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

Abstract

The efficacy of chlorine dioxide (ClO 2) 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 ClO 2 gas at 21°C 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 ClO 2 concentration and treatment time. At least 3-log reductions were achieved after the following ClO 2 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 ClO 2 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 ClO 2 gas treatment was a promising non-thermal pathogen reduction technique for apples.