Lightly grease a 9" X 5" loaf pan. In a large bowl, combine the sorghum and amaranth flours, cornmeal, quinoa flour, tapioca starch, brown sugar, xanthan gum, yeast & salt. Mix well and set aside. In a separate bowl, using a heavy duty electric mixer with paddle attachment, combine the eggs, egg white, water, oil and vinegar until well blended. With the mixer on it's lowest speed, slowly add the dry ingredients until combined. Stop the machine and scrape the bottom and sides of the bowl with a spatula. With the mixer on medium speed, beat for 4 minutes. Pour the batter into the prepared loaf pan. Let rise, uncovered, in a warm, draft-free place for 60 to 75 minutes, or until dough has risen to the top of the pan. Meanwhile, preheat the oven to 350 degrees F (175 degrees C). Bake in the center of the oven for 35 to 45 minutes, or until loaf sounds hollow when tapped on the bottom. Remove from pan immediately and let loaf cool completely on a wire rack before slicing.