Peach packinghouse trials gauge sanitizer efficacy

September 20, 2023 - Packinghouses routinely run peaches over a spray bar brush bed, applying water to which sanitizer has been added. But Meijun Zhu, Ph.D. with Washington State University, said little data is available about sanitizer efficacy, contact times or treatment durations.

Her research comes as the industry has for some time recognized the challenges with brush beds and the limitations of fruit washing as a pathogen-reduction systems.

A 2020 outbreak associated with Salmonella-contaminated peaches possibly tied to nearby animal agriculture only reinforced the need for additional research to optimize wash systems. To that end, Zhu is leading a two-year CPS-funded research project that combines laboratory studies and packinghouse trials to produce science-based data on sanitizer efficacy on which the industry can base preventive control programs. It is titled “Control of Salmonella and Listeria monocytogenes on peaches through spray-bar brush bed sanitizer intervention.”

“We’re not just doing research in the lab; we’re also going to the facilities,” she said. “That’s another real benefit, gathering the data in test settings so the industry can use it immediately.”

Zhu said her project would only be successful with Steven Pao, Ph.D. with California State University, Fresno, who will collaborate with the in-plant testing. She also praised stone fruit industry representatives and produce food safety consultant George Nikolich for their input and guidance.

“George has been a significant contributor,” Zhu said. “He’s shipping the peaches every week to us, which has been immensely helpful. He makes the connections, offers valuable advice and provides fruit to us from PrimaWawona. Without his help, it would be very difficult.”

In a typical peach packinghouse, the fruit first are washed, which also includes defuzzing. They then move over a brush bed where overhead nozzles spray sanitizer-treated water.

Zhu said she decided to focus on peaches because the application of sanitizers to their pubescent, or fuzzy, skin is much more challenging compared to smooth-skinned stone fruit, such as nectarines, plums, and pluots.

“Some fuzz is removed before the sanitizer spray, but it will probably still be different from nectarines because they have a smooth surface,” she said.

If they identify a sanitizer program that works on peaches, Zhu said the odds are good that it will be equally or more effective on smooth-skinned stone fruit.

She will use peaches sent weekly from commercial California packinghouses to her Pullman, Washington, lab to inoculate them with a specific quantity of bacteria. Zhu will then test the efficacy of the most common sanitizer — chlorine — as well as a few other food-grade antimicrobials. As part of her work, she will also look at different sanitizer concentrations and contact times.

Conducting the same tests on peaches from weekly shipments throughout the season should account for any potential varietal responses, she said. As with many other crops, specific peach varieties mature during different parts of the summer.

In addition, she hopes to validate a nonpathogenic bacterium — Enterococcus faecium NRRL B-2354 — as a surrogate for both Salmonella and Listeria.

The goal is to collect as much lab data as possible this season in preparation for the large-scale trials at three California packinghouses in 2024. This fall, Zhu’s team visited the facilities to scope out variables and to make preliminary measurements to firm up her experimental design.

She said conducting these trials under real-world conditions is critical to account for natural variations in packinghouse operations.



“Every packing facility probably has a different design,” Zhu said. “In the lab, we can control concentrations and contact time, but we’re unable to mimic the processing flow because it’s dynamic. We add some fruit but cannot mimic the flow or cross contamination. This will give us real-world data on different practices, and the data will have broader coverage and be more representative.”

Dr. Zhu will present her final report at the 2024 CPS Research Symposium.

You can learn more about CPS-funded projects here.