Project Summary

The Incidence and Characterization of Listeria Monocytogenes in the Processing Environment of Fully Cooked, Ready-to-Eat Beef Products

Principle Investigator(s):
T.L. Frederick, C.A. Turner, G.R. Bellinger, R. Nickelson II, S.P. Wood, and W.D. Warren
Food Safety Net, Inc.
Completion Date:
July 2001


Layman’s Summary

The purpose of this study was to determine and evaluate the incidence of Listeria monocytogenes in the processing environments of fully cooked, ready-to-eat beef products. A total of 6,030 environmental samples were collected and tested for Listeria monocytogenes. These samples were collected from five facilities producing ready-to-eat beef products including hot dogs, roast beef, sliced deli meats, taco meat, tacos, and pepperoni. Each facility was divided into four “environmental” zones based on the contamination risk to the finished product. Samples were collected on three different visits at approximately thirty-day intervals between visits. During each visit, samples were collected at each of the three sampling times: (1) Pre-operational; (2) six hours after the start of operations for the first shift (Operational); and (3) six hours after start of operations for second shift (End). Of the samples positive for L. monocytogenes, 600 were selected for ribotype identification. The sampling plan and site selection used in this study was aggressive and targeted harborage areas for potential positives to establish a realistic overview of the incidence of Listeria ssp. in the processing environment. Overall, L. monocytogenes was present in 13% while Listeria ssp. were detected in 27% of the environmental samples. In general, there was a reduction noted in the incidence of Listeria ssp. for visits 2 and 3 of each facility. As expected, the Pre-operational samples had the lowest incidence rates of both L. monocytogenes (9% positive) and Listeria ssp. (19%). From the Pre-operational samples to the Operational and End samples, there was an increase in the incidence of L. monocytogenes and Listeria ssp. Of the zones tested, the raw area (Zone 3) clearly had the highest incidence of L. monocytogenes and Listeria ssp. Overall, the Pre-operational, RTE (Zone 1) areas had the lowest incidences; however, over sampling time and period, similar levels of environmental Listeria ssp. were observed for the Operational and End samples. Nineteen different ribotypes were identified amongst the 600 isolates tested. Of the 19 ribotype groups, four ribotypes, designated DUP-1039, DUP-1052, DUP-1059, and DUP 1062, dominated the isolate pool. The ribotypes that persisted in the processing facility were more prevalent among the industrial isolates than among human isolates.