How Can You Tell if Walnuts, Hazelnuts, or Almonds Have Gone Rancid?

In oily nuts such as walnuts, hazelnuts, and almonds, spoilage usually takes the form of rancidity rather than mold. The oils inside oxidize when exposed to air, light, and heat. This process is generally not immediately visible; it first affects the smell and taste. What signs should you look for in the smell and taste? Fresh walnuts, hazelnuts, and almonds have a mildly sweet, oily aroma. Rancid nuts develop a sharp smell similar to paint thinner or old cardboard. An unexpected bitterness or sharp taste that lingers on the tongue may be the clearest sign, even before the smell changes; cooking or adding the nuts to a dessert will not hide this taste. What changes in appearance should you look for? A walnut's inner skin becoming darker brown than usual, an oil stain at the tip of an almond, or a hazelnut's surface becoming dull and matte can indicate that oxidation is progressing. A white or greenish layer of mold on nuts stored in a damp place is a separate and more serious type of spoilage; in this case, there is also a risk from mold and the toxins it produces. Do not smell or taste moldy nuts; discard them. Do shelled and unshelled nuts spoil differently? Walnuts and hazelnuts sold in their shells become rancid much more slowly than shelled nuts because the shell protects the oil from air and light. Unshelled walnuts can stay fresh for up to 6 months in a cool, dry place, while shelled walnuts may start to become rancid within a few weeks at room temperature. For this reason, storing nuts bought in bulk in their shells is a practical way to preserve freshness. Storing shelled nuts in a covered container in the refrigerator considerably slows rancidity and extends their shelf life to a few months; in the freezer, it may extend to a year. Eating a rancid nut generally does not cause serious food poisoning, but oxidized fats may be harmful to health, so a batch with a sharp, unpleasant smell should not be eaten and should be discarded. Roasted nuts become rancid faster than raw nuts because roasting brings the oil to the surface and increases its exposure to air. For this reason, a large package of roasted hazelnuts or almonds should be used soon after opening, or the remainder should be stored in the refrigerator. Roasted nuts left at room temperature may start to become rancid within a few weeks. In packages of mixed nuts, rancidity in one type can affect the smell of the others. If the package has a rancid smell, it is safer to assess the entire package and remove any suspect nuts rather than trying to identify which nut is the source. Checking the production date when buying nuts is a simple but effective way to choose the freshest package on the shelf. Mold may grow on nuts kept in a damp place for a long time and can lead to a mold toxin called aflatoxin, especially in peanuts and almonds. This toxin is heat-stable and is not destroyed by cooking. For this reason, a batch of nuts that smells damp or moldy as well as rancid should not be eaten.