In childhood, just about everyone learns that food dropped on the floor can be retrieved and eaten, if it's done quickly enough (hence, the "five-second rule"). But does the five-second rule work? I'm sorry to report that curious scientists have dashed our hopes and dreams for our floor food. It turns out that germs from the floor or another dirty surface can indeed transfer to food instantaneously (in less than a single second), so the five-second rule definitely won't cut it.
As the two Rutgers University researchers on this project point out, previously we didn't know much about whether the five-second rule works, despite its popularity as folk wisdom and appearances in popular culture. And, unfortunately, our intuition about food safety isn't a very reliable guide to what's actually safe. So, Robyn C. Miranda and Donald W. Schaffner set out to determine with some precision and objectivity whether five seconds is a long enough amount of time for food to become contaminated by contact with a contaminated surface.
The experiment's setup was pretty simple. Miranda and Schaffner chose a variety of foods (watermelon, bread, buttered bread, gummy candy) to place on a variety of surfaces (stainless steel, tile, wood, carpet). The surfaces had first been coated in one of two types of solution designed to grow bacteria. This means that, by the time the food was dropped on them, each surface had been significantly colonized with germs that could potentially move onto the food. The germs colonized were related to Salmonella, an actual food-borne pathogen, so the results would have real implications for food safety (whereas some bacteria are harmless to humans, so we'd care less if they transferred to dropped food).
The results of the experiment pretty fully disprove the five-second rule. In general, it took less than one second for bacteria to transfer to the foods (though longer contact times meant higher bacteria transfer, unsurprisingly). Other factors turned out to be very important too, as explained in Science Daily. Watermelon picked up the most bacteria, probably because it's so wet and water is what facilitates bacterial movement.
Carpet, which seems like it would hold and transfer bacteria easily, actually transferred fewer germs than tile and stainless steel. Wood surfaces varied in how well they transferred bacteria to food. So, while more research may be necessary to figure out exactly how germs, surfaces, and foods interact, one thing's for certain — RIP, the five-second rule. Maybe ignorance was bliss.