The toast with jam flies out of your hand or a piece of banana breaks off - the food lands on the floor. What to do? If you know the famous 5-second rule, you might still eat that slice of toast. It states that food that has landed on the floor can still be eaten if it is picked up within five seconds. Some may also know this rule as the 3-second rule - the principle is the same. However, it is not only we who ask ourselves whether we can still eat fallen food - scientists have also investigated this question.
What happens when food falls on the floor can be explained quite simply: If the toast, watermelon or candy falls on the kitchen floor, the bacteria that are romping around on the floor stick to the food. So far, so clear. But what role does the time that the food stays on the floor play? To answer this question, various researchers have carried out a number of experiments with a very similar structure: food is dropped on a floor with different coverings that has been prepared with bacteria and picked up again after a certain time. Afterwards, the scientists looked at how many bacteria were attached to the food.
A 2014 study of senior biology students, led by microbiology Anthony Hilton, found that time is an important factor in the transmission of bacteria to fallen food. The type of soil and the moisture in the food also play a role. According to this experiment, it's ok to use the 5 second rule.
In the study, the students used the bacteria Escherichia coli and Staphylococcus aureus. They investigated how many bacteria are left on toast, pasta, biscuits and sticky candies after they have been dropped on carpet, tiles or laminate. They observed the contact of the food with the floor between three and 30 seconds. More bacteria stuck to moist food than to dry food if it was left on the contaminated surface for more than five seconds. And: After falling on the carpet, fewer bacteria stick than when it comes into contact with hard floors.
Anthony Hilton told Science Daily that there is always a risk of infection when food falls on the floor and is contaminated with bacteria. But it also depends heavily on which bacteria would be on the floor. He continued, "We found evidence that the transmission of bacteria from indoor floor coverings is incredibly low, with carpet actually posing the lowest risk of transmitting bacteria from fallen food."
However, two very similar studies, one from 2006 at Clemson University and one by a research team led by Donald Schaffner, professor and specialist in food science at Rutgers University, a few years later come to a different conclusion.
When investigating Clemson University, the scientists worked with bread and lyoner and the bacterium Salmonella Typhimurium. Here, too, the 5-second rule was tested on different floor coverings: wooden floor, tiles and carpet. However, the researchers first also investigated how long the salmonellae can survive on the surfaces.
Rutgers University researchers dropped bread, buttered bread, watermelon and gummies on carpet, tile, wood and stainless steel. Here, too, the researchers used a bacterium similar to salmonella. The test durations were less than one second, five, thirty or three hundred seconds. The findings from the two studies: the wetter the food, the more bacteria stick to it. And: After falling on the carpet, fewer bacteria stuck to it than is the case with hard floors. And if you stay longer, there are more bacteria on the food that has fallen down.
"The five-second rule is a gross oversimplification of what actually happens when bacteria are transferred from a surface to food," said Donald Schaffner of Rutgers University in a statement. "Bacteria can contaminate immediately."
The researchers in all three studies basically come to very similar conclusions, but assess them differently. So they disagree on how many attached bacteria should stop eating food that has fallen off.
It should be noted, however, that the experiments pay close attention to how many bacteria stick to the fallen food. Ernst Tabori, medical director at the German Advice Center for Hygiene in Freiburg, told the "Süddeutsche Zeitung" that he thought it was nonsensical to infer the danger from the number of bacteria. The body has the necessary means to "deal with germs".
So what does that mean for us? Not only the time factor plays a role in how many bacteria stick to a food after it has fallen on the floor. Also the humidity of the food and the floor covering. And of course it also depends on which bacteria are on the floor. Whether there is one underneath that could cause an infection. And how clean the floor is.
This means that if we drop a biscuit on the carpet, after a few seconds fewer bacteria will stick to it than to a piece of watermelon that has fallen on tiles. So eating the cookie will probably be less unappetizing.
If you want to be on the safe side, you should better dispose of fallen food - regardless of whether it was on the floor for two or five seconds. Ernst Tabori also points out that in addition to germs, animal hair or dust particles can also adhere to the food - this can lead to immune reactions in allergy sufferers.
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Sources: Science Daily, Rutgers University communication, Rutgers University study, Clemson-University study, Time , Süddeutsche Zeitung