Growth of Listeria monocytogenes in Ready-to-Eat Foods: Re-enactment of Observed Domestic Storage Practices Implemented by Older Adults

Introduction: Internationally, Listeria monocytogenes has the highest rate of reported foodborne disease hospitalizations (< 95%) and related mortality (< 40%). European incidence has doubled almost exclusively among older adults (≥ 60years) since 2001. Majority of incidence is reported to be sporadic; the domestic kitchen is associated with sporadic foodborne disease. Older adults are more susceptible to foodborne disease, and are also reported to frequently consume ready-to-eat (RTE) foods commonly associated with L. monocytogenes. Consequently, implementation of safe food-storage is of paramount importance to reduce the risks of listeriosis in the home.

Purpose: Determine older adults’ food-storage practices of RTE-foods associated with L. monocytogenes in the domestic kitchen and re-enact such practices in a laboratory to assess L. monocytogenes growth.

Methods: Evaluation of older adults (≥ 60 years) (n = 100) food-storage practices in the home (recording of refrigeration temperatures, observation of food-storage practices and microbiological analysis of food contact surfaces) informed the laboratory re-enactment trials. Identified common storage behaviors of older adults were re-enacted with soft-cheese and RTE meat inoculated with ~3.71 log CFU L. monocytogenes, stored at ≤ 5°C (n = 110),  > 5°C (n = 110) and ambient-temperature (n = 55). Food samples were analyzed every 24 hours for up to 21 days.

Results: Key common practices implemented by older adults included prolonged storage of RTE foods and/or refrigeration temperatures > 5°C. Majority (72%) of older-adults’ domestic refrigerators operated at temperatures > 5°C. Most refrigerators (70%) contained foods associated with L. monocytogenes, 54% of which were reportedly stored beyond UK recommendations (2 days after opening) some up to 21 days. Results indicate L. monocytogenes grew at all re-enactment temperatures. Average generation times revealed L. monocytogenes growth was slower at ≤ 5°C (94 h t^-1) than at > 5°C (21.5 h t^-1) and ambient-temperature (11 h t^-1), suggesting older adults’ prolonged storage of RTE-foods at > 5°C/ambient temperature increased L. monocytogeneslevels, making such foods unsafe for consumption.

Significance: Findings indicate that many older adults do not implement practices that concur with food-safety recommendations required to reduce the risk of L. monocytogenes in the home. Re-enactment findings demonstrated L. monocytogenes growth resulting from such practices which may result in RTE-food becoming unsafe for consumption. Findings may be used to inform risk based, targeted consumer food-safety initiatives for older adults to increase implementation of safe food-storage practices to reduce risk of listeriosis in this consumer group.