When a bite of your favorite tasty treat hits the floor, you can’t let it go to waste, right? Grab it within five seconds, and you’ll be fine— after all, that’s the rule.
Not so fast, says a study testing the scientific rigor of the oft-cited guideline.
If you’re a frequent food fumbler yourself, or have ever spent time with a toddler, you’ve witnessed the temptation to eat food off the floor. Perhaps you quickly considered the state of the floor and the stickiness of the food before yelling “Five second rule!” and snatching up the snack.
Food scientist Donald Schaffner set out to test those variables in his lab at Rutgers University. His study found that while the amount of bacteria on food does increase the longer the food sits on a contaminated surface, some of that transfer happens right away. With evidence that bacteria can transfer in less than a second, the study effectively disproves the five-second rule. The findings appeared online in September, ahead of publication in the November 2016 issue of Applied and Environmental Microbiology, the journal of the American Society for Microbiology.
For two years, Schaffner worked with graduate student Robyn Miranda, dropping pieces of watermelon, bread, bread with butter, and gummy candy onto a variety of surfaces including stainless steel, tile, wood and carpet. They completed a total of 2,560 drops to get an accurate picture of what really happens when food falls onto common surfaces in homes, restaurants and food processing plants.
To contaminate the surfaces, Miranda applied drops of a liquid containing E. aerogenes, a microorganism that’s similar to salmonella but doesn’t cause illness. She then dropped the food items on the germy surfaces and left them there for time periods ranging from less than a second to five minutes.
The moisture content of the food as well as the surface area between the floor material and the food seemed to play the biggest role in bacterial transfer. “Bacteria don’t have legs,” Schaffner said in a press release. “They move with the moisture.”
For example, watermelon, which is moist and smooth, picked up the most bacteria no matter how long it sat on the floor surface. Surprisingly, the sticky gummy candy didn’t pick up much at all. Carpet also showed the least transfer, with the bacteria possibly stuck in the absorbent fibers of the carpet, while smooth stainless steel and tile passed much more bacteria on to the dropped food.
But what about those of us who’ve been sneaking food from the floor every now and then with no ill effects?
“Many Americans don’t actually recognize the symptoms of food-borne illness,” said William Hallman, chair of the Department of Human Ecology at Rutgers. “And when we get sick, we often place the blame on the wrong suspect.”
So, even if you did get sick from eating floor food, you might not connect that incident to symptoms that could show up as long as three days later. The kind of pathogens on the surface also play a role in contracting a food-borne illness.
“It’s really got nothing to do with the time,” said Paul Dawson, a food scientist at Clemson University. “It’s like Russian roulette. … It’s just a matter of what’s on the surface.”