Move that oil away from the stove — here's why

Even a health-conscious choice like olive oil can become harmful if stored improperly. Leaving a bottle uncapped on a sun-drenched kitchen windowsill or next to a gas stove — where heat radiates constantly — are among the most common mistakes, and both can speed up oxidation.

Oxidized oil does not just taste and smell worse. It also loses antioxidant content. Antioxidants work by preventing or slowing the oxidation of fats, but as oxidation accelerates, those compounds are consumed in the process of fighting it — reducing levels of polyphenols, vitamin E and other protective components.

The three enemies of cooking oil are light, heat and air, and laboratory research confirms their impact. A study published in the journal Antioxidants in 2022 by researchers at the University of Castilla-La Mancha in Spain stored extra-virgin olive oil in open brown glass bottles at temperatures of 25, 40, 50 and 60 degrees Celsius over an extended period. The higher the storage temperature, the faster the total phenolic compound content and alpha-tocopherol — a form of vitamin E — declined in the air-exposed oil.

The deeper concern is that accelerated oxidation can lead to rancidity. Oxidation is the chemical process by which fats in oil react with oxygen. Rancidity is what happens when those fats break down as a result — producing unpleasant tastes and odors.

Medical experts caution that consuming rancid oil can cause mild digestive symptoms. The breakdown products generated during rancidity may also affect cells in the body. Animal studies have raised the possibility of links to cancer and inflammation.

A study published in Anticancer Research in 2002 by researchers at the University of Pécs Medical School in Hungary found increased expression of cancer-related genes in multiple organs of mice fed rancid corn oil. In a separate experiment published in Poultry Science in 2022, researchers at China Agricultural University replaced 75 percent of the corn oil in chicken feed with oxidized corn oil for 42 days; the chickens showed higher expression of some inflammation-related genes in small intestine tissue compared with a control group. However, both studies were conducted on animals, and further research is needed before the findings can be applied to humans.

To prevent rancidity and preserve beneficial antioxidants, proper storage by oil type is essential. Sesame oil, olive oil and avocado oil are best kept inside a cupboard away from direct sunlight, and should be stored away from the heat of a gas stove or oven. Olive oil is best consumed within three months of opening to maintain its flavor and quality.

Perilla oil requires different treatment. Because it is rich in omega-3 fatty acids — specifically alpha-linolenic acid — which are highly vulnerable to oxidation, it should be kept in the refrigerator at 4 degrees Celsius or below, not at room temperature. Keep it sealed and take it out only when needed.

The universal rule for all cooking oils is to minimize exposure to light, heat and air. Replace the cap immediately after each use and check the expiration date printed on the product.