The five-second rule is based on the not-entirely-scientific belief that bacteria cannot contaminate food within five seconds, so you won’t get sick eating things you have picked up from the floor.
The first person to investigate this urban myth scientifically was Jillian Clarke, an American high-school student, during an apprenticeship in a microbiology laboratory at the University of Illinois in 2003. Clarke and her colleagues inoculated rough and smooth tiles with the bacterium E coli (certain strains of which cause stomach cramps, diarrhoea and vomiting) and put gummy bears or cookies on the tiles for five seconds. She found that E coli was transferred to gummy bears within five seconds, more so from smooth than rough tiles. As a side issue, Clarke also established in her work that university floors are remarkably clean and that people are more likely to pick up cookies from the floor than cauliflower.
Paul Dawson, professor of food science at Clemson University in South Carolina is a five-second-rule expert. His 2007 study, published in the Journal of Applied Microbiology, found that the dirtiness of the floor was more important than how long the food lay on it. His study was a progression from Clarke’s because it measured the amount of contamination. Using bread or bologna, he showed that it was better to drop either of them on carpet inoculated with salmonella, where less than 1% of the bacteria were transferred, than on tiles or wood, where up to 70% got on to the food. A similar study from Aston University found that, as soon as food hit the floor, it became contaminated – especially on smooth surfaces – but that the number of bacteria on the food increased up to 10 times between lying from three seconds to 30 seconds on the floor.
Dawson says that the five-second rule is simply not true because, if food hits a virulent brand of E coli, even the small number of bacteria it attracts immediately will make you sick. He doesn’t eat food when it falls on the floor. The very young or old shouldn’t use the five-second rule as their immune systems may not cope with even tiny amounts of bacteria. If the floor is filthy, then the rule is invalid on the grounds of grossness anyway. But the likelihood is that, for most of us, eating food off the floor isn’t going to hurt us. So if you are very hungry and you must pick food off the floor, then do it quickly, and preferably off a carpet.