Effect of inoculation and washing methods on the efficacy of different sanitizers against Escherichia coli O157:H7 on lettuce Original Research Article

The efficacy of aqueous chlorine dioxide (ClO2) (10 mg l−1, for 10 min), ozonated water (10 mg l−1 for 10 min) and thyme oil (0·1% for, 5 min) on inactivation of mixed strains of Escherichia coli O157:H7 was investigated using different inoculation methods (dip, drop or sprinkle) after 6 and 24 h of incubation at 5±1°C. Different population sizes of E. coli O157:H7 (7·82, 5·95 and 3·71 log cfu g−1), as well as multiple washing, were also used to evaluate the efficacy of these sanitizers. No significant (P\le0·05) growth of E. coli O157:H7 was noticed on lettuce after inoculation by dip, drop or sprinkle methods and incubation for 6 and 24 h at 5±1°C. Drop the inoculated lettuce, after 24 h of incubation at 5±1°C, resulted in maximum log reduction of E. coli O157:H7 (1·42 log10 cfu g−1) after washing with sterile de-ionized water in comparison to dip- or sprinkle-inoculated lettuce (<1 log10 cfu g−1 reduction). Aqueous ClO2, ozonated water and thyme oil washing treatments were also less effective on dip- or sprinkle-inoculated lettuce than on drop-inoculated lettuce. Regardless of the type of treatments applied, a significantly higher log reduction of E. coli O157:H7 was detected on sprinkle-inoculated lettuce after 24 h of incubation at 5±1°C, when the initial population was 3·71 log10 cfu g−1. Increasing the number of washes (to two or three) in sterile de-ionized water did not decrease the microbial numbers further, whereas a second washing in aqueous ClO2 (10 mg l−1 for 5 min), ozonated water (10 mg l−1 for 5 min) and thyme oil (0·1% for 2 min) produced a significant reduction comparable to the first washing. The results obtained from this study indicate that effectiveness of sanitizers depends on the inoculation method, incubation time, population size and multiple washing.