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Cold Atmospheric Gas Plasmas
Cold Atmospheric Gas Plasmas MG Kong, Old Dominion University, Norfolk, VA, USA G Shama, Loughborough University, Loughborough, UK Ó 2014 Elsevier Ltd. All rights reserved.
The Nature of Gas Plasmas and Cold Gas Plasmas Gas plasmas are ionized gases formed by liberating electrons from gas molecules and atoms using external energy sources such as lasers or high electrical voltages. Once ignited, and under the influence of an external energy source, electrons and other charged particles (e.g., ions) are accelerated to acquire considerable kinetic energy and, as a result, become capable of ionizing, exciting, and dissociating gas molecules and atoms to form highly reactive chemical species. When excited gas atoms and molecules relax back into their normal energy state, which is referred to as the ‘ground state,’ they release photons. Most of these are in the visible range, but some are in the ultraviolet (UV) and even vacuum UV (VUV) regions. Gas plasmas then may be thought of as a collection of coexisting chemically reactive species, energetic electrons, and other charged particles, as well as electromagnetic waves including UV photons, in a stationary or flowing gaseous medium. These chemically reactive species, charged particles, and UV photons are generated, lost (e.g., via recombination), and replenished dynamically often in a periodic fashion. Numerous examples of gas plasmas exist all around us. They may be naturally occurring, such as flames, lightning, the auroras, and the sun, or artificially created, as in fluorescent lamps, welding arcs, and plasma television screens. Gas plasmas have been referred to as the fourth state of matter after solids, liquids, and gases – in fact, 99% of the visible universe is made up of plasmas. Gas plasmas span a vast range of physical and chemical properties. Interstellar plasmas, for example, may have a density as low as 10 particles per cubic centimeter. Hence, interparticle collisions are infrequent and particle kinetic energy is inefficiently transferred into the thermal energy of the gas, and therefore the gas temperature of the plasma is low. On the other hand, welding arcs can have electron densities of the order 1015 cm 3 and the frequency of collision with gas molecules is high. This leads to electron kinetic energy being efficiently converted into thermal energy, and gas temperatures that can exceed 10 000 K. As far as the treatment of thermally labile materials (including foods) is concerned, it is necessary to prevent gas temperatures from rising above about 60 C. It is additionally important to ensure that temporal stability of the plasma is maintained. These two operating constraints, low gas temperature and temporal stability, must be achieved without compromising the reactivity of plasma chemistry, which would render the plasma inefficient for its intended applications. This poses a challenge, as a large electrical power input increases the concentrations of reactive plasma species (and hence application efficacy) but accounts for plasma instability and leads to high gas temperatures. One of the main challenges in gas plasma technology is to address the potential mutual exclusion of plasma reactivity and plasma stability. A breakthrough occurred in the late 1980s when lowtemperature plasmas at atmospheric pressure, subsequently
Encyclopedia of Food Microbiology, Volume 1
known as ‘cold atmospheric plasmas,’ were first demonstrated. This became possible through a combination of three different approaches in the way in which the plasma was generated. The first of these was the use of dielectric barriers to the electrodes to limit the rapid growth in the discharge current – and hence prevent the heating up of the gas. Second, was the use of high excitation frequencies so that the electric field changes its polarity quickly to stifle the buildup of a large discharge current, and, finally, the utilization of noble and atomic gases such as helium and argon that have good thermal conductivity and little electron affinity. In general, the excitation frequency for plasma generation is above 1 kHz (10 000 oscillation voltage cycles per second) and extends to radio frequencies of 1–300 MHz and microwave frequencies of 1–50 GHz. Reactive gases such as oxygen, nitrogen, air, or even water vapor usually are mixed in small quantities into the background noble gas to enable the production of reactive oxygen and nitrogen species. These innovations led to the start of a rapid development in cold atmospheric plasma science and technology. Figure 1 shows an example of a cold atmospheric plasma that could potentially be used in the food industry. Perhaps the greatest immediate potential application of cold gas plasmas in the food industry has to do with their ability to inactivate a wide range of microorganisms. This is achieved by means of reactive plasma species, particularly reactive oxygen species (ROS), and reactive nitrogen species. These include hydroxyl radicals (OH ), singlet oxygen (1O2), superoxide (O2 ), ground and excited state oxygen atoms (O/O*), nitric oxide (NO), hydrogen peroxide (H2O2), and ozone (O3). Some l
Figure 1 Schematic of a seven-jet plasma array arranged in a honeycomb configuration with both elevation (left) and plan view (right). Each plasma jet delivers a jet-centric spread of reactive species on the downstream substrate.
