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.

Flour

The qualities of flour are determined by the type of wheat (or grain) used and the method of milling.

Milling is the process by which the wheat kernel is broken down and sifted into small particles. Refined flour is sieved to remove the germ and bran layers from the endosperm of the grain. Bran and germ are rich in nutrients and flavor, but they spoil quickly and interfere with the formation of continuous, strong gluten. In conventional milling, grooved metal rollers shear open the grain, squeeze out the germ, and scrape the endosperm away to be ground, sieved, and reground until it attains a specific particle size. Stone grinding is not as widely used, but it crushes the whole grain more thoroughly before sieving, which allows some of the germ and bran to end up in the refined flour. This makes stone ground flour more nutritious and flavorful, but gives it a shorter shelf life.

Freshly milled flour makes weak gluten and dense loaves. Flour needs to be aged to improve its baking qualities. As it ages, flour becomes exposed to oxygen which enhances its ability to make long gluten chains. Oxygen frees sulfur groups at the end of glutenin proteins that allows them to better react with each other. Manufacturers have supplemented flour with oxidizing agents such as ascorbic acid and azodicarbonamide to speed this process. Traditional air-aging also had the secondary effect of lightening the flour, rendering it whiter as yellow xanthophyll pigments oxidized to colorless compounds. To obtain similar whiteness, bleaching flour with azodicarbonamide and peroxide has become common practice in the United States. However, there have been concerns about this chemical alteration. Hence, bleaching is not allowed in Europe.

Gluten proteins and starch account for 90% of flour weight, but there are two minor components that have important effects, fats and enzymes. Fat accounts for only 1% of flour but it is essential to the development of raised bread. Fats help stabilize the bubble walls, help soften the bread structure, and slow staling. Enzymes break down starch to simple sugars that are more easily digested by yeast. Manufacturers have started to add enzymes extracted and purified from microscopic molds ("fungal amylase") to increase enzymatic activity in a predictable pattern.

Bread 101

What qualifies as bread? On the most basic level, bread is the result from cooking a mixture of milled grains and water. Here is a quick run down of the most common ingredients in bread:

• Flour - Wheat flour is most commonly used in raised bread because it contains two proteins, glutenin and gliadin, which form gluten when combined with water. As the baker kneads the dough, the gluten develops and becomes elastic. This elasticity allows the incorporation of carbon dioxide gas into the dough.
• Starch is a carbohydrate that makes up 70% of the flour by weight. Starch granules release sugars that the yeast feed on. Starch reinforces gluten and absorbs water during baking, helping the gluten contain the carbon dioxide.  
• Water is the most important liquid in bread. It dissolves and activates yeast and blends with the flour to create gluten.  
• Yeast is a live, single-cell fungus that begins feeding on the sugars in flour and releases the carbon dioxide that makes bread rise. Yeast also adds many of the flavors and aromas associated with bread.
• Baking powder and baking soda are chemical leavenings that participate in the reactions between acidic and alkaline compounds that produce the carbon dioxide necessary to inflate dough or batter. These chemical leavenings act much faster than yeast, and are best used in quick breads.
• Salt slows rising time, which allows the flavor of the dough to develop. Salt also adds structure to the dough by strengthening the gluten, which keeps the carbon dioxide bubbles from expanding too rapidly.
• Eggs add food value, color, and flavor. They also make the crumb fine and the crust tender. Eggs add richness and protein.
• Fat in the form of butter, margarine, shortening or oil add flavor and moisture to bread. Fat slows moisture loss and helps bread stay fresh longer.
• Other liquids such as milk, buttermilk, cream or juice may be added for flavor or to enhance texture. Only add warm liquids to dry ingredients. Too cool liquids slow or stop yeast action. Too hot liquids destroy the yeast and prevent bread from rising.
• Sweetners such as sugar, brown sugar, honey, molasses, jams, and dried fruits may be used to add flavor and color to the crust.