To keep muffin batter from turning purple, toss frozen blueberries with flour before adding them to mix. To add more nutrients and fiber to baked goods, substitute half of the all-purpose flour called for whole-wheat flour. If you don’t have buttermilk on hand, you can use 1 tablespoon of white vinegar or lemon juice plus enough milk to measure 1 cup. Stir and let stand for 5 minutes. You can also use 1 cup of plain yogurt or 1-3/4 teaspoons cream of tartar plus 1 cup milk.