air plasma (4.2 min) 113
• Air or sulfur hexafluoride (SF6) 5 times more log reduction was observed in SF6 than in air plasma (5 min)
• Input power – 300W
• Treatment time – 5 to 20 min
Almonds DBD plasma Microbial decontamination 4 to 5 log reduction in E. coli was observed in all almond varieties after 30 min 112
Input – 16 to 30 kV
Discharge gap – 10 mm
(e) Plasma treatment on spices
Saffron Low-pressure cold plasma technology Decontamination 110 W for 30 min – high microbial reduction 207
Input – 70, 90, and 110 W 3.52 log CFU per g – TVC
Treatment time – 5, 10, 15, and 30 min 4.62 log CFU per g – coliforms
2.38 log CFU per g – mold
4.12 log CFU per g – yeast
Black pepper corn Low-pressure cold plasma technology Decontamination 250 W for 20 min showed the highest decontamination 208
Input – 150 W and 250 W 0.88 log CFU per g – aerobic plate
Treatment time – 10 min and 20 min 3.66 log CFU per g – yeast and mould
Black pepper DSCBD – DBD Decontamination B. subtilis count reduced by 5.06 log after 5 min exposure 117
Input – 400 W B. subtilis spores, E. coli and S. Enteritidis D – values were 142, 47 and 58 s
Gas – ambient air
Treatment time – 60 to 300 s
Saffron Plasma jet Bioactive compound Safranal content was reduced by 14% and 21% at 8 kV and 12 kV, respectively 118
Input – 8 and 12 kV However, isophorone (4 and 8%) and 4-ketoisophorone (2 and 6%) increased by 8 min Ar plasma at 8 kV and 12 kV, respectively
Gas – Ar with 5, 10, and 20% O2 (flow rate – 1 L min−1)
Treatment time – 6 and 8 min
Red pepper flake Microwave integrated CP treatment with high (HMCPT) and low microwave density (LMCPT) Decontamination 1.8 and 0.6 log per cm2 reduction was observed in spore count in HMCPT and LMCPT 119
Input – 900 W Based on particle size, the microbial reduction was more for flakes (1.4 and 2.7 log per cm2) than particles (0.8 and 1.2 log per cm2) in LMCPT and HMCPT
Gas – He (flow rate – 1 std L min−1 with 0.7 kPa)
Treatment time – 20 min
Onion (Allium cepa L.) powder Microwave-powered CP treatment with HMCPT and LMCPT Microbial reduction B. cereus spores reduction was more in HMCPT (2.1 log per cm2) than LMCPT (1.2 log per cm2) 120
Input – 400 to 900 W For microbial reduction, vacuum air drying of the sample was superior to hot air drying
Gas – He (flow rate – 1 L min−1 at 0.7 kPa)
Treatment time – 10 to 40 min
Onion powder DBD – CP treatment Microbial reduction E. coli and L. monocytogenes count reduced by more than 1 log after 20 min at 9 kV, while more than 2 log reduction was observed in S. Enteritidis 121
Input – 9 kV
Treatment time – 20 min
Whole black pepper Radiofrequency (G1) and microwave (G2) generated plasma jet Microbial reduction G 2 plasma was more effective against aerobics and spores than G1 plasma 28
G1 used argon (10 L min−1)
G2 used air (18 L min−1)
• Input power – 30 W (G1), 1.2 kW (G2)
• Treatment time – 15 min (G1) and (G2) 30 min
Black pepper Atmospheric pressure plasma jet (APPJ) Microbial reduction Argon and CO2 gas mixer plasma treatment inactivated B. subtilis after 5 min of treatment 122
• Argon plasma (flow rate – 20 L min−1)
• Argon (0.5 L min−1) + CO2 (20 L min−1) plasma 0.5 (argon) + 20 (CO2) 0
• Air plasma (20 L min−1)
• Argon + H2O plasma (20 L min−1)
• Input power – 280 V (8 A)
• Treatment time – 0 to 10 min
Black pepper Fluid bed APPJ treatment Microbial reduction Microbial reduction of 1.5 and 5 log reduction of Salmonella was observed after 20 and 80 s of treatment 123
• Air and argon (flow rate – 20 and 14 L min−1, respectively)
• Treatment time – 0 to 80 s
Black pepper RF – low-pressure cold plasma Microbial reduction H2O2 usage (60 min) reduced microbial count by 1 to 3 logs 124
Input – 300 and 400 W After 60 min, Ar plasma caused higher microbial reduction than air, O2 and N2 plasma (for all microbes)
Gas – O2, N2, air, and Ar (0.3 and 9 mbar)
Treatment time – 15 to 60 min
Red pepper (Capsicum annuum L.) powder Microwave powered CP Microbial reduction 20 min plasma treatment at 900 W reduced Aspergillus flavus (N2 gas at 667 pa) by 2.5 logs 125
• Air and argon (267 to 26[thin space (1/6-em)]680 pa) Bacillus cereus required heating for inactivation
• Input – 300 to 900 W
• Treatment time – 0 to 80 s
[thin space (1/6-em)]
(2) Plasma treatment for safe storage, extraction and quality enhancement
(a) Grain safety and quality
Wheat DBD Degradation of deoxynivalenol 50 kV and 5 min degraded DON by 83.99% 209
Input – 30, 40, and 50 kV
Treatment time – 4, 8, 12, and 16 min
Chickpea Atmospheric cold plasma Increased storage period to study the occurrence of Callosobruchus chinensis In vishal – 40 W, 15 and 20 min – grain damage – 1 to 3% 210
Input – 40, 50, 60 W In other samples, the damage was 0%
Treatment time – 10, 15, and 20 min
Canola grain, canola meal and barley grains ACP DBD and ACP-jet Degradation of Zearalenone ACP-DBD – 3 min degradation was 91.6%, 83.2%, and 64.8% for canola grain, canola meal, and barley grain 211
• ACP – DBD – input – 0–34 kV, 1 A and 300 W, 3500 Hz, duty cycle – 70% and output pulse – 10 µs ACP jet – 85% Ar + 15% O2 – high degradation
• ACP-jet – 1500 Hz, voltage – 0–22 kV, 70% duty cycle, 10 µs
And 0–0.025 A with 75% Ar + 25% N2
• Treatment time – 0.5, 1, 3, 5, and 15 min
Barley Atmospheric cold plasma (DBD) Removal of deoxynivalenol 6 min – 48.9% and 10 min – 54.4% 212
• Input – 0–34 kV, 1 A, and 300 W, 3500 Hz, duty cycle – 70% and output pulse – 10 µs
• Treatment time – 0, 2, 4, 6, 8, and 10 min
Wheat DBD plasma jet T. castaneum mortality for Ar, Ar + N2 (20 sccm) and Ar + N2 (80 sccm) gas mixture were 90, 70 and 46%, and for T. confusum 93%, 63% and 57%, respectively. A similar trend was observed for O2 admixture 126
Input – 90–130 W
Gas – Ar (3000 sccm), O2 (1 to 8 sccm) with Ar, N2 (0 to 80 sccm) with Ar
Treatment time – 10 to 15 min
Wheat, barley CP with DBD Long-time exposure reduced microbes on the surface 127
• Atmospheric air Wheat germination was affected due to the long exposure time
• Input power – 80 kV
• Treatment time – 5 and 20 min
Wheat germ DPD plasma Lipase activity was 27.11 and 25.03% at 20 and 24 kV plasma voltage after 25 min 129
Input voltage – 20 and 24 kV Lipoxygenase activity was 55.18 and 49.98% for the same condition
Treatment time – 5 to 35 min
