Is the Five Second Rule Accurate?

Updated on September 11, 2018
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Kortney has been a clinical physician assistant for 13 yrs. When not being a PA, Kortney’s hobbies include writing, research, and investing.

The Origin of the Five Second Rule

The Five Second Rule says that if you drop food on the floor and you pick it up within 5 seconds, then it is safe to eat. Most of us have applied the Five Second Rule at some point in our lives. We drop something onto the floor and quickly snatch it up, yelling out, "Five Second Rule!" as we shove it right into our mouth. I'm not sure whether the yelling out of the 'Five Second Rule' is for those around you to observe that you are practicing good hygiene or if it's to convince ourselves that it's okay to still eat the food. Maybe it could be both.

I became interested in studying the Five Second Rule a few years ago as I watched my kids and their friends use this rule as if it were an indisputable fact. I knew from my training in medical and health sciences that there was much more to this than most people even cared to think about. Clearly, five seconds is not too short a time interval for harmful bacteria to attach itself to our food. Or is it?

No one really knows where the Five Second Rule came from. Some claim that it originated in restaurants where the chefs were cooking very expensive food and didn't want to have to throw away expensive food that dropped onto the floor. Others say that it originated on a television cooking show wherein the host accidentally dropped the piece of food and claimed on her show that it was still safe to eat if it is picked up quickly. However, what most don't know about that cooking show is that she dropped it on the stove top, not onto the floor. If the Five Second Rule originated from this show, then, well, it's nothing more than a myth or urban legend. Despite this, it has been passed on throughout the years and through generations as being the 'Golden Rule" for dropped food.

As I think back to the possible origins of this rule, I wonder, have we all been duped into believing that this is actually based on a scientific fact? Ask my children and they will tell you that they are 100% positive that this rule is a proven fact. Well, as you may find out below, you shouldn't believe everything that you hear.

The Five Second Rule

If you drop food onto the floor, it is still safe to eat as long as you pick it up within five seconds.

My Experiment

In my quest for knowledge about this topic, I recruited my not so willing kids to help me set up a real scientific experiment. It was more of me convincing them that this was going to be fun for all of this. However, in the end, we all committed to proving (or disproving) our theory that the Five Second rule was false. Given that my kids have an opportunity to enter a science fair at their school, we used this experiment as part of their project. The following is the experiment that we used, including the scientific problem, the question, the hypothesis, the materials, the procedure, the data, the results, and the conclusion.

Purpose

To determine if the Five Second rule is accurate or not

Questions

If a dropped food is picked up within 5 seconds is it safe to eat?

Does the amount of time that the food is on the floor make any difference when determining whether or not it is safe to eat.

Hypothesis

Food that is dropped on the floor is safe to eat if it is picked up within 5 seconds.

Materials

Sterile Gloves

Agar Plates

Timer

Sterile Cotton Swabs

Apple

Cheese

Camera

Incubator

Variables

There are a lot of variables that can affect the results of this experiment, which include

  • Type of food used - For the purposes of this experiment, I have used two foods: Apples and Cheese.
  • Type of floor the food is dropped on - Although I did not attempt this experiment on multiple different types of floors, it is known that the type of floor the food is dropped on can affect the outcome. In this experiment, I used a clean and dirty floor.
  • Length of exposure time - For this experiment, I tested just two time intervals: 5 seconds and 30 seconds

Procedure

  1. Gather materials, including agar plates, sterile swabs, gloves, timer, and food being tested.
  2. Establish a control by taking a swab of the cheese before it is dropped on the floor. Spread the obtained sample from the swab onto an agar plate. The swab should initially be wiped on the center of the agar plate and then spread out towards the edges of the agar plate in a star-like formation. This procedure should be used every time a sample is obtained for this experiment.
  3. Label the agar plates based on the food being tested, type of surface used (dirty or clean), and length of exposure (5 sec, 30 sec).
  4. Begin by dropping the cheese onto the dirty surface. The timer should start as soon as the cheese hits the surface.
  5. After 5 seconds, using sterile gloves, pick up the cheese and swab the surface that was in contact with the floor.
  6. Using the swab, carefully lift the lid on the correctly labeled agar plate and spread the specimen carefully over the entire surface using the procedure noted in step #2.
  7. Replace the lid on the agar plate quickly to avoid contamination. Tape the lid on so that it is secure.
  8. Repeat steps 4 through 7 using the apple.
  9. Repeat steps 4 through 8, but instead of picking up the sample at 5 seconds, leave the sample on the floor for 30 seconds.
  10. Repeat steps 4 through 9, but use the clean surface this time.
  11. Once all samples have been taken, place the agar plates in an incubator. The plates should be placed upside down so that the agar is on the top. The incubator should be kept between 85-100 degrees F to allow optimal growth.
  12. Make observations of findings at regular intervals, including 12 hours, 24 hours, 36 hours, 48 hours, and 72 hours. Record the number of bacterial colonies that can be seen on each plate and also take a picture of each plate for comparison later
  13. After 72 hours, data should be compiled and graphed.
  14. Discard of agar plates in the trash after 72 hours.

Bacterial Colonies Shown on Agar Plates

Type of Floor/Exposure Time
Apple
Cheese
Dirty Floor (5 seconds)
52
28
Dirty Floor (30 seconds
56
41
Clean Floor (5 seconds)
12
4
Clean Floor (30 seconds)
15
6

Results

The hypothesis states that food that is dropped on the floor is safe to eat if it is picked up within 5 seconds. Based on my research, the hypothesis was proven to be incorrect. In my experiment, it was determined that the food picked up within 5 seconds still contained bacteria. In fact, there was a lot of bacteria present on the apple and the cheese. The food that was on the floor for 30 seconds grew more bacteria than the 5 second group. However, because both groups grew bacteria, they would both be considered to be unsafe to eat.

It can be concluded that a food that is dropped on the ground is considered unsafe regardless of whether it is on the ground for 5 seconds or for 5 minutes. The length of time that it was on the floor did not affect the growth of bacteria on the agar plates. Although longer exposure to the floor does appear to result in more bacteria, this would not impact the risks of getting sick.

In order for a food to be considered safe to eat after being dropped on the floor, it would be necessary to see no increase in bacteria compared to the control food that was not dropped on the floor. In my data, it can be seen that the control piece of cheese did have some bacterial growth (2 colonies). When that piece of cheese was dropped on the floor, results showed that there was a significant increase in the number of colonies (it had 11 colonies), even when it was picked up within 5 seconds. The increase from 2 colonies to 11 colonies shows that there is an increased risk of getting sick if the food is eaten.

Unfortunately, these results don't bode well for the billions of people who have used and continue to use the 5 Second Rule on a regular basis. My medical training tells me that it is still quite unlikely that there will be a very harmful bacteria on the floor that attaches to your food. There are some bacteria that won't necessarily make us sick. However, do we really want to take those chances? For me and my kids, the answer to this is a resounding "No!"

I will say that my kids and I did have a great time doing the experiment. If you have any feedback, my kids would love to hear it. At their science fair, they got a lot of attention for this particular project. Thanks for taking the time to follow along with us and we look forward to hearing from you in the comments section below.

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