Rancid remedies: Uncovering ancient repellents from native wisdom

Rancid bacon fat might not sound revolutionary, but at New Mexico State University, it might be the key to creating an all-natural mosquito repellent.

As Texas archeologist Gus Acosta was going through records from the 1700s, he found documents from Spanish conquistadors detailing their journeys in America.

Many of the conquistadors wrote about how Indigenous people would slather themselves with old animal fats, repelling pests such as mosquitoes from the area. According to the documents, many Gulf Coast explorers insisted the practice prevented mosquitos, even in high-density areas like Texas and neighboring states.

“So everybody who traveled to the Americas these days complained about mosquitoes in their journals,” said Immo Hansen, a biology professor at NMSU. “The natives have developed technologies to to deal with this, and they describe them using rancid fats.”

After seeing the studies, Acosta decided to reach out to conduct experiments using the historical documents as a baseline . For nearly five years, Acosta and Hansen gathered various fats and oils for experimenting.

“Normally, fats and oils have relatively no odor because they’re made up of long and fatty acids, which aren’t very volatile,” said Hailey Luker, biology Ph.D student at NMSU. “Over time, if these oils are exposed to air, oxygen, sunlight, heat, these fatty acids become smaller and those become more volatile.”

To create a rancid acid, researchers render fat from tissue, boil it to create an oil and let it rest until the oil becomes rancid. For the oils to be considered rancid, it must taste unpleasant or create a rotten smell.

With that knowledge, NMSU conducted a study using olfactory senses to test each oil. People tested in a double blind study — meaning neither researchers or participants know which sample is which — to determine which oils were the most rancid.

Researchers tested cod, bear, shark and alligator oils. All oils were sourced ethically, meaning every animal was legally and ethically hunted. For the shark and fish oil, researchers received samples from a sustainable fishery.

Each oil has different properties, so oils will all become rancid at different rates. Volunteers tested each oil and ranked them based on the level of rancidity. Luker said research found that alligator oil was the most rancid, with volunteers having a “visceral reaction” to the smell.

“It’s actually astonishing how good data we got from the smell test,” Hansen said. “They scored these oils almost identical. And so, I think our olfactory system, our nose, is evolved very much to detect spoiled food.”

Research found showed the mosquitoes had a reaction to the oils, causing them to avoid testing subjects arms that were coated with rancid fats.

Using this study, NMSU will conduct the same study using plant oils. In the future, NMSU researchers hope to find the combination of molecules that can create an all-natural mosquito solution.

“We want to develop new mosquito repellents, cheap mosquito repellents, easy-to-make mosquito repellents, and people are very interested in what they conceive as nonsynthetic repellent,” Hansen said.

Hansen said this study is important to all people, and increasingly so for New Mexicans.

“Albuquerque has been invaded by yellow fever mosquitos since 2018,” he said. “They transmit diseases like yellow fever, which is a deadly disease. There’s a vaccine against it, but nobody here is immunized against it. But a bigger concern is they also transmit dengue fever, Zika virus and chikungunya virus.

“Temperature is rising and mosquitoes, tropical mosquitoes, are moving north, basically invading us so we need to get serious about mosquitos and well protect ourselves.”