Ultraviolet Reactors for Nonsolid Food Pasteurization: Comparison
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UV-C is an increasingly popular option for the pasteurization of non-solid plant-based foods because it is germicidal, economical, and environmentally-friendly. It is a proven alternative to conventional thermal processes for inactivating microorganisms.UV-C irradiation in nonsolid foods can be performed in equipment that either uses batch or continuous operation modes.

  • fruits
  • vegetables
  • plant-based
  • minimally processed
  • UV-C treatment
  • bioactive
  • nutrients
  • nutrition security

1. Introduction

Batch UV-C processing consist of placing the product in a glass container within a UV-C irradiation chamber. The product is then irradiated for a predetermined amount of time at a given UV-C dose. Continuous operation mode involves pumping the product into a high-permittivity UV light tube, coiled tube, or jacketed reactor that contains lamps that emit UV-C light. In continuous systems, UV-C exposure is performed for minutes to hours during which the product flows around the lamps with or without recirculation [1][2][3]. Continuous UV-C is preferable for industrial applications because it could present advantages over batch processing such as increased productivity [3].
A variety of UV light sources have been used in UV-disinfection systems including pulsed-light (PL), excimer lamps, low-pressure mercury (LPM), medium-pressure mercury (MPM), low-pressure high output mercury lamp-amalgam type, mercury-free amalgam lamps, and so on [4]. LPM lamps currently serve as radiation sources in the majority of UV-based disinfection systems for the treatment of nonsolid foods and beverages [5]. More recently, ultraviolet light-emitting diodes (UV-LEDs) have been employed in treating juices and beverages in continuous reactors [6]. There are various types of UV-C reactors with various flow patterns. The flow pattern in the reactor has a major impact on the UV-C dose required for the inactivation of undesirable microorganisms. Hence, to maximize the homogeneity of UV-C treatment, it is necessary to enhance the flow conditions [7]. In general, four flow types can be identified as follows: Taylor–Couette flow, Dean–Vortex flow, laminar, and turbulent flow. Various reactor designs or systems are often utilized to achieve the aforementioned flow characteristics [8].

2. Laminar and Turbulent Flow Reactors

Laminar flow is a flow type where the fluid travels smoothly or through regular paths, as opposed to turbulent flow where the fluid experiences unstable fluctuations and mixing [9]. Figure 1 is a schematic illustration of laminar and turbulent flows.
Figure 1. Flow through a UV-C reactor with one lamp inside a pipe. (A) 3D view and (B) cross section view of the velocity profile for both the laminar and turbulent regimes.
The low radial mixing in laminar flow systems reduces their efficiency in facilitating a uniform UV dose distribution [10]. Turbulent flow reactors, on the other hand, use higher flow rates to increase turbulence within a UV reactor, thereby enabling close contact between the UV-C light and the product’s constituents during treatment and overcoming product turbidity, which interferes with UV penetration. The turbulent flow mechanism efficiently mixes the fluid, allowing for a more uniform UV-C dose distribution [11][12]. Different types of turbulent flow UV-C reactors have been developed. In the Aquionics UV-C turbulent reactor (Hanovia, Slough, UK), the fluid passes through a cylindrical compartment made of stainless steel in which 12 parallel 42 Watt UV-C low-pressure lamps are housed in a quartz sleeve (Figure 2) [1].
Figure 2. Diagram of a turbulent UV-C reactor.
Laminar and turbulent flow regimes were previously used in thin film reactors. The intended effect of thin film reactors is to shorten the path of UV radiation to maximize the UV-C radiation delivery to the food or beverage, thus providing a solution to the inadequate penetration of UV photons [13]. Using laminar and turbulent flows in continuous thin film reactors, Koutchma et al. (2004) investigated the effectiveness of UV radiation to inactivate E. coli K-12 in apple juice. They observed that the inactivation of E. coli in apple juice increased under turbulent flow conditions due to enhanced mixing [14]. In another study, turbulent flow conditions in an ultra-thin film annular reactor produced a better UV dose distribution and higher microbial inactivation rate compared to a laminar flow regime. The microbial inactivation rates were found to increase as the flow rate increased due to greater turbulence intensity [15]. A diagram of a thin film annular reactor is provided in Figure 3. The annular structure is utilized because it is effective for deactivating microorganisms. It is made of a single lamp that is positioned at the center of the reactor. The product flows in the annular gap. The flow in this gap might be turbulent or laminar depending on the flow rates [16].
Figure 3. Diagram of a thin film annular UV-C reactor. Flow type can be laminar or turbulent.

