Thermal Inactivation of Escherichia coli O157:H7 and Salmonella Agona in Wheat Flour

Contaminated wheat flour has been identified as the probable vehicle of a multi-state outbreak of Escherichia coli O157:H7 associated with ready-to-bake cookie dough. Several cookie dough manufacturers are currently using heat-treated flour for ready-to-bake products, although data on thermal inactivation of foodborne pathogens in wheat flour remains scarce. The objective of this research was to first determine appropriate methods and parameters for bacterial inoculation and thermal treatment of wheat flour, and to subsequently determine the population reductions of E. coli O157:H7 and Salmonella Agona in artificially contaminated wheat flour following thermal treatment for 1, 5, 15 or 30 minutes at 55, 60, 65 or 70°C in a shaking water bath. Flour samples (aw = 0.55) in sterile plastic bags were individually inoculated (~109 CFU/g), pulsified to distribute cultures, and pressed to a uniform thickness (1mm) prior to heat treatment. Following treatment, samples were rapidly cooled and diluted with peptone water; then plated onto Tryptic Soy Agar (TSA) and incubated at 37°C for 24 h prior to enumeration. The minimum heat treatments required for a 5-log reduction in microbial populations (~ 109CFU/g to ~ 104CFU/g) were 5 minutes at 70°C and 30 minutes at 70°C for E. coli O157:H7 and S. Agona, respectively. This research supports the hypothesis that the microbiological safety of ready-to-bake products may be improved by the use of heat-treated flour. Master of Science in Life Sciences