Distance – 1.5 and 2 cm
Winter wheat CP with DBD Total isolated colonies were reduced to 27 from 250 after 10 s treatment 131
• Atmospheric air
• Input power – 80 kV
• Treatment time – 3, 10, and 30 s
Wheat DBD plasma with polymethylmethacrylate 10 kV pulse voltage removed 3 times more endospores than 6 kV treatment 130
Gas – Ar (flow rate – 2.8 nm)
Input – 6 to 10 kV
Time – up to 10 min
Wheat Low-pressure plasma circulating fluidized bed reactor 700 and 900 W with 10% O2 mix reduced 1.91 and 2.59 log microbes after 10 min 132
Gas – Ar and O2 (15 norm. L min−1)
Input – 700 and 900 W
Time – up to 73.5 s
Grains and legumes Low-pressure CP Microbial contamination reduced below 1% for Aspergillus spp. and Penicillum spp. 133
• Air gases or SF6 (300 torr)
• Input power – 300 W
• Treatment time – 30 s to 30 min
(b) Flour safety
Wheat flour Bell jar type of plasma apparatus Pest control 60 W and 30 min showed complete inhibition of larvae, pupae, and egg 213
Input – 40 and 60 W
Treatment time – 20, 25, and 30 min
Refined wheat flour DBD CP treatment Insect inactivation The maximum mortality rate was found for 15 min exposure times for eggs (93.33%), larvae (93.33%) and adults (100%) 29
• Atmospheric air (1 mbar)
• Input power – 1 to 10 kV
• Treatment time – 2 and 20 min
Refined wheat flour DBD atmospheric CP Insect control Complete mortality was achieved by increased voltage or increased time, or reduced electrode distance for Tribolium castaneum adults 134
• Atmospheric air (flow rate – 1 L min−1)
• Input power – 500 to 3000 V
• Treatment time – 0.5 to 7 min
Maida flour DBD plasma Insect control An increase in exposure time and input power or a decrease in electrode distance gave 100 mortality of T. castaneum 135
Input – 500 to 3000 V
Atmospheric air (1 mbar)
Treatment time – 0.5 to 7 min
Electrode distance – 2 to 6 min
(c) Flour quality
Quinoa flour ACP Functionality study Results in protein polymerization and starch depolymerization 214
Input – 50 kV and 60 kV Treatment time and voltage affects technological properties
Treatment time – 5 and 10 min
Jackfruit seed flour Pin-to-plate ACP Functionality modification Higher input voltage and exposure time cause starch and protein modification and loss in crystallinity (25.75% to 21.31%) 215
Input – 170, 200, and 230 V The hydration properties, like water solubility, increased from 9.65 g g−1 to 14.11 g g−1 water absorption – 6.39 g g−1 to 7.66 g g−1
Treatment time – 5, 10, and 15 min Swelling power – 7.28 g g−1 to 8.79 g g−1
Water holding – 2.93 g g−1 to 3.48 g g−1
Little millet flour Multipin electrical discharge atmospheric cold plasma Functional property modification 24 W and 30 min resulted in increased functional properties 216
Input – 13 and 24 W Water absorption, oil absorption capacity, swelling, and solubility increased
Treatment times – 10, 20, and 30 min A decrease in the viscosity of cooked paste, colour, and dispersibility was observed
Wheatgrass flour Radiofrequency CP treatment Functional modification No change in protein solubility 137
Input – 120 W Starch damage increased, and water absorption was affected after treatment
Gas – Ar and CO2 (flow rate of 10 and 25 cm3 min−1) Solvent retention of different flour types increased
Treatment time – 1 h (2 × 30 min)
Rice starch Bell jar CP reactor Functional property modification Amylose content, pH, turbidity, and starch hydrolysis percentage were reduced 138
• Atmospheric air (0.15 mbar) At maximum power level and time, GT is reduced
• Input power – 40 to 60 W
• Treatment time – 5 and 10 min
Wheat flour Continuous atmospheric pressure cold plasma Fatty acid profile FFA and phospholipid were reduced after 60 and 120 s treatment at 20 V 139
Input – 15 and 20 V Pasting properties were unaffected
Treatment time – 60 and 120 s
Rice flour (short and long rice) DBD plasma ozone treatment Functional property modification Higher transmittance and swelling power for treated rice pastes and grains, respectively 140
Input – 60 and 70 kV Syneresis increased for both flours
Gas – atmospheric air
Treatment time – 5 and 10 min
Grain pea (Pisum sativum) Surface DBD air plasma Functional property modification PPI solubility increased with plasma treatment time up to 191% (10 min) in distilled water. PPF solubility reduced from 71 to 33% for the same treatment time 141
• Input – 8.8 kVPP
• Gas – air
• Time – up to 10 min
Wheat flour (soft, hard) DBD atmospheric CP Functional property modification Peak time and peak integral of soft and hard flour increased significantly when voltage and treatment time exceeded 60 kV and 5 min 25
• Atmospheric air (flow rate – 1 L min−1)
• Input power – 60 and 70 kV
• Treatment time – 5 and 10 min
Wheat flour Counter flow – cold plasma generated Functional property modification Wet gluten (34.2 to 32.1%), dry gluten (9.7 to 7.9%) and gluten softening (13 to 10 mm) values decreased after 45 min, while sedimentation (56 to 61 cm3) and bread-making strength index (50 to 56) values increased compared to control 142
Gas – O3 at 1000 ppm (flow rate – 2.5 L min−1)
Treatment time – 30 and 45 min
[thin space (1/6-em)]
(3) Plasma treatment on minimally processed and processed fruits products quality
(a) Minimally processed fruit products
Fresh-cut cantaloupe DBD Quality and flavour CP treatment inhibited the growth of bacteria and mould during 10 days storage 217
Input – 40 kV
Treatment time – 90 s
Banana slices DBD reactor Enzyme activity Polyphenol oxidase and peroxidase activity decreased to 70% and 100% 218
• Input voltage: 4.8 to 6.9 kV and 12 to 22 kHz Total phenol and flavonoid content increased to 50%
• Treatment time: 0.6 to 2.6 min Vitamin B6 increased
Carrot discs DBD reactor Surface decontamination 2.1 log CFU per g reduction in total aerobic mesophiles and yeast and mould at 5 min and 100 KV 219
• Gas: atmospheric pressure plasma
• Input voltage: 60 kV, 80 kV, and 100 kV
Treatment time: 0, 1, 2, 3, 4, 5 min
Fresh-cut pitaya fruit DBD reactor Accumulation of phenolics and antioxidant activity 60 kV for 5 min reduced the total aerobic bacteria and increased safety 220
• Input voltage: 40, 50, 60, and 70 kV The content of individual phenolics increased, resulting in increased antioxidant activity
