Dissolve yeast in warm water in large glass bowl. Stir in flour and sugar. Beat with electric mixer on medium speed about 1 minute or until smooth. Cover loosely; let stand at room temperature about 1 week or until mixture is bubbly and has a sour aroma. Transfer to 2-quart or larger nonmetal bowl with tight-fitting lid. Refrigerate until ready to use.
Care of Sourdough Starter: Use Sourdough Starter once a week or stir in 1 teaspoon sugar. After using starter, replenish it by stirring in 3/4 cup Gold Medal® all-purpose flour, 3/4 cup water and 1 teaspoon sugar until smooth. Cover loosely; let stand in warm place at least 1 day until bubbly. Cover tightly; refrigerate until ready to use.