“You aren’t going to eat that are you?” your friend wonders as you reach down to grab the cookie you just dropped on the floor. “Five-second rule!” you argue, before popping the cookie into your mouth.
According to this popular belief, if you drop a piece of food on the floor and pick it up in less than five seconds, then it’s safe to eat. The presumption is that bacteria on the floor don’t have enough time to hitch a ride on the food. But is it true?
In 2003, Jillian Clarke, a senior at the Chicago High School for Agricultural Sciences in Illinois, put the five-second rule to the test. She inoculated two types of tiles—smooth and rough—with Escherichia coli and dropped gummy bears and cookies on the tiles for five seconds. Clarke and her coworkers saw that bacteria transferred to food very quickly, even in just five seconds, thus challenging the popular belief.
A few years later, food scientist Paul Dawson and his students at Clemson University in South Carolina also tested the five-second rule and published their results in the Journal of Applied Microbiology. When they dropped bologna sausage onto a piece of tile contaminated with Salmonella typhimurium, over 99% of the bacteria transferred from the tile to the bologna after just five seconds. The five-second rule was just baloney, Dawson concluded.
But in 2014, microbiology professor Anthony Hilton and his students at Aston University in the United Kingdom reignited the debate. They studied the transfer of E. coli and Staphylococcus aureus from a variety of indoor floor types (carpet, laminate, and tile) to toast, pasta, biscuit, and a sticky sweet, with contact times ranging between three to 30 seconds. According to their results (which were shared in a press release but not published in a peer-reviewed journal), the longer a piece of food was in contact with the floor, the more likely it was to contain bacteria. This could be interpreted as evidence in favor of the five-second rule, Hilton noted, but was not conclusive.
This prompted food science professor Donald Schaffner and his master’s thesis student, Robyn C. Miranda, at Rutgers University in New Jersey to conduct a rigorous study on the validity of the five-second rule, which they published in the journal Applied and Environmental Microbiology. They looked at bacterial transfer to four different foods (watermelon, bread, bread with butter, and gummies) when dropped on four different surfaces (stainless steel, ceramic tile, wood, and carpet) contaminated with Enterobacter aerogenes. By analyzing bacterial transfer at <1, 5, 30, and 300 seconds, they found that longer contact times resulted in more transfer but some transfer took place “instantaneously,” after less than 1 second, thus debunking the five-second rule once and for all.
Your chance of falling ill after eating food that has touched the floor depends on factors like how contaminated the floor is and the type of bacteria present. “Based on our studies, the kitchen floor is one of the germiest spots in the house,” Charles P. Gerba, a microbiologist and professor of virology at the University of Arizona, tells Popular Science. Believe it or not, “the kitchen is actually germier than the restroom in the home,” he added. This is because, compared to other rooms in a house, the kitchen gets a lot of foot traffic and food debris often falls on the floor, creating an ideal breeding ground for bacteria. While most of the bacteria lurking on kitchen floors are harmless, some—like Clostridium, Campylobacter, Salmonella, and Escherichia—can cause food poisoning.
If you just hate throwing away food, there are safer options than relying on the five-second rule. Rinsing dropped food can reduce contamination—although this method isn’t foolproof. “Rinsing is a good idea if it is a fruit or vegetable, but it’s harder to rinse off microbes from meat due to their rougher surface,” Gerba said. “You should also rinse food off if it falls in the kitchen sink as it is also very germy because of the moisture in the sink,” he suggested.
The next time you’re tempted to eat that cookie you just dropped, remember: bacteria move fast. As hungry as you may be, do you really want to eat a Salmonella-laced cookie?
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.
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