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We’ve all done it: Dropped some food on the floor and quickly picked it up and eaten it, or fed it to our child. The old 5-second rule means bacteria will have no time to climb aboard if the food has been in contact with the floor for less than five seconds, right?
Wrong, according to researchers at Rutgers University, who studied four different foods, dropped onto four different surfaces, for four different contact times. As part of the test, the researchers liberally applied Enterobacter aerogenes, a nonpathogenic “cousin” of salmonella, to each surface.
“At the shortest amount of time we studied, which was a fraction of a second, no matter what food and what surface, we always found some bacteria transfer in at least one of our experimental trials,” says Donald Schaffner, a food science professor at Rutgers University and the lead author of the paper, published in the journal Applied and Environmental Microbiology.
The researchers conducted multiple trials of each combination of food, surface and contact time.
While they showed the 5-second rule to be untrue, time does matter, says Schaffner. For many foods and many surfaces, the longer the food sat on the floor, the more bacteria it collected. That’s probably gravity at work, he says, pressing the food down and expanding the surface area in contact with the floor’s germs.
You may think it's no big deal, but about 12 percent of food-borne illness reported to the Centers for Disease Control and Prevention is the result of cross-contamination of food from surfaces.
To eat or not to eat
In the study, not all foods were bacteria magnets, and not all surfaces easily parted with bacteria.
Carpet can be hard to clean compared to stainless steel, ceramic tile and wood, but of the four, carpet transferred the least bacteria in the study. That’s because the bacteria sank down into the carpet’s fibers and away from its surface, said Schaffner.
As far as the foods, watermelon had the most contamination, while gummy candy had the least. Bread and butter and plain bread were between the two extremes.
“Watermelon was the most moist food we studied, and we saw almost all the bacteria transfer in the fraction of a second,” said Schaffner.
The other contact times were five, 30 and 300 seconds. Bacteria need some way to get from point to point, and water is a great vehicle for that, he says.
The results for wood, stainless steel and tile varied depending on food and contact time.
There have been other, sometimes contradictory, tests of the 5-second rule, but only one other peer-reviewed published paper in 2006.
In that research, Clemson University researchers used bologna and bread and came to basically the same conclusion as the new report: If you eat food dropped on the floor, you’re taking a chance no matter how long it’s sat there, said food safety expert Paul Dawson, lead author of the Clemson study.
“It really depends on what’s on that floor,” said Dawson.
Before you eat the dropped food, stop. Think. Do you have pets who’ve stepped into who knows what or people tramping about your kitchen in dirty shoes?
Another consideration: Who is eating the floor food?
“Is the person very young, very old or immunocompromised? All those things put that person at a higher risk of getting sick,” said Schaffner.