Researchers at Rutgers University have partially disproven one age-old urban myth, which says that food dropped on the ground is safe to eat if picked up within five seconds. Professor of Food Science at Rutgers, Donald Schaffner, published the findings in the American Society for Microbiology’s journal, Applied and Environmental Microbiology.

Schaffner explains, “The popular notion of the ‘five-second rule’ is that food dropped on the floor, but picked up quickly, is safe to eat because bacteria need time to transfer,” and adding that the practice is featured in at least two TV programs, he continues by saying “We decided to look into this because the practice is so widespread. The topic might appear ‘light’ but we wanted our results backed by solid science.”

The research tested four different surfaces, each paired with four different foods. The surfaces included stainless steel, wood, ceramic tile, and carpet. The foods included watermelon, bread, buttered bread, and gummy candy. The researchers tested contact times of less than one second, five seconds, thirty seconds, and three hundred seconds. They grew the bacteria Enterobacter aerogenes, which is a non-pathogenic bacteria, related to Salmonella, which occurs naturally in the human digestive system.

Surfaces were inoculated with the bacteria, and then allowed to dry fully. Food samples were then dropped and left for various amounts of time. Scenarios were tested using each surface type, food type, contact time, and the researchers used two different methods to grow the bacteria, trying each scenario with both methods of bacterial preparation. This totaled 128 scenarios, which were tested 20 times each. This amounted to 2,560 measurements. For each measurement, the food as well as the surface were tested for cross-contamination.

The results varied between the expected and the counterintuitive. Unsurprisingly, watermelon was the most easily contaminated, while gummy candy was the least. Schaffner explained, saying that “Transfer of bacteria from surfaces to food appears to be affected most by moisture. 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.”

Counterintuitively, the carpet had low rates of transfer, whereas the stainless steel and tile rates of transfer were quite high.

Ultimately, the results showed that while the five second rule is realistic, in the sense that longer contact times does increase contamination, it also shows that a variety of other factors determine the results. And of course, the transfer of bacteria can indeed be instantaneous, in certain circumstances.

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