UV-C Treatment for Food Surface Decontamination: Impact of Colony Size on Listeria monocytogenes Inactivation

UV-C light is a promising non-thermal technology for microbial inactivation on food surfaces; however, its efficacy may be compromised by the spatial structure of microbial colonies. The present work investigated the influence of Listeria monocytogenes colony size on UV-C treatment effectiveness using agar-based model systems. Petri dishes were inoculated at defined concentrations and incubated to generate colonies of varying sizes, which were subsequently exposed to a UV-C dose of 0.12 J/cm2. Colony growth was monitored over 48 h using an image-based analysis workflow implemented in MATLAB, combined with individual colony tracking. A neural network model was developed to predict the probability of growth cessation based on colony diameter, and quantitative PCR combined with bead-beating was used to estimate cell counts per colony. UV-C treatment applied immediately after inoculation achieved high inactivation efficacy, consistent with minimal cell aggregation. As colony size increased, treatment effectiveness declined markedly. Bootstrap analysis of the neural-network predictions identified a minimum mean growth cessation probability at a colony diameter of approximately 0.862 mm. At this diameter, the predicted probability was 56.8%, with a pointwise 95% bootstrap interval of 50.3–62.8%, corresponding to approximately 106.14 (viable + non-viable) cells per colony. These findings demonstrate that colony spatial structure substantially limits UV-C efficacy and underscore the importance of early-stage intervention in food surface UV-C decontamination protocols.