Efficacy of chlorine dioxide gas in reducing Escherichia coli O157:H7 on apple surfaces

The efficacy of chlorine dioxide (ClO2) gas in inactivating a mixture of Escherichia coli O157:H7 (C7927, EDL933, 204P) on different apple surfaces was investigated. The bacteria were spotted onto the calyx and stem cavities, and the skin at 3, 5, or 8 log cfu site(-1), air dried, then treated with ClO2 gas at 21degreesC and 90-95% relative humidity. Bacterial populations were determined using surface plating, membrane transfer plating, or most probable number method. Bacterial log reductions increased with both increase of ClO2 concentration and treatment time. At least 3-log reductions were achieved after the following ClO2 gas treatments: 12.0 mg l(-1) for 10 min or 3.3 mg l(-1) for 20 or 30 min on the calyx cavities, and 12.0 mg l(-1) for 10 min, 4.8 mg l(-1) for 20 min, or 3.3 mg l(-1) for 30 min on the stem cavities. A 7.2 mg l(-1) treatment for 30 min showed the highest log reduction on the calyx (6.5 +/- 0.7 log) and stem cavities (4.1 +/- 0.2 log), respectively. More than a 5-log reduction on the skin was achieved by ClO2 gas treatment with 7.2 mg l(-1) for 10 min or 3.3 mg l(-1) for 20 or 30 min. A treatment with 12.0 mg l(-1) for 10 min, 7.2 mg l(-1) for 20 min, or 4.8 mg l(-1) for 30 min completely inactivated the 8 log cfu site(-1) bacteria initially inoculated on the skin, which was determined by an end-point method. It was concluded that ClO2 gas treatment was a promising non-thermal pathogen reduction technique for apples. (C) 2003 Elsevier Ltd. All rights reserved.