3. Taylor–Couette Reactors

The flow between two coaxial cylinders with an inner rotating cylinder is referred to as Taylor–Couette flow [17][18]. As seen in Figure 4, the Taylor–Couette ultraviolet unit is made up of two concentric cylinders: an outer stator (outer stationary cylinder) and an inner rotor (inner rotating cylinder). The fluid product is pumped through the annular space between the cylinders and subjected to UV-C irradiation that originates from lamps positioned around the outer cylinder. The rotation of the inner cylinder creates a Taylor–Couette flow [19]. The vortices produced in the Taylor–Couette UV reactors have the potential to deliver effective radial and axial mixing. Furthermore, the thickness of the fluid boundary layer between the fluid and the UV lamps is minimized, resulting in optimal UV exposure times for the undesired microorganisms and uniform radiation intensities [20]. Several flow regimes can be obtained under different flow and rotation rates in a Taylor–Couette system [17]. Ye et al. (2008) demonstrated that higher log reduction levels of Escherichia coli K12 (ATCC 25253) and Yersinia pseudotuberculosis could be achieved with laminar Taylor–Couette flow as opposed to turbulent or laminar Poiseuille flow. The authors concluded that the Taylor–Couette UV-C reactors are appropriate for the preservation of a variety of juices, particularly those with high absorption coefficients [21]. Similarly, a study by Orlowska et al. (2014) highlighted that the Taylor–Couette UV unit offered effective mixing that could overcome the limited UV-C penetration depth in opaque beverages like carrot juice [22].
Figure 4. Taylor–Couette UV-C reactor.

4. Dean–Vortex-Based Reactors

A Dean flow system (Figure 5) is characterized by secondary flow vortices (also known as Dean vortices) with the primary forward flow caused by the coiled flow channel in coiled tube reactors. The Dean vortices form as a result of centrifugal forces acting on the fluid volume during rotation. This generates effective radial mixing as well as a greater homogeneity of velocity and residence time distribution (RTD) of the liquid products due to higher Reynold numbers and turbulent conditions, resulting in more uniform treatment conditions [23][24]. The fluid product in these reactors passes through a highly UV-transparent fluorinated ethylene propylene (FEP) tube that is coiled in a helix pattern around one or more UV-C lamps [1]. A Dean flow system consisting of a module made up of a polytetrafluoroethylene (PTFE) envelope with a helically coiled tube tightly fitted to a quartz glass cylinder that houses the UV-C light source was previously used to investigate the formation of toxic compounds in UV-C treated cloudy apple juices. No quantifiable alterations were found in the cytotoxic and genotoxic effects of UV-C treated apple juices [25]. The UVivatec Dean–Vortex reactor was used to treat Lactobacillus plantarum BFE 5092 in orange juice. The authors found that increasing the Reynolds number from 86 to 696 led to an increase in the inactivation rate by roughly 2.5 log10 cfu/mL [24]. Barut Gök (2021) exposed apple and grape juice to low doses of UV-C in a Dean–Vortex-based reactor. This study demonstrated the potential of this technology to eliminate relevant microorganisms in opaque fruit juice such as Lactobacillus plantarum NRIC1749 and Saccharomyces cerevisiae NCIB4932 [26]. Orange juice was treated using a modified UV-C reactor based on Dean–Vortex flow. The results indicated that UV-C treatment would be a helpful way to remove or reduce the content of 5-(hydroxymethyl)furfural in orange juice. Additionally, no furan formation was found, and there was no significant alteration in the appearance and color of the juice following UV-C treatment [27]. Cranberry-flavored water was previously treated in a continuous UV-C reactor under a laminar flow regime combined with Dean vortices to ensure suitable mixing, and the treatment enabled a 5-log reduction (99.999%) of Escherichia coli ATCC 700728 and Salmonella enterica ser. Muenchen ATCC BAA 1764 with a UV-C fluence of 12 mJ·cm−2 and 16 mJ·cm−2, respectively. In addition, there was no formation of cytotoxic substances up to a UV-C dose of 120 mJ·cm−2 [28].

Figure 5. Dean–Vortex UV-C reactor.

References

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  3. Ochoa-Velasco, C.E.; Beristain-Bauza, S.C.; Hernández-Carranza, P.; Ruiz-López, I.I. A Reactor Engineering Approach to Describe Bacterial Inactivation during Continuous UV-C Light Processing. Innov. Food Sci. Emerg. Technol. 2021, 74, 102853.
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