A study published in Frontiers in Sustainable Food Systems (May 2023) looked at Listeria monocytogenes (Lm) contamination patterns in three produce processing facilities—one with a cut iceberg lettuce line, one with a cut fruit line and one with a salad bowl line. Lead author Ana Allende, Ph.D., and her team from the CEBAS-CSIC research institute in Spain also tested biocides against resident Lm populations to gauge efficacy and potential loss of sensitivity.

The two-year project was designed to yield practical data about produce facilities’ environmental monitoring plans as well as the efficacy of sanitation programs.

Their first objective was to understand how different factors such as zoning, sanitary design and connectivity affected the probability of contamination in different fresh produce processing facilities. In the case of salad bowls, the ingredients included not only leafy greens and other vegetables but also proteins from meat, fish and cheese, or pastas from different sources.

The researchers divided the processing areas into three zones based on their proximity to contact with the produce. Zone 1 involved areas with direct contact, such as knives and conveyor belts. Zone 2 included surfaces that did not contact food but were in close proximity. Zone 3 included more remote noncontact surfaces, such as drains, floors and ceilings, that could potentially lead to contaminating zones 1 and 2.

The researchers conducted systematic sampling of the facilities at the end of the day before cleaning and sanitizing. They also resampled the three processing lines after the cleaning and disinfection activities. In addition to the more than 600 total samples from the three zones, the researchers collected 45 samples from raw ingredients and end products.

Findings

Regardless of the facility, the highest number of positive Lm samples came from Zone 3. Whole genome sequencing revealed that the same two serotypes of Lm were found on the three processing lines after the two samplings, before and after cleaning.

“This makes us understand that these serotypes are inherent and are moving from zone 3 to zone 1,” said Allende.

When evaluating the efficacy of biocides against resident Lm isolates, “we found, indeed, all of the isolates obtained from the environment after cleaning were sensitive to the biocides,” she said.

While the research aimed to provide relevant results for the three cooperating produce processors, it also has broader implications for the produce industry about how they should conduct environmental monitoring including sampling after processing just before cleaning, Allende said. In addition, it should help processors better understand the main contamination points in zone 1 and how they relate to identical or similar Lm sequence types in zones 2 and 3.

“One of the hypotheses we had was the raw material was introducing much of the Listeria,” she said. “This was before we did sampling and the whole genome sequencing to understand the isolates and that they were not all coming from the raw material. Some of the contamination was probably coming from zone 3 in the different processing facilities.”

Image: Ana Allende, Ph.D.