Effect of Temperature, Water Activity, and Structure on Salmonella Thermal Resistance in Multiple Wheat Products

Introduction: Wheat kernels, meal, and flour are low-moisture products with potential for Salmonella contamination. Therefore, pathogen reduction processes are needed, and must be validated, for cereal products. However, many factors affect Salmonella thermal resistance, such as product structure and water activity (a_w), but little is known about the effects.

Purpose: Therefore, the objective was to quantify the effects of product structure, a_w, and temperature on Salmonella thermal resistance on/in wheat products.

Methods: Wheat kernels were surface-inoculated with Salmonella Enteritidis PT30 and equilibrated in controlled-humidity chambers to 0.25, 0.45, or 0.65 a_w. After equilibration, kernels (50 g/batch) were ground (in the humidity chamber) into whole-grain meal (25 s) and flour (60 s), and then equilibrated, again. Equilibrated samples were treated isothermally (in triplicate) in vacuum-sealed plastic bags (seven kernels/bag, ~0.4 g) and aluminum test cells (flour and meal, ~0.6 g) in a water bath (80, 85, or 90°C) and then removed at eight equal time intervals, immediately cooled in an ice bath, and plated on modified trypticase soy agar. The plates were incubated for 48 h at 37°C, and then enumerated.

Results: As expected, Salmonella thermal resistance decreased (P<0.05) with increasing temperature and water activity. For example, D-values on kernels at 0.45 a_w were 9.3±0.5, 3.3±0.3, and 1.1±0.1 min at 80, 85, and 90°C, respectively. However, there was no effect (P>0.05) of product structure on thermal resistance at 0.45 a_w.

Significance: This is the first known study to report Salmonella thermal inactivation kinetics on an intact cereal grain. The results indicate that a_w affects the thermal resistance of Salmonella Enteritidis PT30, but that structure of cereal grain products may not be a significant factor in application of thermal inactivation parameters.