Inoculum dynamics and leaf surface variability as key variables of Listeria monocytogenes growth on fresh produce

Ensuring the quality and safety of fresh fruits and vegetables is important, not only for food waste minimization but also for building consumer trust in ready-to-eat (RTE) products. Consumers need reassurance that fresh produce do not pose any significant human risk upon consumption and ongoing efforts are being made by food businesses to deliver RTE products which are free of microbial contamination. Pathogenic organisms such as L. monocytogenes can persist in horticultural environments, potentially proliferating and contaminating the end product. This study examined the potential of three food commodities commonly grown in Ireland- strawberries, butterhead lettuce and baby leaf spinach, to support the growth of L. monocytogenes at two different storage temperatures (4 °C and 10 °C) specifically following the EURL Lm protocol for challenge testing. The data was then analysed to identify sources of variability affecting the pathogen's maximum growth rate. Results showed that L. monocytogenes was able to grow on butterhead lettuce and baby leaf spinach at 10 °C, while strawberries did not support its growth at this temperature. None of the tested commodities supported growth at 4 °C. Bayesian analysis of the Baranyi model highlighted inoculum variability as the primary growth determinant, while leaf surface property variations played a secondary role. These findings demonstrate the potential of certain horticultural products to support the growth of L. monocytogenes under common storage conditions, highlighting the importance of appropriate growing, handling and storage protocols to reduce contamination of RTE foods wherever possible, and directly impacting the food business operators’ responsibility to adhere to new EU regulations.