Synergistic effect of chlorine dioxide and drying treatments for inactivating Escherichia coli O157:H7 on radish seeds.

Studies were done to determine whether calcium hypochlorite (Ca(OCl)(2)) and chlorine dioxide (ClO(2)) treatment followed by drying had a synergistic killing effect on microorganisms on radish seeds intended for sprout production. Uninoculated radish seeds and seeds inoculated with Escherichia coli O157:H7 were treated with water, Ca(OCl)(2) (free chlorine concentrations of 50 or 200 microg/ml), or ClO(2) (50 or 200 microg/ml) for 5 min and subsequently dried at 25 degrees C for up to 24 h. Populations of total aerobic bacteria (TAB), molds and yeasts (MY), and E. coli O157:H7 on the seeds treated with Ca(OCl)(2) were not significantly different (P = 0.05) than populations on seeds treated with ClO(2) at the same concentrations. However, populations of microorganisms on seeds treated with ClO(2) decreased more rapidly during drying. Treatment with ClO(2) (200 microg/ml) followed by drying caused reductions in TAB, MY, and E. coli O157:H7 of 3.1, 2.0, and 3.8 log CFU/g, respectively. When seeds were treated with water, Ca(OCl)(2) (50 or 200 microg/ml), and ClO(2) (50 microg/ml) and subsequently dried, reductions in TAB, MY, and E. coli O157:H7 were 0.2 to 2.0, 0.4 to 2.0, and 1.4 to 2.2 log CFU/g, respectively. Results indicate that inactivation of E. coli O157:H7 on radish seeds is greater after treatment with ClO(2) followed by drying than after treatment with Ca(OCl)(2) followed by drying, thus providing a synergistic treatment combination for reducing the safety risk associated with sprouts produced from these seeds.