Puree mashed sweet potatoes, sugar and egg yolks together in a food processor or blender until smooth. Pour into a large mixing bowl. Add cinnamon, nutmeg, salt, vanilla, lemon juice, melted butter and LACTAID(R) Fat Free Milk. Mix well. In separate bowl, whip egg whites using an electric mixer until soft peaks form. Whisk egg whites into pie filling. Pour filling into prepared pie crusts.
Bake pies in middle of the oven for 60-70 minutes or until the center of the pie is lightly firm to the touch. Cool pie on wire rack.