Knowledge Transfer Task Force

Article 13 - Looking ahead at research needs for fresh-cut produce

July 31, 2020

Related Resources:
Lathrop - The effects of storage conditions and the microbiome of non-traditional salad ingredients on the fate of Listeria monocytogenes
Nitin - Rechargeable antimicrobial and antifouling plastics for improved cleaning and sanitation of plastic bins and totes
Suslow - Resolving postharvest harborage sites of Listeria protects Zone 1 surfaces
Wiedmann - Listeria whole genome sequence data reference sets are needed to allow for improved persistence assessment and source tracking
Zhang - Non-fouling food contact surfaces - prevention of biofilm and surface-mediated cross-contamination
Zhu - Control of Listeria monocytogenes on apple through spray manifold-applied antimicrobial intervention

This article originally appeared in The Packer, and is reprinted here with permission. © 2020 The Packer

When history appears to repeat itself, we must ask ourselves what we can learn.

According to the Food and Drug Administration (FDA), fresh produce has been recalled approximately 40 times since 2017 – mostly fresh-cut, and mostly involving Listeria monocytogenes (Lm).

Consumer demand has been growing for processed salads and pre-cut produce, primarily veggie items, in convenient pack types. So it is in our industry’s best interest to safeguard this demand. As science and technology have evolved that enable us to detect low levels of pathogens, it provides us with the opportunity – and responsibility – to use this knowledge to improve our systems.

What is unique to fresh-cut processing that contributes to episodic foodborne illness outbreaks and recalls?  We can point to several factors – and for some answers, we can look to research from the Center for Produce Safety (CPS), which focuses solely on answering our industry’s food safety questions.

Risk factors

Equipment design is one factor. Older processing equipment can be more difficult to clean and sanitize properly.

"If equipment can’t be completely disassembled for deep cleaning, then those inaccessible harborage sites can allow Lm to gain a strong foothold."

Crop type is another factor. Crops grown on the ground are more likely to be contaminated by pathogens like Lm, which can be soil-borne.

Contaminated raw product can contaminate a facility from end to end, including bins and other containers, postharvest cooling and washing systems, and processing equipment.

The mere fact that produce is processed, versus sold whole, is another factor. 

"When fresh produce is cut, that presents an entry point for pathogens and can easily become a nourishing environment where microbes can flourish, especially in warmer temperatures, higher humidity and pH levels above 4."

And there’s more on temperature: One clear risk factor for fresh produce facilities is that Lm can continue to grow, although more slowly, even at lower temperatures common to our receiving and processing rooms, and in pre-shipping cold storage.  This enables it to outcompete the normal microflora at refrigeration temperatures.

Left unchecked, microbes including Lm can form biofilms that are increasingly difficult to eliminate over time. Once established, Lm can easily take up residence in a facility and be readily spread by human and equipment traffic.

Processing facilities are suited well to Lm residence, as Lm biofilms can survive even when exposed to acidic environments, and to sanitation procedures if they are not done correctly, consistently and in a timely manner.

Putting the research to work

Carter McEntire owns McEntire Produce, a major fresh-cut processor, wholesaler and repacker in the Southeast. He vividly recalls the devastating 2006 North American foodborne illness outbreak traced back to packaged spinach.

When Taylor Farms’ Bruce Taylor publicly advocated for harnessing the power of science to help solve the fresh-cut industry’s food safety problems, McEntire decided to get involved with CPS. He did so shortly after it was formed in 2007, as he saw an opportunity to develop better food safety programs to minimize pathogens throughout the fresh-cut supply chain. 

"Using learnings from CPS research and other best practices that have since emerged, McEntire has built a state-of-the-art processing facility that applies hygienic and sanitary design principles." 

According to McEntire, the company’s primary focus is preventing contamination in the first place by ensuring contracted farmers are complying with food safety regulations and best practices.  

Recognizing that Lm is a major threat to fresh produce safety, CPS has prioritized funding of related research, including research on Lm’s relatives in the listeria family, who serve as indicators for the pathogen.

What we've learned about listeria

For example, CPS has funded work by Cornell University’s Martin Wiedmann, Ph.D., who studies the prevalence and ancestry of listeria in farmland and in cold storage and processing facilities.

Dr. Wiedmann recently reported that from over 1,000 soil samples taken from fresh produce farmland across the country, 11.8% tested positive for Lm specifically and 31% were positive for listeria species in general.  (It should also be noted that there are region-specific strains of listeria.) Understanding this prevalence can help industry members evaluate whether their facilities might have transient or resident Lm contamination.

"CPS-funded research also lends insight into how to reduce Lm risk."

Trevor Suslow, Ph.D.’s 2017-19 research at University of California-Davis on citrus packing sheds concluded that Lm must be controlled by aggressive cleaning and sanitation.  Meijun Zhu, Ph.D., from Washington State University conducted Lm research on apples that showed the efficacy of antimicrobial sanitizers.

Meanwhile, Amanda Lathrop, Ph.D. from California Polytechnic State University, showed how storage temperature conditions affect Lm on several produce items used in fresh-cut salads. 

CPS has also funded research on how to prevent biofilms from forming in the first place.  Nitin Nitin, Ph.D., from the University of California, has just developed rechargeable antimicrobial and antifouling plastics for plastic bins and totes. He is now seeking partners to commercialize that technology. Boce Zhang, Ph.D., from University of Massachusetts studied biofilm prevention and found Dursan coating to be the best application to prevent biofilms, especially on stainless steel surfaces.

Getting proactive

McEntire comments, “Listeria is a real produce safety hot button, and the more scientific research that is done, the better industry is armed to develop best practices.” 

"He strongly believes that a robust environmental monitoring program (EMP) in the processing facility should be deployed as part of an overall prevention program. If Lm is detected, then immediate corrective action should be taken to resolve the contamination."

McEntire recognizes the implications of testing for Lm, since if a company finds a food product is positive for Lm then the company is required to report with FDA’s Reportable Food Registry. That in turn might potentially trigger a recall of any implicated product that has already left the facility. 

This is why FDA and other experts suggest a more comprehensive evaluation of a facility by looking for listeria species, which is an indicator for Lm. Aggressively acting upon a positive for any listeria is protective of public health and does not require notification to FDA.

While some in industry have been reluctant to deploy a robust EMP that tests for listeria in facility zones 1-3, such programs have become more common and EMP strategies have improved significantly.

McEntire pointed to lessons the fresh-cut industry can glean from other industries that have faced food safety challenges. “Much can be learned from the meat industry, for example the Jack-in-Box e-coli outbreak years ago,” he says. “Their industry pulled together and made significant improvements to food safety.” 

A positive precedent

In light of the success of USDA Food Safety and Inspection Service’s listeria program for the ready-to-eat meat industry, in which companies do not need to report initial Zone 1 positives for listeria species as long as corrective actions are effective, FDA aligned their policy for FDA-regulated products including produce. 

"The USDA strategy has improved listeria control in the ready-to-eat meat industry, and with FDA now also embracing this approach, it should have the same result in produce."

The fresh-cut industry can find helpful direction and additional details in FDA’s 2018 “Control of Listeria monocytogenes in Ready-To-Eat Foods: Guidance for Industry

Given the challenges faced with fresh-cut sector, they must remain diligent with food safety best practices and apply CPS and other science to fuel real change in fresh-cut produce safety.