Survival and thermal resistance of Salmonella in dry and hydrated nonfat dry milk and whole milk powder during extended storage.

Foodborne pathogens such as Salmonella can endure dry environments of milk powders for extended periods due to the increased adaptability at a low water activity (a) and proliferate when powders are hydrated. This study compared the survivability and the thermal resistance of a 5-serovar Salmonella cocktail in dry and hydrated nonfat dry milk (NFDM) and whole milk powder (WMP) stored for 180 days at ambient temperature (~20 °C). This study was designed as two factorial (storage days and milk powder type) randomized complete block design with three replications as blocks. The milk powders were spray inoculated with 5-serovar Salmonella cocktail and dried back to the original pre-inoculation a. The D-values of Salmonella in inoculated NFDM and WMP were determined periodically (every 30 days, starting from day one). The milk powders were also individually hydrated on each analysis day to determine D- and z-values of Salmonella in hydrated powders. The D-values were determined using thermal-death-time disks and hot-water baths at 80, 85 and 90 °C for milk powders, and 59, 62 and 65 °C for hydrated powders. The D- and z-values of Salmonella at specific temperatures within dry or hydrated powders during the storage period were compared at P ≤ 0.05 using two-way ANOVA and Tukey's Test. The D-values of Salmonella in WMP on day 1 were 18.9, 9.9 and 4.4 min at 80, 85 and 90 °C, respectively, which increased to 29.4, 13.6 and 6.5 min at 80, 85 and 90 °C, respectively, on day 180. Whereas, D-values of Salmonella in NFDM on day 1 were 17.9, 9.2 and 4.4 min at 80, 85 and 90 °C, respectively, and stayed similar during the storage. The D-values of Salmonella in milk powder remained similar throughout the storage once hydrated. The overall z-value of Salmonella in NFDM and WMP was 16.3 °C, whereas in hydrated NFDM and WMP, the overall z-value was 6.4 °C.