The scientific basis of the “5-second rule”

How long can a food stay on the ground and be eaten? The 5-second rule finally explained by science

We have all heard it invoked at least once: the "rule" that if a food touches the ground for less than 3, 5 or 10 seconds, germs and bacteria would not have time to contaminate it. A theory that has ancient origins, never supported or disproved by scientific evidence.

At least until a few months ago: Paul Dawson and Brian Sheldon, professors of food science and microbiology and authors of a book on food safety that tries to dispel myths and legends such as that of 5 seconds.

The 5-second rule

The fact that you can pick up dropped food on the floor and eat it without consequence, especially if it's a particularly desired food, is comforting, to say the least. And it may have some scientific basis, according to Dawson and Sheldon's findings.

"If it's a piece of chocolate or a candy that you drop, you tend to want it more than other foods," Dawson says, so it often happens that a particular treat may enjoy a 10-, 20- or even 30-second rule - whereas we'd tend to give less time to any food, for which we often only need 3 seconds.

The issue is complex, and involves a belief that's much older than you might think: for Dawson and Sheldon, the 5-second rule would even date back to banquets held at the court of Genghis Khan in the 13th century.

As Dawson explains, "there are ancient texts by Genghis Khan in which he himself tells his subjects that any food prepared for him can be eaten even if it falls on the ground": the Mongolian ruler's assumption was based on the concept that food prepared for him was special.

But the notion that germs and bacteria on surfaces need a certain amount of time to contaminate food had not yet been addressed from a genuinely scientific point of view, if we don't consider the episode of Mythbusters dedicated to the subject.

There was only one 2014 study from Aston University that seemed to demonstrate the scientific basis of the 5-second rule: it was picked up by major international news outlets, but it was a paper that wasn't even submitted for review, which is why it can't be considered scientific.

"I remember some studies where you would pick up food and throw it on the ground, randomly, on the college campus," Dawson recalls, "but in those studies they didn't even analyze what was present on the surfaces."

The Experiment: watermelons vs. candy

Dawson and Sheldon's study is the first scientific investigation of the 5-second rule published in a peer-reviewed scientific journal: published in 2016 in the Journal of Applied Microbiology, it is being explained today by the two food scientists at the launch of their newly published book, "Did You Just Eat That?".

Dawson and Sheldon tested 128 different scenarios, using different foods, surfaces and floors, and performed for the sake of science over 2,500 measurements.

They tested watermelons, gummy candies and the classic buttered white bread that perhaps inspired Murphy's Law. Each food was dropped to the floor from a distance of about 13 inches, on steel, ceramic, wood and carpeted floors.

The results are pretty clear: watermelon collects by far the highest number of organisms, while gummy candies remain nearly unharmed by bacteria. Un tempo di contatto prolungato con il pavimento effettivamente è causa di maggiori quantità di microrganismi sul cibo.

La cosa più chiara è che il cibo è in grado di raccogliere germi, batteri e altri agenti contaminanti in maniera eccezionalmente veloce. Soprattutto se presenta un alto tasso di umidità.
"Quando una delle due superfici è umida" spiega Dawson "il trasferimento dei batteri è molto facilitato, perché i batteri viaggiano proprio insieme all’umidità del cibo".

"Anche se abbiamo provato che un contatto più lungo provoca più trasferimento di germi, abbiamo anche rilevato che la natura del cibo e delle superfici influiscono ugualmente, se non di più" sulla sicurezza del cibo caduto a terra.

Sorprendentemente, la moquette è risultata il tipo di superficie più sicura su cui lasciar cadere la propria colazione, forse a causa della facoltà delle fibre di tessuto di trattenere i microrganismi.
As Sheldon says, "As we explain in the book, microorganisms don't have legs but are attracted to certain characteristics of food surfaces, such as moisture and fat content."

In short, bad news: the juiciest, sugariest foods dropped on the shiniest floor are the very ones for which the 5-second golden rule ultimately doesn't apply.