Buttermilk, like other types of milk, can last up to 2 weeks in the refrigerator. Buttermilk will keep fresh longer the cooler it is, so store it in the coolest part of the refrigerator.
Buttermilk is produced when lactic acid bacteria are added to fresh skim or part-skim milk. It was once made as a byproduct of butter-making. This tangy milk product is typically used in baking and cooking, and some people even drink it. Buttermilk adds moisture and tenderness to baked goods. It can also be used to leaven bread, and it is often preferred by bakers to baking powder.