Reduction of Listeria monocytogenes populations during exposure to a simulated gastric fluid following storage of inoculated frankfurters formulated and treated with preservatives

The effect of a simulated gastric fluid (adjusted to pH 1.0 with HCl) on Listeria monocytogenes, inoculated postprocessing on pork frankfurters formulated with sodium lactate (SL) and sodium diacetate (SD) and not dipped or dipped in solutions of lactic acid or acetic acid, was evaluated during storage of the frankfurters at 10 °C for 40 days. Pork frankfurters containing 1.8% SL, 0.25% SD, 1.8% SL+0.125% SD, or 1.8% SL+0.25% SD were inoculated with 102–103 CFU/cm2 of a 10-strain preparation of L. monocytogenes and were not dipped or dipped for 2 min in solutions of 2.5% lactic or acetic acid before they were vacuum-packaged and stored. Survival of L. monocytogenes was determined after exposure of frankfurters for 0, 20, 40, and 60 min to the simulated gastric fluid after storage for 0, 10, 20, 30, or 40 days. Growth of L. monocytogenes on frankfurters formulated with antimicrobials was inhibited in the order control <SL<0.25% SD<SL+0.125% SD<SL+0.25% SD. Survival of L. monocytogenes during exposure to simulated gastric fluid was higher on those frankfurters on which there had been extensive growth of the pathogen than on frankfurters on which growth had been inhibited. Inclusion of single antimicrobials in the formulation of not dipped frankfurters appeared to increase resistance of L. monocytogenes to simulated gastric fluid provided the initial level of the pathogen exposed to the simulated gastric fluid was higher than 5 log CFU/cm2. Although the combinations of SL and SD used in the formulation of not dipped frankfurters inhibited growth of L. monocytogenes, prolonged (40 days) storage of such frankfurters induced slight resistance to simulated gastric fluid. The frankfurter formulation of 0.25% SD and dipping in 2.5% lactic acid appeared to increase resistance of L. monocytogenes to simulated gastric fluid following growth during prolonged (days 20 to 40) storage. Use of 2.5% acetic acid dipping combined with antimicrobials in the formulation inhibited growth of L. monocytogenes on frankfurters and subsequent detectable survival after exposure to simulated gastric fluid.