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

•Management of free chlorine (FC) levels is necessary during fresh-cut washing.•Maintenance of minimum FC residual levels are needed to avoid cross-contamination.•In fresh-cut spinach, an FC residual level of 7 mg/l inhibited Escherichia coli O157:H7.•The experiments were performed under progressive accumulation of organic matter.•FC levels should be tested under dynamic tests, similar to the industrial situation.

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.