Clean greens

Photo © Adobestock

Leafy greens are a go-to crop in CEA production, and their popularity among growers is only increasing. But when it comes to public safety concerns, products like lettuce and spinach have been getting a bad reputation, finding themselves at the center of some high-profile food safety recalls.

In 2018, for instance, the Centers for Disease Control and Prevention issued a recall of romaine lettuce just before Thanksgiving, suggesting that consumers should not trust any green salad on the holiday table. But while reports of leafy greens recalls make for good sensational local news coverage, the concerns aren’t necessarily overblown. An April 2024 report, “Foodborne Illnesses from Leafy Greens in the United States: Attribution, Burden, and Cost,” by researchers from The Ohio State University found that leafy greens accounted for 9.2% of known pathogen-caused foodborne illnesses, affecting some 2.3 million people with an annual cost of $5.28 billion.

But it’s important to put CEA-grown leafy greens’ potential share of those illnesses into context. According to USDA statistics, about 90% of leafy greens crops are grown outdoors. And according to a January 2024 Economic Research Service report from the agency, lettuce only accounted for about 6% of the total crop area in CEA production (in 2019). In other words, the consumer risk from foodborne illness caused by CEA-grown leafy greens is vanishingly small — though certainly not zero. The fact is that the way leafy greens are produced in controlled environments comes with a unique set of concerns that can’t, and shouldn’t, be ignored.

“They are definitely not inherently more susceptible by any means,” says Kristen Gibson, director at the Center for Food Safety at the University of Arkansas. “But their risk, and where the risks are coming from, looks different than conventional outdoor production of produce.”

Food production or food processing?

Gibson’s research is predominantly focused on food safety in CEA, particularly around leafy greens. She notes that the industry’s food safety challenges are largely due to the unique space leafy greens occupy somewhere between food processing and food production. Unlike traditional outdoor growers, leafy greens are produced using hydroponics systems that are as much factory as farm. Deep water culture and nutrient film technique production methods are just two of the more popular ways to produce greens, and both are a bit more sophisticated than standard soil-based outdoor agriculture. And these production processes introduce more points for pathogens to come into contact with the product.

“Growing in this built environment is leaning more toward that traditional food processing side of things,” Gibson says. “It’s not as easy to figure out exactly what needs to be focused on.”

From seed to harvest, growers need to be aware of these vulnerable places in their operations, Gibson says. But that hasn’t always been the case. She notes that in the past, many companies put their plant pathologist in charge of food safety. But plant health doesn’t always translate to human health. There are plenty of pathogens that will not affect the health of a plant but will wreak havoc on the human body.

Fortunately, the paradigm is shifting, according to Gibson. “Since my time working with the industry, I think the thought process has changed. Who they have in place in their companies has changed,” she says. “Companies have been putting the right people in place: pulling from the food industry, pulling from processing and hiring people able to implement some of the measures that need to happen.”

Protection from pathogens

Having the right people in place to monitor the safety of crops is just one crucial area for CEA growers to focus. But it’s not the only measure that will help ensure consumer health and boost confidence in the industry. Gibson notes that her research and the growers that she works with are focusing on a few key areas.

Substrate: While the majority of indoor leafy greens crops are grown without substrate in pure hydroponic systems, propagation still occurs in substrate. But contaminated substrate can introduce pathogens to the system at the start.

“Depending on the supplier and how they’re stored, substrate can be susceptible to contamination,” Gibson says. “Not only with human pathogens but with plant pathogens as well.”

That makes careful storage, monitoring and handling of propagation and even growing substrates a first step in keeping production systems protected.

Surfaces: Unlike outdoor growing, when leafy greens are transferred into hydroponics systems to grow, their leaves come into contact with the plastic or polystyrene surfaces that support the plants during production. If the surfaces aren’t properly sanitized between growing cycles, it’s possible that pathogens can proliferate and transfer to leaves. If that happens, it’s possible that processing areas, even if they are well away from growing areas, can become contaminated as well.

“Environmental pathogens like listeria can grow in little niches or reservoirs. It’s not going to happen overnight, but it could happen over a long period of time,” Gibson says. “This is similar to what you would find in the traditional food processing environment. How to clean and sanitize a production environment remains an interesting conversation.”

Water: The biggest concern for food safety in both CEA and traditional farming is the health and quality of the water. The post-outbreak report from the fall 2018 incident, for instance, found the spread of the pathogen started at an outdoor pond where water was sourced. It’s enough of a concern that the USDA has strengthened rules around how and when growers need to monitor agricultural water.

Things get a bit tricky with water in CEA, Gibson says. There is some debate on whether the “nutrient solution” used in hydroponics even qualifies as agricultural water. Regardless, any wet area can be a breeding ground for pathogens. That means growers are looking to manage their nutrient solution in ways that keep the nutrients while maintaining safety. These techniques include diligent monitoring, filtering, UV light treatment or even chemical intervention, such as introducing ozone into the system.

The future of safety

Gibson notes that food safety in CEA is an ongoing and robust conversation. There are no real hard and fast rules for what should be done. Part of that is because every enclosed agricultural system is different. Some are older greenhouses retrofit for indoor food production. Some are brand-new, multi-million-dollar automated builds. And even within those categories, processes and product flow are unique and diverse. This creates obvious challenges for finding any sort of one-size-fits-all solution. Nevertheless, there are innovations on the horizon that could become part of the fabric of CEA food safety.

Some of the more exciting developments are centered around the materials used in building hydroponics systems, including ways to make them easier to sanitize and more sustainable. There is also work being done to address substrate health. Some of that experimentation is centered on creating substrates that actively inhibit pathogen growth.

Many of these efforts are focused on the microbiome. “There’s a lot of interest in understanding that microbial community and how we can manipulate it and utilize it,” Gibson says. “Getting more information to tell if there’s imbalance based on microbial markers and trying to get more understanding of what micro-communities look like and how they indicate plant pathogens or human pathogens.”

Whatever the future holds, those in the business of CEA are taking food safety seriously, Gibson says. “I do think they’re listening,” she says. “I think they’re trying to respond. But then there’s this idea of over-response and sending too much, testing a lot of product when it’s not necessary. I think there was a little bit of an under response, and now there’s an over-response. This isn’t a bad thing, but there are still a lot of questions.”

Patrick Alan Coleman is editor of Produce Grower magazine. Contact him at pcoleman@gie.net.