Effect of reheating on viability of a five-strain mixture of Listeria monocytogenes in vacuum-sealed packages of frankfurters following refrigerated or frozen storage.

The purpose of this study was to assess consumer preferences for storing and reheating frankfurters and to use this information to assess the effect of product formulation and storage times and temperatures on the viability of Listeria monocytogenes after reheating of frankfurters. Individual links were inoculated with about 8.0 log CFU per package of a five-strain mixture of the pathogen, vacuum sealed, and stored at 4 degrees C for 3 and 15 days and at -18 degrees C for 30 days. Frankfurters formulated with and without 2% added potassium lactate were heated to a surface temperature of 60, 70, 80, or 90 degrees C for up to 8 min by submersing the packages in a thermostatically controlled circulating water bath. Surviving bacteria were recovered and counted by rinsing the contents of each package with sterile peptone water and plating this solution directly onto modified Oxford selective agar plates. In general, the results revealed that about a 5-log unit reduction was achieved by reheating to a surface temperature of 70 degrees C for about 2 min or 80 or 90 degrees C for about 0.6 min regardless of storage conditions or formulation. Product formulation did not appreciably affect the viability of the pathogen after heating; there was no appreciable difference in the number of cells surviving the heat treatment in product prepared with or without potassium lactate. These findings can be used to establish reheating guidelines for consumers to ensure that frankfurters, which may become contaminated with low levels of L. monocytogenes prior to packaging and after unpackaging, are adequately reheated prior to consumption.