The Science Behind The Five-Second Rule

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Did You Just Eat That?: Two Scientists Explore Double-Dipping, the Five-Second Rule, and other Food Myths in the Lab

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The following is an excerpt of Did You Just Eat That? by Paul Dawson and Brian Sheldon.

Suppose you just dropped a piece of Godiva dark chocolate on the floor and quickly picked it up. Your mind instantly starts weighing your options, the pros and cons of eating the chocolate—the angel on one shoulder and the devil on the other. The good angel tries to convince you to discard the chocolate because it may have picked up potentially dangerous bacteria and you may get sick—or worse. The devil responds that it doesn’t even matter if there are dangerous bacteria on the floor, because you can apply the infamous five-second rule.

Known as the five, ten, (you fill in the blank)–second rule, this urban myth proposes that if food is removed from a contaminated surface quickly enough, the microorganisms on the surface won’t have time to transfer, or “jump,” onto the food. In the food production and service industries, it’s common practice to throw away food intended for human consumption if it has been dropped onto unsanitary surfaces. However, there is a perception that if dropped food is picked up quickly enough from a dirty surface, the food may be “okay” to eat. In fact, some scientists have postulated that allowing children to practice the five-second rule might improve their immune systems. Yeah, and drinking from the sewer might also improve your immune system—if you survive. Some urban myths have a scientific basis, whereas the origins of others are simply unknown. Are seconds a critical time frame that determines if food is safe to eat, or aren’t they? As you’ll see, this food myth is affected more by how dirty the floor is than by how long the food lies there.

Origin Of The Five-Second Rule

The rules about eating food off the floor are sometimes attributed to Genghis Khan (1162– 1227), who is said to have instituted the “Khan Rule” at banquets for his generals: If food fell on the floor, it could stay there as long as Khan allowed, because food prepared for Khan was so special that it would be good for anyone to eat no matter what.

In reality, people had little basic knowledge of microorganisms and their relationship to human illness until much later in our history. Thus, eating dropped food was probably not taboo before we came to this understanding. People could not see the bacteria, so they thought wiping off any visible dirt made everything fine.

Television’s original queen of the culinary arts, Julia Child (1912– 2004), may have contributed to the urban myth by routinely picking up dropped food while preparing delectable meals. Jessie Schanzle, a writer for the news website “The Conversation,” discovered that a well-known but inaccurate story about Child had contributed to the myth.

Viewers of her cooking show, The French Chef, have claimed they saw Child drop lamb (some say chicken or a turkey) on the floor and pick it up, advising viewers that if they were alone in the kitchen, their guests would never know. In fact, it was a potato pancake that fell onto the stovetop, not on the floor. Child returned it to the pan saying, “But you can always pick it up, and if you are alone in the kitchen, who is going to see?” But in popular culture the misremembered story persists.

Five-Second Rule Studies

Tests of the five-second rule have been presented on several television shows, in academic news releases, and in only two published research studies—one of which was conducted in our laboratory.

The first research study directly addressing the five-second rule was announced in a 2003 press release from the University of Illinois. In that study, gummy bears and fudge- striped cookies were dropped on vinyl floor tiles inoculated with E. coli. The E. coli was transferred from the tiles to the gummy bears and fudge-striped cookies within five seconds, but the researchers did not report on the number of bacteria transferred.

Based on surveys of college students, the authors also found that the gender of the person eating off the floor and the type of food on the floor affected the chance of eating food off the floor:

​•​ 70 percent of females and 56 percent of males were familiar with the five-second rule, and most of them use it to make decisions about tasty treats that slip through their fingers.

​•​ Women are more likely than men to eat food that’s been on the floor. Who would have thought? Maybe females are less wasteful than males?

​•​ Cookies and candy are much more likely to be picked up and eaten than cauliflower or broccoli. No surprise here!

The popular Discovery Channel television series MythBusters got in on the act in 2005 with an episode that aired on October 19. After conducting a few tests, the show’s hosts, Jamie Hyneman and Adam Savage, concluded that contact time (2 or 6 seconds) was not a determining factor in the transfer of bacteria to food. They dropped wet food (pastrami) and dry food (crackers) onto contaminated surfaces and found that the pastrami picked up more bacteria than the crackers did. Jamie admitted that they needed to run more tests to see if there was a difference between 2 and 6 seconds. As a mini-test, they placed contact petri dishes at various locations around the shop and found that toilet seats were cleaner than floors. Like most MythBusters episodes, these were not statistically designed research studies. What does cleaner even mean without some kind of context or control group?

