Determination of thermal inactivation parameters of Salmonella in nonfat dry milk and whole milk powder during four months storage

Primary Author: Amninder Singh Sekhon

Faculty Sponsor: Minto Michael


Primary College/Unit: Agricultural, Human and Natural Resource Sciences

Category: Agricultural and Natural Resource Sciences

Campus: Pullman




Principal Topic- Foodborne pathogens such as Salmonella can endure dry environments of milk powders for extended periods of time due to increased adaptability at low water activity (aw) levels and proliferate when powders are hydrated. This study focused to compare survivability and thermal resistance of Salmonella in NFDM and WMP stored for 120 days.


Methods- This study was designed as completely randomized with three replications. Milk powders were spray inoculated with 5-serovar Salmonella cocktail and dried back to original pre-inoculation aw. The D-values of Salmonella in inoculated NFDM and WMP were determined periodically (every 30 days, starting from day-one). The D and z-values were also performed for hydrated forms of NFDM and WMP. Rehydration (13% total solids w/v) was performed on days of performing D- and z-value study. Five separate thermal death time disks containing 5g or 5 mL of respective sample were transferred into five thermal-death-time, sealed and placed in hot-water baths set at 80, 85 and 90°C for inoculated powder, and 59, 62 and 65°C for hydrated inoculated powder. Samples were held for 0 to 56 minutes in hot-water baths and quickly transferred to cold-water baths at pre-determined time intervals. The samples were enumerated using injury-recovery media, and D- and z-values were calculated.


Results- During storage, heat resistance of Salmonella in WMP was statistically higher than NFDM. D-values of Salmonella on day 120 in WMP were 24.6, 11.4, and 7.0 minutes at 80, 85 and 90°C respectively as compared to 21.9, 9.2, and 5.2 min in NFDM.