Non-thermal atmospheric pressure plasmas for post-harvest application

Scientific article

Non-thermal

atmospheric

pressure

plasmas for post-harvest application of fruit and vegetable sanitation

Uta Schnabel, Mathias Andrasch, Jörg Stachowiak, Thomas Weihe, Jörg Ehlbeck and Oliver Schlüter uta.schnabel@inp-greifswald.de

2017

Scientific article

Postharvest

Non-thermal atmospheric pressure

AUTHORS

plasmas for post-harvest application of fruit and vegetable sanitation

Index Title 1. Motivation

2. Traditional technologies

Leibniz Institute for Plasma Science and Technology ▪ Uta Schnabel ▪ Mathias Andrasch ▪ Jörg Stachowiak ▪ Thomas Weihe ▪ Jörg Ehlbeck

3. Alternative sanitation methods

Page 3

4

5

4. Non-thermal atmospheric pressure plasmas

5. Examples from science

6. Conclusion

8

10

13

Table 1: A short overview of different types of cold plasma

Leibniz Institute for Agricultural Engineering and Bioeconomy ▪ Oliver Schlüter

Table 2: Technical characteristics and influencing parameters to describe plasma treatment.

References

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1. Motivation Fresh and fresh-cut produce have a limited shelf life of several days, which only allows a short distribution (locally). The limited shelf life and the associated losses of fresh produce have various causes, but particularly depend on a microbial contamination at all stages in the value chain. The microbial contamination may also cause foodborne illnesses, which occur annually and worldwide. Especially, produce like fresh-cut and fresh vegetables and fruits (e.g. leafy greens, potatoes, tomatoes, sprouts, berries) are frequently affected. The U.S. Food and Drug Administration (FDA) listed them all under the ten riskiest foods in their Center for Science in the Public Interest (CSPI) Report 2009. Whereby leafy greens are on the top [1].

The incidence of all foodborne outbreaks in the USA was enhanced from 0.7 % in the 1970s to 33 % in 2012 [2]. The European Food Safety Authority (EFSA) described in their zoonoses report of 2011, 5,648 reported food-borne outbreaks for 2011 with more than 200,000 confirmed human cases [3]. Among others, the outbreaks were caused by Bacillus toxins, Campylobacter, Clostridium, E. coli mainly Verotoxinproducing Escherichia coli (VTEC) and Listeria, Yersinia. Ready-to-eat as well as fresh-cut fruits and vegetables were also contaminated with these microorganisms.

In the E. coli outbreaks, 50 % of the involved products were vegetables and juices and other related products, which is most notably traced back to a large outbreak of haemolytic-uraemic syndrome (HUS) and bloody diarrhea associated with shiga-like toxin-producing E. coli (STEC) O104:H4 infections occurred primarily in northern Germany from May to July 2011.

Examples for treatable specimens and food produce, like fresh fruits and vegetables as well as packaged ones. In addition, grains, seeds and other dry bulk products are possible

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This outbreak is the largest recorded to date in Germany and, based on the number of cases of HUS, the largest outbreak of this sort worldwide. Fenugreek sprouts were identified as the most likely vehicle of the infection. In total, 3,793 STEC cases, including 827 HUS cases with death of 35 patients and 2,966 cases of acute gastroenteritis with death of 18 patients, were reported. Related to the outbreak, 137 cases, including 54 HUS cases, in 15 European and non-European countries were documented internationally.

A large outbreak of 10,952 Norovirus cases were reported in Germany in 2012 associated with consumption of imported Chinese frozen strawberries [4, 5]. More than one hundred (103) cases of hepatitis A were counted in 2012-13 in Denmark, Finland, Norway and Sweden associated with frozen strawberries [6, 7].

These are just examples of the remarkable impact of foodborne disease on the health of consumers and resulting direct and indirect consequences, costs and losses.