http://dx.doi.org/10.1016/B978-0-12-384730-0.00366-9
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of these species (e.g., OH and 1O2) are extremely short lived, particularly in moist environments, whereas others, such as H2O2 and O3, are considerably more stable. In cold atmospheric plasmas, electrons can have kinetic energies as high as 10 eV (which is equivalent to 110 000 K). As a result, they produce numerous ROS at higher concentrations than would be possible with conventional oxidizing agents. To quote specific values, the plasma species OH and 1O2 typically are present at concentrations above 1015 cm 3 (or w 100 ppm) in liquid-containing gas plasmas. The presence of these highenergy electrons along with ROS will lead to synergistic oxidation effects. It has been demonstrated that it takes less than 60 s for cold atmospheric plasmas to achieve more than 6 log reductions in Bacillus subtilis spore viability. In fact, this is more impressive than the quoted plasma treatment time of 60 s because both electrons and plasma ROS are produced in a train of short bursts each lasting for 1–5 ms for one half-period of the applied voltage. At 20 kHz, this is equivalent to 4–20% of the voltage on-time and the actual on-time of plasma ROS is only 2.4–12 s of 60 s of the plasma treatment. This very short on-time for electrons and plasma ROS is beneficial for controlling plasma stability and maintaining low gas temperature. It is also useful in minimizing potential damage to the integrity of the material that is undergoing treatment. Electron-enabled temporal modulation of plasma ROS allows them to be applied at high concentrations for a short period of time, achieving high microbial inactivation efficacy with little damage to the material (e.g., a foodstuff) associated with the microorganisms. This is distinctively different from microbial inactivation using conventional chemical disinfectants that may produce one or two relatively stable ROS such as H2O2 or O3. The relatively low oxidation potentials of the latter combined with the absence of synergy with other (plasma-produced) ROS necessitates long contact time with the contaminated material, thus posing greater risk of material damage. Plasma chemistry offers an arguably unique route to biological decontamination with advantages of application efficacy and process control. Different arrangements for treating foods with plasmas are possible. For example, the light-emitting part of the plasma either may be allowed to make direct contact with the surface of the food undergoing treatment or, alternatively, may be placed remotely from it so that direct contact does not occur. These two different methods of configuring treatment may be used to modulate both the quantities and the types of plasma species impinging on the food. Direct treatment brings about efficient bacterial inactivation; however, if the food undergoing treatment is particularly labile, then indirect treatment is recommended to avoid unacceptable changes occurring to the food. In addition, the reaction chemistry at the surface of the food will be determined by the particular configuration employed. Choice of contact mode therefore offers a further means of bringing about optimization of cold plasma technology as a food preservation technology. l
l
The Biological Effects of Gas Plasmas Gas plasmas have been shown to be capable of inactivating a wide range of microorganisms, including bacteria and their
spores, fungi and fungal spores, and viruses. The absence of other microorganisms associated with water and food contamination, such as protozoan cysts and the eggs of helminths, is a reflection that they have not featured in previous studies and not necessarily that they are resistant to plasmas. There are also reasons to be optimistic that plasmas may inactivate prions as more than one study has shown that they have the ability to destroy amyloid aggregates, which are widely accepted to be models for the highly infective prion proteins. Despite the steady accumulation of evidence of the lethality of gas plasmas toward microorganisms in general, relatively little is known about the mechanisms of inactivation, cell injury, and recovery, and in particular which of the many plasma species are the most active. The current situation is complicated by the fact that there are many different types of gas plasmas and a number of different ways of operating them. There is a practical incentive in identifying the most lethal plasma species. By altering plasma operating conditions or the gases used to generate the plasma, it should in theory be possible to alter the composition of the plasma to favor the formation of desired individual species. This would effectively enable plasmas to be tuned and thus operated more effectively. Much of the work on elucidating mechanisms has made use of bacterial spores, and in particular spores from the closely related genera Bacillus and Geobacillius. Spores possess a distinctive structure that is quite different from vegetative bacteria, and although certain spores do present a threat to foods, the risk is perhaps not as great as that posed by certain nonsporulating bacteria. Some initial clues to possible inactivation mechanisms have been obtained by analysis of the form of inactivation curves, which are conventionally plotted in the form of the logarithm of survivors against time. These are rarely monotonic in gas plasma inactivation studies, and in practice may be bi- or even triphasic. Additionally, useful information has been gained from close observation by scanning electron microscopes of the physical appearance and dimensions of spores following treatment by different types of plasmas. Conclusions drawn from this type of analysis can only ever be tentative and need to be confirmed by means of additional studies. Notwithstanding these reservations, a consensus seems to have formed that inactivation – principally among spores – is brought about by a number of individual mechanisms. UV photons are thought to exert a direct lethal effect by damaging DNA, but they may also participate in so-called photodesorption, which results in the release of volatile compounds as the surface of the microorganism is gradually eroded. A different form of surface erosion is also thought to take place under the influence of oxygen atoms – and possibly radical species – and has been termed ‘etching.’ An alternative approach aimed at definitively identifying the participation of individual plasma species has involved the use of mutants of Escherichia coli deficient in particular genes. This approach succeeded in identifying oxygen atoms as the principal cause of cell inactivation with only minor participation from UV photons, OH radicals, and nitric oxide. These apparently conflicting conclusions as to the identity of the most lethal species illustrate that it is unlikely that a single unified mechanism is in operation and that lethality will depend on
Cold Atmospheric Gas Plasmas
the target organism as well as being influenced by the type of plasma and its mode of operation. Different gases favor proportionally efficient production of different plasma species. For example, argon mixed with oxygen tends to produce more UV photons than helium mixed with the same amount of oxygen, and helium–oxygen plasmas tend to produce more atomic oxygen and less ozone than air plasmas. While oxygen atoms, ozone, and UV photons are all known to be effective against bacteria, the extent to which they bring about damage to plant and animal tissues are quite different. Less is currently known about damage to animal tissue, and this needs to be taken into consideration in assessing the potential of this technology and its potential applications. An additional consideration in the treatment of foods is the depth to which the generated plasma species are able to attain beyond the surface of the food. In the treatment of either plant or animal tissue, or foods constituted from either of these, water will nearly always be present. The plasma species will need to penetrate through this water to reach any contaminating microorganisms. Oxygen atoms are known to be short lived in the liquid phase whereas hydrogen peroxide (generated due to presence of water vapor in the air and in the food) is relatively long-lived. The evidence from studies conducted with
Table 1
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deionized water is that antimicrobial activity can be obtained at depths in the range of 5–30 microns. Heat generation within the plasma can significantly enhance mass transfer from the gas phase to the liquid phase, thus increasing the penetration depth. In addition, plasma-induced changes to the pH of water present in foods have also been shown to affect the extent of penetration.
