Introduction

In the current era, there have been major changes in the eating habits and diet patterns among consumers. Today’s consumers are comparably more aware of health and meeting their nutritional needs. Also, various international health organizations have stressed the proven benefits of a diet rich in fresh foods or with minimal alterations. As a consequence, there is a rise in the consumption of fresh and minimally processed foods.¹ Such foods contain higher water and nutritional content and also as they are mostly consumed in raw forms without much processing, they are an active carrier of various food-borne illnesses.² Shelf-life extension of the fresh and/or minimally processed foods are conventionally achieved by thermal techniques and the addition of the permitted preservatives but the originality and fresh flavors are lost. So, there is a continuous quest for a technique that provides safe, nutritious food with minimum effect over the sensory and functional properties along with an increase in the storage life of the product.

Conventional thermal techniques like-freezing, drying, cooling, pasteurization, are used to enhance shelf-life of foods but employing non-thermal techniques has gained much importance in recent times.^3,4 Plasma technique is one such non-thermal techniques used for food processing.⁵ Plasma technology offers major benefits to heat-sensitive products by having minimum deteriorative effects over quality. It has major applications including retaining nutritional properties, functional value, and sensory properties thus ensuring a fresh appearance or characteristics. It is also used for structural modifications of food and even packaging materials. This review investigates and discusses the published articles on cold plasma processing of foods or their products, and the data on the impact of such a treatment on the quality and safety aspects of different food groups.

How cold plasma is generated?

Firstly, the term ‘Plasma’ was coined by Irving Langmuir an American physicist in 1928.^1,5 Universally, ‘Plasma’ is mentioned to as the fourth state of matter. It’s quite simple to generate with a basic principle involving energizing a neutral gas system. Adding sufficient energy to a gas system leads to excitation of the gas molecules commonly referred to as ‘ionization’ which is due to collisions among the gas molecules. As a consequence, a combination of varied reactive species like electrons, ions (+ and -) , free radicals, molecules in the ground and/or excited states, photons are generated and this constitutes the ‘plasma’.^5,6,7,8 Thus, such quasi-neutral gases have sufficient energy and are chemically very interactive, and confer varied chemical and biological effects. Distinctively, plasmas are differentiated into ‘thermal plasma’ and ‘low-temperature plasma’ based upon thermodynamics. Typically, ‘thermal plasma’ is generated by subjecting a gas to temperatures greater than 20,000 K; in such a setup, all the generated reactive species, electrons and ions are in a thermodynamic temperature equilibrium; thus, the plasma is at a very high temperature.⁵ On the contrary, ‘low temperature’ plasma is commonly generated using electrical discharges at near room temperature. Here, the cooling of the varied gas species/molecules is more effective than energy transfer from electrons i.e. thermodynamic non-equilibrium thus, the gas temperature remains at low temperature. Thus, such non-equilibrium plasma generation is assigned as ‘cold plasma’ or non-thermal plasma (NTP).^7,8,9,10 In the current, review we are only concerned with ‘cold plasma’ applications on various foods.

In the available literature, cold plasmas have been generated using varied configuration systems. Widely reported methods to induce ionization are corona discharges, dielectric barrier discharges, microwave discharges, and plasma jets.^1,5,9 In all systems, the feed gas is energized into plasma. Different types of feed gases can be used in plasma generation such as air or nitrogen (N₂) or noble gases like- Helium (He), Argon (Ar), and Neon (Ne) or their mixture with oxygen (O₂) and moisture.^2,9,11 The antimicrobial nature of plasma is attributed to the generated reactive gas species of oxygen (ROS) and nitrogen (RNS) owing to collisions of molecules on energizing.^1,8,9 Commonly identified and reported ROS and RNS include O, O₃, OH₃, NO, NO₂ and these plays a significant role in microbial decontamination.^2,7,9,12 Overall, it been suggested that the preservation efficacy of plasma effects increases when a feed gas is supplemented with O₂.⁷

Mechanism of microbial inactivation by cold plasma

Figure 1: Flowchart representing the impact on microbes mediated by cold plasma treatment. Click here to view Figure

