Antimicrobial coatings for controlling Listeria monocytogenes based on polylactide modified with titanium dioxide and illuminated with UV-A

The anti-listerial properties of biodegradable polylactide coatings modified with titanium dioxide have been studied. Free standing films were prepared by casting solutions prepared from titanium dioxide and previously extruded polylactide. It was demonstrated that polylactide alone could support 2.84 ± 0.10 log CFU reduction of Listeria monocytogenes when incubated at 23 °C for 2 h. However, the log reduction for Listeria could be increased to >4 log CFU with titanium dioxide:polylactide composites illuminated with UV-A. The inactivation kinetics of L. monocytogenes followed a diphasic die-off with an initial 30 min lag period then a progressive decline in bacterial levels over a further 90 min period. The anti-listeria effect of polylactide:titanium dioxide films was dependent on illumination with UV-A but independent on the concentration of TiO2 incorporated in the film within the range of 1-5% w/w. The mode of L. monocytogenes inactivation was via direct contact of the pathogen with the polylactide, in addition to the generation of oxygen radicals produced by excitation of the titanium dioxide. The composite film illuminated with UV-A was equally effective against Salmonella Typhimurium and Shiga toxin producing Escherichia coli. The coating was stable to 5 repeated sanitation cycles consisting of detergent and sodium hypochlorite rinses. The polylactide-titanium dioxide coating shows potential as an antimicrobial coating although further work is required to assess if the protective film can function under commercial conditions.