Cold Plasma in Food and Agriculture: Fundamentals and Applications

Cold Plasma in Food and Agriculture: Fundamentals and Applications is an essential reference offering a broad perspective on a new, exciting, and growing field for the food industry. Written for researchers, industry personnel, and students interested in nonthermal food technology, this reference will lay the groundwork of plasma physics, chemistry, and technology, and their biological applications.

Food scientists and food engineers interested in understanding the theory and application of nonthermal plasma for food will find this book valuable because it provides a roadmap for future developments in this emerging field. This reference is also useful for biologists, chemists, and physicists who wish to understand the fundamentals of plasma physics, chemistry, and technology and their biological interactions through applying novel plasma sources to food and other sensitive biomaterials.

• Examines the topic of cold plasma technology for food applications
• Demonstrates state-of-the-art developments in plasma technology and potential solutions to improve food safety and quality
• Presents a solid introduction for readers on the topics of plasma physics and chemistry that are required to understand biological applications for foods
• Serves as a roadmap for future developments for food scientists, food engineers, and biologists, chemists, and physicists working in this emerging field

• Language: English
• Publisher: Academic Press
• Release date: Jul 15, 2016
• ISBN: 9780128014899

Book preview

Technology

Chapter 1

Plasma in Food and Agriculture

N.N. Misra*; O. Schlüter†; P.J. Cullen‡,§    * GTECH, Research & Development, General Mills India Pvt Ltd, Mumbai, India

† Leibniz-Institute for Agricultural Engineering Potsdam-Bornim, Potsdam, Germany

‡ Dublin Institute of Technology, Dublin, Ireland

§ University of New South Wales, Sydney, NSW, Australia

Abstract

With the planet’s population projected to reach almost 10 billion by 2050, innovative approaches to both food production and processing will be required to meet food demands. Decontamination of foods and minimization of food spoilage are critical issues to ensure food safety and sustainability. While thermal and chemical approaches remain a cornerstone for food processing, there is an ongoing search for nonthermal solutions for the treatment of food. Such approaches should contribute to improved food safety and quality profiles. Cold plasma technology has brought a new dimension to the concept of decontamination under ambient conditions, in that it truly is an ensemble of both physical and chemical decontamination methods. This innovation stems from the development of methods to generate plasma at atmospheric pressures and temperatures. The recent developments in cold plasma sources, the confirmation of strong antimicrobial action and the ability to plasma treat foods with the retention of their quality has led to the emergence of a new subject area within food science.

Keywords

Nonthermal; Plasma; Agriculture; Food; Sustainability

Acknowledgments

We are grateful to many people who contributed to this book, both directly and indirectly. We thank all the authors for their contributions and excellent cooperation. We would also like to thank the many staff members of Elsevier, particularly, Patricia Osborn, Marisa LaFleur, and Karen Miller. Our special thanks to Karen for her patience and constant encouragement during the preparation of this book.

1 Challenges and Trends in Food Production

The food industry, from farm to fork, must continually adapt to meet the demands of a growing population, both in terms of nutrition and consumer expectations. This must be achieved within the confines of the resources available and regulatory requirements. Innovation with regard to food production and processing is required to meet the emerging challenges of global food security and the complexities of the modern food chain. The key drivers of novel processing technologies, such as cold plasma, within the food and agricultural industries are discussed in the following sections.

1.1 Food Security

With the planet’s population projected to reach almost 10 billion by 2050, innovative approaches to both food production and processing will be required to meet food demands. One of the greatest challenges remains the global provision of safe food, which meets consumer demands of nutritional intake. Almost one-third of worldwide food production for human consumption is lost or wasted (Galanakis, 2015) along the food production, processing, and supply chain as outlined in Fig. 1 (Gustavsson et al., 2013). It is imperative that novel and smart solutions are developed for sustainable food-consumption patterns and global food security. Environmentally friendly intervention strategies that protect food crops or food products from decay or pests, which lead to reduced losses and/or extension of shelf life, are a key component in addressing global food security.

Fig. 1 Food wastage encountered between the harvest, supply chain, and consumption stages. (From UN FAO.)

