Eating food that has fallen on the floor isn't safe -- even if you pick it up right away. So much for the five-second rule/ Bacteria can transfer to food in less than one second. Led by Donald Schaffner, a professor and extension specialist in food science, the team tested four different foods dropped on four types of surfaces and then left on the surfaces for varying lengths of time to determine how quickly bacteria transferred to the food.

The tests:
The surfaces: stainless steel, ceramic tile, wood and carpet
The foods: watermelon, bread, bread and butter and gummy candy
The times: less than one second, five seconds, 30 seconds and 300 seconds

The researchers used two media -- tryptic soy broth and peptone buffer -- to grow Enterobacter aerogenes, a nonpathogenic "cousin" of Salmonella naturally occurring in the human digestive system. Transfer scenarios were evaluated for each surface type, food type, contact time and bacterial preparation; surfaces were inoculated with bacteria and allowed to completely dry before food samples were dropped and left to remain for specified periods. All totaled 128 scenarios were replicated 20 times each, yielding 2,560 measurements. Post-transfer surface and food samples were analyzed for contamination.

The results:
Watermelon had the most contamination of the four foods tested. This was not surprising since the wetter the food, the higher the risk of bacteria transfer.
The gummy candy had the least contamination.
Carpet had very low transfer rates of bacteria compared with tile or stainless steel. Wood was more variable. The topography of the surface played an important role in bacterial transfer.

The longer the dropped food remained in contact with the surface, the greater the number of bacteria that adhered to it. The takeaway: "The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food," Schaffner said. "Bacteria can contaminate instantaneously."


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