1. In a large bowl, mix flour, sugar, baking soda, and salt. In a medium bowl, whisk buttermilk, egg yolks, and lemon peel to blend. Stir buttermilk mixture into flour mixture just until evenly moistened. Gently stir in ricotta cheese.
2. In a bowl, with a mixer on high speed, beat the egg whites until soft peaks form. With a flexible spatula, gently fold the whites into the batter just until they are incorporated.
3. Place a nonstick griddle or a 12-inch nonstick frying pan over medium heat (350°); when hot, coat lightly with oil and adjust heat to maintain temperature. Drop batter in 1/2-cup portions onto the griddle and cook until pancakes are browned on the bottom and edges begin to look dry, about 2 minutes; turn cakes with a wide spatula and brown other sides, 1 1/2 to 2 minutes longer. Coat pan with more oil as necessary to cook remaining pancakes.
4. Serve pancakes as cooked, or keep warm in a single layer on baking sheets in a 200° oven for up to 15 minutes.