Fast, efficient, economical – nearly perfect plasma

• Cold plasma was initially designed for sectors other than the food industry.
• Plasmation can be a preliminary or autonomous method for preserving food products.
• Scientists have tested the impact of plasma on color, texture, antioxidant activity, and nutrient content in the studied products.
• The new method allows for time and energy savings.
• Before Cold Plasma (CP) technology is introduced to the food industry, additional research is necessary.

Although plasma was described a century ago, its application in food preservation has only been explored since the beginning of this century. –It's a very young method; there aren't many studies. There's still a lot to do, we're just paving the way. That's exactly why I chose this topic – says Dr. Klaudia Masztalerz from the Institute of Agricultural Engineering at UPWr. An article titled "Cold plasma as a new, energy-efficient method for preliminary food drying: latest achievements, mechanisms, and considerations for industrial applications" was published in "Trends in Food Science & Technology," a prestigious journal mainly concerning food processing technology.

– Conventional drying is one of the most energy-intensive processes in the food industry. It requires a lot of energy and time, leading to greenhouse gas emissions – adds the author. – Hence, the necessity to develop more environmentally friendly technologies. Non-thermal methods, such as the application of cold plasma, are hopeful.

Dr. Masztalerz is the main author from our university; co-authors include Prof. Antoni Szumny, Prof. Adam Figiel, and Mohsen Gavahian from the National Pingtung University of Science and Technology of Taiwan. The publication is the result of a collaboration that began a year ago. Dr. Gavahian, a distinguished specialist in designing energy-efficient new food processing technologies, was invited to UPWr as part of the PROM program of the National Agency for Academic Exchange (NAWA).

Inside stars and laboratories

Plasma – the term was first used by Nobel laureate, American physical chemist Irving Langmuir in 1928. It is ionized gas, which, alongside solid, liquid, and gas, is the fourth state of matter. High-temperature plasma is the primary building block of the Universe, filling the interiors of stars. On Earth, it is rarely seen, for example, during auroras and atmospheric discharges accompanying storms. Low-temperature plasma, ranging from 20 to 40 degrees Celsius, can be obtained in the laboratory by applying a strong electromagnetic field to a gas, causing its ionization. It contains electrons, ionized and non-ionized molecules, atoms in the ground and excited states, and free radicals. Plasma conducts electricity well and, depending on the type of gas and the energy supplied, can be more or less ionized. It varies in color: plasma using low-intensity current is colorless, while high-intensity current produces fluorescent violet light. The chemical composition of cold plasma (CP), radical concentration, and the number of emitted photons depend on voltage, temperature, gas flow intensity, duration of exposure, and gas composition. Nitrogen, oxygen, air, and even expensive noble gases like argon or helium can be used.

– The key component of cold plasma that contributes to its beneficial properties is the active free radicals, which neutralize various pathogens – says the scientist. These reactive molecules damage the cells of microorganisms, which is utilized in medicine, for treating hard-to-heal wounds and skin diseases, sterilizing medical equipment, and in dentistry for cleaning and whitening teeth. Plasma has proven especially effective during the COVID pandemic for disinfecting surfaces. CP technology was first designed for various industry sectors: it is used for cleaning gases and wastewater, waste disposal, pigment production. It can improve packaging materials by altering their surface to reduce water vapor and oxygen permeation, which is particularly important in packaging plant products. Plasma can treat food inside packaging, potentially extending shelf life. For example, strawberries treated with cold plasma directly in the packaging maintain freshness even three days longer.

The application of cold plasma in food processing is particularly promising. This is what the authors from Wroclaw and Taiwan focused on – their review paper covers peer-reviewed articles from journals from 2017-2023, collected through academic search engines, including Google Scholar, PubMed, Science Direct, etc. They analyzed the combination of plasmation and drying by different methods: convective hot air drying, microwave drying, and lyophilization.

