Improving Food Safety of Hydroponic Leafy Greens

Hydroponic farming is experiencing rapid growth worldwide, offering a sustainable and efficient method of producing fresh, nutrient-rich crops. However, the unique conditions of hydroponic systems also present complex food safety challenges. Dr Sanja Ilic and Dr Melanie Lewis Ivey, researchers at The Ohio State University, are at the forefront of efforts to understand and mitigate the risks of human pathogen contamination in commercial hydroponic production. Their pioneering work is providing crucial insights and practical guidance to help ensure the safety and nutritional value of hydroponically grown leafy greens.

The Rapid Expansion of Hydroponic Farming

In recent years, hydroponic farming has emerged as an increasingly popular and important method of crop production around the globe. Unlike traditional field-based agriculture, hydroponic systems involve growing crops in nutrient-rich water solutions without the use of soil. This innovative approach offers numerous advantages, including higher yields per unit area, more efficient use of water and other resources, and the ability to grow crops year-round in controlled greenhouse environments.

Leafy greens, such as lettuce and herbs, are particularly well-suited to hydroponic cultivation and are among the most commonly grown crops using this method. The demand for fresh, locally produced leafy greens has soared in recent years, driven by consumer interest in healthy eating and sustainable food production. As a result, the hydroponic leafy greens sector is experiencing rapid growth, with many new commercial operations emerging across the USA and further afield.

However, the unique conditions found in hydroponic systems, including warm temperatures, high humidity, and the use of recirculating nutrient solutions, can also be conducive to the growth and spread of harmful bacterial pathogens. These pathogens can enter hydroponic systems through contaminated water sources, growing media, or inadequate sanitation practices, and once established, they can be difficult to eradicate. In fact, several high-profile outbreaks of foodborne illness in recent years have been linked to contaminated hydroponic produce, underscoring the urgent need for robust food safety strategies tailored to this production system.

Despite the growing importance of hydroponic production, there has been limited research on the specific food safety risks and best practices for this sector. Many of the current guidelines and regulations for produce safety were developed based on field-grown crops and may not be directly applicable or sufficient for hydroponic operations. As Dr Ilic and Dr Lewis Ivey point out, this lack of science-based guidance leaves hydroponic growers without the tools and knowledge they need to prevent contamination and ensure the safety of their crops effectively.

Investigating Pathogen Dynamics in Commercial Hydroponic Systems

The nutrient film technique (NFT) system is one of the most common methods used for hydroponic leafy greens production. It involves growing crops in shallow channels through which a thin film of nutrient solution is continuously circulated, providing the plants with water, nutrients, and oxygen. While this method is highly efficient and productive, the recirculating nature of the system means that if pathogens are introduced, they can quickly spread throughout the entire crop.

Recognising the critical need for more research in this area, Dr Ilic and Dr Lewis Ivey have focused their efforts on understanding the complex dynamics of human pathogen contamination in real-world hydroponic production systems. Their group investigated the survival and spread of Salmonella Typhimurium and Listeria monocytogenes, two major bacterial pathogens responsible for foodborne illness, in commercial hydroponic lettuce production using the NFT system.

To simulate real-world contamination scenarios, Dr Ilic and Dr Lewis Ivey inoculated the nutrient solution in experimental NFT systems with different concentrations of Salmonella and Listeria, representing both low-level sporadic contamination events and worst-case high-concentration scenarios. They then tracked the pathogens’ persistence and spread in the recirculating nutrient solution, the rockwool growing medium, and on the lettuce roots and leaves over the course of a typical 28-day crop production cycle.

Pathogen Accumulation and Risks to Consumers

The results of this study were concerning, as both Salmonella and Listeria were able to persist in the hydroponic system throughout the entire production cycle, even when introduced at low initial concentrations. The pathogens were found to accumulate to high levels in the rockwool growing medium and on the lettuce roots, likely forming protective biofilms that made them difficult to eradicate. Most worryingly, the pathogens were also able to transfer from the nutrient solution and roots to the edible lettuce leaves, presenting a serious risk of foodborne illness for consumers.

While the levels of Salmonella and Listeria in the recirculating nutrient solution did decline over time, especially in the first 24 hours after inoculation, the pathogens remained detectable in the system even at the end of the crop cycle. This finding highlights the importance of preventing contamination from occurring in the first place, as once pathogens establish a foothold in a hydroponic system, they can be extremely difficult to eliminate.

