Makes about 3 cups
Sprinkle yeast and sugar over water in large bowl; stir to dissolve. Let stand until foamy, about 5 minutes. Add flour and whisk until smooth. Cover bowl with plastic wrap. Let stand in warm draft-free area until sour aroma develops, stirring daily, 2 to 3 days. Cover tightly and refrigerate. (If not used at least once a week, pour off 1 cup starter and replenish as below. ) Bring to room temperature and stir before using.
To replenish starter: For each 1 cup used, add 1 cup warm water, 1 cup all-purpose flour and 1 tsp sugar to starter. Whisk until smooth. Cover with plastic wrap. Let stand in warm draft-free area overnight. Stir. Cover and refrigerate.