**1. Introduction**

For many years *Salmonella* spp. has been one of the most important foodborne pathogens. The European Food Safety Agency (EFSA), in 2018, reported 91,857 confirmed cases of salmonellosis in the European Union (EU). The most prevalent serovars were: *S. enteritidis*, *S. typhimurium* and *S*. Infantis [1]. The significant source of human infections are eggs and egg products, which in 2018 accounted for 45.6% of salmonellosis foodborne outbreaks [1]. A multi-country outbreak of *S. enteritidis*, linked to eggs, has been ongoing in the EU for several years. From 1 February 2017 to 14 January 2020, 15 EU countries reported 656 confirmed cases and 202 probable cases [2].

**Citation:** Grudlewska-Buda, K.; Wiktorczyk-Kapischke, N.; Wałecka-Zacharska, E.; Kwieci ´nska- Piróg, J.; Gry ´n, G.; Skowron, K.J.; Korkus, J.; Gospodarek-Komkowka, E.; Bystro ´n, J.; Budzy ´nska, A.; et al. Effect of Radiant Catalytic Ionization and Ozonation on *Salmonella* spp. on Eggshells. *Foods* **2022**, *11*, 2452. https://doi.org/10.3390/ foods11162452

Academic Editors: Dapeng Peng and Yongzhong Qian

Received: 13 July 2022 Accepted: 12 August 2022 Published: 14 August 2022

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There are two possible routes of bacterial contamination of eggs shell: either vertically or horizontally. Horizontal transmission occurs during the laying of eggs and depending on the eggshell architecture and bacterial serotype. Vertical transmission can originate from the hen reproductive tract. *S. enteritidis* is usually transmitted vertically, while *S*. Infantis contaminates the egg via a horizontal route [3]. Minor flaws of the eggshell favor bacterial colonization and transmission [4]. Bacterial contamination of eggshells depends on various environmental factors such as the presence of food, water, feces, dust, litter, the type of birds' housing system, the laying rate and/or cuticle state [5]. The thickness of particular layers, pore distribution, ultrastructure and transparency affect the eggshell penetration [6]. Furthermore, workers, domestic animals, rodents, contaminated feed, litter and water [7] or food production environment (transfer belt, packaging materials) can be the source of eggs contamination [8].

Cleaning and disinfection are the most common methods used to remove microbiological contamination from the egg surface [9]. Cleaning with chemical agents, e.g., alkaline solutions or sodium hydroxide removes the cuticle layer resulting in a visually clean egg [10]. Cleaning significantly reduced the number of Enterobacterales bacteria [11]. Regularly applied disinfectants include agents based on chlorine, iodine, hydrogen peroxide, ozone and quaternary ammonium compounds [12].

Methods of egg disinfection must limit the growth of microorganisms outside the shell and limit the penetration of microorganisms into the inside of the egg [13]. The ozonation may use for the disinfection of hatcheries, eggs and poultry carcasses. Ozone has a good bactericidal effect and causes a quick inactivation of microorganisms. The effectiveness of ozonation increases with the extension of the exposure time of the eggs. It is related to the increase in ozone concentration over time [14]. The hydrophobic protein layer (cuticle) hinders bacterial penetration. This thin outermost layer desiccates immediately after eggs laying and protects eggs against bacterial invasion and water loss [15].

Researchers are still searching for innovative technologies allowing the eradication of pathogens from the egg surface. Such methods should eliminate microbiological contaminants ensuring egg freshness and consumer safety. New technologies include physical and chemical processes such as high hydrostatic pressure, ionizing radiation, ultrasounds, pulsed electric field, UV radiation and plasma, which inactivate microorganisms at ambient or sublethal temperatures [16]. Additionally, some preparations are applied directly to the eggshell, e.g., colloidal silver, substances of natural origin (propolis) or plant extracts such as thyme and cinnamon, allicin, oregano oil or red grapefruit juice [17]. An innovative solution, successfully applied in the air purification system, is radiant catalytic ionization (RCI) [18]. This technology uses the photocatalysis phenomenon in the presence of UV radiation and photocatalysts, such as TiO2, which form a hydrophilic coating of the matrix surface in the RCI module [19]. The RCI cell consists of matrices forming a honeycomb structure. The coating of the dies also includes clusters of other elements such as rhodium, silver and copper. On the opposite site a broad-spectrum UV light source of 100 and 367 nm is located [19,20]. Catalytic oxidation, stimulated by UV radiation, at the boundary of heterogeneous phases (gas-solid), leads to reactive oxygen forms generation (ROS): hydroxyl radicals (OH•), hydrogen peroxide (H2O2) and superoxide anion (O<sup>2</sup>−). The total number of generated ions is about 5.0 × 104 ions × cm−<sup>3</sup> of air [21]. ROS interact with DNA, lipids and proteins, contributing to the destruction of genetic material, lipid peroxidation and amino acid degradation [22–24].

To date, RCI has been used primarily in the air purification industry. The method successfully eliminated biofilm and planktonic cells from various surfaces, indicating its applicability for disinfecting food processing surfaces and healthcare equipment [21]. Studies by Ortega et al. [23] and Skowron et al. [25] have demonstrated the utility of using RCI against various pathogens, including *Salmonella* spp. from different surfaces. The discussed technology has also proven effective when removing biofilms from abiotic surfaces contaminated with food pulp [21] and from vegetable and fruit surfaces [22], suggesting its possible application in food disinfection.

This study aimed to assess and compare the efficacies of radiant catalytic ionization (RCI) and ozonation against *Salmonella* spp. on the eggshell, with different initial inocula, exposure time and processing temperature.
