The 5 Second Rule: should you eat food if it drops on the floor?
Recent research by has hit the press this week, reported as concluding that bacteria may not transfer quickly to some foods when dropped on the floor, and therefore the 5 second rule may apply. I carried out an experiment and put some special powder on the floor to simulate bacteria, and then dropped some bread on the floor and picked it up immediately. The bread looked fine in daylight (see the left of the image), but under the UV light showed up all the powder that had transferred (right of image) – showing what can be picked up if you drop food on the floor!
However, there is published work which gives a completely different conclusion.. (Dawson, P, Han, I, Black, C and Simmons L: 2006 The Society for Applied Microbiology, Journal of Applied Microbiology).
These scientists found that Salmonella typhimurium, bacteria that are found in chickens, eggs, raw meat and the guts of animals including pets and pests, “could survive for up to 4 weeks on dry surfaces in high enough proportions to be transferred to foods.”
Furthermore, they found that Salmonella could be transferred to foods tested “almost immediately on contact”. The authors of this study had considered many other research papers together with another undergraduate study that had suggested that the five second rule may be true.
Dawson et al found that whilst carpet may have been found to have a lower percentage transfer rate than other surfaces probably due to the “wicking” effect (as found in the Aston study), this effect may have created an environment that allowed bacteria to survive longer, and higher levels were found on carpet than hard surfaces after 24 hours.
Overall the study concluded that even with very short contact times, there could be the transfer of large numbers of bacteria from surfaces to foods.
So what should be made of the most recent study?
I would say proceed with extreme caution. Many factors will affect whether there is a transfer of harmful bacteria to food dropped on the floor:
- the type of surface
- the type of food (moisture levels)
- the numbers of bacteria on the surface
A number of factors will affect whether this transfer of bacteria could be harmful:
- whether you have pets, or people wear outdoor shoes in the house
- whether the bacteria on the floor are dangerous to humans (pathogens)
- how many bacteria are needed to make you ill (for example, E. coli O157 is infectious in doses as low as 10 organisms).
- whether the person eating the food is from a vulnerable group (pregnant, immuno-compromised, over 6o or very young)
- whether you are likely to cook the food or whether it is ready-to-eat
So unless you fancy carrying out a risk assessment each time you drop food on the floor, or have a resident microbiologist to check out what bacteria are on your floor before you make a decision, my advice would be, if it falls on the floor, and it is ready-to-eat, then don’t eat it. Simple.
Apart from anything else, do you like fluff, dirt or pet hair on your food?
Also see other blogs on the subject.
102 (2007) 945–953