Purpose: This study aimed to compare thermal resistance of Enterococcus faecium and Salmonella in peanut butter, almond meal, wheat flour, nonfat dried milk powder, date paste, and ground black pepper; and to determine reproducibility of results across five laboratories.
Methods: Each of six low-moisture foods were assigned to two laboratories, with the lead laboratory inoculating samples, testing homogeneity, and equilibrating to the desired water activity (aw) prior to shipping. The products were inoculated with E. faecium NRRL B-2354 or a five-strain Salmonella cocktail (Agona, Typhimurium, Montevideo, Mbandaka, Enteritidis - all from low-moisture products). Both laboratories subjected samples to three isothermal heat treatments and survivors were enumerated on differential media.
Results: Overall, Salmonella showed lower (P<0.05) thermal resistance than E. faecium in almond meal, peanut butter, and nonfat dried milk powder. Although almond meal and peanut butter have a similar fat content, E. faecium and Salmonella showed less thermal resistance in almond meal (D90°C Ef: 8.55±0.13 min; D90°C Sal : 7.24±0.08 min at 0.45 aw) than in peanut butter (D90°C Ef: 21.17±0.21; D90°C Sal: 11.45 ± 0.25 min at 0.25 aw). Milk powder, at 0.25 aw , showed greater thermal resistance of E. faecium (D90°C: 25.20 ± 0.94 min) and less thermal resistance for Salmonella (D90°C: 6.69±0.13 min at aw0.25) than in almond meal and peanut butter.
Significance: Overall, E. faecium appears to be a robust surrogate for Salmonella spp. across multiple low moisture products (with multiple cross-laboratory validations); however, product characteristics significantly impact that relationship. It is, therefore, necessary to consider product composition when validating thermal processes for low aw foods.