Food industry challenges
Consumers expect that the food they consume is safe to eat. In addition, the consumer also wants the food to have high nutritional value with minimal preparation times, as evidenced by the growth in products such as convenience ready-to-eat (RTE) foods and minimally processed fresh produce. In order to meet these demands, food manufacturers are looking for new methods and technologies. A new technology which may meet manufacturers’ needs is cold plasma. This article discusses this relatively new technology, how it could be used in the future to potentially address some of these issues, and some of the work being conducted at Campden BRI.
Plasmas are referred to as the fourth state of matter. A plasma state is achieved when sufficient energy (such as heat or electricity) is applied to a gas. It is estimated that 99.9% of the universe is in a plasma state, however very few natural plasmas are generated here on Earth. The northern aurora, southern aurora and lightning were recognised as naturally-occurring plasmas and have been described and studied since the 17th and 18th Centuries.
The development of energy storage devices and vacuum systems in the 19th Century allowed the generation and significant understanding of plasma discharges. By the beginning of the 20th Century, techniques for producing plasmas were well established as were the means of controlling them. It was further understood that a partially ionised gas consisted of neutral, positive, and negative species. Much of this early work involving plasmas was conducted at low pressures as it was easier to generate and control a plasma discharge at these pressures. Today, plasma systems are already used for many applications that affect our daily lives. Examples include: computer chips; textiles and polymers; treating artificial joints and arterial stents for biocompatibility; plasma TVs; fluorescent and high-intensity-discharge lamps; plasma spray coatings for jet engines; production of nanoscale materials; plasma remediation of greenhouse, toxic gases; and the destruction of hazardous wastes.
Plasma generation and chemistry
For the generation of manmade plasmas described in this article, electrical energy is always used. Applying a voltage to a gas generates an electric field that can accelerate any free electrons in the gas. Accelerated electrons will collide with neutral gas atoms, resulting in excitation or ionisation. Ionisation releases more free electrons to be accelerated, causing an ‘avalanche effect’ generating a rich abundance of highly reactive, short-lived chemical species that are capable of inactivating a wide range of microorganisms, including foodborne pathogens and spoilage organisms.
Cold atmospheric plasmas can be generated using direct current (DC) or alternating current (AC) power supplies. A range of different frequencies for AC power supplies from low kHz frequencies through to Radio frequencies (MHz) all the way up to Microwave (GHz) frequencies have been used to generate plasma discharges. Typically, noble gases such as helium or argon are used to generate a plasma because lower voltages are required to break down the gas and sustain a discharge. Other gases can be added (such as oxygen or nitrogen) to provide the type of reaction chemistries required. Plasmas have also been generated using only nitrogen as the operating gas and with the correct electrode configuration and power supply, plasmas can also be generated using air.
Cold plasmas are composed of a cocktail of different chemical species such as positive ions, negative electrons, excited atoms, UV photons, radicals and reactive neutral species such as reactive oxygen (ROS) and Nitrogen species (RNS) (Figure 1). Taken on their own, many of these chemical species are known to inactivate microorganisms. Cold plasma systems can produce these chemical species simultaneously and directly at the point of need, thus maximising the antimicrobial potential of this technology. The potential of atmospheric plasmas for inactivation of microorganisms was fully understood around twenty years ago (2). Since then there has been an exponential increase in the number of publications reporting the inactivation of a variety of pathogens, viruses, fungi, yeasts and moulds on a range of different surfaces. Much of the early atmospheric plasma research focused on inactivating pathogens on heat sensitive abiotic materials but more recently, research has focused on treating food products.
The use of cold plasma has not yet been fully realised in the food industry but the antimicrobial properties of plasma systems make it an attractive tool for food manufacturers in the fight against cross-contamination, microbiological spoilage and reduced shelf-life.
The most obvious application is the disinfection of surfaces in processing equipment, packaging, food contact surfaces and, potentially, food itself.
Cold plasma treatments are based on a non-thermal dry process, which requires low input powers and can be built to adapt to current processes. When the electrical supply is switched off, all the reactive plasma species return to their neutral ground state. The dominating reactive gas species can be significantly altered depending on the type of power supply, how the power is applied (continuous vs. pulsed), the configuration of the electrodes and the type of gases used. This means the technology has the potential ability to tailor reaction chemistries for specific applications.
The diffuse nature of plasma allows a greater chance that the reactive chemical species can inactivate bacteria in pores, crevices or harder-to-reach areas of equipment and surfaces. This offers significant advantages over alternative techniques, such as UV light where microbes can be protected by ‘shadowing’ effects.
Dry processing technologies such as cold plasma could be an ideal tool for disinfecting processing equipment and the environment of manufacturers of low aw food products. A reduced need for the use of chemicals and water, could also allow cost savings for manufacturers. A dry process also means that the technology can be operated during food production to treat problematic areas of the factory, processing line or equipment to maintain low bacterial levels. This would reduce the chances of cross-contamination and bacteria attaching and developing into biofilms.
