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I’ve visited a laboratory or museum before and admired a pristine eyeball or a tiny deceased living being floating in a glass jar, you’ve seen the preservative power of alcohol. The formal name of this technique is liquid preservation. Scientists have relied on it to preserve their curious specimens since the 17th century. And if done right, it can last a specimen for hundreds of years, according to the American Museum of Natural History.
But how does it work?
“Most importantly, it is toxic to the types of microorganisms that would cause the decay,” Bill Carroll, associate professor of chemistry at Indiana University Bloomington, told Live Science. He cited wine as an example. It’s made when yeast eats sugar from grapes and then excretes alcohol. But the yeast excretes so much alcohol that the concentration becomes toxic and kills the yeast, he said. And that alcohol content – around 14% – helps delay the growth of bacteria for years, according to California Wine Advisor (many wines also contain additional preservatives like sulfur).
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Preserving other organic materials – like DNA, tissues, or even whole animals – requires higher levels of alcohol, said Katherine Maslenikov, director of fish collection at the Burke Museum in Seattle. Maslenikov typically relies on alcohol, especially ethanol, for long-term storage.
For example, Maslenikov could take a fish sample, take some tissue samples for DNA analysis, and inject formalin (a solution of formaldehyde gas dissolved in water) into the fish to stop internal biological processes such as enzymatic reactions and tissue breakdown. Then she could dip the fish specimen in a glass with 70% alcohol and 30% water. For long-term storage, “70% seems like that magic number,” Maslenikov said. There is enough water in the solution to keep the tissue hydrated, which will help the animal or sample hold its shape, and there is enough alcohol to prevent mold and bacterial growth, she said.
Alcohol in even higher concentrations, for example 95% ethanol, has a dehydrating effect, ie it removes and replaces the water in the cell, in the tissue or in the whole body sample with alcohol. The lack of water leads to changes in water-sensitive proteins; They unfold or denature and harden side by side, which fixes the shape of the sample, according to Ask a Biologist, a series from Arkansas State University. This technique is a common method of preserving DNA, according to a 2013 study in the journal PLOS One.
It can be difficult to decide what percentage of alcohol to use. Using too much or too little can affect the shape and flexibility of the sample, or even affect its ability to preserve the sample in solution. It preserves high alcohol concentrations used to dehydrate a sample. However, Maslenikov said that this process can also leave a specimen wrinkled (due to the loss of water) and brittle (due to the hardened proteins). Sometimes that’s fine; it all depends on what you’re trying to preserve.
Meanwhile, a specimen can spoil quickly if it holds back too much water.
“If an organism has enough water in its tissues, it can dilute the alcohol,” Christopher Rogers, associate research professor at the University of Kansas Biological Survey and Center for Ecological Research, told Live Science in an email. When this happens, the alcohol concentration may not be high enough to kill any lurking microorganisms that may be deeper in the sample, such as the intestines of a whole animal sample. These overlooked bacteria can degrade the sample. “That’s why it’s important to change your alcohol [about] 24 hours after placing the creature, “because it increases the alcohol concentration of the solution, Rogers said.
When it comes to using alcohol as a preservative, Carrol looks for a concentration sweet spot: “A concentration where you inhibit microorganisms but don’t destroy the cellular structure of what you are looking at.”
Originally published on Live Science.