Survival and growth of Escherichia coli O157:H7 inoculated onto cut lettuce before or after heating in chlorinated water, followed by storage at 5 or 15°C

This study determined the effects of mild heat and chlorine treatment followed by storage for up to 18 days at 5 degreesC or 7 days at 15 degreesC on the survival and growth of Escherichia coli O157:H7 inoculated onto fresh-cut iceberg lettuce. The efficacy of treatment with 20 ppm chlorine in killing the pathogen on lettuce at 50 degreesC was determined. Treatment of lettuce with 20 ppm chlorine at either 20 or 50 degreesC did not result in significantly greater reductions in populations of E. coli O157:H7 compared to respective treatments in water without chlorine. The pathogen steadily decreased in viability on treated lettuce throughout subsequent storage at 5 degreesC for 18 days. The population increased by 2.3 to 3.2 log(10) CFU/g within 2 days, then continued to increase at a slower rate through 7 days of storage at 15 degreesC. At 4 and 7 days, significantly (alpha = 0.05) higher populations were reached on lettuce that had been treated at 50 degreesC, compared to respective samples that had been treated at 20 degreesC, regardless of the presence of 20 ppm chlorine in the treatment water. Treatment of lettuce with 20 ppm chlorine at 50 or 20 degreesC before or after inoculation with E. coli O157:H7 did not have a marked influence on behavior of the pathogen during subsequent storage at 5 or 15 degreesC.