Cold plasma in food processing: Design, mechanisms, and applications

allergens (Ekezie et al., 2019b; Venkataratnam et al., 2019) in food products.However, CP treatment is still an emerging process regarding adverse effects in foods (e.g., lipid oxidation), safety evaluation, and regulatory approval. In the last years, several studies focused on improving CP treatment by designing new plasma equipment and testing different process variables in many situations (Andrasch et al., 2017; Feizollahi et al., 2020; Misra and Jo, 2017; Zhao et al., 2020; Ziuzina et al., 2016). The growing literature presents many reviews discussing the consequences of CP application to different food types (Ekezie et al., 2017a; Feizollahi et al.,2020; Gavahian and Khaneghah, 2020; Muhammad et al., 2018b; Pan-kaj et al., 2018). However, it should be noted that there is a lack of comprehensive evaluation about the parameters affecting CP generation and their impact on food processing, such as electrode material, system geometry, and shape. Therefore, this review presents a comprehensive analysis of the current state of the art concerning CP operating parameters and application in the food sector. The main mechanisms and factors influencing plasma efficiency are presented and discussed,including their relationship in the most enlightening studies of the CP effect in food products.

2. Cold plasma generation: mechanism and methods
2.1. Townsend theory and Paschen’s law Gas breakdown and electron avalanche refer to the fundamental mechanisms for transforming a gas from non-conductive into a conductive medium for electrons. The formation and multiplication of the so-called electron avalanches throughout the gas breakdown are criteria for discharging all kinds of plasma, as described by the Townsend theory (Xiao, 2016). According to Townsend’s theory, as sketched in Fig. 1a, (i) when the energy applied between two electrodes is sufficient, the molecule kinetic energy increases, and electrons are released from the cathode surface in opposition to the electrical field. Electrical current increases as the voltage increases, reaching saturation, and (ii) a current becomes constant. The electrons are accelerated towards the anode. Under these conditions, the collisions are elastic (without altering internal energy),and electron energy is little to ionize or excite other molecules. (iii) The ,electrons continue to collide until acquiring energy to ionize atoms, with the inelastic collisions, which are more efficient for transferring energy.If the collisions have enough energy, they can dissociate the molecules and atoms, transforming them into ions and electrons. The migration of electrons and ions forms the current. (iv) The electrons formed are accelerated in the electric field, colliding and ionizing other atoms and molecules, generating many positive ions, electrons, and the electron avalanche. Due to less mass and higher speed, electrons (105–106 m/s) move to the avalanche head, while positive ions (50–500 m/s) are the tail. The ions extract new electrons from the cathode surface, which will form subsequential avalanches. When a sufficiently intense ionization occurs, the gas disrupts completely and becomes conductive (Bruggeman et al., 2017; Conrads and Schmidt, 2000; Misra et al., 2016b; Xiao, 2016).A glow discharge (GD) can be generated at low pressure into the electrode gap after the breakdown, such as micro-discharges. However,a streamer discharge with a filamentary appearance can be generated at atmospheric pressure, as shown in Fig. 1b. This type of discharge occurs when the (v) anode captures the electrons, and it forms a volume of positive ions between the electrodes (space charge). The ions recombine with free electrons, and photons are emitted, causing the nearby gas photoionization, and generating more electrons. Thus, new avalanches are formed (secondary avalanches). (vi) The secondary avalanches join the main avalanche, as the electrons recombine with their positive ions. (vii) A consecutive and rapid process occurs, with photons release and new avalanches formation creating a highly conductive channel, known as streamer discharge (Bruggeman et al., 2017; Xiao, 2016). From Townsend’s theory, the avalanche condition derived Paschen’s law, which is traditionally used to predict gas breakdown (Garner et al., 2020). Paschen’s law defines that the voltage necessary to ignite a plasma between two electrodes for a specific gas depends on the product pressure (p) and electrode gap distance (d). This voltage leads to an equilibrium between the electrons’ generation that creates volumetric electron avalanches and secondary electron emission processes,     with electrons losses on the surfaces (Garner et al., 2020). For low values of the product pd, the breakdown voltage is high due to the few collisions that occur, and therefore more energy to generate plasma is necessary. For high pd values, the breakdown voltage is also heightened due to numerous collisions that cause the particles to lose energy quickly, being essential to increase the energy supplied (Nehra et al., 2008). The shape of the curve p vs. d for different gases is similar, presenting a minimum pd value in the range of 130–1300 Pa cm (Bruggeman et al., 2017).2.2.CP sources suitable to food  application .The plasma generation methods most applied for food processing are categorized into dielectric barrier discharge (DBD), plasma jet (PJ), corona discharge (CD), radiofrequency (RF), and microwave (MW) (Bermudez-Aguirre, 2020). Specificities for each of them are given and discussed in the following.

2.2.1.Dielectric barrier discharge (DBD)

The plasma production with DBD is growing in importance due to its low costs at the industrial scale. This technology is one of the most convenient forms of plasma generation that provides several applica- tions due to its configuration and flexibility for the electrode shape and the dielectric material used (Misra et al., 2019b; Ziuzina et al.,  2013).DBD plasma is generated by a high voltage applied between two metal electrodes (a powered electrode and a ground electrode). One or both electrodes are covered with a dielectric material, such as a polymer, glass, quartz, or ceramic, separated by a variable gap ranging from 0.1 mm to several centimeters (Fig. 2a) (Becker et al., 2005; Kogelschatz, 2003). The typical range of parameters for DBD operation is (i) gas pressures between 1 × 104 and 1 × 106 Pa, (ii) frequency band  varying between 10 and 50 MHz, (iii) alternating current (AC) or pulsed direct current (DC) with voltage amplitude oscillating between 1 and 100 kVrms (Feizollahi et al., 2020).An application that opens many possibilities for the DBD system is the food treatment in-package, with CP generation inside the sealed package. This procedure allows to extend the action time of the reactive species on microorganisms and prevents post-process contamination. An example is the DBD reactor developed by Ziuzina et al. (2016) for in- dustrial operation in food production. This prototype used ACP for continuous in-package decontamination of fresh cherry tomatoes, evaluating E. coli and L. innocua counts. The plasma system consisted of two parallel 1 m-long electrodes with an applied output voltage of 0–100 kV, an adjustable discharge gap of up to 4.5 cm, maximum consumed power of 900 W, and a discharge current of 2.2–5.0 A. The authors observed a reduction in 5 log and 3.5 log in E. coli and L. innocua counts, respectively, after 150 s of treatment. Another piece of equip- ment at a pilot scale was proposed by Zhao et al. (2020); their ACP-DBD prototype was used to inactivate S. aureus on the apricot surface. It consisted of a copper mesh as a high voltage electrode, a quartz tube as a dielectric barrier, and a grounded copper foil. A pulsed DC power supply drove this device. The applied voltage, frequency, and voltage pulse width were 17 kV, 1 kHz, and 3 μs, respectively. The authors observed a 1.57 log reduction of S. aureus in 15 s of treatment.