Cold Plasma Technology: An Introduction
An emerging technology in recent years is cold plasma technology as a process for killing microorganisms (Niemira, 2012a) especially pathogens on delicate surfaces. It is also referred to as cold atmospheric plasma (CAP), cool plasma or non-thermal plasma technology in other research circles.
What makes it attractive is that as a dry technology it is applied at ambient temperatures so doesn’t rely on cold or heat processing unlike the thermal plasma methods used in arc-welding or other high-temperature applications (Laroussi & Akan, 2007; Niemira and Gutsol, 2010). It is being applied with great promise as a food safety tool and for minimal processing meaning it is a good example of a non-thermal process technology.
Examples exist on the treatment of fresh and freshly-cut fruits and vegetables, nuts and other fresh and even low-moisture foods (Thirumdas et al., 2015a). Salads and fruit desserts are particularly suited to this treatment One of its key benefits is that it generally preserves the quality characteristics of the food and drink and ensures shelf-life is maintained.
It has also been applied to the disinfection of material surfaces and so finds application in treating medical devices and surgical implements.
What is Plasma?
The basic process is the application of high-voltage electricity or some other focused energy to ionize gas molecules that become highly reactive. This is a plasma which is best described as a partially ionized gas where a whole range of species are present. They include ions, electrons, free radicals and molecules in various types of excited states.
As well as solid, liquid and gas states, these excited gas molecules act as a unique fourth state which makes them ideal for surface treatments especially the destruction of biofilms, pathogens and spoilage organisms.
The History Of Studies On Plasma
The fourth state of plasma was identified by Sir William Crookes in 1879 in his Crookes tubes. It was then described as ‘cathode ray’ matter. Nearly 20 years later, Sir J.J. Thomson refined its description further in 1897 but it was not described as plasma until 1928. In 1928, Langmuir and Tonks were investigating electric discharges at the General Electric Research Laboratory where ionized gas was thus described.
Plasma can occur as two types. There is a Thermal Plasma (Hot Plasma) which is generated by very high temperature and pressure with so called heavy electrons. The second and more useful type is cold plasma a.k.a. non-thermal plasma also known as near ambient temperature plasma (NTP). This type is generated either at atmospheric pressure or under vacuum or low-pressure conditions under relatively low ambient temperatures between 30 and 60 Centigrade and only relying on low energy input. Hence the term cold plasma.
The mode of production is also a method of classification. These can take the form of microwaves, a gliding arc, a corona or a dielectric barrier discharge.
The pressure is also another form: plasma can be generated at low pressure, atmospheric pressure and high pressure.
Generating Cold Plasmas
Cold plasmas or NTP can be generated a number of ways but the most common is to use electricity or electromagnetic waves as with microwaves with a gas at a relatively low pressure. You can produce cold plasma discharges with stationary, direct current (DC) and alternating current (AC) fields.
A number of electrical power supplies are used to generate these plasma discharges and they include direct current, DBD (corona), radiofrequency waves and microwaves.
How Does Cold Plasma Work?
The mode of action is due to:-
- chemical reactions with cellular structures which disrupt cell function,
- ultraviolet (UV) damage of cellular components,
- UV-mediated DNA strand breakage which disrupts cell growth, development and population growth.
The reactive species present in the cold gas plasma are ultraviolet photons and other highly energetic species, such as electrons, singlet oxygen, OH radicals, and NO radicals ions, other free radicals based on the gas used, and other ‘excited’ molecules and atoms (Kovačević et al., 2016). All these species are very strong oxidizers which can rapidly destroy other inorganic and organic compounds. The reaction mechanisms involved in chemical decomposition have not been clearly established; however, they fall into 2 main categories: chemical (free radical-promoted) attack and direct electron impact (Becker et al., 2005).
The level, types and range of activity of all these components depends mainly on the type of gas used and then on plasma source, voltage applied, flow-rate, frequency and method of generation (Ramos et al., 2013; Pankaj et al., 2014).
Either one or a mix of outcomes described above is enough to destroy the microorganisms and make the technology highly efficacious.
How Does It Compare With Other Processing Methods?
The process needs to be placed in context with other non-thermal processes and the use of chemical methods. It’s also an application which has impact on the products produced too let alone the destruction of micro-organisms. This will also include modified packaging and atmospheric packaging, irradiation, pulsed light, high pressure processing, ultrasound and so on.
The main claims for the process in context with other competing methods are best summed up (Ramos et al., 2013) :-
- rapid with sterilization taking place within a few minutes
- produces minimal increases in temperature of a food
- high efficiency and effectiveness in microorganism destruction and that includes viruses
- a low impact on the internal product structure and matrix
- little impact on nutrients especially vitamins
- no residues left
- efficient with use of resources including running costs for natural gas and electricity
- most effective on surfaces
- no ‘shadow effects’ meaning all product is treated
- can be part of a process especially packaging.
