| E. coli O157:H7 contamination in fresh produce originated on farm can spread during postharvest washing. Industry and government guidelines have suggested the use of chemical disinfectants such as sodium hypochlorite in wash water to prevent microbial cross-contamination and have recommended that wash water disinfectants be monitored. The goal of this study was to determine the minimal effective level of chlorine needed to prevent E. coli O157:H7 cross-contamination during post-harvest washing of fresh-cut lettuce with wash water containing different levels of organic load and to evaluate the way to monitor the organic load in wash water by correlating measurements of Total Organic Carbon (TOC) with turbidity measurements. Eight grams of cut romaine lettuce inoculated with 8 log cfu/g of E. coli O157:H7 were washed with 800 g of uninoculated lettuce in 40 L of sterile tap water at 3 for 2 min. Washing trials were performed in water with different levels of organic load (with the addition of 0, 3, 6, 12, or 30 g of lettuce juice powder) and at different levels of chlorine treatment (0 - 40 ppm of sodium hypochlorite). 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 TOC. 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 3.61 +/- 0.13 log cfu/ml and 3.34 +/- 0.23 log cfu/g, respectively. A minimal free chlorine level of 5, 10, 20, 30, or 40 ppm was needed to prevent E. coli O157:H7 cross-contamination during postharvest washing of fresh-cut lettuce in water with a TOC level of 20+/-3, 26+/-2, 41+/-6, 74+/-4, or 180+/-4 mg/L, respectively. TOC is a good indicator of organic load in wash water, but it took hours to get results. Turbidity measurement only takes minutes and it correlated well with TOC, indicating that turbidity has the potential to indicate the organic load in wash water. |