Treatment time: 1, 3, 5, and 7 min
Fresh-cut pear Atmospheric cold plasma Microbial inactivation 65 kV for 1 min was effective in achieving a shelf-life of 7 days 221
Input voltage: 45 kV, 65 kV
Treatment time: 1 and 5 min
Fresh-cut apples (Pink Lady®, Fuji, Modì®, Red Delicious (RD)) Double barrier discharge type gas plasma with three brass electrodes Enzymatic browning Browning area – 17% reduction in RD and 50% in other samples (30 min). Whereas 60 min treatment gave 86% and 58% reduction in Pink Lady® and Modi® 143
• Atmospheric air
• Input power – 150 W
• Treatment time – 30 (15 + 15) and 60 (30 + 30) min
Apples (Malus domestica cv.) Dielectric barrier discharge (DBD) CP Polyphenol content No significant change in total phenolic content 145
• Atmospheric air Extracted polyphenol did not affect the cell viability
• Input power – 150 W
• Treatment time – 30 (15 + 15) and 120 (60 + 60) min
Fresh-cut apples Dielectric barrier discharge (DBD) chamber with brass electrodes Polyphenol content Polyphenol oxidase activity – reduced by 12, 32, and 58% after 10, 20, and 30 min treatment 144
• Atmospheric air
• Input power – 9 V (15 kV measured potential difference)
• Treatment time – 10, 20 and 30 min (5 + 5, 10 + 10 and 15 + 15 min each major slice side)
(b) Processed fruit products
Apple juice DBD and glow discharge plasma Browning enzyme activity, antioxidant capacity, and total phenolic content Both systems inactivated polyphenol oxidase and peroxidase and increased total phenolic content and antioxidant activity 222
DBD DBD – better at increasing total phenolic and antioxidant capacity
Input – 50–900 Hz
Glow discharge
Input – air plasma flow rate: 10–30 mL min−1
Treatment time – 10–30 min
Cashew apple juice ACP Bio accessibility of vitamin C Malic acid and phenylalanine increased for ACP 700 223
Increased the vitamin C bio accessibility at ACP 700
Camu–camu juice DBD Bioavailability of vitamin C Higher excitation frequency increased the availability of ascorbic acid 224
Input – 24 kV and frequency – 200, 420, 583, 698, and 960 Hz Degradation of anthocyanins and peroxidase enzyme at a higher frequency
Kiwi turbid juice DBD Microbial inactivation and quality changes • 12 kV, volume – 18 mL and discharge time – 1 min was the optimum condition 225
Input – 10, 25, and 40 kV • Polyphenol content – 0.18 mg g−1, chlorophyll content – 3.47
Treatment time – 1, 3 and 5 min • Sterilization rate – 18.03%
Volume – 10, 15, and 20 mL
Siriguela juice Low-pressure plasma processing Bioactive component Enhanced the bioactive component content 226
Nitrogen gas flow rate – 10–30 mL min−1 Highest pigment increase – 10 min – 10 mL min−1
Treatment time – 5 to 15 min Highest antioxidant activity – 15 min – 20 mL min−1
Blueberry juice Cold plasma jet Bioavailability of vitamin C 2 min exhibited high anthocyanin 227
Input – 11 kV, 1000 Hz and oxygen gas concentration – 0, 0.5 and 1 Long treatment decrement of vitamin C
Treatment time – 2, 4, and 6 min 1% concentration of O2 resulted in the reduction of Bacillus by 7.2 logs CFU per g
Apple juice Dielectric barrier discharge (DBD) plasma chamber with 2 mm electrode distance from sample surface at 23 ± 2 °C temperature Microbial inactivation 4.0 log CFU per mL at 30 W (40 s) 146
• Atmospheric air 4.2 log CFU per mL at 40 W (40 s)
• Input power – 30, 40, 50 W 4.34 log CFU per mL at 50 W (30 s)
• Treatment time – 0 to 40 s
Banana starch Corona electrical discharge (CED) plasma Starch modification No changes in amylose and resistant starch contents. The crystalline to amorphous ratio increased after CED plasma treatment 150
Input – 30, 40, and 50 kV
Treatment time – 3 min
Sour cheery nectar APPJ setup E. coli – 3.34 log reduction at 650 W after 120 s 148
Tomato juice • Atmospheric dry air (flow rate – 3000 L h−1) Microbial inactivation E. coli – 1.43 log reduction at 650 W after 120 s
Apple juice • Input power – 650 W E. coli – 4.02 log reduction at 650 W after 120 s
Orange juice • Treatment time – 0 to 120 s E. coli – 1.59 log reduction at 650 W after 120 s
White grape juice High voltage atmospheric pressure CP with plexiglass and polypropylene dielectric barrier Microbial inactivation S. cerevisiae population reduced to 7.4 logs (80 kV for 4 min) 149
• Atmospheric dry air (Ziploc bag)
• Input power – 80 kV (peak)
• Treatment time – 1 to 4 min
[thin space (1/6-em)]
(4) Plasma treatment on essential oil extraction and quality
Hyssop DBD Changes in essential oil content during storage Trichomes were vulnerable to plasma treatment 228
Input – 17, 20, and 23 kV Increased treatment duration reduced the EO content
Treatment time – 1, 5 and 10 min
Fennel and spearmint leaves DBD Essential oil yield 19 kV and 10 min yield 1.89% (v/w) for fennel and 1.81% (v/w) for spearmint leaves 229
Input – 17–23 kV
Treatment time – 5–15 min
Lemon peel Low-pressure DBD cold plasma Essential oil yield Plasma treatment for 10 min at 2.5 kV increased the extraction yield by 149.34% 152
• Gas – atmospheric air (1 mbar)
• Input – 1 to 2.5 kV
• Treatment time – 10 min
Lemon verbena (Lippa citriodora Kunth.) Low-pressure CP connected with vacuum pump Essential oil yield Essential oil (EO) content increased by 36.7% during the first 1 min of treatment and later decreased due to higher exposure time 153
• Nitrogen, argon, oxygen
• Treatment time – 0, 1, 3, and 5 min
Sweet basil (Ocimum basilicum) APPJ CP Essential oil yield Plasma treated basil EO produced more antioxidant activity (94.82%) than untreated (90.64%) basil at a 250 µg mL−1 concentration 27
• Air/helium
• Treatment time – 5 s
Lemon peel DBD plasma treatment Essential oil yield Maximum EO yield was obtained in the argon plasma (flow rate – 15 mL min−1) after 1 min 154
• Argon, oxygen, nitrogen, or air (10–20 mL min−1) Argon plasma of 1 × 1 × 1 cm size lemon peel produces maximum EO extraction efficiency
• Input power – 30 kV (peak)
• Treatment time – 1 to 15 min
Clove oil Plasma jet with pyrex glass tube at atmospheric pressure Essential oil yield The minimum concentration required for microbial inhibition (A. niger, Penicillium sp., and Rhizopus sp.) was reduced to 20 times for clove oil and 8 to 9 times for eugenol after 10 min at 40 W 151
• Argon (flow rate of 10 L min−1)