The results of this study provide a critical wake-up call for the hydroponic industry, demonstrating that even low levels of pathogen contamination can pose significant risks in these systems. They also underscore the urgent need for effective strategies to prevent contamination and to rapidly detect and mitigate any pathogens that do enter the system.

Evaluating Sanitation Strategies for Hydroponic Systems

Building on their findings about pathogen persistence in hydroponic systems, Dr Ilic and Dr Lewis Ivey next turned their attention to evaluating potential sanitation strategies. Effective sanitation is a cornerstone of food safety in any production system, but there has been little research on which specific sanitisers and methods are most effective in hydroponic settings.

Many hydroponic growers currently use chemical sanitisers such as chlorine (bleach), hydrogen peroxide, or peroxyacetic acid to clean and disinfect system components and equipment. However, the efficacy of these sanitisers against human pathogens like Salmonella, and their potential impacts on sensitive hydroponic crops are not well understood. Some sanitisers may also pose risks to worker safety or contribute to the formation of harmful disinfection byproducts.

To address this knowledge gap, Dr Ilic and Dr Lewis Ivey conducted a comprehensive study comparing a range of commonly used sanitisers for their ability to eliminate Salmonella from key surfaces in NFT hydroponic systems. These surfaces included the growing channels, channel lids, pipes, and nutrient solution tanks, all of which can serve as reservoirs for pathogen growth and spread if not properly sanitised.

Food Safety and Quality – A Delicate Balancing Act

In addition to assessing sanitiser efficacy against Salmonella, their team also investigated the impacts of these treatments on the yield, appearance, and nutritional quality of hydroponically grown lettuce and basil. Leafy greens are known to be sensitive to chemical treatments, and any negative impacts on crop health or nutrient content could undermine the benefits of hydroponic production.

The results of the sanitiser evaluation revealed clear differences in efficacy among the tested products. Peroxyacetic acid-based sanitisers and certain quaternary ammonium compounds were found to be the most effective, eliminating Salmonella from all tested surfaces when used at the recommended concentrations and contact times. In contrast, the chlorine-based sanitisers performed poorly, proving no more effective than water alone in reducing Salmonella contamination on most surfaces.

However, the study also found that all of the tested sanitisers had some negative impacts on crop health and quality. Lettuce and basil plants grown in nutrient solutions treated with the sanitisers had lower overall yields and reduced levels of key nutrients like chlorophyll and carotenoids compared to untreated control plants. The 200 ppm peroxyacetic acid treatment was found to have the least detrimental effects overall, particularly for basil, but still resulted in some reductions in crop performance.

These findings highlight the complex challenges of balancing food safety and crop health in hydroponic production. While effective sanitation is essential for preventing pathogen contamination, the use of harsh chemicals can also have unintended consequences for the yield and quality of sensitive crops like leafy greens. Growers need science-based guidance on which sanitisers and application methods can effectively control pathogens while minimising negative impacts on their crops.

The results of Dr Ilic and Dr Lewis Ivey’s sanitiser evaluation provide an important starting point for such guidance, identifying peroxyacetic acid as a promising option for hydroponic leafy greens production. However, they also underscore the need for further research to fine-tune sanitation protocols and explore alternative strategies that can achieve robust food safety without compromising crop performance.

Toward a Safer and More Sustainable Future for Hydroponics

Building on their initial findings, the team is now working to develop more targeted, evidence-based recommendations for water and nutrient quality management, sanitation protocols, and other key food safety practices in commercial hydroponic operations. They are also collaborating with industry partners to translate their research findings into practical tools and resources that growers can easily implement in their own operations. Drs. Ilic and Lewis Ivey have been awarded federal funding from the United States Department of Agriculture (USDA) to further explore the complexities of biofilm formation in hydroponic leafy green production systems. The goal of this new research is to develop effective strategies for mitigating biofilm formation to enhance food safety and production efficiency.

Ultimately, the goal of this work is to help the hydroponic industry realise its full potential as a sustainable, efficient, and safe source of fresh, nutritious produce. With its many environmental and economic benefits, hydroponic farming has the potential to play a vital role in meeting the growing global demand for healthy food while reducing pressure on scarce land and water resources. But to achieve this potential, the industry must prioritise food safety and invest in the research and innovation needed to ensure that hydroponic crops are consistently safe and of the highest quality.

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