Food: the science behind the 20-second rule

Do you directly discard food that has fallen to the ground, or do you live according to the 20-second rule? Apparently, there is a 5-second rule, but for some reason 20 seconds seem to be the general rule in student circles. Not sure why.

I am of course talking about the idea that food is safe to eat as long it has not stayed more than 20 (or 5) seconds on the floor.

A paper was published in 2006 where the authors tested the 5-second rule and transfer of the pathogen Salmonella typhimurium to bits of food. It might sound like a ridiculous thing to spend research money on, but if you put it into perspective: each year there are 76 million cases of food borne illnesses in the United States alone and 5200 people die annually due to food poisoning.

Still, I am not entirely sure how many of these cases were caused by people picking up food from the ground and eating it.


Salmonella can be beautiful! By National Institutes of Health (NIH)

What the authors found was that yes, the longer the food has contact with the surface, the more bacteria there were on the food, but 5 seconds were definitely long enough for the bacteria to contaminate the food. Interestingly, if you drop food on a carpet, there is less transfer of bacteria to the food compared with tile or wood.

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Still, I think it might be good not to worry too much. Also, I also think some people might be more susceptible than others. I can eat any old stuff and never be affected, whereas others start puking very easily. I also think that too much worry can be bad for you too.

If I dropped my last piece of liquorice, no bacterial fear would ever hold me back from picking it up and putting it in my mouth!

Science Safely!

Dr. Anna of The Imaginarium

...can eat very old food.

…can eat very old food.


Dawson, P., et al. “Residence time and food contact time effects on transfer of Salmonella Typhimurium from tile, wood and carpet: testing the five‐second rule.” Journal of applied microbiology 102.4 (2007): 945-953.

Posted in Bacteria, Biology, Medicine, The Biology of Disgusting and tagged , , , .


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