Using an electric beater, combine sugar with prune puree, margarine, and almond and vanilla extracts, beating for 2 minutes; beat in egg whites and egg for 1 minute.
Mix together flour, baking powder, allspice, and salt in a bowl. Beginning and ending with flour mixture, add into prune mixture alternately with milk, fully combining after each addition.
Spoon batter into a cooking spray coated 8 1/2 x 4 1/2-inch loaf pan. Bake at 350 degrees F for 1 hour or until a toothpick comes out clean. Let cool in the pan for 15 minutes and then turn out and let cool on a wire rack.