P1-144 The Effect of Water Velocity on Escherichia coli O157:H7 Transfer from Inoculated Lettuce to Wash Water in a Closed Pipe System

Monday, August 4, 2014
Exhibit Hall D (Indiana Convention Center)
Lin Ren, Michigan State University, East Lansing, MI
Beatriz Mazón, Michigan State University, East Lansing, MI
Bradley Marks, Michigan State University, East Lansing, MI
Elliot Ryser, Michigan State University, East Lansing, MI
Introduction: Cross-contamination of fresh-cut leafy greens during commercial flume washing continues to pose major food safety concerns for the industry.

Purpose: This study aimed to quantify Escherichia coli O157:H7 transfer from lettuce to water at different velocities in a closed pipe system.

Methods: One ml of a 4-strain nontoxigenic E. coli O157:H7 cocktail containing 8.1 log CFU/ml, or  5 x 5 cm pieces of iceberg lettuce dip-inoculated to contain E. coli O157:H7 at ~7 log CFU/g, were inserted into continuously flowing water pumped through a 10-cm dia pipe at 0.0756, 0.108 and 0.189 l/s. Water and lettuce samples were collected at 5- or 10-sec intervals for 2 min and quantitatively examined for E. coli by plating appropriate dilutions with/without membrane filtration on TSAYE with ampicillin. Data from triplicate experiments were assessed for significance using the Kenward-Roger correction.

Results: In the water study, cumulative populations of E. coli O157:H7 recovered were inversely related to flow rate. Using inoculated lettuce, E. coli O157:H7 populations in the water were significantly higher at a flow rate of 0.189 l/s (2.5 log CFU/ml) (P < 0.05) compared to 0.0756 l/s (2.1 log CFU/ml) and 0.108 l/s (1.9 log CFU/ml). E. coli O157:H7 populations in water peaked in the water/lettuce experiments at 50/45, 35/40 and 20/25 s for flow rates of 0.0756, 0.108, and 0.189 l/s, respectively, with no E. coli detected after 75/95, 65/100 and 30/80 s.

Significance: Bacterial removal rate from the lettuce increased significantly with water velocity, which is important to consider in future models of bacterial transfer and/or associated risk analyses.