Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2002
Publication Date: N/A
Interpretive Summary: Lettuce, including pre-prepared salad, is occasionally contaminated with pathogenic bacteria such as Escherichia coli O157:H7. Ionizing radiation can eliminate E. coli O157:H7 from lettuce, but its efficacy can be influenced by the product type. The radiation resistance of E. coli O157:H7 that was inoculated onto four different varieties of lettuce (Iceberg, Red leaf, Green Leaf, Boston) surfaces or homogenate was determined. The ionizing radiation dose required to kill 99.999 percent of E. coli O157:H7 on lettuce ranged from 0.535 to 1.70 kGy. No effect of ionizing radiation on lettuce texture was observed at doses up to 0.5 kGy. Processors can use the information provided to successfully eliminate E. coli O157:H7 from lettuce and lettuce mixes using ionizing radiation and consumers will benefit from a safer food product.
Technical Abstract: An outbreak strain of Escherichia coli O157:H7 was inoculated onto closely related but structurally distinct types of lettuce (Lactuca sativa): Boston (butterhead lettuce), Iceberg (crisphead lettuce), Green leaf and Red leaf (colored variants of looseleaf lettuce). Inoculation was either onto the surface of cut leaf pieces, or into a homogenized leaf suspension. Samples were gamma-irradiated and the radiation sensitivity of the inoculated bacteria expressed as a D-value (the amount of ionizing radiation necessary to reduce the bacterial population by 90 percent, in kGy). Recovery of bacteria from nonirradiated leaf pieces was also measured. When inoculated on the leaf surface, E. coli O157:H7 had a significantly greater radiation sensitivity on Red leaf (D = 0.119) and Green leaf (D = 0.123) than on Iceberg (D = 0.136) or Boston (D = 0.140). When inoculated into a homogenized leaf suspension, the sensitivity was significantly greater on Iceberg (D = 0.092) than on Green leaf (D = 0.326), Boston (D = 0.331) or Red leaf (D = 0.339), a difference of approximately 350 percent. Significantly fewer bacteria were recovered from the surface of Iceberg vs. the other types examined. Following radiation doses up to 0.5 kGy, the texture (maximum shear strength) of lettuce leaves was measured along the midrib and the leaf edge for each lettuce type. There was little significant change in texture for any type of lettuce, for either section of leaf examined, at any dose up to 0.5 kGy. These data show that relatively subtle differences between lettuce types can significantly influence the radiation sensitivity of associated pathogenic bacteria.