Science debunks food myth that stretches back to Gengis Khan

It is perhaps the greatest agony: Watching food you paid good money for — or worse, painstakingly cooked — tumble to the floor. Children may be traumatized by their ice cream slipping off the cone to the curb, but food lovers carry this fear with them right into adulthood. There is one known salve: the 5-second rule.

This so-called rule is hard-baked into society. Essentially, it encourages you to tempt your fate and pluck favorite, fallen morsels back from beyond the point of no return — all predicated on the notion that bacteria and pathogens lurking on our kitchen floors, city streets, and the backs of cars can’t possibly have infiltrated our food in less than five seconds.

This notion may be comforting, but it’s not necessarily scientifically sound, Paul Dawson and Brian Sheldon tell Inverse. Dawson is a professor of food science at Clemson University, while Sheldon is a professor emeritus of food microbiology at North Carolina State University, Dawson and Sheldon are also co-authors of a book on food safety called Did You Just Eat That?

Together, they help Inverse explore the science behind the idea — and reveal what beloved American chef Julia Child has to do with it.

What is the five-second rule?

The idea that if you save your food fast enough, it is still ok to eat goes by many names: the 3-second rule, the 5-second rule, even the 10-second rule for the especially daring sorts among us. Unlike many scientific theories or hunches, which can be difficult to parse, the 5-second rule (or however many seconds you swear by) means exactly what it says on the tin: It is the specific amount of time you can allow fallen food to sit on the ground and still remain edible. The other way of saying this is that you have five seconds to act before the food ostensibly becomes contaminated with invisible and harmful microorganisms.

In practice, most people apply the rule unilaterally, regardless of different food types and floor surfaces — but most especially if the food they dropped is particularly delicious, Dawson says.

“If it’s a piece of chocolate or candy [that drops], you might cherish that a little more than something else,” he says. You might even be tempted to extend the time frame — perhaps a morsel of truffle deserves a 10-second rule, rather than the more meager 3-second rule, before you write it off.

Where did the five-second rule come from?

Like many things to do with food, humans tend to learn by example. Perhaps you were taught the 5-second rule by an older sibling, or maybe the street-smart white blood cell Osmosis Jones told you all about it in his titular 2001 children’s movie. Or you saw it tested on Mythbusters on cable TV (spoiler: they didn’t come to a conclusion as to whether it was a “bustable” myth). In truth, the roots of the rule go far deeper than that.

Dawson and Sheldon tell Inverse that this rule may originate in the lavish court banquets of Gengis Khan. Khan is one of the most infamous military leaders in all of history and established the Mongol Empire in the early 13th century.

“There were some ancient writings of Gengis Khan... that he told his subjects that any food that was on the floor that was prepared for him was fine to eat as long as it stays there,” Dawson explains. The “Khan Rule” was based on the idea that food prepared for Khan was so special it was impervious to all harm.

Julia Child’s beloved cooking TV show, The French Chef, may also have added to the mythos, Dawson says. In one episode of the show, aired in the 1960s, the famous chef drops a potato pancake mid-flip. It bellyflops down onto the stovetop, where Child lets it sit for several seconds.

Child doesn’t cite the 5-second rule directly, but she does encourage any viewers who might do the same to simply place their pancakes back in the pan. After all, Child reasons, none of the guests — all safely out of the kitchen — would be any the wiser once the pancake was dished up to them. No harm done. Right?

Is the five-second rule scientific?

History aside, how scientific is the 5-second rule, really? The answer is that it depends on whom you ask, Dawson and Sheldon say. Despite being a keystone of public consciousness for decades, there’s very little published research that actually explores whether or not there’s any truth to the rule.

“Some [studies] in particular that were done early that I recall were just taking food and dropping it randomly around a college campus,” Dawson says. “[But] they really didn't know what was on that surface.”

“That was not a controlled study and there are a lot of things out there on the internet, or even in print, that didn’t get closely reviewed,” he adds.

This causes problems for well-meaning journalists who want to give their readers the facts about the rule. One notable press release from Aston University, published in 2014, reads as particularly misleading. The release claims scientists at the university have evidence proving the 5-second rule. The news was picked up by the popular press, including publications like Slate and the New York Post. The big problem with the “evidence” however was that the findings were not published in a peer-reviewed journal — the standard for good scientific research. In turn, the claims in the release are basically moot. The research has still not undergone peer review, to Inverse’s knowledge.