4. Plasma applications in post-harvest stages

4.1. Plasma treatment of different agriculture produces

Some of the edible products, such as leaves, roots, fruits, nuts, and spices, are either contaminated by microbes or will have allergens. Plasma treatment can decrease the contaminant level and increase the active component percentage in food products. However, based on the surface morphology of different commodities, the effect may change and usually produces a smooth surface that will have more microbial removal rate than the unevenly surfaced one, and this could be due to the improper penetration of plasma species into the porous structures.93 Decontamination and disinfestation of grains, higher extraction of EO, and functional modification of flours are some advantages of plasma treatments that will be discussed in this section. However, like any other treatment, plasma produces minor undesirable changes that will also be covered here. Different post-harvest applications of plasma are shown in Fig. 4.
image file: d2fb00014h-f4.tif
Fig. 4 Plasma application in post-harvest stages of agri-food chain.
4.1.1. Leaves. Interaction of plasma species with microbes causes itching, and thus inactivation occurs, while DNA damages caused by UV photons further enhance this effect (Fig. 5). So, microbial contamination on freshly harvested leaves can be reduced by plasma treatment. Pasquali et al.94 reported the microbial decontamination of radicchio leaf (Cichorium intybus L.) after DBD plasma treatment and found a significant removal of microbial load. However, the sample stored after treatment had less chroma (C*) value at 4 °C. In order to improve the inactivation efficiency, different gas mixes were suggested, and the Ar–O2 gas mix is one among them. In romaine lettuce, Ar and O2 gas mix gave better decontamination efficiency without altering the texture of the produce.93 However, this gas combination is not ideal for decontaminating all microbes since they have different physical and morphological characteristics. For example, black and green tea were treated with an argon plasma jet, where E. coli and coliform were removed within 5 min, while yeast and mould took more time. Further, it also increased the total phenolic content (TPC) of black tea (10.77 to 11.38 g GAE/100 g) and green tea (14.94 to 16.02 g GAE/100 g), but caffeine and colour values were not affected significantly.95 Similarly, microwave plasma reduced the microbial (viable) count of dried peppermint, stinging nettle, and lemongrass leaves by up to 1.6 logs within 7.5 min of treatment time.96 Though decontamination of leaves was the primary goal of these studies, Baier et al.97 found a permanent reduction in photosynthetic efficiency (Fv/Fm) of corn leaves when a higher power level was used. Nevertheless, in the case of lamb's lettuce, the interaction of reactive species caused the reduction in bioactive phenolic components and not in the photo- or thermo-desorption processes. Further reductions such as protocatechuic acid (16%), chlorogenic acid (29%), and caffeinic acid (35%) were observed during the treatment, along with some structural changes.98 Similar reductions were observed in flavonoids, phenolic acids, and the contact angle of lamb's lettuce.99
image file: d2fb00014h-f5.tif
Fig. 5 Effect of plasma treatment on microbial cell.
4.1.2. Roots. Surface morphology is one of the critical factors in deciding the decontamination efficiency of plasma treatment, Bermúdez-Aguirre et al.93 reported that the surface morphology of baby carrots restricted the penetration of plasma into the porous surface areas, and hence the microbial log reduction was found to be less.
4.1.3. Fruits. Freshly harvested fruit surfaces can either be contaminated by microbes or by pesticides. For microbial removal, similar to Amini and Ghoranneviss's95 study, strawberries were treated in DBD atmospheric cold plasma at 60 kVrms voltage in high oxygen (G1) and high nitrogen (G2) environments. The results showed that G2 was more efficient against mesophiles, whereas G1 was effective on yeast/mould. To maintain more firmness and L* value, G1 was preferred since it produced more ozone (G1 – 2800 ppm) than G2 (600 ppm).100 For cocktail tomatoes, along with E. coli inactivation, an increase in a* value was observed with respect to the treatment time. However, at a fixed treatment time of 10 min at 12.83, 11.18, and 6.90 kV power levels, a slight reducing trend of a* value was observed.93 However, to achieve the desired microbial reduction from fruit surfaces, it is essential to select a suitable feed gas. A recent study revealed that using air plasma on red currants was only able to reduce fungal population (1.28 log) and not aerobic mesophilic. At the same time, using nitrogen plasma induced up to 1.11 log microbial reduction in aerobic mesophilic without causing any lethal effect on yeast and moulds.101 Among the various plasma types, cold atmospheric pressure plasma (CAPP) is commonly employed for decontamination of these kinds of fruit and berries.102 In addition to plasma treatment and PAW, plasma activated air (PAA) also helps in the decontamination of fresh fruits and vegetables (apples cucumbers, tomatoes, and carrots). Interestingly, the commodity with a rough surface (carrot) achieved a higher microbial reduction of up to 5 logs within 5 min of PAA treatment compared to smooth surfaced apple and tomato samples (∼3 log reduction).103

As pesticides are used extensively in agricultural commodities, their residues remain on the surface of the commodities even after primary and secondary processing. Hence, Ranjitha Gracy et al.37 applied plasma treatment to tomatoes to validate the chlorpyrifos reduction in DBD plasma with different dosage levels. The results showed around 90% pesticide reduction due to the conversion of the phosphorothiol group (P[double bond, length as m-dash]S) into the phosphoryl group (P[double bond, length as m-dash]O). After the treatment, considerable changes in firmness, total phenolic content, and colour index were observed. Earlier, Sarangapani et al.104 studied the same with blueberries contaminated with boscalid and imidacloprid pesticides and found at least a 75% reduction in both pesticides. Along with this, ascorbic acid content increased from 8.91 to 14.01 mg/100, while time and power level increases reduced the total flavonoids, anthocyanin, and TPC. Different interactions of plasma with water are shown in Fig. 6.

image file: d2fb00014h-f6.tif
Fig. 6 Interaction of reactive species with water in plasma treatment and PAW generation.

Apart from plasma treatment, PAW also helps in fruit processing. For example, PAW water can clean the fungicide contaminated tomatoes and can cause up to 85.3% and 79.47% reduction in the chlorothalonil and thiram contents.105 Further, they are also capable of reducing microbial contamination of fruits and vegetables.106

4.1.4. Nuts. Nuts are high-quality oil and nutritive food source; hence, it is susceptible to microbial contamination. In order to remove the contaminants from the surface, it needs to be processed without significant quality deterioration. However, plasma treatment provides surface decontamination with no or minimal quality changes. In unpeeled almonds, air plasma increased the inactivation of salmonella; however, air mixed with N2 caused browning.107 A similar decontamination study on hazelnuts showed Aspergillus spp. Reduction even after 30 days of storage due to the presence of RS.108 The effect of moisture content and different cultivars on microbial removal of nuts were experimented with walnut fruits in a plasma jet using argon gas and observed that microbial removal was high in dried walnuts than the fresh ones for the given treatment time. It is due to the low water activity, moisture content, and rugged texture of dried walnuts. After treatment, no further microbial growth was observed till 30 days (4 °C) except in a few (Shahmirzad and Taleghan) walnut types.109 Apart from product variables, the nature of the microbes present in nuts also has varying responses to the treatment. In Dasan et al.110 report on hazelnuts, Aspergillus flavus reduced up to 4.5 log per g, whereas A. parasiticus reduction was 4.19 log per g under the same conditions. The difference in morphology, toxic production, and geographical range of these microbial species could not be the factor for the difference in microbial removal. However, molecular research is needed to get more information about this effect. To evaluate the feed gas effect, Niemira111 reported the microbial reduction in almonds with air and nitrogen plasma and found higher microbial removal in air-treated almonds than the nitrogen treated due to ROS in air plasma. With this, almonds' decontamination can also be increased by higher treatment time, input voltage, and frequency.112 Likewise, hazelnut, peanut, and pistachio nut were treated in low-pressure air or sulfur hexafluoride (SF6) cold plasma. Though SF6 showed higher microbial reduction than air plasma, aflatoxin removal was higher (50%) in air plasma treatment than SF6 (20%) after 20 min treatment. Peanuts and pistachio nuts were not entirely sterilized after 20 min SF6 treatment, and hazelnut microbial load reduced linearly till 5 min of SF6 treatment.113

