You may be like a lot of people and associate foodborne illness with eating improperly cooked meat. Did you know that raw vegetables can also carry foodborne illness? Bacteria and viruses get on produce in a variety of ways: by food handlers, contaminated water, or soil fertilized with untreated manure.
Romaine lettuce grown in Arizona made the news in 2018 because of widespread E. coli contamination. Sprouts, another popular health food, have been involved in 74 outbreaks of (mostly) Salmonella since 1973. Turns out these nasty pathogens are not just reserved for chicken and beef! There is no cooking step to kill the bacteria or virus on produce before it goes into your salad. Sometimes washing doesn’t even help, so prevention is key.
Salmonella enterica and Listeria monocytogenes are two bacteria that can make you sick when eating contaminated produce. It turns out, some microgreen grow mats might be a breeding ground for these bacteria.
Microgreens are a hot new leafy green on the market. A microgreen is the first 2 to 3-inch (5 to 7-cm) tall shoot from a germinating vegetable seed. They are grown indoors in trays or hydroponics systems in soil, soil-substitutes, or without any rooting medium at all. Scientists understand a lot about how bacteria get to leafy greens from the soil, but little about contamination in indoor farms. Are indoor farms safer if they don’t use dirt? We wanted to find out.
Microgreen growers do use soil. However, they also use materials such as coco coir (made from coconut husks), Biostrate(TM) mats, plastic, perlite, rice hulls, and hemp in soil-free indoor systems. Our hypothesis was that if soil can transfer bacteria to lettuce, other growing materials can too. E. coli and Salmonella survived better in hydroponic nutrient solution compared to soil, so we wondered if there would also be differences among soil-free materials.
Within the last few years, there have been close to 10 microgreen recalls over diarrhea-causing Salmonella and Listeria monocytogenes found during routine testing. So, we decided to compare the survival of these two pathogens among popular soil-free growing materials to see if the bacteria lived longer on any of them.