Nuts are the top dogs in my kitchen. I love pressing almonds, with their little brown jackets, into buttery cookies. Craggy walnuts toasted with maple syrup and spices are my go-to salad toppers. And I ration the globular macadamias from parents in Australia with a tight fist. All of the above makes this harder to admit: I’ve been storing nuts all wrong.
The pantry, where I’ve typically always kept nuts, seems like their natural habitat—a sensical continuation of the supermarket shelf where I grabbed them. But I recently learned that it can actually be the archenemy of that roasty, sweet flavor. Nuts are full of unsaturated fats, which makes them nutritious and delicious, but that also means they become rancid quickly in warm light-and-oxygen-filled environments.
You’ve definitely eaten a rancid nut. “It’s kind of like old moldy wood,” says Mark Overbay, cofounder and president of Big Spoon Roasters, a small-batch nut butter company based in Hillsborough, North Carolina. An unspoiled nut, on the other hand, is a tiny luxury. Here’s how to keep your precious stash at peak freshness.
Why do nuts go rancid?
Early rancidity is barely perceptible; like slightly stale crackers, you’ll probably push through. Karen Schaich, ScD, a food scientist at Rutgers University, often brings rancid nut products into class for her undergraduate students to smell. When it comes to peanut butter, most can’t even perceive the musty notes. They’ll respond, “Oh, that’s just normal,” says Schaich.
But truly rank nuts can be “bitter, sour, and potentially cheesy or vomit-like,” says food scientist Topher McNeil. The science behind this unfortunate transformation is a complicated chemical reaction called oxidation. Fueled by exposure to air, light, and heat, unsaturated fats break down and release free radicals. These highly reactive molecules sound like regulars in the mosh pit and are just as annoying: They form compounds like hydroperoxides, which can speed up the breakdown of fats, as well as certain aldehydes and ketones that are responsible for all the tastes and smells we associate with rancid nuts.
Nuts “definitely have a period of peak freshness,” says Overbay. Generally, a safe shelf-life at room temperature is between 3–6 months, says McNeil. But it’s complicated: Use-by dates are determined based on a slew of external factors, such as processing and packaging. The type of nut is also important. Nuts that have a large surface area, like walnut halves, with their complex network of ridges and valleys, will go rancid faster because of a greater exposure to oxygen. And fattier nuts, like macadamia or Brazil nuts, will oxidize quicker than leaner almonds.
Is it dangerous to eat rancid nuts?
“If a nut is really oxidized, I guarantee you you’re not going to get it past your mouth because it just tastes so awful,” says Schaich. But if you do, for some reason, eating the occasional spoiled nut is “no big deal,” because we have enzymes in our stomachs that can essentially incinerate the oxidation byproducts.
