Effect of Turbidity on Chlorine Disinfection of E. coli O157:H7 and Salmonella in Leafy Green Wash Water

Introduction: Prevention of microbial cross-contamination in produce wash water is critical in the prevention of foodborne disease outbreaks. The most commonly used sanitizer for produce washing is free chlorine, which is known to be affected by the pH and temperature of the wash water. However, the organic load in wash water increases with the cumulative amount of produce processed through a flume, and this increased organic loading may affect the efficacy of free chlorine in wash water.

Purpose: The purpose of this study was to evaluate the effect of turbidity (as an indicator of organic load) on free chlorine inactivation of Salmonella and E. coliO157:H7 strains in leafy green wash water.

Methods: Chlorine disinfection experiments were carried out for Salmonella and E. coli O157:H7 using leafy green wash water at varying turbidity levels. Wash water from three commercial produce production facilities was collected at maximum turbidity after washing cut iceberg lettuce and diluted to create a range of turbidity levels for testing. Experiments were conducted at 5°C and pH 6.5-7, depending on the water source. Free chlorine concentrations were monitored throughout the course of each experiment to account for depletion. Salmonella and E. coli O157:H7 concentrations were enumerated by the most probable number (MPN) technique. Inactivation data for each microbe and water source was compared at the varying turbidity levels to determine the effect of increasing turbidity on chlorine disinfection efficacy.

Results: The inactivation data generated from disinfection experiments showed that as turbidity increased in the wash water, chorine disinfection efficacy decreased. Median inactivation rates for maximum turbidity water (40-80 NTU) were 3.5 and 6 times slower for Salmonella and E. coli O157:H7, respectively, compared to non-turbid wash water.

Significance: The results of this study can help guide produce production facilities to develop wash water management approaches and water quality monitoring strategies based on turbidity.