Keep your produce safe

Features - Cover Story

Here’s what greenhouse growers can do to lower the risk of contamination.

January 22, 2019

Greenhouse-grown romaine lettuce was not implicated in the late 2018 E. coli outbreak. In fact, in a Dec. 17, 2018, update, FDA specifically noted that there was no evidence linking hydroponically and greenhouse-grown romaine to the outbreak, and thus, no recommendation was made to avoid the lettuce from these sources.

That does not mean, however, that greenhouse and hydroponic produce is not susceptible to contamination by foodborne pathogens. Rather, according to Ohio State University assistant professor and food safety state specialist Sanja Ilic, current growing processes are not designed to prevent contamination.

“Greenhouse production centers around controlling environmental conditions; however, current practices are not designed to reduce microbial contamination,” she says. In fact, those very conditions that allow intensive, year-round greenhouse/hydroponic production are unfortunately conducive to the survival, growth and spread of foodborne pathogens, she explains.

It is for such reasons that Virginia Tech fresh produce food safety team coordinator and extension specialist Amber Vallotton recommends that all greenhouse growers follow the framework of the Produce Safety Rule of the Food Safety Modernization Act (FSMA). “Whether or not the grower falls under the Produce Safety Rule, every grower is required to produce the safest product they can,” she says. So she gives the same advice to all growers — field or controlled environment — teaching a preventive approach to make produce as safe as possible.

“You can’t eliminate contamination altogether, but you can put practices in place to reduce and mitigate risk,” she says.

Greenhouse pathogenic protection

Some key practices that can prevent foodborne-pathogen contamination of greenhouse-produced fresh produce include:

Although greenhouses are controlled environments, factors like worker hygiene, soil and irrigation water must be monitored for possible contamination.
Photo: Adobe Stock

Worker hygiene

Handwashing. “One of the most effective ways to reduce the risk of contamination — and one of the most difficult practices to enforce — is handwashing,” Ilic says. Ranked as the most effective strategy to prevent contamination with foodborne pathogens in the greenhouse, handwashing is especially effective for the pathogens that originate from humans such as norovirus, Ilic says. “Our survey found that workers in the majority of tomato greenhouses — 78 percent — do not wash their hands prior to harvest.” To prevent contamination, workers should thoroughly wash and dry their hands before starting work, before wearing gloves, after toilet use, and after breaks or absences from workstations. However, handwashing is difficult to enforce; to be effective, proper equipment must be present and workers must receive adequate training.

Footwear. Another aspect of worker hygiene is clean clothing, especially footwear, Ilic says. “Although this aspect is not discussed for field production, contaminated footwear can track pathogens in the greenhouse and lead to contamination of produce.” In fact, research by her team isolated Listeria monocytogenes (Lm) from greenhouse workers’ shoes. Footbaths containing sanitizer can be a good strategy to prevent the entry of pathogens in the greenhouse and should be installed and maintained properly, she says. 

Health. Adequate health policies also need to be in place. “If a greenhouse worker is sick, there are the same risks as in the field,” Vallotton says. For example, if a worker has norovirus, he shouldn’t be allowed to work in a field or a greenhouse.

Irrigation water

Contaminated water has been implicated in numerous foodborne outbreaks. Thus, Ilic says, “Management of irrigation water in the greenhouse, and especially [in] hydroponic production, is critical to prevent contamination with foodborne pathogens.” In hydroponic greenhouses, pathogens can rapidly spread via water to surfaces, the environment and the plants. Water treatment should occur prior to mixing with additional source water, when water is recirculated, or when fertilizers or pesticides are dosed. 

Growers also should consider the water source risks and testing — whether it is from a public system, a well or surface source, Vallotton says. “The same for the spray system — it has to be potable water.” Additionally, she says, if the controlled-environment growers are adding acid to reduce the pH in the water, they need to be very careful with the storage of the acid. For example: Never leave a vat of acid sitting next to a packing table. “It’s not a foodborne pathogen, but it is a risk to people and product,” she says.

According to experts, establishing food safety requires assessing risk in your system, and setting a plan for prevention and corrective action.
Photo: Adobe Stock


Although soil is more controlled in greenhouses, it, and any soil amendments, can carry risk, Vallotton says. Such risk can vary by the grower’s use of compost or biological amendments and the kind of produce being grown, e.g., whether it is leafy greens or other high-risk foods. “If you’re bringing in compost amendments, it’s not much different at all from field-grown produce,” she says, adding, “I advise growers to not use manure-based amendments for leafy greens.” If you are, get a certificate of analysis (COA) on how it was made and cured; the process used to kill any pathogens, etc. The grower also needs to consider the architecture of the crop, such as, she says, “Will the edible portion touch the ground?”

Environmental sanitation

Foodborne pathogens are commonly found throughout the greenhouse environment, and a study by Ilic’s team isolated Lm from 37.5 percent of surface samples, including harvest bins and boxes, scales, tarp floor covers and doorknobs in tomato greenhouses. “Sanitation of equipment, tools and surfaces during propagation, growing and post-harvest handling is critical to ensure produce safety,” she says.

According to Ilic’s team survey, few greenhouse operations conducted sanitation of floors, gutters and other surfaces. Additionally, some hydroponic growers do not do a cleanup between the crops at all, but interplant for continuous production.

To prevent contamination, written sanitation standard operating procedures (SSOPs) should be developed for pre-growing cleanup and cleaning at certain periods during growing, she says. Tools used in the greenhouse, such as clippers, should be cleaned and sanitized after each use. There are a number of sanitizer options for greenhouses with different properties, including chlorine bleach, quaternary ammonium compounds, hydrogen peroxide and peroxyacetic acid.

Overall, food safety is a matter of looking at your system, assessing the risk, and setting a plan for prevention and corrective action, Vallotton says. For example, if you have overhead lights, do you have supplemental protection in place in case of breakage? If you’re starting seeds or transplanting plants, have you washed the containers or using new ones? If you’re using tools to cut plants, have you ensured the tools are washed and sanitized? “Follow the flow of the food,” she says, from before the start to harvest and distribution.

It is not difficult to develop a safety program based on testing, environmental controls and sanitation. But, Ilic says, “Because there is in general lack of understanding among the growers about the requirements for the Food Safety Plan, this task is perceived by many greenhouse growers as more difficult and time-consuming that it is.” As such, her team is working to develop sanitation protocols for hydroponic growers of leafy greens to enable them to quickly customize for the needs in their operation. Also, a Food Safety Plan (FSP) workshop has been developed and will be piloted in early 2019 to help growers write their own plan working directly with educators. “This should make the development of a food safety program much easier for the growers,” she says.

“A lot of it is about following [good agricultural practices]; and since lettuce has been implicated so much, be super meticulous,” Vallotton says.

Lisa is the editor of Quality Assurance & Food Safety (QA) magazine.