Gas Plasmas in the Food Industry The potential for the application of gas plasmas in the food industry falls broadly into two distinct areas: treatment of foods (including food packaging) and treatment of equipment used in food processing – possibly also extending to the premises in which food-processing operations are conducted. Much of the work that has been done in this field has been directed toward achieving microbial inactivation, but the potential also exists for the removal of allergens as well as microbial endotoxins from the surface of food-processing equipment. Tables 1 and 2 show the range of foods that have been treated using gas plasmas. The range of foods is expanding and includes both plant-derived foods and also meat and a variety of dairy products. The data used to compile these tables come
Air-based gas plasma treatment of plant-derived foods
Foodstuff
Targeted microorganisms
Effects of treatment
Almonds Apples
E. coli E. coli O157:H7 Salmonella Stanley
Apples Lettuce Mango
E. coli O157:H7 Listeria monocytogenes E. coli Saccharomyces cerevisiae Pantoea agglomerans Gluconobacter liquefaciens Salmonella (Unspecified serovars) E. coli Saccharomyces cerevisiae Pantoea agglomerans Gluconobacter liquefaciens Aspergillus parasiticus
5 log reductions after 30 s Salmonella; 2.9–3.7 log reductions after 3 min E. coli O157:H7 2.6 to 3 log reductions after 3 min >2 log reductions after 2 min 1 log reduction after 1 min P. agglomerans and G. liquefaciens >3 log reductions after 2.5 s E. coli >3 log reductions after 5 s S. cerevisiae >3 log reductions after 30 s
Melon (cantaloupe) Melon (honeydew)
Nuts (hazelnuts, peanuts, and pistachios)
Table 2
>2 log reductions after 1 min P. agglomerans and G. liquefaciens >3 log reductions after 2.5 s E. coli >3 log reductions after 5 s S. cerevisiae >3 log reductions after 10 s 1 log reduction after 5 min 5 log reductions in the presence of SF6
Air-based gas plasma treatment of dairy products and meat
Foodstuff
Targeted microorganisms
Effects of treatment
Bacon
Listeria monocytogenes Salmonella typhimurium E. coli Listeria monocytogenes Listeria innocua Listeria monocytogenes Salmonella enteritidis Salmonella typhimurium Listeria monocytogenes E. coli
4.6 log reductions after 1.5 min (results reported as total aerobic counts)
Cheese Chicken (raw) Chicken (cooked) Eggs Ham Pork (raw)
>8 log reductions after 2 min >3 log reductions after 4 min 4.7 log reductions after 2 min S. enteritidis 4.5 log reductions after 90 min S. typhimurium w3.7 log reductions after 90 min 1.7 log reductions after 2 min 6 log reductions after 0.5 min
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Cold Atmospheric Gas Plasmas
from a variety of sources, and it is difficult to make comparisons between individual studies because not only are the target organisms frequently different but so too are the types of plasma-generating equipment and conditions of operation. As a general rule, comparisons can be safely made at this stage only between inactivation data within the same study. Another important consideration that affects microbial survival is the physical nature of the surface of the foodstuff and the distribution of the microorganisms associated with it. One study showed that bacteria applied to the surface of freshly cut fruit surfaces could migrate into the interior of the food and as a result find themselves beyond the reach of active plasma species. In another study that compared the treatment of bacteria on the surface of chicken flesh and chicken skin, it was found that greater reductions in viability were obtained in the former case. This presumably indicated that when deposited on the surface of chicken skin, some bacteria could become lodged inside feather follicles and as a result become immune from the effects of the plasma. Surface topography should not be assumed fixed even for a single type of food, and atomic force microscopy has revealed, for example, that changes can occur to the surfaces of fruit during ripening. Gas plasmas have been used to sterilize the interior of bottles and various other forms of food packaging, such as plastic trays and films. An innovative approach currently under development is the use of gas plasmas to bring about the deposition of thin films directly onto the surface of foods – typically fruits – to extend their shelf life. Gas plasmas also have potential applications in the treatment of food-processing surfaces. Quite conventional plasmagenerating configurations could be used to effect this. A recent innovation was the permanent incorporation of a plasmagenerating device into an item of food processing (a circular slicing blade). Blades of this type have been shown capable of transmitting contamination between foods, and as proposed in the study, it was intended that the device would be activated periodically to deal with any accumulation of microorganisms at the surface of the blade. This represents a quite radical approach to the maintenance of hygienic conditions. Gas plasmas could be used to remove allergens, and possibly endotoxins as well, such as lipopolysaccharides from E. coli, from the surface of food-processing equipment. As mentioned, it has been shown that plasmas were effective in destroying protein fibrils that had been generated on the surface of inert materials. This is clearly one area in which more work is required.
