Science says germs are faster than the 5-second rule for eating food that’s fallen on the floor

Photo: Jonathan Daniel/Getty Images

If your watermelon hits the ground, never pick it up and eat it.

Researchers at Rutgers University in New Jersey, USA have found that bacteria transfers to food in less than a second, and that the more moisture it contains, the higher the level of contamination.

Bacteria can transfer in less than a second, according to professor Donald Schaffner, who found that the type of surface and contact time also contribute to cross-contamination, sometimes in less than a second. His Rutgers research team debunked the widely held view of a “safe” five-second window in findings published online in the American Society for Microbiology’s journal, Applied and Environmental Microbiology.

“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,” Schaffner said.

“We decided to look into this because the practice is so widespread.”

The good news is that while watermelon had the most contamination, gummy lollies – think snapes and jelly babies – had the least number of germs.

The researchers tested four surfaces – stainless steel, ceramic tile, wood and carpet – and four different foods (watermelon, bread, bread and butter, and gummy candy). They also looked at four different contact times – less than one second, five, 30 and 300 seconds, then grew a nonpathogenic “cousin” of Salmonella from the foods to measure the results.

Surfaces were inoculated with bacteria and allowed to completely dry before food samples were dropped and left to remain for specified periods.

Surprisingly, the researchers found that carpet has very low bacteria transfer rates compared to tiles and the stainless steel food safety officials insist should be used in commercial kitchens.

“The topography of the surface and food seem to play an important role in bacterial transfer,” Schaffner said.

“Transfer of bacteria from surfaces to food appears to be affected most by moisture. Bacteria don’t have legs, they move with the moisture, and the wetter the food, the higher the risk of transfer.”

And yes, the longer the contact time, the more bacteria present, which is some ways plays in favour of the five-second rule, except that by then your food is already contaminated, even if other factors are also at play.

“The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food,” the professor said.