Vulnerability of Microgreens to Pathogens Characterized by Researchers
One of the varieties of microgreens tested was sunflower. Pictured are eight-day-old sunflower microgreens being grown on a biostrate growing mat – a soilless substrate.
Microgreens, although tiny, are packed with nutrients and flavor. Their use in salads has increased, but researchers have not yet focused on their susceptibility to outbreaks of foodborne illnesses – common in the salad greens industry.
Kristen Gibson, professor of food safety and microbiology for the Arkansas Agricultural Experiment Station and director of the Center of Food Safety, led a team of researchers to understand the possible food safety risks associated with the production of micro -greens.
The results so far show that the pathogenic potential depends on the variety of microgreens and the type of soil alternative.
The Agricultural Experiment Station is the research arm of the Agriculture Division of the U of A System.
Microgreens are “immature seedlings of edible plants,” according to a paper by Gibson and other researchers.
They are sometimes confused with sprouts due to their similar characteristics, but sprouts grow for less time, according to the article. A sprout only has its first set of leaves, the cotyledon, while a microgreen sprouts a second set, the true leaves.
Unlike more mature salads, microgreens are usually grown indoors and don’t use traditional soil. Instead, growers use grow mats, peat with vermiculite, hemp mats or other recycled plant materials, Gibson said.
Microgreens have yet to cause outbreaks, Gibson said. But “over the past three years, there have probably been six to eight microgreens recalls due to the potential for contamination of Salmonella Where Listeria.”
“These recalls are indicative of whether the product may be contaminated, or of the environment in which the product is grown,” Gibson said.
In salad greens, the most recent outbreak of foodborne illness ended on April 4, according to the Food and Drug Administration. Listeria monocytogenes was detected on lettuce harvesting equipment.
Eighteen illnesses have been reported, including 16 requiring hospital care and three resulting in death, according to the FDA.
Former Gibson student Gina Misra, a researcher on this project and owner of a microgreens operation in Delaware, pitched the idea to him after realizing the knowledge gaps in microgreens production, said said Gibson.
“It’s just an environment that hasn’t been characterized,” Gibson said. “Controlled-environment agriculture, in general, has not been characterized.”
Virus persistence test
The researchers used pea sprouts and sunflower microgreens — the most commonly grown varieties according to a nationwide survey by Gibson’s team.
They grew both varieties on peat with vermiculite and grow mats. Half of the soil alternatives contained no plants. They introduced Tulane virus, a norovirus surrogate, into the soil alternatives at different times during the 10-day growth cycle.
“Human noroviruses are the number one cause of foodborne illness,” Gibson said. But they cannot be easily cultured or cultured in the lab, unlike the Tulane virus.
To test the persistence of the virus and its transfer to edible tissues, they added Tulane virus to the zero-day soil alternatives. To test for contamination near harvest, the researchers inoculated the soil alternatives with Tulane virus on the seventh day.
The researchers also tested virus persistence on leaf surfaces by direct inoculation three days before harvest.
Tulane persisted in both soil alternatives, but virus counts increased from 10 million to 100,000 over the 10-day grow cycle.
It’s promising that the virus doesn’t persist at the original experimental level over time, Gibson said. Some aspects of the ground alternatives cause the virus to no longer be infectious.
“When you think of contamination in a production system, you think of low-level contamination,” Gibson said. Maybe 100 or 1000 viruses would be in the system.
“Technically, if you have low level contamination and you have seen this decrease, then perhaps the risk would not be very high of the virus ending up in the final product or becoming contaminated in any way” , Gibson said.
“But the other thing is you don’t need a lot of virus to cause an infection,” Gibson said. It could be 1,000 or even 18 virus particles that cause disease.
Viruses also persisted on edible material after direct inoculation. Pea shoots had about 50,000 more viruses per plant than sunflower microshoots, which had only six viruses per plant.
Although production parameters such as temperature, humidity and lighting can be controlled, “the indoor environment can be a source of pathogens that could pose food safety concerns,” Gibson said.
“Indoor production gives this idea, maybe a misconception, that you can control things better,” Gibson said.
“You can easily get pathogens if you don’t practice cleaning and sanitation practices or good agricultural practices that would be applicable to those environments,” Gibson said.
Personal hygiene is also important as microgreens are a crop with “direct contact handling” during harvest time.
Even before planting the microgreens, it’s important to make sure that supplies like seeds and growing material are from reliable sources, Gibson said. Both of these are at risk of contamination.
“Implementing all of these practices and being vigilant about them is a really good way to ensure your product is as safe as possible,” Gibson said.
Many microgreen growers are on smaller farms, so they might not follow all the farming standards of large farms, Gibson said. Regular testing for contamination may not be feasible for some.
Gibson wants to be able to provide food safety protocols for small microgreen growers more specific to their operations.
Gibson is the principal investigator of a $550,000 United States Department of Agriculture grant from the National Institute of Food and Agriculture to develop microgreens training materials. They didn’t use grant funds for this project, but this research feeds into the overall goal, Gibson said.
Wenjun Deng, who earned his Ph.D. in July 2021, was a researcher on this project. She used funds from a U of A Congress of Graduate and Professional Student Research Grant to support the experiment, Gibson said.
Adam Baker, a postdoctoral fellow, also helped in the lab.
Gibson is exploring sanitizing products for use on edible fabrics and plans to test other varieties of popular microgreens and possibly other microorganisms.
Gibson is also interested in seeing how viruses and bacteria interact with each other in soil alternatives.
To learn more about Agriculture Division research, visit the Arkansas Agricultural Experiment Station website: https://aaes.uada.edu/. Follow us on Twitter at @ArkAgResearch.
For more information about the Cooperative Extension Service, visit https://www.uaex.uada.edu and follow us on Twitter at @AR_Extension.
To learn more about the Division of Agriculture, visit https://uada.edu/. Follow us on Twitter at @AgInArk.
About the Agriculture Division
The mission of the University of Arkansas Division of Agriculture System is to strengthen agriculture, communities, and families by connecting trusted research with the adoption of best practices. Through the Agricultural Experiment Station and the Cooperative Extension Service, the Division of Agriculture conducts research and extension work within the country’s historic land grant education system.
The Division of Agriculture is one of 20 entities in the University of Arkansas system. It has offices in all 75 counties of Arkansas and faculty at five system campuses.
The Agricultural System Division of the University of Arkansas offers all of its extension and research programs and services without regard to race, color, sex, gender identity, sexual orientation, national origin , religion, age, disability, marital or veteran status, genetic information, or other legally protected status, and is an Affirmative Action/Equal Opportunity Employer.