Future Prospects From the perspective of food processing, cold gas plasmas must be classed as an emerging technology. Any new process for the treatment of foods is required by regulatory agencies to demonstrate definitively that it does not bring about any harmful effects in the food undergoing treatment. This covers both the generation of compounds that could harm human health as well as the destruction of compounds naturally present in the food that are beneficial to human health – a prime example being vitamins. To date, relatively few foodrelated studies employing plasmas have extended to these
considerations. Such investigations, however, will need to be undertaken in the future if gas plasma technology is to be adopted by the food industry. Consumers will automatically reject foods that appear different to their preconceived idea of what a food should look, smell, and taste like. Again, relatively few studies have been conducted to confirm that the organoleptic properties of the food have not been adversely affected. Encouragingly, those few studies that have addressed this issue have not reported adverse effects, but more work is clearly necessary to confirm this. The uses of gas flows, for example, could result in moisture losses from foods undergoing treatment and it would be relatively simple to amend processing conditions to counter this possibility. Cost of treatment with gas plasmas remains an area on which little information has been made openly available. The use, for example, of noble gases will add to processing costs, but it might be possible to bring about some form of gas recycling with the aim of lowering operating costs if the use of noble gases rather than, say, air or nitrogen was shown to be essential for a particular application. Scale-up is another issue that needs to be addressed if the technology is to be translated into the commercial sector. There are no fundamental restrictions as to the scale at which plasmas can be generated, what is needed however is the demonstration of this capability and that it can be achieved at an acceptable cost.
See also: Minimal Methods of Processing; Non-Thermal Processing.
Further Reading Deng, S., Ruan, R., Mok, C.K., Huang, G., Lin, X., Chen, P., 2007. Inactivation of Escherichia coli on almonds using nonthermal plasma. Journal of Food Science 72, M62–6. Kogelschatz, U., 2003. Dielectric-barrier discharges: their history, discharge physics, and industrial applications. Plasma Chemistry and Plasma Process 23, 1–46. Kong, M.G., Kroesen, G.G., Morfill, G., Nosenko, T., Shimizu, T., van Dijk, J., Zimmermann, J.L., 2009. Plasma medicine: an introductory review. New Journal of Physics 11, 115012. Leipold, F., Kusano, Y., Hansen, F., Jacobsen, T., 2010. Decontamination of a rotating cutting tool during operation by means of atmospheric pressure plasmas. Food Control 21, 1194–1198. Lieberman, M.A., Lichtenberg, A.J., 1994. Principles of Plasma Discharge and Materials Processing. John Wiley & Sons, New York. Liu, J.J., Kong, M.G., 2011. Sub-60 C atmospheric helium-water plasma jets: modes, electron heating and downstream reaction chemistry. Journal of Physics D: Applied Physics 44, 345203. Moisan, M., Barbeau, J., Moreau, S., Pelletier, J., Tabrizian, M., Yahia, L.H., 2001. Low-temperature sterilization using gas plasmas: a review of the experiments and an analysis of the inactivation mechanisms. International Journal of Pharmaceutics 226, 1–21. Perni, S., Shama, G., Hobman, J.L., Lund, P.A., Kershaw, C.J., Hidalgo-Arroyo, G.A., Penn, C.W., Deng, X.T., Walsh, J.L., Kong, M.G., 2007. Probing bactericidal mechanisms induced by cold atmospheric plasmas with Escherichia coli mutants. Applied Physics Letters 90, 073902. Perni, S., Shama, G., Kong, M.G., 2008. Cold atmospheric plasma disinfection of cut fruit surfaces contaminated with migrating microorganisms. Journal of Food Protection 71, 1619–1625. Vleugels, M., Shama, G., Deng, X.T., Shi, J.J., Kong, M.G., 2005. Atmospheric plasma inactivation of biofilm-forming bacteria for food safety control. IEEE Transactions in Plasma Science 33, 824–828.
