Researchers Debunk 5-Second Rule

For many kitchens, the “five-second rule” about the safety of eating food dropped on the floor is a given: Pick it up before it’s been on the floor five seconds and it’s good to go.

Researchers at Rutgers University-New Brunswick have looked into the validity of that kitchen mythology and found that bacterial cross-contamination of food actually depends on the surface it contacts, the type of food, and the length of contact time. In other words, for food as moist as watermelon, any time in contact with the floor is probably too long.

“Cross-contamination risk is greatest if the food is wet, and in the case of watermelon, virtually all of the bacteria were transferred from the surface to the food even in a fraction of a second,” says Donald Schaffner, PhD, extension specialist in food science and distinguished professor at Rutgers University. He and his colleague’s research is published online in Applied and Environmental Microbiology.

Four surfaces were tested in the research: stainless steel, ceramic tile, wood, and carpet. In addition to watermelon and gummy candy, bread and bread and butter were also dropped on the different surfaces that had been contaminated with an equal quantity of Enterobacter aerogenes in two matrices, tryptic soy broth or peptone buffer. All possibilities were tested for four contact times: less than one second, five seconds, 30, and 300 seconds. The 128 scenarios were tested 20 times each.

Even with food least prone to transfer of bacteria—gummy candy in this research—from a surface least likely to transfer bacteria—which turned out to be carpet in this trial—at least one bacterial cell was observed. “In other words, there was no condition where we could definitely say there was zero risk,” says Dr. Schaffner.

The smaller degree of transfer from carpet indicated that the bacterial contamination had sunk down into the lower levels of the carpet where they were not available to contact the food. “I certainly don’t advocate going out and carpeting restaurant kitchens or food processing plants,” he says.

The take-home message for the food industry is that cross-contamination is a “very real phenomenon. While the degree of cross-contamination may vary, the default assumption should be that it always occurs,” says Dr. Schaffner. “I suspect that most restaurant kitchens and food processing plants have rules about not using food that has fallen to the floor, and my research confirms that those rules are there for a reason and should be observed.”