Submitted to: Journal of Food Protection
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/19/2003
Publication Date: 8/1/2003
Citation: PORTO, A.C., CALL, J.E., LUCHANSKY, J.B. EFFECT OF RE-HEATING ON VIABILITY OF A 5-STRAIN MIXTURE OF L. MONOCYTOGENES IN VACUUM-SEALED PKGS OF FRANKFURTERS,COMMERCIALLY PREPARED WITH AND WITHOUT 2.0% POTASSIUM LACTATE, FOLLOWING REFRIGERATED OR FROZEN STORAGE. JOURNAL OF FOOD PROTECTION. 2003. V. 67(2). Interpretive Summary: Listeria monocytogenes is an established foodborne pathogen responsible for about 50% (31 of 62) of recalls of cooked meat in 1999, with ready-to-eat (RTE) meats as a primary vehicle for listeriosis. Given that about 7 billion frankfurters are consumed during the summer months and that about 20 billion frankfurters are consumed per year in the U.S., and due to its association with outbreaks/cases from frankfurters, there are numerous investigations underway to develop strategies to decrease the risk of listeriosis in such RTE products. An informal survey of consumers to gain insight on storage and re-heating preferences revealed that individuals were just about as likely to refrigerate as they were to freeze their frankfurters in the home, and that most individuals preferred grilling over microwaving and/or boiling. Based on these responses, experiments were designed to store vacuum-sealed packages of frankfurters that were contaminated with L. monocytogenes at refrigeration or freezing conditions and then to re-heat these frankfurters at near-boiling temperatures. As expected, refrigeration or freezing had little effect on L. monocytogenes, but pathogen numbers decreased as the re-heating times and temperatures increased. In fact, it was possible to eliminate at least 100,000 cells per package by heating at near-boiling temperatures for as little as 15 seconds. These findings may be useful to establish consumer guidelines for re-heating frankfurters and, thus, for decreasing the risk of listeriosis.
Technical Abstract: The purpose of this study was to assess the effect of product formulation and storage times and temperatures on the viability of Listeria monocytogenes after re-heating of frankfurters. Individual links were inoculated with about 8.0 log10 CFU/package of a five-strain mixture of the pathogen, vacuum-sealed, and stored at 4° or -18°C. Next, frankfurters were heated to a surface temperature of 60°, 70°, 80°, or 90°C for up to 8 minutes by submersing the packages in a thermostatically-controlled circulating water bath. Survivors were recovered and enumerated by rinsing each package with sterile peptone water and direct plating onto MOX selective agar plates. The data for two trials were averaged: D-values at 60°, 70°, 80°, and 90°C were 2.5, 0.3, 0.2, and 0.06 minutes, respectively, for frankfurters containing potassium lactate as an ingredient, and 2.4, 0.3, 0.1, and 0.06, respectively, for frankfurters prepared without potassium lactate. Similar results were obtained using frankfurters inoculated and then stored at 4°C for 3 or 15 days or at -18°C for 30 days prior to re-heating. Statistical analyses revealed a significant (p<0.05) decrease in D-values with an increase in temperature. The z-values ranged from 3.64° to 4.39°C for frankfurters formulated with potassium lactate and 3.51° to 3.87°C for frankfurters formulated without any added potassium lactate. The results also revealed that a 5-log10 unit reduction was achieved within 20 seconds at 80° or 90°C. Product formulation did not appreciably affect D-values for any of the parameters tested. These findings establish re-heating guidelines that can be followed by consumers to ensure that frankfurters, which may become contaminated with low levels of L. monocytogenes after unpackaging, are adequately re-heated prior to consumption.