Preheat the oven to 325 degrees.

Butter and flour the inside surface of a standard loaf pan. Set aside.

In medium bowl, sift together the flours, baking soda, baking powder and salt. Set aside.

In a large bowl, and using an electric mixer on high speed, whip together the sugar and butter until light and fluffy, about 5 minutes.

Mix in the eggs one at a time until each one is well incorporated.

Mix in the vanilla and lemon zest. On low speed, mix in half the flour, then half the sour cream. Alternate between the two until all of the flour and sour cream have been mised into the batter. Be sure to frequently scrape down the sides with a rubber spatula to ensure that all of the batter gets evenly incorporated.

Pour the batter into the loaf pan. It will be a little thick so be sure to use a rubber spatula to get any remaining batter from the bowl and to spread it evenly in the pan.

Place the pan in the oven on the center rack.

Bake for approximately 1 1/2 hours, or until a toothpick placed in the center comes out clean.