Most of us have heard it: if you drop food on the floor, it’s still okay to eat it, as long as you act quickly and pick it up within five seconds of it hitting the ground. But does the so-called “five-second rule” have any scientific backing? Anecdotally, most of us would agree—it depends on the kind of food. I’m more likely to eat a cracker that I’ve dropped on the floor than I am, say, a buttered bagel that lands spread-side down. But does timing also make a difference? Is there really a delay or grace period before which germs can find their way to your fallen food?
The Science Behind the 5-Second Rule
Avid listeners know that I’m not against eating moldy food, but sometimes even I have to draw the line at food that has fallen on the floor. Living in a house with dogs and children means my floors are never going to satisfy the June Cleavers of the world, so is there any amount of time that is OK for food to sit on the floor?
In a study recently published in the Journal of Applied and Environmental Microbiology, microbiologists subjected the five-second rule to some rigorous scientific tests. The scientists dropped four different foods (watermelon slices, strawberry gummy candy, bread and buttered bread) onto four different kinds of surfaces (steel, ceramic tile, wood, and carpet) that were contaminated with bacteria. They then left the foods there for <1, 5, 30, or 300 seconds before testing them to see if they had also become contaminated. With 128 possible combinations of food type, surface type, and contact time each replicated 20 times each, the researchers took > 2500 measurements.
As predicted by the five-second rule, the longer the food was in contact with the germy surface, the more contaminated it became. However, anything that fell on the contaminated surface itself became contaminated, no matter how long it was left there. Moisture also played a role: watermelon was most easily contaminated, while gummy candy had the lowest chance of picking up the bacteria.
The surface also mattered: non-absorbent surfaces like steel and tile reliably transferred bacteria more effectively than carpet and the more porous wood. So what you are dropping and where you are dropping it should be considered just as much if not more than how long your dropped item has lingered on the floor.
Should You Eat Food That Falls on the Floor?
Other studies have reached similar conclusions. Bacteria can be transferred immediately so a quick reaction is not going to help keep you germ free. The CDC notes germ-laden surfaces as one of the leading causes of the transfer of foodborne illnesses.
However, some researchers note that our concern over the kitchen floor is misplaced. For example, Dr. Aaron Carroll, a professor of pediatrics at Indiana University, points out that there are other surfaces in our house and even our kitchen that are far more germ-infested than the kitchen floor.
A 1998 study examined the average number of bacterial colonies per square inch on different surfaces in the kitchen. The floor turned up 2.75 colonies per square inch but that number was about doubled on the refrigerator handle (5.37 colonies per square inch) and the kitchen counter (5.75 colonies per square inch). Most people don’t think twice about eating food off of a “clean” dish, but those sponges were found to harbor 20 million colonies per square inch. Gross!
Part of the problem is that what we perceive as clean is not always really all that clean. Cell phones, for example, which we put up to our face without hesitation, are known to host more bacteria than toilet seats. We also sometimes rely on past experience to assess risk. With the first child, you may sterilize the pacifier every single time it falls on the ground, but once you know that your first kid survived just fine, and perhaps once you see just how dirty kids can get, your second child may not get the same treatment. There is even some evidence that gender plays a role, with women being more likely to adhere to the five-second rule and thus less likely to eat food off the floor.
So should we eat food that falls on the floor? Bacteria, it seems, are unavoidable so there is unfortunately no grace period for food to be contamination free once dropped on a germy surface. However, given the ubiquity of bacteria, perhaps our desire to not waste food, and our desire to avoid alarm every time our toddler eats a cheerio off the floor, are important to factor in as well. In any case, I think I’ll go scrub my kitchen counter now—and sterilize that sponge.
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.
Photo courtesy of shutterstock.
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