After using your powdered buttermilk, make sure to place it in an airtight container before putting it away. You can usually use the container it came in, as that's generally resealable, but you sometimes may need to transfer the buttermilk to a Tupperware container or something similar. Powdered buttermilk lasts longer if it's kept away from heat and light. Find a cooler place in your kitchen, like the back of a pantry that's far away from the oven, to store your buttermilk. When powdered buttermilk is expired, it will be discolored and give off a strong sour odor. Discard powdered buttermilk that's no longer good. In the original container, it should last for two years. However, if you transfer buttermilk to an airtight plastic container it can last up to 10 years.

Summary:
Keep your powdered buttermilk in an airtight container. Store your powdered buttermilk in a cool, dry place. Throw it out when it looks discolored or smells bad. Store buttermilk in a plastic container for a longer shelf life.