For the Sourdough Bread, combine the warm water and fresh yeast in the bowl of a stand mixer. Stir to dissolve the yeast fully. Add the Sourdough Starter, flours and salt. (NOTE: If refrigerated, bring Sourdough Starter to room temperature before using.) Mix on low speed until the dough is fully developed. Remove the dough from the mixing bowl. Roll the dough into a smooth ball and place in a medium mixing bowl lightly brushed with olive oil. Cover and refrigerate for 16 hours. Remove the bowl from the refrigerator and ferment at room temperature for 6 hours.
Divide the dough into 2 equal pieces and form into loaves. Places the loaves on the counter or in a proofing basket and cover with a warm, damp cloth. Proof the loaves at room temperature for 2 hours. Preheat the oven to 400°F Continue proofing the loaves at room temperature for 1 hour.
Score the loaves with a sharp knife, spray with water and bake for 30 to 40 minutes, or until the crusts are a deep golden brown and the middle of the loaves is 190-200°.
Remove the bread from the oven and place on a cooling rack for 30 minutes. If the bread was baked in loaf pans, remove the bread from the pans before cooling.
Note: For more intense sour flavor, extend the refrigeration time of the dough an additional 6 to 8 hours.
For the Sourdough Starter, Stage 1: Combine the warm water and fresh yeast in a medium mixing bowl. Stir to dissolve the yeast fully. Add the buttermilk, plain yogurt, all-purpose flour and semolina flour to the bowl and stir until the ingredients are fully incorporated. Wrap the unwashed red grapes securely in a clean cheesecloth or small kitchen cloth and submerge the cloth in the mixture. Ferment at room temperature for 12 hours.
Stage 2: Carefully remove the cloth with the grapes from the mixture, using a spatula to scrape any remaining starter off the cloth and back into the bowl. Squeeze the cloth containing the grapes over the bowl, capturing the juice in the bowl. Discard the cloth and the grapes, and stir the starter until the grape juice is fully incorporated. Add the all-purpose flour and semolina flour to the starter. In a separate bowl, add the yeast to the warm water, stirring until the yeast is fully dissolved, then add this mixture to the starter. Mix the starter until all ingredients are fully incorporated. Ferment at room temperature for 4 hours before using, or cover and place in the refrigerator for future use.