Researchers at the Agricultural Research Service and the University of Georgia have identified the primary source of Listeria monocytogenes contamination in commercial chicken cooking plants: incoming raw poultry.
Led by Agricultural Research Service microbiologist Mark E. Berrang, PhD, the researchers tested a newly constructed commercial chicken cooking plant before and after processing began.
“We were able to show that, prior to operation, a properly constructed poultry further processing plant is free of L. monocytogenes,” Dr. Berrang told Food Quality. “But this is a ubiquitous organism that is very common in the natural environment. As processing commenced and the plant started to run, the organism was eventually introduced, which we would expect in any kind of food plant that [handles] raw product.”
For their 21-month study, the researchers sampled the floor drains of the food processing plant. “It is the most sensitive spot to sample in a food plant, because everything is eventually washed down the drain during production and sanitizing shifts,” Dr. Berrang said. “If a plant ever gets Listeria, it will show up in the floor drain.”
What’s interesting is that it didn’t take it very long for a new plant that had no Listeria to become colonized.
Stan Bailey, PhD, director of scientific affairs, bioMérieux
Roughly once a month, the researchers sampled the inside surface of plant drainpipes. They used sterile sponges moistened with a liquid that neutralized the sanitizers used in the processing plant, then cultured the sponges for Listeria. In addition, they swabbed the incoming raw meat, tested the incoming fresh air in the air vent filters, and took samples from the area outside the plant, including dirt and water from creeks and ditches. To determine if the employees were carrying the organism into the plant, Dr. Berrang and colleagues swabbed the floors the employees walked across as they entered the plant.
Within one month, the researchers found Listeria in the drains. By four months, they found the organism was still present after sanitization, indicating that the plant was colonized with a persistent strain, Dr. Berrang said.
J. Stan Bailey, PhD, director of scientific affairs at bioMérieux Inc., was not shocked that the researchers found Listeria in the plant or even that raw chicken was the vector. “It’s been known for quite a few years that raw poultry carries some level of Listeria,” he told Food Quality. “As a matter of fact, I did my PhD work on the subject in the 80s, and I saw at that time [that] 25% to 30% of chicken had Listeria.”
What surprised Dr. Bailey, past president of the International Association for Food Protection, was how quickly the organism appeared. “What’s interesting is that it didn’t take it very long for a new plant that had no Listeria to become colonized.”
To combat the Listeria threat, food-processing plants need a solid sanitation program. “One would have to establish [the program] early on and be diligent with a proper, well thought out, scientifically valid sanitation program, where you use the right sanitizers,” Dr. Bailey said. “You wouldn’t want to use high pressure, because that would run the risk of knocking the bacteria that’s in the drains into the air and getting it on equipment. You’d use low pressure foams and the like.”
Listeria is ubiquitous and will eventually colonize nearly every kind of food plant, according to Dr. Berrang. “And, once in the plant, it can become a persistent, long-term resident,” he said. This does not rule out safe food production, however. “It’s certainly possible with careful, correct and … proper proactive sampling and testing to produce safe, ready-to-eat meat out of these plants.”
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