The mechanism behind microbial decontamination involves the interaction of generated reactive species viz. radicals and reactive molecules with their cell membranes and cellular functions. The major species of interest conferring decontaminating effect includes ROS [atomic oxygen (O), singlet oxygen (¹O₂), excited state of oxygen (O₂^*) and superoxide anion (O₂^– )] and RNS [atomic nitrogen (N), nitrogen positive ion (N₂⁺) and excited state of nitrogen (N₂^*), nitric oxide free radical (NO•), and nitric oxide (NO)]. And in cases where humidity is involved during plasma generation species like H₂O⁺, OH⁻ anion, OH• radical, and hydrogen peroxide (H₂O₂) are generated.¹³ NO, and NO₂ damages cells by causing lipid peroxidation, denaturation of proteins and enzymes, and interaction with genetic material.^7,10,12 Additionally, the generation of UV photons has also been linked to plasma-mediated microbial inactivation.² The photons impair DNA replication by forming thymine dimers, nucleotide-base modifications, and nucleotide oxidation.⁵ Cell damage, cell bursting, pore formation, cell leakage, cytoplasm shrinkage, etc. have also been observed for cold plasma treated microbes under electron microscopy.^14,15,16,17 The foregoing literature indicates that cold plasma proves to be an effective non-thermal strategy to achieve microbial decontamination. However, much more areas need to be further explored especially in terms of quality impact post plasma treatment on food products.^8,11

Applications of cold plasma in food processing

Figure 2: Applications of cold plasma technology in food sector. Click here to view Figure

Cold Plasma has found effective applications as anon-thermal technology for mild surface decontamination in foods as well as packaging materials. As it employs the use of gases, which could reach every nook and cranny, it is one of the most effective ways of disinfection as compared to cleaning with water or chemicals. When microbes are present on the food surface, cold plasma can be used for inactivating both vegetative cells and spores (Disinfection of foods). The packing material used for food products plays a crucial role in retaining food quality. Cold plasma can also be utilized for surface sterilization of packaging material eliminating microbes and their spores (Disinfection of packaging material). For heat-sensitive materials, it is the best way for controlling disinfection. Cold plasma is a gas so it can easily treat irregularly shaped packages and bottles which is otherwise a difficult task. This technology has found application in not only controlling the microbial contamination of food or food products but also able to improve or maintain the quality of raw/processed foods from various food groups.

Cold plasma treatment of cereals

Cereals form a major source of the diet of the people of India. Plasma, a non-thermal technology has proven applications in the cereal industry (Table 1). Brown rice has higher nutritional values²⁶ but its poor cooking and eating properties limits its desirability. Few experiments with plasma technology suggest improving both cooking and eating properties of rice. A low-pressure cold plasma treatment of parboiled rice resulted in higher water uptake ratio and the cooking time was reduced to 8 min.²⁷ Textural properties like hardness and stickiness were also improved and a decreasing trend was observed on increasing power and treatment time. A similar low-pressure plasma treatment (1-3 kV-10 min) on germinated brown rice led to an increase in the germination rate, seedling length, and water uptake.²⁸ The 3 kV plasma treatment for 10 min gave the best results and α-amylase activity was also higher in the rice as compared to the control rice.

Table 1: Cold plasma treatment impact on cereal grains/products.

DBD: Dielectric barrier discharge

Plasma-treatment of brown rice resulted in an increase in the antioxidant activity and the levels of gamma-aminobutyric acid by 32 %. Cold atmospheric plasma treatments were employed to inhibit the Aspergillus flavus growth on brown rice cereal bars .¹⁹ A plasma treatment (40 W-20 min) when applied on rice-based cereal products resulted in decrease of surface mycelium by around 4 log₁₀ CFU/g and delayed the mycelium growth for 20 days. Plasma treatment also has a positive impact on wheat seeds.²⁹ The plasma treatment using a surface discharge reactor on wheat seeds led to an increase in the roots and sprouts of the wheat seeds with little effect on germination rates.

Cold plasm generated by a Dielectric barrier discharge (DBD) method has shown changes in the structure, granule morphology, crystalline properties, and rheological properties of corn starch.³⁰ Treatment using DBD not only affected the surface morphology of the starch granules, creating pinholes. Also, the plasma treatment caused oxidation of partial hydroxyl groups to carbonyl groups, and molecular degradation occurred resulting in a decrease in the degree of crystallinity. Thus, opening up avenues for development of chemical-free starch modifications. Modified starch finds great applications in food industries as an emulsifier and for the formation of starch-based films. Hence, cold plasma can be an alternative to chemical modifications of starch.

