Meet the cold plasma technology

Cold plasma food processing

Can you imagine a technology capable of disinfecting surfaces, packaging, food contact surfaces or even food itself? Well, stop imaging. Cold plasma exists, is innovative, and offers some interesting benefits for the food sector.

 

But, what is indeed the cold plasma technology?

After lockdowns and restrictions due to COVID-19, I am sure that almost every person has had more time for themselves. In my case, I took advantage of being at home by creating this website, getting interested in physics and watching TV shows. Regarding the TV shows, I completed Chernobyl and Breaking Bad. For a person not interested in science at all, words that often appear in these shows such as radiation, nucleus, and chemical synthesis, among others, might sound science-related curious words that tend to draw the viewers’ attention. Surely, Cold Plasma may be one of those unusual words. This has been recently associated with the food sector!

To understand this emerging technology, we should first understand what plasma itself means. Plasma is, in fact, one of the four fundamental states of matter. It consists of gas made of ions and free electrons.

The four states of matter. Plasma requires the highest energy to be stable. Photo credit: Britanicca.

For the non-physics lovers or non-science people, let’s briefly explain what ions and free electrons are. In a neutral atom, the amount of positively charged protons equals the number of negatively charged electrons. This means that their charges cancel out, leading to a zero net charge (no overall charge). There are cases, however, in which an atom loses or gains one or more electrons. This causes the atom’s net charge to shift to negative when it ends up having more electrons than protons, and to positive when it ends up having more protons than electrons. These charged atoms are called cations if they have more protons and anions if they have more electrons. Overall, they are called ions to differentiate them from neutral atoms.

 

Cation and anion. Photo credit: Link.

Electrons, on the other hand, are the subatomic particles that orbit the nucleus of an atom. They are generally negative in charge and are much smaller than the nucleus of the atom. Electrons are thought to be elementary particles because they have no known components or substructure. Free electrons, in contrast, are not permanently attached to any atom.

A simple model of an atom with the nucleus made of protons (which have a positive charge), and neutrons (which are neutral). The electrons (which have a negative charge) orbit the nucleus. Photo credit: Shutterstock.

As we said earlier, plasma is a gas made of ions and free electrons.

Based on the thermal equilibrium, plasma can be classified as thermal and low-temperature plasma. The low-temperature plasma can be further divided into other two categories. Nevertheless, the type of plasma the food industry is interested in is referred to as cold plasma (CP) or non-equilibrium plasma. This is not in thermodynamic equilibrium because the electron temperature is much hotter than the temperature of the present heavy species (ions and neutrals).

Plasma can be generated using any kind of energy which can ionize the gases, such as electrical, thermal, optical (UV light), radioactive (gamma radiation) and X-ray electromagnetic radiation. However, for CP generation, electric or electromagnetic fields are used. The versatility of CP generation sources offers unique designs that are compatible with current food industry equipment.

CP is generally considered a tool for surface treatments. In fact, this technology has been used by the polymer and packaging industries for decades for surface modification and functionalization of polymers.

Over the past decade, CP has gained significant interest for use as a non-thermal technology in food processing. The novelty of this technology lies in its non-thermal, economical, versatile and environmentally friendly nature.

Pin reactor discharging CP in the open air to treat blueberries. Photo credit: PlasmaLeap.

The applications of CP for food industries have been demonstrated for food decontamination, enzyme inactivation, toxin removal, food packaging modifications, and wastewater treatment. Particularly for food processing, CP is effective against significant food-borne pathogenic microorganisms such as Escherichia coliSalmonella typhimuriumStaphylococcus aureus, and Listeria monocytogenes.

CP has inactivation effects on bacteria via bacterial cell wall/membrane or extracellular polymeric substance disruption, or via action on intracellular components, metabolic activity, or virulence factors. For instance, preventing biofilm formation leading to cell death. Furthermore, CP treatment also results in enhanced physicochemical, physiological, and functional properties of foods.

Cold Plasma interactions with microbial contaminants and food. Photo credit: Trends In Biotechnology.

From a food processing perspective, plasma source, electrode design, pressure, voltage, treatment time, the distance between electrodes and reactive gas all play essential roles in determining the gas speciation, reactive species concentration, discharge characteristics and overall efficiency of the process.

CP is a novel, non-thermal technology that has shown good potential for food decontamination. However, most of the research is largely focused on microbial inactivation studies, with limited emphasis on food quality. Accordingly, the precise understanding of the mechanisms and control over the quality attributes are required for cold plasma technology to realize its full potential at a commercial scale.

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