Heat resistance comparison of Salmonella and Enterococcus faecium in cornmeal at different moisture levels.

Adequate surrogate identification is critical for validating in-plant thermal process controls for Salmonella inactivation in different food matrices. This study compared the thermal inactivation parameters (D- and z-values) and evaluated the heat resistance of Enterococcus faecium (8459) as a surrogate for a 5-serovar Salmonella cocktail in cornmeal. The cornmeal was spray inoculated with the respective bacteria to achieve ~9 log CFU/g population and set to the desired moisture contents (16, 22, and 28% w.b.). The inoculated cornmeal was then heat-treated at pre-determined temperatures (60, 64, and 68 °C) in sealed aluminum thermal-death-time disks in hot water baths for pre-determined time intervals. Injury-recovery media [brain heart infusion (BHI) agar overlaid with xylose lysine deoxycholate (XLD) agar for Salmonella or BHI agar overlaid m-enterococcus agar for E. faecium] were used for microbial enumeration to account for thermally injured bacterial cells. The D-values of Salmonella in cornmeal at 16, 22, and 28% moisture content were 37.5, 8.4, and 2.4 min at 60 °C, 19.9, 3.5, and 1.1 min at 64 °C, and 10.1, 1.4, and 0.5 min at 68 °C, respectively. The D-values of E. faecium in cornmeal at 16, 22, and 28% moisture content were 140.4, 18.9, and 3.3 min at 60 °C, 78.4, 7.1, and 1.6 min at 64 °C, and 37.3, 2.8, and 0.8 min at 68 °C, respectively. The z-values of E. faecium and Salmonella in cornmeal at 16, 22, and 28% moisture content were 13.9, 9.7, and 12.5 °C, and 14.0, 10.4, and 11.7 °C, respectively. These results indicated similar or higher thermal resistance (D-values) and equivalent thermal sensitivity (z-values) of E. faecium compared to Salmonella at different moisture contents and respective temperatures (P ≤ 0.05). Therefore, E. faecium could be used as a surrogate for Salmonella during thermal process validation of cornmeal processing.

Validation of brownie baking step for controlling Salmonella and Listeria monocytogenes.

Pathogens, such as Salmonella and Listeria monocytogenes, can survive under the dry environment of flour for extended periods of time and could multiply when flour is hydrated to prepare batter or dough. Therefore, inactivation of these pathogens during the cooking/baking step is vital to ensure the microbiological safety of bakery products such as brownies. The aim of this research was to validate a simulated commercial baking process as a kill-step for controlling Salmonella and L. monocytogenes in brownies and to determine thermal inactivation parameters of these pathogens in brownie batter. Independent studies were conducted in a completely randomized design for each pathogen. All-purpose flour was inoculated with a 5-serovar Salmonella and 3-strain L. monocytogenes cocktails. For baking validation, brownie batters were prepared from inoculated flour, and cooked in the oven set at 350°F (176.7°C) for 40 min followed by 15 min of ambient air cooling. For calculating D-values, brownie batter was transferred into thermal-death-time disks, sealed, and placed in hot-water baths. The samples were held for pre-determined time intervals in hot-water baths and immediately transferred to cold-water baths. Microbial populations were enumerated using injury-recovery media. At the end of baking, Salmonella and L. monocytogenes populations decreased by 6.3 and 5.9 log CFU/g, respectively. D-values of Salmonella and L. monocytogenes cocktails were 53.4 and 37.5 min at 64°C; 27.2 and 16.9 min at 68°C; 10.7 and 9.1 min at 72°C; and 4.6 and 7.3 min at 76°C; respectively. The z-values of Salmonella and L. monocytogenes cocktails were 11.1 and 16.4°C, respectively. This study can be used as a supporting document for the validation of similar brownie baking processes to control Salmonella and L. monocytogenes. The data from this study can also be employed for developing basic prediction models for the survival and thermal resistance of these pathogens during brownie baking step.

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 (aw) 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 aw. 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.