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Title: Potential of E.coli O157:H7 to grow on field-cored lettuce as impacted by postharvest storage time and temperature

item McEvoy, James
item Luo, Yaguang - Sunny
item Conway, William

Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/19/2008
Publication Date: 11/28/2008
Citation: Mcevoy, J.L., Luo, Y., Conway, W.S., Zhou, B., Feng, H. 2008. Potential of E.coli O157:H7 to grow on field-cored lettuce as impacted by postharvest storage time and temperature. International Journal of Food Microbiology. 128:506-509.

Interpretive Summary: Foodborne illnesses due to E. coli O157:H7 contaminations are a major health concern in the United States. A better understanding of how E. coli can serve as a source of contamination and how the bacterium grows on lettuce at field and refrigerated temperatures may lead to better sanitary practices and reduce the number of food borne illnesses. In this paper we report that E. coli can be transferred from a contaminated knife used in the field during harvest to remove the cores of iceberg lettuce heads. We determined that quick cooling of the lettuce after harvesting is critical in keeping growth of any contaminating E. coli to a minimum if pathogen contamination should occur. Consumers of fresh leafy vegetables as well as the produce growing and packaging industries will benefit from this research.

Technical Abstract: A recent development in iceberg lettuce production is field coring where the outer leaves and the cores of the lettuce heads are removed at the time of harvesting in order to reduce shipping waste and maximize production yield. Using a coring knife contaminated with 2 x 105 cells of E. coli O157:H7, the transfer of the pathogen to lettuce heads and subsequent growth of the pathogen and background microorganisms at simulated field and refrigerated temperatures (30 and 5 C) were examined. At 5 °C no significant growth or loss of viability of E. coli, total aerobes or yeasts and molds were noted during an 8 h incubation period. At 30 °C however, significant increases of all three classes of organisms occurred between 0 h to 4 h and 4 h to 8 h. E. coli numbers, regardless of whether the bacteria were cold stressed prior to use as inoculum, increased by approx. 2 logs10 cfu/g at 30 °C from 0 h to 8 h. The numbers of all three classes of microorganisms were significantly higher in the 30 °C samples compared to the 5 °C samples at both the 4 h and 8 h sampling times. A single contaminated coring knife was found to successively inoculate at least three lettuce heads with E. coli as determined following 6 h incubation at 30 °C.