Salmonella survives plastic crate washing test, transfers to fresh cucumber

John Mullinder

A scientific study just published in the international peer-review journal, Food Control, poses some serious questions about the sanitation of the re-usable plastic crates (RPCs) sometimes used to distribute fresh produce to retailers.

The study shows how Salmonella can become established on RPCs and survive the typical sanitation cycles that are applied to decontaminate the crates between uses. The surviving Salmonella then transferred to and from fresh produce on the RPC, underscoring the potential for crates to spread the pathogen throughout the supply chain.

Salmonella infection can cause vomiting, diarrhea and dehydration in humans, and can lead to reactive arthritis or even death in susceptible hosts such as the young, old, pregnant or immune-compromised.

Food safety commentators have long suspected that there was a link between ineffective washing and cleaning of crates before their next use, and the transfer of virulent pathogens such as SalmonellaE. coli and Listeria to fresh produce sold at retail. The risk of transferring devastating plant pathogens such as tomato brown rugose fruit virus between farms is also a recognised risk factor for RPCs.

Back in 2013, University of Guelph food scientist, Dr. Keith Warriner, the corresponding author of this latest study, found damaged and visibly dirty crates being re-used in Ontario and Quebec. It was even suggested that some crates were being given a quick hose down and then simply transferred from farm to retailer and then on to another farm, rather than being shipped to the closest wash facility, which is what is meant to happen in a re-use system. A more robust study the following year found worse results, including E. coli on 13% of the crates tested.

Typical industry cleaning procedures didn’t completely sanitize the crates either. Research led by Dr. Steven Ricke at the University of Arkansas showed that Salmonella cells remained on crates after cleaning. Ricke suggested that bacterial biofilms were hiding in the cracks and crevices of the crate’s surface, making it harder for industrial sanitizers to reach them.

Dr’s Siyun Wang (University of British Columbia) and Warriner (University of Guelph) and their associates have now taken this research a step further, sampling more than 160 crates at grower/packer operations in three Canadian provinces (Ontario, Quebec, and British Columbia). Laboratory-based trials were undertaken to simulate the conditions under which Salmonella could persist and even grow on residues left by damaged produce.

The researchers then simulated a typical industrial sanitation cycle (water rinse followed by a caustic wash and then peracetic acid sanitizer shower) to see if the Salmonella survived that treatment. It did, the authors concluding that “if present at sufficient levels, Salmonella can (both) survive sanitation and (then) contaminate subsequent produce batches when crates are redistributed’’ to a new grower.

“These findings, taken in combination with the relatively poor sanitary status of re-usable crates sampled within packer/grower facilities, highlight the potential food safety risks represented by re-using crates.”

A summary of the peer-reviewed study can be found at Food Control – V110. You can get the entire report here.

Please Note: PPEC, which represents the Canadian corrugated box industry on environmental issues, co-funded this University of Guelph project in the interests of getting all the facts on the table. The traditional corrugated box system for the produce industry provides a fresh box for each delivery. The boxes are recycled several times over the course of their lives and meet rigid process control standards in their remanufacture. In a typical mill recycling process, the temperature of the paper sheet reaches 220-240 degrees Fahrenheit, well above 100 degrees Celsius, the boiling point of water and the temperature required for sterilisation. The converting process also involves high temperatures and other hygiene controls. Having a fresh box every time minimizes the potential for undesirable pathogens and bacteria being carried forward to the consumer. A recent independent study of corrugated produce boxes showed that the corrugation process destroys bacteria.

A 2019 peer-reviewed comparative life cycle analysis conducted by Quantis showed that neither corrugated containers nor reusable plastic containers had an advantage in the environmental impact categories studied. Much depended on the commodity being shipped, transport distances, and other variables.

John Mullinder
Executive Director
The Paper & Paperboard Packaging Environmental Council (PPEC)