The 5-second rule of food is a cultural belief that if you drop food on the floor, as long as you pick it up within 5 seconds it is safe to eat. The reason people believe this is because bacteria need time to transfer to food so as long as the food is not on the floor too long, the bacteria will not have time to get on it. It’s important to realize that most sources make much to big a deal out of this rule, as we usually use it jokingly. We know from experience that we are not likely to get sick from a piece of food that sits on the floor for a few seconds and we refer to the 5-second rule in a tongue-in-cheek way. There may actually be bacteria on the food, we just know that it’s not likely to make us sick. For that matter, we know there may be bacteria present on our food at any time, but we don’t often become ill from it. So, with all that said, is the 5-second rule true? The answer is YES and NO.
Is the 5-Second Rule True or False?
The 5-scond rule is not completely untrue. Scientific evidence shows us that bacteria can transfer to food immediately. It doesn’t need 5 seconds. The type of surface and the type of food affects how quickly bacteria will move onto the food. And just as most of us know through experience, we aren’t likely to become ill from it. Let’s unpack this myth further and tease out some kernels of truth from it.
Before we even begin, we have to set some parameters. Let’s assume that when we drop a food item on the floor, the floor is dry. If we have to deal with both wet and dry floor conditions, there are too many variables to consider. As the mythbusters would say, let’s test the spirit of the myth, and I think that most people are assuming that the floor is dry. If a dirty floor stays wet for a long period of time, there are going to be a lot more active bacterial colonies living on it and I do not think most people would eat a piece of food after it dropped on a wet floor, 5-second rule or not.
Three Basic Questions About the Five-Second Rule
As I’ve already explained, the basic premise of the rule, that bacteria need several seconds to get onto food when it is dropped, is generally not true. But, there is more to consider! When trying to figure out if the 5-second rule, we actually have to ask three basic questions: 1) Can harmful, pathogenic bacteria survive on a dry surface for a long period of time 2) Is there harmful bacteria on our kitchen floor, 3) How fast can pathogenic bacteria transfer onto a piece of food after it is dropped, assuming such bacteria are present, and 3) Does it matter whether the food is wet or dry. We’ve eliminated a wet floor as an option, but we cannot eliminate the possibility of the food itself being wet. Intuition would tell us that a dry piece of bread or a cracker would be less prone to picking up bacteria than buttered bread or a wet piece of bologna, but this must be tested.
To thoroughly investigate the 5-second rule for food, researches would need to deal with all these questions and more. In fact, it would be appropriate to divide any experimental investigation into the 5-second rule into discreet individual experiments as described here: (1) the presence of microbes on various surfaces onto which food might be dropped, (2) the effect of food type on microbial transfer and (3) the effect of time of surface exposure on microbial transfer.
Type of Surface and Moisture Level Effects Bacterial Transfer Rate to Food
The most often-cited study concerning the 5-second rule did not ask most of these questions. Researchers at Rutgers University, instead, bypassed the question of whether common household floors harbored pathogenic bacteria and actually inoculated their test surfaces with Enterobacter aerogenes, a nonpathogenic “cousin” of Salmonella naturally occurring in the human digestive system. They tested stainless steel, tile, wood, and carpet with watermelon, bread, bread with butter, and gummy candy. They also used four different contact times – less than one second, five, 30 and 300 seconds.
The study found that found that moisture, type of surface and contact time all contribute to cross-contamination. In some cases, bacterial transfer begins in less than one second. 1Miranda RC, Schaffner DW.2016.Longer Contact Times Increase Cross-Contamination of Enterobacter aerogenes from Surfaces to Food. Appl Environ Microbiol82:.https://doi.org/10.1128/AEM.01838-16
It shouldn’t surprise you to learn that dropped watermelon had the most contamination, and gummy candy the least.
According to Donald Schaffner, professor and extension specialist in food science at Rutgers, “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.”
The Kernel of Truth in the 5-Second Rule
So, already we see a kernel of truth. Longer contact times result in more bacterial transfer but wet conditions also cause more contamination, something I believe most of us intuitively know. Remember what I said about a wet floor. You likely wouldn’t eat something after it dropped onto a wet floor.
The research also found that carpet had low transfer rates compare to tile or stainless steel. The rough topography of the carpet caused less contact with the food. The “topography” of the food matters as well. A bumpy and rough food, especially a dry one, will probably experience less bacterial transfer than a smooth one.
So, even according to this research, the 5-second rule is not completely false. Often, shorter contact times result in decreased bacterial transfer to food. However, Schaeffer summed up the findings by saying “The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food. Bacteria can contaminate instantaneously.”