Berries in a plasma bath

Plasma can be applied to food products in various ways. Either by direct gas blow under pressure or by immersion in plasma water, i.e., water treated with plasma, with reduced pH.

 – The latter method is more precise because, for example, a fruit or vegetable immersed in such water is entirely subjected to the action of cold plasma. However, the blow does not reach every part of their surface – notes Dr. Masztalerz.

Example: we immerse berries in a plasma bath for a few seconds or place them under a gas nozzle in a special chamber for a few minutes.

– The chamber ensures the safety of the operator. It isolates her because a lot of free radicals, which are not beneficial to humans, are produced during this process, accelerating cell degeneration – explains the scientist. Plasmated berries will be sterile, and their microorganism-free skin will become rough. Micro-damages will form on its surface, leading to electroporation, making them easier to dry. Free radicals cleanse the fruit surface and destroy the protective wax layer, increasing moisture diffusivity. Changes also occur inside. Ions and reactive forms of oxygen or nitrogen (RNS) alter the cell structure. They disrupt intracellular bonds, reduce the strength of the cell wall network, create intracellular spaces and micropores in the cell wall, significantly facilitating drying.

Plasmation can be an autonomous method, only for sterilizing the surfaces of directly consumed products that will be eaten within a few days or weeks. However, if we want to ensure longer shelf life, we need to intervene more in the interior of the product. Plasmation is then a preliminary method combined with other methods, usually drying.

Another classic processing method is blanching with hot water/steam, i.e., short-term immersion in a hot agent, followed by rapid cooling. Other methods, such as curing, candying, or salting, are based on osmotic phenomena. Osmotic dehydration involves removing water from products using a hypertonic solution, most often a solution of salt or sugar. Due to the osmotic pressure between the product and the solution, there is a bidirectional exchange: water contained in the product permeates into the osmotic solution, and particles contained in the solution – into the product. All these methods allow for shortening the drying time but are not without flaws. They can cause nutrient loss, involve the use of chemical substances, and high energy consumption. Plasma is a much better preliminary treatment.

Some gain, others lose

Scientists have tested how CP affects color, texture, antioxidant activity, and nutrient content, including phenols and vitamin C. They experimented on tomatoes, strawberries, grapes, saffron, melons, potatoes, lettuce, shiitake mushrooms, raw meat, deli meats, sprouts, nuts, spices (including pepper), fruit and vegetable salads, milk, and even juices, which can be plasmated instead of pasteurized. In food preserved this way, there are no thermal changes. Their temperature does not increase, so the nutritional and sensory properties do not deteriorate as much. However, each food product behaves differently. Some benefit from contact with plasma, while others lose a bit. It depends, among other things, on the duration. If it is too long, the quality of the dried product may deteriorate. Treatment may negatively affect several easily oxidized nutrients, such as polyunsaturated fatty acids, heat-sensitive dyes, and nutrients.

– Therefore, it is crucial to perfect the conditions of preliminary drying treatment – says Dr. Masztalerz. Plasma helped dry caraway, but extending the blow time caused the seeds to harden. Similarly, the hardness of fresh pasta increased during drying after CP preliminary treatment. Plasmation did not change the color of chili peppers or tangerines, but raw pork changed to a more yellow color, thus its commercial value deteriorated. Berries and chokeberries unfortunately lost their purple color, along with health-promoting anthocyanins, sensitive to oxygen. The level of carotenoids in dried tomatoes and tannins in millet flour also decreased. In contrast, the level of beneficial polyphenols in pomegranate juice or blueberries increased under the influence of plasma. In jujube slices, the content of valuable procyanidins, flavonoids, phenols, and antioxidant activity increased by about 54, 34, 14, and 37 percent, respectively.

– The shortened drying time at high temperatures due to plasma leads to greater retention of bioactive components – explains the author.

