Combine flour and undissolved yeast, In large plastic, ceramic or glass bowl (don’t use metal). Gradually add warm water to flour mixture and beat until smooth. (Batter may be stiff, but will relax as it ages or when brought back to room temperature after refrigeration.) Cover with a cloth towel or cheesecloth; let stand in warm place until mixture is bubbly and sour-smelling, about 2 to 4 days. Starter may darken, but if it changes to another color, discard and start over. Transfer to a 2-quart or larger plastic container with tight-fitting lid. Refrigerate until ready to use.
To keep starter alive: Once a week, stir in equal amounts of all-purpose flour and warm water (100° to 110°F). Beat until smooth. Cover with a cloth towel or cheesecloth and let stand in warm place until mixture is bubbly and sour-smelling, about 12 to 24 hours. Use immediately or cover tightly and refrigerate until ready to use.
To replenish the starter: Add equal amounts of all-purpose flour and water (100° to 110°F) to the remaining starter. (For example, for every 1 cup of starter removed, replenish with 1 cup flour and 1 cup water.) Beat until smooth. Cover with a cloth towel or cheesecloth and let stand until bubbly, 12 to 24 hours. Use immediately or cover tightly and refrigerate until ready to use.
Tip: To achieve a stronger sourdough flavor in recipe, allow sourdough starter to ferment 4 to 6 weeks.
Variation: For a whole wheat starter, substitute the all-purpose flour with 100% whole wheat or a 50-50 blend of all-purpose and whole wheat flour. Starter will appear beige in color and have a brown layer of liquid instead of clear. Please note that the 50-50 blend will provide more sourdough flavor in the finished product than the 100% whole wheat starter.