Subject: 3. What is the protein or gluten content of various flours?
Cake flour is typically 7-9% protein; pastry, or cookie, ~9-10%; all purpose, 10-12%, bread, 12.5-13.5%, clear and high gluten, 14-15%; gluten "flour" (actually refined gluten), 45%. The protein consists of ~80% gluten, and the gluten of cake flour is weakest, and bread and high gluten flour the strongest, and the intermediate ones increasingly stronger. Gluten is more of less made up of equal parts of gliadin and glutenin.
Gluten strength definition and measurement are not entirely well understood, even by cereal chemists. Generally, you want the most protein and strongest gluten for bagels and breads that also use other poor or non-gluten flours such as rye or oat; moderately strong for all wheat bread; weaker for pastries and cookies; still weaker for cakes such as pound cakes; yet still weaker for "high-ratio," rich cakes; and weakest for angel food cakes. One of the quality tests for soft wheat flour is the "cookie spread test, which is one measure of this. There are also farinographs, elastographs, and whatnot to further attempt to measure this elusive property. Fortunately, with most flours, increasing protein content goes along with increasing gluten strength.
Flour -- A Treatise: http://www.theartisan.net/Flours_One.htm
Flour Test -- Flours of America: http://www.theartisan.net/flour_test.htm
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