Researchers Test the 5-Second Rule

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One hundred twenty-eight transfer scenarios were evaluated: each contact surface type, each food type, 4 contact times (>1 second, 5, 30, and 300 seconds) and 2 inoculum matrices.
One hundred twenty-eight transfer scenarios were evaluated: each contact surface type, each food type, 4 contact times (>1 second, 5, 30, and 300 seconds) and 2 inoculum matrices.

Multiple factors play a role in surface-to-food bacterial transfer, including food topography and moisture level, according to research published in Applied and Environmental Microbiology.1

Using watermelon, white bread, unsalted butter, and gummy candy, Donald W. Schaffner, PhD, food science specialist and professor in the department of food science at the School of Environmental and Biological Sciences, Rutgers University-New Brunswick, New Jersey, and colleagues set out to debunk the “5 second rule.” The 5 second rule is a popular notion that states that food dropped on the floor for 5 seconds is safe from causing gastrointestinal complications because certain organisms need time to transfer.

Cultures of enterobacter aerogenes B199A—a nonpathogenic, food-grade microorganism—were prepared, as were 4 common domestic surfaces: 16 gauge stainless steel, ceramic glazed tile, maple laminate wood, and indoor/outdoor carpet. Each surface material was cut into a 5x5cm square, disinfected prior to experimentation, and spot inoculated with 1 ml of inoculums—8 to 10 drops spread over the surface.

One hundred twenty-eight transfer scenarios were evaluated: each contact surface type, each food type, 4 contact times (>1 second, 5, 30, and 300 seconds) and 2 inoculum matrices. Tryptic soy broth (TSB) or peptone buffer were used to grow enterobacter aerogenes on each surface, and surfaces dried completely before food samples were dropped from a height of 12.5cm and left for the prespecified periods. The researchers replicated each scenario 20 times, for a total of 2560 measurements.

The researchers noted  that more bacteria transferred to watermelon (~0.2% to 97%) compared to other foods; gummy candy had the least bacteria transfer (~0.1% to 62%), while transfer of bacteria to bread and bread with butter were similar (~0.02% to 94% and 0.02% to 82%, respectively).

“Our study shows that bacterial transfer is dependent on the surface, food type, contact time, and inoculum matrix,” Dr Schaffner noted in a press release about the study.2 “Although this research shows that the 5-second rule is ‘real' in the sense that longer contact time result in more transfer, it also shows that other factors including the nature of the food and the surface are of equal or greater importance.”

References

1.      Miranda RC, Schaffner DW. Longer contact times increase cross-contamination of Enterobacter aerogenes from surfaces to food. Appl Environ Microb. 2016. doi: 10.1128/AEM.01838-16.

2. Rutgers researchers debunk “five-second rule”: eating food off the floor isn't safe [news release]. Rutgers Today. New Brunswick, NJ: Rutgers University. Published September 8, 2016. Accessed September 16, 2016.

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