Good Agricultural Practices (GAP), implementation and compliance of standards and certificates are basic steps in terms of food safety. Sporadic presence of pathogens on fresh produce, however, requires effective sanitation techniques to reduce microbial loads without any negative effects on product quality. Because of the significant economic importance, a great demand regarding gentle sanitation in the production and processing of fresh produce exists generally.

2. Traditional technologies Conventional methods of decontamination and cleaning of fresh food are based on heating, pasteurization, cooling and freezing or rinsing with water.

Some of these methods rely on lethal heat treatment such as steam pasteurization, autoclaving, ohmic heating, etc. Where thermal technologies have side effects on nutritional, sensory and functional properties of the treated foods.

Freezing is an important processing method for imported fruits and vegetables especially berries. For example, 95 % of the raspberries produced in Serbia, one of the main raspberry producers in the world and the main exporter to EU, are frozen [8].

This technology used for extending the shelf life of berries is not able to reduce microbial load, which occur during harvest and growing steps before.

During

processing and consumption of the fruits, these contaminations can outbreak and cause foodborne illnesses.

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For example, Salmonella has the capability to survive freezing [9], and it has been shown that Salmonella survived on frozen whole strawberries [10] as well as in juices and purĂŠes of strawberries [11, 12].

The presence of Norovirus in frozen raspberries has been linked to outbreaks of gastroenteritis in Finland [13] and in the case of frozen strawberries to a large outbreak of the similar symptoms in Germany [4]. The fruits implicated in these outbreaks had been frozen, which demonstrates the capacity of these viruses to survive and remain infectious after undergoing a freezing process. Outbreaks of hepatitis A infection have also been linked with frozen berry fruit [14, 15].

Therefore, the development of environmentally friendly alternative disinfection and cleaning methods is important, but also the product compatibility, costs, environmental impact, impact on product quality and regulatory provisions have to be taken into account [16].

3. Alternative sanitation methods Alternative non-thermal sanitation methods with chemical sanitizers like ozone, chlorine, electrolyzed water and peracetic acid or physical treatments such as high hydrostatic pressure, pulsed electric field, oscillating magnetic field, UV- or gammairradiation and high-power ultrasound have been developed and studied in recent years [17-19]. These processes retain quality of foods better than conventional methods; however, they have their own drawbacks. They are costly, require specialized equipment as well as trained personnel, are either not fully harmless for consumers or could not achieve the desired success, or even negatively affected the produce quality [19].

Moreover, consumer acceptance and safety issues should be considered [20]. The FDA (Food and Drug Administration of the United States) approved the use of sodium hypochlorite, chlorine dioxide, hydrogen peroxide, peracetic acid and ozone as sanitizers for fresh and minimally processed fruits and vegetables [21, 22].

Sanitization with chlorine products is popular and has been intensively used in food industry [23]. However, increasing public health concerns in terms of the possible formation

of

chlorinated

organic

compounds

such

as

chloramines

and

trihalomethanes, have raised doubts for the use of chlorine especially by the fresh-cut industry [24, 25].

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Trihalomethanes were classified as possible human carcinogens by the United States Environmental Protection Agency (EPA) [26]. Furthermore, the prolonged exposure to chlorine vapors could result in the irritation to the skin and respiratory tract of workers [27].

The safety and efficacy of chlorine might eventually be the reason of the implementation of restrictions by regulatory agencies in USA [28]. Some European countries including Germany, the Netherlands, Switzerland and Belgium prohibited the use of chlorine in convenience produce [29]. In some countries, also the use of sodium hypochlorite is restricted.

In Germany, residual chlorine or its reaction products must be absent in minimally processed vegetables at the consumer level and not adversely causing effects on odor and flavor of the produce. Beside the impairment of the produce, it can be a danger to the consumer health. In France, its use is not actually authorized but only tolerated for disinfection of minimally processed products and the subsequent rinsing with potable water is obligatory. The same process should be done in United States, which allows the use of this chemical product but only with subsequent washing to remove product excess [30]. The trend is that such prohibitions will be expanded, so some alternatives to minimize the use of chlorine or replace this sanitizer are preferable.

