The influence of modified-atmosphere packaging, storage temperature, and time on survival and growth of Escherichia coli 0157:H7 inoculated onto shredded lettuce, sliced cucumber, and shredded carrot was determined. Growth of psychrotrophic and mesophilic microorganisms and changes in pH and sensory qualities of vegetables, as judged by subjective evaluation, were also monitored. Packaging under an atmosphere containing 3% oxygen and 97% nitrogen had no apparent effect on populations ofE. coil 0157:H7, psychrotrophs, or mesophiles. Populations of viable E. col 0157:H7 declined on vegetables stored at 5°C and increased on vegetables stored at 12 and 21°C for up to 14 days. The most rapid increases in populations ofE. coli 0157:H7 occurred on lettuce and cucumbers stored at 21°C. These results suggest that an unknown factor(s) associated with carrots may inhibit the growth ofE. coli 0157:H7. The reduction in pH of vegetables was correlated with initial increases in populations of E. coil 0157:H7 and naturally occurring microfloras.

A study was undertaken to determine the fate of Escherichia coli 0157:H7 in ground, roasted beef as influenced by the combined effects of pH, acidulants, temperature, and time. There was essentially no change in the viable population of E. coli 0157:H7 when beef salads (pH 5.40 to 6.07) containing up to 40%o mayonnaise were incubated at 5°C for up to 72 h. At 21 and 30°C, significant (P 5 0.05) increases in populations of the organism occurred in salads containing 16 to 32% mayonnaise (pH 5.94 to 5.55) between 10 and 24 h of incubation. Death was more rapid as the pH of acidified beef slurries incubated at 5°C was decreased from 5.98 to 4.70. E. coli 0157:H7 grew in control slurries (pH 5.98) and in slurries containing citric and lactic acids (pHs 5.00 and 5.40) incubated at 21°C for 24 h; decreases occurred in slurries acidified to pHs 4.70, 5.00, and 5.40 with acetic acid or pH 4.70 with citric or lactic acid. At 30°C, populations decreased in slurries acidified to pHs 4.70 and 5.00 with acetic acid. Citric and lactic acids failed to prevent significant increases in populations in slurries at pH 4.70 to 5.40 between 10 and 24 h of incubation. The order of effectiveness of acidulants in inhibiting growth was acetic acid > lactic acid. citric acid. The same order was observed for inactivation of E. coli 0157:H7 in acidified (pH 5.00) beef slurry heated at 54°C.

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