CONTINUOUS UV LIGHT IRRADIATION: GENERAL CONCEPT

UV light is a spectrum of light not visible to the human eye. UV rays can be classified based on the emission wavelength, as vacuum UV (100-200 mn), UV-A (315 and 400 nm), UV-B (280-315 nm), and UV-C (200 to 280 mn). The active part of the electromagnetic spectrum is located between 200 and 300 nm (UV-C) (Shama, 2007) (Figure 1.3). Several factors can influence the efficacy of UV treatment such as absorption coefficient, flow rate, fluence, intensity, sample depth, and turbidity. The fluence is the amounts of UV light emitted from the lamp which depend upon the intensity of light and exposure of the time. The intensity of light is an intrinsic characteristic of the lamp. Furthermore, the intensity of the light that is generated in the sample depends on the geometiy of the reactor and the strength of the lamp (distance between the UV source and sample).

MECHANISMS OF ACTION ON MICROBIAL INACTIVATION

Microbial inactivation by UV-light involves DNA mutation (Guerrero- Beltran and Barbosa-Canovas, 2004). The main result of the UV-C light is the photochemical damage to nucleic acids in the cellular structure that can cause cross-linking of the pyrimidine bases with the formation of a new configuration, the photoproducts are the cyclobutyl pyrimidine dimers. The cross-linking number is proportional to the UV light dose. The damaged cells depend on the dose of absorbed UV energy. The resistance of microorganisms to UV radiation varies considerably, in fact, the most sensitive to treatment are vegetative bacteria followed by yeasts, bacterial

Schematic representation of UV-light technology and the germicidal wavelengths of the light spectrum involved in the treatment

FIGURE 1.3 Schematic representation of UV-light technology and the germicidal wavelengths of the light spectrum involved in the treatment.

spores, fungi, and viruses. The technology is easy to use, cheap in terms of energy, and has low maintenance, installation, and operation costs. UV treatments do not perform to produce any kind of chemical substances and toxic materials. In addition, it works at ambient temperature. Despite the known germicidal efficiency of UV light, this technology is not suitable for products with high levels of soluble organic matter, suspended solids, or turbid (Guneser and Karagul-Yuceer, 2012). In solid foods, the irregular shapes and the presence of crevices on the food surface reduce the decontamination efficacy of UV-C light. Ultimately, the effectiveness of the treatment also depends on the shape of the food product due to the low penetration power.

APPLICATION OF CONTINUOUS UV-LIGHT IRRADIATION TO DAIRY SECTOR

Several researchers’ reports are available on the efficiency of UV light in microbial inactivation. Milk and dairy products pose a great opportunity for UV treatment. Mataket al. (2005) achieved very significant log reductions of pathogens in goats’ milk using a UV apparatus (158 ± 16 J/ m2 for 2 seconds) which allows the passage of fluid as a thin film on the UV exposed lamp, to completely penetrate the UV light into the fluid (Gomez-Lopez et ah, 2012). This condition is very important to increase the efficacy of UV penetration into the liquid. Reinemannet al. (2006) reported a 2-3 log reduction of fungi and bacteria in UV-treated milk. Another study on milk was realized by Choudhary et al. (2011) who examined the efficacy of a coiled tube UV reactor, for inactivation of Bacillus cereus and E. coli spore in raw whole milk and pasteurized skimmed milk (111.87 J/nffor 11.3 seconds). UV light treatment allowed obtaining more than 7 log reductions of E. coli in skimmed milk and 4 log reductions in raw whole milk. Gupta (2011) who observed great microbial reductions in brine, sweet, and acid whey, has reached a similar result. Bandlaet al. (2012) and Orlowskaet al. (2013) have also used UV light treatment on whole milk; the results showed the efficacy of the process without having changes in color, pH, soluble solid contents, and viscosity. In a recent study, UV-C light was also assessed on Fior Di Latte cheese (Lacivita et al., 2016). The authors evaluated the UV-C light technology to extend the shelf-life of this fresh cheese. A preliminary test with inoculated cheese (Pseudomonas spp.) was carried out and UV-C light penetration depth in the samples was also evaluated. Then, shelf- life test on treated and untreated Fior Di Latte cheese was carried out. A microbial reduction of about 1-2 log cycles (Pseudomonas spp. and Enterobacteriaceae) was found with a consequent shelf-life prolongation. Furthermore, in this study was observed that UV-C light penetrates into a very thin surface layer of the product; therefore, the technology does not provoke any changes in color, texture, and appearance.

 
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