Food science professor and extension specialist Donald Schaffner found that cross-contamination can occur in less than one second, dependent upon certain conditions--moisture level, surface time and contact time.

“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,” Schaffner said, adding that while the pop culture “rule” has been featured by at least two TV programs, research in peer-reviewed journals is limited.

“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,” said Schaffner, who conducted research with Robyn Miranda, a graduate student in his laboratory at the School of Environmental and Biological Sciences, Rutgers University-New Brunswick.

The research team tested a number of different controls, including:

• Four different foods--bread, bread and butter, gummy candy and watermelon
• Four contact surfaces--carpet, ceramic tile, stainless steel and wood.
• Four contact times--less than 1 second, 5 seconds, 30 seconds and 300 seconds
• Two media--trypic soy broth and peptone buffer (to grow Enterobacter aerogenes, a nonpathogenic “cousin” of Salmonella naturally occurring in the human digestive system)

A total of 128 transfer scenarios were conducted and replicated 20 times each, creating 2,560 measurements. Post-transfer surface and food samples were then analyzed for contamination. Here are some of the findings:

• Watermelon had the most contamination; gummy candy had the least
• Carpet had very low bacteria transfer rates compared to tile and stainless steel

“Transfer of bacteria from surfaces to food appears to be affected most by moisture,” says Schaffner. “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.”

Overall, the oversimplified concept of 5-second rule has been overshadowed by the fact that the nature of the food and the surface it falls on are of greater importance. Bacteria can transfer from surface to food instantly.

Rutgers’ findings have been published by Applied and Environmental Microbiology, a journal of the American Society for Microbiology.

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