Cold plasma treatment in dairy products

Milk and milk products are important dietary requirements worldwide. Being highly nutritive dairy and dairy products are very prone to microbial and chemical putrefaction. Although thermal treatments like pasteurization or sterilization no doubt ensure microbial safety and extend shelf-life, negatively impact the nutritive and sensory attributes.³¹ Thus, the benefits of non-thermal technologies are quite attractive for the dairy sector.

Table 2: Impact of cold plasma treatments on dairy and dairy products.

DBD: Dielectric barrier discharge

Cold plasma has demonstrated remarkable efficiency in the removal of undesirable microorganisms in the dairy sector while retention under many circumstances, the nutritional, functional, and sensory aspects of the products when compared to conventional approaches (Table 2).

The dielectric barrier discharge (DBD) method has been predominantly tested for milk and milk products. A DBD method (80 V-120 s) caused 100 % fatality to Staphylococcus aureus and E. coli and about 98 % fatality to L. monocytogenes in milk.³⁶ Also, an increase in voltage and treatment time induced higher lipid peroxidation. However, the increased lipid peroxidation was comparable to that of UHT-treated milk. In another study, with DBD treatment of milk at 2 kV under reduced pressures (0.16 mbar) resulted in approximately 1 log reduction in background microflora without having any detrimental impact on the physicochemical parameters.³⁷ An interesting study, wherein chocolate milk drinks were subjected to cold plasma technology (varying nitrogen plasma flow rate and treatment time) altered the physical and thermal properties.³⁸ Milder and severe treatment resulted in higher particle size, consistency, and altered melting profile indicative of protein denaturation. However, intermediate processing levels resulted in chocolate drinks having similar characteristics to that of pasteurized ones. Thus, it can suitably be recommended that CP technology can be further tuned for better results in maintaining attributes. Cheese is being a recognized source of food-borne illnesses. Experiments on sliced cheddar cheese, inoculated with L. monocytogenes, E. coli O157:H7, and Salmonella thyphimurium on thin-layer DBD N₂+O₂ plasma (100 W-10 min) exposure respectively decreased the counts by 3.2, 2.1, and 5.8 log₁₀ CFU/g.³³ However, this treatment led to significant reductions in flavor and overall acceptance along with changes in physiochemical parameters. Queso fresco is a cheese that is prone to Listeria contamination. Researchers³⁹ subjected the Queso fresco cheese package to a DBD in package treatment with moisture free air at 60-100 kV for 5 min. A 5 min treatment resulted in respective reductions by 1.4 and 3.5 log₁₀ CFU/g reductions for L. innocua and E. coli K-12. It was also noted that the treatment inflicted slight changes in the pH, color, and lipid oxidation of the cheese but non-significant impact on the texture. Interestingly, subjecting whey protein isolate to plasma treatment lead to mild oxidation and changed surface hydrophobicity of proteins, in turn improving the foaming and emulsifying property and also causing proteins to aggregate.⁴⁰ From a technological standpoint, this opens avenues for modulating protein functionality for further research and industrial applications. Overall, CP applications have suggested promising results on microbial inactivation however, fine-tuning of process parameters is critical to safeguard the quality attributes in dairy and dairy products.

Cold plasma treatment of meat and meat products

The meat industry is a major player in food manufacturing and processing sector. Historically, a surge in meat consumption occurred worldwide between 1960 and 2000, and an increase in the per capita meat consumption from 10 kg to 26 kg annually.⁴¹ Meat being an excellent source of nutrients and is easily contaminated by microbes, such contaminations lead to spoilage and loss of meat and meat products. Overall, losses are also generated due to consumption, processing and packaging (around 21%), distribution, handling and storage.⁴² Meat spoilage is dominated by three factors a) microbial proliferation, b) lipid oxidation, and c) enzyme action.⁴¹ Meat spoilage leads to its discoloration, formation of undesirable odors, flavors, and textural losses (softening) which results in rejection by consumers. Pathogens that affect meat include Salmonella spp., Escherichia coli, Listeria monocytogenes, Staphylococcus aureus, and Clostridium botulinum.^43,44 The most common meat spoiling microorganism being Pseudomonas spp. it results in development of putrefactive odors and slime when their population levels exceed 10⁷ CFU/cm².⁴³ The use of low-pressure plasma technology serves to be advantageous in the meat industry (Table 3) and can treat wide areas uniformly for decontamination.

