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Appl. Environ. Microbiol. doi:10.1128/AEM.02811-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Vacuum cooling process and internalization of Escherichia coli O157:H7 into lettuce tissue

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

^* To whom correspondence should be addressed. Email: hpli{at}ucdavis.edu.

	Abstract

Vacuum cooling is a common practice in the California leafy-green industry. This study addressed the impact of vacuum cooling on the internalization of Escherichia coli O157:H7 in lettuce as part of the risk assessment responding to the E. coli O157:H7 outbreaks associated with leafy-green produce from California. Vacuum cooling increased the internalization of E. coli O157:H7 into the lettuce tissue significantly (2.65E+06 CFU/g) compared to the non-vacuumed condition (1.98E+05 CFU/g). A stringent surface sterilization and quadruple washing could not eliminate the internalized bacteria in 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 slower reduction rate of E. coli O157:H7 in stored lettuce leaves, compared to the non-vacuumed condition.