Friday Flashback: If you ever eat food dropped on the floor, read this.

By Seriously Science | February 28, 2014 6:00 am
Photo: flickr/Sebastian Anthony

Photo: flickr/Sebastian Anthony

You’ve probably heard about the five-second rule: the idea that if you drop food on the floor, you have five seconds to pick it up and eat it before it gets too “contaminated” to safely ingest. But does this “rule” actually have any scientific basis? Here, researchers tested how long it took for bacteria to be transferred to food (specifically, bologna and bread) resting on different surfaces (tile, wood, and carpet). They found that 5 seconds of contact was enough to transfer 99% of bacteria from the tile to the bologna, but a much smaller number of bacteria were transferred when the food was dropped on carpet. So there you have it: if you drop your food on the carpet, feel free to pick it up, brush off the lint, and enjoy. But if you drop it on tile, you might want to go ahead and grab another slice of bologna.

Residence time and food contact time effects on transfer of Salmonella Typhimurium from tile, wood and carpet: testing the five-second rule.

Three experiments were conducted to determine the survival and transfer of Salmonella Typhimurium from wood, tile or carpet to bologna (sausage) and bread.
Experiment 1. After 28 days, 1.5 to 2.5 log(10) CFU cm(-2) remained on tile from and the more concentrated media facilitated the survival of S. Typhimurium compared with the more dilute solutions. Experiments 2 and 3. The bacterial transfer rate to food decreased as the bacterial residence time on the surface increased from 2, 4, 8 to 24 h with transfers of 6.5, 4.8, 4.6 and 3.9 log CFU ml(-1) in the rinse solutions, respectively. Over 99% of bacterial cells were transferred from the tile to the bologna after 5 s of bologna exposure to tile. Transfer from carpet to bologna was very low (<0.5%) when compared with the transfer from wood and tile (5-68%).
(i) Salmonella Typhimurium can survive for up to 4 weeks on dry surfaces in high-enough populations to be transferred to foods and (ii) S. Typhimurium can be transferred to the foods tested almost immediately on contact.
This study demonstrated the ability of bacteria to survive and cross-contaminate other foods even after long periods of time on dry surfaces, thus reinforcing the importance of sanitation on food contact to minimize the risk of foodborne illness.”

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  • Jeffrey Slater

    you need to get out more

  • Infiniteworld

    I saw this same study done a while back (no link, sorry) but they also did the obvious and tested the floor and found no bacteria. Which is more to the point anyway.

  • Wayne101

    Can we see where dropping an item that is covered with “glue” eg bologna or moist white bred, might collect and retain bacteria better than say a dry saltine cracker? Then the question isn’t just a function of surface and contact time. It also includes consideration of donor and receptor surfaces as well. A dry saltine dropped on a hard dry surface will not collect and retain the same number of bacteria that that same saltine would collect when first covered with a layer of butter, squeeze-cheese, or pate.

    One might guess that a wet surface contacting a wet surface would be an optimal transfer situation. Then a wet surface contact with a dry surface, followed by a dry to dry pairing. Dry to dry “might” meet the 5-second rule of food contamination.

    The collect and retain properties may also vary depending on the moisture type (eg water v oil?) and any anti-bacterial properties (eg honey, vinegar, salt)

    So the testing does not disprove the rule. It only proves that the 5-second rule doesn’t apply in all situations.


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