Two projects look at effects of pear storage on microbes

Two researchers are leading complementary CPS-funded projects that use genomic sequencing technology to examine how storage conditions may influence microbial communities, including Listeria, on whole pears.

With previous research mostly focused on whole apples, they point to a general lack of knowledge about the fate of Listeria monocytogenes on pears in storage. In the end, they hope their results will provide pear growers and packers with science-based information they can use to enhance food safety programs.

Laura Strawn, Ph.D., with Virginia Tech, is leading the project titled “A metagenomic approach to food safety risk mitigation in pears.”

Meijun Zhu, Ph.D., with Washington State University, is heading the project titled “Interaction of resident microbiome and Listeria on pears during cold storage.”

Both are using metagenomic sequencing, where microorganisms residing on the pear’s skin are sampled and their DNA analyzed. Using specialized computer software, they can then sort out and match DNA to that in existing libraries to identify microorganisms. Strawn said metagenomics allows researchers to understand the diversity of both bacterial and fungal species. “So much of our prior knowledge is based upon organisms that we can culture in the lab, which is heavily biased by the methods we use,” she said. “Metagenomics allow us to see a more complete picture of what microorganisms are there and how their populations change throughout storage. We’re so excited to apply metagenomic analyses, as we believe this allows for a more holistic approach to enhancing food safety and quality”.

A metagenomic approach to food safety risk mitigation in pears

Strawn’s project looks at shifts in the microbial community on whole pears beginning in the orchard before harvest through nine months of controlled atmosphere (CA) storage. The research team also has focused on how key microbial players may affect food safety risks during storage.

Joining her as co-investigators are Faith Critzer, Ph.D., and Hendrik C. den Bakker, Ph.D., both with the University of Georgia; and Alexis M. Hamilton, Ph.D., with Virginia Tech.

Strawn and her team are using the green Anjou pear variety, since it has a long storage life. They also are looking at pears stored in bulk bins as well as paper-wrapped individual fruit in cartons.

The researchers first identified culturable microbes, including yeasts, molds and lactic acid bacteria, on whole pears before storage. They then sampled microbial communities on marketable and unmarketable pears under the two different storage regimes during long-term CA storage at three, six and nine months. Using metagenomics, they created a profile to track microbial population shifts over time.

The researchers also wanted to understand how Listeria monocytogenes populations behaved under refrigerated or CA storage over nine months when co-inoculated with a common decay-causing mold species and other microorganisms found on pears stored for six and nine months. Although the results are preliminary, Strawn said, “our research is showing that pears are not a very hospitable environment for Listeria.”

She said a side benefit may be identifying the succession of decay-causing microorganisms. This can help the industry better understand how postharvest storage practices ultimately drive what microbial populations they’re likely to see. “We want to know changes in the microbial profiles so we can figure out why fruit are going bad,” Strawn said.

She praised the CPS industry advisory group for helping ensure the project is relevant to the industry. “I think it’s really enhanced the project,” she said. “They have very insightful comments and different perspectives that have helped the team in our exploration and interpretation of the data.”

Interaction of resident microbiome and Listeria on pears during cold storage

Zhu and her team are investigating potential shifts in resident microbial populations on pear surfaces during long-term CA and cold storage and how that might influence the persistence of foodborne pathogens.

“It’s very important to provide information on the fate and dynamic changes of Listeria die-off because to this moment, we don’t have any data,” Zhu said. “It will definitely help industry food safety plans.”

Joining her as co-investigator is Manoella Mendoza, M.S., with the Washington Tree Fruit Commission.

The trial involves three pear varieties: Bartlett, D’Anjou and Bosc. Zhu said she chose Bartlett because it’s the most popular and a summer variety. Both D’Anjou and Bosc are winter varieties. While Bartlett and D’Anjou have smooth skins, Bosc has rough skin. In addition to conventionally grown pears, they included two varieties of organically produced pears: Bartlett and D’Anjou.

Organic production practices restrict the use of certain fertilizers and chemicals, prohibiting the use of synthetic compounds. As a result, she said the set of microbes found on organic pears may differ from those on conventionally grown fruit. Zhu said the inclusion of organic pears was motivated by the continued growth of this category.

For the study, they inoculated unwaxed pears with a non-pathogenic strain of Listeria in a food-grade laboratory. They also included uninoculated fruit as checks.

With the help of industry cooperators, they transferred the treated fruit to commercial CA and cold storage and removed samples every three weeks over a nine-month period.

Each time, they measured Listeria die-off as well as potential shifts in microbial populations. In the beginning, Listeria populations quickly decreased and then eventually stabilized, Zhu said, citing preliminary results.

They also assessed fruit quality attributes and hope to be able to correlate those results to microbial data. Researchers are still in the process of running metagenomic analyses of the microbial samples they collected. “We’re hoping our microbial sequencing will empower us to identify, based on interactions, potential biological control agents for decay and pathogens,” Zhu said.

Simultaneously in their laboratory, they inoculated pear samples with a three-strain cocktail of L. monocytogenes, and incubated them for 12 weeks under conditions that mirrored industry cold storage. Zhu said they were able to validate the non-pathogenic strain was an appropriate surrogate for the pathogen.

Both project investigators are continuing sample collection throughout 2024 and will undertake metagenomic sequencing and bioinformation analyses in the same year.

About CPS

The Center for Produce Safety (CPS) is a 501(c)(3), 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.