A DBD type plasma exposure for 10 min on chicken breast decreased the total aerobic counts, L. monocytogenes, E.  coli, and S. typhimurium by 3.36, 2.14, 2.73, and 2.71 log₁₀ CFU/g.⁴⁹ Also in-package DBD plasma treatment of inoculated pork butt inactivated L. monocytogenes, E. coli 0157:H7 and Salmonella by 2.04, 2.54, and 2.68 log₁₀ CFU/g respectively and inoculated beef loin by 1.90, 2.57, and 2.58 log₁₀ CFU/g, respectively for the same microbes.⁵⁰ Cold plasma treatment of modified atmosphere gas (high O₂ and CO₂) filled packages increased the shelf-life of chicken meat along with a reduction in microbial loads compared to air-filled packages.⁵¹ Modified atmosphere (65 % O₂, 30 % CO₂,5 % N₂) gas packages treated with cold plasma resulted in restricting microbial load > 4 log₁₀ and extended shelf-life of chicken for up to 14 days.

Table 3: Effect of cold plasma treatment on meat and meat products.

DBD: Dielectric barrier discharge

Rosemary (Rosmarinus officinalis) extracts have high antioxidant.⁵² Rosemary extract addition alongside plasma treatment has shown promising results in reducing lipid oxidation in meats. When chicken breast patties containing 1 % rosemary extracts were plasma treated at 70 kV for 180 s and stored for 0 to 5 days, the addition of rosemary extract prevented lipid oxidation in treated meat and also inhibited the microbial growth.⁵³ It increased the pH of the treated samples, but altered color reported by a decreased L* values, and increased a* and b* values. Research on natural extracts from plants as additives along with cold plasma treatment can be further pursued to control the microbial quality and lipid oxidation of plasma processed foods. Cold plasma treatment of meat and meat products has shown promising results in decreasing the psychrophilic or mesophilic microbial population without changing the appearance of meat and increasing the nitric content in meat batter (Table 3). Overall, there is promising application of plasma technology in meat processing.

Cold plasma treatment of egg and poultry products

There has always been a great demand for egg and egg-based products worldwide. Today consumers have become more aware towards food safety issues and prefer eggs with clean shells and free from contamination. Eggs can be contaminated by many micro-organisms during production, processing, food preparations, and consumption.⁵⁴ Salmonella spp. contamination presents a major concern for the egg industry. Commercial egg cleaning four steps: wetting eggs to soften the debris on the shell; washing eggs with brushes; rinsing with clean hot water, and drying by air jet.⁵⁵ However, chemical washing utilizes chlorine (sodium hypochlorite), and it is capable of destroying a wide spectrum of pathogens but due to its carcinogenic effects its use has become limited.^56,57 Thus, cleaning or treating eggs without chemicals or with reduced water usage is a preceding need, and cold plasma technology presents itself as such a technology.

Table 4: Cold plasma impact on eggs and poultry products.

DT: Direct treatment; IDT: Indirect treatment

A high voltage pulsed jet plasma treatment (20 kV-5 min) exposure sufficiently inactivated all the S. enterica serovar typhimurium on eggshells.⁵⁸ Also He-gas plasma jet treatments on L. monocytogenes at 2 kV input voltage for 2 min on steam cooked egg yolk and egg white resulted in a decrease of the microbial population by 5 logs in cooked egg white. Whereas with O₂+ N₂ mixtures reductions in microbial counts by 6.7 logs were reported.⁵⁹ Additionally, direct application pattern of plasma was found to be more effective than the indirect plasma application for decontamination of egg shells. An exposure time of 15 min reduced the Salmonella counts by about 6.4 and 4.3 log cycles respectively for direct and indirect treatments.⁶⁰ Overall, it can be noted that decontamination efficacy is dependent upon the carrier gas compositions employed, treatment duration, and exposure modes. Cold plasma treatment has been successful in controlling the Salmonella population without apparent changes in egg and poultry product’s quality (Table 4).

Cold plasma treatment of nuts and seeds

Table 5: Effect of cold plasma treatment on nuts.

