Impact of plasma processed air (PPA) on phenolic model systems: Suggested mechanisms and relevance for food applications

https://doi.org/10.1016/j.ifset.2020.102432Get rights and content

Highlights

  • Plasma processed air is generated on demand using compressed air and electricity.

  • Plasma-related formation and polymerization of reaction products were evidenced.

  • Reaction kinetics of PPA-treated phenolic compounds correlate with process intensity.

  • Plasma-induced reaction product might serve as process indicators.

Abstract

Cold plasma is considered to be a novel, non-thermal, chemical-free and eco-friendly disinfection and surface modification technology. Plasma treatment of air to generate the so called plasma processed air (PPA) induces the formation of reactive oxygen (ROS) and nitrogen species (RNS). As a result, PPA has a different chemical composition compared to untreated air and suits therefore as an alternative method for microbial disinfection. However, depending on the product properties of the food matrix and its composition, a number of plasma-induced reactions also need to be taken into consideration.

This necessitates also the elucidation and understanding of the basic interactions of plasma species with bioactive compounds. The intention here is to avoid the degradation of these valuable substances and to prevent other undesirable effects in future food related applications.

In the present study, the effects of PPA treatment on selected antioxidants such as pyrocatechol and derivatives of hydroxycinnimic acid were investigated in model systems to specify possible reactions induced. Antioxidant capacity, pH value, UV–Vis spectroscopy, RP-HPLC and LC-MS analysis were applied to identify reaction products providing information on possible changes induced in food matrices by PPA treatment.

Exposure to PPA caused a perceptible color change towards yellow-brown accompanied by a strong reduction of the pH and the formation of insoluble sediments in the model solutions. The accumulation of nitrate, nitrite, but not of hydrogen peroxide was shown. LC-MS analysis demonstrated the formation of plasma-modified derivatives in all tested systems. The main reactions in liquid model solutions exposed to PPA were attributed to oxidation, nitration and polymerization of the phenolic compounds.

Introduction

Recently, cold atmospheric pressure plasma (CAPP) technology has attracted extensive attention among food scientists and has evolved to be a promising technique with great potential for application to food. The direct, semi-direct or indirect application of plasma is of interest in the food sector as it can be used to treat food at temperatures below 70 °C (Schlüter et al., 2013). The combination of its nonthermal character with its ability to operate at atmospheric pressure makes CAPP a suitable approach for the treatment of heat-sensitive products.

Whereas the penetration depth of plasma-immanent species is limited during treatment of solids, in liquid foods, however, it is supported by convection and in this case most of the bulk comes into contact with the applied plasma or at least with the subsequent reaction products. This implicates that all components contained may react with plasma-immanent species, and are thereby affected. Due to their interaction with water and other molecules, plasma reactive oxygen (ROS) and nitrogen (RNS) species initiate several chain reactions, resulting in a large variety of different reactive molecular entities. Dependent on the plasma source and the process parameters, most important ROS substance class formed in liquids are hydroxyl radicals (OHradical dot), which are able to react and form hydrogen peroxide (H2O2). Depending on the pH value, the latter can be very stable and remain active in the liquid far beyond the plasma exposure time. In addition, hydroperoxy radicals (OOH) or superoxides (O22−) can be formed in the presence of OH radicals, which can also be quite stable (Ikawa et al., 2010; Zhang et al., 2006).

Using a microwave driven plasma discharge and air as the process gas to produce plasma processed air (PPA), longer-living RNS come into contact with the matrix to be treated. Composed of 78.08% nitrogen, 20.95% oxygen, and 0.97% argon, Bacri and Raffanel (1987) calculated that exposure of standardized air to a plasma source induces the formation of 28 species (including electrons) contained in the excited plasma state and divided the plasma-specific species into eight families: atomic nitrogen (N, N+, N+++, N+++), dinitrogen (N2, N2+), nitric oxide (NO, NO+, NO), nitrogen dioxide (NO2, NO2+, NO2), nitrous oxide (N2O, N2O+, N2O) atomic oxygen (O, O+, O++, O+++, O), dioxygen (O2, O2+. O2), and argon (Ar, Ar+, Ar2+, Ar3+). These species are not stable and depending on the treatment conditions recombine to more stable reaction products. By treating liquids with dry plasma gas, many chemical reactions can take place in the gas phase, the liquid phase and at the interface. The main resulting products could be NO, NO2, NO3, O2, O- and N2O3 in the gas phase, HO, HNO2 and HNO3 in the gas and liquid phase and H2O2, ONOOH, HNO2 and HNO3 in the liquid phase (Schnabel et al., 2015).

PPA generated by a microwave-driven plasma torch has been applied to successfully inactivate enzymes (Bußler et al., 2017), as well as microorganisms (Baier et al., 2013) and bacterial spores (Hertwig et al., 2015a) on the surfaces of fresh and dry food (Baier et al., 2014; Baier et al., 2015a), spices and herbs (Hertwig et al., 2015b) and meat (Fröhling et al., 2012). Results obtained in studies reporting PPA specific browning reactions in fruits and vegetables notwithstanding enzyme inactivation indicate that oxidation and polymerization reactions of secondary plant metabolites as phenolic compounds occur as a result of PPA treatment under certain process conditions (Bußler et al., 2017).

