Purpose: The purpose of this study was to investigate the post-processing behavior of Salmonella spp. when inoculated onto hard and semi-hard natural cheeses with elevated temperature storage.
Methods: Ten varieties of commercial, pre-sliced natural cheeses were purchased from the supermarket. Slices (20 g) were surface inoculated with 100 μl of a 6-strain cocktail of 107 log CFU/mL Salmonella spp. using an L-shaped spreader. Slices were air dried under a hood, then folded to enclose the inoculum and placed into standard cheese bags. Bags were vacuum sealed and stored aerobically at 25 °C up to 16 days, with samples taken every 2-3 days. At each sampling point, a slice was diluted 1:9 with Butterfield’s Phosphate Diluent and stomached 2 min. Samples were serially diluted in BPD and plated on Modified Eosin Methylene Blue agar. Plates were incubated at 37 °C for 24 h, after which counts were taken and log CFU/g was calculated for each sample. Duplicate trials were completed for all cheeses. Death curves for each cheese were fit linearly using Combase DMFit software.
Results: Death rates ranged from –0.07 to –0.01 log CFU/g per day, representing Swiss and reduced-sodium Provolone cheeses, respectively, with total reduction over storage ranging from –1.2 to –0.3 log CFU/g, again in Swiss and reduced-sodium Provolone. Change in sample pH over storage ranged from –0.55 in Provolone to +0.08 in Colby cheese.
Significance: This research confirmed that Salmonella spp., when introduced post processing, is unable to grow at 25 °C in the following cheese slices: Cheddar (mild, medium, and sharp), Colby, Gouda, Havarti, Muenster, Provolone, reduced-sodium Colby Jack, and reduced-sodium Provolone. In conjunction with cheese challenge studies investigating behavior of Listeria monocytogenes, Staphylococcus aureus, and Escherichia coli O157:H7, this data will be significant in identifying cheeses safe for unrefrigerated storage.