Preheat oven to 350°F. Coat 8-cup fluted cake pan with cooking spray or oil. Set aside.
In a medium bowl, combine flour and baking powder.
In a large mixing bowl, using an electric mixer on medium speed, beat together sugar and butter until light and fluffy. Beat in eggs and egg white. Mix in Orange juice, milk, sour cream or yogurt, poppy seeds and orange zest until blended.
Reduce mixer speed to low and add flour mixture, mixing until just blended. Spoon batter into prepared pan; smooth top.
Bake until top is golden brown and a toothpick inserted near the center comes out clean, about 55 minutes.
Place pan on a wire rack and cool 10 minutes. Turn cake out of pan onto rack and cool completely. If desired dust with confectioners' sugar. Garnish with red and green grapes in center.