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The growth of Listeria monocytogenesinoculated on frankfurters at four inoculum levels (0.1, 0.04, 0.01, and 0.007 CFU/g) was examined at 4, 8, and 12°C until the time L. monocytogenespopulations reached a detectable limit of at least 2 CFU/g. A scaled-down assumption was made to simulate a 25-g sample from a 100-lb batch size in a factory setting by using a 0.55-g sample from a 1,000-g batch size in a laboratory. Samples of 0.55g were enriched in PDX-LIB selective medium, and presumptive results were confirmed on modified Oxford agar. Based on the time to detect (TTD) from each inoculum level and at each temperature, a shelf life model was constructed to predict the detection or risk levels reached by L. monocytogeneson frankfurters. The TTD increased with reductions in inoculum size and storage temperature. At 4°C the TTDs (±standard error) observed were 42.0 ± 1.0, 43.5 ± 0.5, 50.7 ± 1.5, and 55.0±3.0 days when the inoculum sizes were 0.1, 0.04, 0.01, and 0.007 CFU/g, respectively. From the same corresponding inoculum sizes, the TTDs at 8°C were 4.5 ± 0.5, 6.5 ± 0.5, 7.0 ± 1.0, and 8.5±0.5 days. Significant differences (P< 0.05) between TTDs were observed only when the inoculum sizes differed by at least 2 log. On a shelf life plot of ln(TTD) versus temperature, the Q10(increase in TTD for a 10°C increase in temperature) values ranged from 24.5 to 44.7 and with no significant influence from the inoculum densities. When the observed TTDs were compared with the expected detection times based on the data obtained from a study with an inoculum size of 10 to 20 CFU/g, significant deviations were noted at lower inoculum levels. These results can be valuable in designing a safety-based shelf life model for frankfurters and in performing quantitative risk assessment of listeriosis at low and practical contamination levels.

This research was conducted to study the growth of Listeria monocytogenesinoculated on frankfurters stored at different conditions as a basis for a safety-based consume by shelf life date label. Three L. monocytogenesstrains were separately inoculated at 10 to 20 CFU/cm2onto frankfurters that were previously formulated with or without high pressure and with or without added 2% potassium lactate (PL) and 0.2% sodium diacetate (SD). Inoculated frankfurters were air or vacuum packaged; stored at 4, 8, or 12°C; and L. monocytogenesand psychrotrophic plate counts were determined for 90, 60, and 45 days, respectively, or until the stationary phase was reached. The data (log CFU per square centimeter versus time) were fitted using the Baranyi-Roberts model to determine maximum growth rates and lag-phase time. The maximum growth rates and the lag time under each growth condition were used to calculate the time to reach 100-fold the initial Listeriapopulation. In frankfurters lacking PL and SD, the count of all strains increased by 2 log after 18 to 50 days at 4°C and 4 to 13 days at 8°C. The growth was inhibited at 4 and 8°C in frankfurters containing PL and SD, but one ribotype was capable of growing, with the time to reach 100-fold the initial Listeriapopulation ranging from 19 to 35 days at 12°C. In most cases, the time to reach 100-fold the initial Listeriapopulation of L. monocytogeneswas significantly longer in vacuum-packaged frankfurters as compared with air-packaged samples. Inclusion of PL and SD also inhibited the growth of psychrotrophs, but at all temperatures the psychrotrophic plate counts were greater than 4 log CFU/cm2at the end of the experiments. These results indicated that despite the use of antimicrobials, certain L. monocytogenesstrains could be capable of growing under storage-abuse conditions. Growth kinetics data could be useful for establishing a shelf life date label protocol under different handling scenarios.