Title: The effects of free chlorine concentration, organic load, and exposure time on the inactivation of Salmonella, Escherichia coli O157:H7 and non-O157 STEC Authors
Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 20, 2012
Publication Date: March 1, 2013
Citation: Shen, C., Luo, Y., Nou, X., Wang, Q., Millner, P.D. 2013. The effects of free chlorine concentration, organic load, and exposure time on the inactivation of Salmonella, Escherichia coli O157:H7 and non-O157 STEC. Journal of Food Protection. 76(3):386-393. Interpretive Summary: The water used for fresh-cut produce washing during commercial operations often contain high levels of organic materials which reduce chlorine concentrations rapidly. In this study, we investigated the effects of organic load, initial and residual free chlorine concentration, and exposure time on the ability of chlorinated wash water to inactivate major food-borne human pathogens. The presence of organic materials from cut tomato and lettuce in wash water caused rapid depletion of free chlorine; this in turn led to significant increase in exposure time needed to inactivate pathogens. Furthermore, when free chlorine concentration fell below a critical limit, pathogens survived in the wash solution, despite the increase in exposure time. Results from this study provide critical information for the produce industry to develop practical solutions for improving food safety of fresh and fresh-cut produce.
Technical Abstract: This study evaluated the effects of free chlorine (FC) concentration, contact time, and organic load on the inactivation of Salmonella, E. coli O157:H7, and non-O157 STEC in suspension. Four strains each of Salmonella, E. coli O157:H7, or non-O157 STEC cells were inoculated separately or as a multi-strain cocktail into 12-well microplates containing a range of FC concentrations. Lettuce or tomato juice extract was used to simulate the organic load commonly encountered during commercial fresh and fresh-cut produce wash operations. Pathogens were exposed to the treatment solutions for 5, 10, 15, 30, 45, 60, 90, or 120-sec. Surviving pathogens were enumerated on XLT-4 or sorbitol MacConkey agar, and water quality changes and bacterial cellular cytoplasm leakage were tested. Results showed that pathogen inactivation is a function of FC concentration (P <0.0001), exposure time (P < 0.0001), and vary significantly (P < 0.0001) among pathogen strains. In general, no survival of pathogens was detected in the solutions when exposed to residual FC concentrations greater than 0.5 mg/L for more than 30-sec, or to residual FC concentration greater than 1.0 mg/L for more than 5-sec. When the FC concentration and contact time were less than or equal to the above conditions, survival of pathogens was strain dependant and ranked as: Salmonella > E. coli O157:H7 > non-O157 STEC. Increasing tomato or lettuce juice extract concentration in wash water significantly increased water turbidity and chemical oxygen demand, and decreased residual FC concentration, resulting in a significant reduction in the efficacy of chlorinated water to inactivate pathogens. The pathogen inactivation efficacy was specifically shown to be dependent on the residual FC chlorine concentration, which is a function of both the initial chlorine concentration and the organic load. These results indicate that maintaining sufficient FC concentration in the wash solution is of critical importance to prevent pathogen survival and cross-contamination during commercial produce wash operations. The introduction and accumulation of organic materials in the wash solution, as commonly occurs during produce wash operations can lead to rapid loss and eventual depletion of FC, and thus potential for pathogen survival and spread. Maintaining an adequate level of FC is critical for pathogen inactivation and prevention of cross-contamination.