Combine 1/2 cup flour, yeast, sugar, and salt in a medium bowl, stirring well. Gradually add water, sour cream, and vinegar to flour mixture, beating well at low speed of an electric mixer until blended. Beat an additional 2 minutes at medium speed. Gradually stir in enough of the remaining 3/4 cup plus 2 tablespoons flour to make a soft dough.
Sprinkle 1 tablespoon flour evenly over work surface. Turn dough out onto floured surface, and knead until smooth and elastic (about 8 minutes). Cover and let rest for 10 minutes.
Punch dough down, and knead lightly 4 or 5 times. Divide dough in half.
Roll 1 portion of dough into a 4- x 5-inch rectangle.
Roll up dough, starting at long side, pressing firmly to eliminate air pockets; pinch ends to seal.
Place dough, seam side down, on a baking sheet coated with cooking spray. Repeat procedure with remaining dough. Cover and let rise in a warm place (85), free from drafts, 20 minutes or until doubled in bulk.
Gently make 3 or 4 slits, about 1/4-inch deep, diagonally across each loaf, using a sharp knife coated with cooking spray.
Combine egg white and 1 tablespoon water; brush loaves with egg white mixture, and sprinkle with sesame seeds.
Bake at 375 for 15 minutes or until loaves are golden and sound hollow when tapped.
Remove from baking sheet, and let cool on a wire rack.
