Effects of gas atmosphere, antimicrobial dip and temperature on the fate of Listeria innocua and Listeria monocytogenes on minimally processed lettuce

Authors


Gillian A. Francis Fax: +353 61 331490. e-mail jillian.francis@ul.ie

Abstract

The growth and survival of inoculated strains of Listeria innocua and L. monocytogenes on minimally processed lettuce were studied. The effects of package atmospheres (lettuce sealed within packages after flushing with 100% N2 or without flushing with N2, 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 temperatures (3°C and 8°C) were investigated. Populations of L. innocua and L. monocytogenes on undipped lettuce stored at 3°C gradually decreased (by 1–1.5 log cycles) during a 14 day storage period. By contrast counts on lettuce stored at 8°C did not change significantly (P > 0.05). Flushing packages of lettuce with 100% N2 followed by storage at 8°C resulted in a significant increase (P < 0.05, by 2–3 log cycles) in L. innocua and L. monocytogenes counts during storage. L. innocua, strain NCTC 11288, behaviour was similar to that of L. monocytogenes (strains ATCC, 19114 and NCTC 11994) under these storage temperatures and atmospheres. Using L. innocua as a model for L. monocytogenes, it was found that dipping lettuce in a chlorine or citric acid solution followed by storage at 8°C resulted in a significant increase (P < 0.05, by 2 log cycles) in L. innocua populations compared with undipped samples. It is concluded that N2 flushing or use of antimicrobial dips combined with storage at 8°C, both enhanced the survival and growth of Listeria populations on shredded lettuce.

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