In fact, the process of contamination can begin in under a second, especially if the food is moist or wet, such as a watermelon.
Scientists from Rutgers University in New Brunswick tested four foods—bread and butter, bread on its own, watermelon and a sticky sweet—on different surfaces, including carpet, wood, tiles and stainless steel.
The watermelon started to show contamination in less than a second, while a gummy sweet was the most resistant to bacteria. This is because bacteria move with water, and “the wetter the food, the higher the risk of transfer,” said researcher Donald Schaffner.
The surface also has something to do with the speed of contamination. Carpets have low transfer rates, while contamination happens the quickest from tiles and stainless steel.
Of course, the longer you leave any food on the floor, the more it will get contaminated, say the researchers, but the five-second rule is an over-simplification. Once food has hit the floor, you’re better to just bin it, they say.