Comedian Elayne Boosler touched on a great deal of the human experience thusly: “My mother was so proud of her housecleaning. She always said, ‘You could eat off my floor’. You can eat off my floor, too. There're thousands of things down there.” Homer Simpson, spotting a piece of pie on the floor, said, “Mmmm, floor pie!” And then there was the episode of Friends where Rachel and Chandler are picking at a slab of cheesecake that's fallen on the hallway floor when Joey walks in—and sits down, pulls a fork out of his pocket and says, “Alright, what are we having?”
As these three popular culture examples clearly show, people often eat food that has fallen on the floor. Of course, most people try to pick the food up as quickly as possible after it has hit the deck. That practice has been codified as the five-second rule: it's safe to eat any comestibles retrieved from the floor within five seconds. Actually, I remember it from when I was a kid as the 15-second rule, but we were on a budget.
Back in March 2014, my Scientific American colleague Larry Greenemeier wrote a Web story about research at Aston University in England that appeared to confirm the five-second rule. (The study results were announced by the institution but were not published in any peer-reviewed journal.) “Food retrieved just a few seconds after being dropped is less likely to contain bacteria than if it is left for longer periods of time,” Greenemeier summarized. “The Aston team also noted that the type of surface on which the food has been dropped has an effect, with bacteria least likely to transfer from carpeted surfaces. Bacteria is much more likely to linger if moist foods make contact for more than five seconds with wood laminate or tiled surfaces.”
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At this point, I'm reminded of the famous story of the old Jewish man perturbed by the fact that when he dropped a piece of buttered bread, it landed with the buttered side up. Now, the sticky butter avoiding the floor might seem like good luck. But as life is a vale of tears, the man was troubled that the universe did not appear to be functioning in accordance with the Creator's vast, eternal plan. So he consulted his rabbi. And the rabbi, after days of study and reflection, arrived at a scientific explanation: the bread was buttered on the wrong side.
Again, the Aston University work, which found contamination by Escherichia coli and Staphylococcus aureus bacteria to be a function of food's time spent on the floor, was interpreted as supportive of the five-second rule. But not so another study that came out online in September 2016 in the journal Applied and Environmental Microbiology.
That work, by scientists at Rutgers University, tracked Enterobacter aerogenes transfer from various surfaces to different foods. And the researchers state: “Although we show that longer contact times result in more transfer, we also show that other factors, including the nature of the food and the surface, are of equal or greater importance. Some transfer takes place ‘instantaneously’ at times [less than one second], disproving the ‘five second rule.’”
And that's how the Rutgers study was reported by numerous news outlets—as the debunking of the five-second rule. But what is fascinating to this observer is that both studies basically found the same thing: the degree of bacterial contamination is dependent on contact time, surface type and what we'll call food Elmeritude, or glueyness. The coverage echoed the different ways the two studies' conclusions were couched. (Don't drop food on the couch.)
But what's truly bothering me is, When did the five-second rule come to pertain to bacterial transfer? Unless I'm misremembering my misspent youth, the key factor in edibility of fallen food was whether it had schmutz all over it. If you picked it up and it was free of dust bunnies or cat hair, bombs away for your stomach acid and immune system to deal with. Anyway, that's the side my bread is buttered on.