All ingredients should be at room temperature. Combine ricotta cheese and sour cream in mixing bowl. Beating slowly, add cream cheese, sugar and butter. Increase speed to medium and add eggs, flour, cornstarch, vanilla and lemon juice. Beat on highest speed possible without splattering for 5 minutes. Pour into 10 inch springform pan. Bake in preheated 350 degree oven for 1 hour. Turn off heat and leave in closed oven for 1 more hour. Cool on rack.