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.