New research continues to demonstrate that typical cleaning doesn’t actually sanitize the reusable plastic containers (RPCs) used to transport fruits, vegetables, poultry and other foods.
In a study publicized earlier this year, Steven Ricke, director of the University of Arkansas Center for Food Safety, and his team grew Salmonella, Listeria monocytogenes, and E. coli O157:H7 on RPC surfaces and then subjected them to cleaning and sanitizing practices typical in the industry. They found that bacterial biofilms could still survive on the surface of RPC material.
The new set of studies that Ricke conducted measured the actual number of Salmonella cells remaining on RPC surfaces after cleaning. At the maximum concentrations the Food and Drug Administration allows for sanitizing food contact surfaces, and at levels much higher, the residual number of Salmonella organisms ranged from 2,700 to 5.1 million after sanitization.
The first two studies evaluated the effectiveness of sodium hypochlorite and peracetic acid each at 200 ppm (parts per million) – the maximum concentrations allowed for each chemical.
It’s hard to say what constitutes a safe number of bacterial cells, but scientists typically look for a 5-log reduction, or a 100,000-fold reduction. The 200 ppm of sodium hypochlorite led to an average 2.73-log reduction per surface, and the 200 ppm of peracetic acid led to an average 2.50-log reduction.
To put it another way, the surfaces consistently exceeded the 1,000-organism limit expected on clean RPCs.
In a third study, Ricke tested 200,000 ppm of sodium hypochlorite, a concentration 1,000 times higher than the maximum. Some surfaces had nearly a 5-log reduction in Salmonella at these levels, but the average was a 3.77-log reduction.
One reason for the persistence of bacterial cells is the strength of biofilms, but another is that the roughness of RPC surfaces give bacteria places to hide.
“When you really look at scanning electron microscope pictures of these surfaces, they’re not as smooth as they appear to the naked eye,” Ricke says. “They look like a lunar landscape with lots of nooks and crannies.”
Both Ricke and Maryann Sanders, a microbiologist and regulatory compliance specialist at Haley & Aldrich, who conducted a third-party review of Ricke’s research for the corrugated packaging industry, recommend that shippers and retailers use single-use containers to eliminate the contamination risk.
While his research was funded by the corrugated packaging industry (a single-use container option), Ricke says that in working for a public institution, he has to “serve the taxpayers” and always has to “follow the science.”
It’s important to consider all of the foods that are transported in reusable plastic containers, Sanders says, because “after they’re used, they’re basically collapsed and placed together in one spot to be taken back to the depot to be cleaned.”
This collection of containers means “there’s a very high probability of cross-contamination,” she explains. A container that transported apples may not harbor any pathogens, but if it’s stacked under one that held raw chicken, pathogens on the second container could transfer. And once a biofilm is introduced, it’s very difficult to remove.
No foodborne illness outbreak has been directly linked to pathogens lingering on a RPC, but tracing an infection all the way back to one container would be impossible.
Sanders says single-use containers are her ideal.
“I’m not going to say that every plastic container that gets reused is contaminated, but I wouldn’t want to take that chance if I was a manufacturer or producer,” she says.
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