Gaseous ozone treatment inactivates Listeria innocua in vitro

Aims: To investigate the effects of ozone on inactivation of Listeria innocua on solid media. Methods and Results: Suspensions of L. innocua ranging from 4.5 x 10(4) - 6.4 x 10(4) CFU ml(-1) were inoculated onto potato dextrose agar (PDA, pH 5.6 and 6.8) and nutrient agar (NA, pH 6.0 and 6.8), then exposed to gaseous ozone. Variable factors included postinoculation standing time at 20 degrees C before exposure to ozone, ozone concentration, treatment duration and treatment temperature (5 or 20 degrees C). The interaction among ozone concentration, treatment duration, media and temperature in effecting changes in colony-forming units (CFU) was significant. The 100 nl l(-1) ozone treatment for 2 h reduced the microbial populations by 2-3 log CFU ml(-1). Cell viability decreased more rapidly on PDA than on NA. The average time to obtain a 2 log CFU ml(-1) reduction was 1.3 h at 20 degrees C and 2.5 h at 5 degrees C (P < 0.001). Conclusions: Gaseous ozone effectively inactivates L. innocua at concentrations of 50 and 100 nl l(-1) during short exposure times at both 5 and 20 degrees C. The Gompretz model can be utilized for determining the response of L. innocua to ozone over time. Significance and Impact of the Study: This study provides useful information on ozone inactivating Listeria spp., which may be imposed on ensuring quality and safety of horticultural produce and food products.