Purpose: To assess survival and virulence of Salmonella in low moisture foods.
Methods : A two-strain mixture of Salmonella enterica serotypes Typhimurium and Enteritidis was used to inoculate two model LMF, chocolate and inshell pistachios. Products (100 g) were inoculated with four ml of the Salmonella cell suspension, dried for one to three hours until aw approximated that of the uninoculated product, aliquoted into 15-ml centrifuge tubes and stored in the dark at 22°C. Salmonella populations immediately after inoculation, after drying and at 1, 4, 6, 10, 15, and 21 days were determined in triplicate on nonselective (TSA-YE) and selective (XLD) media. Virulence was tested in the insect Galleria mellonella model by injecting 10 µl of rinsate from products at one day into the last left proleg of 10 larvae. Rinsate from uninoculated products were also injected as controls. The larvae were incubated at 37°C and larval mortality was daily monitored.
Results: The population of Salmonella in the inoculated products was eight and nine log CFU/g immediately after inoculation and 7.3 and 8.0 log CFU/g after drying for chocolate and pistachios, respectively. Populations decreased to 4.5 and 7.5 log CFU/g by 21 days in chocolate and pistachios, respectively. Recovery of Salmonella from either product on TSAYE and XLD was similar. Larvae inoculated with 3.4 log CFU/ml of the cocktail had a mortality of 100% after 24 h compared to 80% of larvae inoculated with 3.9 log CFU/ml cells from chocolate and 30% for larvae inoculated with 3.7 log CFU/ml cells from pistachios. The mortality of controls was 0 and 10% for uninoculated pistachios and chocolate, respectively.
Significance: Findings show that Salmonella can survive in chocolate liquor and pistachios and that cells adapted to the LMF environment retained virulence.