1.2 Food Safety

In the food production chain, food safety remains a major challenge; this is compounded by the emergence of pathogens with low infectious doses and increased virulence. New food-safety intervention strategies are required to manage food safety across the increasingly complex global supply chain. Efficient strategies are required to reduce the microbiological safety risks of food products, while maintaining product quality characteristics with the combined goal of shelf-life extension. Given the vast array of crops and food commodities produced, along with their associated pathogens, no single universal technology is likely to meet all requirements. Consequently, it is important to provide the industry with variable options to meet its specific needs.

1.3 Minimal Processing

Two of the most noticeable developments in food processing over the past 25 years are the rise of minimally processed foods and the more recent nonthermal processing technologies. The former arose as a consequence of the consumer demands for fresh-like fruits and vegetables requiring minimum effort and time for preparation. The latter, a class of ambient temperature technologies, evolved in search of alternatives to conventional thermal processing. Nonthermal processing technologies for food preservation have the potential to address the demands of the consumer and deliver high-quality processed foods with an extended shelf life that are additive-free and have not been subjected to extensive heat treatment. Because of the relatively mild conditions of most nonthermal processes compared with heat pasteurization, consumers are often satisfied by the more fresh-like characteristics, minimized degradation of nutrients, and the perception of high quality.

1.4 Consumer and Regulatory Acceptance

Consumers are not only concerned about the ingredients within the foods they consume, but also the processes which are employed along the farm to fork food chain. Paradoxically, consumers are demanding foods which are minimally processed, meet their nutritional and taste desires, yet require minimal preparation. Understanding and addressing consumer issues related to novel food processes is one of the most important challenges facing the developers of innovative food products and processing aids. Research suggests that acceptance of new technologies is based to a great extent on public perceptions of the associated risks, and that perceptions of risk are influenced by trust in information and the source which provides it. Several consumer research studies have consistently shown that consumers have poor knowledge and awareness levels toward most novel food-processing techniques, which serves as a major impediment to their acceptance. Consumer awareness of plasma is in general low, and especially so with regard to food-treatment applications. There are both potentially negative and positive aspects with regard to the acceptability of plasma for agricultural and food applications. The association of terms including ionization, radiation, radicals, and reactive species may be associated by the public with food irradiation, leading to potentially negative impressions of the approach. Conversely the proposal to use common activated gases like air to treat foods, instead of approaches which leave chemical residues (pesticides, fumigants, chlorine, etc.), would be an attractive solution. It is also likely that the proposed application may also influence consumer acceptance, with differences expected between agricultural applications, waste-water treatments, food packaging, and the various food commodities, for example.

2 The Emergence of Nonthermal Solutions

Fig. 2 displays a timeline marking some of the historic milestones in the development of thermal and nonthermal food processing. One of the major developments in the history of food science was the invention of canning by Nicholas Appert in 1809–10, which remains one of the most widely used methods of food preservation. Similarly the discovery of pasteurization by Louis Pasteur in 1864 revolutionized food safety and preservation and the intervention remains a cornerstone of the food industry. Despite the fact that thermal processes such as canning and pasteurization effectively inactivated microorganisms and enzymes, their detrimental impact on color, flavor, and nutritional quality of foods has not always met consumer demands.

Fig. 2 Timeline depicting milestones in development of nonthermal food-processing and nonthermal plasma technologies.

Consequently the food industry has sought alternative or synergistic approaches to provide the treatment objectives. Nonthermal technologies have been designed to meet the required food-product safety or shelf-life demands, while minimizing the effects on the nutritional and quality attributes (Cullen et al., 2012). Nonthermal technologies can be defined as preservation treatments that are effective at ambient or sublethal temperatures, thereby minimizing negative thermal effects on nutritional and quality parameters of foods. The nonthermal processing technologies to receive most attention to date include: high-pressure processing (HPP), irradiation, ultrasound, ozonation, and electrical methods, such as pulsed electric fields (PEF), light pulses, electrolyzed oxidizing water, and oscillating magnetic fields. Most of these topics have been well researched and a wealth of information is available (Knorr et al., 2011), including some recent books (Cullen et al., 2012; Koutchma et al., 2010; O'Donnell et al., 2012; Zhang et al., 2010).

