Minimum free chlorine residual level required for the inactivation of Escherichia coli O157:H7 and trihalomethane generation during dynamic washing of fresh-cut spinach

Even though chlorination of wash water in the fresh-cut industry is a common practice, little is known about the minimum free chlorine residual level required to keep water free from foodborne pathogens and its concomitant generation of disinfection by-products. This work aimed to provide insight to these issues. Two kinds of tests were performed to simulate a fresh-cut industry washing tank of spinach. In the first, a washing tank containing clean water was continuously filled with concentrated process wash water with high organic matter obtained from spinach and inoculated with an Escherichia coli O157:H7 cocktail (5 log CFU/ml). A peristaltic pump dosed a chlorinated solution to the washing tank during the test in order to adjust free chlorine (FC) concentration to 1 and 3 mg/l. In the second test, the washing tank already contained process wash water from the beginning of the experiments and the FC concentration was adjusted to 3 and 5 mg/l. Results showed that the maintenance of a FC concentration of ca. 5 mg/l (until a maximum of 7 mg/l) during washing of fresh-cut spinach kept the wash water free from the pathogen during one hour although trihalomethane levels > 1000 μg/l were generated. Results indicate that a minimum residual level of ca. 7 mg/l FC is an effective treatment to inactivate E. coli O157:H7 under industrial conditions. Validation of sanitizer efficacy by using a dynamic system might facilitate the implementation of selected treatments in the food industry.