T4-02 Fate of Listeria monocytogenes in Lettuce Wash Water during Chlorine Replenishment

Monday, August 4, 2014: 1:45 PM
Room 203-204 (Indiana Convention Center)
Bin Zhou, University of Maryland, College Park, MD
Yaguang Luo, U.S. Department of Agriculture-ARS, Beltsville, MD
Xiangwu Nou, U.S. Department of Agriculture-ARS, Beltsville, MD
Shuxia Lyu, Shenyang Agricultural University, Shenyang, China
Introduction: Listeria monocytogenes is a foodborne pathogen frequently associated with fresh produce. For commercial fresh-cut produce wash, effective management of the chlorination process for high organic loading rates requires determination of the lower limit of free chlorine concentration necessary to control pathogenic microorganisms on produce.

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.