Inactivation of Listeria monocytogenes on the Surface of Smoked Salmon by Riboflavin-based, 460nm Light Emitting Diode Illumination

Introduction: It is known that the antimicrobial effect of blue light emitting diodes (LED) could be enhanced by the addition of photosensitizers, such as riboflavin. However, little information is available on the effectiveness of blue LED with the addition of photosensitizers in eliminating foodborne pathogens on ready-to-eat fish products.

Purpose: The aim of this study was to compare the bactericidal efficacies of 460nm LED illumination, with various photosensitizers, against Listeria monocytogenes in phosphate buffered saline (PBS) and to evaluate the effectiveness of riboflavin-based LED illumination on the inactivation of L. monocytogenes on the surface of smoked salmon at chilling temperatures.

Methods: A bacterial suspension (10⁷ CFU/ml) in PBS containing each photosensitizer [5-aminolevulinic acid (ALA), chlorogenic acid (CA), and riboflavin (RF)] was illuminated with 460nm LED at 20°C for 30 min (0.2 kJ/cm²) to find the most effective photosensitizer. For food application, the smoked salmon inoculated with L. monocytogenes cocktail culture (10⁴ CFU/cm²) was treated with RF and then illuminated by LED for 48 h (12.9 kJ/cm²) at 4 and 10°C.

Results: A 30 min treatment with LED illumination, alone, reduced L. monoctyogenes in PBS 1.5 log CFU/ml, while illumination with ALA, CA, and RF inactivated L. monocytogenes by 2.8, 2.8, and 4.2 log CFU/ml, respectively; revealing that RF was the most effective photosensitizer. On the surface of smoked salmon, LED illumination (48 h at 4^oC) with and without RF decreased 0.7 and 0.4 log CFU/cm² of L. monocytogenes populations, respectively. At 10°C, bacterial populations on non-illuminated and illuminated samples increased up to 5.7 and 4.4 log CFU/cm², respectively; no growth was observed in LED-illuminated salmon with RF for 48 h.

Significance:   These results suggest that riboflavin-based 460nm LEDs could be a potential technology to preserve smoked salmon at chilling temperatures, minimizing the risk of listeriosis.