Inactivation of Shiga-toxin-producing Escherichia coli, Salmonella enterica and natural microflora on tempered wheat grains by atmospheric cold plasma

Abstract

Bacterial foodborne disease outbreaks implicating raw wheat products have focused much attention on decontaminating wheat grains used for manufacturing flour and other milled products. The present study investigated the effectiveness of atmospheric cold plasma (ACP) for destroying Shiga-toxin-producing Escherichia coli, Salmonella enterica and natural microflora on tempered wheat grains. Portions (10 g) of grains were each inoculated with a 5-strain mixture of E. coli or S. enterica to obtain an initial count of ∼7.0 log10 CFU/g. Inoculated or non-inoculated wheat grains were sealed in plastic bags filled with atmospheric air and exposed to ACP (44 kV) dielectric barrier discharge for 0 (control), 5, 10, 15 and 20 min. Pathogen survivors were evaluated by washing wheat grains with sterile diluent, plating the wash solution on thin agar layer (TAL) media and appropriate selective (SEL) agar, and counting bacterial colonies after 48 h of incubation (35 ᵒC). Non-inoculated grains were analyzed for yeast and molds, mesophiles, psychrotrophs, and Enterobacteriaceae. After 20 min, initial viable counts (log CFU/g) on TAL medium and SEL agar, respectively, decreased by 3.09 and 4.84 (for E. coli), and 4.40 and 4.32 (for S. enterica). A higher level of sub-lethal injury occurred in E. coli compared to S. enterica (P < 0.05). After ACP treatment for 20 min, log CFU/g reductions of mesophiles psychrotrophs, and Enterobactericeae were 0.96, 2.14 and 1.38, respectively. In contrast, yeast and molds were totally destroyed (3.29 log CFU/g reduction) after only 10 min. ACP has good potential for killing both enteric pathogens and spoilage microorganisms on tempered wheat grains destined for the manufacture of wheat flour. Further research on the baking characteristics of flour manufactured from ACP-treated wheat grains is warranted.