Cold plasma does not need liquids like water for contact and uses no invasive chemical additives. The technology has been exploited successfully in other applications such as the surface treatment of materials used in polymers, textiles, electronics and printing (Niemira and Gutsol, 2010).
Issues With The Cold Plasma Technology
At this moment in time the limitations of research focus on a lack of understanding about the nature of the inactivation and how the gas plasma components actually cause destruction although these effects can be speculated upon. There are certainly physicochemical changes which may or may not be benefits which we will discuss at various points. Generally, by its nature cold plasmaIt’s not known what the long-term impact is of the treatment on food which is unusual given that the short- and immediate-term benefits are known.
As we shall see the type of equipment needed to produce cold plasma is many and varied and in some cases extremely complex and complicated. It is still big investment and this be because of the many different types of equipment available. For a producers, choosing the right type of equipment is at the moment difficult and applications are on a case by case. There is little flexibility in approach and the equipment is not durable.
Types Of Equipment
The variety of equipment that generates cold plasma is constantly being developed and refined. In general, plasma is generated from a gas which is ionised and allowed to flow around and through the food being treated. Operational success and efficiency depend on the type of gas ionisation process which on the face of it is complex but once managed properly is highly successful and becomes routine.
The technologies used to produce cold plasma vary widely, but fall into three general categories according to Niemira & Sites (2008): (1) electrode contact where the target is in contact with or between electrodes, (2) direct treatment, in which active plasma is deposited directly on the target, and (3) remote treatment in which active plasma is generated at some distance, and plasma is moved to the target.
There are also two classes of plasma generating technology – those that operate in a partial vacuum and others at atmospheric pressure. Gases are better ionized at lower pressures which allows a greater volume of plasma to be generated for a given voltage. The advantage in power is offset by the capital needed to create partial-pressure chambers and associated controls.
The gas to be used for cold plasma process is also a critical factor. Gases such as helium, argon, and neon, ionize more readily than other gases, such as oxygen, nitrogen, and air. Thus, helium cold plasma can be created with less electricity than air plasma. In this case, the trade-off according to Niemira (2012) is improved antimicrobial efficacy of cold plasmas, which contain oxygen ions and monoatomic oxygen singlets.
Gas plasmas are generated using voltage differences of 9kV to 16kV. Mango and melons were treated using a 16kV generating device to destroy both E.coli and L. monocytogenes (Perni et al., 2008 a & b).
Destruction Of Microorganisms
One of the main proponents of the technology is Brendan A. Niemira, who is the research leader of the Food Safety and Intervention Technologies Research Unit at the U.S. Dept. of Agriculture’s Eastern Regional Research Center in Wyndmoor, Pa. USA. His team regularly investigates the technology for its academic and commercial exploitation.
The success of the technology is based on how well it damages cells. The effect of damage is down to the generation of plasma material which interacts with the cells in myriad ways. To be brief, the level of damage depends on the microorganisms itself as in the type of microorganism, the physiological state of the cells, the concentration and number of cells, as well as the physical properties of the gas plasma including intensity, type of gas etc.
Whilst it is advocated as a ‘dry technology’, the impact of cold plasma on cells depends on the presence of water within the cell. The best or most effective damage appears to occur to a microbial cell when it has a relatively high moisture content. It is not as effective against ‘dry’ microorganisms although it still has a profound and significant impact.
It is thought that cold plasma produces malondialdehyde (MDA) in microbial cells which triggers the production of damaging DNA adducts that result in irreparable damage to the microbial cell. Other biochemical mechanism of destruction include the role of charged particles and reactive species in wholesale damage to cell membranes and to DNA itself usually through the disruption of covalent bonds. The mechanisms of damage are still being examined and we shall see that these destructive mechanisms are still being interpreted.
Cold plasma is now touted as an alternative to chlorine gas decontamination of fruits and vegetables, especially leafy vegetables because of these microcidial properties.
Typical examples of the application have been the destruction of notorious food poisoning organisms such as Salmonella, Listeria monocytogenes and Escherichia coli O157:H7 on dessert apples (Niemira and Sites, 2008), melons, lettuce, (Critzer et al., 2007), strawberries, potato (Fernandez et al., 2013) and almonds (Niemira, 2012b).
