Combine the first 5 ingredients in a large bowl; beat at medium speed of a mixer for 2 minutes. Add egg whites and egg; beat 1 minute.
Combine flour, baking powder, allspice, and salt; stir well. Add flour mixture to prune mixture alternately with milk, beginning and ending with flour mixture; mix well after each addition.
Pour batter into an 8 1/2 x 4 1/2-inch loaf pan coated with cooking spray. Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Let cool in pan 15 minutes on a wire rack; remove from pan, and let cool on wire rack.