P3-104 The Impact of Organic Load on the Minimal Level of Chlorine Needed to Prevent Escherichia coli O157:H7 Cross-contamination during Washing of Fresh-cut Lettuce

Wednesday, July 31, 2013
Exhibit Hall (Charlotte Convention Center)
Chao Zhou, Illinois Institute of Technology, Bedford Park, IL
Jianfeng Wang, Illinois Institute of Technology, Bedford Park, IL
Mingxia Zang, Illinois Institute of Technology, Bedford Park, IL
David Laird, U.S. Food and Drug Administration-USPHS, Bedford Park, IL
Tong-Jen Fu, U.S. Food and Drug Administration, Bedford Park, IL
Introduction: Fresh produce grows in natural environments and is susceptible to microbial contamination. The contamination can spread during postharvest washing. Industry and government guidelines have suggested usage of chemical disinfectants such as sodium hypochlorite in wash water to prevent microbial cross-contamination and have recommended that wash water disinfectants be monitored. However, specific standards regarding the levels of sanitizer needed have yet to be established. As wash water is often recycled during commercial processing, the impact of the increased organic load on the effective level of sanitizer needed to prevent microbial cross-contamination remains to be determined.   

Purpose: To determine the minimal effective level of sodium hypochlorite needed to prevent Escherichia coli O157:H7 cross-contamination during washing of inoculated fresh-cut lettuce as affected by the organic load of wash water.

Methods: Eight g of cut romaine lettuce inoculated with 7 log CFU/g of E. coli O157:H7 were washed with 800 g of uninoculated lettuce in 40 l of sterile tap water at 3ºC for 2 min. Washing trials were performed in water with different levels of organic load (addition of 0, 3, 6, 12, or 30 g of lettuce juice powder) and at different levels of chlorine treatment (0 - 50 ppm of sodium hypochlorite, NaOCl). The degree of cross-contamination was determined by measuring the presence of E. coli O157:H7 in the wash water and uninoculated lettuce after washing. Wash water samples were also analyzed for total/free chlorine, turbidity, and total organic carbon. 

Results: In the absence of chlorine, E. coli O157:H7 transfer occurred at all levels of organic load and resulted in the contamination of wash water and uninoculated lettuce at levels of 3.7 ± 0.3 log CFU/ml and 3.3 ± 0.2 log CFU/g, respectively. Without the addition of lettuce juice powder, cross-contamination was prevented in wash water containing 5 ppm of NaOCl. With the addition of 3 g of lettuce juice powder, transfer of E. coli O157:H7 was observed when the washing run was conducted with 5 ppm of NaOCl but was prevented with 10 ppm of NaOCl. With the addition of 30 g of lettuce powder, 50 ppm of NaOCl was needed to prevent cross-contamination. It was difficult to have an accurate determination of the residual free chlorine level as it continued to decrease during the washing run.

Significance: Measurements of the organic load of wash water are needed to determine the effective level of sanitizer required to prevent microbial cross-contamination during postharvest washing of fresh-cut produce.