2.1 Related Nonthermal Technologies

There are a number of nonthermal technologies which are directly relevant to plasma for food applications given their commonalities including:

2.1.1 Pulsed Electric Field Processing

The use of electricity in food processing was introduced in the early 1900s and was first applied for the pasteurization of milk by ohmic heating. In 1960, Heinz Doevenspeck, an engineer in Germany, patented PEF equipment (Doevenspeck, 1960). PEF processing involves the application of pulses of high voltage (typically 20–80 kV/cm) to foods placed between two electrodes. In 1995, the CoolPure® PEF process developed by PurePulse Technologies (4241, Ponderosa Ave., San Diego, CA, 92123, United States) was approved by the US Food and Drug Administration (FDA) for the treatment of pumpable foods. Despite few successful industrial applications in the area of tuber crops and meat preprocessing (ELEA, 2016), PEF is commonly suitable for pumpable liquid foods, and similar to HPP, the high equipment cost could be a concern depending on the desired energy input.

2.1.2 Pulsed Ultraviolet-Light Processing

The bactericidal effect of ultraviolet light was first demonstrated by Gates (1928). Pulsed light technology involves the application of a series of very short, high-power pulses of broad spectrum light to the foods. The use of pulsed light from a xenon lamp with emission between 200 and 1000 nm wavelengths, with a pulse width not more than 2 ms and the cumulative level of the treatment not exceeding 12 J/cm², is permitted for food decontamination by the FDA. Pulsed technology is still an emerging technology; one of the only known commercial applications in the food industry includes the decontamination of bottle caps. Sample heating, light penetration issues, shadowing effects, and the absolute necessity for contact between microorganisms and photons are some of the major challenges that have limited the widespread use of pulsed light technology.

2.1.3 Ozone Processing

Interest in ozone has expanded in recent years in response to consumer demands for greener food additives, regulatory approval, and the increasing acceptance that ozone is an environmentally friendly technology (O'Donnell et al., 2012). The multifunctionality of ozone makes it a promising food-processing agent. Excess ozone auto decomposes rapidly to produce oxygen, thus leaving no residue in foods from its decomposition. The US FDA’s ruling on ozone usage in food as an antimicrobial additive for direct contact with foods of all types (FDA, 2001) has resulted in increased interest in potential food applications worldwide.

2.1.4 General Remarks

While the list of nonthermal technologies extends beyond what is discussed in this section, based on the examples provided, it is fair to remark that there is currently no ideal method to achieve sterilization at ambient temperature. Under this scenario, nonthermal plasma (NTP) technology has emerged as a promising method and has recently gained considerable attention of food scientists and researchers. It has shown potential for solid as well as liquid foods. This is evident from Fig. 3 which shows the increase in the number of scientific publications concerning developments in cold plasma science and its applications toward decontamination in general, and foods in particular. A list of selected research groups active in study of cold plasma technology for food applications can be found in Table 1.

Fig. 3 Number of publications dealing with plasma technology, plasma-based microbial inactivation (in general) and plasma-based microbial inactivation in foods during the last two decades. (Data accessed from Web of Science on Apr. 2014.)

Table 1

Investigators Exploring Applications of Cold Plasma Technology in Food Industry (by Alphabetical Order of Name)

Most nonthermal technologies were originally developed and employed in other fields, and were later extended for food-processing applications. For example, HPP was originally employed for processing of ceramics and materials, while ultrasound for diagnostics and noninvasive monitoring. Likewise, plasma has a long history of applications in semiconductor processing and electronics, and has recently spread to biological and food applications.

3 What Is Cold Plasma?

Plasma is often referred to as the fourth state of matter, according to a scheme expressing an increase in the energy level from solid to liquid to gas, and ultimately to an ionized state of the gas plasma, which exhibits unique properties. The hierarchy of increasing energy levels is illustrated in Fig. 4. Thus, any source of energy which can ionize a gas can be employed for generation of plasma. Plasma is comprised of several excited atomic, molecular, ionic, and radical species, coexisting with numerous reactive species, including electrons, positive and negative ions, free radicals, gas atoms, molecules in the ground or excited state, and quanta of electromagnetic radiation (UV photons and visible light). The free electric charges—electrons and ions—make plasma electrically conductive, internally interactive, and strongly responsive to electromagnetic fields (Fridman, 2008). Most active chemical species of plasma are often characterized by very efficient antimicrobial action.

