You may want to think twice the next time you invoke the five-second rule for food that's fallen on the floor.

According to a new study in the US, it takes less than a second for dropped food to become contaminated by bacteria.

Researchers say people are only fooling themselves by pretending the accident never happened and quickly picking up the food and popping it into their mouths.

Debunking the rule as an urban myth, Donald Schaffner, a professor in food science at Rutgers University in New Jersey, insisted: "The 'five-second rule' is a significant oversimplification of what actually happens when bacteria transfers from a surface to food. Bacteria can contaminate instantaneously."


The study found that the amount of moisture present, the type of surface, and how long the food is actually on the floor all contribute to cross-contamination.

"The popular notion of the 'five-second rule' is that food dropped on the floor, if picked up quickly, is safe to eat because bacteria need time to transfer," Professor Schaffner told Rutgers Today​.

He continued, "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 scientists tested four surfaces - stainless steel, ceramic tile, wood and carpet - and four different foods- watermelon, bread, bread and butter and gummy sweets.

They coated the different surfaces with Enterobacter aerogenes, a microorganism similar to Salmonella.

And they then looked at four different contact times - less than one second, five seconds, 30 seconds and 300 seconds.

Watermelon garnered the most contamination, while, somewhat surprisingly, sweets had the least.

Buttered and plain bread had similar transfer levels, meaning it doesn't really matter whether your buttered toast falls butter up or butter down.

Carpet had very low contamination transfer rates compared to tile and stainless steel.

"Transfer of bacteria from surfaces to food appears to be affected most by moisture," said Professor Schaffner.

"Bacteria don't have legs, they move with the moisture, and the wetter the food, the higher the risk of transfer."

He did note, however, that longer food contact times do usually result in the transfer of more bacteria from each surface to food.

The findings were published in the American Society for Microbiology's journal, Applied and Environmental Microbiology.

So, could eating food off the floor be good for you?

Research led by Anthony Hilton, a professor of applied microbiology at Aston University, Birmingham, shows 87 per cent of people say they would eat food dropped on the floor, or have done so. The majority follow the five-second rule.

So why isn't the UK suffering huge epidemics of floor-acquired illness? Professor Hilton feels confident it is safe to pick up dropped food at home - as long as you move quickly enough.

"I have three young boys who have grown up dropping toast on the floor and picking it up again," he has said. "In my own home, which I know to be hygienically clean, the risk of them picking up anything nasty with the toast is very, very low."

His research team demonstrated that while some bacteria get transferred to food as soon as it lands on the floor, it takes time for them to transfer in enough numbers to cause illness.

"The initial transfer of bacteria is insufficient to contaminate the food," says his report.

Hence, he concludes, the five-second rule does hold true for normally hygienic home floors - although street pavements may well be another matter, he adds.