Cutting fruits and vegetables doesn't just leave a knife slick with juice and pulp; it can spread viruses to the knife if the produce isn't clean, according to a new study. Scientists already knew that bacteria could contaminate utensils in this way, but the new study is the first to look at hepatitis A virus and norovirus, the most common cause of foodborne illness in the United States. The findings could help food safety investigators pinpoint the source of foodborne illnesses and develop new methods to prevent outbreaks.
To carry out the new study, researchers at the University of Georgia's Center for Food Safety in Griffin set up a test kitchen. The scientists developed techniques to strip viruses off knives and graters to quantify their abundance. They measured the amount of hepatitis A virus or norovirus transferred from contaminated honeydew melons, cantaloupes, tomatoes, strawberries, and cucumbers to the knives used to cut the produce.
For all types of produce, regardless of the material and sharpness of the knife used, more than half of all samples transferred viral particles to the knife. Some combinations of viruses and produce were more likely to contaminate a knife. Norovirus was transferred more often from strawberries and cucumbers than was hepatitis A virus, for example, but melons and tomatoes transferred more hepatitis A virus than norovirus. Similar experiments on viral transfer from carrots to a grater found cross-contamination the majority of the time, the scientists report this month in Food and Environmental Virology.
The researchers then tested whether a contaminated knife could transfer viruses to clean fruits and vegetables. "What was really surprising was that at least seven produce items could be contaminated by a single knife that was contaminated," says virologist and food safety researcher Jennifer Cannon, the leader of the study. And that's just a lower estimate, she adds, because the study only tested seven items cut sequentially with the same knife. The results suggest that foodborne viruses are just as likely to spread to utensils as bacteria are. And based on previous studies showing that fewer than 20 norovirus particles are enough to make someone sick, the viral knife contamination could cause illness.
The new findings aren't surprising in light of previous studies on bacteria, says food scientist Donald Schaffner of Rutgers University in New Brunswick, New Jersey, who was not involved in the new work. But having quantitative results on viruses offers a starting place for understanding in more detail the factors that influence the spread of food pathogens.
"This kind of data is absolutely essential in helping us design best management practices," Schaffner says. Having methods in place to study viral cross-contamination will allow future studies to test the effectiveness of sanitizers and cleaning methods, he says. And knowing which pathogens are likely to jump from produce items to knives can help investigators trace outbreaks of foodborne illness.
The lessons on viral contamination of utensils are most useful for commercial kitchens, Cannon says, where many produce items bound for different meals are being simultaneously chopped. But the finding is also a warning for home cooks. "Anything that you handle, whether it's a spoon, knife, or bowl, those can all be points of cross-contamination from your hands to food and between food items," she says. "I think recommending that people not only wash their hands and produce but wash their utensils in between produce items is reasonable."