Some quality changes are also associated with nuts during treatment. For instance, peanuts were treated with coplanar DBD plasma; though antioxidant capacity was not changed due to short exposure to ROS, the TPC increased from 200.23 to 341.15 mg (GAE/100 g) after 25 W treatment for 8 min (0.5 L min−1) since phenols protect cells against the damaging effects UV and ROS. However, lipid oxidation and moisture reduction were observed due to the oxidation of strong ROS species.114 Another problem with peanuts is the Ara h 1 allergens present in them; however, it was reduced in the whole peanut (WP) and defatted peanut flour (DPF) through binding epitope modifications caused by protein and lipid functionality changes after the treatment.115 Aflatoxins are commonly found in nuts, which is a hazardous toxin, though it was removed about 70% from hazelnut (Aflatoxin B1-AFB1 and total Aflatoxin-AFs) by DBD plasma treatment at 1150 W. Changes in the lactone ring were considered to be the reason for degradation and this effect was increased with power and time.116

4.1.5. Spices. Mošovská et al.117 experimented with plasma on black pepper and found that B. subtilis spores were more resistant than E. coli and S. Enteritidis. After treatment, minimal surface damage and moisture loss with unchanged sensory properties were observed. Amini et al.118 reported that the ester and volatile compounds of saffron in response to the plasma treatment. Safranal was degraded by plasma treatment and increased (from 21% to 33% at 12 kV) by adding O2 (5%) to the plasma gas composition. In contrast, isophorone and 4-ketoisophorone increased by O2 gas mixer. Plasma treatment reduced crocin esters, and an O2 gas mixer enhanced the effect in Ar plasma.

Kim et al.119 reported decontamination of red pepper flakes from B. cereus in high microwave density (HMCPT) and low microwave density (LMCPT) CP treatment. It was observed that IR drying and flaked samples were suited for microbial reduction. Higher sample water activity (aW) produced a more lethal effect on microbial reduction. Kim et al.120 treated onion powder in microwave-powered plasma. Microbial inactivation was high in HMCPT than in LMCPT. In addition, the treatment drying method influenced microbial reduction. Onion Aw reduced from 0.26 to 0.16 and 0.12 in LMCPT and HMCPT. Plasma treatment with low temperature (4 °C) storage prevented microbial growth. Quercetin concentration and DPPH activity followed a reducing trend in HMCPT samples, and for the control sample, an increasing trend was observed during storage at both temperatures (4 °C and 25 °C). Won et al.121 reported the effect of He-plasma on onion preservation. In plasma treatment, increased E. coli, L. monocytogenes and S. Enteritidis inactivation by about 2 log per cm2 when aW increased from 0.4 to 0.8 was noted, and also, the increase in particle size positively influenced inactivation.

Hertwig et al.28 reported the effect of radio frequency (G1) and microwave (G2) generated plasma jet on black pepper. G1 plasma treatment for 15 min caused 0.7 logs and 0.6 log inactivation of aerobic and spore count, whereas G2 plasma treatment caused 1.7 logs and 1.4 inactivation of an aerobic and spore count for the same time interval. The piperine content of both plasma-treated samples was reduced slightly, and G2 plasma was effective against S. enterica. Takemura et al.122 reported the microbial reduction in black pepper in plasma treatment. Argon and CO2 mixed plasma treatment reduced microbial load better than other combinations. The growth retardation of untreated samples was less than air and argon + CO2 mixer plasma. Similarly, Sun et al.123 reported APPJ plasma treatment of black pepper on surface microbial reduction. A sample stored at a higher temperature (37 °C) and low RH (33%) reduced the initial microbial count more than that stored at a low temperature and RH. During the treatment, peppercorn surface temperature reached above 120 °C after 80 s exposure. Combined storage condition and 80 s plasma treatment reduced an average of about seven microbial log counts. Grabowski et al.124 treated black pepper with plasma for decontamination. Spore & non-spore forming aerobic bacteria and anaerobic spore-forming bacteria were removed during the treatment. Increased treatment time increased microbial reduction but resulted in water loss. Lumps on pepper were observed after treatment except in O2 and H2O2 plasma. Kim et al.125 reported the effect of microwave-powered CP on the decontamination of A. flavus from red pepper powder. The treatments performed at higher power levels reduced the water activity of pepper due to evaporation, which further increased the ROS concentration. Colour change was insignificant, and He and O2 gas mix with heating up to 90 °C inactivated B. cereus spores.

Research has shown a considerable microbial reduction in most agricultural products and enhancement in active components. However, specific issues, such as colour change, weight loss, and bioactive quality losses, must be addressed by selecting proper plasma characteristics and treatment time. Apart from that, the mode of action of RS or UV produced from plasma treatment on the food components needs to be explained to understand the reason for positive and negative impacts on quality.

4.2. Plasma treatment for safe storage, extraction, and quality enhancement of agricultural products

Removal of microbes and pests during storage is essential to increase the quality of goods. Fumigation and other thermal treatments result in either residue formation or quality deterioration. Plasma treatment facilitates microbial removal and disinfestation without affecting product quality. Meanwhile, it also inactivates the enzymes that reduce the product quality or shelf life. In some cases, plasma treatment improves the functional properties of food constituents.
4.2.1. Grain safety and quality. Carpen et al.126 reported the effect of nitrogen and oxygen gas mixture with Ar plasma on the mortality of Tribolium castaneum and T. confusum in wheat. When O2 was added with Ar plasma at the concentration levels of 2.8 and 8.4 sccm, mortality reduced from 88% to 65% and 1.3% for T. castaneum and 96% to 51% and 0% for T. confusum, respectively. The reduction in mortality was assumed to be due to temperature reduction caused by gas mixing. Similarly, when wheat and barley were subjected to plasma treatment to remove microbial contamination, bacteria and fungi in barley were reduced to 2.4 logs and 2.1 logs after 20 min (24 h retention), while in wheat, it was reduced to 1.5 logs and 2.5 logs after 20 min for bacteria and fungi, respectively. It was found that germination was increased after 5 min treatment with 2 h retention in wheat, and the contact angle was reduced.127 Apart from insects, microbes and their secondary metabolites also affect grain quality. Hence, Durek et al.128 studied the importance of plasma treatment and its processing variable (feed gas: CO2 + O2, CO2; time: 1–3 min) effect on Aspergillus niger and Penicillium verrucosum contaminated barley. Though both microbe's counts were reduced in all plasma treatments, the P. verrucosum's produced mycotoxin (ochratoxin A) content reduced only in the CO2 + O2 plasma treatment (49.0 ng g−1 to 27.5 ng g−1 after 1 min and 23.8 ng g−1 after 3 min). When CO2 alone was used on barley, it increased the mycotoxin content upon storage.

