Project looks at adding antimicrobials to stone fruit coatings

Stone fruit are considered relatively low risk for foodborne pathogens because they grow off-ground in trees and undergo a good portion of packing under dry conditions.

That prompted Qixin Zhong, Ph.D., with the University of Tennessee, to consider whether enhancing the wax and fungicidal coatings typically applied to stone fruit with food-grade antimicrobials might reduce foodborne pathogen risk while extending shelf life.

“Stone fruit generally have low risk of contamination by pathogens, but there have been multi-state outbreaks and recalls due to pathogens,” he said. “So we need to enhance the safety of stone fruit.”

Joining Zhong as co-principal investigator is Thomas Denes, Ph.D., also with UT. “Dr. Denes is a microbiologist, so his expertise is instrumental to this project,” said Zhong, who focuses on food ingredient science and technology.

New this year with CPS-funded projects are Industry Advisory Councils, one of which is paired with each research group. As the name implies, the councils meet periodically with researchers to discuss their progress and provide guidance from an industry perspective. “It’s an excellent mechanism and provides much support to the project,” Zhong said.

Packinghouses typically apply an edible wax coating to stone fruit to slow moisture loss or dehydration during storage and extend shelf life. A regulatory-approved fungicide is usually added to help control native yeasts, molds, and fungi found on stone fruit. While this mixture addresses the respective targets, it isn’t effective against Salmonella enterica and Listeria monocytogenes, Zhong said.

Based on literature reports of food-grade antimicrobials — some of which may not have not been used commercially for fresh produce — the researchers chose several candidates. They first examined each antimicrobial's physical and chemical properties, such as water solubility, and how they might affect compatibility with the coatings. They then screened the antimicrobials individually and in combination for their efficacy in controlling Salmonella enterica and Listeria monocytogenes under laboratory conditions. The theory was that combining antimicrobials might result in synergies with greater results than the sum of their individual effects.

The researchers also looked at how adjusting pH might enhance the antimicrobials. They identified the ones that performed the best and then examined how they worked when added to the coatings.

The second part of the project, which the researchers started during the 2023 summer, was to screen the performance of antimicrobial-enhanced coatings on peaches. Thanks to Prima® Wawona and the California Fresh Fruit Association (CFFA), Zhong received overnight shipments of freshly picked peaches that had yet to run through a packinghouse. “Without the industry support, I don’t know where we’d get the peaches,” he said.

The researchers washed and brushed the fruit to simulate a commercial packinghouse. Then, they inoculated the peaches with one of the two pathogens and applied a coating. They also left some of the fruit uncoated as an untreated check.

The peaches were put in a 32-degree Fahrenheit cooler to mimic packinghouse storage. Beginning at day five and running up to day 20, the researchers pulled samples every five days to measure pathogen die-off. They also weighed the fruit to determine moisture loss, along with fungal, mold, and/or yeast growth.

Once the California peach harvest begins, Zhong said they plan to repeat the trial in 2024 to validate their initial results. “We hope this research will give the industry some great information to think about possibly different practices,” he said.

About CPS

The Center for Produce Safety (CPS) is a U.S. tax-exempt, charitable organization focused exclusively on providing the produce industry and government with open access to the actionable information needed to continually enhance the safety of produce.