Purified Chlorine Dioxide as an Alternative to Chlorine Disinfection to Minimize Chlorate Formation During Postharvest Produce Washing

Abstract

The washwater used to wash produce within postharvest washing facilities frequently contains high chlorine concentrations to prevent pathogen cross-contamination. To address concerns regarding the formation and uptake of chlorate (ClO 3 − ) into produce, this study evaluated whether switching to chlorine dioxide (ClO 2 ) could reduce chlorate concentrations within the produce. Because ClO 2 exhibits lower disinfectant demand than chlorine, substantially lower concentrations can be applied. However, ClO 3 − can form through several pathways, particularly by reactions between ClO 2 and the chlorine used to generate ClO 2 via reaction with chlorite (ClO 2 − ) or chlorine that forms when ClO 2 reacts with produce. This study demonstrates that purging ClO 2 from the chlorine and ClO 2 − mixture used for its generation through a trap containing ClO 2 − can scavenge chlorine, substantially reducing ClO 3 − concentrations in ClO 2 stock solutions. Addition of low concentrations of ammonia to the produce washwater further reduced ClO 3 − formation by binding the chlorine produced by ClO 2 reactions with produce as inactive chloramines without scavenging ClO 2 . While chlorate concentrations in lettuce, kale, and broccoli exceeded regulatory guidelines during treatment with chlorine, ClO 3 − concentrations were below regulatory guidelines for each of these vegetables when treated with ClO 2 together with these two purification measures. Switching to purified ClO 2 also reduced the concentrations of lipid-bound oleic acid chlorohydrins and protein-bound chlorotyrosines, which are exemplars of halogenated byproducts formed from disinfectant reactions with biomolecules within produce.