Inactivation by chlorine dioxide gas (ClO2) of Listeria monocytogenes spotted onto different apple surfaces Original Research Article

The bactericidal effects of chlorine dioxide (ClO2) gas treatments on a mixture of three strains of Listeria monocytogenes (Scott A, F5069 and LCDC 81-886) spotted on to the calyx, stem cavity and pulp surface of apples were investigated at 21°C and 90% relative humidity (RH). ClO2 gas was more effective for inactivating the bacteria attached to pulp skin than those attached to the calyx or stem cavity at a ClO2 level of 4·0 mg l−1 and a treatment time of 10 or more minutes. After treatments of 1·0–4·0 mg l−1 ClO2 for 10 min, a 2–3 log reduction of L. monocytogenes were observed on both cavities. There were 4·3±0·2 and 4·3±1·1 log reductions of L. monocytogenes achieved on the calyx and stem cavities, respectively, by a 4·0 mg l−1 ClO2 gas treatment for 30 min. After treatment of 8·0 mg l−1 ClO2 for 30 min, no survivors were detected using an end point method for 3·6–5·3 log cfu spotted site−1 on the calyx cavity and 3·5–5·0 log cfu spotted site−1 on the stem cavity. No significant differences (P>0·05) were found between bacterial log reductions on pulp skin at 1·0 or 3·0 mg l−1 ClO2 gas treatment for 10 min. When the ClO2 level was increased to 4·0 mg l−1 for 10 min, a 5·5±1·0 log cfu spotted site−1 bacteria on pulp skin was inactivated. After 4.0 mg l−1 ClO2 gas for 30 min, L. monocytogenes levels on the pulp skin could be decreased by 3·9±0·0 to 6.5±0.1 log cfu spotted site−1 using an end point determination method. ClO2 gas was a potentially powerful sanitizer for inactivating L. monocytogenes on each apple surface, especially those attached to the pulp skin.