AP-CCP: atmospheric pressure capacitive coupled plasma; DC-DP: direct current diode plasma; and ICP: inductively coupled plasma

Cashew nuts (Anacardium occidentale) are an important source of nutrition and are widely consumed. They are regarded as excellent sources of protein, fats, and vitamins.⁶⁴ Their antioxidant activity is attributed to phenolic composition mainly constituted by a mixture of anacardic acids (monoene, diene, and triene). The anacardic acids have bactericide, insecticide, fungicide, cytotoxic and anti-inflammation properties.⁶⁵ Cashew nuts also possess allergic reactions in some individuals. Allergy due to cashew nut is a commonly reported tree nut allergy specially in the US.⁶⁶

Influence of atmospheric plasma on the cashew nut composition and allergenicity was investigated using glow discharge plasma at 80 W and 50 kHz power supply.⁶⁷ 30 min treatments resulted in an increase in anacardic acids content from 0.2 to 0.55 µg/mg and a decrease in sucrose levels from 33 to 18 mg/g. The plasma processing however did not affect the binding of human cashew allergic IgE. In another work, cold plasma application on almonds dipped levels of E. coli culture broth, and the most effective treatment was at 30 kV, 2000 Hz for a duration of 30 s which resulted 5 log reduction in bacterial populations.⁶⁸ The bactericidal effects on E. coli inoculated on almonds increased linearly with the applied plasma treatment voltage and frequency.

Hazelnuts (Corylus avellana) are widely consumed nuts, and they can be consumed in-shell and shelled. Commonly such nuts are contaminated by aflatoxins which are also found on other nuts, cereals, spices, etc.⁶⁹ The genus Aspergillus contaminates hazelnuts and produce aflatoxins (AFs), specifically A. parasiticus produces four main aflatoxins viz. AFB1, AFB2, AFG1, and AFG2 whereas A. flavus produces AFB1 and AFB2. AFs are as carcinogenic agents to humans and animals. A detailed study on cold atmospheric pressure plasma treatment effect on mycotoxin detoxification was conducted on hazelnuts.⁷⁰ Firstly, different gases concentrations were tested (N₂, 0.1 % O₂ and 1 % O₂, 21 % O₂) and then power and exposure time were optimized. Power of 1000 W for 12 min effectively reduced AFB1 and total aflatoxins concertation by approximately 65 % and 74 % respectively. It was also found that plasma treatments were more effective for degrading AFB1 and AFG1 compared to AFB2 and AFG2. Hence, cold plasma applications seem viable for effectively detoxifying nuts containing aflatoxins, and reduce contaminating microbial populations without affecting the sensory qualities (Table 5).

Cold plasma treatment of fruits and vegetables (F&Vs)

Recommendation by major governmental agencies has fueled the increased consumption of F&Vs among consumers. Additionally, the consumers are also aware of the primary health-benefiting components in F&Vs.^75,76 Considering, the fact that F&Vs are typically ingested raw, concerns of safety and quality is paramount. CP can be employed as an in-package treatment method for a variety of F&Vs, including surface disinfection and even for fresh-cut produce.^77,78,79

A CP treatment at 60 kV for 5 min lowered innate aerobic bacterial populations and increased cutting-induced phenolic buildup in newly cut dragon fruit, according to one study.⁸⁰ In sliced carrots about 2 log reductions of total mesophiles, yeast and molds were observed on CP treatment (100 kV-5 min) while the process retained textural qualities and the carotenoid content.⁷⁹ In pilot-scale studies, interesting results have been observed for CP treatments. In a recent study, aerosolized H₂O₂ ionized using cold plasma under a laboratory setting resulted in ≥ 5 log₁₀ reductions in L. innocua and S. thyphimurium in grape tomatoes, apples, cantaloupe, and Romanian lettuce.⁸¹

The same group further ran a pilot-scale investigation following the same methodology and demonstrated reduction of S. typhimurium and L. innocua in apples, tomatoes, and cantaloupe below detection limits (<0.7 log CFU/pc).⁸² Additionally, the treatment did not significantly affect quality attributes such as appearance, color, texture, pH, ascorbic acid content, soluble solids content and antioxidants in the fruits. Another pilot plant study employing the DBD method involving humid air was efficient in minimizing E. coli and L. monocytogenes populations in strawberries and spinach by 2-2.2 logs and 1.3-1.7 logs respectively.⁷⁸

Corona discharge air plasma (CDAP) treatment employed for post-harvest storage of onions resulted in lower infections of Fusarium spp. and Alternaria sp. during cold storage.⁸³ CP has been widely studied for the treatment of other fruits and vegetables like cabbage, cherry tomatoes, cucumber, baby kale, radicchio leaves, peas to name a few.⁵ Interestingly, cold plasm’s ability to inactivate norovirus in foods such as blueberries, Tulane virus in Romanian lettuce has opened further possibilities^84,85,86 in attaining viral food safety.

