Effectiveness of Ultra-High Irradiance Blue-Light-Emitting Diodes to Control <i>Salmonella</i> Contamination Adhered to Dry Stainless Steel Surfaces

Controlling <i>Salmonella</i> contamination in dry food processing environments represents a significant challenge due to their tolerance to desiccation stress and enhanced thermal resistance. Blue light is emerging as a safer alternative to UV irradiation for surface decontamination. In the present study, the antimicrobial efficacy of ultra-high irradiance (UHI) blue light, generated by light-emitting diodes (LEDs) at wavelengths of 405 nm (841.6 mW/cm²) and 460 nm (614.9 mW/cm²), was evaluated against a five-serovar cocktail of <i>Salmonella enterica</i> dry cells on clean and soiled stainless steel (SS) surfaces. Inoculated coupons were subjected to blue light irradiation treatments at equivalent energy doses ranging from 221 to 1106 J/cm². Wheat flour was used as a model food soil system. To determine the bactericidal mechanisms of blue light, the intracellular concentration of reactive oxygen species (ROS) in <i>Salmonella</i> cells and the temperature changes on SS surfaces were also measured. The treatment energy dose had a significant effect on <i>Salmonella</i> inactivation levels. On clean SS surfaces, the reduction in <i>Salmonella</i> counts ranged from 0.8 to 7.4 log CFU/cm², while, on soiled coupons, the inactivation levels varied from 1.2 to 4.2 log CFU/cm². Blue LED treatments triggered a significant generation of ROS within <i>Salmonella</i> cells, as well as a substantial temperature increase in SS surfaces. However, in the presence of organic matter, the oxidative stress in <i>Salmonella</i> cells declined significantly, and treatments with higher energy doses (>700 J/cm²) were required to uphold the antimicrobial effectiveness observed on clean SS. The mechanism of the bactericidal effect of UHI blue LED treatments is likely to be a combination of photothermal and photochemical effects. These results indicate that LEDs emitting UHI blue light could represent a novel cost- and time-effective alternative for controlling microbial contamination in dry food processing environments.