Survival of Listeria monocytogenes on Frozen Vegetables during Long-term Storage at -18 and -10°C

Two recent listeriosis outbreaks have occurred in the United States and Europe due to contaminated individually quick-frozen (IQF) vegetables. While one of the outbreaks was due to frozen vegetables considered ready-to-eat (RTE), the other was linked to frozen corn whose packaging contained cooking instructions and was considered not-ready-to-eat (NRTE). However, consumers may thaw certain frozen vegetables and consume without cooking. Since no data is available on the survivability of L. monocytogenes on IQF vegetables during frozen storage, this study examined the population of six different strains (comprising lineages 1/2a, 1/2b, and 4b) on IQF vegetables during long-term storage. Individual strains were inoculated onto an IQF vegetable mix at 4 log CFU/g and stored at -18 or -10°C for 360 days. Although fluctuations in populations of all strains were observed on the vegetables during storage, no significant differences based on strain, lineages, or temperature were observed. Overall, L. monocytogenes populations were only reduced by up to 0.47 and 0.59 log CFU/g after 360 days at -18 and -10°C, respectively. Results from this study suggest that L. monocytogenes is able to persist on IQF vegetables for extended time periods with no significant reduction in population. Future studies could evaluate the survival and growth of L. monocytogenes on IQF vegetables during thawing and storage.