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.