Tolouie et al.129 reported the plasma effect on wheat germ enzyme activity to enhance its shelf life. Lipase and lipoxygenase activities were reduced more in 24 kV plasma treatment, while antioxidant activity and total phenols were unchanged. Lipase and lipoxygenase activities were recovered by 1.18 and 6.52 U g−1, respectively, after 30 days. Butscher et al.130 reported bacterial inactivation of plasma treatment in wheat grain where water content was reduced by 10.48 to 9.35% gluten, and the falling number was unaffected. Kordas et al.131 reported around ten times reduction in the fungal colonies of wheat seeds when exposed to up to 10 s of plasma treatment, and it also resulted in more than 98% germination energy. However, the root and leaf length decreased during the same treatment time. Butscher et al.132 reported the effect of power level and O2 concentration on microbial destruction in wheat. Higher O2 levels and power levels positively affect microbes, while the extensograph and amylograph parameters such as elasticity, strain resistance, energy, P/L ratio, and gelatinization increased after plasma treatment. Selcuk et al.133 reported the surface structure of grains and legumes on microbial disinfection of air and SF6 plasma. Cooking quality and water absorbance had no significant effect due to treatments. However, the seed germination rate was maintained above 85% after treatment. Wheat disinfection was 99% in SF6 plasma treatment.

4.2.2. Flour safety and quality. Ramanan et al.29 reported the effect of CP on the mortality of T. castaneum at different life stages (egg, larva, and adult). Low voltage levels and less electrode distance were found not to affect mortality. However, higher voltage (3000 V) mortality was affected by distance. T. castaneum mortality prediction model showed no significant difference with experimental mortality rate values (egg, larva, adult) at a 95% confidence level. At 2500 V, maximum mortality was found at a 3.7 cm electrode distance. Eggs, larvae, and adults had 93.33%, 93.33%, and 100% mortality, whereas the predicted model showed 100%, 80%, and 100% mortality, respectively. Similarly, Radhakrishnan et al.134 demonstrated T. castaneum disinfestation using CP. Refined wheat flour treated with plasma produces 100% mortality at 1750 V for 7 min and 3 min with 4 cm and 2 cm electrode distances. While, at 3000 V voltage and 4 cm electrode distance, even 3 min treatment achieved the same mortality. It was also found that there were no significant changes in the colour of the flour after treatment. Likewise, for maida flour, mortality of T. castaneum after plasma treatment was reported by Mahendran et al.135 Results indicated that 1750 V at 2 cm for 3 min, 1750 V at 4 cm for 7 min, and 3000 V at 4 cm for 3 min treatments produced 100% mortality. Different mechanisms of insect mortality are shown in Fig. 7. Apart from disinfestation, plasma treatment also assisted in reducing the microbial load (7.22 CFU per g to 4.73 CFU per g) of tenebrio flour and altered the flour properties such as solubility, water and oil binding capacity.136
image file: d2fb00014h-f7.tif
Fig. 7 Mortality of plasma treated insect.

Studies have found the effect of plasma treatment on rheological properties and changes in flours. Held et al.137 reported the flour and dough properties of hard and soft wheat. Secondary structure contribution to protein (β-turns, α-helices, random and β-sheet) was significantly affected after plasma treatment. Peak maximum hard, soft and intermediate flour time increased after plasma treatment while extensibility values decreased. Thirumdas et al.138 reported the changes in rice starch after CP treatment. At 60 W power level, gelatinization temperature (GT) reduced after 10 min of treatment. At the same time, the pasting temperature was reduced for all treatments. An increase in peak viscosity, storage modulus (except 60 W to 5 min) and loss modulus (except 60 W to 5 min) was found. Bahrami et al.139 studied the effect of plasma on the functional properties of wheat flour. It was found that protein fractions in flour changed with a higher dosage, while non-polar lipids and glycolipids fractionation had no difference after treatment, and the dough became strong due to protein and lipid modifications. When short and long rice flours were treated with plasma, it showed an increase in transmittance, swelling power, and gel syneresis. Though rice protein was not affected by plasma treatment, the pasting properties of long rice flour increased due to starch molecules' cross-linkage by ozone oxidation. In contrast, after treatment, amino acids were reduced.140

Pea protein-rich flour (PPF), pea starch-rich four (PSF), pea testa flour (PTF), and pea protein isolate (PPI) fractions of pea flour were treated in surface DBD plasma, which led to a mass loss of 2.1, 1.2, 1.3, and 1.1% for PPF, PSF, PTF, and PPI, respectively. Colour changes were increased for PPF and PPI and reduced for PSF when exposure time increased. PPF and PTF water and fat binding increased as treatment time increased to 10 min.141 Misra et al.25 reported plasma's effect on wheat flour's rheological properties. Peak time and peak integral of soft and hard flour increased significantly when voltage and treatment time exceeded 60 kV and 5 min. Ozone concentration increased with respect to time and input voltage increased. α-helix, β-turn + β-sheet percentage were reduced at 60 kV for 5 min exposure in both samples. However, they increased at 70 kV after 10 min time in soft wheat flour.

Plasma-induced flour functionality changes also resulted in end product quality changes. Menkovska et al.142 reported the effect of plasma on wheat flour and dough quality changes. An increase in farinograph values (except dough softening) was observed with respect to treatment time. Similarly, alveograph values increased (except elasticity) with respect to time. Shape formation ratio, specific and total loaf volumes increased from 0.47 to 0.58 h per day, 3.79 to 4.55 cm3, and 560 to 657 cm3, respectively, after 45 min of treatment due to gas retention and crumb cell increase.

Few research studies have focused on the mode of action of ROS on insects, while the effect of UV and RNS on insects is unclear. In addition, PTW and CAW have not produced the same mortality, even though they produced similar pH. Thus, the actual reason behind the mortality of insects in PTW is unclear. Industrial use of PTW has many practical issues since it hydrates/increases the moisture content of produce.

4.3. Processed and minimally processed fruits products quality

The primary reason for minimal processing is to preserve the food with minimal quality changes. However, as non-thermal processing, plasma can help retard browning and microbial activity without adding preservatives and retains the quality of the product. Browning in cut fruit surface is the major problem in preservation. In the case of apples (Pink Lady®, Fuji, Modì®, Red Delicious (RD)), the browning area reduced significantly after plasma treatment due to less residual PPO activity of 10% and 50% in Fuji and Modi, respectively, after 60 min. This could be due to the changes in the secondary structure of proteins and modification of the amino acid side chain of enzymes.143 Along with browning control, other parameters such as soluble solids, dry matter, acidity, firmness, and rupture strength were increased;144 however, antioxidant capacity and total phenol index (TPI) were reduced in apples.145 When the juice was extracted from apples, more than 4 logs of microbial reduction were achieved at 30 W (40 s), 40 W (40 s), and 50 W (30 s) without TSS change. However, the treatment affected total phenolic content (41.7 to 32.4 GAE mg/100 g), antioxidant capacity, pH, and colour values.146 These data confirmed the study of Surowsky et al.147 in which a plasma jet was applied to reduce Citrobacter freundii loads in apple juice by about 5 log cycles after a plasma exposure of 480 s using argon and 0.1% oxygen plus a subsequent storage time of 24 h. Similar effects were found in sour cheery nectar and other juices (tomato, apple, and orange) with a minimum of 1 log reduction in E. coli at 650 W after 120 s treatment time. However, in this case, TPC increased by more than 14% except in orange juice (9.52%).148 However, for white grape juice along with S. cerevisiae inactivation, phenols (720.62 to 445.02 GAE µg per mL), flavonoid (265.21 to 231.04 CE µg per mL), DPPH (88.16 to 82.24), and antioxidant capacity (679.35 to 637.61) were reduced after plasma treatment. The degradation of aromatic rings in phenols due to RS causes a reduction in total phenolic content. Thus it is also reflected in DPPH and antioxidant capacity reduction.149

Wu et al.150 reported the effect of corona electrical discharge (CED) plasma on banana starch property changes. DSC's onset temperature increased to 60.0, 60.5, and 61.2 °C from 57.2 °C at 30, 40, and 50 kV cm−1 intensities due to the cross-linkages created by RS in the polymeric chain of starch granules. This effect was seen in peak and conclusion temperatures increase and gelatinization enthalpy reduction. In terms of pasting behaviour, peak viscosity reduced from 100.4 (control) to 44.4 RVU (50 kV cm−1) while pasting temperature reached 92.1 °C from 74.4 °C (control) due to higher crystallinity and swelling resistance of starch.

