New research funded by the Center for Produce Safety (CPS) has found that Listeria monocytogenes contamination in produce processing facilities is most likely to occur in non-contact areas such as drains, floors, and ceilings, rather than in direct contact areas like knives and conveyor belts. 

Ana Allende, Ph.D., and her team from the CEBAS-CSIC research institute in Spain conducted a two-year project to examine Listeria monocytogenes contamination patterns and related sanitation programs in three produce processing facilities and identify the main contamination points.

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 non-contact surfaces that could potentially lead to contaminating zones 1 and 2. 

They conducted a systematic sampling of the facilities at the end of the day before cleaning and sanitizing, as well as after cleaning and disinfection activities.

The researchers collected more than 600 samples from the three zones and 45 samples from raw ingredients and end products. The team then used whole genome sequencing (WGS) on 100 samples to understand whether the Listeria was transient or persistent. They found that the same two serotypes of Listeria monocytogenes were present on the three processing lines after the two samplings, before and after cleaning, suggesting that these serotypes were inherent and were moving from zone 3 to zone 1.

“We started to become interested in the role of environmental contamination following years of attending the CPS Symposium where some researchers, such as Dr. Martin Wiedmann and Dr. Laura Strawn, focused on Listeria control in produce packinghouses and processing facilities,” said Allende. “We're trying to bring our experiences from another point of view. The facilities we're able to sample here could also help us understand the significance of this problem.”

In addition, the researchers evaluated the efficacy of biocides against resident Lm isolates and found that all of the isolates obtained from the environment after cleaning were sensitive to the biocides, allaying concerns that the pathogens were becoming resistant to the sanitizers.

Allende said that the research aimed to provide relevant results for the three cooperating produce processors, but it also has broader implications for the produce industry about how they should conduct environmental monitoring, including sampling after processing just before cleaning. The findings should help processors better understand the main contamination points in Zone 1 and how they relate to identical or similar Listeria monocytogenes sequence types in Zones 2 and 3.

“One of the hypotheses we had was the raw material was introducing much of the Listeria,” said Allende. “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.”

While several studies have examined Listeria monocytogenes prevalence in dairy and meat processing facilities, few have looked at Listeria monocytogenes contamination patterns and related sanitation programs in produce processing facilities. The research project conducted by Allende and her team is designed to yield practical data about produce facilities’ environmental monitoring plans, as well as the efficacy of sanitation programs.

The U.S. Food and Drug Administration has a zero-tolerance policy for Listeria monocytogenes in processed produce samples, while the European Commission has set a threshold of up to 100 colony-forming units per gram.