As the phrase goes — you can’t manage what you don’t measure. But in that same vein, how you measure something matters. When it comes to pathogen contamination in leafy greens, it really matters.
That was a key take away at the most recent Center for Produce Safety webinar, held Oct. 6. During the webinar, researchers presented the findings of a study into the food safety of leafy greens grown in different controlled environment agricultural systems. The study was partially funded by CPS.
“We were trying to identify if CEA systems are safer than open fields, and we can conclude that they are not inherently safe,” says Ana Allende, from the University of Spain’s Center for Soil Science and Applied Biology and the study’s principal investigator.
“What we can do to make it safer is not magic,” she continues. “We recommend implementation of an environmental monitoring program. It will be essential for early detection and prevention of any microbiological contamination.”
About the study
According to Allende, there is a perception among consumers that CEA-grown produce is inherently safer. However, illness as a result of CEA-grown leafy greens contaminated with salmonella has happened, and listeria is a pressing food safety concern for field-grown leafy greens. Allende says there was a knowledge gap in the CEA industry regarding risk for listeria that this study, in part, tried to address.
The research was conducted at commercial CEA growing facilities in Spain, though the facilities selected mirrored U.S. and Canadian growing practices. The study looked at three different types of CEA growing conditions — soil-based, hydroponic and substrate-based systems — growing a variety of leafy greens for listeria and salmonella contamination.
Researchers also looked at persistence of pathogens in the environment and on common surfaces such as harvesting crates after intentional inoculation. They also looked at the efficacy of crate washing systems for removing contamination.
The study’s main findings included:
- Water is a primary contamination vector: Salmonella was found in the substrate-based system’s irrigation water and nutrient solutions, and listeria was found in the hydroponic system’s drainage water.
- Soil or growth media can serve as pathogen reservoirs: Listeria was found in the soil-based and hydroponic systems, especially on workers’ boots.
- Normal work in CEA systems can spread pathogens: Workers’ boots and cart wheels inoculated with listeria could spread the pathogen to other parts of the CEA facility, though persistence after 24 hours was low, particularly for cart wheels.
- Dirty harvest crates can harbor pathogens: Insufficient cleaning of harvest crates that allows organic matter to remain can protect pathogens.
One of the most surprising findings according to Allende came from the portion of the study that looked at the cleaning of the harvesting crates.
The study collected swabbed samples from harvesting crates before and after being run through a commercial tunnel washing system. While the researchers didn’t find listeria on either the dirty or clean crates, they did find other bacteria. A lot of them. The study reported that “the total bacterial levels were very high” and washing did almost nothing to change the total bacterial load on the crates.
“I must say that the experiment dealing with the crates was really surprising because the growers were using these automatic systems, which seemed to be very advanced, but they are failing in the most basic task,” Allende says.
Key takeaways for growers
The researchers had a few key recommendations to CEA leafy green growers that came out of the study. The primary one was to have a rigorous environmental monitoring program that focuses on key contamination vectors, or hotspots as Allende calls them.
“We identified soil and floor surfaces as a hotspot, as well as the reusable plastic crates. Also, the water is a key risk factor for contamination, and of course the recirculating nutrient solution that is used in the hydroponic and the substrate-based systems.”
When it came to monitoring water, which the study identified as the primary contamination vector, Allende stresses you cannot do initial tests and then assume the water will stay safe.
“You have to check as the days go on,” she says.
Mabel Gil, a co-principal investigator on the study and also from the University of Spain, adds that the volume of water sampling sizes is important. The bigger the better.
“For irrigation water, our growers were surprised about the results because the labs normally have a very small sample size like 100 milliliters or something similar,” she says. “In our case it was 10 liters. When you increase the sample size, you have more possibility of finding positives.”
When it came to the apparent failure of the automatic washing systems for reducing bacterial load on the harvesting crates, Allende says there are a lot of potential advantages to automation. However, she says growers must both implement the systems properly and validate their efficacy.
Ultimately, however, both researchers stress the need to monitor for contamination and to monitor properly based on the risk factors of growers’ specific facilities and growing systems.
“If you don’t want to find, you will not find, so you really have to look,” Allende says. “If you don’t use the more sensitive sampling methods, you probably will think that you don’t have any positives in your system. But you have to look carefully to really detect the contamination points.”
