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

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 degrees 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 less than or equal to 0.05) 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 degrees C. Drop the inoculated lettuce, after 24 h of incubation at 5 +/- 1 degrees 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 degrees 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.