Combine yeast and 3/4 cup warm water in a 2-cup liquid measuring cup; let stand 5 minutes.
Stir together flour and next 3 ingredients in a large bowl; cut in shortening with a pastry blender until mixture is crumbly. Stir in yeast mixture and sweet potatoes just until blended.
Turn dough out onto a lightly floured surface, and knead until smooth and elastic (about 5 minutes). Place dough in a well-greased bowl, turning to grease top. Cover and chill 8 hours, if desired.
Roll dough to 1/2-inch thickness; cut with a 2-inch round cutter. Freeze up to 1 month, if desired. Thaw biscuits; place on ungreased baking sheets. Cover and let rise in a warm place (85°), free from drafts, 20 minutes or until doubled in bulk.
Bake at 400° for 10 to 12 minutes or until lightly browned.