Bacteria can begin to transfer to food dropped on the floor in less than one second, according to research from New Brunswick, N.J.-based Rutgers University, effectively disproving the so-called "five-second rule."
The research team published its findings in Applied and Environmental Microbiology.
Bacteria transfer time depends both on floor surface type and food type, the researchers found. They dropped watermelon, bread, bread and butter and gummy candy on four surfaces — stainless steel, ceramic tile, wood and carpet — and looked at bacteria transfer after less than one second, five seconds, 30 seconds and 300 seconds.
One of the main influences of bacteria transfer is moisture, they found. "Bacteria don't have legs, they move with the moisture, and the wetter the food, the higher the risk of transfer," said Donald Schaffner, PhD, a professor and food science specialist — watermelon had the most contamination, while gummy candy had the least.
"Also, longer food contact times usually result in the transfer of more bacteria from each surface to food," Dr. Schaffner said.
In terms of surface type, carpet had much lower bacteria transfer rates than stainless steel and tile, while transfer from wood was more variable.
Overall, the research shows the popular notion that food on the floor for less than five seconds is safe to eat isn't a hard-and-fast rule. "The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food," said Dr. Schaffner. "Bacteria can contaminate instantaneously."