CP treatments impact quality attributes differently in different F&Vs. Existing literature suggesting detrimental impacts of CP treatment on quality attributes both physical and chemical⁸⁷ somewhat raises concerns over its potential. However, fine-tuning of CP treatment approaches viz., employing milder treatments, intermittent treatments, and the use of plasma activate water for treating F&Vs have resulted in better preservation of quality attributes.

Several studies (Table 6) have been conducted for cold plasma treatment of fruits and vegetables, and reported that cold plasma treatment can enhance antioxidant activity, phenolic compounds extraction, total sugar and microbial reduction.

Table 6: Impact of cold plasma treatment on fruits and vegetables.

DBD: Dielectric barrier discharge

Cold plasma treatment of spices and herbs

Herbs, spices and their essential oils are quite widely used in varied sectors ranging from food to cosmetics. However, herbs and spices are predominantly used in the dried powdered form. Conventionally processed under open environments e.g. sun/shade drying, resulting in contamination with pathogenic bacteria, bacterial spores and toxigenic fungi raising health concerns.^95,96 However, counteracting those using thermal treatments is not practical, as the key active volatile and aromatic constituents, and color responsible for quality and price are lost. Furthermore, use of chemical fumigants like ethylene oxide is under scanner or has been banned by some countries. Prevailing research is indicative of cold plasma applications as a non-thermal decontaminating technique for herbs and spices without potential toxic residues.

A microwave driven plasma setup forming a remote plasma by Hertwig and others⁹⁷ applied for decontamination of black pepper seeds, crushed oregano and paprika powder. Result of the treatment suggested effective inactivation of native microflora in each of the samples. Greater reduction was observed in pepper and paprika powder (~3 log CFU/g in 60 min) compared to oregano (1.6 log CFU/g). However, a considerable loss in paprika color was observed within 5 min of treatment while, lesser impact on oregano and pepper seeds were observed. CP treatment when integrated with microwaves or mild heat has been effective in inactivating Bacillus cereus and spores in red chilli flakes and powder reaching destruction levels till 2.6 logs for spores and around 4.4. logs for vegetative cells. Importantly, a combined treatment better preserved the color of the samples^98,99 compared to plasma treatment alone. A similar combined treatment involving pulsed light with CP, non-thermally inactivated spores of A. flavus (~1.3 log spores/g), B. pumilus (~2.3 log spores/g), and E. coli O157:H7 (>3.8 log CFU/g) in red pepper flakes  while retaining the color and moisture content.¹⁰⁰ A radiofrequency low pressure plasma process (50 W, 20 min, 40 mTorr) with oxygen as working gas proved to be efficient in eradicating E.coli O157:H7 in dried pepper mint, maintained its quality, improved its antioxidant activities and total phenolics and darkening of the sample post treatment was observed.¹⁰¹  Low pressure radio frequency oxygen plasma (60 W, 15 min, 8.5 mTorr) also proved to be efficient in eliminating toxigenic fungi viz. Aspergillus spp., Rhizopus spp., and Penicillum spp. from saffron strands. On comparing plasma-treated saffron to untreated samples, the crocin ester, picrocrocin, and safranal component of treated saffron exhibited no significant decreases. Additionally, the treated sample showed visible etching under SEM which possibly led to accelerated color release.¹⁰² Plasma generated using ambient air using a coplanar surface barrier discharge system led to ~5 log CFU/g reduction in vegetative cells of B. subtilis and and ~ 2 log CFU/g for spores on black pepper corns. The treatment also eliminated E. coli and S. enteritidis below detection levels (1 log CFU/g) and no significant changes in chemical bonds and morphology of the pepper corns were reported.¹⁰³ Argon based microwave plasma resulted in complete eradication of A. niger molds on black pepper corns, juniper berries, and allspice within 45 s but the treatment was inefficient for inactivating B. subtilis spores. Further, experiments suggested antioxidant activities of the three samples increased with plasma treatment but again color loss was notable.¹⁰⁴

Thereby, available research shows promising aspects of CP in microbial safety of herbs and spices (Table 7). However, color loss is a distinct parameter under a negative impact which needs to be looked into further.