Though plasma can reduce the microbial load of food products, it also causes certain quality deteriorations in it. Microbial reduction can be correlated with the changes in the acidity of the plasma-treated sample, but matrix effects have to be considered for clear identification of the inactivation mechanisms, and a focus on evaluating the correlation between the plasma treatment parameter and the subsequent product quality changes is needed.

4.4. Essential oil extraction and quality

Essential oils naturally have anti-bacterial properties; when treated with plasma, the properties can be enhanced and reduce the quantity of EO required to perform the same effect. While the treatment also affects the EO components due to higher oxidation in a few studies.

EO obtained from plasma-treated sweet basil produced more antioxidant activity with higher eugenol content, while the treated seed growth after a month was 40 mm higher than the control.27 Similarly, an increase in the antimicrobial property of clove oil was achieved after plasma jet processing. Due to this, the minimum concentration required for microbial inhibition (A. niger, Penicillium sp., and Rhizopus sp.) was reduced for clove oil and eugenol.151 Other than improving EO quality, the extraction yields also increased in microwave pretreated lemon peel at different plasma power levels (1.0 kV, 1.5 kV, and 2.0 kV), and it was reported that the increase in lemon peel EO yield is due to the rupture that happened on the oil glands by etching, and it was confirmed by SEM images.152 Similarly, Ebadi et al.153 reported the same in lemon verbena (Lippa citriodora Kunth.) without any pretreatment after short-time plasma exposure and found an increase in spathulenol (8.1%) and globulol (7.3%) content after 5 min treatment. Earlier, Kodama et al.154 studied the lemon peel with different gases and peel sizes to understand the effect of process and product variables on extraction. The results found that DBD used with Ar gas induced damage in lemon peel oil glands and increased the EO extraction in the initial stages of treatment. However, the surface area increase (size reduction) reduced the limonene, γ-terpinene, and β-pinene concentrations in EO due to overexposure. Similarly, plasma treatment induced both negative and positive effects in the bioactive compounds of essential oil extracted from turmeric powder. However, the treatment reduced the microbial count from turmeric powder by 1.5 logs.155

5. Effect on the food products of animal origin

5.1. Dairy products

Milk and its products are highly perishable and often contaminated by bacteria such as Salmonella spp., Streptococcus, Coliforms, Enterococcus, Bacillus, etc. Subjecting the milk to plasma treatment generates free radicals inside the sample and offers antibacterial properties. However, the antibacterial nature of the milk relies on the plasma treatment intensity. Therefore, DBD plasma is the most studied plasma treatment for milk and milk products (fat-free- dry powder and cheese). The treatment was also effective against most milk-contaminating microorganisms (Ex: E. coli, L. monocytogenes, and Salmonella typhimurium). Further, the plasma treatment produced superior quality products compared to pasteurized ones. However, its effectiveness is not superior to UHT treatment.156,157

H2O2 reactive species are the major cause for the increase of milk's antibacterial properties following plasma treatment. However, few studies suggest direct damage in the microbial cell walls and protein leakage due to plasma species.158–160 Nevertheless, based on the plasma discharge type, the mechanism of microbial inactivation will vary. Specifically, the microbes present in the liquid of the thin layer will observe more direct cell wall damage than the ones present in the larger volumes. The indirect plasma species effect in the high-volume liquid samples is the plasma species's high reactivity and dissolving property. Therefore, the secondary reactive species generated in the liquid samples will have a greater influence on the microbes when using a larger volume of samples.

Plasma treatment tends to reduce the pH of liquid samples due to the generation of H+ ions. However, in milk samples, the phosphate and milk casein buffering effect limits the pH change and causes only a mild reduction. Short-time plasma treatments did not affect the milk and milk products' colour. However, long exposure resulted in the reduction of yellowness and lightness. Further, the treatment also increases the saturated fatty acid content in the milk by oxidizing the unsaturated fatty acids. Apart from that, lipid oxidation also increases in milk products due to the oxidation of plasma species. Similarly, the reactive species oxidize sulfonate and nitrate the side chains of amino acids.156

5.2. Seafood

Cold plasma is effective against most spoilage-causing microorganisms such as L. monocytogenes, Staphylococcus aureus, and Enterobacteriaceae. However, depending on the packing conditions, the treatment effectiveness varies for different seafood products. For example, using CO2 and O2 gas inside the package of the seafood product enhances CP-assisted microbial reduction. In addition, the natural humidity inside the packaging material allows the plasma reactive species interaction and subsequently provides a bactericidal effect to the microbes post the plasma treatment.161

During plasma treatment, the hydrogen in the sample dissociates into H+ ions and reduces the pH of the sample. The increase in acidity subsequently triggers protein breakdown. In addition, reactive species also denature proteins through oxidation and fragmentation. The oxidation process causes the proteins to form aggregates with cross-linkages. Like other lipid sources, the seafood lipids oxidize due to plasma treatment and induce off-flavor upon prolonged exposure. However, CP treatment can retort the activity of several enzymes (α-amylase, lipase, alkaline phosphatase, peroxidase, and lipoxygenase) that spoil seafood. Furthermore, CP improved the colour value of the seafood samples by increasing the L* and b values through lipid oxidation and pigment production (Fig. 8).161

image file: d2fb00014h-f8.tif
Fig. 8 Impact of cold plasma on animal based food products.

5.3. Meat and poultry

Salmonella spp., E. coli O157:H7, S. aureus, L. monocytogenes, and Campylobacter jejuni contaminate most meat and poultry products. These contaminants can be removed effectively using CP treatment. In addition, studies have shown the decontamination efficiency of cold plasma in many types of meat and poultry products (i.e., pork, beef, poultry meat, and egg). The mechanism of action is similar to that discussed in the earlier sections. However, the quality characteristics vary based on the product's nature.162

At minimal exposure level, CP treatment does not induce any colour changes. However, at longer exposure times, the L* (reduce), a (increase), and b (increase) values will change significantly. However, the treated sample partially recovered from the colour degradation during the storage period. Similarly, most of the plasma-treated meat and poultry samples observed a minimal effect on the off-odor and water content. However, it can be avoided by minimizing the treatment duration. When it comes to pH, inert gas CP treatment did not cause any considerable changes in meat or poultry products. However, excess nitrogen reactive species reportedly reduced the pH of the CP-treated samples. Nevertheless, the lipids in meat and poultry products also are oxidized due to the reactive plasma species. Further, the treatment also results in the degradation (oxidation) of proteins due to the acidic compounds (H2O2) produced by CP treatment.162

6. Sensory and consumer perception of plasma treated-food products

Though non-thermal technologies preserve the freshness of food products, some techniques impair the sensory qualities. For example, the oxidation of lipid compounds during plasma treatment results in the production of off-flavor-producing compounds. On the other hand, CP treatment also improved the sensory qualities of certain food products. For instance, cut cantaloupe exhibited better sensory characteristics after 40 kV plasma treatment (90 s). Whereas 15 kV CP treated dairy beverage did not cause any sensory attribute changes to the product. However, products like tender coconut had a negative impact (chemical odour) on the sensory quality due to CP (49.4 kJ kg−1) treatment.163 Therefore, understanding the CP effect on the sensory properties of different food products will be a breakthrough in plasma research. Nevertheless, consumer perception of the plasma-treated products also needs to be counted for the success of plasma-treated food products. In this regard, consumers' perceptions of food products can be analyzed through the brain-computer interface method to obtain unbiased results from the consumers.164 Therefore, Coutinho et al.165 surveyed the perception of chocolate milk with 1085 participants. The study revealed that consumers are more concerned about the CP-treated product price and feel the plasma-treated product will be costlier than the conventionally processed products.

