Lettuce tissue damage affects the growth and/or survival of Escherichia coli O157:H7
M. R. WACHTEL, Y. Luo, Y. Huang, and J. L. Mc Evoy. Produce Quality & Safety Lab, USDA-ARS-Beltsville Area, 10300 Baltimore Ave., Bldg. 002, Rm. 117, Beltsville, MD 20705
Field coring is a recent industrial development designed to increase production yield of iceberg lettuce. This process generally increases in-plant production yield from the traditional 60 - 70% to nearly 100%. However, this process involves some tissue damage when lettuce heads are dropped from a height of ~ 6 feet as they pass from a conveyor belt to the field holding bin. Our objective was to assess the severity of lettuce tissue damage upon dropping and its possible correlation with the capacity to sustain growth and/or survival of Escherichia coli O157:H7. Lettuce heads were inoculated in a 106 CFU/ml E. coli O157:H7 NalidixicR solution. Heads were dropped 6 feet onto a wooden pallet covered with cardboard, simulating commercial harvesting of field cored lettuce. Pathogens were enumerated by plating on LB-Nal agar. The relative conductivity of iceberg lettuce heads increased in correlation with the falling distance, indicating that severe membrane damage was induced when heads fell from greater distances. Damaged lettuce tissue allowed the survival or growth of ~ 0.5 log E. coli O157:H7 relative to the control after lettuce heads were stored at ambient temperature for 4 hours followed by 4°C storage for 48 hours. The immediate spray application (fine mist) of 100 ppm total chlorine did not significantly reduce pathogen numbers on both damaged and non-damaged heads. Dipping lettuce pieces in 100 ppm chlorine for 1 minute, 48 hours post-inoculation resulted in a ~ 0.5 log E. coli decrease compared to the non-dipped control. Larger numbers of pathogens were recovered on the damaged heads compared to non-damaged heads, regardless of the chlorine treatment. These results suggest that tissue damage incurred in the field increases the potential risk for pathogen contamination. Chlorine treatment was of limited effectiveness in combating this problem.
Session 15D, Food Microbiology: Fruits and vegetables