Grandma Christensen's Blueberry Muffins

2-1/2 cups sifted flour
1/2 cup sugar
2-1/2 teaspoons baking powder
1/2 teaspoon salt
2 egg yolks, slightly beaten
1/2 cup cooking oil
1 cup milk
2 egg whites, stiffly beaten
1 cup fresh or frozen blueberries

Sift flour once, measure, add baking powder, sugar, and salt and sift again. Combine egg yolk, and milk, then add oil. Add to flour mixture. Mix only enough to dampen flour. Fold in egg whites and blueberries. Bake in greased or paper lined muffin pans in hot oven at 425 degrees for 25 minutes, or until done. Makes 12 large muffins.