Microgreens are a quick turnaround, high-value crop which can extend the growing season for indoor and outdoor growers alike. While microgreens can add “green” to your wallet year-round, keeping them free of pathogenic bacteria requires some effort and adherence to food safety practices.
Sprouts, which are harvested prior to the development of true leaf emergence, are responsible for the highest risk of food safety concerns for edible crops. Microgreens, harvested after the emergence of the true leaf and cut above the soil line rather than pulled out of the soil with seeds and roots attached, as are sprouts, account for the next highest category of risk.
Why are microgreens risky? Because they are grown and harvested close to the substrate and require moist, warm environments to thrive. They may be grown without a lot of direct sunlight, and sunlight decreases pathogen levels.
Even when grown in sunlight, microgreens are harvested young, at one to three weeks, so there isn’t a lot of time for sunlight to bring down populations of pathogenic bacteria. And microgreens are eaten raw.
“It’s a challenge to manage bacteria in these environments,” said Jeff Stoltzfus, Penn State Extension farm and food safety educator.
Keeping Clean
Bacteria thrive in moist environments – including growing substrates – and it’s easy for them to splash up onto the microgreens or to be moved by equipment or hands during the growing or harvesting steps. It is extremely important for anyone working with microgreens to follow some basic food safety protocols.
First, no workers should come to work sick. Hands must be washed for a full 30 seconds to eliminate pathogens. Shoes and clothes need to be clean and not be carrying contaminants into the production area.
Assessing the surrounding area for debris and weeds, which can harbor rodents or pests, and eliminating these, is crucial. Standing water is also a potential contamination source.
Bird droppings or animal feces are other contamination concerns. If any microgreens show signs of droppings, do not look healthy or have been touched without proper cleanliness standards, they should not be harvested and the areas immediately around them should be quarantined.
Inputs – substrates, water and the equipment used in the growing and harvesting process – can all introduce contamination. Growing trays, growing media, surfaces, tools, and even buildings themselves can all be possible routes of microgreen contamination. Cleaning and sanitizing exposed surfaces regularly will reduce risk.
Cleaning surfaces means scrubbing dirt off of them. Sanitizing is an additional step – treating a cleaned surface with a disinfectant to further reduce the presence of microorganisms.
Ventilation is a concern. If the air is blowing through contaminated vents, or the water used in a cooling system is contaminated, the crop will be at risk.
Listeria is a primary concern in cooling systems. For example, if air blows across an area of moisture (such as a wet area beneath coolers), that area should be cleaned at least between crop cycles to avoid pathogenic growth and contamination, even though the crop does not come in direct contact with the surface.
Hose ends can become contaminated if left on the ground or if they come in contact with growing media or dirty hands. Connections, nozzles and other pieces of the watering system can harbor E. coli too. Water can become contaminated just before it is used to water the crop.
Pre- and post-harvest water needs to be high-quality for safe microgreen production. It’s less risky to water via drip irrigation directly to the roots than to water overhead and risk splashing bacteria from substrates to the edible crop.
All water should be tested, with generic E. coli being the standard indicator organism. Any amount of E. coli indicates that the water is not safe to use. An ultraviolet light system, placed as close to the crop as possible, is the best means of keeping E. coli from the crop. The bulb will need to be changed at least annually for ongoing effectiveness.
Even tap water can have E. coli so do not assume that because you safely drink it that it is safe for the crop.
Testing water sources, and doing so as close to the product as possible, is critical. The source of water needs to be assessed, as well as any contamination during its delivery. If water is from a public system, it will already have been tested at the source for E. coli, but water from a pond or private well will need to be assessed at the source. All water will need to be assessed as close to the point of delivery to the crop as possible.
“We need to think about water specifically to the crop we’re growing,” as per the Food Safety Modernization Act, Stoltzfus said. “How we grow sweet corn is going to be very different than how we grow microgreens as it relates to water. In a situation where we have water in close contact with the growing crop, and the edible part of that crop, then it becomes a bigger issue.”
Pre-harvest water includes irrigation water, cooling water and water for applying nutrients or pesticides. Post-harvest water is used for washing the produce or washing food contact surfaces. In the growing of microgreens, both categories of water need to be equally free from contaminants.
Stoltzfus does not recommend that growers wash microgreens upon harvest. Any contact with water increases the risk of contamination. Customers should wash the greens at home.
Sanitizers can be used in water for microgreen production, but they need to be food safe. Chlorine, peracetic acid or quaternary ammonium are some possibilities. Mixing them properly and being aware of potential impacts on crop growth or health, including potential root damage, is imperative. The shelf-life of mixed products can vary, and the effectiveness will decrease over time.
Soil media and amendments can be another source of contamination. Media that contain biological soil amendments of animal origin are of the most concern. Fish products, bone meal and blood meal can all impart risk. Compost teas, composted vegetable waste and soil media that has been exposed to the elements also can harbor pathogens.
Commercial soilless mixes should be vetted with the supplier to ensure they’ve been properly treated to reduce pathogens. Peat, coir and hemp are generally safe substrates.
Ultraviolet light can reduce a significant amount of bacteria. Trays can be put out into sunlight to reduce the risk of pathogens. Growing microgreens in direct sunlight can reduce overall risk.
Fungal spores and mycotoxins aren’t as much of a concern, as the risk is low in microgreens production. If fungi are present, possibly from contaminated seeds, the microgreens should be discarded. Growing in high temperatures and high humidity environments can increase the risk.
“Bacteria are an important part of growing crops. Ninety-nine percent of them are not pathogenic to humans,” Stoltzfus said. “They are often important in crop production. We need some bacteria.”
It’s the pathogenic bacteria that cause concern. Testing for E. coli in the water and taking precautions to avoid introducing bad bacteria during any part of the growing, harvesting or post-harvest process by following best food safety practices, is the key to growing microgreens safely.
