Butter should be refrigerated at or below 40°F (4.4°C) to retain its quality and to guard against rancidity. Under these conditions, it can be stored about a month. Butter that has an unpleasant smell or sour taste is likely to be rancid and should not be eaten, as the odour corresponds to a build-up of butyric acid. Opened butter cartons and unwrapped sticks of butter should be kept in a separate refrigerator compartment or in a covered butter dish to keep the butter from absorbing the odours of other foods. To soften it, remove butter from the refrigerator 30 minutes before you want to use it. Frozen butter can be stored for extended periods of time. At 20 to 30°F (–6.6 to –1.1°C), it can be kept for up to four months. At –10°F (–23.3° ), it can be kept for up to a year. To freeze it, place the packaged or wrapped butter in a plastic freezer bag, or wrap it in heavy foil. To thaw the butter, place unopened cartons or sticks in the refrigerator compartment. Thaw only enough for immediate use or to be used within one month.