Survival of Staphylococcus aureus and Listeria monocytogenes on vacuum-packaged beef jerky and related products stored at 21 degrees C.

In the manufacture of beef jerky, a thermal lethality step is followed by drying to prevent growth of pathogenic bacterial postprocessing contaminants on the finished product. Recent guidelines from the U.S. Department of Agriculture have raised the question of the maximum water activity (a(w)) in jerky products that will inhibit growth of pathogenic bacteria. The survival of the potential postprocessing contaminants Staphylococcus aureus and Listeria monocytogenes was evaluated on 15 vacuum-packaged beef jerky and related products with a(w) values ranging from 0.47 to 0.87, just below the 0.88 limit reported for anaerobic growth of S. aureus. Small individual product pieces were inoculated on the outer surface with five strains each of S. aureus and L. monocytogenes, repackaged under vacuum, and stored at room temperature (21 degrees C) for 4 weeks. Pathogen numbers were determined before storage and after 1 and 4 weeks. None of the 15 jerky products supported growth of either pathogen. Counts of S. aureus fell by 0.2 to 1.8 log CFU after 1 week of storage and by 0.6 to 5.3 log CFU after 4 weeks of storage. Numbers of L. monocytogenes fell by 0.6 to 4.7 log CFU and by 2.3 to 5.6 log CFU after 1 and 4 weeks of storage, respectively. Although factors other than a(w) may have some effect on pathogen survival, the results of the present study clearly support drying beef jerky to an a(w) of < or = 0.87 to ensure that bacterial pathogens cannot grow on vacuum-packaged product stored at room temperature.