Bowl of walnuts
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It’s pretty hard to find a snack food that’s unquestionably healthy, yet still binge-worthy. Nuts probably come as close as anything could. High protein? Check. Lots of fiber? Check. Numerous, well-documented health benefits? Check. All the rich, crunchy, fatty, salty, tasty goodness you could possibly want? You bet. The only real flaw with nuts as a snack food is that they’re prone to spoilage, and rancid nuts are nobody’s idea of a good time.

What Happens When Nuts Go Rancid?

Nuts are a high-fat food, which is both a good thing and a bad thing. It’s good because the oils you find in most nuts are primarily the healthy, unsaturated kind. These are what dietitians and doctors mean when they speak of “good fats,” because they’re heart-healthy and provide an excellent way to get fat-soluble vitamins into your digestive system.

The bad news is also that the oils you find in most nuts are primarily the healthy, unsaturated kind, which means they’re quite perishable. Given the opportunity, their oil molecules will bond with stray oxygen atoms and become “oxygenized,” which gives them unpleasant flavors and odors. Saturated, solid fats – the kind you find in animal fat and some tropical oils – don’t do that nearly as easily.

In practical terms, nuts that have turned rancid don’t just taste bad. They’ve also gone through chemical changes that can have unpleasant effects on your health if you eat them regularly. Eating a handful of rancid almonds won’t send you to emergency, but eating old almonds or other nuts over a period of years can eventually cause problems.

Recognizing Rancid Nuts

You can’t see or smell or taste hazards like salmonella or E. coli, but rancidity is relatively easy to recognize. When the oils in your nuts oxidize, they create a range of spoilage molecules with funky names – pentanal, 2-ethyl furan, hexanal, trans-2-octenal – and even funkier smells.

If you open a container of nuts and take a sniff, it should smell pleasantly nutty. If your first impression includes odors similar to paint, nail polish remover or really old plastic containers, that’s your nose’s way of telling you that your nuts have gone rancid. If the nuts are flavored in some way, with spices or smoke or a crunchy coating, it can mask the smell but usually won’t hide it completely.

If you’re uncertain whether you smell rancidity or just something weird in the way the nuts are seasoned, go ahead and try one. If it’s rancid, it’ll taste nasty. Just spit it out and discard the rest of the package.

Don’t Forget Mold

Rancidity isn’t the only thing you need to watch for. Mold is another common problem with nuts. You’ll see it more often in fresh, unprocessed nuts than in the commercial, packaged varieties, but it can show up in processed nuts too. On plain varieties of nuts, you’ll recognize mold pretty easily because it makes a powdery coating on them. If you’re buying seasoned nuts that are meant to have a powdery coating, it’s a bit harder. Trust your nose, instead, to recognize that distinctively sharp mold smell.

Don’t taste nuts you think are moldy. Some molds produce potent toxins, and even a bit or two might make you sick.

Some Nuts Are More Vulnerable to Spoilage

As with any other food, you’ll find that some nuts spoil more quickly than others. Usually, the longer-lasting nuts have higher overall fat content – and therefore, a lower percentage of the quick-spoiling fats – and often thicker skins that offer more protection to the nut inside.

Shelf life depends partly on the form your nuts take, too. The more intact the nut, the less of it that’s been exposed to oxygen. Out-of-date ground almonds will spoil more quickly than slivered almonds, which will spoil more quickly than whole almonds.

Proper Storage Guidelines

It’s always best to store nuts in a cool, dark place. If they’re in airtight packaging, store them that way. Manufacturers add antioxidants and other preservatives to the nuts and their packaging, and the packaging itself can block out oxygen, so that’s how they’ll keep best. If you bought your nuts in bulk, transfer them to airtight containers or jars as soon as possible.

Most whole nuts are good for 3 to 6 months in your pantry, and a few can last longer. In your fridge, they’re generally good for up to a year because refrigeration slows oxidative damage pretty dramatically. For really long-term storage, or for sliced and ground nuts, your freezer is the best bet. In a vacuum-sealed bag, most nuts will last up to two years before they begin to lose quality.