Most manufacturers recommend storing margarine tubs in the refrigerator promptly after use. If it is not refrigerated, soft margarine loses its shape, flavor and texture and eventually becomes rancid.
Margarine contains polyunsaturated and monounsaturated fats that contribute to maintaining its shape, flavor and texture. These fats oxidize if the margarine is kept outside the refrigerator. When these fats oxidize, they become liquid, and if the margarine is left at room temperature for an extended period of time, it becomes stale. It is not easy to determine how long margarine can be kept outside the refrigerator without affecting its quality, as many factors contribute to the oxidation process.