The answer is yes, there are no preservatives in suet, so over time the fat will harden and become rancid. Suet goes bad quickly when exposed to air or heat because of its high-fat content. It can also develop mold on the surface, which has a distinctive smell. When storing suet, you should keep it in a container or tightly wrapped and sealed. To prevent suet from going bad, store it in the freezer or refrigerator.