Project assays Listeria survival 
on extensive array of produce

Key Take-Aways
* Research results are expected to aid risk assessments and commodity-specific food safety programs.
* Research is examining 14 different whole and fresh-cut produce items.
* Produce pH appears to play a role in Listeria growth.
* Project is focused only on Listeria monocytogenes. However, several other pathogens, including
Salmonella and hemorrhagic E. coli, should be included when developing food safety strategies.
April 9, 2020 -  A multi-faceted project is testing Listeria monocytogenes survival and growth on more than a dozen fresh-cut and whole produce items under normal and temperature abusive storage conditions.

The project, "L. monocytogenes growth potential, kinetics, and factors affecting its persistence on a broad range of fresh produce" is led by Xiangwu Nou, Ph.D., with the U.S. Department of Agriculture's Agricultural Research Service. The project seeks to provide the produce industry with additional information so producers and processors can make better informed risk assessments. The data also should help commodity-specific food safety programs.

"We're not trying to develop production technology for the industry - we want to provide information to them," Nou said.

Xiangwu Nou, PhD, USDA, Agricultural Research Service
He was quick to point out that the research deals only with Listeria; however, several other pathogens, including Salmonella and hemorrhagic E. coli, should be included when developing food safety strategies.

Joining Nou as Co-PI's are Patricia Millner, Ph.D., and Yaguang "Sunny" Luo, Ph.D., also with the USDA ARS Beltsville (Maryland) Agricultural Research Center.  Nou said both Millner and Luo have been invaluable with the experiment's design and interpretation of results. Millner has been an ARS microbiologist for more than 40 years. With an extensive background in food technology, Luo has more than 20 years' experience in food safety research.  "We complement each other with different perspectives," Nou said.

The testing involves inoculating produce items with a known quantity of Listeria, then storing the inoculated items at the temperatures that are typical for the commodities during commercial storage, transportation or retail display for extended times. During that time, researchers remove samples periodically to assay for Listeria as well as background microorganisms. Another test involves storing samples at elevated temperatures to simulate temperature abuse for different numbers of days.

The researchers have collected data about Listeria growth on 11 of 14 commodities selected for the project. Among them are whole or fresh-cut fruits and a few vegetables, which include avocados, blueberries, grapes, mangoes, cantaloupes, pineapples, fresh-cut carrots, and cauliflowers.

"Our purpose is not simply testing the growth of Listeria on a bunch of random products. We want to learn why Listeria tends to grow or not grow on these types of fresh produce," Nou said.

So far, the results show commodity pH plays a critical role in Listeria survival and growth, as all products with low pH show rapid declines in Listeria counts.  However, the outcomes are much more complicated. For example, while both have near-neutral pH, fresh-cut cantaloupe supported Listeria growth at all temperatures tested, but fresh-cut carrots completely prevented Listeria recovery from the cut surface.  In addition, both cantaloupe and carrot juices supported Listeria growth extremely well. The researchers are examining fresh-cut commodities that significantly promote or inhibit pathogen growth to try to identify intrinsic properties, such as nutrient availability and anti-Listerial compound production, responsible for the variations.

"We want to see which ones we might be able to manipulate for Listeria control," he said.
On some items, Listeria populations decreased while populations of background microorganisms increased. But Nou said it's too early to know how to interpret that, although it is something they want to research further with microbiome analyses.

"When we see the decline of Listeria along with an increase in background microflora, we tend to think that is due to competition," he said. "On some other commodities you see Listeria grows well, along with big increases in background microflora."

Input from industry partners has been invaluable, Nou said.  "We might be familiar with one, two, maybe three produce items but not all we proposed to look at," he said. "So we rely on our industry partners to provide the information on how they handle these different commodities at the production and distribution processes. That information is essential for our study design."

Industry partners have shipped fresh produce directly to the researchers, so they can simulate the entire commercial storage and distribution processes. Although laboratory trial scales are small due to the use of food-borne pathogens, the researchers nonetheless try to simulate real-world conditions as closely as possible so that the results are relevant to the industry. 

Project abstract:

Xiangwu Nou, PhD, 
 USDA Agricultural Research Service
CPS minute video with Dr. Nou may be found on the  CPS YouTube channel as well as the Videos page on the CPS website

Learn more about this project and other CPS funded research by attending the 2020 CPS Research Symposium On Tuesday, April 14, CPS will announce a change in the date and format for the annual event. Please check the CPS website for updates.  
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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.   
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