The objective of this work was to compare organoleptic and microbiological spoilage with the survival of Escherichia coli0157:H7 in modified atmosphere (MA) stored shredded lettuce. The rates of growth ofE coli0157:H7, increase in aerobic plate counts (APC g−1), and rates of visual spoilage of shredded lettuce held under air or MA at 13 and 22°C were compared. Samples were inoculated with nalidixic acid-resistantE coli0157:H7 (ATCC 35150) and placed in a chamber which was continuously flushed with gas mixtures of 0/10/90, 3/0/97, 5/30/65, 20/0/80 (O2/CO2/N2, v/v) and held at 13 or 22°C. APC growth was inhibited in 5/30/65 (O2/CO2/N2) at 13°C compared to all other atmospheres which were not significantly different from each other. The growth rates for bothE coli0157:H7 and APC were greatest in air at 22°C. Carbon dioxide concentration had no significant effect on the growth ofE coli0157:H7 at either temperature. The shelf-life of shredded lettuce, as judged by appearance, was extended in atmospheres containing 30% CO2 by approximately 300% compared to air. The APC were similar at the time when the shredded lettuce samples were judged unacceptable regardless of shelf-life. However, the extended shelf-life provided by the MA allowedE coli0157:H7 to grow to higher numbers compared to air-held shredded lettuce.
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Diaz et al. (1996) studied this question.