One study established that cold plasmas are more effective against gram-negative organisms such as Pseudomonas aeruginosa, Burkholderia cenocepacia and E. coli than against gram-positive organisms. They found differences in resistance among the gram-positive organisms such as Streptococcus pyogenes, Staphylococcus aureus, and Staphylococcus epidermidis (Ermolaeva et al., 2011). Another study found an order in decreasing sensitivity to be Listeria monocytogenes then Salmonella and then E. coli O157:H7 (Critzer et al., 2007).
The Niemira and Sites paper (2008) provides examples of each application:
electrode contact – a reduction of E. coli 12955 on almonds by 5 log after a 30-second treatment (30 kV, 2 kHz) (Deng et al., 2007). This system involved almonds which were placed between the 10-mm gap between the two electrodes of a dielectric barrier discharge system.
Salmonella
Salmonella is a major health issue and one microorganism that this technology appears to be able to tackle. The interest in treating various produce to destroy Salmonella has been extensive (Fernandez et al., 2013). Generally, at least 2 log CFU/10 cm2 reduction is possible after just 2 minutes exposure and 5 log CFU/10 cm2 reduction after 5 minutes. In that study changes in growth phase, growth temperature and chemical treatment regime were examined using CAP.
One study assessed the reduction of S. enteritidis and S. typhimurium on eggshells. In that study they showed (1) reduction of 2.5 and 4.5 log CFU per eggshell in S. enteritidis following a 90 minute burst of treatment at 35% and 65% RH respectively, (2) a reduction of 3.5 log CFU per eggshell for S. typhimurium after a 90 minute treatment.
It is thought that charged particles do not have a role in killing Salmonella (Fernandez et al., 2013).
E.coli
Cold plasma was used to reduce E. coli O157:H7 on spinach by 3 to 5 log cfu/mL through in-package ozone generation (Klockow and Keener 2009).
Treatment of rice has also recently been effectively investigated (Thirumdas et al., 2015b; Sarangapani et al., 2015; Lee et al., 2016). Rice is notorious for allowing bacteria like Bacillus subtilis to grow, thrive and produce enteric toxins. The application is highly effective in the destruction of this microorganism. An excellent review of the impact of the technology on treatment of packaging is to be found by Pankaj et al., (2014).
Pathogens associated with apple and orange juice hit the national headlines back in the 1990s when it was found that E. coli O157: H7, Salmonella, and Cryptosporidium were linked to serious food poisoning outbreaks. In 1998, the U.S. Food and Drug Administration (FDA) altered the labelling laws requiring a warning on the label describing the risk of pathogen exposure if a fruit juice was unpasteurized or the process did not produce a 5-log reduction. In recent years atmospheric cold plasma has been demonstrated to inactivate E.coli K12 that has either been acid adapted or not (Ozen et al., 2021) at the University of Georgia.
Treatment To Destroy Viruses
Food poisoning by viruses is still a major cause of illness and death in the food industry. Norovirus for example caused almost 5.5 million cases of food borne illness in 2020 in the USA alone. That is a staggering amount of just one type of food-borne illness in a country noted for its stringent food safety requirements. What is also surprising is that level of illness is still an issue.
A number of viruses are implicated.
Cold Plasma and Food Quality
One of the issues associated with its application with a whole range of foodstuffs is that whilst it can damage microbial cells, these same types of reactions will also be affecting food biomolecules. It does have an impact to a limited extent on a range of qualitative parameters such as taste and flavour as well as more analytical parameters such as soluble solid content, acidity, dry matter, colour changes etc. especially with food storage. Again, this is very much a subject for research.
Notable and significant effects that have been noticed in stored food treated with plasma compared to a control include an increase in dry matter content with a concomitant decrease in the soluble solids content and a significant decrease in titratable acidity.
One of the issues of the technology is the potential increase in oxidation of lipids because of the generation of free radicals. There is also loss of colour in some instances because of attack on anthocyanins and carotenes although some researchers do not think this is a major issue as we shall see. There is also a loss of firmness in fruit which may be due to damage to pectin or loss of water and a rise in acidity. Again, the results are variable.
Cold Plasma Technology and Destruction Of Aflatoxins
Aflatoxins are often found in food products if they have been contaminated with fungi. Chili powder is problematic in a number of countries because of the presence of mycotoxins such as aflatoxins. The maximum allowable limit of aflatoxin for chili set by the USDA is 10 ppb whilst the Food safety and Standards Authority of India has set it at 30 ppb. Unfortunately in India at the moment over 40% of chili powder sold in that country’s markets is contaminated with aflatoxin. Treatment using a low-pressure radio frequency cold plasma technology can produce a 43% reduction in this aflatoxin content. The degree of reduction depends on radio frequency power as the time of exposure. The mechanisms for this reduction have still to be understood (Rajendran et al., 2021).
