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Formation of trichloromethane in chlorinated water and fresh-cut produce and as a result of reaction with citric acid

Xuetong Fan, Kimberly J. Sokorai
Postharvest biology and technology 2015 v.109 pp. 65-72
chemical oxygen demand, chlorine dioxide, citric acid, fresh-cut foods, lettuce, onions, organochlorine compounds, pH, sodium hypochlorite, washing
Chlorine (sodium hypochlorite) is commonly used by the fresh produce industry in the U.S. to sanitize wash water, fresh and fresh-cut fruits and vegetables. However, possible formation of harmful chlorine by-products is a concern. The objectives of this study were to compare chlorine and chlorine dioxide in trichloromethane formation, determine trichloromethane levels in chlorinated wash water, cut-lettuce and diced onions, and evaluate the reaction of chlorine with citric acid, a chemical often used to adjust pH of chlorine solution. Results showed that little trichloromethane (≤3μgL−1) was produced from chlorine dioxide solution even at concentrations up to 200mgL−1 compared with the trichloromethane level (∼40μgL−1) in solutions of chlorine mixed with lettuce extract. The formation of trichloromethane in 1L of 100mgL−1 chlorine wash water increased from 155 to 284μgL−1 after repeated use of the wash water to wash six batches of 100g cut lettuce. Levels of trichloromethane in the washed cut lettuce were in the range of 14–22μgkg−1, and were reduced to less than 8μgkg−1 after being rinsed with water. Chlorine solution used to wash diced onions produced much less trichloromethane (32μgL−1) compared with that for washing cut lettuce despite higher chemical oxygen demand and turbidity in the wash water. Citric acid reacted with chlorine and produced trichloromethane. Over 1000μgL−1 trichloromethane was produced after 30min reaction with chlorine at 22°C, while less than 35μgL−1 trichloromethane was produced when Na-phosphate was used to adjust pH of chlorine. The amount of trichloromethane increased with reaction time and concentration of citrate. Our results demonstrated that formation of trichloromethane in wash water depended on type of cut-vegetables, and citric acid contributed to a significant amount of trichloromethane formation.