Purpose: The objectives were to: (1) compare thermal resistance of E. faecium and Salmonella in nonfat milk powder (aw 0.25) at 85, 90, and 95°C and (2) evaluate the repeatability of the protocol by comparing results across two laboratories.
Methods: Nonfat milk powder was inoculated with either E. faecium or a five-strain Salmonella cocktail (Agona, Enteritidis PT30, Tennessee, Montevideo, Mbandaka; all linked to low-moisture foods) and equilibrated to aw 0.25 at 23°C for three to four days at Washington State University (WSU). After vigorously shaking to eliminate caking/clumping, ten 1-g subsamples were plated to assess homogeneity of the inoculum. Half of the samples (108.0±0.2 CFU/g) were shipped to Michigan State University (MSU) for isothermal treatment in parallel with WSU. Inoculated samples (~0.7g) isothermally heated (85, 90, or 95°C) in sealed aluminum test cells were periodically removed, cooled, and plated for survivors to calculate thermal inactivation parameters (D- and z-values).
Results: Both organisms exhibited log-linear kinetics at all temperatures tested. At all temperatures, D-values for E. faecium were significantly higher (P<0.05) than those for Salmonella. D-values for E. faecium/Salmonella were D85°C = 48.8±3.9/13.9±0.3, D90°C = 2.52±0.9/6.7±0.1, and D95°C = 13.2±0.6/3.5±0.1 min, respectively. Enterococcus faecium exhibited a higher z-value (17.6°C) than Salmonella(16.6°C).
Significance: Enterococcus faecium appears to be a valid surrogate for assessing thermal inactivation of Salmonella in nonfat milk powder, based on a higher surrogate thermal resistance.