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

The bactericidal effects of chlorine dioxide (002) 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 21degreesC 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 mgl(-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. (C) 2002 Elsevier Science Ltd All rights reserved.