1/2 cup whole wheat flour
Preheat the oven to 350 degrees F. Coat an 8 1/2- by 4 1/2-inch loaf pan with baking spray. Whisk together the flour, baking powder and salt in a medium bowl. Put the sugar and lemon zest in another bowl and rub the lemon zest into the sugar with your fingers. Add the yogurt, milk, olive oil, vanilla, egg whites and whole egg and vigorously whisk until well blended. Add the flour mixture into the egg mixture and fold until just incorporated. Transfer to the prepared pan. Bake until a cake tester inserted in the center comes out clean, about 50 minutes. Cool in the pan on a wire rack for 5 minutes, and then un-mold and cool to room temperature.