Olive oil can turn for several reasons, says Danielle Duboise, co-founder of the meal delivery service Sakara and co-author of Eat Clean, Play Dirty. When cooked at 350 degrees F or higher in the oven or on the stove, its heart-healthy polyphenols are destroyed, studies show. If it's stored in a clear container (exposing it to light) or with a loose cap (which lets air in), the oil can oxidize and create free radicals. When taken in consistently over time, free radicals are linked health problems such as advanced aging, heart disease, and cancer, says Stanley Omaye, Ph.D. a professor of nutrition and toxicology at the University of Nevada, Reno.
Smell is usually the telling factor, Duboise adds. Good olive oil will have a peppery, grassy, and earthy scent; if it’s rancid, it’ll have a waxy, crayon-like odor.
The bottom line:
Optimizing your oil starts in the store. Choose a high-quality product, which will have a “harvest” date on the label rather than a “bottled on” date, Duboise says. (It’s best within 18 months of being harvested.) At home, keep it in an opaque container in a cool, dark place—but never in the fridge. Make a habit of smelling the oil before each use to make sure it’s still at top quality.