Bootstrap Kill-ratio Analysis: Enterococcus faecium NRRL B-2354 Surrogate Suitability for Salmonella in Apple Drying

Dried apple products have been implicated in recalls and outbreaks, but limited data are available on Salmonella thermal inactivation across the range of water activities (aw) encountered during apple drying, and on the suitability of Enterococcus faecium (E. faecium) as a surrogate under these conditions. This study investigated the thermal inactivation kinetics of Salmonella Montevideo 488275 and Enterococcus faecium NRRL B-2354 on apple cubes at four aw levels (0.45, 0.60, 0.75, 0.90) and thermal treatment temperatures from 50.0 to 82.5 °C under isothermal conditions. Apple cubes were dried and equilibrated to target aw, inoculated with Salmonella or E. faecium, packed in aluminum test cells, and then isothermally treated in a water bath. D-values decreased with increasing temperature and increasing aw. The global regression model estimated a lower z_(a_w ) for Salmonella (0.33) compared with E. faecium (1.07), indicating that Salmonella’s thermal resistance was more sensitive to changes in water activity. Conversely, E. faecium exhibited a higher z_T (15.11 °C) than Salmonella (14.07 °C), suggesting slightly lower sensitivity to temperature. At aw 0.90, E. faecium was significantly more thermally resistant than Salmonella (p < 0.05); while across most conditions at aw 0.45 to 0.75, Salmonella and E. faecium exhibited statistically similar D-values (p > 0.05). Surrogate suitability was further evaluated using the kill ratio (KR) and a bootstrap KR distribution, which indicated relatively low variability at low and intermediate aw levels and conservative surrogate performance at high aw. Overall, E. faecium can potentially serve as a surrogate for Salmonella in dried apple processing within the tested range, with the incorporation of a safety margin at low and intermediate water activity levels.