In a large sifter, combine flour, sugar, cornstarch , baking powder and salt. Sift, in batches, into a large bowl. Use a pastry blender to a heavy-duty mixer to blend in shortening until mixture resembles cornmeal in texture. Spoon into a 20-cup container with a tight-fitting lid. Seal container. Label with date and contents.

Store in a cool, dry place. Use within 10 to 12 weeks. Makes about 17 cups of mix.