The Five-Second Rule
September 29, 2016
Chemistry has given us many useful materials, but laboratory chemicals need to be handled carefully because of their toxicity and other hazards. The US Occupational Safety and Health Administration has published a useful introduction to safe laboratory practice. Such a guide, supplemented by additional safety information for the specific chemicals being used, should be required reading before working in a laboratory.
Acids are a common laboratory chemical. When working with acids, it's important to use the proper personal protective equipment (PPE) that includes goggles, gloves, a lab coat, and possibly a protective apron. Most acids need to be diluted before use, and the rule is to always add the acid to the diluting water. Otherwise, the heat evolved during the dilution would cause added water to vaporize and sputter into your face.
The reason for this is the large enthalpy of mixing, also called the heat of mixing, of the acid and water. This is a thermodynamic effect related to entropy, and the Flory–Huggins solution theory is often used to estimate the heat of mixing. This theory was originally developed for polymer mixtures, but it's applicable to many solutions when appropriate values of solubility parameters are used.
This acid-water dilution rule guides us in our laboratory life, but there are other rules that guide us in everyday life. One of these, known to early career scientists, is the mortgage/rent rule that states that not more than about 25% of your monthly income should go to rental or mortgage payments. While some dissenting opinions have scaled this up to above a third of your income, since we're talking safety, 25% would be more prudent.
The rule of life that's gotten a lot of attention lately is the so-called "five-second rule." I hadn't heard of this rule until about five years ago, when it started to appear frequently in many television sitcoms. The rule is that food dropped on the floor is safe to eat if retrieved within five seconds. Such a rule is easily tested by experiment.
In 2007, scientists from the Clemson University Department of Food Science and Human Nutrition tested the five-second rule in a series of experiments focused specifically on the Salmonella Typhimurium bacterium. They examined the transfer of this bacterium from wood, tile, and carpet to bologna sausage and bread.
The Clemson research team found that the Salmonella Typhimurium bacterium survived for up to four weeks on dry surfaces in populations significant enough to be transferred to foods. It was further found that such bacteria will transfer almost immediately to the bread and bologna on contact. For bologna, a five-second contact was enough to transfer 99% of bacterial cells on a tile surface. Interestingly, transfer from carpet material was very small, being less than 0.5%, as compared with tile and wood surfaces (5-68%).
In a recent follow-up study, scientists from the School of Environmental and Biological Sciences of Rutgers University (New Brunswick, New Jersey), did experiments on bacterial transfer to bread, bread with butter, watermelon, and gummy candy.[5-6] The tested surfaces were stainless steel, tile, wood, and carpet, and the contact times tested were about 1 second, 5 seconds, 30 seconds and 300 seconds.
This research was conducted at the Rutgers School of Environmental and Biological Sciences by professor Donald W. Schaffner and his graduate student, Robyn Miranda. They used two different bacterial growth media, tryptic soy broth and peptone buffer, to grow the nonpathogenic Enterobacter aerogenes, that's similar to Salmonella. The surfaces were spot inoculated with a milliliter of the bacteria-contaminated media, and they were allowed to dry for 5 hours, yielding an approximate concentration of 107 colony-forming units/surface.[5-6]
The foods, with a 16 square centimeter contact area, were dropped from a 12.5 cm height and left on the surfaces for the required time. In these experiments, there were 128 scenarios (4 foods x 4 surfaces x 4 durations x 2 bacterial media), and these were replicated 20 times each, for a total of 2,560 measurements. The post-transfer contamination was estimated by analysis of bacterial growth rates on tryptic soy agar.[5-6]
Watermelon had the most contamination, and gummy candy had the least, a consequence of the availability of a transfer fluid. Says Schaffner
"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."
As in the previous Clemson experiments, carpet had a very small transfer rate compared with tile and stainless steel. Wood surfaces gave various rates. "The topography of the surface and food seem to play an important role in bacterial transfer," says Schaffner. Transfer of bacteria to bread (∼0.02-94%) and bread with butter (∼0.02-82%) were similar, so it doesn't matter whether your bread falls butter-side down. Up to 97% of bacteria were transferred to watermelon, while gummy candy had a range of ∼0.1-62%.
There's some truth to the five-second rule, since longer contact times result in more bacterial transfer, but the type of surface and the type of the food are generally of greater importance.[5-6] As Schaffner summarizes,
"The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food... Bacteria can contaminate instantaneously."
- Laboratory Safety Guidance, Report OSHA 3404-11R, Occupational Safety and Health Administration, U.S. Department of Labor, 2011.
- D.M. Gualtieri, "Flux Growth of (Ca,Ge)-Substituted Rare-Earth Iron Garnets in the Regular Solution Approximation," J. Appl. Phys., vol. 50, no. B3 (March 1, 1979), pp. 2170-2172, http://dx.doi.org/10.1063/1.327041.
- Karen Weise, "Housing's 30-Percent-of-Income Rule Is Nearly Useless," Bloomberg.
- P. Anupriya, I. Han, M. Cox, C. Black, and 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, vol. 102, no. 4 (April, 2007), pp. 945-953, DOI: 10.1111/j.1365-2672.2006.03171.x.
- Robyn C. Miranda and Donald W. Schaffner, "Longer Contact Times Increase Cross-Contamination of Enterobacter aerogenes from Surfaces to Food," Appl. Environ. Microbiol., Advanced Online Publication (September 2, 2016), doi:10.1128/AEM.01838-16.
- Rutgers Researchers Debunk 'Five-Second Rule': Eating Food off the Floor Isn't Safe, Rutgers University Press Release, September 8, 2016.
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