Can Butter be Softened or Melted in the Microwave Oven?
 
Yes. To soften butter, microwave one stick of butter on LOW for 1 to
1 1/2minutes. To melt, place butter in a glass or plastic cup
and microwave on HIGH for about 1 minute per stick (1/2 cup).
 
How Can I Prevent Butter from Going Rancid?
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Properly storing butter in the refrigerator or freezer and tightly
wrapping butter to minimize its exposure to air help keep butter
from going rancid. Exposure to oxygen increases the risk of
rancidity. When oxygen comes in contact with the unsaturated
fatty acids in butter, off-smelling and off-tasting compounds can
develop. Rancid butter is not unsafe to eat.
 
Source: www.dairyfarmingtoday.org