"The popular notion of the `five-second rule' is that food dropped on the floor, but picked up quickly, is safe to eat because bacteria need time to transfer,'' said study author Professor Donald Schaffner, of New Jersey's Rutgers University.
"We decided to look into this because the practice is so widespread. The topic might appear 'light' but we wanted our results backed by solid science.''
The team dropped watermelon, bread, bread and butter, and gummy sweets on stainless steel, ceramic tile, wood and carpet surfaces for less than a second, five, 30 and 300 seconds.
Watermelon had the most contamination and gummy candy the least. Carpet had very low transfer rates compared with those of tile and stainless steel, whereas transfer from wood was more variable.
They found that moisture, the type of surface and contact time all contributed to cross-contamination and, in some instances, the transfer began in less than one second.
Although they found the five-second rule is "real'' in the sense that longer contact time resulted in more bacterial transfer, it also showed other factors, including that the nature of the food and the surface it fell on, were of equal or greater importance.