Table 7: Effects of cold plasma treatment on spices and herbs.

DBD: Dielectric barrier discharge, LMCT: Low microwave cold plasma treatment, HMCT: High microwave cold plasma treatment, UV: Ultra-Violet.

Cold plasma treatment of fish and fishery products

Sea foods are a rich source of proteins and lipids (e.g. omega-3-fatty acids).¹¹² But due to the presence of endogenous enzymes and microbial contamination, sea foods have limited shelf life. As per the United Nations Food and Agricultural Organization (FAO), fish products are demarcated as nutritious food source.¹¹³ Prawns and Shrimps contribute about 16 % of export value and are popular worldwide.¹¹⁴ As the demand for fresh sea foods is increasing, it is quite necessary to develop novel processing and preservation methodology with higher decontamination efficacy.

High voltage cold atmospheric pressure plasma (HVCAP) treatments of Asian Sea Bass slices with argon and oxygen gas mixtures for 5-10 min resulted in a decline in the total viable cell counts by 1.0 log₁₀ CFU/g.¹¹⁵ Also a 5 min treatment extended the shelf-life of the sea Bass slices with refrigeration.  However, longer duration of treatments elevated levels of lipid oxidation and caused protein denaturation. In fresh shrimp preservation, plasma activated water (PAW) greatly impacted the inhibition of microbes on a storage time of 4-8 days.¹¹⁶ PAW treated shrimps did not change shrimp proteins but the pH reduced below 7.7 during storage. Also, there was a delay in change in color and hardness upon treatment, and production of volatile basic nitrogen diminished to below 20 mg/100g during PAW ice storage.  Microbial and quality parameters were also tested on Mackerel fillets with DBD treatment at 70 kV and 80 kV for time 1-5 min.¹¹⁷ The treatment resulted in sufficient reduction in the number of spoilage causing bacteria (aerobic, psychotropic, Pseudomonas and lactic acid bacteria). On quality parameters no changes in the color and pH were observed, however, lipid oxidation got elevated.

Limitations associated with Cold Plasma Technology

Plasma technology offers some limitations as its use is still limited over laboratory scale. Due to this it has not been commercialized in the food sector yet. The use of plasma technology to treat nuts has been reported to cause an increase in the peroxide values. With increase in power and treatment time a 20 % increase in the peroxide values was observed for plasma treated walnuts, this may be due to fact that the free radicals present in cold plasma induced auto-oxidation of fats and lipids present in the foods.¹¹⁸ A decrease in the color parameters like L*, a* and b* of plasma treated strawberries was reported.⁸⁸ Another disadvantage is that cold plasma technology cannot be used for complete inactivation of endogenous enzymes like polyphenol oxidases and peroxidases that causes enzymatic browning and deterioration of outer surfaces as it is a surface phenomenon. An optimum flow rate and process time of treatment required, otherwise there may be decrease of the bioactive compounds; change is fatty acid composition (more saturated fats, and low MUFA and PUFA); and health indices in case of dairy products.¹¹⁹ More studies are required on plasma gas compositions, treatment times and mode of treatment (indirect or direct plasma treatment) for efficient use of the cold plasma technology for decontamination and quality maintenance of food products.

Conclusion and Future Prospects

Cold plasma technology is one of the most recent and widely explored non-thermal techniques. The ability to generate various reactive gases at room temperature assists in achieving microbial decontamination, inactivation of endogenous enzymes, and preservation with minimal impact on the physical, physicochemical, and functional properties of the treated food. Although remarkable results have been put forwarded for its use in varied food industry, viz, cereals, fruits and vegetables, dairy, meat processing among others, still it needs to overcome certain challenges. Much research is required in the mechanism of action of the reactive ion species with food components. The effect of cold plasma on allergens and anti-nutritional factors is still lesser known and is an area of great interest for further exploration. More scientific work is encouraged for clear and depth knowledge of the cold plasma technology and its future aspects.

Acknowledgement

The authors would like to thank the Departments of Food technology, Jamia Hamdard University and Rajiv Gandhi University for contributing to the work.

Conflict of Interest

The authors declare that there is no conflict of interest.

Funding Sources

No funds were availed from any funding agencies or sources for preparing this review article.

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