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

Introduction: Contaminated wheat flour has been identified as the probable vehicle of a multi-state outbreak of Escherichia coli O157:H7 associated with consumption of refrigerated, raw cookie dough.  Several cookie dough manufacturers are using heated-treated flour for ready-to-bake products. Published data on thermal inactivation of foodborne pathogens in wheat flour remains scarce.

Purpose: To determine the effect of thermal treatment of artificially contaminated wheat flour on populations of E coli O157:H7 and Salmonella Agona.

Methods:  E. coli O157:H7 and Salmonella Agona were added to wheat flour (1 g) in sterile plastic bags. Inoculated wheat was pulsified (15 s) to distribute cultures and pressed to a uniform thickness (1 mm).  Bagged, inoculated wheat samples (approx. 9 log CFU/g) were submerged in a preheated shaking water bath for 1, 5, 15 or 30 minutes at 55, 60, 65 or 70 °C. Following thermal treatment, samples were submerged in ice water for 30 seconds and diluted with 0.1% peptone water. Samples (0.1 ml) were plated onto TSA and incubated at 37 °C for 24 h prior to enumeration.

Results:   During heat treatment (30 min), populations of E. coli O157:H7 decreased by 2.9, 4.4, 5.7 and 5.7 log CFU/g, and Salmonella Agona populations decreased by 3.9, 4.3, 5.1 and 5.2 log CFU/g  at 55, 60, 65, and 70 °C, respectively.  Both pathogens remained detectable by direct plating or enrichment at 30 min of heat treatment.

Significance: Thermal processing of wheat flours may help improve the microbiological safety of ready-to-bake products.