Following the Food Safety Modernization Act of 2011 in the U.S., guidelines for disinfection washes in food packaging facilities are under consideration to control pathogen risks. However, disinfectant exposures may need optimization because the high concentrations of chlorine disinfectant promote the formation of high levels of disinfection byproducts (DBPs). When chlorine doses up through the 200 mg/L as Cl range relevant to the current practice were applied to spinach and lettuce, significant DBP formation was observed, even within 5 min at 7 °C. Concentrations of volatile chlorinated DBPs in washwater were far higher than typically observed in disinfected drinking water (e.g., 350 μg/L 1,1-dichloropropanone). However, these DBPs partitioned to the aqueous phase and so represent a greater concern for the disposal or reuse of washwater than for consumer exposure via food. The volatile DBPs represent the low-yield, final products of chlorination reactions with multiple biomolecular precursors. The initial, high-yield transformation products of such reactions may represent a greater concern for consumer exposure because they remain bound within the biopolymers in food and would be liberated during digestion. Using protein-bound tyrosine as an example precursor, the concentrations of the initial 3-chlorotyrosine and 3,5-dichlorotyrosine transformation products from this one precursor in the leaf phase were comparable to, and, in the case of some lettuces, exceeded, the aggregate aqueous concentration of volatile DBPs formed from multiple precursors. Chlorotyrosine formation increased when spinach was shredded due to the greater accessibility of chlorine to proteins in the leaf interiors. The cytotoxicity of chlorotyrosines to Chinese hamster ovary cells was higher than any of the trihalomethanes regulated in drinking water.

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The covalent modifications resulting from chlorine reactions with peptide-bound amino acids contribute to pathogen inactivation and disinfection byproduct (DBP) formation. Previous research suggested that histidine is the third most reactive of the seven chlorine-reactive amino acids, leading to the formation of 2chlorohistidine, 2-oxohistidine, or low-molecular-weight byproducts such as trihalomethanes. This study demonstrates that histidine is less reactive toward formation of chlorine transformation products (transformation time scale of hours to days) than five of the seven chlorine-reactive amino acids, including tyrosine (transformation time scale of minutes). Chlorine targeted tyrosine in preference to histidine within peptides, indicating that chlorine reactions with tyrosine and other more reactive amino acids could contribute more to the structural modifications to proteins over the short time scales relevant to pathogen inactivation. Over the longer time scales relevant to disinfection byproduct formation in treatment plants or distribution systems, this study identified β-cyanoalanine as the dominant transformation product of chlorine reactions with peptide-bound histidine, with molar yields of ∼50% after 1 day. While a chlorinated histidine intermediate was observed at lower yields (maximum ∼5%), the cumulative concentration of the conventional low-molecular-weight DBPs (e.g., trihalomethanes) was ≤7%. These findings support the need to identify the high-yield initial transformation products of chlorine reactions with important precursor structures to facilitate the identification of unknown DBPs.

High chlorine doses (50–200 mg/L) are used in postharvest washing facilities to control foodborne pathogen outbreaks. However, chlorine can react with biopolymers (e.g., lipids) within the produce to form chlorinated byproducts that remain in the food. During chlorination of micelles of oleic acid, an 18-carbon alkene fatty acid, chlorine added rapidly across the double bond to form the two 9,10-chlorohydrin isomers at a 100% yield. The molar conversion of lipid-bound oleic acid to 9,10-chlorohydrins in chlorine-treated glyceryl trioleate and produce was much lower, reflecting the restricted access of chlorine to lipids. Yields from spinach treated with 100 mg/L chlorine at 7.5 °C for 2 min increased from 0.05% (0.9 nmol/g-spinach) for whole leaf spinach to 0.11% (2 nmol/g) when shredding increased chlorine access. Increasing temperature (21 °C) and chlorine contact time (15 min) increased yields from shredded spinach to 0.83% (22 nmol/g) at 100 mg/L chlorine and to 1.8% (53 nmol/g) for 200 mg/L chlorine. Oleic acid 9,10-chlorohydrin concentrations were 2.4–2.7 nmol/g for chlorine-treated (100 mg/L chlorine at 7.5 °C for 2 min) broccoli, carrots, and butterhead lettuce, but 0.5–1 nmol/g for cabbage, kale, and red leaf lettuce. Protein-bound chlorotyrosine formation was higher in the same vegetables (5–32 nmol/g). The Chinese hamster ovary cell chronic cytotoxicity LC50 value for oleic acid 9,10-chlorohydrins was 0.106 mM. The cytotoxicity associated with the chlorohydrins and chlorotyrosines in low masses (9–52 g) of chlorine-washed vegetables would be comparable to that associated with trihalomethanes and haloacetic acids at levels of regulatory concern in drinking water.

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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.