Everyone has dropped food on the floor at some point in their lives, and most likely following this someone probably yelled out, “Five second rule!” This refers to the notion that food can remain on the ground for five seconds until people deem it unsafe to eat. Although, the question I pose is this actually true? Is this common phrase just a natural reaction to dropping food or is there any truth to it?
It would be impossible to test all types of bacteria on all types of foods while accounting for all possible surfaces. Although, several specialized experiments have been conducted to give a better insight as to what happens between the food and the bacteria.
Paul L. Dawson of Clemson University conducted an experiment where the bacteria Salmonella Typhimurium was used to observe how different types of food react when on different surfaces. Three surfaces were analyzed and they were tile, wood, and carpet. To go along with this bologna was placed onto each of the surfaces as a determinant of how much bacteria would be transferred. The bacteria was suspended in a re-suspended pellet overnight, then was placed onto the surfaces with a glass rod. For each food, a 10×10 cm slice of the same weight was placed onto the surface and was left there for time intervals of 5, 30, and 60 seconds. This process was repeated for every 2,4,8, and 24 hours the bacteria was left on the surface to determine long term effects as well. The food was then sliced up into equally sized pieces and then placed in a rinse solution, in which the S. Typhimurium cells were recovered through decimal dilution. The bacteria cells were counted and converted to CFU, which is the number of S. Typhimurium cells per milliltre.
This study concluded multiple things. First and foremost, the amount of bacteria that was transferred from the surface to the food exponentially decreased over time. For example in the first 5 seconds the bologna was placed on the tile surface, approximately 99% of the bacteria was transferred to the bologna. This trend also held true for the transfer of bacteria from the wood surface. As time went on, there was an exponential decrease in the amount of transferred bacteria for tile and wood. Figures showing this are presented below.
A major difference was found in the transfer of bacteria from the carpet to the bologna. In the first five seconds of exposure, less than .05% of bacteria was transferred onto the bologna. A figure from the experiment is shown below.
The lines in this graph are much more linear, and less exponential as shown in the previous figures.
Overall, the study concluded that the amount of time the food was in contact with the bacteria did not necessarily matter, but rather how long the bacteria had been on the surface prior was a major determinant. For all three surfaces, at the 8 hour mark there was a spike in the amount of bacteria transferred. Why this happened is still a mystery but it shows how bacterial time on the surface plays a larger role then food contact on bacteria. In simple terms, the five second rule only remains true if the bacteria had also been exposed for that first five seconds. If one were to drop a piece of food onto bacteria that had been exposed for 24 hours, then in the first five seconds the food would be exposed to it, a significantly less amount of bacteria would be transferred to onto the food.
Flaws in this experiment include only testing bologna, when different types of foods could contract different mounts of bacteria. The study referenced another study where
Another flaw was that only salmonella was used, although its impossible to test all possible strains of bacteria. And even though the experiments were done in sterile petri dishes, it is impossible to account for all types of bacteria, so there is a chance there was foreign bacteria that could have skewed the results.