September/October 2021

What’s your food safety nightmare?
By Drew McDonald, Center for Produce Safety

What fresh produce food-safety topics keep industry leaders awake at night? A pair of recent Center for Food Safety webinars examined two prominent risks.

Matt Miles, process engineer with apple grower/packer Allan Bros., revealed one of his nightmares during a Center for Produce Safety (CPS) Research Symposium webinar on June 22: “VBNCs. Even our negative (pathogen) tests might not actually be negatives.”

Drew McDonald, Center for Produce Safety and Taylor Fresh Foods

Miles and other industry leaders joined me to share how we’ve applied learnings from CPS’s produce-specific research to our businesses’ food safety programs. He teed up a presentation during that webinar by Spain’s Ana Allende, Ph.D. Her team studied whether exposure to common industry wash water sanitizers can cause pathogenic bacteria to become “viable but nonculturable” (VBNC)— alive but unable to grow, so they can’t be detected by culture tests, but can later resuscitate to pose a food safety threat.

Sanitizers vs. VBNCs

“We know this is quite a controversial topic,” Allende said. “We wanted to help answer industry’s questions.”

Allende explained that pathogens, including Listeria monocytogenes and E. coli, can become VBNC when stressed — such as being exposed to sanitizers used in fresh produce operations’ wash water. Her CPS-funded research shows that common sanitizers, including sodium hypochlorite, chlorine dioxide and peroxyacetic acid, can stress pathogens into a VBNC state. Further, Allende found that VBNC pathogens can attach to produce during washing. They can then not only survive on produce in storage, but in 20% of trials they recovered to a viable, culturable state when stored at even a chilly 7 degrees Celsius (23 Fahrenheit), presenting a potential food safety concern.

“Is the antimicrobial effectiveness of commercial sanitizers in produce wash water overestimated by conventional plate-count tests? Under the conditions we examined, the efficacy of some sanitizers can be overestimated,” Allende told attendees.

Specifically, for each of the sanitizers tested, Allende found that whether inoculated pathogens are inactivated, or VBNCs are induced, is directly related to the dose level of the sanitizer, and how long the sanitized water is in contact with the pathogen-inoculated fresh produce being washed.

Click to enlarge

For example, she found that pathogens aren’t completely inactivated and some level of VBNC is induced at sodium hypochlorite (chlorine) levels below 5 ppm; she noted a dose of 10-20 ppm is needed to successfully kill pathogens.

Critically, no dose of peroxyacetic acid tested — up to the U.S.-allowed maximum of 80 ppm — completely inactivates pathogens, and some level of VBNC is induced.

“Are foodborne pathogens in a VBNC state a safety concern?” she continued. “Based on these results, that is a low probability… More research is needed to establish critical limits” for abusive storage conditions, she added.

Mitigating listeria

Allende’s research put a spotlight on Martin Wiedmann’s, Ph.D., report at CPS’s June 29 symposium webinar. While to date research has largely focused on preventing listeria contamination, Wiedmann and his Cornell University team studied cleaning and sanitizing to mitigate listeria once it is found in produce packing and processing facilities.

Wiedmann’s team first reviewed 1,600-plus studies on listeria and fresh produce. Armed with those learnings, they then sought to identify likely locations of both persistent and persistently transient listeria in several packinghouses, taking 1,000-plus swabs at each. They also tested ways to mitigate listeria on-site — using a computer model, because introducing pathogens into real environments is, as we all know, risky.

Wiedmann reported he was surprised at some of the places where listeria was persistent – including forklifts, forklift stops, even stickers on equipment. Other common areas included catch pans, wax area equipment and junctures where floors meet walls. He noted deep cleaning alone is not sufficient.
“Breaking down equipment, and identifying hard to reach areas, is more important than just throwing different types of sanitizers at the problem,” he said.

Allan Bros.’ Miles stressed the value he’s taken away from the experiences of and conversations around CPS research events. To him, “having the ability to attend a symposium and speak with principal investigators are the most important parts of being involved with CPS,” even virtually.

My experience is that CPS-funded researchers are indeed incredibly responsive when industry contacts them — just ask.

For more news from this year’s CPS Research Symposium — including webinar recordings, and written key learnings — visit the Events section of CPS’s website at

— Drew McDonald is senior vice president of quality and food safety for Taylor Fresh Foods, and the longtime chair of Center for Produce Safety’s Technical Committee. This group of industry volunteers provide the scrutiny and controls needed to ensure that the research CPS funds is practical, and translatable to the real world.


Be sure to check out our other specialty agriculture brands

Organic Grower