Better to Steer Clear
Perhaps you’ve used an old olive oil or eaten a nut that tasted completely and utterly disgusting. That item had likely become rancid. When fatty or oil-containing foods decompose, the result is often unpleasant odors or flavors. Light, air, moisture, and outside bacteria can hasten this process. Unsaturated fats are more prone to rancidity, which means that fish, poultry, and game birds will become rancid before meats that are higher in saturated fats, such as beef.
Nuts, especially walnuts, pecans, cashews, and peanuts, are also high in unsaturated fats and therefore particularly susceptible. It’s best to store any of these products at cool temperatures and in opaque and airtight containers or wrapping. (See the Directory for more specific storage suggestions.) Though they will rarely make you sick, rancid foods are something you’re just plain not going to want to eat because of their bitter taste or off-smell.