Purpose: The purpose of this study was to assess the lethality of Salmonella spp. in low to intermediate moisture foods employing dry heat. Currently, very limited information is publically available regarding the lethality in such products.
Methods: Nine strains of Salmonella were grown on TSA, harvested, and inoculated into soy protein and wheat flour products. The moisture levels of the products utilized were 5-8%, 12-15%, and 20-25%. Five-gram portions of product were placed in aluminum cups and baked for various time in a Fisher Isotemp 351 convection oven at temperatures ranging from 200 °F to 325 °F. Salmonella levels on untreated inoculated controls and baked samples were established by plating triplicate samples in duplicate on TSA overlayed with XLD agar. Survivor curves were plotted and D-values calculated and compared.
Results: Log reductions for each product and treatment were calculated. Log reductions found varied significantly based on product type, moisture level, temperature and time of heat exposure. Low moisture (5%-8%) products processed at temperatures ranging from 200° to 325 °F varied considerably requiring 8 minutes up to 2,400 minutes of dry heat treatment to attain a 5+ log reduction whereas the intermediate moisture (20-25%) products showed less variability and were able to achieve a similar reduction in 180 seconds to 18.5 minutes.
Significance: These results show that the degree of variability in log reduction of Salmonella spp. under dry heat treatments decreases significantly as the moisture level increases. However, factors inherent to the product type (e.g., fat levels) can also impart variability.