What Does Science Say About the Five-Second Rule? It’s Complicated

The real world is a lot more nuanced than this simple rule reflects

How badly do you want those fries?
How badly do you want those fries? lofilolo / iStock

Many people of all ages agree: Food, when dropped on the floor, remains “good” for five seconds. But this pillar of American folklore, the so-called “five-second rule,” is now under attack from scientists at Rutgers University.

Though the five-second rule may seem like a silly line of inquiry, food safety is a major health burden in the United States. The Centers for Disease Control and Prevention estimate that every year, one in six Americans (roughly 48 million people) get sick from foodborne illness, 128,000 are hospitalized, and 3,000 die.

“We decided to look into this because the [five-second rule] is so widespread. The topic might appear ‘light,’ but we wanted our results backed by solid science,” Donald Schaffner, food scientist at the School of Environmental and Biological Sciences, told Rutgers Today.

Schaffner and his graduate student Robyn Miranda tested different bacteria transfer scenarios using four surfaces (stainless steel, ceramic tile, wood, and carpet) and four foods (watermelon, bread, bread and butter, and gummy candy).

They inoculated each surface with Enterobacter aerogenes—a nonpathogenic “cousin” of Salmonella bacteria that occurs naturally in the human digestive system—and dropped the food on each surface for differing lengths of time (less than one second, five, 30, and 300 seconds). The food samples were then analyzed for contamination. In total, the different combinations of surface, food, and length of contact yielded 128 scenarios, each of which was replicated 20 times. The pair published their results in the journal Applied and Environmental Microbiology.

The duo didn’t necessarily disprove the five-second rule, showing that bacteria transfer does increase with contact time. However, their findings reveal a more nuanced reality than that imparted in common playground wisdom.

“The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food,“ Schaffner said. “Bacteria can contaminate instantaneously.”

By food, watermelon collected the most bacteria, and gummy candy the least. According to Schaffner, moisture drives the transfer of bacteria from surface to food; the wetter the food, the higher the risk of transfer.

Looking at the surfaces, tile and stainless steel had the highest rates of contamination transfer. Somewhat surprisingly, carpet had the lowest rate of transfer, and the rate was variable on the wood surface. In the end, they found that many factors contribute to contamination: The length of contact, the characteristics of the surface and the moisture of the food all play a role.

Schaffner and Miranda are the not the first to investigate the five-second rule, but peer-reviewed research is limited. In 2013, the popular MythBusters duo also found that moist foods collected more bacteria than drier foods, and an undergraduate research project tested the rule in an unpublished 2003 study from the University of Illinois. Interestingly, the Illinois study found that women are both more familiar with the rule than men and more likely to eat food off the floor.

Unsurprisingly, the Illinois researchers also found that cookies and candy were more likely to be picked up and eaten than cauliflower and broccoli, which raises an important question. If we really want that food, does it matter how long it has been on the floor?

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