2 cups instant yeast
1 cup white sugar
1 tablespoon salt
1 tablespoon all-purpose flour
1 cup warm milk
2 teaspoons lemon zest
Mix yeast, sugar and salt; let stand about 5 minutes.
Mix milk, lemon juice and flour into 1 cup of warm milk. Gradually stir in lemon zest, stirring until well blended.
Bring 2 cups of this mixture to a boil in small bowl. Stir in lemon-lime flavored carbonated beverage, if desired. Pour into greased 6-quart or 9x13-inch baking pan.
Bake in preheated oven for 60 minutes, until very slightly browned.