The five-second rule has got to be really high on any list of things that people believe are true even if they don’t know where they actually learned it. As ideas go, its appeal is straightforward and damn-near universal: After all, it’s a ready-made excuse not to have to get rid of perfectly good food just because it happened to hit the floor for a few seconds there. But it shouldn’t come as any surprise that bacteria don’t actually take precisely five seconds to transfer from the ground to food. Turns out they can make the move in no time at all, which means any food that hits the ground is probably best left uneaten.
That’s the finding of researcher Donald Schaffner and his team at Rutgers University. While a few other studies have looked at the five-second rule, theirs looked at more foods on more surfaces for more varying lengths of time than any study before, making this humanity’s most authoritative exploration yet of the five-second rule.
“The five-second rule is wrong”
“I think the key take-home point is that the five-second rule is wrong,” Schaffner tells Inverse. “If you drop a moist food on a hard surface you can get rapid transfer of most of the bacteria present on that surface.” For instance, watermelon picked up the most bacterial contamination, more than plain bread, buttered bread, and gummy candy. The moisture of the watermelon gave bacteria an easier path to transfer from the surface to the food.
One seemingly counterintuitive result is that a hard, smooth surface like tile actually saw much faster transfer of bacteria than carpet. We might think of carpets as more naturally dirty than tile, but that’s thinking in terms of the macroscopic realm of trapped crumbs or stray hairs, rather than the microscopic realm of potentially harmful bacteria.
“We studied microscopic cross-contamination which is different than macroscopic cross-contamination,” Schaffner says. “If I drop a piece of wet candy on the floor and it lands on a hair, the hair will stick to the candy and then I’ll get that hair in my mouth, which is disgusting but not necessarily unsafe unless there are bacteria on that hair.”
But why was there less transference of bacteria in carpet than in tile in the first place? Schaffner says that may to some extent be an artifact of how they designed the study, though he says he’s confident the results they obtained here are a good match for real-world conditions.
“When you inoculate bacteria onto a surface and that surface is a hard surface like stainless steel the bacteria spread out evenly and remain on the surface and available for transfer,” he said. “When you inoculate bacteria onto a more absorbent surface like carpet many of the bacteria sink down into the carpet. Those bacteria that sink down into the carpet are not available for transfer, because they are physically separated from the top surface of the carpet which is what contacts the food.”
As for why some foods have slower transfer rates when in contact with bacteria, Schaffner says they aren’t sure. He says one hypothesis is that more gradual transfers are a result of foods slowly “relaxing” into the surface — as, say, a piece of bread gradually unfolds and flattens while lying on the ground, the total area of contact increases, and so more bacteria move over.
It’s worth keeping in mind that food doesn’t necessarily become unsafe the instant it comes in contact with bacteria, as a certain amount of bacteria needs to transfer over before there’s a serious health risk. But that amount can vary depending on the kind of bacteria and how strong the person’s immune system is.
Is there any hope left for the five-second rule? Well, even if the “five-second” bit doesn’t have any scientific basis, the data does support the idea that a food becomes more dangerous the longer it stays on the ground, at least in some cases.
“The five-second rule is kind of right, especially when you have a food with an uneven surface that is not moist and a surface which is also uneven,” said Schaffner. “In these situations, longer times seem to promote more transfer.”
The problem lies in trying to figure out how long a food can be on the ground before it becomes too dangerous. While it’s possible to get more precise times in the laboratory, we’re left only to guess with foods in the kitchen and the living room. A five-second rule might really be a five-microsecond rule. So, when in doubt, it’s better to play it safe with food on the ground.