Application of an Eco-sustainable technology: use of direct and photodynamic UV light for the microbial inactivation in food industries

This study investigates visible light, particularly blue light (BL), as a sustainable, non-thermal, and chemical-free approach for microbial inactivation in food settings. The research initially focuses on utilizing endogenous photosensitizers (PS) within bacterial cells, optimizing treatment parameters, and determining effective dose ranges. Experiments were conducted on substrates like agar, liquid media, and mature biofilms to mimic real-world food-related conditions. Tests conducted on agar and liquid substrates highlighted the greater susceptibility of Gram (+) bacteria compared to Gram (-) bacteria. However, variability was observed even within the same bacterial class, probably due to differences in the quantity, type, and intracellular localization of the PS, as well as to the bacteria's specific oxidative stress response mechanisms. Biofilms were cultivated over periods of 5 and 13 days on different surfaces, and their reduction, along with morphological changes post-irradiation, was validated using CLSM. The findings consistently demonstrated that lower BL wavelengths (405 nm) exhibit greater antimicrobial efficacy than higher wavelengths (450 nm). The research also compared 365 nm (UVA) and 405 nm (BL) wavelengths to examine their effects on Reactive Oxygen Species (ROS) production, known to mediate cellular damage and death during light irradiation. Lipid peroxidation and membrane permeabilization were assessed, revealing that additional ROS-mediated mechanisms contribute to microbial inactivation. In partnership with Electrolux Italia S.p.A., a moke-up system for 365 nm and 405 nm light irradiation was developed and optimized for potential use in consumer appliances. The system was evaluated for irradiance uniformity, light dose, microbial inactivation, and energy efficiency. Results showed consistent microbial inactivation across both wavelengths at room and refrigerated temperatures, with light dose emerging as the critical factor for effective treatment. The study further explored exogenous photosensitizers (PS) in solution, using Methylene Blue (MB) as a synthetic PS and curcumin as a natural alternative. Experiments with MB revealed that factors like liquid substrate characteristics and microbial gram type significantly impacted the required PS concentration. Curcumin showed high microbial reduction efficacy for both Gram (+) and Gram (-) bacteria, albeit with differing kinetic profiles. Monochromatic light enabled efficient microbial inactivation using low PS concentrations and minimal light doses, demonstrating its effectiveness. The study examined 4MeP porphyrin at 5% and 10% concentrations, supported in a vanillin-based material, Valpol. Results showed that Gram (+) bacteria required a lower light dose for effective inactivation at equivalent PS concentrations. Reusing the material improved microbial inactivation as pores formed, increasing PS exposure to light. Interestingly, the relationship between higher PS content and microbial inactivation was non-linear. This thesis explores light-based technologies for the food sector, emphasizing their practical applications and technological transfer for implementation. Key factors for effective use include light dose, microbial type, wavelength, and substrate characteristics. The study highlights the need to tailor conditions to specific applications for successfully deploying light-based treatments in food contexts.