Purpose: The objective of this study was to determine and compare the heat resistance of Salmonella Tennessee and Typhimurium DT104 with that of E. faecium, a potential surrogate for thermal inactivation of Salmonella in model peanut pastes (two levels of aw (0.3 and 0.6) and two concentrations of fat (47 and 56%)).
Methods: Four peanut pastes containing 47% or 56% fat at 0.3 or 0.6 aw, were inoculated with dry inoculum on talc (~1x106 CFU/g) and screened for thermal resistance at 75°C. A thermal death test method using copper plates to compress the peanut paste samples at ≤ 1 mm thickness was used. Counts of survivor cells were used to generate the inactivation curves using the Geeraerd and Van Impe Inactivation Model Fitting Tool in the xla program; for statistical analysis Minitab Release 14 software was used.
Results: There was no significant (P ˃ 0.05) difference between the heat resistance of Salmonella Tennessee and Salmonella Typhimurium in tested conditions (e.g., the minimum calculated time for 2 decimal reductions (2D) was 25.4 min for Tennessee and 24.2 for Typhimurium at 47% fat and 0.3 aw). Maximum log-reduction (4.0) of Salmonella Typhimurium occurred at 56% fat at both aw, implying that fat level might be a more critical factor than aw in heat inactivation of this organism under these conditions. The inactivation rates of E. faecium were consistently lower (e.g., 74.7 min for 2D at 47% fat and 0.3 aw).
Significance: Understanding how fat and aw can affect the heat resistance of Salmonella in food will allow a better control of this pathogen; E. faecium might be an appropriate and conservative surrogate for thermal process validation.