SCIENCE

Why The Five-Second Rule Is 'Sort Of' Right

It's all about the kind of food and the kind of floor.

10/09/2016 8:20 AM AEST | Updated 13/09/2016 12:44 AM AEST
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If you’ve ever had your doubts about the “five-second rule,” a new study won’t come as much of a surprise. It shows that food dropped on the floor can get pretty germy even if you scoop it up within the proverbial five seconds.

“Bacteria can contaminate instantaneously,” Dr. Donald Schaffner, a professor of food science at Rutgers University and co-author of the study, said in a written statement.

For the study, which was published online on Sept. 2 in the journal Applied and Environmental Microbiology, Schaffner and a collaborator dropped four foods (watermelon, gummy candy, bread and buttered bread) onto four different surfaces (steel, ceramic tile, wood and carpet) that had been contaminated with a harmless relative of disease-causing Salmonella bacteria.

The food was allowed to remain in contact with the surfaces for one, five, 30 or 300 seconds before being removed and tested to see if it had become contaminated with the germs.

What did the study show? Steel and tile surfaces tended to transfer more bacteria, while transfer rates from wood were more variable.

More to the point, the longer the food remained in contact with the germy surface, the greater the contamination tended to be. Watermelon showed the greatest contamination, gummy candy the least. That’s not especially surprising given that bacteria thrive in moisture.

“Bacteria don’t have legs, they move with the moisture, and the wetter the food, the higher the risk of transfer,” Schaffner said in the statement. 

This isn’t the first time scientists have taken on the five-second rule.

A 2014 study conducted at Ashton University in England used similar methodology but employed E. coli and Staphylococcus bacteria. The research, which wasn’t subjected to peer review, found time on the floor to be a “significant factor” in determining the level of contamination ― and moist food on a tiled or laminated surface was the diciest proposition. 

And earlier this year, the Discovery Science Channel aired a segment in which the narrator said that “moist foods left longer than 30 seconds [on the ground] collect 10 times the bacteria than those snapped up after only three.”

So where does that leave us?

Schaffner said the key take-away from his research was that the five-second rule simply doesn’t hold true for for very moist foods that fall onto non-absorbent surfaces. But, he added, “for some foods dropped on other surfaces, the transfer rate does increase with time, making this five-second rule ‘sort of’ right.”

Maybe just hold onto your food.

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