The researchers observed minimal pathogen growth at 4 °C but found that even modest temperature increases promoted microbial growth, particularly for L. monocytogenes. Dr. Fan said the findings underscore the importance of maintaining proper cold-storage temperatures throughout processing.

Some Produce May Naturally Inhibit Listeria

The study also found that some produce items, particularly carrots, naturally inhibited L. monocytogenes. Populations on carrots declined by 2–3 log colony forming units (CFU) to nearly undetectable levels, although similar reductions were not observed for E. coli or Salmonella. Dr. Fan noted that additional research is needed to better understand the compounds responsible for carrots' anti-Listeria activity and emphasized that processors should continue following established preventive controls for all produce.

Peracetic Acid Could Facilitate Listeria Growth During Storage

Researchers also examined the effects of sanitizer washes before storage. Although peracetic acid (PAA) was more effective than chlorine at reducing initial L. monocytogenes populations on some produce items, PAA-treated samples sometimes exhibited greater Listeria growth during storage. The researchers suggested this may result from PAA reducing competing background microbiota, allowing L. monocytogenes to proliferate more readily.

In a separate experiment, contaminated storage containers transferred pathogens to fresh batches unless they were thoroughly cleaned and sanitized, indicating that simple rinsing was insufficient to eliminate contamination.

Developing Pathogen Growth Predictive Models

Data generated by the project are being used to develop predictive models that estimate pathogen growth based on storage temperature, holding time, sanitizer treatment, and produce type. Bryan Vinyard, Ph.D., USDA-ARS, developed mathematical models from the dataset, and researchers are collaborating with Luyao Ma, Ph.D., of Oregon State University to apply machine learning to improve risk prediction.