In a 4-quart nonmetallic container, dissolve yeast in warm water; let stand 5 minutes. Add whole wheat flour and sugar. Stir until blended. The mixture will be thick; any remaining lumps will dissolve during the fermentation process.
Cover loosely with plastic wrap. Let stand in warm place for 5 days, stirring 2 to 3 times each day. The starter will rise and fall during the fermentation period; it becomes thinner as it stands. A temperature of 80° to 85°F is best for developing the sour flavor.
When the starter is developed, it is bubbly and may have a yellow liquid layer on top; stir starter before using. The starter can now be used for baking or placed in the refrigerator for later use.
To use the starter, measure out desired amount as specified in the recipe. Let refrigerated starter come to room temperature before using; this will take about 4 hours.
Replenish remaining starter with 3 parts whole wheat flour to 2 parts water and 1 teaspoon sugar. Stir until blended, some lumps may remain. Cover loosely and let stand in warm place for 10 to 12 hours or overnight. The starter will rise and become bubbly. Stir and store in refrigerator.
If the starter is not used every week, stir in 1 teaspoon sugar to keep it active.