Inactivation of Escherichia coli O157:H7 from Pak Choi Seeds Using Sequential Treatments of Chlorine Dioxide, Drying, and Dry-heat

Introduction: Vegetable seed sprouts have a high risk of food poisoning because they are frequently consumed raw.  Even if sprout seeds are contaminated with low levels of foodborne pathogens, the population of pathogens can be increased to high levels during cultivation of the seeds.  Therefore, it is important to eliminate foodborne pathogens from sprout seeds.

Purpose: The objectives of this study were to screen the seeds that are resistant to aqueous chlorine dioxide (ClO₂) and to develop a method to eliminate Escherichia coli O157:H7 from the seeds.

Methods: To screen seeds resistant to ClO₂, 15 types of sprouts seeds were treated with ClO₂ (200 µg/ml) solution for 5 min and then their germination rates were evaluated.  To eliminate E. coli O157:H7 from sprout seeds, seeds were treated with ClO₂ (0, 50, or 200 µg/ml) for 5 min, dried at 45°C with 23% relative humidity (RH) for 24h, and heated at 80°C with 23% RH for up to 24h.  After the treatments, the population of E. coli O157:H7 on the seeds and the germination rate were measured.

Results: Pak Choi seeds showed the highest germination rate among 15 types of seeds after treatment with ClO₂.  When Pak Choi seeds containing E. coli O157:H7 (6 log CFU/g) were treated with 0, 50, and 200 µg/ml ClO₂ for 5 min, the numbers of E. coli O157:H7 decreased to 5.0, 2.4, and 1.0 log CFU/g, respectively.  When those seeds were dried and dry-heated, E. coli O157:H7 were completely inactivated from the seeds and the germination rate was 89%.

Significance: This study showed that E. coli O157:H7 on Pak Choi seeds were completely inactivated by sequential treatment of ClO₂, drying, and dry-heat without decreasing the germination rate.