Purpose: In this project, we investigated dynamic changes in water quality and sanitation efficacy during chlorine replenishment of fresh-cut lettuce wash water.
Methods: Sodium hypochlorite was incrementally added into simulated lettuce wash water containing preset lettuce latex concentrations. Changes in water quality, including pH, free chlorine, total chlorine, and oxidation reduction potential (ORP), were closely monitored throughout the process. Sanitization efficacy of wash water was evaluated by measuring inactivation of a three-strain cocktail of Escherichia coli O157:H7.
Results: A 3-stage sequence in measured total and free chlorine concentrations was observed in response to incremental addition of NaClO to simulated wash water. A typical chlorine-breakpoint curve was composed of 1) an initial peak, 2) a valley, and 3) steadily increasing concentration of measured free chlorine. The boundaries of these three phases define the combined hump and chlorination breakpoint, respectively. Free chlorine concentration and cumulative NaClO input were correlated only during phase 3. The positions of the free chlorine peaks and valleys shifted as a function of the organic content in the wash water and the cumulative NaClO input. When cumulative NaClO input exceeded the breakpoint level, i.e., free chlorine was approximately 5 mg/l. E. coli O157:H7 population was reduced to undetectable level (<0.75MPN/ml).
Significance: This study provides critical information in establishing performance standards of produce wash water sanitation and developing improved chlorine dosing systems to maintain stable chlorine concentration during commercial product wash operations.