Influence of Surface Topography on the Effectiveness of Pulsed Light Treatment for the Inactivation of Listeria innocua on Stainless-steel Surfaces

Pulsed light (PL) treatment has been proven effective for killing a wide variety of microorganisms on foods and food contact materials. However, there is concern regarding how shading may impact the effectiveness of PL when applied to imperfect surfaces. The main objective of this work was to examine how surface properties, particularly topography, influence the microbicidal effect of PL. Four types of stainless-steel surfaces were inoculated with Listeria innocua and treated with up to 12 pulses of light. The highest level of inactivation achieved was about a 4-log reduction. Initially, an increase in inactivation with increasing treatment intensity was observed, but the inactivation curves tailed off above 3 light pulses. The differences in inactivation levels among the 4 finishes at specific treatment levels were rather insignificant, but some interesting trends were observed. At low treatment levels, inactivation on the smoothest finish was slightly lower than for the other surfaces, due to clustering of the cells on the highly hydrophobic smooth surface and to its reflective nature. For the roughest surface, scanning electron microscopy (SEM) imaging confirmed the preferential location of the cells inside surface features, which also promoted a relatively uniform distribution of the cells across the surface. This counter balanced to some extent of the shading effects, and as a result inactivation on the roughest surface was comparable to inactivation on the smoother surfaces. These results demonstrate that PL can be effective on both smooth and rough surfaces, but also indicate a complex effect of various surface properties on inactivation.