Purpose: In this study, we investigated the survival of Listeria monocytogenes affected by the dynamic changes in water quality during the chlorine replenishment of fresh-cut lettuce wash water.
Methods: Sodium hypochlorite was added incrementally into simulated lettuce wash water containing pre-set lettuce latex concentrations. Sanitization efficacy of wash water was evaluated by measuring inactivation of a three-strain cocktail of Listeria monocytogenes. Changes in water quality, including pH, free chlorine, total chlorine, and oxidation reduction potential (ORP), were also closely monitored throughout the process.
Results: As NaClO was incrementally added to simulated wash water, chlorination proceeded in three stages with regard to total and free chlorine concentrations. The boundaries of these stages define the combined hump and chlorination breakpoint. When the cumulative NaClO input exceeded the combined chloramine hump level and approached the breakpoint, i.e., free chlorine was approximately 3 mg/l, the Listeria population was reduced to an undetectable level (<0.75MPN/ml).
Significance: This study established the correlation between chlorination parameters and survival of Listeria monocytogenes in simulated produce wash water. It highlighted the importance of establishing performance standards for produce wash water sanitation and developing improved strategies to maintain a stable chlorine concentration during commercial produce wash operations.