Why do nuts and grains go bad?

Even when stored as directed — in cool, dry places — your canister of macadamia nuts or container of rice can go bad. While not as obvious or off-putting as a moldy tomato or a blueish cut of meat left too long in a refrigerator, grains and nuts do expire. But why does this happen? 

Just like other foods, these groups are made of fat, carbohydrate and protein molecules. Over time, these macronutrients mix with each other and their surroundings, changing their taste, texture and your appetite for them.

It's important to note that "going bad" can mean a lot of things. Obviously, nuts and grains can spoil. If not properly stored, these foods can be contaminated by mold or yeast, according to Julien Delarue, a sensory and consumer scientist at the University of California, Davis. But grains and nuts can also expire — meaning they lose their desirable sensory properties over time, Delarue said. 

Related: Why aren't peanuts, pecans and almonds real nuts?

For nuts and nut-like foods, it's the fat content that leads them down the road to expiration. Nuts have a high polyunsaturated fatty acid, or PUFA, content. That's one reason you might include them in a healthy diet because PUFA are important for brain function and heart health, according to the Centers for Disease Control and Prevention (CDC). But PUFAs are especially sensitive to a process called oxidation, in which oxygen effectively breaks double bonds in the molecule. As oxidation changes the structure of the fats, it also alters the smell and taste. And once the oxidation process has started, it can spread through a bag or jar pretty rapidly, according to Delarue. Walnuts have the highest content of PUFAs, so your best bet is to store them in the refrigerator — or the freezer if you plan to have them for more than a month — to keep oxidation at bay, according to a blog post from the University of Florida's Institute of Food and Agricultural Sciences. 

You'll be able to recognize the nuts that have gone bad by their smell, Delarue told Live Science. Everyone has different sensitivities to sensory properties, though. "Unfortunately I am part of the sensitive group" that picks up on even subtle whiffs of rancid, oxidized nuts, Delarue said. If you aren't quite as sensitive to the smell and don't want to risk a tiny taste, the expiration label is there to help, Delarue said. 

Eating oxidized foods is never recommended, because oxidation is related to many diseases, such as cancer and heart disease. But if expired nuts are stored properly, meaning there's nothing hazardous, such as mold growing on them, then occasionally eating them shouldn’t be harmful to your health, as it's a relatively small exposure, Delarue said. It just might be unpleasant. 

Grains, such as quinoa and oats, can last much longer than nuts, Delarue said. But they will still change over time thanks to what's known as staling. "Stale is a generic word meaning there's an effect on the texture," Delarue said. 

In the case of grains, staling is caused by molecular reorganization in the starch and proteins, namely gluten, that make up grains. The starch and gluten molecules "rearrange slightly and bind to each other, and make the texture more coarse or harder," he said. This added rigidity means that the grains hydrate or gelatinize — the process that makes them soft, delicious and digestible — less efficiently.

This is why in much of Asia, consumers are mindful of the harvest year when they purchase rice, Delarue said. Fresher rice is preferred because it cooks most efficiently. And if these consumers do have older rice, they adjust the cooking methods to account for the fact that the rice is not as fresh. 

However, there's really no harm in grains that have gone a little stale. "Grains can be stored for years as long as they are stored in proper dry conditions and cool temperatures. So, don't waste your food," Delarue said. 

Originally published on Live Science.