Fate of post-processing inoculated Listeria monocytogenes on vacuum-packaged pepperoni stored at 4, 12 or 25 °C

If present, Listeria monocytogenes may not be eliminated during processing of pepperoni or may be introduced during peeling, slicing, or packaging. We evaluated the fate of the pathogen on sliced inoculated pepperoni during vacuum-packaged storage, and potential differences in survival among three types of inocula, including nonacid-adapted, acid-adapted and pepperoni extract-habituated cultures. Commercial pepperoni (two replicates, three samples per treatment) was sliced and inoculated (3 to 4 log CFU/cm²), before vacuum-packaging and storage for up to 180 days at 4, 12 or 25 °C. Samples were periodically analyzed for pathogen counts (PALCAM agar) and total bacterial counts (tryptic soy agar with 0.6% yeast extract). The pH of the product was relatively stable (4.50–4.81) throughout storage. Overall, levels of the pathogen (all inocula) and total counts decreased continuously during storage at all temperatures. The pathogen died slower at 4 °C than at 12 and 25 °C, while at 12 and 25 °C the death rates were similar. Death rates depended on type of inoculum and generally decreased in the order: acid-adapted, extract-habituated and nonacid-adapted inoculum. At day 60, pathogen levels were below the detection limit and remained undetectable throughout the rest of the 180-day storage period, regardless of inoculum type and storage temperature. Therefore, storage of sliced vacuum-packaged pepperoni, especially at ambient temperature, prior to consumption may reduce the potential risk of listeriosis.