Purpose: The objective of this study was to evaluate effects of fat concentrations of frankfurters on survival of Listeria monocytogenes under simulated gastric fluid and heat process, and invasion of the pathogen on Caco-2 cell.
Methods: Three frankfurters formulations (10% fat, F10; 20% fat; F20 and 30% fat, F30) were prepared, and a 10-strain mixture of L. monocytogenes was inoculated on frankfurters to obtain 4-5 log CFU/cm2. The inoculated samples were then vacuum packaged, and they were stored at 10 °C for 30 days. The samples were analyzed every 10 days to evaluate survival of L. monocytogenes from simulated gastric fluid challenge at 37 °C for 120 min, and heat challenge at 63 °C for 60 min. Survival of total bacteria and L. monocytogenes were enumerated on tryptic soy agar plus 0.6% yeast extract and PALCAM agar, respectively. The isolated cells from F10, F20, and F30 were subjected to the invasion assay using Caco-2 cells.
Results: Cell counts of L. monocytogenes on F30 samples were higher (P < 0.05) than those of F10 and F20 during storage, and the pathogens on F30 samples were more resistant to heat than other treatments on day 10. L. monocytogenes on frankfurters with higher fat concentration showed higher (P < 0.05) resistance to gastric fluid. Moreover, some relationship between fat concentration and cell invasion was observed.
Significance: These results suggest that high fat concentration in frankfurters may increase resistance of L. monocytogenes to various stresses.