Milk should be stored at a temperature below 40°F to maintain its quality and safety. By law, Grade A milk must be maintained at a temperature of 45°F or below, but temperatures well below 40°F are necessary to protect the milks quality. Ideally, milk should be stored at around 37°F, which is the best temperature for most refrigerators. Storing milk at a temperature above 40°F can cause it to develop signs of spoilage, including sour odor, off-flavor, and curdled consistency. It is important to refrigerate milk promptly after purchase and each use, and to store it in the coldest part of the refrigerator, not in the door where it will be exposed to outside air every time someone opens it. Milk should not be left out at room temperature for more than two hours, or one hour in temperatures above 90°F, as bacteria can start to grow. Pour milk to be used into a serving container and return the original container to the refrigerator. Unused milk that has been sitting out should not be returned to its original container where it could contaminate the remaining milk. Properly refrigerated, milk will last about two weeks, and the colder it is kept, the longer it lasts.