Because different flours have not only different protein contents, but different protein qualities, it’s not really possible to turn all-purpose flour into pastry flour or vice versa. However, it’s possible to dilute the gluten proteins of a given flour by the addi
tion of cornstarch or another pure starch, or strengthen them by adding powdered vital gluten. To approximate pastry flour with all-purpose, add one part by weight of starch to two parts of all-purpose flour; to approximate all-purpose flour with pastry flour, add one-quarter part of gluten to two parts of pastry flour. (Purified gluten loses a little less than half its strength in the drying process.) With its chlorine-altered starch and fats, cake flour is inimitable.