Combination of Blue Light and Chemical Sanitizers for Inactivation of Listeria monocytogenes Dried Cells on Inert Surfaces

Environmental contamination of ready-to-eat (RTE) foods with Listeria monocytogenes is a major food safety concern. Treatment of surfaces with antimicrobial blue light (aBL) has recently emerged as a technology that can supplement current sanitation practices. Chemical sanitizers are used extensively by the food industry, but their combination with aBL at 405 nm has not been evaluated. This project was undertaken to determine the combined effect of sanitizers with aBL to inactivate L. monocytogenes dried cells on inert surfaces. Peracetic acid (PAA), benzalkonium chloride (BAC), and sodium hypochlorite (NaClO) at time/concentrations that delivered less than 2 Log CFU viability reductions were combined with limited aBL treatments applied simultaneously or sequential on cells dried on stainless steel (SS) and polytetrafluoroethylene (PTFE) coupons. When five-strain mixtures were dried on SS and treated with aBL alone (575 J/cm²), average viability reductions of less than 2 Log CFU/coupon were observed. Single treatments with NaClO (20 ppm, 60 min) and PAA (30 ppm, 30 min) caused less than 1.5 Log CFU/coupon inactivation on SS. During independent experiments that combined the same concentration/time of NaClO and PAA applied simultaneously with the above aBL dose, viability reductions of 5.4 and 4.7 Log CFU/coupon, respectively, were measured. The results of sequential treatments of dried cells with sanitizers and aBL were variable depending on the sanitizer, concentration, surface, and the sequence order. Measured reductions of sequential treatments varied from 1.5 Log CFU/coupon with BAC (40 ppm/30 min) to 5.5 Log CFU/coupon with NaClO (75 ppm/60 min) applied before aBL on PTFE. The comparison between the results obtained at low sanitizer concentrations simultaneously used with aBL to the sum of the single results (3.5 vs. 5.4 Log CFU/coupon or 3.5 vs. 4.7 Log CFU/coupon) resulted in statistically significant differences (p < 0.05). These findings suggested that there was a synergistic effect between sanitizers and aBL.