When you
drop a piece of food on the floor, is it really OK to eat if you pick up within
five seconds? This urban food myth contends that if food spends just a few
seconds on the floor, dirt and germs won’t have much of a chance to contaminate
it. Research in my lab has focused on how food and food contact surfaces become
contaminated, and we’ve done some work on this particular piece of wisdom.
While the
“five-second rule” might not seem like the most pressing issue for food
scientists to get to the bottom of, it’s still worth investigating food myths
like this one because they shape our beliefs about when food is safe to eat.
So is five
seconds on the floor the critical threshold that separates an edible morsel
from a case of food poisoning? It’s a bit a more complicated than that. It
depends on just how much bacteria can make it from floor to food in a few
seconds and just how dirty the floor is.
Where did the five-second rule come from?
Wondering if
food is still OK to eat after it’s been dropped on the floor (or anywhere else)
is a pretty common experience. And it’s probably not a new one either.
A
well-known, but inaccurate, story about Julia Child may have contributed to
this food myth. Some viewers of her cooking show, The French Chef, insist they
saw Child drop lamb (or a chicken or a turkey, depending on the version of the
tale) on the floor and pick it up, with the advice that if they were alone in
the kitchen, their guests would never know.
In fact it
was a potato pancake, and it fell on the stovetop, not on the floor. Child put
it back in the pan, saying “But you can always pick it up and if you
are alone in the kitchen, who is going to see?” But the misremembered story persists.
It’s harder
to pin down the origins of the oft-quoted five-second rule, but a 2003 study
reported that 70% of women and 56% of men surveyed were familiar with the
five-second rule and that women were more likely than men to eat food that had
been dropped on the floor.
So what does
science tell us about what a few moments on the floor means for the safety of
your food?
Five seconds is all it takes
The earliest
research report on the five-second rule is attributed to Jillian Clarke, a high school student
participating in a research apprenticeship at the University of Illinois.
Clarke and her colleagues inoculated floor tiles with bacteria then placed food
on the tiles for varying times.
They
reported bacteria were transferred from the tile to gummy bears and cookies within
five seconds, but didn’t report the specific amount of bacteria that made it
from the tile to the food.
But how much bacteria actually transfer in five
seconds?
In 2007, my
lab at Clemson University published a study – the only peer-reviewed
journal paper on this topic – in the Journal of Applied Microbiology. We wanted
to know if the length of time food is in contact with a contaminated surface
affected the rate of transfer of bacteria to the food.
To find out,
we inoculated squares of tile, carpet or wood with Salmonella. Five minutes
after that, we placed either bologna or bread on the surface for five, 30 or 60
seconds, and then measured the amount of bacteria transferred to the food. We
repeated this exact protocol after the bacteria had been on the surface for
two, four, eight and 24 hours.
We found
that the amount of bacteria transferred to either kind of food didn’t depend
much on how long the food was in contact with the contaminated surface –
whether for a few seconds or for a whole minute. The overall amount of bacteria
on the surface mattered more, and this decreased over time after the initial
inoculation. It looks like what’s at issue is less how long your food
languishes on the floor and much more how infested with bacteria that patch of
floor happens to be.
We also
found that the kind of surface made a difference as well. Carpets, for
instance, seem to be slightly better places to drop your food than wood or
tile. When carpet was inoculated with Salmonella, less than 1% of the bacteria
were transferred. But when the food was in contact with tile or wood, 48%-70%
of bacteria transferred.
Last year, a
study from from Aston University in the UK used nearly identical parameters to
our study and found similar results testing contact times of
three and 30 seconds on similar surfaces. They also reported that 87% of people
asked either would eat or have eaten food dropped on the floor.
Should you eat food that’s fallen on the floor?
From a food
safety standpoint, if you have millions or more cells on a surface, 0.1% is
still enough to make you sick. Also, certain types of bacteria are extremely
virulent, and it takes only a small amount to make you sick. For example, 10
cells or less of an especially virulent strain of E. coli can cause
severe illness and death in people with compromised immune systems. But the
chance of these bacteria being on most surfaces is very low.
And it’s not
just dropping food on the floor that can lead to bacterial contamination.
Bacteria are carried by various “media,” which can include raw food, moist
surfaces where bacteria has been left, our hands or skin and from coughing or
sneezing.
Hands, foods
and utensils can carry individual bacterial cells, colonies of cells or cells
living in communities contained within a protective film that provide
protection. These microscopic layers of deposits containing bacteria are known
as biofilms and they are found on most surfaces and objects.
Biofilm
communities can harbor bacteria longer and are very difficult to clean.
Bacteria in these communities also have an enhanced resistance to sanitizers
and antibiotics compared to bacteria living on their own.
So the next
time you consider eating dropped food, the odds are in your favor that you can
eat that morsel and not get sick. But in the rare chance that there is a
microorganism that can make you sick on the exact spot where the food dropped,
you can be fairly sure the bug is on the food you are about to put in your
mouth.
Research
(and common sense) tell us that the best thing to do is to keep your hands,
utensils and other surfaces clean.