Life is complex and to get through it without having to think about everything you do along the way, you make up rules. These rules to live by usually make sense, like “look before you cross the road” but sometimes we hold onto rules without any evidence that they are actually valid. That is why researchers wanted to test the “5-second rule” which holds that if you drop a piece of food, so long as you pick it up within 5 seconds then it is safe to eat.
The basis of the 5-seconds rule is that not enough bacteria can transfer from the surface on which the food is dropped to the food within that time to cause you harm. To test this the researchers constructed a number of scenarios.
They used four different surfaces; stainless steel, ceramic tile, wood, and carpet. They also used four different foods; watermelon, bread, bread with butter, and “gummy candy”. As a last variable they altered the amount of time that the food stayed in touch with the surface; less than one second, 5 seconds, 30 seconds, and 300 seconds.
They used a type of bacteria called Enterobacter aerogenes which is a cousin to Salmonella. The bacteria were prepared in a “broth” and then spread on the various surfaces which were allowed to dry before the food was dropped on them.
The results showed that the time the food spent on the surface was less important than the type of food and the nature of the surface.
Watermelon showed the most bacterial contamination whereas the candy had the least. Bacteria move with moisture and the wetter the food the higher the risk of transfer. Additionally, compared with stainless steel and tiles the carpet showed much less bacterial transfer, while wood was variable.
In essence, although bacteria can transfer instantaneously, the longer a food stayed on a surface the more the bacterial contamination but the researchers say that other factors like the nature of the food and the surface on which it is dropped are of equal or greater importance. So the 5-second rule…not really one to live by.
Source: Applied and Environmental Microbiology