Fig. 4 Pictorial representations of the four states of matter.

Plasmas can be subdivided into equilibrium (thermal) and nonequilibrium (low-temperature) plasma. If a gas is heated to sufficiently high temperature (typically in the order of 20,000 K) for achieving the ionization of the gas, such plasma would be referred to as thermal plasma. In thermal plasma, all the constituent chemical species, electrons, and ions exist in thermodynamic temperature equilibrium. The low-temperature plasma can be further branched into quasiequilibrium plasma (typically 100–150°C) and nonequilibrium plasma (< 60°C). In the former type, a local thermodynamic equilibrium among the species exists, whereas in the latter, cooling of ions and uncharged molecules is more effective than energy transfer from electrons, and the gas remains at low temperature; for this reason nonequilibrium plasma is also called NTP or cold plasma. Nonequilibrium plasmas are typically obtained by means of electrical discharges in gases.

Within the physics and engineering domains, the descriptors of cold plasmas may operate at temperatures of hundreds or thousands of degrees above ambient. Consequently, the term cold plasma has recently been employed to distinguish one-atmosphere, near room-temperature plasma discharges from other NTPs. The generation of spatially uniform, well-controlled cold plasma at atmospheric pressures has now become a reality, thereby creating an opportunity to safely and controllably apply plasma to foods and biological surfaces, including medical applications.

Cold plasma is obtained at atmospheric or reduced pressures (vacuum) and requires less power input. Cold plasma can be generated by an electric discharge in a gas at lower pressure or by using microwaves. Typical illustrations for plasma generation at atmospheric pressure include corona discharge, dielectric barrier discharge (DBD), radio-frequency plasma, and the gliding arc discharge. In contrast, thermal plasmas are generated at higher pressures and require high-power inputs.

4 History

Plasmas in nature, such as cloud-to-ground lightning and polar lights, have always intrigued people. The earliest investigations in electrical discharges were conducted by Ernst Siemens, who in 1857 reported about the phenomena of DBDs. In 1928, the American scientist Irving Langmuir proposed that the electrons, ions, and neutrals in an ionized gas could be considered as corpuscular material entrained in some kind of fluid medium. He termed this entraining medium plasma, similar to the plasma (meaning formed or molded in Greek) introduced by the Czech physiologist Jan Purkinje to denote the clear fluid which remains after removal of all the corpuscular material in blood. However, it emerged that there was no fluid medium entraining the electrons, ions, and neutrals in an ionized gas. Nevertheless the name prevailed and the term plasma now refers to any system with electrons and ions, where charged particles determine the properties of the system.

Following Langmuir’s seminal work, plasma physics emerged as an important research field. Townsend was the first to describe the flow of current through a gas, by describing the principle of self-consistency due to the ionization balance during the gas discharge process (Townsend, 1915, 1925). Plasma processing has been used since the 1970s for etching semiconductor materials (Manos and Flamm, 1989). The application of plasmas within the evolving computer industry started in the 1980s, particularly for the fabrication of miniaturized circuits. Since the last decade of 20th century, the development of and advancements in atmospheric pressure plasma has eliminated the need for expensive vacuum chambers and pumping systems. In subsequent years, several other applications of plasma have emerged including plasma medicine, water treatment, and food preservation. More recently, cold plasmas have also been generated inside sealed plastic packages in various configurations, and this has been referred to as the in-package plasma technology (Misra et al., 2014a; Patil et al., 2014).