In 2006, our Clemson University study was the only scientifically peer-reviewed paper on this topic published to that date. We investigated whether the length of time that food is in contact with a contaminated surface does in fact affect the transfer of bacteria to the food. You’ll find more details on this study later in this chapter. Briefly, though, here is the process: We (1) inoculated square samples of tile, carpet, and wood with a strain of salmonella; (2) dropped food on these surfaces; and (3) then measured the number of bacteria transferred from the surface to the food.

One year later, in 2007, two undergraduate microbiology students at Connecticut College reported that Skittles were safe to eat after 30 seconds of contact with the floors located in the university dining hall and snack bar, whereas apple slices were safe to eat after more than 1 minute. Since the levels of surface contamination (including pathogens) were not reported, their results most likely depend on whether the surface was contaminated or not. The article “To Eat or Not to Eat: Seniors Prove ‘Five- second Rule’ More Like 30” concluded that “no bacteria were present on the foods that had remained on the floor for 5, 10 or 30 seconds.” The conclusions of this report are somewhat confusing and contrary to nearly all other reported studies focused on bacterial adhesion to wet and dry food surfaces. In a 2014 press release, researchers from Aston University in Birmingham, England, reported that contact time significantly affected the transfer of E. coli and Staphylococcus from inoculated carpet, tile, and laminate to toast, pasta, and sticky candy. They also reported that 87 percent of people they queried either would or have eaten food dropped on the floor.

In January 2016, the Science Channel’s television series The Quick and the Curious showed NASA engineer Mick Meacham offering cookies to strangers after dropping them on the ground in a park. The show’s host stated that moist foods left on the ground for 30 seconds picked up ten times more bacteria than moist foods left for only 3 seconds, but no data or tests with moist foods were reported to support this statement. 

In 2016, nine years after the Connecticut College study, a second peer-reviewed paper was published on the topic from Rutgers University. The results were similar to those reported in our 2006 study, although the researchers included a wider range of foods and bacteria. They tested watermelon, bread, bread with butter, and gummy bears on tile, stainless steel, wood, and carpet for 0, 5, 30, and 300 seconds. They found that bacteria transfer to food ranked from highest to lowest in the following order:

watermelon > bread = bread with butter > gummy bears

Generally, though, they observed the same trends reported in our 2006 study.

So, What Does The Research Tell Us About The Five-Second Rule?

Can you drop food on the floor, pick it up, and then eat it with no risk of ingesting pathogenic microorganisms? Until recently, only one peer-reviewed research study actually tested the rule. Then an even more recent publication pretty much nailed down the answer. The five-second rule appears to be an old wives’ tale. The differences in the conclusions drawn from these previous studies are attributed to how the studies were designed and conducted. For example, the Connecticut College researchers applied their test to a real-world scenario by choosing surfaces at the university where people dine. Were they really testing the five-second rule, or were they testing the chance that the surfaces would be contaminated? We believe they were testing the latter, since they didn’t appear to determine the contamination level of the surface on which the food was being dropped.

There is conclusive evidence that when food comes into contact with a contaminated surface, bacteria are transferred almost immediately. Eating food that has been dropped on the floor could be compared to driving a car without wearing a seat belt. You could drive your whole life without wearing a seat belt and never have an accident, but it doesn’t prove that wearing a seat belt won’t prevent injury in case of an accident. Similarly, eating food from a non-contaminated surface poses no risk. However, many factors affect the associated safety risk of eating food that has contacted a surface. They include but are not limited to the dose/population and type of microorganisms present, presence of pathogenic or nonpathogenic organisms, composition/characteristics of the contact and microorganism surfaces (charge, hydrophobicity, etc.), and general health status of the consumer.

Excerpted from Did You Just Eat That? by Paul Dawson and Brian Sheldon. Copyright © 2019 by Paul Dawson and Brian Sheldon. Used with permission of the publisher, W.W. Norton & Company, Inc. All rights reserved.