Ozone is a powerful oxidizing and sterilizing agent used in small concentrations and short contact time to the produce. Since the product of degradation is oxygen, ozone has the great advantage of not being a source of pollution, [31].

Ozone can be used as a sanitizer in the form of a gas or dissolved in water. When ozone is utilized as a gas, the length of exposition is longer (1-4 h) than ozone dissolved in water (1-10 minutes) [32].

In the 1990th, Japan, France and Australia permitted the use of ozone in the food industry and the FDA declared ozone as a GRAS product (Generally Recognized As Safe) [32, 33]. Later the U.S. regulation was modified to allow the use of both aqueous and gaseous phases in food treatment, conservation and transformation [32]. Treatments of vegetables and fruits showed a considerable decrease in mold and bacterial counts without causing any change in their chemical composition and sensory quality [34, 35].

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Fresh-cut lettuce, washed with ozonated water and packed in ozone, showed an extension of shelf life [36]. Although ozone is an efficient sanitizer, it has some limitations when used on food. Moreover, due to its short life span, ozone must be generated on site as storage is not possible [37]. Ozone is a highly instable and corrosive gas. The decomposition of ozone requires elaborate processes depending on the types of radicals formed in solution and on several types of organic matter in the medium that induce, promote or inhibit the reaction chain [38]. In addition, low doses of ozone, which can inactivate pure microbial cultures, can be inefficient against viruses, spores and cysts.

In recent years, acidic electrolyzed water is investigated as one of the advanced technologies for non-thermal food processing. Acidic electrolyzed water was developed in Japan and is used as a sanitizer in the food industry to reduce or eliminate bacterial populations on food products, food-processing surfaces, and non– food contact surfaces. This technology was approved as a food additive by Japan and the USA more than 10 years ago [39, 40]. Electrolyzed water is prepared by electrolysis of an aqueous solution of sodium chloride at low concentrations of 0.2% or lower in an electrolytic cell with a diaphragm between a cathodic side and an anodic side. Another possibility is the use of hydrochloric acid solution at a concentration of 2 to 6 % in an electrolytic cell without a separating diaphragm [4144]. Many scientific studies have confirmed the strong antimicrobial effect against most viable bacteria cells, pathogenic and non-pathogenic bacteria and spore, virus and fungus both in vitro and on different food and agricultural produces (fruits and vegetable surfaces) [45-49]. Acidic electrolyzed water has many advantages, especially against chlorine sanitizers.

It impacts the environment less adversely, because it is environment friendly as it is generated by the electrolysis of only water and a dilute salt solution [40, 45, 50-52].

When it is exposed to organic matter, or diluted by tap water, it becomes ordinary water again.

Since no concentrated chemicals are required during its production, it has been reported to be safer to the user [53-55]. It is more effective, less dangerous and less expensive than most traditional preservation methods such as glutaraldehyde [56, 57], sodium hypochlorite and acetic acid [58]. The main disadvantage of acidic electrolyzed water is the rapidly loss of its antimicrobial activity in a short time because of chlorine loss [59, 60].

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This is obvious when the solution is not continuously supplied with H+, HOCl and Cl2 by electrolysis [61] at a lower pH of 2.5. Especially, high hydrostatic pressure is increasingly being used to stabilize non-thermally treated fruit juices and obtain products with superior sensory quality [62]. In strawberry purée, a reduction of at least 5 lg of Salmonella enterica (a mixture of 4 serovars Montevideo, Newport, St-Paul and Stanley) was obtained for pressures equal or higher than 300 MPa applied for 20 min at 21 °C [63].

