When we drop a piece of chocolate on the floor, most of us wouldn’t hesitate to pick it up and eat it, citing the ‘five-second rule’. According to scientists at Rutgers University, however, it is usually unsafe to eat something that’s been on the ground even for only a few seconds, since bacteria can transfer into almost instantaneously. Recently published in the Applied and Environmental Microbiology journal, the study reveals that the total contact time, amount of moisture present as well as the type of surface all affect the extent of cross-contamination. Speaking about the find, Donald Schaffner, an expert in food science and a professor at the university, said:

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… 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.

For the project, the team tested the so-called ‘five-second rule’ on four different surfaces, including wood, stainless steel, carpet and ceramic tiles. Among the foods used for the research were watermelon, bread and butter, gummy candy and also just bread. The researchers chose four different contact times: one second, five seconds, half a minute and finally, 300 seconds. What’s more, they grew the bacterium Enterobacter aerogenes in the laboratory, using soy broth and peptone buffer.

Occurring naturally in our digestive system, the microorganism is a gram-negative, nonpathogenic version of Salmonella. Armed with powerful microscopes, the scientists observed the transfer scenarios of each of the different food items, surface types, bacterial prep and lastly, contact time. Once the bacteria were introduced in it, the surface was allowed to dry completely, and only then were the edible samples dropped on it.

The food, as pointed out by the team, was left for particular amounts of time. The experiments yielded a total of 128 scenarios, each of which was in turn reproduced up to 20 times for greater accuracy. A total of 2,560 measurements were then examined, while the dropped food samples were thoroughly analyzed to determine the extent of bacterial contamination. As it turns out, the watermelon was found to be the most contamination, with the gummy candies being the least. Schaffner added:

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. Also, longer food contact times usually result in the transfer of more bacteria from each surface to food.

Additionally, the carpet boasted significantly lower transfer rates than steel as well as tile. Interestingly, the transfer rates of wood seemed to vary with different scenarios. Although it’s true that longer contact times inevitably lead to greater contamination, the latter actually depends on a variety of factors, including the kind of surface the food falls on, the nature of the food and so on. Schaffner went on to say:

The topography of the surface and food seem to play an important role in bacterial transfer… The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food. Bacteria can contaminate instantaneously.

Source: Rutgers University