The Nature of Gas Plasmas and Cold Gas Plasmas Gas plasmas are ionized gases formed by liberating electrons from gas molecules and atoms using external energy sources such as lasers or high electrical voltages. Once ignited, and under the influence of an external energy source, electrons and other charged particles (e.g., ions) are accelerated to acquire considerable kinetic energy and, as a result, become capable of ionizing, exciting, and dissociating gas molecules and atoms to form highly reactive chemical species. When excited gas atoms and molecules relax back into their normal energy state, which is referred to as the ‘ground state,’ they release photons. Most of these are in the visible range, but some are in the ultraviolet (UV) and even vacuum UV (VUV) regions. Gas plasmas then may be thought of as a collection of coexisting chemically reactive species, energetic electrons, and other charged particles, as well as electromagnetic waves including UV photons, in a stationary or flowing gaseous medium. These chemically reactive species, charged particles, and UV photons are generated, lost (e.g., via recombination), and replenished dynamically often in a periodic fashion. Numerous examples of gas plasmas exist all around us. They may be naturally occurring, such as flames, lightning, the auroras, and the sun, or artificially created, as in fluorescent lamps, welding arcs, and plasma television screens. Gas plasmas have been referred to as the fourth state of matter after solids, liquids, and gases – in fact, 99% of the visible universe is made up of plasmas. Gas plasmas span a vast range of physical and chemical properties. Interstellar plasmas, for example, may have a density as low as 10 particles per cubic centimeter. Hence, interparticle collisions are infrequent and particle kinetic energy is inefficiently transferred into the thermal energy of the gas, and therefore the gas temperature of the plasma is low. On the other hand, welding arcs can have electron densities of the order 1015 cm 3 and the frequency of collision with gas molecules is high. This leads to electron kinetic energy being efficiently converted into thermal energy, and gas temperatures that can exceed 10 000 K. As far as the treatment of thermally labile materials (including foods) is concerned, it is necessary to prevent gas temperatures from rising above about 60 C. It is additionally important to ensure that temporal stability of the plasma is maintained. These two operating constraints, low gas temperature and temporal stability, must be achieved without compromising the reactivity of plasma chemistry, which would render the plasma inefficient for its intended applications. This poses a challenge, as a large electrical power input increases the concentrations of reactive plasma species (and hence application efficacy) but accounts for plasma instability and leads to high gas temperatures. One of the main challenges in gas plasma technology is to address the potential mutual exclusion of plasma reactivity and plasma stability. A breakthrough occurred in the late 1980s when lowtemperature plasmas at atmospheric pressure, subsequently
Encyclopedia of Food Microbiology, Volume 1
known as ‘cold atmospheric plasmas,’ were first demonstrated. This became possible through a combination of three different approaches in the way in which the plasma was generated. The first of these was the use of dielectric barriers to the electrodes to limit the rapid growth in the discharge current – and hence prevent the heating up of the gas. Second, was the use of high excitation frequencies so that the electric field changes its polarity quickly to stifle the buildup of a large discharge current, and, finally, the utilization of noble and atomic gases such as helium and argon that have good thermal conductivity and little electron affinity. In general, the excitation frequency for plasma generation is above 1 kHz (10 000 oscillation voltage cycles per second) and extends to radio frequencies of 1–300 MHz and microwave frequencies of 1–50 GHz. Reactive gases such as oxygen, nitrogen, air, or even water vapor usually are mixed in small quantities into the background noble gas to enable the production of reactive oxygen and nitrogen species. These innovations led to the start of a rapid development in cold atmospheric plasma science and technology. Figure 1 shows an example of a cold atmospheric plasma that could potentially be used in the food industry. Perhaps the greatest immediate potential application of cold gas plasmas in the food industry has to do with their ability to inactivate a wide range of microorganisms. This is achieved by means of reactive plasma species, particularly reactive oxygen species (ROS), and reactive nitrogen species. These include hydroxyl radicals (OH ), singlet oxygen (1O2), superoxide (O2 ), ground and excited state oxygen atoms (O/O*), nitric oxide (NO), hydrogen peroxide (H2O2), and ozone (O3). Some l
Figure 1 Schematic of a seven-jet plasma array arranged in a honeycomb configuration with both elevation (left) and plan view (right). Each plasma jet delivers a jet-centric spread of reactive species on the downstream substrate.
