The Center for Produce Safety (CPS) has highlighted important takeaways for industry based on recently concluded research projects, which were presented at the 2024 CPS Research Symposium.

CPS underlined findings from these projects that merit immediate industry action, findings that reinforce current best practices and present opportunities for redoubled efforts, and findings worth monitoring that may have practical value in the future.

Immediately Actionable Findings: Wild Bird Management, Sanitation Best Practices, and E. coli Mitigation and Enrichment

A project that compiled pathogen data for wild birds and characterized wild bird behavior near farming operations found that pathogen prevalence on wild birds is generally low, and that bird feces are not typically found directly on plants. At the same time, larger bird species known to interact with and flock in animal farming environments produce larger feces, increasing pathogen survival, especially in higher humidity. Buffer zones around potential contamination sites are recommended as management tools.

Regarding cleaning and sanitation, various research presented at the 2024 CPS Research Symposium underlined key components of effective programs. These cleaning and sanitation programs must be:

• Considered a universal component of any food safety program, and any equipment used to harvest, pack, process, transport, or hold fresh produce must be cleaned and sanitized
• Made a priority by management, who should ensure that sanitation teams are properly staffed, trained, and equipped, and given enough time to effectively complete the job
• Based on written standard sanitation operating procedures (SSOPs) to ensure consistency and comprehensive training
• Started with top-down scrubbing to loosen dirt, biofilms, and bacteria and prevent recontamination of already cleaned surfaces
• Verified with targeted indicator testing and data analysis
• Based on operation-specific risk assessments considering areas that have been identified as Listeria harborage sites
• Performed on a daily schedule.

A study examining the survivability of Escherichia coli on forward-processed leafy greens demonstrated that, in the absence of temperature abuse, forward processing does not significantly increase the risk of E. coli growth, survival, or physiological changes of concern. At the same time, the findings underlined the critical importance of maintaining strict temperature control across the entire supply chain to mitigate pathogen growth. The study did find slightly higher numbers of viable but not culturable (VBNC) E. coli under simulated forward processing conditions. Again, rigorous temperature management is important to keep VBNC E. coli dormant.

Also related to VBNC E. coli, a separate laboratory simulation project explored the factors contributing to E. coli entering the VBNC state, as well as what the VBNC state means in terms of pathogen virulence, detection, and public health risk. The project showed that, although the VBNC state can impede pathogen detection, increasing the enrichment period for VBNC E. coli to 24 hours dramatically improves detectability. Current commercial enrichment periods are shorter than 24 hours.

Findings Reinforcing Current Best Practices: Mitigating Salmonella Contamination on Onions, and Surface-Specific Sanitation

Two studies focusing on Salmonella contamination of onions identified areas in which producers should check that best practices are being met.

First, field trials in Oregon and Washington affirmed the significant risk of crop contamination through irrigation water, especially near harvest. The trials replicated agricultural practices that possibly contributed to a 2020–2021 foodborne illness outbreak linked to Salmonella-contaminated onions produced in California. After contaminating irrigation water with a Salmonella surrogate and applying it to onions via overhead drip, the researchers found the onions to be contaminated. Although the Salmonella surrogate died off rapidly after irrigation, contamination levels on the onions remained at one percent 28 days after the initial application.

A second set of field trials in Texas inspired by a 2021 multistate salmonellosis outbreak associated with red onions sought to understand how Salmonella colonizes and internalizes in onion bulbs, as well as to identify production practices that may reduce plant susceptibility. An important takeaway from the research was that onion curing is not a pathogen elimination step. Although curing can result in some reduction of Salmonella levels, the efficacy was dependent upon environmental conditions during curing. Die-off was less impactful when curing periods overlapped with periods of rain, cool temperatures, or diminished sunlight.

On another note, one project evaluated the efficacy of commercially available sanitizers against common foodborne pathogens, applied to a variety of materials that make up containers and equipment used by the tree fruit industry. The results showed that, in comparison to harder, smoother materials (i.e., stainless steel and hard plastics), softer, more porous materials (i.e., wood, polyethylene plastics, and nylon) pose a greater sanitation challenge due to harboring microorganisms in niches that help microbes avoid detergents, scrubbing, and penetration by water and sanitizers.

Looking to the Future: Cyclospora Detection and New Cleaning and Sanitation Approaches

Several projects have progressed work toward a reliable detection method for Cyclospora cayetanensis, a protozoan parasite that is uniquely challenging due to the difficulties of culturing it in the laboratory and the complicated microscopy required to confirm it in samples. Specifically, research demonstrated the ability of zero valent iron sand filters to capture Cyclospora oocysts from water, which can be subjected to DNA extraction and genetic sequencing for the development of improved detection methods. Another study combined aptamers, which can bind Cyclospora oocysts, with colorimetric, paper-based rapid detection tests that can be used in-field; however, work is still underway to determine whether the tests are selective only for C. cayetanensis and no other species of Cyclospora.

Other Cyclospora research showed the promise of shotgun metagenomics for simultaneously detecting the Cyclospora and other harmful microorganisms in water samples. Finally, one study keyed into Cyclospora’s chemical fingerprint using micro-balancing, fluorescence, and infrared spectroscopy to distinguish the parasite from a complex microbial mixture, and to identify different lineages of oocysts, which may be useful in traceback investigations during foodborne illness outbreaks.

Regarding cleaning and sanitation, progress was made on two novel approaches using Listeria-specific bacteriophages and superheated dry steam, although more work is required to make the solutions commercially viable.