If walnut kernels are wrinkled and dry but have no unpleasant odor or bitter taste, they are generally considered safe to eat; wrinkles usually result from moisture loss and are not by themselves a sign of spoilage. The main thing to check is whether the taste has become bitter.
Why do walnut kernels wrinkle and dry out?
After walnuts are removed from their shells, their natural moisture slowly evaporates as they come into contact with air, causing the kernels to shrink and become wrinkled over time. This moisture loss happens faster in a warm, dry place, while kernels may stay plumper longer in a cool, humid place. Wrinkling alone does not mean walnuts are inedible; many dry walnuts are wrinkled even though their flavor and smell have not changed. The main concern is the oil inside the walnut oxidizing and turning rancid, a process that can occur with or without wrinkling.
How can you tell whether they are safe to eat?
Split a walnut kernel into 2 parts and smell it. Fresh walnuts have a mildly sweet, slightly woody aroma, while rancid walnuts may have a sharp odor resembling paint or old oil. If the smell is noticeably unpleasant, discard it without tasting, because rancid oil usually tastes bitter and sharp as well. If the smell seems normal, you can taste a small piece; if it has an unpleasant, metallic, or sharply bitter taste, inspect the rest of the package because it may be in the same condition. If there are visible mold spots, black specks, or a powder-like coating, discard the walnuts without smelling or tasting them; these signs mean they are no longer safe to eat.
Does the same apply to other nuts?
Almonds and hazelnuts can also turn rancid because they are high in fat, but the process is slower than with walnut kernels when they are stored in their shells; the shell offers some protection from air. Rancidity is uncommon in lower-fat nuts such as pistachios, and wrinkling is less noticeable. This general pattern applies to all nuts in shells, but each type spoils at its own rate. Storing walnuts in an airtight container in the refrigerator or freezer greatly slows both wrinkling and rancidity. Walnut kernels left exposed at room temperature may dry out and lose flavor noticeably within a few weeks. Buying walnuts in the shell slows the process considerably compared with buying shelled kernels; as long as the shell remains intact, the oil inside is not exposed to air. Divide bulk walnuts into small portions and leave out only the amount you plan to use so the rest stays fresh longer. Walnut kernels that are wrinkled but smell and taste normal can be used in cooked dishes or baked goods, where the wrinkles become less noticeable during cooking.