Chlorine (sodium hypochlorite) solution at pH 6.5 is currently the most common sanitizer used in the fresh-cut produce industry in the U.S. because it is cheap and easy to use. Chlorine is used in the concentration range of 50–200 mg/L, even if recent studies demonstrated that lower concentrations of free chlorine were sufficient to inactivate bacterial pathogens in water and to prevent cross-contamination.
However, there is a concern about the chlorine use in the fresh produce industry and other industries due to its potential environmental and health risks associated with the formation of carcinogenic halogenated disinfection by-products (DBPs). Chlorine reacts with organic matter and forms trichloromethane. Chlorine solution pH is commonly adjusted by adding citric acid during washing.
The use of chlorine in fresh-cut produce washing is prohibited in European countries.
Chlorine dioxide (ClO2) produces fewer potentially carcinogenic halogenated DBPs and is less corrosive than chlorine and is allowed at a maximum concentration of 3 mg/L in the U.S.
Xuetong Fan and Kimberly Sokorai, USDA scientists, investigated the formation of trichloromethane in wash water, cut lettuce and diced onions, to compare sodium hypochlorite with chlorine dioxide in producing trichloromethane, and to evaluate the impact of citric acid on trichloromethane formation.
Results showed that little trichloromethane (≤3 µg/L) was produced from the chlorine dioxide solution even at concentrations up to 200 mg/L compared with the trichloromethane level (about 40 µg/L) in solutions of chlorine mixed with lettuce extract. The formation of trichloromethane in 1 L of 100 mg/L chlorine wash water increased from 155 to 284 µg/L after repeated use of the wash water to wash six batches of 100 g cut lettuce.
Levels of trichloromethane in the washed cut lettuce were in the range of 14–22 µg/kg, and were reduced to less than 8 µg/kg after being rinsed with water. Chlorine solution used to wash diced onions produced much less trichloromethane (32 µg/L) 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 1000 µg/L trichloromethane was produced after 30 min reaction with chlorine at 22°C, while less than 35 µg/L 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.
Scientists conclude: "Our results suggested that trichloromethane was produced in chlorine solution for washing cut lettuce, although the levels of trichloromethane in cut lettuce were relatively small. After being rinsed with water, trichloromethane in the lettuce were reduced to levels close to detection limit. Much lower levels of trichlormethane were produced in wash water for diced onions compared to that for cut lettuce. ClO2 produced little trichloromethane compared with chlorine. Our results also demonstrated that citric acid reacted with chlorine and contributed to the formation of high levels of trichloromethane in wash water. To lower the formation of trichloromethane in water, citric acid may be replaced with other pH adjustors such as phosphate."
