All white flour looks more or less alike at a casual glance, so it’s sometimes difficult to understand why so many kinds line the bakery aisle at your supermarket. In most cases, the major difference lies in a flour's percentage of gluten-forming proteins. For example, bread flour is specially formulated to produce a lot of gluten, while all-purpose flour aims for a versatile, mid-range gluten level.
The Tale of the Tape
Grain farmers describe wheats as either "hard," meaning they contain lots of gluten-forming proteins, or "soft," meaning they have relatively lower levels. Millers purchase both hard and soft wheats, and blend them in specific proportions to arrive at a flour with a consistent character. In most of the United States, all-purpose flour is blended to result in a gluten level of roughly 10 to 12 percent by weight. In the South and Pacific Northwest, where softer wheats flourish, it's often as low as 8 or 9 percent. Bread flour, in contrast, weighs in at a robust 13 to 16 percent gluten.
The Tool for the Job
Each of these flours is well-suited to some forms of baking and less so for others. High-gluten bread flour makes a wonderfully light and airy loaf, with a strong crumb and a crisp crust. Unfortunately, its strong proteins make it unsuitable for cakes or delicate pastries, where gluten results in a tough, chewy crumb. Southern all-purpose flour, with its soft wheat, is the opposite. It makes good cakes and biscuits but poor bread. Ordinary all-purpose flour strikes a middle ground, making acceptable bread but remaining soft enough for other uses.