http://dx.doi.org/10.1016/B978-0-12-384730-0.00366-9
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Cold Atmospheric Gas Plasmas
of these species (e.g., OH and 1O2) are extremely short lived, particularly in moist environments, whereas others, such as H2O2 and O3, are considerably more stable. In cold atmospheric plasmas, electrons can have kinetic energies as high as 10 eV (which is equivalent to 110 000 K). As a result, they produce numerous ROS at higher concentrations than would be possible with conventional oxidizing agents. To quote specific values, the plasma species OH and 1O2 typically are present at concentrations above 1015 cm 3 (or w 100 ppm) in liquid-containing gas plasmas. The presence of these highenergy electrons along with ROS will lead to synergistic oxidation effects. It has been demonstrated that it takes less than 60 s for cold atmospheric plasmas to achieve more than 6 log reductions in Bacillus subtilis spore viability. In fact, this is more impressive than the quoted plasma treatment time of 60 s because both electrons and plasma ROS are produced in a train of short bursts each lasting for 1–5 ms for one half-period of the applied voltage. At 20 kHz, this is equivalent to 4–20% of the voltage on-time and the actual on-time of plasma ROS is only 2.4–12 s of 60 s of the plasma treatment. This very short on-time for electrons and plasma ROS is beneficial for controlling plasma stability and maintaining low gas temperature. It is also useful in minimizing potential damage to the integrity of the material that is undergoing treatment. Electron-enabled temporal modulation of plasma ROS allows them to be applied at high concentrations for a short period of time, achieving high microbial inactivation efficacy with little damage to the material (e.g., a foodstuff) associated with the microorganisms. This is distinctively different from microbial inactivation using conventional chemical disinfectants that may produce one or two relatively stable ROS such as H2O2 or O3. The relatively low oxidation potentials of the latter combined with the absence of synergy with other (plasma-produced) ROS necessitates long contact time with the contaminated material, thus posing greater risk of material damage. Plasma chemistry offers an arguably unique route to biological decontamination with advantages of application efficacy and process control. Different arrangements for treating foods with plasmas are possible. For example, the light-emitting part of the plasma either may be allowed to make direct contact with the surface of the food undergoing treatment or, alternatively, may be placed remotely from it so that direct contact does not occur. These two different methods of configuring treatment may be used to modulate both the quantities and the types of plasma species impinging on the food. Direct treatment brings about efficient bacterial inactivation; however, if the food undergoing treatment is particularly labile, then indirect treatment is recommended to avoid unacceptable changes occurring to the food. In addition, the reaction chemistry at the surface of the food will be determined by the particular configuration employed. Choice of contact mode therefore offers a further means of bringing about optimization of cold plasma technology as a food preservation technology. l
l
The Biological Effects of Gas Plasmas Gas plasmas have been shown to be capable of inactivating a wide range of microorganisms, including bacteria and their
spores, fungi and fungal spores, and viruses. The absence of other microorganisms associated with water and food contamination, such as protozoan cysts and the eggs of helminths, is a reflection that they have not featured in previous studies and not necessarily that they are resistant to plasmas. There are also reasons to be optimistic that plasmas may inactivate prions as more than one study has shown that they have the ability to destroy amyloid aggregates, which are widely accepted to be models for the highly infective prion proteins. Despite the steady accumulation of evidence of the lethality of gas plasmas toward microorganisms in general, relatively little is known about the mechanisms of inactivation, cell injury, and recovery, and in particular which of the many plasma species are the most active. The current situation is complicated by the fact that there are many different types of gas plasmas and a number of different ways of operating them. There is a practical incentive in identifying the most lethal plasma species. By altering plasma operating conditions or the gases used to generate the plasma, it should in theory be possible to alter the composition of the plasma to favor the formation of desired individual species. This would effectively enable plasmas to be tuned and thus operated more effectively. Much of the work on elucidating mechanisms has made use of bacterial spores, and in particular spores from the closely related genera Bacillus and Geobacillius. Spores possess a distinctive structure that is quite different from vegetative bacteria, and although certain spores do present a threat to foods, the risk is perhaps not as great as that posed by certain nonsporulating bacteria. Some initial clues to possible inactivation mechanisms have been obtained by analysis of the form of inactivation curves, which are conventionally plotted in the form of the logarithm of survivors against time. These are rarely monotonic in gas plasma inactivation studies, and in practice may be bi- or even triphasic. Additionally, useful information has been gained from close observation by scanning electron microscopes of the physical appearance and dimensions of spores following treatment by different types of plasmas. Conclusions drawn from this type of analysis can only ever be tentative and need to be confirmed by means of additional studies. Notwithstanding these reservations, a consensus seems to have formed that inactivation – principally among spores – is brought about by a number of individual mechanisms. UV photons are thought to exert a direct lethal effect by damaging DNA, but they may also participate in so-called photodesorption, which results in the release of volatile compounds as the surface of the microorganism is gradually eroded. A different form of surface erosion is also thought to take place under the influence of oxygen atoms – and possibly radical species – and has been termed ‘etching.’ An alternative approach aimed at definitively identifying the participation of individual plasma species has involved the use of mutants of Escherichia coli deficient in particular genes. This approach succeeded in identifying oxygen atoms as the principal cause of cell inactivation with only minor participation from UV photons, OH radicals, and nitric oxide. These apparently conflicting conclusions as to the identity of the most lethal species illustrate that it is unlikely that a single unified mechanism is in operation and that lethality will depend on
Cold Atmospheric Gas Plasmas
the target organism as well as being influenced by the type of plasma and its mode of operation. Different gases favor proportionally efficient production of different plasma species. For example, argon mixed with oxygen tends to produce more UV photons than helium mixed with the same amount of oxygen, and helium–oxygen plasmas tend to produce more atomic oxygen and less ozone than air plasmas. While oxygen atoms, ozone, and UV photons are all known to be effective against bacteria, the extent to which they bring about damage to plant and animal tissues are quite different. Less is currently known about damage to animal tissue, and this needs to be taken into consideration in assessing the potential of this technology and its potential applications. An additional consideration in the treatment of foods is the depth to which the generated plasma species are able to attain beyond the surface of the food. In the treatment of either plant or animal tissue, or foods constituted from either of these, water will nearly always be present. The plasma species will need to penetrate through this water to reach any contaminating microorganisms. Oxygen atoms are known to be short lived in the liquid phase whereas hydrogen peroxide (generated due to presence of water vapor in the air and in the food) is relatively long-lived. The evidence from studies conducted with
Table 1
495
deionized water is that antimicrobial activity can be obtained at depths in the range of 5–30 microns. Heat generation within the plasma can significantly enhance mass transfer from the gas phase to the liquid phase, thus increasing the penetration depth. In addition, plasma-induced changes to the pH of water present in foods have also been shown to affect the extent of penetration.
