Weigh or lightly spoon flours into dry measuring cups; level
with a knife. Combine flours and next 4 ingredients (through
salt) in a medium bowl, stirring with a whisk. Combine
buttermilk, 1/4 cup juice, oil, and 1 egg in a small bowl,
stirring with a whisk. Add buttermilk mixture to flour mixture,
stirring just until moist; let stand 15 minutes. Place egg white in
a medium bowl; beat with a whisk until medium peaks form.
Gently fold egg white into batter.