A new study in the journal Applied and Environmental Microbiology finds that the five-second rule, which most parents know and love, is probably, in lay terms, mostly bogus. Food can pick up bacteria almost as soon as it hits the floor, the study reports, particularly depending on the type of food and the surface it's hit. There have been other studies on the topic, but they’ve largely been unpublished, or carried out by science television shows, so it’s hard to know what to believe about the “rule.” But this new study seems to be more rigorous, and has actually been published in a peer-review journal—so it may mean that it’s time to tweak our (magical) thinking about food safety.
"We decided to look into this because the practice is so widespread,” said study author Donald Schaffner, in a statement from Rutgers University. “The topic might appear 'light' but we wanted our results backed by solid science.”
The team looked at four types of food: watermelon, bread, bread and butter, and gummy candy. They dropped these foods onto four different types of floor—stainless steel, ceramic tile, wood and carpet—and then observed how many of a certain type of bacteria (Enterobacter aerogenes) affixed themselves to the foods after varying amounts of time (less than one second, five seconds, 30 seconds and 300 seconds).
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Watermelon generally picked up the most bacteria, and gummies the least. Bread and butter, and plain bread, were somewhere in the middle. This isn’t surprising since bacteria move best in liquid, and don’t attach so well to smooth, dry surfaces. "Transfer of bacteria from surfaces to food appears to be affected most by moisture," Schaffner said. "Bacteria don't have legs, they move with the moisture, and the wetter the food, the higher the risk of transfer. Also, longer food contact times usually result in the transfer of more bacteria from each surface to food."
The type of floor also mattered. The carpet had the lowest transfer rate, while stainless steel and ceramic tile transferred more. Wood was a little more variable. And the time the food came into contact with the surface also affected the uptake of bacteria, with longer contact times responsible for more bacteria. But in some cases, the transfer happened very quickly—or almost instantaneously—which suggests the five-second rule may not be so reliable. Most of the findings are fairly intuitive, but it's nice to have science back it up.
“Although this research shows that the five-second rule is ‘real’ in the sense that longer contact time result in more transfer,” the authors write in their paper, “it also shows that other factors including the nature of the food and the surface are of equal or greater importance. The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food.”
Some may remember a study a couple of years ago which determined that the five-second rule was generally legit—unfortunately, the study wasn’t published in a peer-review journal, so we knew then that it might not be worth basing food decisions on. The only other peer-reviewed study out there on the five-second rule found that increased contact time with the floor did increase the amount of bacteria (they used Salmonella), but only when the bacteria had been sitting on the surface in question for over eight hours. The TV shows—MythBusters and The Quick and the Curious on the Discovery Science Channel—didn’t find any conclusive evidence.
One possible counter to the message of the study is the "hygiene hypothesis," which suggests that our obsession with cleanliness, overuse of antibacterial soaps, and general lack of exposure to dirt is responsible for the rise in certain health problems like asthma and allergies. But it's unlikely that this would apply so well to foodborne illness, and few experts would likely recommend that we try to up our intake of pathogens through food, since this can be deadly.
But, according to the CDC, in the U.S. each year, there are some nine million cases of foodborne illnesses that can be attributed to specific foods—and of these, surface cross-contamination accounts for at least 12%. Many of these probably don’t come from our errant reliance in the five-second rule, but some certainly may. Since no one wants to end up with food poisoning, practicing good food hygiene is probably wise. It may be OK to rely on the rule under certain circumstances—but if the food is moist and the surface questionable, it might be your best bet just to toss it, rather than risk taking in a mouthful of stowaway bacteria.