Purpose: The objective was to quantify the impact of five different inoculation methods on thermal resistance of Salmonella Enteritidis PT30 in wheat flour, and subsequent repeatability in a two-laboratory comparison study.
Methods: Batches of wheat flour (100 g) were inoculated with Salmonella Enteritidis PT30 by five different methods: (A) high-concentration, low-liquid volume (HCLV) broth culture, (B) HCLV suspended lawn culture, (C) pelleted and resuspended lawn culture, (D) direct contact with a lawn culture, and (E) fomite transfer of a lawn culture. After inoculation, samples were equilibrated (~5 d) to ~0.45 aw in a controlled-humidity chamber, subjected to isothermal (80°C) inactivation trials in aluminum test cells in a water bath (11 durations in triplicate), immediately cooled in ice water, serially diluted, and plated on modified trypticase soy agar with yeast extract. D-values were computed from the resulting log CFU/g data by linear regression.
Results: Post-equilibration and post-come-up Salmonella populations ranged from 8.7 to 6.3 and 7.7 to 3.7 log CFU/g, respectively. Method A yielded the largest population decline during equilibration (~3 log) and come-up (~2.5 log) and also the highest D-value (504.9 s), compared to the other methods (P < 0.05). The MSU-generated D-values for methods B, C, and D were clustered (250.9, 285.9, and 226.7 s, respectively), but statistically different (P < 0.05).
Significance: Based on these findings, careful consideration should be given to the inoculation method, which can significantly impact thermal resistance of Salmonella in low-moisture foods, and the inherent uncertainty, which can significantly affect utility of resulting models.