Stability of Listeria monocytogenes in wheat flour during extended storage and isothermal treatment

Foodborne pathogens including Salmonella have been implicated in recent recalls of low-water activity (aw) foods, such as peanut butter, almond flour, wheat, flour and dry milk powder, and are primary concerns for the microbiological safety of dry food products. Although there are an increasing number of studies on Salmonella thermal resistance conducted in low-moisture foods, little information is available on Listeria monocytogenes thermal resistance in those products. This study evaluated the survival of L. monocytogenes in wheat flour during long-term storage as well as its thermal resistance in wheat flour equilibrated to aw 0.30, 0.45, and 0.60. L. monocytogenes survived in wheat flour at both aw 0.31 and 0.56 during 6 months of storage at room temperature, with populations decreasing about 2.52 and 6.27 logs at aw 0.31 and 0.56, respectively. Equilibration in low-aw flour enabled L. monocytogenes to become more resistant to thermal treatment. At treatment temperature between 70 and 80 °C, D-values increased with decreasing aw. For aw 0.30. 0.45, and 0.60 (measured at room temperature), respectively, D-value (in min) ranges for 70-80 °C were 37.10-7.08, 17.44-3.13, and 16.85-1.59. The z-values were 12.9, 14.2, and 9.9 °C for aw 0.30, 0.45, and 0.60, respectively. These data highlight the need for vigilance when processing dry foods, and provide valuable information for the industry to validate thermal processing for control of L. monocytogenes in low-moisture foods. This study also offers insight into the development of thermal inactivation strategies to control L. monocytogenes and other foodborne pathogens in foods with similar matrices.