Don’t count on the “five-second rule” to keep dropped food free of bacteria. An exhaustive test of the notion that food touching the floor for no more than five seconds is still safe to eat debunked it with more than 2,500 measurements. “The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to a food,” said Rutgers researcher Donald Schaffner. “Bacteria can contaminate instantaneously.”
Researchers dropped watermelon cubes, gummy candy, plain bread and buttered bread from varying heights on carpet, tile, stainless steel and wood contaminated with bacteria. They tested contamination after intervals from less than a second to five minutes. Longer exposure did increase contamination, but the most important factors were the composition of the food and the surface. Watermelon soaked up the most bacteria, while the candy was least contaminated. Carpet transferred fewer bacteria than tile or steel, while the results for wood varied. The study was published in Applied and Environmental Microbiology.