Antifungal effect of riboflavin combined with 405 nm light-emitting diode illumination on Rhizopus stolonifer and application for tomato preservation

Although the antifungal effect of 405 nm light-emitting diode (LED) illumination has been documented, its limited efficacy necessitates additional strategies to enhance its performance. We aimed to improve the antifungal effect of 405 nm LED illumination by incorporating riboflavin, an exogenous photosensitizer, and evaluate its efficacy against Rhizopus stolonifer both in vitro and in vivo using tomatoes as a model food. The impact of the combined riboflavin and LED treatment on the physicochemical quality of tomatoes was also assessed. When three strains of R. stolonifer (KACC 44501, 45159, and 45160) were treated with 100 μM riboflavin in phosphate-buffered saline and exposed to LED illumination at a total dose of 2.62 kJ cm^-², only strain KACC 45160 was not reduced below the detection limit, indicating strain-dependent effects. Additionally, antifungal efficacy improved with increasing riboflavin concentrations, with the highest effect observed at 50–100 μM. Riboflavin treatment and LED illumination completely inhibited the mycelial growth of KACC 45160. LED illumination, with or without riboflavin, significantly reduced mitochondrial activity in spores, suggesting this as a potential mechanism of action. When applied to tomatoes inoculated with KACC 45160 spores, riboflavin treatment and LED illumination achieved an additional 0.8 log colony-forming units/tomato reduction compared to LED alone, without adversely affecting the physicochemical quality attributes of the tomatoes, such as color, firmness, and antioxidant capacity. These findings highlight the potential of riboflavin combined with 405 nm LED illumination as a preservation strategy to mitigate fungal spoilage in tomatoes during storage and transportation.