Stir the yeast into the warm water in a large bowl & let proof for 10 minutes. Add the rest of the water & the flour 1 c at a time, mixing vigorously after each addition. Continue to mix for about 6 minutes. The mixture will clear the sides of the bowl but remain soft & sticky. Cover with plastic wrap & leave to rise at room temperature for 24 hours. The starter will triple in volume & then collapse a little. Cover & refrigerate until ready to use. To use the biga, scoop out the needed amount while it is still cold & hasn't begun to grow & expand at room temperature. Note: Ensure that you use golden durum flour & not durum integrale which contains the whole husk of the wheat berry & is not right for this recipe.