Inactivation of Enzymes
Generally, a number of enzymes associated with quality changes in produce are affected. These are mainly enzymes associated with enzymatic browning such as polyphenoloxidase and peroxides. Given that one of the main problems with freshly cut vegetables and fruit is enzymatic browning producing discolouration, plasma treatment is highly beneficial in cutting down this effect. The produce retains its freshness because we tend to measure the age of the produce by the level of browning.
The level of enzyme inactivation is approximately 70%.
Microwave cold plasma (CP) treatment on potato is shown to cause inactivation of polyphenol oxidase (PPO) (Kang et al., 2019).
The Effects of Cold Plasma On Seed Germination
Cold plasma treatment of seeds improves their germination rate by 50%. This is due to high energy ion particles etching the seed coat which weakens the integrity of the outer layers. The benefits have been reported for a range of vegetable seeds such as peas, soybeans, buckwheat, wheat and maize/corn.
It is also possible to delay germination using cold plasma treatment when the seed in coated with CF4 an octadecafluorodecalin.
Effects On Starch And Modified Starch
The technology is touted as an alternative for dry etching and surface modification by altering the behaviour of various polymers including starch. It has a specific effect in changing the crystallinity of solid starch granules which impacts gelatinization and the formation of pastes.
One of the main chemical reactions that plasma can induce is a form of graft-polymerization of ethylene with rice and sweet potato starch. Plasma can also induce a type of homopolymerization of ethylene onto starch. This has been seen for starch from waxy corn, corn, potato ,cassava and potato.
Cold Plasma Technology and Treatment of Packaging
One of the many beneficial applications is the impact of plasma on food packaging. It helps modify the polymerization of the packaging polymers which improves adhesion and strength as well as enhancing its printability. The impact of treatment includes modifications based on chemical and physical changes as well as biological changes. It can also modify the headspace inside the packaging.
Applications In Extraction Of Bioactives and Ingredients
Cold plasma technology shows potential for improving the extraction of bioactive materials from fruit, roots, seeds and other materials. We’ve already mentioned effects on fruit juice production and on anthocyanin release.
A couple of studies have demonstrated that cold plasma treatment of fruits enhances anthocyanin and overall colour release from juices for processing. The study on pomegranate juice production showed an increase in colour content from between 21% and 35%. Pomegranate juice can be notoriously weak in colour so any improvements are welcome (
In this study, the sugar concentration of the juice was lower. The researchers also looked at two processing frequencies (200 and 700 Hz). They also examined the impact of treatment in a model stomach by looking at in vitro digestion of the juices at 37°C/6 h. All componentry such as polyphenol release including vitamin C produced in CAP treated samples seemed to perform better in the simulated digestion system.
The range of applications is ever expanding and whilst this short item only breaks the surface, there will be plenty of opportunities to develop the literature relating to this technology.
For example it is known that ginsengosides are effectively extracted from ginseng root compared to conventional techniques.
Cold Plasma Technology: Treatment of Whole Fruit
Plasma reduces peroxidase (POD) activity in tomatoes which produces a delay in ripening and extends the shelf-life by 4 weeks. It can also raise the rate of germination by 11 per cent. It appears to have a knock-on effect of improving root morphology and nutrient absorption.
The effect on kiwi fruit is also marked following plasma treatment. It helps quality retention by maintaining the critical green colour and minimising any darkening. There are no changes in antioxidant activity or its content and any changes in texture. The significant benefit is extended storage shelf-life of a week.
Strawberries are prone to rapid microbial spoilage especially from fungi. In one example the strawberries were treated inside a package using a 60kV dielectric barrier discharge (DBD) pulsed at 50 Hz, across a 40 mm electrode gap within the package (Misra et al., 2014). A 5 minute treatment produced just a 2 log reduction in aerobic microbes but enough to extend shelf-life. They also claimed no impact on the strawberry’s colour or firmness.
Plasma treatment of oranges reduces mould contamination and growth as well as reducing weight loss.
Production of Trans-Fat Free Soybean Oil
Researchers at Purdue University have developed a process for hydrogenation using cold plasma technology. In this application, the cold plasma is used to produce a solid form of soybean oil that has no trans fats. Trans fats are linked by their presence to stroke and other serious heart complaints.
The process involves high-voltage atmospheric cold plasma (HVACP). The process occurs at room temperature, which avoids the high temperatures that cause trans fats to form.
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I think you could say more about this emerging technology because it’s attracting researchers to its power across the world. We used it in printing and in electronics and it works fantastically well. I know that people use it a lot more in the food processing world because it has some unusual properties which you din’t find anywhere else in physics. It has to be as good as some of the more traditional techniques.