Fate of Listeria monocytogenes serotypes on frozen mixed vegetables during consumer-simulated thawing and storage

Authors: SALAZAR, JOELLE - Us Food & Drug Administration (FDA); PHELPS, JASMIN - Us Food & Drug Administration (FDA); FAY, MEGAN - Us Food & Drug Administration (FDA); KHOUJA, BASHAYER - Us Food & Drug Administration (FDA); ZHOU, XINYI - Illinois Institute Of Technology; STEWART, DIANA - Us Food & Drug Administration (FDA); Juneja, Vijay; DATTA, ATIN - Us Food & Drug Administration (FDA)

Submitted to: Journal of Food Safety
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/12/2025
Publication Date: 5/21/2025
Citation: Salazar, J.K., Phelps, J., Fay, M.L., Khouja, B.A., Zhou, X., Stewart, D.S., Juneja, V.K., Datta, A.R. 2025. Fate of Listeria monocytogenes serotypes on frozen mixed vegetables during consumer-simulated thawing and storage. Journal of Food Safety. https://doi.org/10.1111/jfs.70022.
DOI: https://doi.org/10.1111/jfs.70022

Interpretive Summary: Listeria monocytogenes is capable of surviving for months or even years on frozen food products. Food poisoning outbreaks linked to frozen vegetables have occurred in the United States. We evaluated the survival and growth of the pathogen on frozen vegetables once thawed and held at different temperatures simulating consumer handling and storage. The results indicate that thawed mixed vegetables are an environment conducive to the growth of L. monocytogenes. Results from this study will aid to fill a data gap associated with L. monocytogenes survival on thawed vegetables and aim to provide science-based guidance on the safe handling and storage of these products.

Technical Abstract: Recent outbreaks and recalls associated with frozen vegetables in the United States and Europe have been linked to Listeria monocytogenes. Although frozen vegetables do not support the growth of L. monocytogenes while in the frozen state, the pathogen can survive on these products for long periods of time. Evidence suggests that consumers may thaw frozen vegetables and consume them without cooking. This study aims to understand the extent to which frozen vegetables support the growth of L. monocytogenes once thawed and held at different temperatures. Six L. monocytogenes strains were used in this study, two of each from serotypes 1/2a, 1/2b, and 4b. Individual strains were inoculated onto frozen vegetables and stored at -18C for 7 d to acclimate the cells to the environment. After 7 d, the vegetables were thawed and stored at 5 or 10C for up to 14 d or at 25C for up to 7 d. L. monocytogenes was enumerated from the thawed vegetables throughout the storage period. Population data were fitted to the primary Baranyi model to estimate growth rates and lag phase durations; the secondary Ratkowsky square root model was used to model the relationship of the growth rates with storage temperature. Five of the L. monocytogenes strains survived and grew on the thawed vegetables (population increases of more than 1 log CFU/g) stored at 5C and all six of the strains proliferated at 10 and 25C (population increases of more than 3 log CFU/g after 14 d and more than 4 log CFU/g after 7 d, respectively). A secondary model was successfully generated based on the growth rates of the six L. monocytogenes strains on the thawed vegetables (r2=0.8888, RMSE=0.2057). Results from this study fill a data gap associated with L. monocytogenes survival on thawed vegetables and can be used to determine safe handling and storage practices for these products to protect public health.