When used to decontaminate strawberry purée before freezing, the same reduction was obtained after a few days storage at -1 °C for lower pressures, between 200 and 300 MPa [63]. According to this study, high hydrostatic pressure should reduce the risk of Salmonella in processed strawberries, to a similar extent as for thermal treatments usually applied to fruit juices. However, the impact of high hydrostatic pressures on Salmonella depends greatly on the Salmonella serovars and on the fruit substrate [62]. Another possible alternative method could be the application of nonthermal atmospheric pressure plasma.

4. Non-thermal atmospheric pressure plasmas

What is physical plasma? – A short definition

Alternative sanitizers, which guarantee a low microbial load in combination with retaining high produce quality during shelf life, are of great interest. A promising physical approach is the application of non-thermal plasma (NTP) generated at atmospheric pressure. Plasmas are ionized gases containing neutral particles and a high proportion of free charged particles such as ions and electrons. The application of non-thermal atmospheric pressure plasma is an innovative discipline with increasing attention in the field of food processing and an emerging non-thermal technology for reducing microbial load on the surface of fresh and processed foods.

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Thus, the potential applications of non-thermal atmospheric pressure plasma for the food industry are manifold and it has specific potential for the treatment of foods [6466]. Dry decontamination of food surfaces, granular and particulate foods, and sprouted seeds could be carried out with that method. Furthermore, the surface of packaging material could be sterilized [67-69].

Non-thermal plasma is implemented in the food industry for the decontamination of raw agricultural products such as apples, lettuce, almonds, mangoes, melons, egg surfaces, cooked meat, and cheese [64, 70]. Non-thermal plasma is also suitable for processes, in which high temperatures are not recommended [68, 71]. In food processing, the direct application of so-called â&#x20AC;&#x153;cold plasmaâ&#x20AC;? (see Table 1), as well as semi-direct or indirect treatment with thermal plasma is of interest as these can be used to treat the food at low temperatures (<70 °C).

For applications in the food sector, preference should be given to processes carried out at atmospheric pressure (e.g. plasma jet, dielectric barrier discharges) because they allow continuous process control and do not accelerate undesirable phase transitions, compared to applications at reduced pressure (p < 1013 mbar) or low pressure (p < 10 mbar) [72].

Physical and chemical components of plasma with antimicrobial/biological effects

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5. Examples from science Food being treated with plasma is exposed to the reactive components in the same way as the contaminating microorganisms. Therefore, the aim is to achieve the highest possible reduction of the microbe count with the lowest possible effect on food quality. Investigations regarding changes of food-related substances have been carried out with isolated compounds.

Substance losses were observed depending on the plasma system and exposure time [73-75]. The impact on the chemical composition of plant systems has only been studied with lambâ&#x20AC;&#x2122;s lettuce (Valerianella locusta) [76, 77]. After plasma treatment, an increased flavonoid content was reported [77]. The reason behind this observed increase has not yet been elucidated. Scanning electron micrographs of plant surfaces treated with low-temperature plasmas revealed changes due to erosion phenomena in the upper epidermis. Plasma treatment of fresh spinach leaves and subsequent cold storage (24 h) caused discolorations [78, 79]. Possible sensory changes have been rarely investigated to date [80, 81]. The inactivation kinetics of microorganisms due to plasma treatment are also greatly influenced by the surface structure [82-88] and thus strongly varies depending on the food surface [85, 88, 89].

Therefore, investigations using model systems cannot be simply transferred to the conditions prevailing on complex food surfaces.

Examples for food treatment by non-thermal atmospheric pressure plasma. Left side: Plasma treatment of lambâ&#x20AC;&#x2122;s lettuce by plasma jet kINPen09. Right side: Plasma treatment of radish seed by plasma jet kINPen09

The efficiency of the method also depends on the specific properties of the product.

Thus, the specific energy input, heating of the product and temperature distribution are as important as material properties, composition, geometry and whether the material being treated is uniformly shaped, in pieces, powdered, or a liquid.

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Pores, capillary openings, a high-water content and the buffering capacity are influencing the inactivation efficiency of the plasma. The process temperature is a particularly suitable parameter for comparative assessment of plasma methods.