The non-thermal properties of plasma make it potentially suitable for treating the surface of delicate raw and fresh produce as well as other foods, as long as the plasma reactive gas itself does not damage, alter or degrade any key food nutrients. Studies on treating strawberries suggest that cold plasma can inactivate spoilage organisms and significantly extend the shelf-life of samples (3, 4). Although only preliminary trials, shelf-life extension of fresh produce, such as strawberries, could significantly reduce product waste and therefore increase manufacturers’ and retailers’ overall profitability as well as generating greater convenience for consumers.
Over the past decade, research has been focused on the potential for cold atmospheric pressure plasma to be used for inactivating pathogens and spoilage organisms on the surface of food products. To date varying log reductions have been achieved on the surfaces of melons, mangoes, apples, strawberries, tomatoes, lettuce, potatoes, cheese, almonds, nuts, seeds egg shells, ready-to-eat meats, bacon, chicken and pork (5, 6).
Some plasma systems may not actually be suitable for certain foods. It is therefore essential to determine the best plasma system for treating a specific food product. For the process to become commercialised and more widely used, it is important to characterise the reactive chemistry of the plasma system in question. Defining the plasma chemistry is essential to understand how it interacts with the food and whether the treatment impacts on nutritional quality.
Suitable plasma systems could be adapted to current processing lines to treat foods. In addition to adapting plasmas to processing lines, plasmas could be used to treat foods after they have been packaged. Treating foods after they have been packaged could prevent any re-contamination of the food product after processing. Plasma devices have demonstrated that packed foods could be treated in aerobic or modified atmospheres to obtain specific reaction chemistries for surface decontamination (4). However, further work is still needed on packaged foods, not only to assess food quality changes but also to ascertain the impacts on the packaging material.
A greater understanding is also needed for manipulating plasma parameters to change the reaction chemistry. This could be used to fine tune the process to minimise or eliminate any negative effects on food quality should they arise.
In addition to treating food contact surfaces, equipment, packaging or food products, plasma also has potential applications for use in treating liquids. There are several different methods for generating plasmas in liquids. Research has revealed that plasma-treated liquids are capable of inactivating microorganisms as well as degrading a wide range of organic contaminants (7). Plasma treatment of liquids could be used alone or adapted to be used in combination with other technologies to treat water effluents. Cleaning water effluents could provide a huge saving for food manufacturers. If the plasma treatment cleans up water effluent to a potable quality, then food manufacturers could also use this technology to reduce their use of fresh water, representing a significant potential saving for those wanting to reduce their water consumption and be more efficient with its use. Again, more research is needed for plasma-treated liquids to ensure that the degradation of organic contaminants does not produce any toxic metabolites. The quality of water after treatment also needs to be well characterised to see if this technology is suitable for recycling process water.
Work at Campden BRI
A feasibility study in collaboration with the University of Liverpool on cold plasma for surface disinfection is currently underway as part of Campden BRI’s member subscription funded research programme 2013-2015 (8). This feasibility study is focused on testing cold plasma against soiling conditions more closely associated with a food production environment but is also available for private contract work.
Bacteria deposited onto steel coupons and a polyurethane conveyor belt material have been tested. Bacteria were mixed with a low organic load (protein) prior to being deposited to simulate a cleaned factory surface. Good log reductions have been achieved for plasma treatments of 2 minutes with Escherichia coli, Salmonella typhimurium and Listeria monocytogenes inoculated onto stainless steel surfaces (Table 1). Five-minute plasma treatments were found to reduce the bacterial levels for all three strains to below the detection limit. S. aureus however, proved to be more resistant to plasma treatments with lower log reductions (Table 1). Bacteria were shown to be more resistant to plasma treatment when deposited onto the polyurethane conveyor belt compared with steel coupons. Plasma trials are also being conducted on food samples such as strawberries. Preliminary results suggest that plasma treatment can extend the shelf-life of strawberries.
Campden BRI in collaboration with the University of Liverpool are also involved in a separate feasibility study for the Food Standards Agency (FSA) to assess cold atmospheric plasma as an effective disinfection tool for sprouting seeds without affecting germination or quality attributes. This project runs from June 2013 to April 2014 and more information about the project can be found on the FSA website (9).
Cold atmospheric plasma offers many benefits as a new technology for food manufacturers. It has many varied applications, from controlling environmental contamination and cross-contamination during food production, through surface pasteurisation of foods to cleaning water effluents. Further research is needed to scale-up the technology before it is suitable for industrial processes. In order to commercialise this technology for food treatment, further research is also needed to characterise the reactive chemistry to understand the effects on food quality.
Danny Bayliss, 2014