The five-second rule is off by about five seconds in many cases, and not in the direction you hoped. That's what Rutgers researchers say in a new study debunking the classic kitchen rule, which declares it safe to eat food off the floor if scooped up in a flash. "We decided to look into this because the practice is so widespread," despite little research to back it up, Donald Schaffner tells Rutgers Today. After dropping watermelon, bread, bread with butter, and gummy candy on stainless steel, tile, wood, and carpeted surfaces crawling with salmonella-like bacteria about 2,500 times—and leaving the food untouched for one, five, 30, or 300 seconds—Schaffner and his team found that food could become contaminated "instantaneously," they report in Applied and Environmental Microbiology.
The longer food sat on the surface, the more bacteria was usually transferred. But in certain cases, contamination occurred immediately, Schaffner says. That's because bacteria "move with the moisture, and the wetter the food, the higher the risk of transfer." Watermelon, for example, was contaminated most often, and gummy candy least often. To sum up, "the five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food," Schaffner says. If you are going to eat food off the floor, carpet is your best bet, since it was least likely to transfer bacteria, say researchers—who point out a 2005 Mythbusters episode found no differences between food left on the floor for two or six seconds, per CTV News. (When it comes to pesticide residue, strawberries are the worst.)