The application of cold plasma to caraway seeds improved their germination ability. In milk contaminated with Escherichia coli, bacteria were eliminated by plasma, and the color, pH, and fat content did not change. CP can not only remove bacteria, yeasts, fungi, viruses. It can even reduce the content of pesticides or carcinogenic harmful 5-hydroxymethylfurfural, which forms during the thermal processing of food products, especially in acidic environments, by half.

A few seconds save several hours

Drying assisted by cold plasma is an excellent alternative to traditional drying methods also in terms of cost. The most popular convective drying is very energy-intensive and thus expensive. Fruits, vegetables, or meat are exposed to a stream of hot air for many hours.

– The duration varies depending on the raw material: for herbs, a few hours are sufficient, for other products, it can be even 20 hours – says the author. Temperatures also vary: herbs are dried at 30-40 degrees, fruits, and vegetables even at 70 degrees. A higher temperature shortens the process but lowers product quality. If we preliminarily treat them with plasmation, then this process is halved: we save time and energy. A few seconds of CP preliminary treatment can save several hours of drying.

– Plasma fits into sustainable development, which is particularly close to me – recounts the scientist, who completed her engineering and master's studies in renewable energy sources and waste management at UPWr. In her doctorate, she dealt with osmotic dehydration in concentrated fruit juices with the addition of herbal extracts used as an osmotic solution. The application of health-promoting osmotic solutions not only reduces the water content in plant raw material. It also increases its bioactive potential due to the penetration of active compounds from the solution into the dehydrated raw material. The osmotic dehydration process alone is not sufficient to ensure the stability of food products. It is necessary to combine it with other food preservation methods, such as drying, which is, however, very energy-intensive and time-consuming. Plasmation is a more economical solution.

To err on the side of caution

Why has the method not yet moved beyond laboratories into the food sector?

– Before it reaches the industry, we need to test the effect of plasma on the chemical and physical properties of a larger number of products to minimize risk. There are still no necessary regulations for new technologies regarding food processing using CP. It is also necessary to check the impact of cold plasma on allergens and toxicity. Depending on the method of plasma generation, different types of free radicals are produced, often in a manner and quantity difficult to predict. This means that the impact on food products is not entirely controlled. Further research is needed to translate this method to a production scale – says Dr. Masztalerz. She conducted research on this topic during internships at the University of Agriculture in Krakow and the University of British Columbia in Canada. Together with Prof. Figiel and other employees of the Department of Thermal Technology and Process Engineering, she cooperates with the Wroclaw company Plazmatronika, which manufactures devices using cold plasma. The downside of plasmation is the equipment: costly and difficult to scale up.

– Presumably, this will change as plasma technology develops. Currently, research is usually conducted on a small scale, applying plasma to surfaces of a few square centimeters. Work is needed to scale up this method. Besides, the equipment is too expensive. It should be more accessible, easy to use, and operate with various gases, not just expensive noble gases – says the publication's author. She emphasizes the need to test different plasma sources, which can arise, for example, as a side effect during microwave-vacuum drying. It is also worth continuing research on the possibilities of using plasma water for food preservation. A more developed technology that uses non-thermal processing methods is pulsed electric fields (PEF). Compared to cold plasma, it is much more predictable. PEF technology involves treating a sample immersed in water with a pulsed electric field. This leads to the formation of micro-openings on the surface and the release of active components in cell walls, increasing the intensity of evaporation during drying and the efficiency of the extraction process.

– However, plasmation, as we write about it, seems almost perfect – says Dr. Masztalerz. She lists: that it is efficient and environmentally friendly because it is energy-saving and without toxic by-products. It improves food safety because it is very effective in destroying pathogens, which can reduce poisoning in the case of easily perishable food, for example, salmonella. It is beneficial for food products because it extends shelf life, preserving nutritional and sensory properties without the use of preservatives. It is economically beneficial; even plasmatrons, i.e., devices for generating ionized gas, do not require a large amount of electric power for operation. – We still need some time to dispel doubts – she adds.

Aneta Augustyn

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