Brazil nuts, hazelnuts, walnuts, pecans, and peanuts rattle freely in the shell if they're old and dry. When choosing nuts out of the shell, look for plump, unbroken nutmeats. Avoid those that are discolored or shriveled. Airtight containers preserve freshness and flavor, but they make it impossible to taste the nuts until you leave the store. When you are able to taste, choose nuts that are sweet and crunchy.
Rancid nuts have a bitter, unpleasantly oily taste.
A rancid nut can ruin an otherwise perfectly prepared dish, so always taste several nuts from the batch before you use them. But be careful when you taste black walnuts. Their characteristically strong, exotic flavor may seem wrong if you've never had them before. Store all nuts in a cool, dry place. Shelled nuts, unless they're vacuum-packed in cans, should be wrapped tightly and stored in the fridge or freezer.