Researchers look at reducing Salmonella risks for bulb onions

Research funded by the Center for Produce Safety (CPS) is hoping to identify production practices that may contribute to Salmonella contamination of bulb onions and steps growers can take to reduce food safety risks.

Salmonella on onions is a growing concern. This past year a Salmonella Oranienburg outbreak linked to whole, fresh onions imported from Mexico sickened more than 1,000 consumers. The outbreak came just a year after more than 1,000 consumers were sickened by Salmonella Newport from red onions.

Production practices impact on dry bulb onion safety
The first project, titled “Assessing the potential for production practices to impact dry bulb onion safety,” is led by Joy Waite-Cusic, Ph.D., with Oregon State University. 

“We’re just trying to figure out what possibly occurred so we’re able to understand the risks,” Waite-Cusic said, referring to Salmonella outbreaks in 2020 and 2021 linked to bulb onions. 

This year, the researchers conducted onion field trials in Pasco, WA, and Ontario, OR, using a three-strain E. coli cocktail as a Salmonella surrogate. These sites were chosen to represent diverse climatic conditions.

In their Washington trials, which used a yellow onion variety, they made the season’s last overhead irrigation application using E. coli-contaminated water. In the Oregon trials, using red and white onion varieties, the researchers used E. coli­-contaminated water to mix the season’s last pesticide or clay sunscreen application.

In both trials, the researchers monitored pathogen levels after application as the onions matured and cured in the field for 30 days.

“Spoiler alert,” Waite-Cusic said, “We just collected the last samples, and everything died. Over both field trials, only one onion out of 440 tested at the end of field curing was still positive for E. coli.” Field curing did a great job of mitigating contamination that happened in the water applications, she said.

“This doesn’t tell us the answer to the outbreaks, but it’s great news for the industry and food safety.”

This winter, the researchers plan to conduct laboratory dye studies to see if water applied to the leaves of younger plants can move into the tissue and possibly pose a contamination entry route.

More about the study can be found here.

Strategic mitigation
The second project, titled “Strategic approaches to mitigate Salmonella contamination of bulb onions,” is led by Vijay Joshi, Ph.D., with Texas A&M AgriLife Research. 

Joshi’s project seeks to better understand how Salmonella colonizes and enters onion bulbs. He also plans to identify production practices that may reduce plant susceptibility. 

The project proposes developing an onion-specific risk reduction plan by investigating Salmonella’s survival and growth on onion bulbs using different genetic backgrounds, nutritional compositions influenced by agronomic practices, seasons, and managed stress environments.

“Unlocking the genetic potential of plants to combat Salmonella is a novel avenue in improving food safety,” he said. 

Additionally, to understand the potential of onions as a transient host for transmission, the researchers investigated the effects of genetic, nutritional, and physiochemical characteristics on Salmonella survival and growth.

The researchers hope to provide the industry with tools to identify factors in onion production and supply chain that may allow Salmonella to persist, identify varieties and agronomic practices that would minimize or eliminate its persistence, and help the produce industry in developing guidelines to manage risks of Salmonella in onions effectively.

The researchers plan to conduct field trials to determine how bulb quality traits — including nitrogen content, moisture level, and macro- and micro-elements — influence Salmonella internalization.

Their goal is to develop recommendations on onion crop management, harvesting, curing, storage, and distribution that will help the industry minimize the risk of Salmonella contamination.

More about this study can be found here.

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 fresh produce.

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