In a covered large bowl, let egg whites stand at room temperature for 30 minutes. Sift the powdered sugar and flour together three times; set aside. Preheat oven to 350 degrees F.
Add cream of tartar to egg whites in bowl; beat with an electric mixer on medium speed until soft peaks form (tips curl). Add juice concentrate. Gradually add granulated sugar, about 2 tablespoons at a time, beating until stiff peaks form (tips stand straight).
Sift about one-fourth of the flour mixture over beaten egg white mixture; fold in gently. (If the bowl is too full, transfer mixture to a larger bowl.) Repeat, folding in the remaining flour mixture by fourths.
Pour batter into an ungreased 10-inch tube pan; spread evenly. Bake on the lowest oven rack for 40 to 45 minutes or until the top springs back when lightly touched. Immediately invert cake (leave in pan); cool thoroughly. (If the cake has risen higher than the pan, invert it over a jar or bottle so the top of the cake does not touch the countertop.)
To serve, loosen side and center from pan with a narrow spatula; remove cake. Using a serrated knife, cut into wedges.