Environmental monitoring of a freezing tunnel and its close surroundings in the potato processing industry : insights into Listeria monocytogenes contamination, cleaning and disinfection efficacy, and transmission risks

Produce is a known vehicle for Listeria monocytogenes, with the number of produce-related outbreaks increasing. Consumers occasionally misuse frozen vegetables as ready-to-eat instead of ready-to-cook, raising the possibility of L. monocytogenes contamination. Environmental monitoring studies focusing on the freezing step in frozen produce are relatively scarce, yet essential, as this step could cause post-contamination. In this study, conducted between February 2021 and December 2021, a total of 607 samples were collected and analyzed for the presence of L. monocytogenes and surrounding microflora (sulfite-reducing Clostridia, Escherichia coli, coliforms, Pseudomonas spp., lactic acid bacteria, thermophiles, and anaerobic spores). These samples were taken after heat treatment, within and around a freezing tunnel in a frozen potato processing facility. Overall, 33.7% (n = 486) of the fixed infrastructure (food contact surfaces, non-food contact surfaces, and surroundings) and 48.8% (n = 121) of the mobile elements (e.g., staff, cleaning utensils, and transport trolleys) tested positive for L. monocytogenes. The cleaning and disinfection (C&D) could reduce L. monocytogenes to undetectable levels, but its execution proved inconsistent. C&D reduced the positive samples by 26.2% (n = 122) inside the freezing tunnel and by 30.9% (n = 184) outside the freezing tunnel. The main areas of concern proved to be the conveyor belt area for post-contamination risk and the staff as a transmission vehicle with cell counts up to 4.8 log CFU normalized over a surface of 50 cm2. During production, L. monocytogenes positive samples increased in the production environment outside the freezing tunnel going from 27.7% (n = 65) to 46.7% (n = 76) on the fixed elements and from 31.8% (n = 22) to 75.0% (n = 24) on mobile elements. Areas prone to L. monocytogenes contamination contained higher cell counts of coliforms, sulfite-reducing Clostridia, and lactic acid bacteria. Monitoring the presence of L. monocytogenes assists in enhancing the overall hygiene within the processing plant by discovering hard-to-reach places susceptible to contamination, potential transmission routes, and the verification of the C&D procedure. It demonstrates how environmental monitoring can contribute to the overall hygiene of the frozen produce industry.

Publication year:2024

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