Kinds of flour

Most flours available are labeled according to purpose and usually do not include information about the type of wheat they contain. Flour compositions tend to be blends and can vary from region to region. For example, "all-purpose" flour in the South and Pacific Northwest tends to have lower protein content than flour in other parts of the US and Canada. For this reason, recipes developed in the South turn differently when prepared in other regions unless care is taken to approximate the original. Still, here are some guidelines:

• Whole wheat flours are high in protein; with a high concentration of that protein steming from the germ and aleurone layer, which do not form gluten well. Therefore, they make flavorful but dense bread.
• Bread flours are high in strong gluten proteins. They give the lightest, highest, chewiest loaf breads.
• Pastry flours contain low levels of weak gluten protein. They make tender baked goods.
• Cake flour is soft, low-protein, finely-milled, and strongly bleached with chlorine dioxide or chlorine gas. This treatment enhances the starch granules' ability to absorb water and swell in high-sugar batters. It also helps fat bind more readily to the starch granules' surface, which helps disperse the fat more evenly. Lastly, chlorination also gives the flour an acidic pH. Packaged cake mixes have a high sugar to flour ratio; sugar can outweigh flour by as much as 40%. This allows cakes to have a distinctively light and moist texture.
• "Self-rising" flours contain baking powder (1 1/2 tsp baking powder per cup) and therfore do not require added leavening.
• "Instant" flours are low protein flours whose starch granules have been precooked and dried. Water is more able to penetrate the granules during cooking, such that they are well suited for tender pastries and last-minute thickening of sauces and gravies.  

It is almost impossible to substitute one type of flour for another due to the different protein contents and qualities of specific flours. However, a baker can attempt to dilute gluten proteins and add starch to produce the following:

• For pastry flour, add one part by weight of starch to two parts of all-purpose flour.
• For all purpose flour, add one-quarter part of gluten to two parts pastry flour.

Please note that cake flour is inimitable due to its chlorine-altered starch and fats.