I'm sure most of us have dropped a piece of food on the ground before and quickly grabbed it, cunningly quoting the five second rule before taking a bite. Most of us also know intuitively that there isn't a whole lot of rhyme or reason to the rule, but it is great for justifying our appetite, or maybe we had just been cooking for a really long time and it would be such a waste.
Rutgers University researchers put the 5 second rule to the test and found contamination can occur pretty quickly, even less than five milliseconds. Donald Schaffner a food science professor found "The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food." The researchers dropped a huge array of different foods on a variety of surfaces includding ceramic tile, stainless steel, wood, and carpeting. The researchers then contaminated the different surfaces with a salmonella like bacteria known as Enterobacter aerogenes. The researchers examined bacteria transfer in different time intervals and assessed 128 different scenarios 20 times for 2,560 measurements.
Overall, the researchers found that "Bacteria can contaminate instantaneously", but longer exposure to dirty surfaces as well as moisture made the contamination even worse. The key explanatory variable was moisture, which amplified the transfer of germs from floor to food. Additionally, the topography of the surface played a significant role as food items dropped on tile and stainless steel has more contamination than those foods dropped on carpet.
These results aren't actually that surprising to us. Due to the number of bacteria on our floors and the rate of germ transfer it is silly for us to believe in such a rule as shown by the graphic below. The origin of the rule is unclear; however, to nutritionists delight it seems as though most view the rule as a myth.