7. Other novel applications

7.1. Waterless (hydroponic) plant cultivation

Plasma activated water not only assists plant growth in stress (saline, drought, or heavy metal) conditions but also in soilless conditions. Generally, the nitrogen required for plant growth is supplied directly from the soil or from soil microbes. Hence, when the plants are grown in hydroponic conditions, substrates like coir peats, rice husk, etc., are used to support the plant root and provide nutrients. However, PAW itself contains RNS that act as a nitrogen source for plants and eliminate the requirement of substrates. In addition, ROS species also produce H2O2 in PAW, which helps in controlling infections on the plant roots.166 Nevertheless, the PAW effect on plants may not always be positive, as few studies mentioned a negative effect on plant growth (Lettuce); however, after PAW supply, the secondary metabolites (Epicatechin, rutin, quercetin, total phenolic) were found to be high in both roots and leaves.167,168 However, a recent study utilized the plasma jet generated PAW solely for fulfilling the nitrogen requirement of corn and lettuce plants and assisted in seed germination and plant growth.169,170

7.2. Cold plasma as green fertilizer

The rapid climate change and increased population will have an impact on food shortage in the future. Therefore, increased crop production is needed, which is done with the utilization of chemical fertilizers for agricultural crops. Hence a sustainable technology with less energy consumption and reduced chemical fertilizer use are of interest to researchers and producers to meet the demands.171 Cold plasma has the potential to be used as a green fertilizer in the future due to its increased efficiency and eco-friendly production. As the plasma treatment will lead to the production of reactive oxygen species and reactive nitrogen species in water, it acts as a fertilization liquid facilitating plant development. The H2O2 formed during the plasma treatment initiated the abscisic acid hormone and gibberellin, which is responsible for seed germination and dormancy. Similarly, the NO3 generated in PAW has a direct impact on the growth rate of the plant, as nitrate is a crucial element for plant growth. One of the major advantages of PAW as a green fertilizer is the presence of nutrients as ions rather than salts, which helps in easy absorption.170 Matra et al.172 have found that the PAW spray generated from the gliding arc plasma generator enhanced the amount of nitrogenous fertilizer in treated water. Similarly, Subramanian et al.173 have reported that PAW generated from DBD has a higher specific energy of nitrogen, which is desirable for agricultural purposes. Stoleru et al.168 have also concluded that the nitrites and nitrates from PAW worked as a fertilizer and enhanced the growth of plants. Though there are many studies on PAW as green fertilizer, the appropriate quantity of water used and concentration of nitrates could be a major breakthrough in this application.

7.3. Shape transformation

3D printing and sessile drop drying are the two commonly used additive manufacturing practices to produce shape-changing 2D food structures (xerogels). Both these techniques produce flat 2D food structures that can transform their existing structure into a defined 3D structure under external stimuli (water, oil, drying, or pH) contact.174–177 However, to achieve the desired shape-shift, flat 2D structures need to be coated with food-grade constraint materials (i.e., ethyl cellulose). These constraints act as a barrier between stimuli and food surfaces and limit the interaction in the coated areas. However, the uncoated areas interact with stimuli and induce stress gradients between coated and uncoated areas. As a result, the relative expansion increase causes the food structure to fold or bend in a specific pattern to produce a desirable 3D structure.178–180 Nevertheless, surface wettability (i.e., water absorption and oil absorption) and the binding nature of food surface with constraint material decide the success of these shape-transforming foods. Both these factors can be improved by treating these 2D foods in plasma. Plasma etching increases the surface area of food, thereby its binding behaviour and water/oil absorption.30,180 Hence, to utilize the advantages of plasma treatment in shape transformation, Gupta et al.181 treated wheat xerogels using glow discharge plasma (power: 7.32 W, duration: 5 min) and then coated them with ethyl cellulose in a defined pattern. Due to plasma treatment, the swelling gradient created between coated and uncoated areas of xerogels increased and resulted in better shape transformation when immersed in 90 °C hot water. Similar results were obtained in Stephen et al.182 study on oil-triggered shape transformation, where the author treated corn xerogels in cold plasma (voltage: 1 kV, duration: 5 min) prior to cellulose acetate linear strip coating. As a result, the 2D structure curled into a spiral shape within 2 s in hot (220 °C) coconut oil due to the high relative extension and constraint material binding. Likewise, Cheeyattil et al.183 obtained flower shape and samosa shape in the barley flour xerogel through oleomorphic shape shifting. However, same xerogel can be used to obtain water and oil-based shape transformation. Therefore, Jaspin et al.179 tried obtaining complicated flower shapes from plasma-treated flat xerogels using hot water and oil stimuli and succeeded in both attempts.

8. Conclusion

Plasma treatment significantly affected the agri-food chain in terms of maintaining the quality and safety of products, increasing germination, extraction efficiency, plant growth, and removing hazardous contaminants from water, soil, and agricultural produces. The interaction of plasma with food products, microbes, allergens, toxins, enzymes, insects, and other constituents varied depending on process variables such as gas composition, gas flow rate, power level, frequency, treatment time, and type of plasma chamber; product variables such as cultivar type, moisture, and other compositions; and other variables such as nature of pesticide, microbes, chemical contaminants, and their initial concentrations. Different mechanisms were involved in different plasma applications, such as microbial inactivation by surface etching and DNA modification, chemical removal by oxidation, allergen removal by protein and lipid functionality modification, germination by surface modification (wettability and surface etching), seed tolerance by heat shock protein production, disinfestation by nerve toxin effect, higher EO extraction by oil gland damages, browning retardation by changes in the secondary structure of proteins and modification of amino acid side chain, etc. However, many other reasons, such as species and cultivar-dependent efficiency change of treatment in microbes and agricultural commodities, extract quality improvement, uncertain quantity changes in food properties (i.e., increase and decrease in TPC, colour changes, etc.), the effect of RS on consumption to human health and plant growth needs to be explained in further research studies. Also, plasma treatment's large-scale application in these areas is still not stated. Laboratory experiments were mostly done for a lesser quantity of samples. Cost, efficiency, safety, and productivity will be the concern when plasma treatments are used for large-scale operations. Understanding plasma treatment and RS mode of action can increase the chance of productive utilization of plasma treatment by avoiding deterioration during treatment. Some studies have mentioned the long-term effect of RS on product quality and safety.

Conflicts of interest

The authors also declare that there are no conflicts of interest.


The authors wish to acknowledge the funding received from European Union COST Action: PlAgri (Grant No: CA19110) and Science Foundation Ireland (SFI) under grant number 17/CDA/4653.


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