Gas Plasmas in the Food Industry The potential for the application of gas plasmas in the food industry falls broadly into two distinct areas: treatment of foods (including food packaging) and treatment of equipment used in food processing – possibly also extending to the premises in which food-processing operations are conducted. Much of the work that has been done in this field has been directed toward achieving microbial inactivation, but the potential also exists for the removal of allergens as well as microbial endotoxins from the surface of food-processing equipment. Tables 1 and 2 show the range of foods that have been treated using gas plasmas. The range of foods is expanding and includes both plant-derived foods and also meat and a variety of dairy products. The data used to compile these tables come
Air-based gas plasma treatment of plant-derived foods
Foodstuff
Targeted microorganisms
Effects of treatment
Almonds Apples
E. coli E. coli O157:H7 Salmonella Stanley
Apples Lettuce Mango
E. coli O157:H7 Listeria monocytogenes E. coli Saccharomyces cerevisiae Pantoea agglomerans Gluconobacter liquefaciens Salmonella (Unspecified serovars) E. coli Saccharomyces cerevisiae Pantoea agglomerans Gluconobacter liquefaciens Aspergillus parasiticus
5 log reductions after 30 s Salmonella; 2.9–3.7 log reductions after 3 min E. coli O157:H7 2.6 to 3 log reductions after 3 min >2 log reductions after 2 min 1 log reduction after 1 min P. agglomerans and G. liquefaciens >3 log reductions after 2.5 s E. coli >3 log reductions after 5 s S. cerevisiae >3 log reductions after 30 s
Melon (cantaloupe) Melon (honeydew)
Nuts (hazelnuts, peanuts, and pistachios)
Table 2
>2 log reductions after 1 min P. agglomerans and G. liquefaciens >3 log reductions after 2.5 s E. coli >3 log reductions after 5 s S. cerevisiae >3 log reductions after 10 s 1 log reduction after 5 min 5 log reductions in the presence of SF6
Air-based gas plasma treatment of dairy products and meat
Foodstuff
Targeted microorganisms
Effects of treatment
Bacon
Listeria monocytogenes Salmonella typhimurium E. coli Listeria monocytogenes Listeria innocua Listeria monocytogenes Salmonella enteritidis Salmonella typhimurium Listeria monocytogenes E. coli
4.6 log reductions after 1.5 min (results reported as total aerobic counts)
Cheese Chicken (raw) Chicken (cooked) Eggs Ham Pork (raw)
>8 log reductions after 2 min >3 log reductions after 4 min 4.7 log reductions after 2 min S. enteritidis 4.5 log reductions after 90 min S. typhimurium w3.7 log reductions after 90 min 1.7 log reductions after 2 min 6 log reductions after 0.5 min
496
Cold Atmospheric Gas Plasmas
from a variety of sources, and it is difficult to make comparisons between individual studies because not only are the target organisms frequently different but so too are the types of plasma-generating equipment and conditions of operation. As a general rule, comparisons can be safely made at this stage only between inactivation data within the same study. Another important consideration that affects microbial survival is the physical nature of the surface of the foodstuff and the distribution of the microorganisms associated with it. One study showed that bacteria applied to the surface of freshly cut fruit surfaces could migrate into the interior of the food and as a result find themselves beyond the reach of active plasma species. In another study that compared the treatment of bacteria on the surface of chicken flesh and chicken skin, it was found that greater reductions in viability were obtained in the former case. This presumably indicated that when deposited on the surface of chicken skin, some bacteria could become lodged inside feather follicles and as a result become immune from the effects of the plasma. Surface topography should not be assumed fixed even for a single type of food, and atomic force microscopy has revealed, for example, that changes can occur to the surfaces of fruit during ripening. Gas plasmas have been used to sterilize the interior of bottles and various other forms of food packaging, such as plastic trays and films. An innovative approach currently under development is the use of gas plasmas to bring about the deposition of thin films directly onto the surface of foods – typically fruits – to extend their shelf life. Gas plasmas also have potential applications in the treatment of food-processing surfaces. Quite conventional plasmagenerating configurations could be used to effect this. A recent innovation was the permanent incorporation of a plasmagenerating device into an item of food processing (a circular slicing blade). Blades of this type have been shown capable of transmitting contamination between foods, and as proposed in the study, it was intended that the device would be activated periodically to deal with any accumulation of microorganisms at the surface of the blade. This represents a quite radical approach to the maintenance of hygienic conditions. Gas plasmas could be used to remove allergens, and possibly endotoxins as well, such as lipopolysaccharides from E. coli, from the surface of food-processing equipment. As mentioned, it has been shown that plasmas were effective in destroying protein fibrils that had been generated on the surface of inert materials. This is clearly one area in which more work is required.
