EFFECTS OF GAS ATMOSPHERE, ANTIMICROBIAL DIP AND TEMPERATURE ON THE FATE OF LISTERIA-INNOCUA AND LISTERIA-MONOCYTOGENES ON MINIMALLY PROCESSED LETTUCE

The growth and survival of inoculated strains of Listeria innocua and L. monocytogenes on minimally processed lettuce were studied. The effe cts of package atmospheres (lettuce sealed within packages after flush ing with 100% N-2 or without flushing with N-2, lettuce sealed within perforated packages), antimicrobial dips (100 p.p.m. chlorine solution for 5 min, 1% citric acid solution for 5 min) and storage temperature s (3 degrees C and 8 degrees C) were investigated. Populations of L. i nnocua and L. monocytogenes on undipped lettuce stored at 3 degrees C gradually decreased (by 1-1.5 log cycles) during a 14 day storage peri od. By contrast counts on lettuce stored at 8 degrees C did not change significantly (P > 0.05). Flushing packages of lettuce with 100% N-2 followed by storage at 8 degrees C resulted in a significant increase (P < 0.05, by 2-3 log cycles) in L. innocua and L. monocytogenes count s during storage. L. innocua, strain NCTC 11288, behaviour was similar to that of L. monocytogenes (strains ATCC, 19114 and NCTC 11994) unde r these storage temperatures and atmospheres. Using L. innocua as a mo del for L. monocytogenes, it was found that dipping lettuce in a chlor ine or citric acid solution followed by storage at 8 degrees C resulte d in a significant increase (P < 0.05, by 2 log cycles) in L. innocua populations compared with undipped samples. It is concluded that N-2 f lushing or use of antimicrobial dips combined with storage at 8 degree s C, both enhanced the survival and growth of Listeria populations on shredded lettuce.