Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 27, 2011
Publication Date: February 5, 2012
Citation: Yang, Y., Luo, Y., Millner, P.D., Turner, E.R., Feng, H. 2012. Assessment of Escherichia coli O157:H7 transference from soil to Iceberg Lettuce via a contaminated harvesting knife. International Journal of Food Microbiology. 153(3):345-350. Interpretive Summary: Previous laboratory studies have shown that a field harvesting knife can facilitate the transfer of disease causing microorganisms like E. coli O157:H7 from soil to harvested lettuce. However, specific scientific information that is suitable for use in food safety risk assessments and reduction is not available. In this study, the researchers comprehensively evaluated the effect of various realistic lettuce harvesting practices and soil contamination levels on pathogen transfer. The scientific information provided will assist the industry in performing risk assessment of lettuce field-coring harvesting, and support immediate and near-term improvements in lettuce harvesting operations relative to reducing potential risks from pathogen contamination and transfer to product.
Technical Abstract: The potential for coring knives to cross-contaminate lettuce heads with pathogens was evaluated for both ring and blade ends. Rings and blades artificially contaminated with Escherichia coli O157:H7 (EHEC), were used to core three successive heads of iceberg lettuce. The coring rings and blades were inoculated by dipping into soils containing EHEC at concentration ranges of 1-105 MPN/g soil. Factors that influenced EHEC transference from soil to iceberg lettuce via contaminated coring knife blade, included water content (WC) of clay and sandy soils, EHEC concentration, and degree of blade contact (stem, medium, and high) with edible tissue. High moisture content clay soil was positively associated with high pathogen transference. EHEC were recovered on all three consecutive lettuce heads cut with knives soiled with 30% WC clay, or on the first- and second-cut lettuce heads when the knife blade was dipped into 25% WC clay soil contaminated with EHEC 105 MPN/g. No EHEC were detected on any cut heads when clay soil WC was 20% or less, or when the knife blade was dipped into sandy soil contaminated with EHEC at the same level, regardless of percent WC. The extent to which the harvesting knife blade cut across edible lettuce tissues was also an important factor in the amount of pathogen transference that occurred. All studies used a stem cut method except for one in which the extent of blade contact with edible tissues was examined. EHEC were detectable on first-cut lettuce heads with medium- and heavy-contact cutting scenarios when the blade was contaminated with 104 cfu/g EHEC in clay soil (25% WC) and sandy soil (10% WC). However, when the blade, contaminated at the same soil EHEC level, was used to cut only the stem and had no contact with the edible portion of the lettuce head, no pathogen transference was detected.