The challenges and opportunities of cold plasma for the agri-food industry

What is cold plasma?

Lightning, the aurorae, neon signs and solar flares are everyday examples of plasma phenomena. The fourth fundamental state of matter following solids, liquids and gases is plasma; a partially or wholly ionised gas containing an array of active species including electrons, ions, free radicals, UV photons, atomic, highly reactive oxygen (ROS) and nitrogen (RNS) species. When generated at or close to ambient temperatures, such plasmas are considered ‘cold’, ‘non-thermal’ or ‘low-temperature’. Unlike their thermal counterparts, cold plasma utilises smaller volumes of water and operates at lower levels of pressure and power without the need for any vacuum systems or chemicals. It is therefore deemed very environmentally friendly. Its recent increased attention within the agri-food industry is partly driven by its interactions with food products, where it is thought to only operate at low penetration depths compared to other decontamination procedures, ensuring degradation occurs solely on the surface layer of any product. Accordingly, it is believed that the nutritional benefits of food products are protected during the decontamination process. An additional application of cold plasma is in extending the shelf life of food products, making it potentially useful in the drive to reduce food waste.

Solar flares are an example of plasma phenomena

Cold plasma history

Favourable traits like non-thermality, economic viability and environmental friendliness has enabled cold plasma to successfully venture into the world of dentistry, cancer treatment and the aerospace industry, among others. Such has been the captivation and development of cold plasma, that it is predicted to expand at a compound annual growth rate of 16.3 percent between 2018 and 2026, to an overall market value of approximately $51.3 billion. The advancement in scientific research outputs indicates that the projected growth of cold plasma will penetrate into the agri-food sector.

The recent launch of the Centre for Plasma in Agri-Food (AgriPlas) within the Institute for Global Food Security at Queen’s University, Belfast illustrates why. This centre aims to study the ability to reduce harmful chemicals and antibiotics within the food chain, lessen the threat of anti-microbial resistance (AMR) species and mitigate the ingress of other key risk factors in the food supply chain leading to enhanced food safety and sustainability.

Agri-food opportunities

The realisation of having to produce safer, more nutritious and shelf-stable food at a faster rate for a growing global population is rapidly dawning on the public. New and safe technologies are required to accommodate such demands and recent cold plasma studies have identified that the technology may have a prominent role to play. Enhanced degradation of pesticides using cold plasma compared to other common procedures such as photocatalysis, hydrogen peroxide oxidation, ultrasonication, adsorption and membrane technology has been observed. Azoxystrobin, boscalid, chlorpyrifos, cyprodinil, diazinon, dichlorvos, endosulphan, fludioxonil, imidacloprid, iprodione, malathion, pyrimethanil and pyriproxyfen are just some of the pesticides that have been successfully degraded within water and various fruit and vegetable products without any physical damage incurred. Although successful and understandable as to their use, the overuse of pesticides has led to an increase in runoff drinking water, which poses significant long-term health implications. Hence, the ability to degrade such compounds and decontaminate food and drink products is of huge importance and perhaps, the supply of ‘cleaner’ water will be cold plasma’s greatest benefit.

Microbial inactivation of salmonella, mycotoxins, E. coli and Listeria within fruits, vegetables, herbs, spices, grains, nuts, fish and meat have been investigated. The decontamination of mycotoxins on many feed and food products, particularly cereals, is hugely important. Cereals are a major global food export, especially for low-income countries within Africa and Asia. They are a key component of an infant’s diet and, therefore, food safety is imperative. A recent study has shown that between 60-80 percent of all cereal crops are mycotoxin contaminated. High decontamination rates in very short periods of cold plasma exposure is achievable, ensuring that the proteins, fibres, fat and ash within feed and food commodities are not significantly affected.

