Effects of oxygen-depleted atmospheres on survival and growth of Listeria monocytogenes on fresh-cut Iceberg lettuce stored at mild abuse commercial temperatures

The effects of oxygen-depleted atmospheres, 0.25% O2 + 12% CO2 (balance N2) and 2% O2 + 6% CO2 (balance N2), on growth of Listeria monocytogenes on fresh-cut Iceberg lettuce were determined. The study was carried out at mild abuse temperatures using controlled atmosphere chambers. During storage at a constant temperature of 7 °C, growth was enhanced at the lower oxygen level of 0.25% O2 by Day 10. Over 17 days of storage at temperatures designed to mimic mild abuse commercial conditions, there were again significantly higher counts under 0.25% O2 from Day 10 onwards. These were 0.9 and 0.7 log cycles higher on Days 14 and 17, respectively. When a model lettuce agar medium was used to eliminate possible interactions with competing flora the direct effects of the atmosphere enhancing the growth of L. monocytogenes was also observed. It is concluded that use of very O2-depleted atmospheres for control of enzymatic browning of fresh-cut Iceberg lettuce may introduce a potential hazard under some commercial conditions. There is a need for greater vigilance and possibly additional measures to ensure consumer safety.