Inactivation of Escherichia coli O157:H7 on radish seeds by sequential treatments with chlorine dioxide, drying, and dry heat without loss of seed viability

Abstract

We developed and validated a treatment to inactivate Escherichia coli O157:H7 on radish seeds without decreasing seed viability. Treatments with aqueous ClO(2) followed by drying and dry-heat treatments were evaluated for efficacy to inactivate the pathogen. Conditions to dry radish seeds after treatment with water (control) or ClO(2) were established. When treated seeds with high water activity (a(w)) (>0.99) were stored at 45°C and 23% relative humidity (RH), the a(w) decreased to <0.30 within 24 h. Drying high-a(w) seeds before exposing them to dry-heat treatment (≥60°C) was essential to preserve seed viability. The germination rate of radish seeds which had been immersed in water for 5 min, dried at 45°C and 23% RH for 24 h, and heated at 70°C for 48 h or at 80°C for 24 h was not significantly decreased (P ≤ 0.05) compared to that of untreated radish seeds. Sequential treatments with ClO(2) (500 μg/ml, 5 min), drying (45°C, 23% RH, 24 h), and dry heating (70°C, 23% RH, 48 h) eliminated E. coli O157:H7 (5.9 log CFU/g) on radish seeds and, consequently, sprouts produced from them without decreasing the germination rate. These sequential treatments are recommended for application to radish seeds intended for sprout production.

Fig. 1.

Water activity of radish seeds dried at 25°C (○) or 45°C (□). Radish seeds were immersed in water for 5 min and incubated at 25 or 45°C for up to 48 h at 23% relative humidity (RH).

Fig. 2.

Germination rate of radish seeds after sequential ClO₂, drying, and heat treatments. Radish seeds were immersed in water (white bars) or 500 μg/ml of ClO₂ (gray bars) for 5 min, dried at 45°C and 23% relative humidity (RH) for 24 h, and dry heated at 70°C (A) or 80°C (B) and 23% RH for 24 or 48 h. Bars indicate standard deviations. For the same dry-heat treatment time, bars not noted by the same letter are significantly different (P ≤ 0.05).