Dawson and his team at Clemson University were the first to publish an investigation of the 5-second rule in a peer-reviewed journal, The Journal of Applied Microbiology. The study finds — contrary to the Aston release — that exposure time, essentially time on the floor, significantly changes the number of microorganisms then found on food.

Donald Schaffner and Robyn Miranda, Extension Specialist in Food Science and Distinguished Professor at Rutgers University and then master’s of food science candidate respectively, expanded on these findings in 2016. They tested 128 scenarios, including a mix of foods and floor types, and took more than 2,500 measurements.

They tested:

• Watermelon
• Plain white bread
• Buttered white bread
• Gummy worms

Each food was dropped from about five inches above the ground. As to the “ground,” the researchers looked at each food dropped on stainless steel, ceramic tile, wood, and carpet flooring. The study’s results were published in the journal Applied and Environmental Microbiology.

The number of microorganisms each food picked up when it hit the floor varied based on the food and the surface, but there are some stand-out results:

• Watermelon picked up the most organisms, no matter what kind of flooring it hit
• Gummy worms picked up the least number of organisms, no matter the flooring
• Longer contact times do result in more organisms on the food
• Food picks up contaminants from the floor exceptionally quickly

Schaffner tells Inverse that these results largely come down to what surfaces or foods are wettest.

“Based on our research, I think wetness has the greatest impact,” he says. “When at least one of the surfaces is wet, this facilitates bacterial transfer, as the bacteria are just carried along with the moisture.”

In the study, the authors write that their results reliably debunk this food myth.

“Although we found that longer contact times result in more transfer, we also found that other factors, including the nature of the food and the surface, are of equal or greater importance,” write the authors. “Some transfer takes place ‘instantaneously,’ at times of [less than] 1 s, disproving the five-second rule.”

Counterintuitively, the team identifies carpeting as one of the safest surfaces to drop food and then pick it up and eat it — at least in terms of the number of organisms picked up with the food, that is. The team speculates that carpets’ tight fibers may contribute to this unexpected result.

What happens when food hits the ground?

The Rutgers findings may put to bed one aspect of this food science myth, but another facet remains: What’s actually happening when the microorganisms jump ship from floor to food?

According to Sheldon, it all comes down to the attraction.

“Within our book, we talked a lot about this — about organisms and their ability to [move],” Sheldon explains. While most microorganisms don’t have legs, perse, they are attracted to different surface properties of food, like moisture or fat content. If the microbe in question loves the water in food or hates it, that will make a difference, Sheldon says.

“All of these play a role in terms of when you drop a piece of bologna on a surface, how many organisms are going to transfer from one surface to the other,” Sheldon says.

Should you follow the 5-second rule?

The U.S. Centers for Disease Control and Prevention states that roughly 12 percent of foodborne illnesses (of which there are some 9 million instances in the U.S. every year) can be attributed to surface contamination — for example, dropping your food on the floor and eating it anyway. This doesn’t necessarily mean that every time you follow the 5-second rule you will get sick, Dawson says, but it does mean you’re playing a game of Russian roulette with your health. Maybe you will be totally fine — and maybe you won’t be.

Sheldon adds that this could be an especially dangerous game for someone who already has a compromised immune system.

The 5-second rule shouldn’t be banished altogether, Dawson says. It can still play a role in bringing awareness of food contamination to consumers, he says. But as for whether or not Dawson or Sheldon follows it and eats food off the floor (especially raw ones) within five seconds? That’s a solid, unambiguous “no” from these scientists.

But Schaffner, the Rutgers scientist, says he’s not looking to judge you either way — for the most part:

“I'm not really interested in telling people what to do,” he says. “After all if you drop a dry chocolate chip onto a freshly mopped floor and then eat it, I certainly won't judge you. On the other hand if you drop an ice cream onto a New York City subway platform, and then proceeded to eat it I might judge you a little bit.”

CHECK, PLEASE is an Inverse series that uses biology, chemistry, and physics to debunk the biggest food myths and assumptions.

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Note: This story has been updated to more accurately reflect Robyn Miranda’s affiliation with Rutgers. Inverse regrets the error.