Since phenolic compounds as one group of antioxidants contained in fresh plant food matrices determine the path of reaction cascades initiated by ROS and RNS, these substances are of particular importance with regard to the occurring molecular radical interactions between plasma-immanent species and molecules themselves as well as consequent reactions of the subsequently formed intermediates. Phenolic compounds are secondary metabolites formed via the shikimate pathway in higher plants and microorganisms as well as via pentose phosphate pathway by phenylpropanoid metabolism (Randhir et al., 2004). They contain benzene rings substituted with one or more hydroxyl substituents and vary from simple phenol molecules to highly polymerized compounds (Velderrain-Rodríguez et al., 2014). Plant polyphenols have gained considerable interest as dietary antioxidants based on active reports of their alleged contribution to the containment of a variety of human diseases (Fiorentino et al., 2008; Hoper & Cassidy, 2006; Pu et al., 2013). Fruits, vegetables and beverages are the main sources of phenolic compounds in human nutrition.

Excitation of nitrite or nitrate ions in PPA may induce oxidation, nitration, nitrosation or di-/polymerization of phenolic derivatives at the liquid-gas interface. These reactions result from the direct excitation of aromatic compounds in the presence of nitrate ions (Suzuki et al., 1982), the plasma-immanent species or the excitation of nitrate ions formed in the liquid (Jones et al., 2000; Mulvaney et al., 1998). Due to the fact, that specific antioxidants are commonly found in fruits, vegetables, grains and sugar beets frequently used in beverage applications, it is very likely that the formation of phenolic plasma reaction products occur during plasma treatment of liquid media as juices or smoothies or in cells injured by the cutting of fresh cut products.

In this context, the purpose of this study was to investigate the effects of PPA on different food related phenolic compounds in aqueous model solutions. Chlorogenic and caffeic acid were selected on basis of previous work on fresh sliced apples and potatoes (Bußler et al., 2017). The fact that in case of PPA treatment of apple but not for potato a browning reaction was observed was the main reason for taking into account chlorogenic acid as the main phenolic compound in these products. In the same context, caffeic acid was selected, since it represents the phenolic part of the chlorogenic acid molecule. Further, pyrocatechol was included, as it has been used as standard compound in many studies representing a simple phenolic moiety containing two hydroxyl groups in ortho-position allowing the formation of o-quinone by oxidation, which in turn leads to formation of browning pigments. The study further aims to specify the reaction products and investigates antioxidant capacity, pH value and UV–Vis spectra as exemplary markers providing information on the changes caused by the PPA treatment. Special emphasis was set on developing methods for separation and identification of reaction products based on reverse phase high performance liquid chromatography (RP-HPLC) and liquid chromatography/mass spectrometry (LC-MS) in order to gain first insights into the occurring reaction mechanisms of identified plasma-specific reaction products.

Section snippets

Sample preparation

Three model food compounds served as test material: pyrocatechol (benzene-1,2-diol, Carl Roth, Karlsruhe, Germany), chlorogenic acid ((1S,3R,4R,5R)-3-[(E)-3-(3,4 dihydroxyphenyl)prop-2-enoyl]oxy-1,4,5-trihy-droxycyclohexane-1-carboxylic acid, Sigma Aldrich, Steinheim, Germany) and caffeic acid ((E)-3-(3,4-dihydroxyphenyl)prop-2-enoic acid, Sigma Aldrich, Steinheim, Germany). The hydroxycinnimic acid derivates used were dissolved in distilled water and each diluted to a final concentration of

Accumulation of nitrate, nitrite and hydrogen peroxide in pure water

The results obtained in this study demonstrate the accumulation of nitrate and nitrite in pure water induced by treatment with PPA for up to 10 min (Fig. 1, left and center). The concentration of hydrogen peroxide was in the range of 4 to 6 mg L−1 in treated and control samples (Fig. 1, right). The nitrate concentration increased strongly from 10 to 1084 mg L−1 upon exposure to PPA for 1 min. Prolonging the treatment time to 10 min decreased it to a final concentration of about 880 mg L−1.

Conclusion

Results of this study provide evidence for the PPA-induced formation of oxidized, nitrated and polymerized phenolic compounds for all three focal model substances. The results clearly indicate that the plasma-induced reactions, as well as the formation and degradation of reactants are dependent on the process duration. However, further experiments and analyses are necessary to clarify their chemical composition. Underlying reaction mechanism could not be fully elucidated and need to be

Author agreement statement

We the undersigned declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere.

We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

We understand that the Corresponding Author is the sole contact

Declaration of competing interest

All authors concur with the submission, the work in its present form has not been submitted to a scientific journal before, and there is no commercial conflict of interest.

Acknowledgements

We gratefully acknowledge the technical assistance of Arved Jeltsch (University of Potsdam) and the scientific support of Jörg Ehlbeck in plasma application (Leibniz Institute for Plasma Science and Technology).

This work was partially supported by the competence cluster “NutriAct-Competence Cluster Nutrition Research Berlin-Potsdam” funded by the Federal Ministry of Education and Research (Grant No. 01EA1408A-G), by the project “Plasma-based decontamination of dried plant related products for

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      However, specific toxicological aspects have to be assessed firstly on pilot-scale to evaluate the industrial relevance. Phenolic compounds have shown reactions after plasma treatment into nitr(os)ated products, which have to be examined for a possible human endangerment (Bußler, Rawel, & Schlüter, 2020). Plasma induced browning effects on apple pieces could be possibly also attributed to these nitration or polymerization reactions (Bußler, Ehlbeck, & Schlüter, 2017).

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