P1-105 Method to Determine Differences in Thermal Tolerance of Salmonella Serotypes at Low Water Activity

Monday, July 29, 2013
Exhibit Hall (Charlotte Convention Center)
Nathan Anderson, U.S. Food and Drug Administration-IFSH, Bedford Park, IL
Susanne Keller, U.S. Food and Drug Administration-IFSH, Bedford Park, IL
Dana Gradl, U.S. Food and Drug Administration-IFSH, Bedford Park, IL
Shannon Pickens, Illinois Institute of Technology, Bedford Park, IL
Haiping Li, U.S. Food and Drug Administration-IFSH, Bedford Park, IL
Introduction: FSMA requires that facilities identify and evaluate foreseeable biological hazards as well as identify and implement preventive controls to provide assurances that those hazards will be minimized or prevented.  To validate processes that may involve low moisture conditions to achieve safety targets it is necessary to identify the most resistant target pathogen.

Purpose: To develop a simple method to study the thermal resistance of Salmonella serotypes inoculated in oat flour at low water activity.

Methods: Outbreak and non-outbreak-associated salmonellae (n = 16) were used for the inoculation of oat flour. Dry matrices in Whirlpak bags were inoculated with lawn-grown cells at a 1:1 ratio (~18 g total) and massaged by hand until thoroughly mixed. Inoculum was packed in aluminum crucibles and stored open at 11% relative humidity and ambient temperature (23 ± 2°C).  Crucibles were sealed after equilibration was achieved (3 to 6 days).  A differential scanning calorimeter (DSC) was used to heat treat crucibles at 90°C for 5, 7 and 10 min and followed by cooling to 20°C for 2 min. Lids were aseptically removed from crucibles and samples diluted in buffered peptone water (BPW), then  plated on tryptic soy agar with yeast extract (TSAYE) and incubated aerobically at 37°C for 24 h. Counts were expressed as CFU/g.

Results: Triplicate samples showed little variation (mean SD = 0.2 log). All serotypes exhibited enhanced thermal resistance at lower aw (~0.2) and linear inactivation behavior from which D-values were calculated that were comparable to published data.  Salmonella serotypes Tennessee and Agona appeared to be among the most heat resistant with D90°C-values of 16.0 and 15.5 min, respectively.

Significance: Results indicate that a DSC can be used to generate reliable thermal kinetic data for Salmonella serotypes under low-moisture conditions.  This simplified method may allow for rapid identification of the most thermally tolerant pathogens for validation of preventive controls.