Dissolve yeast in 1/2 cup warm water in a large mixing bowl; let stand 5 minutes. Stir in molasses, salt, caraway seeds, shortening, 1 3/4 cups water, rye flour, and whole wheat flour. Beat at medium speed of an electric mixer until smooth. Gradually stir in enough all-purpose flour to make a soft dough.

Turn dough out onto a floured surface, and knead until smooth and elastic (about 8 to 10 minutes). Place in a well greased bowl, turning to grease top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in bulk.

Punch dough down; roll dough into a 19- x 16-inch rectangle. Roll dough jellyroll fashion, starting at long side; pinch seam and ends to seal. Place loaf, seam side down, on a greased baking sheet. Cover and let rise in a warm place, free from drafts, 1 hour or until doubled in bulk.

Brush loaf with egg white. Bake at 400°F for 35 to 40 minutes or until loaf sounds hollow when tapped. Let cool on wire rack.

Marble Bread: Prepare dough for Whole Wheat-Rye Bread and for Basic White Bread, letting both doughs rise the first time.

Punch whole wheat-rye dough down; divide into 2 equal portions. Place each portion onto a lightly floured surface. Cover and let rest 15 minutes. Repeat procedure with white dough.

Combine 1 portion whole wheat-rye dough and 1 portion white dough; lightly knead 12 to 15 times.

Roll combined dough into a 19- x 16inch rectangle. Roll dough jellyroll fashion, starting at long side; pinch seam and ends to seal. Place loaf, seam side down, on a greased baking sheet. Repeat procedure with remaining dough. Cover and let rise in a warm place, free from drafts, 1 hour or until doubled in bulk.

Brush each loaf with half of a slightly beaten egg white. Bake at 400° for 35 to 40 minutes or until loaves sound hollow when tapped. Let cool on wire racks.