Combine yeast and water and let sit 5 minutes. Using a food processor or blender, puree beans and 1 cup water until smooth. Combine bean puree, skim milk, margarine, and sugar in a large bowl. Add in yeast, 4 1/2 cups all-purpose flour, whole wheat flour, and salt. Add in enough of the rest of the flour to form a soft dough.
Knead dough until elastic, about 5 minutes. Lay dough in a greased bowl and let rise, covered in a warm place 1 hour, dough should be doubled in size. Punch down dough.
Separate dough into 3 even portions. Form each piece into a loaf and place on a greased cookie sheet. Let rise, covered, 45 minutes.
Coat loaves with skim milk and bake at 375 degrees F for 1 hour. Let cool before slicing.