The growth of Listeria monocytogenes in cheese packed under a modified atmosphere

The effect of Modified Atmosphere Packaging (MAP) on the growth of Listeria monocytogenes in mould ripened cheeses was studied at refrigeration temper atures (2-8.3 degrees C) over a storage period of 6 weeks. Control experime nts in cling film with no atmospheric modification produced a lag time befo re growth of up to 1 week and rapid subsequent growth. MAP with a CO2 conce ntration of less than 20% allowed growth to occur but when O-2 was incorpor ated; the lag time was reduced from 3 to 2 weeks and subsequent growth was also faster, producing an increase in cell numbers of 1.4 log cycles over t he incubation period. N-2-MAP in the absence of O-2 increased the lag time to 3 weeks and slowed growth, while the inclusion of CO2 extended the lag t o 3 weeks and slowed subsequent growth even more. In MAP with 80:10:10 (v/v /v) N-2:CO2:O-2, there was a lag period of 2-3 weeks before growth of L. mo nocytogenes occurred, while the total viable aerobic count (TVAC) decreased by 2-3 log cycles and the total Lactobacillus count showed little change. It was concluded that MAP was not suitable for preventing the growth of L. monocytogenes in such cheeses.