Future Prospects From the perspective of food processing, cold gas plasmas must be classed as an emerging technology. Any new process for the treatment of foods is required by regulatory agencies to demonstrate definitively that it does not bring about any harmful effects in the food undergoing treatment. This covers both the generation of compounds that could harm human health as well as the destruction of compounds naturally present in the food that are beneficial to human health – a prime example being vitamins. To date, relatively few foodrelated studies employing plasmas have extended to these
considerations. Such investigations, however, will need to be undertaken in the future if gas plasma technology is to be adopted by the food industry. Consumers will automatically reject foods that appear different to their preconceived idea of what a food should look, smell, and taste like. Again, relatively few studies have been conducted to confirm that the organoleptic properties of the food have not been adversely affected. Encouragingly, those few studies that have addressed this issue have not reported adverse effects, but more work is clearly necessary to confirm this. The uses of gas flows, for example, could result in moisture losses from foods undergoing treatment and it would be relatively simple to amend processing conditions to counter this possibility. Cost of treatment with gas plasmas remains an area on which little information has been made openly available. The use, for example, of noble gases will add to processing costs, but it might be possible to bring about some form of gas recycling with the aim of lowering operating costs if the use of noble gases rather than, say, air or nitrogen was shown to be essential for a particular application. Scale-up is another issue that needs to be addressed if the technology is to be translated into the commercial sector. There are no fundamental restrictions as to the scale at which plasmas can be generated, what is needed however is the demonstration of this capability and that it can be achieved at an acceptable cost.
See also: Minimal Methods of Processing; Non-Thermal Processing.
Further Reading Deng, S., Ruan, R., Mok, C.K., Huang, G., Lin, X., Chen, P., 2007. Inactivation of Escherichia coli on almonds using nonthermal plasma. Journal of Food Science 72, M62–6. Kogelschatz, U., 2003. Dielectric-barrier discharges: their history, discharge physics, and industrial applications. Plasma Chemistry and Plasma Process 23, 1–46. Kong, M.G., Kroesen, G.G., Morfill, G., Nosenko, T., Shimizu, T., van Dijk, J., Zimmermann, J.L., 2009. Plasma medicine: an introductory review. New Journal of Physics 11, 115012. Leipold, F., Kusano, Y., Hansen, F., Jacobsen, T., 2010. Decontamination of a rotating cutting tool during operation by means of atmospheric pressure plasmas. Food Control 21, 1194–1198. Lieberman, M.A., Lichtenberg, A.J., 1994. Principles of Plasma Discharge and Materials Processing. John Wiley & Sons, New York. Liu, J.J., Kong, M.G., 2011. Sub-60 C atmospheric helium-water plasma jets: modes, electron heating and downstream reaction chemistry. Journal of Physics D: Applied Physics 44, 345203. Moisan, M., Barbeau, J., Moreau, S., Pelletier, J., Tabrizian, M., Yahia, L.H., 2001. Low-temperature sterilization using gas plasmas: a review of the experiments and an analysis of the inactivation mechanisms. International Journal of Pharmaceutics 226, 1–21. Perni, S., Shama, G., Hobman, J.L., Lund, P.A., Kershaw, C.J., Hidalgo-Arroyo, G.A., Penn, C.W., Deng, X.T., Walsh, J.L., Kong, M.G., 2007. Probing bactericidal mechanisms induced by cold atmospheric plasmas with Escherichia coli mutants. Applied Physics Letters 90, 073902. Perni, S., Shama, G., Kong, M.G., 2008. Cold atmospheric plasma disinfection of cut fruit surfaces contaminated with migrating microorganisms. Journal of Food Protection 71, 1619–1625. Vleugels, M., Shama, G., Deng, X.T., Shi, J.J., Kong, M.G., 2005. Atmospheric plasma inactivation of biofilm-forming bacteria for food safety control. IEEE Transactions in Plasma Science 33, 824–828.