Purpose: This study compared thermal resistance of Enterococcus faecium and a Salmonella cocktail in almond meal (water activity (aw) 0.45) at 80, 85, and 90°C and determined the reproducibility of results across two laboratories.
Methods: A batch of whole almonds (450 g) was inoculated with either E. faecium NRRL B-2354 or a five-strain Salmonella cocktail (Agona, Enteritidis PT30, Tennessee, Montevideo, Mbandaka, all linked to low-moisture foods) at ~ eight log CFU/g, conditioned to 0.45 aw(±0.025) at 25°C, ground into almond meal, reconditioned to the target aw and tested for homogeneity before sample dissemination. Isothermal treatments were performed by heating ~ one gram inoculated and equilibrated samples in aluminium test cells. Survivors were enumerated on designated differential media after incubation.
Results: Overall, E. faecium was more thermally resistant than Salmonella (P< 0.05) across all three temperatures. Enterococcus faecium and Salmonella, respectively, yielded D80°C of 34.01±0.41 and 26.53±0.24 min, D85°C of 17.34±0.29 and 13.17±0.10 min, D90°C of 8.55±0.13 and 7.24±0.08 min. Although the z-values were close, that of E. faecium (16.69±0.25°C) was lower (P<0.05) than that of Salmonella(17.73±0.08°C). These results were also reproducible between laboratories, with statistically similar (P>0.05) z-values for both E. faecium (MSU: 16.61±0.29°C; WSU: 16.68±0.40°C) and Salmonella (MSU: 17.95±0.28°C; WSU: 17.56±0.22°C).
Significance: Although E. faecium already is an accepted Salmonella Enteritidis PT30 surrogate for almonds, this study confirms the robustness of the surrogate for a broader cocktail of Salmonellae isolated from low-moisture foods, and reproducibility across laboratories.