Effects of acid adaptation on the survival of Listeria monocytogenes on modified atmosphere packaged vegetables

The survival and growth of Listeria monocytogenes and L. innocua on ready-to-use (RTU) packaged vegetables (lettuce, swedes, dry coleslaw mix, bean-sprouts) were studied. The effects of acid adaptation of Listeria spp. on their survival during subsequent storage were also investigated. Listeria innocua behaviour was similar to that of L. monocytogenes on all vegetables examined. The survival and growth patterns of Listeria varied with the packaged product. Populations on packaged lettuce and swedes significantly increased (P < 0.05, by 1-1.5 log cycles) during a 14-day storage period. During the same period, Listeria counts gradually decreased (by 1-1.5 log cycles) on coleslaw mix. Acid adaptation enhanced survival of Listeria spp. during storage in packages of vegetables which had relatively high in-pack CO2 levels (i.e. 25% in packaged coleslaw, bean-sprouts). It is concluded that adapting listerial cells to mildly acidic conditions rendered cultures more resistant to relatively high (25-30%) CO2 atmospheres.