One study identified that a range of mycotoxins such as deoxynivalenol, enniatins, fumonisins, sterigmatocystin and zearalenone can be successfully decontaminated within rice extracts, and aflatoxins have shown 62-82 percent rates of degradation in corn samples. Decontaminating herb and spice samples containing fungi such as Aspergillus flavus or Aspergillus brasiliensis has been achieved through cold plasma. In addition, the cold nature of the plasma ensures no loss of volatiles within the samples, thus protecting their nutritional benefits. This has been a huge problem with several thermal processes trialled to date.

As well as decontamination, cold plasma can positively modify food properties such as protein-rich isolates in grain peas, which allows increased water and fat-binding capabilities. Additionally, increased gel and flour hydration properties have been observed in basmati rice flour and cold plasma treatment appears to reduce required cooking times, increase water uptake and decrease the hardness of basmati and brown rice. This rapid progress has induced speculation that cold plasma could be used to combat other agri-food issues, such as hydrogenation of edible oils, wastewater management, seed germination performance, mitigation of food allergens and anti-nutritional factors. The latter two present momentous applications for cold plasma as global warming postulates significant challenges to farming, cultivation and seed growth, while issues regarding allergen labelling continuously plague the food industry.The shelf-life of meat and fish products may also benefit from cold plasma treatment by reducing mould or yeast populations as well as killing Listeria innocua in ready-to-eat meals – a bacterium that has recently received significant media attention in the UK due to six deaths resulting from consumption of contaminated sandwiches.

Cold plasma challenges

A consumer preference study of poultry decontamination methods in Ireland identified that cold plasma is currently the fourth preferred method (39 percent) behind forced air chilling (55 percent), crust freezing (48 percent) and steam ultrasound (47 percent). Only organic acid wash (37 percent) and chemical wash (16 percent) were considered less favourable.
In the short period of time that CP has been used, it is encouraging to witness a gradual appreciation on an island so reliant on agriculture and food production. At a time when consumer confidence towards the global food industry is perhaps at an all-time low, most recently due to authenticity and fraud scandals, persuasion of the public could be cold plasma’s greatest hurdle. The need to bring social scientists into the CP arena is therefore of fundamental importance; consumers need to have correct information presented in a way that is easy to access and understand.

Cold plasma offers numerous opportunities for the agri-food industry, yet the infancy of the technique means that further scientific studies are still required. While surface decontamination is easily achievable, studies are needed to establish the following:

• What the targeted compounds degrade in to post plasma treatment
• What chemical mechanisms occur throughout the process
• Whether the toxicity of the degraded compounds are more pernicious than the initial compounds, thus compromising food safety.

This type of work will be central to the AgriPlas centre research portfolio. Additionally, despite being energy efficient with a low energy output, the processing costs of cold plasma are largely dictated by the expensive noble gases of helium and argon. If nitrogen can be identified as a prosperous alternative, then such costs would be greatly reduced. Also, appropriate destruction and exhaustion of gases are required as well as the necessary safety measures when using very high voltages.

Cold plasma has the potential to alleviate allergen, pesticide, microbial and further public health concerns as well as treating infected animals, enhancing seed germination and cleansing surfaces of bacteria. At a time when food safety and reducing food waste are experiencing very high levels of public scrutiny, cold plasma technology may be one of the most important emerging techniques to help restore consumer trust. 

About the authors

Chris Elliott is Professor of Food Safety and Founder of the Institute for Global Food Security, Queen’s University, Belfast. He led the independent review of Britain’s food system following the 2013 horsemeat scandal and is a member of the New Food Advisory Board.

Chris Elliott

Connor Black has most recently moved to Wm Morrison Supermarkets after completing both a PhD and 18-month postdoctoral research fellow at the Institute for Global Food Security at Queen’s University, Belfast under the supervision of Professor Christopher Elliott. Connor’s PhD was focused on innovative methods to detect food fraud, which involved identifying mass fraud of oregano products within the UK as well as achieving near-instantaneous identification of fish fraud. His postdoctoral research studies were then aimed at identifying potential applications of cold plasma technology within the food industry.

Connor Black