Purpose: Investigate how water flow rate may affect the extent of E. coli O157:H7 cross-contamination during washing of inoculated lettuce in chlorinated water.
Methods: Cut romaine lettuce (8 g) inoculated with 7 log CFU/g of E. coli O157:H7 were added into 40 liters of water together with uninoculated lettuce (800 g) and washed for 2 min. Washing trials were performed at 3°C with various levels of chlorine (0, 5, and 10 ppm) and wash water flow rates (fast, medium, or slow by setting the valve pressure at 3.5, 5, or 7 psi, respectively). The degree of cross-contamination was determined by measuring the presence of E. coli O157:H7 in wash water and uninoculated lettuce.
Results: In the absence of chlorine, the spread of E. coli O157:H7 occurred and resulted in the contamination of wash water and uninoculated lettuce at levels of 3.0 ± 0.0 log CFU/ml and 3.1 ± 0.1 log CFU/g, respectively. At 10 ppm of chlorine, regardless of water flow rate, no E. coli O157:H7 was found in wash water or uninoculated lettuce. At 5 ppm of chlorine, while no E. coli O157:H7 was found in wash water or uninoculated lettuce samples (0/6) collected during washing at a slow flow rate (valve pressure at 7 psi), cross-contamination of E. coli O157:H7 was observed in washing runs operated at higher flow rates, with the pathogen detected in 2/6 wash water and 1/6 uninoculated lettuce samples collected during washing at the fast flow rate (3.5 psi).
Significance: Under certain conditions, wash water flow rate may have an impact on the extent of E. coli O157:H7 cross-contamination during lettuce washing.