Low-energy X-ray irradiation was assessed as a means of inactivating Escherichia coli O157:H7 on baby spinach. Round-cut (2.54 cm diameter) baby spinach samples were dip-inoculated in a 3-strain E. coli O157:H7 cocktail, and irradiated on each side of the leaf using a prototype low-energy (70 kV) X-ray irradiator to achieve X-ray doses of up to 0.176 kGy per side. The D10 value for E. coli O157:H7 on baby spinach was 0.033 ± 0.001 kGy. Irradiated bags of baby spinach (225 g) yielded a dose rate distribution of 1.93 ± 0.75 Gy/s at the mid-plane to 7.18 ± 3.85 Gy/s on the bag's surface. Predicted log reduction for E. coli O157:H7 in the bag was 4.10 log CFU, while the observed mass averaged value was 3.99 log CFU. Next, pre-irradiated, round-cut baby spinach leaves were inoculated with an E. coli O157:H7 cocktail pre-exposed to a peracetic acid-based (PAB), chlorine-based (CB) or quaternary ammonium compound-based (QACB) sanitizer to obtain 90 to 99% injury, and subjected to X-ray doses of up to 0.063 kGy. Compared to control values of 0.021 ± 0.001 kGy, D10 values for PAB-, CB-, and QACB-injured E. coli O157:H7 on baby spinach were 0.0136 ± 0.000, 0.0223 ± 0.001, and 0.0242 ± 0.001 kGy respectively, with exposure to PAB significantly (P < 0.05) enhancing E. coli O157:H7 susceptibility to X-ray irradiation, while exposure to CB significantly (P < 0.05) reduced susceptibility. Based on these findings, low-energy X-ray irradiation appears to be most effective against E. coli O157:H7 when spinach is flume tank-washed using a PAB sanitizer prior to irradiation.
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