5 Cold Plasma in Food Processing—A Paradigm Shift

An overview of the applications of gas plasma technologies in various areas of science and technology is presented in Fig. 5. It is evident that the range of applications covers many aspects of everyday life and almost all major industries. However, some of the first applications, as mentioned earlier, include the use of plasmas in electronics, especially for material processing, such as the etching of semiconductor surfaces, and later the plasma chemical vapor deposition process as well. DBD-based plasma televisions are probably the best well-known application of plasmas. Other material processing applications include the use of plasma in the polymer and textile industries for surface modification. Cold plasmas are gaining increasing importance in nanotechnology, especially for the synthesis of nanoparticles following the well-established bottom-up approach. Cold plasmas also represent an alternative technology for gas phase depollution of volatile organic compounds emitted by various industries and are being tested for liquid-phase destruction of pollutants in industrial effluents (Misra, 2015). The latter follows the success of ozone application, which is a predecessor to cold plasma for decontamination processes, typically generated using corona discharges. Apart from various industrial uses, plasmas in general are useful tools in analytical chemistry, especially for ionization of molecules and atoms for optical spectroscopy and mass spectrometry (Kadam et al., 2016). A critical analysis of the developments within plasma science and technology spanning the last decade clearly reveals plasma applications in biology as one of the most exciting and multidisciplinary fields. This marks the shift from treatment of inanimate objects to living or cellular objects. Such applications include the treatment of foods, plant materials, and in animal and human medicine. With the emergence of plasma medicine, research in the use of cold plasmas for wound healing, skin treatment, cancer treatment, and bone growth have seen an upsurge (von Woedtke et al., 2013).

Fig. 5 An overview of applications of nonthermal plasma in various areas of science and technology.

Cold plasma as a food technology is a newcomer to this field. In order to appreciate the potential opportunities that cold plasma presents for the food industry, one can compare the limitations of nonthermal technologies and the advantages of cold plasma. Based on the scientific, nonscientific, and patent literature, the described advantages of cold plasma treatment for food preservation can be summarized as follows:

1. Cold plasma offers high microbial inactivation efficiency at low temperatures (generally < 50°C). This allows it to extend shelf life, thereby improving the efficiency of the supply chain.

2. Almost all plasma sources available until now allow in situ production of the acting agents, just on demand, and in a range of gases. Therefore, cold plasma is compatible with most existing packaging and modified atmospheres.

3. The active chemical species of plasma are characterized by high diffusivity and therefore act rapidly and access the entire food surface in most cases.

4. Cold plasma is seemingly benign to many food products, if not all, and generally has negligible impact on the product matrix. In addition, it could also reduce preservative use.

5. The application of the cold plasma technology is free of water or solvent; thus, it is also considered environmentally friendly.

6. In general, cold plasma leaves no residues, given sufficient time is provided for the recombination reactions to proceed. However, this may not be universally true, and requires comprehensive validation studies.

7. Most cold plasma sources require only a low energy input; therefore, cold plasma technology is energy efficient.

Another important point to be noted here is that the technology is applicable for both solid as well as liquid foods. The various configurations of plasma sources enable generating plasmas in different gas atmospheres (to treat a range of produce), and also underwater (or liquid) discharges. While we summarized the advantages of cold plasma technologies, we also wish to mention that it is not a universal technology for decontamination of all classes of foods.

Some of the limitations of cold plasma technology include:

1. At present one of the major problems associated with cold plasma treatment of foods is associated with precisely controlling the chemistry of the gas plasma reactions, especially due to the varying levels of humidity introduced by foods.

2. Cold plasma in oxygen containing gas mixture may not be suitable for treating high-fat foods, as the reactive oxygen species formed could lead to oxidation.

3. The cost of the plasma processing is largely dictated by the cost of the gas or gas mixture in which the plasma is generated. The overall process could turn out to be expensive if operated using noble gases.

4. The plasma generation, when carried out using very high voltages, requires additional safety measures. Appropriate measures for destruction and exhaust of the gases are also required.

6 Objective of the Book

The primary objective of this book is to provide insights into the current state of the art and review the emerging applications of cold plasma technology in the food and agricultural industries. The fundamentals of plasma science—its physics and chemistry, process diagnostics, microbial inactivation principles, the effect on microorganisms in various food matrices, and the retention of nutritional and physico-chemical quality of plasma-treated foods and agricultural products—are detailed. A separate chapter is dedicated to discuss the future research needs and the plausible future applications of cold plasma technology in the food industry.

This book is aimed at researchers, students, and industry personnel interested in the area of nonthermal food technology. It is primarily intended for food scientists and food engineers interested in understanding the theory and application of cold plasma for food applications. On the other hand, the book will serve as a good reference for plasma physicists interested in applying novel plasma sources for treatment of foods or biological materials. The contents of the book are self-contained, and explain the fundamentals of plasma physics, chemistry, and technology, before moving on to applications in food processing.

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