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

Abstract

This study determined the effects of mild heat and chlorine treatment followed by storage for up to 18 days at 5°C or 7 days at 15°C 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°C was determined. Treatment of lettuce with 20 ppm chlorine at either 20 or 50°C 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°C for 18 days. The population increased by 2.3 to 3.2 log10 CFU/g within 2 days, then continued to increase at a slower rate through 7 days of storage at 15°C. At 4 and 7 days, significantly (α = 0.05) higher populations were reached on lettuce that had been treated at 50°C, compared to respective samples that had been treated at 20°C, regardless of the presence of 20 ppm chlorine in the treatment water. Treatment of lettuce with 20 ppm chlorine at 50 or 20°C 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°C.