Though it was “busted” by the Myth Busters years back when they were still on the air, researchers at Rutgers University School of Environmental and Biological Sciences (extension), Professor Donald Schaffner and his graduate student Robyn Miranda recently put the “5 second rule” to rigorous scientific test. The “5 second rule” is the popular notion that food dropped on the floor, but picked up quickly, is safe to eat because bacteria need time to transfer.
Bacterial cross-contamination from food coming into contact with surfaces can contribute to foodborne disease. The cross-contamination rate of Enterobacter aerogenes was used as a proxy for disease carrying bacteria and measured on household surfaces of stainless steel, tile, wood and carpet. The food types were watermelon, bread, bread with butter and gummy candy. The transfer times tested were under 1 second, 5 second, 30 second and 300 seconds. Transfer scenarios were evaluated for each surface type, food type, contact time and bacterial prep; surfaces were inoculated with bacteria and allowed to completely dry before food samples were dropped and left to remain for specified periods
What the researchers found was that bacteria can transfer essentially immediately on contact with food that is dropped; and the wetter the food, the higher the risk of transfer― watermelon had the most contamination, gummy candy the least. Also, longer food contact times usually result in the transfer of more bacteria from each surface to food
All totaled 128 scenarios were replicated 20 times each, yielding 2,560 measurements. Post-transfer surface and food samples were analyzed for contamination. The researchers concluded that the longer food was in contact with a surface the in more bacterial transferred, they also found that other factors are to be considered such as the nature of the food and the surface it falls on, are of equal or greater importance. Surprisingly, they found that carpet has very low transfer rates compared with those of tile and stainless steel, the transfer rate from wood is more variable. “The topography of the surface and food seem to play an important role in bacterial transfer,” Dr. Schaffner said. “Bacteria don’t have legs, they move with the moisture, and the wetter the food, the higher the risk of transfer. Also, longer food contact times usually result in the transfer of more bacteria from each surface to food.”
The bottom line is that contamination can transfer almost immediately. It is essential to clean your working surfaces and floors regularly to prevent cross-contamination of food on your counters, and don’t eat whatever it was that fell on the floor, throw it out. Donald Schaffner is a professor and extension specialist in food science at the School of Environmental and Biological Sciences, Rutgers University-New Brunswick. Robyn Miranda is a graduate student in his laboratory there. Their study appears online in the American Society for Microbiology’s journal, Applied and Environmental Microbiology (only the abstract is free).