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It’s a baffling smell, straddling the line between stale peanuts and off-gassing paint, or a sandwich of stale bread and old crayons. It’s the powerfully gross fragrance of rancid sesame oil—the aroma of disappointment.
Standing in front of your stir-fry mise en place, you might wonder if there’s any way to rescue the fouled bottle before you. I hate to be the one to break it to you, but that oil is a goner.
It’s not your fault—rancidity is the ultimate fate of every cooking fat, be it extra-virgin olive oil or ghee or, yes, sesame oil. It’s a product of the oxidation process, during which oil molecules come into contact with oxygen and begin to break down. This process creates free radicals: molecules with an unpaired electron, making them highly chemically reactive and ultimately responsible for the color and flavor changes in rancid oil. Heat, light (in the form of UV exposure), water molecules, and certain enzymes speed the oxidation process along.
Unsaturated fatty acids tend to oxidize more quickly than saturated ones. Sesame oil is about 80% unsaturated fat, so it’s more prone to rancidity than, say, coconut oil, which is rich in saturated fat. And because the sesame oil that we typically use for cooking is unrefined—which gives it its nutty, rich, irreplaceable flavor—its shelf life is even more precarious.
Do I have to refrigerate sesame oil?
Surely some version of this question is what brought you to this article in the first place. The short answer: It’s complicated.
Must you refrigerate sesame oil? Absolutely not. It’s shelf-stable—for a time, at least—and it’s been a part of the human diet far longer than refrigeration has been around. (Sesame has been cultivated since the Bronze Age.) Should you refrigerate sesame oil? That largely depends on how quickly you plan to use it.
As with other oils, it’s best to store sesame oil in a cool, dark place, away from heat, and in a well-sealed container. For some people, the refrigerator might easily meet all of those requirements. Storing sesame oil in the fridge, especially if you don’t use it all that often, will prolong its shelf life without any ill effect, outside of some temporary cloudiness. It’s difficult to say with certainty how much you can delay rancidification; it will depend on the brand, the quality of the sesame seeds used to make the oil, and the amount of time it spent on a shelf before arriving in your kitchen.
Realistically, you’ll have to rely on best-by dates and your senses to tell whether your sesame oil has gone off. “The shelf life is two years from the manufacturing date,” says Amalissa Uytingco of Yun Hai, an importer and retailer of Taiwanese ingredients that counts the cold-pressed sesame oils from Dong He Oil mill among its offerings. “It is likely safe to consume afterward as well, but the shelf life reflects how long the oil will retain peak flavor. After opening, it’s best to use as soon as possible and ensure that the cap is sealed tightly when stored.”
More conservatively, food writer Max Falkowitz suggests using up a bottle of sesame oil within a year of purchase, regardless of when you opened it. (Yes, even unopened oil will eventually go rancid.)
Our verdict? Erring on the side of caution is always your best bet with perishable foods. Oil isn’t wine—it doesn’t tend to improve with age.
Can rancid oil make me sick?
Perhaps you’ve already doused your chicken and garlic chive stir-fry with a glug of sesame oil, realizing only later that it’s gone off. Worry not, because it won’t make you sick—not right away, at least. Spoilage in other types of foods involves bacteria and molds that can cause illness, but rancid oil is a different story. Its flavor might not appeal, but it’s unlikely to cause any immediate harm.
As for the longer-term health effects of rancid oil, some researchers have suggested that consuming the free radicals in oxidized foods could lead to cellular damage and chronic disease over time. There haven’t been any large-scale, longitudinal studies to confirm this, however—most likely because it’s difficult to track the long-term effects of something that people don’t consume on a regular basis.
The late food critic Mimi Sheraton claimed in a 1983 New York Times article that rancid oils “destroy the body’s own supply of vitamins B and E,” but there is little clinical evidence for that in humans (there is also no single “vitamin B,” but rather a complex of B vitamins). One study from 1938 did find that rancid fats destroyed vitamin A, but only in rats. Another rat study published in 1939 had similar findings for vitamin E. Meanwhile, a more recent Polish study found that thermally oxidized cooking oils (that is, oils that have been degraded by cooking at very high heat) may contribute to “various disease conditions” due to the presence of free radicals. And while we don’t know for sure if the free radicals in rancid oils persist long enough in the body to cause harm, we do know that other types of free radicals can cause oxidative stress—a condition that is believed to be linked to a host of serious ailments.
So, there’s little reason to play the odds with old oil. Unless you use sesame oil frequently, it’s best to stick with small bottles—which also provides the opportunity to try a few different brands to see what works best for your tastes.
For a good workaday sesame oil, you can’t go wrong with Kadoya and Ottogi, both of which have wide distribution and are easily found in Japanese and Korean grocery stores. Use them for marinades and dressings in which the sesame flavor doesn’t have to play center stage (but they’re perfectly all-purpose too). For a finishing oil, opt for smaller producers with more nuanced offerings, like Momofuku’s toasted sesame oil, Wadaman’s deeply nutty black sesame oil, Dong He’s cold-pressed white sesame oil, or the oils by Queens Bucket, which lightly toasts its sesame seeds using infrared light.
Uytingco notes that whatever you do, don’t keep your sesame oil above or near the stovetop. “The ideal place to store the sesame oil is in a cool and shaded place away from sunlight,” she says. “However, if this is not a possibility, storing the sesame oil in the fridge works fine as well.”
