Survivability and thermal resistance of Salmonella and Escherichia coli O121 in wheat flour during extended storage of 360 days

Foodborne pathogens like Salmonella and Escherichia coli O121 can endure the harsh low water activity (a_w) environment of wheat flour for elongated periods of time and can proliferate when hydrated for baking or other purposes. This study determined the survivability and thermal tolerance (D- and z-values) of Salmonella and Escherichia coli O121 in wheat flour and muffin batter (prepared from inoculated flour on the days of analyses) during the storage period of 360 days. The Salmonella and E. coli O121 studies were conducted as two independent experiments. Both studies were designed as randomized complete block with three replications as blocks. All experimental data were analyzed using one-way ANOVA and Tukey's test in Minitab® software, and P ≤ 0.05 was considered significant. The wheat flour was spray inoculated individually with 7-isolate Salmonella or 3-isolate E. coli O121 cocktail and then dried back to the original a_w levels. On each analysis day, inoculated wheat flour (~5 g) or muffin batter (~2.5 g) was placed inside the TDT disks, heat treated at set temperatures in hot water baths, and sampled at predetermined time intervals for determining the survival microbial population. The population of E. coli O121 and Salmonella cocktails in wheat flour at day 1 were 7.6 ± 0.18 and 7.8 ± 0.07 log CFU/g, respectively, which decreased to 2.0 ± 0.40 and 2.8 ± 0.59 log CFU/g on day 360, respectively. The D-values of Salmonella and E. coli O121 cocktails in inoculated flour and muffin batter prepared from inoculated flour (on the day of analysis) were determined on days 1, 30, 90, 180, 270, and 360 [given enough surviving bacterial population (~3 to 4 log CFU/g) was present in the flour]. The population of Salmonella and E. coli O121 in wheat flour decreased by 5.0 and 5.6 log CFU/g, respectively, during the storage period of 360 days. The D_70°C, D_75°C, and D_80°C values of Salmonella in wheat flour remained similar during the storage period. Whereas, for E. coli O157:H7 in wheat flour, the D_70°C value decreased from 20.3 ± 2.82 to 7.1 ± 2.82 min, and D_75°C decreased from 10.2 ± 2.14 to 2.7 ± 0.27 min, during the storage period of 180 days. The z-values of Salmonella or E. coli O157:H7 remained similar during the storage period. The D- and z-values from this research can be employed for validation of thermal process to ensure safety of wheat flour.