Is There Harmful Pathogenic Bacteria On Our Floors
The research did not consider something that could, in a way, determine the rule moot and pointless. It did not consider whether our floors usually harbor colonies of pathogenic bacteria. There is no point in worrying about harmless bacteria. Some believe that the average floor is a bacterial horror. Nicole Nomides, M.T., M.S., CIC, an infection preventionist at the University of Michigan Health System, was quoted as saying:
Such bite-sized castoffs ought to go straight to the trash can or compost pile. As far as I’m concerned, the world is one big petri dish. I get why people do it; it’s just human behavior. If people don’t see the germs, they don’t believe they’re there.
It would be nice if scientists wouldn’t say things like “as far as I’m concerned” instead of “according to the evidence.” We get into enough trouble listening to lay folks talk about their non-evidenced opinions. Is the world a great big petri dish? Sure. Do we survive it just fine? Yes. The real question is, what is the evidence that your floor is a great big harmful petri dish? Can your dry floor harbor harmful bacteria? Yes, depending on how long it has been dry. In other words, it depends on how long ago it was wet.
Depending on your cleaning practices and other factors, your kitchen may be full of pathogens such as Escherichia coli, Staphylococcus aureus, Salmonella, Campylobacter, and Listeria. Likely, as in other parts of you home, it also harbors Fecal coliforms, the bacteria from human feces. 2Patricia A. Borrusso, Jennifer J. Quinlan, Prevalence of Pathogens and Indicator Organisms in Home Kitchens and Correlation with Unsafe Food Handling Practices and Conditions, Journal of Food Protection, Volume 80, Issue 4, 2017, Pages 590-597, ISSN 0362-028X, https://doi.org/10.4315/0362-028X.JFP-16-354. (https://www.sciencedirect.com/science/article/pii/S0362028X22100050)
However, the worst areas for such pathogens are wet areas and cloths used for wiping surfaces and/or drying dishes such as washing cloths sponges, and tea towels. You sink and the area around it are way worse than your floor and your countertops may be, at times, also more dangerous. Still, harmful bacteria can persist and survive on dry surfaces, sometimes for significant periods of time. How long depends on the type of bacteria.
According to a systematic review of literature by Kramer et al.:
- Most gram-positive bacteria, such as Enterococcus spp. (including VRE), Staphylococcus aureus (including MRSA), or Streptococcus pyogenes, survive for months on dry surfaces.
- Many gram-negative species, such as Acinetobacter spp., Escherichia coli, Klebsiella spp., Pseudomonas aeruginosa, Serratia marcescens, or Shigella spp., can also survive for month
- A few others, such as Bordetella pertussis, Haemophilus influenzae, Proteus vulgaris, or Vibrio cholerae, however, persist only for days. Mycobacteria, including Mycobacterium tuberculosis, and spore-forming bacteria, including Clostridium difficile, can also survive for months on surfaces. Candida albicans as the most important nosocomial fungal pathogen can survive up to 4 months on surfaces.
Harmful Bacteria Can Survive for Months on Dry Floors and Other Surfaces
In other words, unless you clean and disinfect your floor regularly, harmful bacteria, when present, can survive for months even if the floor is dry. If you drop wet food onto the floor, this bacteria can transfer before even a second has passed. If you drop dry food on the floor, it may take longer for this to occur.
If you drop food onto a wet floor or wet food onto a dry floor, consider the 5-second rule to be bunk! If you drop dry food onto a dry floor, realize that some transfer of harmful bacteria may take place, and this will increase depending on contact time. It does help to pick up the food faster!
A dropped cookie, cracker, or piece of unbuttered toast is no big deal. A dropped piece of bologna is going to pick up much more bacteria and can do so almost immediately. After 5 seconds, almost 100% of the bacterial cells will be on the bologna. Another study in the Journal of Applied Microbiology bears out the ability of bacteria to survive and cross‐contaminate other foods even after long periods of time on dry surface. 3P. Dawson, I. Han, M. Cox, C. Black, L. Simmons, Residence time and food contact time effects on transfer of Salmonella Typhimurium from tile, wood and carpet: testing the five‐second rule, Journal of Applied Microbiology, Volume 102, Issue 4, 1 April 2007, Pages 945–953, https://doi.org/10.1111/j.1365-2672.2006.03171.x
We know that we are not likely to become sick from such food. However, based on direct evidence, it is not proper to consider the food completely safe. How you proceed is up to you.
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