Many recipes for cookies and cakes call for room-temperature butter, and it's vital when you want to slather a slice of toast with it. You may keep commercially prepared butter on the counter for a time, but knowing how to store it, when to pop it back in the refrigerator, and when to throw it out will ensure that you have fresh butter available whenever you need it.
Butter can be kept unrefrigerated for several days, but conservative estimates put the shelf life of room temperature butter at one to two days, at most. The ambient temperature of the room will determine how quickly the butter spoils; keeping it at 65 degrees Fahrenheit or below is best. In addition, butter absorbs odor easily, so keep it sealed in an airtight container to preserve freshness and taste; this will also help keep bacteria away.
Unless you need softened, room temperature butter on a daily basis, keep your butter in the refrigerator to extend its life. Butter will last as long as 1 month after the sell-by date on the package if kept continuously refrigerated. To further guarantee freshness, do not store your butter in the refrigerator door, where it is subject to greater temperature variations. Rather, store it in an airtight container in the back of your refrigerator.
If you've got an excess of butter and need a long-term option, store it in the freezer. Simply wrap it in aluminum foil or seal it in a heavy-duty freezer bag and freeze for as long as 6 months if unsalted--also called sweet cream--or 9 months if salted. To thaw, set the butter in the refrigerator or on the counter, or quick-thaw it by cutting it into small pieces and leaving it on a baking sheet at room temperature.
When to Toss
Discard butter if it begins to discolor, grow mold or develops an off-odor much like cheese--or worse. In addition, spoiled butter may be difficult to cut or spread. Of course, if your butter tastes sour, bitter or off in any way, do not use it. Note that homemade butter has a considerably shorter shelf life--up to 3 weeks in the refrigerator.