In large bowl, beat shortening with electric mixer on medium speed 30 seconds. Gradually add granulated sugar, about 1/3 cup at a time, beating after each addition and scraping bowl occasionally. Beat 2 minutes longer. Add egg whites, one at a time, beating well after each addition. Beat in lime peel and vanilla. On low speed, alternately add flour mixture, about 1/3 at a time, with milk, about 1/2 at a time, beating just until blended. Stir in lime juice. Divide batter evenly among muffin cups, fill each about 2/3 full.