The secret to refrigerated muffin batters is the double-acting baking powder. It reacts first with liquids and then with heat during baking. Because baking powder loses its ability to leaven baked goods over time, use it before the expiration date on the package.
To substitute for buttermilk, use 1 tablespoon plus 1 1/2 teaspoon vinegar or lemon juice plus milk to make 1 1/2 cups.
Substitute 1 1/2 cups orange juice for the buttermilk, and add 1 teaspoon grated orange peel to the batter.
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