Photodynamic Inactivation of Food Borne Microbes on Tomato Fruit Surfaces Using Blue Visible Light

Abstract

Photodynamic inactivation (PDI) employs light energy to activate photosensitizer molecules on food surfaces, generating reactive oxygen species that kill microbes without inducing resistance. For experimental purposes, the photodynamic inactivation (PDI) effect on tomato samples under different conditions like room temperature, 10℃, and under blue visible light were studied. Curcumin solution, acting as a photosensitizer (PS), was prepared in two concentrations (200 μl/L PS and 400 μl/L PS) and sprayed once daily on the samples. The tomatoes were then subjected to blue visible light for an hour each day. Following the light exposure, the samples were kept at room temperature or 10℃ for the remainder of the day. In the second phase of experiment, the tomato samples were treated with PS on the first day only and were exposed to blue visible light for one hour each day. After four days of observation, tomato samples placed at room temperature showed very mild growth of microorganism. In the blank sample, very mild microbial growth was visible. In sample 1 and 2, colour degradation was visible but it is negligible. Similarly, sample 1 had some black spots on the surface which indicates the growth of microbes but not so effectively. However, there was no growth of microbes on sample 2. Tomato samples kept under low temperature appeared fresh with no microbial growth or color change after four days of observation, similar to samples exposed to blue visible light. After seven days, while the blank sample kept at room temperature decomposed due to mold growth, sample 1 and sample 2 showed no significant difference compared to their fourth-day condition. The same trend was observed in samples kept under low temperature and blue visible light. After 15 days, tomato samples exposed to blue visible light showed significant decomposition. However, sample 1 (200 μl/L PS) and sample 2 (400 μl/L PS) exhibited less degradation compared to the blank sample. While the observable PDI effect on microbial inactivation with tomatoes under blue visible light is promising, further experiments are necessary to fully understand the complete effect of PS on vegetables. These experiments should include the assessment of physicochemical properties, as well as the study of bacterial or fungal colonies and their log reduction on the tomato surface following PDI treatment.