Why do nuts and grains go bad?

For nuts, it's the fat content that leads them down the road to expiration.

Even when stored in cool, dry places, your bottle of nuts or container of rice can go bad, though not as obvious as a tomato or a bread left too long in a refrigerator.

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. In others words, either these foods can be contaminated by mold or yeast or they lose their desirable sensory properties over time.

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For nuts, 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. 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.

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.

Eating oxidised 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. It just might be unpleasant.

Grains, such as quinoa and oats, can last much longer than nuts. But they will still change over time thanks to what’s known as staling.

In the case of grains, staling is caused by molecular reorganisation 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. This added rigidity means that the grains hydrate or gelatinize — the process that makes them soft, delicious and digestible — less efficiently.

However, grains can be stored for years as long as they are stored in proper dry conditions and cool temperatures.

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