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Impact of Vacuum Cooling on Escherichia coli O157:H7 Infiltration into Lettuce Tissue

Haiping Li,^* Mehrdad Tajkarimi, and Bennie I. Osburn

Western Institute for Food Safety and Security, School of Veterinary Medicine, University of California Davis, Davis, California

Received 12 December 2007/ Accepted 10 March 2008

Vacuum cooling is a common practice in the California leafy green industry. This study addressed the impact of vacuum cooling on the infiltration of Escherichia coli O157:H7 into lettuce as part of the risk assessment responding to the E. coli O157:H7 outbreaks associated with leafy green produce from California. Vacuum cooling significantly increased the infiltration of E. coli O157:H7 into the lettuce tissue (2.65E+06 CFU/g) compared to the nonvacuumed condition (1.98E+05 CFU/g). A stringent surface sterilization and quadruple washing could not eliminate the internalized bacteria from lettuce. It appeared that vacuuming forcibly changed the structure of lettuce tissue such as the stomata, suggesting a possible mechanism of E. coli O157:H7 internalization. Vacuuming also caused a lower reduction rate of E. coli O157:H7 in stored lettuce leaves than that for the nonvacuumed condition.

Published ahead of print on 14 March 2008.

Present address: Master Program of Preventive Veterinary Medicine, University of California Davis, One Shields Ave., Davis, CA 95616.