Heat oven to 350F. Beat together margarine, sugar, and salt. Add egg yolk, lemon juice, lemon peel and cinnamon. Stir in sweet potatoes and milk. Mix well.
Fold in egg whites; pour into pie shell. Bake for 40 to 50 minutes or until knife inserted in center comes out clean. To reduce saturated fat, use a homemade pie shell made with vegetable oil.