Other parameters, e.g. electron energy distribution, plasma composition, and the specific energy input may also be used; however, they have been difficult to determine so far. Selected relevant parameters are listed in Table 2. Most studies carried out so far have used particulate food products. However, liquid foods, e.g. juices, can also be plasma treated [90, 91].

Example for food treatment by non-thermal atmospheric pressure plasma. Dill (Anethum graveolens) seeds treated with dielectric barrier discharge (DBD). Small photo: DBD with plasma turned on, ringshaped electrode configuration

Currently available data reveal that it is possible to achieve microbial count reductions in food of up to 6 lg units, in some cases even up to 8 lg units [72]. However, general conclusions cannot be drawn from these individual observations. The use of plasma to decontaminate the surface of sensitive products, e.g. fresh-cut produce is also being investigated [92, 93]. It is to be taken into consideration that bacterial cultures can also grow invasively into the food (e.g. through stoma of plant leaves) or migrate into food tissues so that plasma treatment may not reach them [87].

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At industrial scale, two studies were recently published. Within one publication a DBD set-up was used for packed fresh-cut lettuce with significant reductions and the other work used a microwave discharge indirectly for the sanitation of fresh-cut lettuce during washing and rinsing with a reduction of 2 lg steps of native load [94, 95].

Possible microorganisms used for artificial inoculation are Gram-negatives, Gram-positives, spores of fungi or bacteria as well as viruses and fungi itself. Right side: Untreated and plasma treated agar plates with Pectobacterium carotovorum

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6. Conclusion Products or product groups treated with a plasma must be subjected to a case-bycase assessment. The plasma process must be described with respect to its technical parameters. No investigations have been conducted so far on whether toxic compounds are formed because of plasma treatment. In the case that plasma treatment leads to significant changes and affects the nutritional value, constituent composition and/or content of undesirable substances in the food, the treated products must be considered within the scope of the Novel Food Regulation. The possible impact on the allergenicity of foods also requires investigation.

Plasma treatment opens new perspectives for lowering the microbial count on food surfaces. For a health assessment, sufficiently substantiated microbiological data are still too scarce. According to the present state of knowledge, plasma-treated products have to be assessed case by case.

Further investigations of inactivation pathways and chemical impact of non-thermal atmospheric pressure plasmas on plant tissue are needed and depend on interdisciplinary and international work and cooperation as well as networks dealing with this research topic. Last but not least this innovative research relies on the support of industry and politics/governments.

Non-thermal plasmas can support a solution for a safe and sufficient nutrition.

Illustration of the vision how plasma treatment of fresh food for sanitation aspects can be realized in future

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Table 1: A short overview of different types of cold plasma Type

Description

Examples

Plasma is in direct contact with the substrate.

Direct

Interaction based on irradiation (VUV, UV),

plasma jet, dielectric barrier

charged molecules, radicals and reactive

discharge (DBD)

particles

Distance between plasma and substrate is much larger than the mean free particle

SemiDirect

path.

surface-DBD with gap,

No interactions with charged particles.

Sterrad process

Antimicrobial effect due to irradiation,

with plasma-activated

longlived

hydrogen

radicals as well as metastable and

peroxide

inhibitory substances

Irradiation with UV and VUV light. Plasma is Indirect

enclosed in a UV/VUV-transparent reactor.

UV lamps

No interaction with plasma particles

ozone generator e.g. for drinking Indirect

Plasma is used to treat gas or liquids

water treatment, plasma processed air (PPA) based on microwave discharge

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Table 2: Technical characteristics and influencing parameters to describe plasma treatment.

Individual Systems

Category

Example Parameters

plasma parameters

current gas composition geometry pressure type of plasma generation

System

voltage applicator parameters

volume of chamber pressure of treatment

product parameters

dosage process temperature area/volume to be treated

radiation

spectral power distribution

charged particles

electron density ion density ion energy distribution

Plasma

neutral particles

density lifetime reactivity temperature type

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