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%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 (Pless than or equal to0.05) growth of E. coli O157:H7 was noticed on lettuce after inoculation by dip, drop or sprinkle methods and incubation for 6 and 24h at 5 +/- 1degreesC. Drop the inoculated lettuce, after 24h of incubation at 5 +/- 1degreesC, resulted in maximum log reduction of E. coli O157:H7 (1.42 log(10) cfu g(-1)) after washing with sterile de-ionized water in comparison to dip- or sprinkle-inoculated lettuce (<1 log(10) 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 24h of incubation at 5 +/- 1degreesC, when the initial population was 3.71 log(10) 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 10 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. (C) 2002 Elsevier Science Ltd. All rights reserved.