3.9 ounces potato starch (about 3/4 cup)
3.45 ounces brown rice flour (about 3/4 cup)
2.1 ounces sweet white sorghum flour (about 1/2 cup)
1.05 ounces tapioca flour (about 1/4 cup)
1/3 cup sugar
1 teaspoon xanthan gum
1 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1/4 cup chilled butter, cut into small pieces
1 1/3 cup low-fat buttermilk
1 teaspoon grated orange rind
1 large egg
1/2 cup dried currants
Preheat oven to 350°.
Weigh or lightly spoon potato starch and flours into dry measuring cups; level with a knife. Combine potato starch, flours, sugar, and next 4 ingredients (through salt) in a large bowl, stirring with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal.
Combine buttermilk, orange rind, and egg in a medium bowl, stirring with a whisk. Add egg mixture to flour mixture; stir just until moist. Gently fold in currants.
Pour batter into a 9 x 5-inch loaf pan coated with cooking spray. Bake at 350° for 55 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.