Life's Little Mysteries

Why Does Cooking Oil Go Rancid?

cooking oil
(Image credit: Serghei Starus |

Cooking oil is an essential kitchen ingredient — that is, until it goes rancid.

The culprits behind this unsavory expiration can be found in pretty much every kitchen. They include exposure to light, heat, water, certain microbes and the very air people breathe, chemists told Live Science.

Luckily, the perpetrators behind this sour, musty smell (and taste) can be forestalled, increasing the shelf life of cooking oil, the experts said. [Can Peanut Butter Go Bad?]

Breaking bonds

Any oil that contains unsaturated or polyunsaturated fats — that is, fats that allow the oil to be liquid at room temperature — can go rancid, said Susan Richardson, a professor of chemistry at the University of South Carolina.

Unsaturated fats have a carbon-carbon double bond in their structure, Richardson told Live Science. However, these bonds can be broken by oxygen in the air, she said. This is called oxidation.

If someone forgets to put the cap on a bottle of oil, such as vegetable, sesame or olive oil, the oil within will be exposed to oxygen.

In this case, "an oxygen comes bumbling along and hits a carbon-carbon double bond," said John Malin, a retired associate professor of chemistry at the University of Missouri. "[The oxygen] attacks that double bond and forms a carbon-oxygen bond."

This carbon-oxygen bond can lead to a number of products, including an aldehyde, ketone or carboxylic acid. Some of these products have rancid odors and tastes, the chemists said.

Water has a similar effect, because H2O has an oxygen atom in it, Malin said. This process, when part of the H2O molecule inserts itself into the carbon-carbon double bond, is called hydration. What's more, rates of oxidation and hydration are enhanced in the presence of light, he said. Ultraviolet light is even more powerful than visible light, because it has more energy, he added.

"That's why cooking oil will preserve better in the dark and when it's capped," Malin said.

Heat also accelerates these chemical processes, and can make oil go rancid more quickly. When they're hot, "these molecules are moving around and wiggling real fast and banging into each other," he said.

However, Malin doesn't recommend storing cooking oil in the refrigerator. That's because the cold temperature will slow down the movement of the liquid's molecules. As they move more slowly, some of the molecules will drop out of the solution and stratify, leading to a cloudy look. It's not unhealthy to consume oil in this state, but most people prefer oil that looks clear, Malin said.

Perhaps unsurprisingly, certain microbes can also make oils rancid. Oils contain triglycerides, a chemical compound that has one glycerol molecule and three fatty acids. Some microbes can chop off those fatty acids from the triglyceride backbone, which, in turn, can make the oil rancid, Richardson said.

"Those [chopped-off] fatty acids can smell and taste bad," she said. [Good Food Gone Bad]

Health effects

By understanding the chemistry behind rancidity, people can figure out which oils will last longer, so long as they're properly stored, Richardson said. For instance, some oils have more carbon-carbon double bonds than others, meaning they will probably spoil more readily. So, in a three-way contest among three oils she randomly picked, corn oil will likely spoil fastest, canola oil next and olive oil last, she said.

But no matter the type of oil, people should throw it away if it's rancid, both chemists advised.

"[Rancid foods] lose their vitamins, but they also can develop potentially toxic compounds," Eric Decker, the head of the Department of Food Science at the University of Massachusetts Amherst, told The Dallas Morning News. These compounds have been linked to advanced aging, neurological disorders, heart disease and cancer, the Morning News reported.

For instance, Richardson found that used oil she had left in a deep fryer had turned sour-smelling and rancid.

"I smelled it and threw it out," she said.

Original article on Live Science.

Laura Geggel

Laura is the archaeology and Life's Little Mysteries editor at Live Science. She also reports on general science, including paleontology. Her work has appeared in The New York Times, Scholastic, Popular Science and Spectrum, a site on autism research. She has won multiple awards from the Society of Professional Journalists and the Washington Newspaper Publishers Association for her reporting at a weekly newspaper near Seattle. Laura holds a bachelor's degree in English literature and psychology from Washington University in St. Louis and a master's degree in science writing from NYU.